Nuclear Envelope

Nuclear Envelope Definition

The nuclear envelope is a double membrane layer that separates the contents of the nucleus from the rest of the cell. It is found in both animal and plant cells. A cell has many jobs, such as building proteins, converting molecules into energy, and removing waste products. The nuclear envelope protects the cell’s genetic material from the chemical reactions that take place outside the nucleus. It also contains many proteins that are used in organizing DNA and regulating genes.

Function of the Nuclear Envelope

The nuclear envelope is a barrier that physically protects the cell’s DNA from the chemical reactions that are occurring elsewhere in the cell. If molecules that stay in the cytoplasm were to enter the nucleus, they could destroy part of the cell’s DNA, which would stop it from functioning properly and could even lead to cell death. The envelope also contains a network of proteins that keep the genetic material in place inside the nucleus.

It also manages what materials can enter and exit the nucleus. It does so by being selectively permeable. Only certain proteins can physically pass through the double layer. This protects genetic information from mixing with other parts of the cell, and allows different cellular activities to occur inside the nucleus and outside the nucleus in the cytoplasm, where all other cellular structures are located.

Parts of the Nuclear Envelope

The nuclear envelope surrounds the nucleus of the cell.

Outer Membrane

Like the cell membrane, the nuclear envelope is a lipid bilayer, meaning that it consists of two layers of lipid molecules. The outer layer of lipids has ribosomes, structures that make proteins, on its surface. It is connected to the endoplasmic reticulum, a cell structure that packages and transports proteins.

Inner Membrane

The inner membrane contains proteins that help organize the nucleus and tether genetic material in place. This network of fibers and proteins attached to the inner membrane is called the nuclear lamina. It structurally supports the nucleus, plays a role in repairing DNA, and regulates events in the cell cycle such as cell division and the replication of DNA. The nuclear lamina is only found in animal cells, although plant cells may have some similar proteins on the inner membrane.

Nuclear Pores

Nuclear pores pass through both the outer and inner membranes of the nuclear envelope. They are made up of large complexes of proteins and allow certain molecules to pass through the nuclear envelope. Each nuclear pore is made up of about 30 different proteins that work together to transport materials. They also connect the outer and inner membranes.

During cell division, more nuclear pores are formed in the nuclear envelope in preparation for cell division. The nuclear envelope eventually breaks down and is reformed around the nuclei of each of the two daughter cells.

The figure below shows a nuclear pore close-up:

Differences Between Nuclear Envelopes in Plant and Animal Cells

Much more is known about animal and yeast cell nuclear envelopes than those of plant cells, but the knowledge gap is decreasing thanks to recent research. Plant nuclear envelopes lack many of the proteins that are found on the nuclear envelopes of animal cells, but they have other pore membrane proteins that are unique to plants. Animal cells have centrosomes, structures that help organize DNA when the cell is preparing to divide; plants lack these structures and appear to rely entirely on the nuclear envelope for organization during cell division. With further research, scientists may better understand the uniqueness of plant cell nuclear envelopes.

Related Biology Terms

• Cytoplasm – all the material in a cell excluding the nucleus.


• Nucleus – central structure in a cell that contains the cell’s genetic material.


• Lipid bilayer – a double layer of lipid molecules; the outer cell membrane and the nuclear envelope are each made up of a lipid bilayer.


• Ribosome – a structure in the cell that makes proteins. Some ribosomes are attached to the outside of the nuclear envelope.

Test Your Knowledge

1. Which is NOT a part of the nuclear envelope?
A. Outer layer
B. Middle layer
C. Inner layer
D. Nuclear pores

Answer to Question #1

B is correct. The nuclear envelope consists of an outer later and an inner layer, with nuclear pores that pass through both layers. It does not have a middle layer.

2. What is the function of the nuclear envelope?
A. To allow different cellular activities to take place in the nucleus and in the cytoplasm at the same time
B. To regulate the transportation of molecules into and out of the nucleus
C. To protect the genetic information
D. All of the above

Answer to Question #2

D is correct. The nuclear envelope performs all of these functions.

3. What does the nuclear lamina do?
A. It organizes and provides structural support for the nucleus, including the chromosomes within
B. It laminates the nucleus, making it easier for molecules to enter during DNA replication
C. It holds the ribosomes in place on the nuclear envelope for protein production
D. It extends out into the cytoplasm to gather chemical information

Answer to Question #3

A is correct. The nuclear lamina is a network of fibers and proteins that provides structural support to the nucleus, anchors chromosomes in place, and regulates events involving organization of the nucleus, such as cell division. It is located on the inside of the inner layer of the nuclear envelope, so it does not have direct contact with ribosomes or cytoplasm.

Nuclear Envelope

Anaerobic Respiration

Anaerobic Respiration Definition

Aerobic respiration is the process by which cells that do not breathe oxygen liberate energy from fuel to power their life functions.

Molecular oxygen is the most efficient electron acceptor for respiration, due to its nucleus’ high affinity for electrons. However, some organisms have evolved to use other oxidizers, and as such, these perform respiration without oxygen.

These organisms also use an electron transport chain to generate as much ATP as possible from their fuel, but their electron transport chains extract less energy than those of aerobic respiration because their electron acceptors are weaker.

Many bacteria and archaea can only perform anaerobic respiration. Many other organisms can perform either aerobic or anaerobic respiration, depending on whether oxygen is present.

Humans and other animals rely on aerobic respiration to stay alive, but can extend their cells’ lives or performance in the absence of oxygen by using forms of anaerobic respiration.

Function of Anaerobic Respiration

Respiration is the process through which the energy stored in fuel is converted into a form that a cell can use. Typically, energy stored in the molecular bonds of a sugar or fat molecule is used to make ATP, by taking electrons from the fuel molecule and using them to power an electron transport chain.

Respiration is crucial to a cell’s survival because if it cannot liberate energy from fuel to drive its life functions, the cell will die.

This is why air-breathing organisms die so quickly without a constant supply of oxygen: our cells cannot generate enough energy to stay alive without it.

Instead of oxygen, anaerobic cells use substances such as sulfate, nitrate, sulfur, and fumarate to drive their cellular respiration.

Many cells can perform either aerobic or anaerobic respiration, depending on whether oxygen is available.

The image below illustrates a test tube test whereby scientists can determine if an organism is:

• An obligate aerobe – an organism that cannot survive without oxygen


• An obligate anaerobe – an organism that cannot survive in the presence of oxygen


• An aerotolerant organism – an organism that can live in the presence of oxygen, but does not use it to grow


• A facultative aerobe – an organism that can use oxygen to grow, but can also perform anaerobic respiration

Where Does Anaerobic Respiration Occur?

Anaerobic respiration takes place in the cytoplasm of cells. Indeed, most cells that use anaerobic respiration are bacteria or archaea, which don’t have specialized organelles.

What do Anaerobic Respiration and Aerobic Respiration Have in Common?

Both aerobic and anaerobic respiration begin with the splitting of a sugar molecule in a process called “glycolysis.” This process consumes two ATP molecules and creates four ATP, for a net gain of two ATP per sugar molecule that is split.

In both aerobic and anaerobic respiration, the two halves of the sugar molecule are then sent through another series of reactions that use electron transport chains to generate more ATP.

It is these reactions that require an electron acceptor – be it oxygen, sulfate, nitrate, etc. – in order to drive them.

What’s the Difference Between Aerobic Respiration and Anaerobic Respiration?

After glycolysis, both the aerobic and anaerobic cells send the two halves of glucose through a long chain of chemical reactions to generate more ATP and extract electrons for use in their electron transport chain.

However, what these reactions are, and where they happen, varies between aerobic and anaerobic cells.

In aerobic cells, the electron transport chain, and most of the chemical reactions of respiration, occur in the mitochondria. The mitochondria’s system of membranes makes the process much more efficient by concentrating the chemical reactants of respiration together in one small space.

In anaerobic cells, however, respiration typically takes place in the cell’s cytoplasm, since most anaerobic cells do not have specialized organelles. The series of reactions is typically shorter, and uses an electron acceptor such as sulfate, nitrate, sulfur, or fumarate instead of oxygen.

Anaerobic respiration also produces less ATP for each sugar molecule digested than aerobic respiration. In addition, it produces different waste products – including, in some cases, alcohol!

Types of Anaerobic Respiration

The types of anaerobic respiration are as varied as its electron acceptors. Important types of anaerobic respiration include:

• Lactic acid fermentation – In this type of anaerobic respiration, glucose is split into two molecules of lactic acid to produce two ATP.


• Alcoholic fermentation – In this type of anaerobic respiration, glucose is split into ethanol, or ethyl alcohol. This process also produces two ATP per sugar molecule.


• Other types of fermentation – Other types of fermentation are performed by some bacteria and archaea. These include proprionic acid fermentation, butyric acid fermentation, solvent fermentation, mixed acid fermentation, butanediol fermentation, Stickland fermentation, acetogenesis, and methanogenesis.

Anaerobic Respiration Equations

The equations for the two most common types of anaerobic respiration are:

• Lactic acid fermentation:

C₆H₁₂O₆ (glucose)+ 2 ADP + 2 phosphate → 2 lactic acid + 2 ATP

• Alcoholic fermentation:

C₆H₁₂O₆ (glucose) + 2 ADP + 2 phosphate → 2 C₂H₅OH (ethanol) + 2 CO₂ + 2 ATP

Examples of Anaerobic Respiration

Sore Muscles and Lactic Acid

During intense exercise, our muscles use oxygen to produce ATP faster than we can supply it.

When this happens, muscle cells can perform glycolysis faster than they can supply oxygen to their electron transport chain.

The result is that lactic acid fermentation occurs within our cells – and after prolonged exercise, the built-up lactic acid can make our muscles sore!

Yeasts and Alcoholic Drinks

Alcoholic drinks such as wine and whiskey are typically produced by bottling yeasts – which perform alcoholic fermentation – with a solution of sugar and other flavoring compounds.

Yeasts can use complex carbohydrates including those found in potatoes, grapes, corn, and many other grains, as sources of sugar.

Putting the yeast and its fuel source in an airtight bottle ensures that there will not be enough oxygen around to interfere with the anaerobic respiration that produces the alcohol!

Alcohol is actually toxic to the yeasts that produce it – when alcohol concentrations become high enough, the yeast will begin to die.

For that reason, it is not possible to brew a wine or a beer that has greater than 30% alcohol content. However, the process of distillation, which separates alcohol from other components of the brew, can be used to concentrate alcohol and produce hard liquors.

Methanogenesis and Dangerous Homebrews

Unfortunately, alcoholic fermentation isn’t the only kind of fermentation that can happen in plant matter. Glucose is fermented into ethyl alcohol – but a different alcohol, called methanol, can be produced from fermentation of a different sugar found in plants.

When cellulose is fermented into methanol, the results can be dangerous. The dangers of “moonshine” – cheap, homebrewed whiskey which often contains high amounts of methanol due to poor brewing and distillation processes – were advertised in the 20^th century during prohibition.

Death and nerve damage from methanol poisoning is still an issue in areas where unskilled people try to brew alcohol cheaply. So, if you’re going to become a brewer, make sure you do your homework!

Swiss Cheese and Propionic Acid

Propionic acid fermentation gives Swiss cheese its distinctive flavor. The holes in Swiss cheese are actually made by bubbles of carbon dioxide gas released as a waste product of a bacteria that uses propionic acid fermentation.

After the implementation of stricter sanitation standards in the 20^th century, many producers of Swiss cheese were puzzled to find that their cheese was losing its holes – and its flavor!

The culprit was discovered to be a lack of a specific bacteria which produce propionic acid. Throughout the ages, this bacteria had been introduced as a contaminant from the hay the cows ate. But after stricter hygiene standards were introduced, this was not happening anymore!

This bacteria is now added intentionally during production to ensure that Swiss cheese stays flavorful and retains its instantly recognizable holey appearance.

Vinegar and Acetogenesis

Bacteria that perform acetogenesis are responsible for the making of vinegar, which consists mainly of acetic acid.

Vinegar actually requires two fermentation processes, because the bacteria that make acetic acid require alcohol as fuel!

As such, vinegar is first fermented into an alcoholic preparation, such as wine. The alcoholic mixture is then fermented again using the acetogenic bacteria.

Related Terms

• ATP – The cellular “fuel” which can be used to power countless cellular actions and reactions.


• Oxidation – An important process in chemistry, where electrons are lost. A molecule which has lost electrons through the process of oxidation is said to have been “oxidized,” or “had its oxidation level increased.”

Test Your Knowledge

1. All cells perform glycolysis.
A. True
B. False

Answer to Question #1

True! All cells split sugars to release some of the chemical energy stored in the sugar molecules.

Some cells stop there, while others go on to use processes of fermentation or aerobic respiration to obtain much more energy from the sugar fragments left over after glycolysis.

2. The process of anaerobic respiration explains how some cells can survive without oxygen.
A. True
B. False

Answer to Question #2

True. Anaerobic respiration enables cells that perform it to survive without oxygen.

3. Cells can live without ATP, as long as they have sugar as a food source.
A. True
B. False

Answer to Question #3

False! All cells must have ATP to survive, as ATP stores energy in a form they can use to power their life processes.

They can turn sugars into ATP, but they require an oxidizer that their cells can use – such as oxygen. fumarate, or sulfur – to do this.

Anaerobic Respiration

Ymir

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Fast Facts:

• Pronunciation: EE-meer


• Origin: Norse


• Other Names: Aurgelmir, Brimir, Blainn


• Children: Generations

Who Is Ymir?

In Norse mythology, Ymir is known as the first being. He was a giant created from drops of water that formed when the ice of Niflheim mixed with the heat of Muspelheim. He was considered the father of all ice giants. The Norse creation narrative says that his hermaphroditic body produced beings that would go on to bear countless generations. His journey ended in tragedy, but because of his evil nature, no one can feel pity for the giant. His demise led to the creation of humans and the Earth.

Origins

In the Norse creation myth, the story starts as many other creation stories do. In the beginning, there was nothing. There was no sand, sea or waves. Neither heaven nor Earth existed. However, long before the Earth was made, Niflheim was created. It contained a spring that flowed into 12 rivers. In the southern part was Muspell, which was incredibly hot and guarded by a giant named Surt who carried a flaming sword. In the north, there was Ginnungagap. The rivers froze here, and everything was covered in ice. The warm air of Muspell reached the coldness of Ginnungagap, causing the ice to thaw and drip. The drops thickened and began to form into the shape of a man. This was the creation of Ymir, the ancestor of all frost giants.

The ice continued to drip and eventually formed a cow. Her name was Audumla and she produced four flowing rivers of milk that Ymir fed from. The cow was nourished by licking the salty, rime-covered stones surrounding her. The myth says that as the days progressed and Audumla began to lick away the stones, a man began to appear. On the first day, the hair of the man was uncovered. On the second, his entire head emerged from the stones. On the third day, he was completely uncovered and came out from the stones. His name was Buri and he eventually had a son named Bor, who proposed marriage to a daughter of a giant named Bestla. They had three sons, Ve, Vili and Odin. Odin would become known as one of the most powerful gods, while the brothers together are known as the rulers of heaven and Earth.

Legends and Stories

The creation of Ymir is fascinating on its own, but he is also responsible for the creation of the Earth, just not in the way that one might think.

The Creation of Earth

Ymir eventually turned into an evil being. Bor’s three sons found themselves in an altercation with the frost giant and were eventually forced to kill him. So much blood flowed from his body that all but one frost giant drowned, and he only survived by building an ark for himself and his family. Odin and his brothers dragged the giant’s body to the center of Ginnungagap, where they made the Earth from his body. His blood became the sea and his bones became the rocks and crags. His hair became the trees. His skull was turned into the sky, where the brothers added sparks and molten rock from Muspell to make the stars. Ymir’s brains were thrown into the sky to form clouds. The earth was flat, so they used Ymir’s eyelashes to block the areas of the earth that they wanted to keep the giants contained in.

Odin and his brothers found two logs on a seashore and made people out of them. One brother gave the people life and breath, while another gave them movement and consciousness. The last brother gave them speech, hearing, sight and faces. This was the beginning of humans. All the generations to follow could be traced back to these two people.

Odin and his brothers left the heavens unlit during their creation. One of the descendants of the two people created from the logs had two children. These children were simply stunning, and the father honored them with the names Sol and Moon. The gods were said to be jealous of the children and when their father seemed less than worthy of them, they took it as a sign to snatch away the children and put them in the sky. Sol was commanded to drive a chariot, carrying the sun across the skies. She drives so fast in the north because she is chased by a giant wolf. Moon takes the same path across the sky after his sister but is not as rushed as she is.

The brothers did make sure that there was a pathway between heaven and the Earth during their creation. A rainbow is meant to serve as a bridge between the two worlds. Its perfect shape and vibrant colors are meant to symbolize its origins from the gods. It is said that the rainbow will break apart when the men of Muspell try to scale it to get to heaven.

Family

Ymir did not marry, or have children in the traditional sense. However, his myth says that while he slept, he perspired. From this perspiration, a male and female emerged from his arms. Together, his legs produced a six-headed son. While Ymir was a giant, his existence is indirectly responsible for the human race as his body was turned into the Earth, and logs on the Earth were turned into humans.

Symbology

Ymir is typically depicted with his cow, which can be said to be his main symbol. The cow was both his companion and his source of nourishment. Other well-known artistic tributes to Ymir show him in the battle with the three brothers that would end with the loss of his life but the creation of the Earth.

The story of Ymir and creation often serves as a lesson for those who have heard it. It is speculated that his death represents the taming of the wild and the unstoppable force in humans that is necessary for the creation of anything progressive. His death also symbolizes how something that is ugly and chaotic can be reformed and reimagined into something beautiful.

Ymir

Sif

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Fast Facts:

• Pronunciation: SIFF


• Origin: Norse


• Role: Goddess


• Symbol: Golden Hair


• Children: Ullr


• Husband: Thor

Who Is Sif?

Sif was a Norse goddess and wife of the warrior god Thor. Her legacy has been overshadowed by that of her husband but she was at one time a highly recognized and important deity. She was the goddess of wheat, fertility and family. There are very few details surround the goddess but what we do know about her shows that she was a very important god to the Norse people.

Origins

The two main texts that depict Sif are the Poetic Edda and the Prose Edda, some of the best known traditional sources on Norse mythology. She is described as a beautiful woman with hair that was as golden as the sun. Her name means “relation to marriage” and she was associated with family caregiving and fertility. Thor was her second husband, with her first being the Giant Orvandil. She is often compared to other goddesses like Freya or Fjorgyn.

Legends and Stories

Myths that involve Sif are many but her roles are passive. This isn’t to say that she wasn’t an important goddess. Her importance to Norse mythology is mainly in her symbolic contributions.

Sif’s Hair

Thor, Sif’s husband, was known for his tough and masculine reputation. But he was absolutely in love with Sif, who was incredibly beautiful. Her most famous physical attribute was her long and thick hair that was the most perfect shade of gold. It flowed far down her back and always appeared to be flawless.

It is said that her long golden hair represented wheat and she was responsible for the Norse people’s crops. She would travel, seeking families and farms, where she would protect the crops from the cold winds and winters.

Sif would brush her hair with a comb encrusted in jewels and wash it in sparkling streams. To get it to dry, she would lay it out on rocks and let the sun’s warmth speed up the process. It was on one of these days that she fell asleep while waiting for her hair to try. Loki, the god of fire and mischief, had cast a spell on her so that he would be able to play with her hair.

Thor treasured his wife’s hair and would often boast of it whenever given the opportunity. Loki knew this and in an effort to upset the god, he chopped off Sif’s hair. Sif was left nearly bald and when she awoke, she found her hair in piles around her. She burst into tears and they fell onto the ground below, flooding the crop fields that she would protect.

Thor called for his wife but was unable to find her. After searching, he finally heard her whisper his name. She declared that she was ashamed and needed to leave the home of the gods and go into hiding. She then let her husband see her and he was immediately struck with sadness for his wife’s suffering.

Thor approached the other gods, demanding that they tell him who did this to his wife. Odin, the chief of the gods, suggested that it was Loki, as no one else had enough mischief in them to do such a thing. Odin called Loki, only after he commanded Thor to not harm him.

Loki admitted to the crime and the gods demanded he find a way to make amends. Loki left to find a way to give Sif her hair back. He traveled to the center of the earth to visit the Gnomes, who were known for their handiwork. He asked them to make a cap of floor-length hair that would match Sif’s hair. The Gnomes worked for days but eventually they presented the cap to Loki, which sported long golden locks as soft as silk made from gold thread.

He brought it to Sif, who put it on. It fit perfectly and gave her back her hair that she had missed so terribly. She was once again happy and was able to help the Norse with their crops once again.

Family

There is very little known about Sif’s own family. While we know she was married to Thor, we don’t know if they had any children. She did have a child from her first husband named Ullr. He was the god of snowshoes, hunting, the bow and she shield. He was described as being incredibly handsome with many warrior-like attributes. He was often called upon for help in battles.

Appearance

Artistic representations of Sif always show a young and strikingly beautiful woman with long, flowing golden hair. In most pictures, the hair is nearly touching the ground. It can be argued that without her long hair, it would be hard to recognize Sif as there is very little description of her otherwise.

Symbology

Sif is said to symbolize fidelity. She is also associated with summer, passion and the sun. Her best symbol though is her hair, which was said to symbolize the crop fields of the Norse population. The health of her hair was directly related to the strength of the crop, specifically wheat according to some sources. An old tradition says that in order to ask Sif for help, one should bake bread with plenty of grains. Sif is also associated with light, as it is said she was able to control the light in the sky and had a hand in the changing of seasons.

Pop Culture

While not proven, it is assumed that Sif is mentioned in the famous Old English poem Beowulf. She is also celebrated at the end of spring every year, as Sif was associated with summer. On the first day of the new season, there seems to be a spark of interest in the story of the goddess in Iceland. Thanks to the written work of a 19th century researcher, the story of Sif was rejuvenated in Scandinavian folklore. Iceland began to build a Norse temple in 2015 to pay tribute to Thor, Odin and Frigg. Because of this, Sif has grown in popularity once again.

Sif

Amphipathic

Amphipathic Definition

An amphipathic molecule is a molecule that has both polar and non-polar parts. Phospholipids, for example, have non-polar fatty acid “tails” and polar phosphate “heads.”

“Polarity” is an important property of molecules that determines how they will interact with other molecules.

Polarity is created when some atomic nuclei in a molecule attract electrons more strongly than others. The result is that the negative charge of the electrons congregates more around one atom than another, while the other atom possesses a slight positive charge because the electrons are closer to the first atom.

Polar molecules often contain elements like oxygen and sulfur, whose nuclei attract electrons very strongly. This allows them to pull some electrons away from their partner atoms.

Water is a good example of a polar molecule – its oxygen atom pulls atoms away from its hydrogens.

Non-polar molecules, on the other hand, are often heavy on elements like carbon, which has a fairly average pull on electrons. This means that carbon molecules are likely to share electrons equally and have a neutral charge.

In the case of polar molecules, “like attracts like” – polar molecules tend to interact strongly with other polar molecules, because their positive and negative ends are attracted to each other.

Non-polar molecules, on the other hand, do not interact strongly with polar molecules and may actually be pushed out of the way by other polar molecules that are attracted to the polar molecules’ partial charges.

Amphipathic molecules are biologically useful because they can interact with both polar and non-polar substances.

This allows them to make things possible that would not be possible with polar and non-polar molecules alone, including the creation of such crucial structures as the cell membrane.

Function of Amphipathic Molecules

Probably the most important function of amphipathic molecules in biology is in the formation of the cell membrane.

For life as we know it to exist, it is crucial that the materials of life – such as DNA, proteins, and energy molecules – are contained within a membrane. This increases the chances of the molecules interacting, and protects them from environmental threats.

Can you imagine a cell existing if its DNA, proteins, and sugars were floating around at random in a lake? Some scientists think that life may have started this way, but it’s not very efficient! Among other things, without cell membranes it would be impossible for living things to develop big structures like the human body that could exist outside of water.

Amphipathic molecules accomplish this remarkable feat in a deceptively simple way. Phospholipids – the type of amphipathic molecule that makes up most cell membranes – are able to form a stable membrane because their “head” is attracted to water molecules, while their “tails” are repelled by them.

That means that phospholipids can form a stable membrane that is impermeable to most substances just by sticking together.

In most cell membranes, the non-polar “tails” of phospholipids congregate together inside of the membrane, while the polar “heads” stay on the outside, interacting with water inside and outside of the cell.

This configuration is stable because the polar heads “want” to interact with polar water molecules at all times, while the non-polar tails “prefer” to interact with other non-polar tails.

Having both polar and non-polar parts is also useful for some proteins, especially proteins that need to span both the polar and non-polar parts of the cell membrane to do their job.

Outside of cells, amphipathic molecules have another extremely useful function: most soaps and shampoos are made of amphipathic molecules!

Soaps work because their molecules combine polar sections, which will stick to water, with non-polar sections, which will stick to other non-polar molecules like grease, oil, and most other substances that won’t wash away with water alone.

Many substances, including grease, won’t wash away with water because they are non-polar. As such, grease molecules have no “desire” to interact with water molecules, so they just kind of sit there while you scrub them.

Adding soap, however, with its amphipathic molecules, gives grease molecules something that they “want” to interact with. Other parts of the soap molecules then stick to water, and the soap molecules take the grease with them when they wash away!

Examples of Amphipathic Molecules

Examples #1: Phospholipids

As described above, phospholipids are molecules whose amphipathic properties make life as we know it possible.

They are the most important component of cell membranes, and also form organelle membranes that allow cells to carry out their metabolic functions more efficiently.

Membranes made of phospholipids inside chloroplasts allow plant cells to harvest energy from sunlight in the process of photosynthesis, which is crucial to life on Earth. Phospholipid membranes in our own mitochondria allow our cells to liberate lots of energy from sugars through the process of aerobic respiration.

Other organelles that use phospholipid membranes to perform life functions more efficiently include the nucleus, the endoplasmic reticulum, the Golgi apparatus, and vesicles.

Examples #2: Soap

Amphipathic molecules allow detergents, soaps, shampoos, and many other cleaning products to carry away substances that don’t wash away with water alone.

Soaps are traditionally made by treating fatty substances, such as vegetable oils or animal fat, with a chemical called lye. Lye – an ionic compound like salt – creates a polar “head” on the fatty acid molecules, resulting in molecules that will both bind to grease and wash away with water.

Examples #3: Membrane Proteins

The most useful function of phospholipid membranes comes from their ability to separate two different chemical mixes. Cells take advantage of that property to create and use energy, including during photosynthesis, aerobic respiration, and the firing of neurons.

However, to create and regulate two different chemistries, cells must be able to selectively move substances back and forth across membranes. This creates the need for transport proteins that cross both the polar and non-polar portions of the cell membrane.

To be stable in their role as gatekeepers of the membrane, membrane proteins themselves must have regions that bond to both the non-polar interior of the membrane, and the polar outer layer.

Receptors – proteins that monitor one side of the membrane for chemical signals, and produce changes on the other side of the membrane if they receive a signal – are another common type of protein that needs to bond with both the polar and non-polar parts of the cell membrane.

Structural proteins that give a cell control over the shape of its membrane must also have this property.

In general, any protein in the cell that must work within the membrane needs to have both polar and non-polar regions.

Related Biology Terms

• Cell Membrane – The membrane that separates the inside of a cell from the outside of a cell.


• Lipid – A non-polar molecule consisting of many carbon and hydrogen atoms which share electrons equally.


• Polar – A term for molecules whose atoms share electrons un-equally, resulting in partial positive and negative charges throughout the molecule.

Test Your Knowledge

1. Which part of a phospholipid is polar?
A. The fatty acid tail
B. The phosphate head
C. Both of the above
D. None of the above

Answer to Question #1

B is correct. Phosphate groups contain several oxygen atoms, which attract electrons more strongly than most atoms. This results in phosphate groups having a negative charge, and being attracted to molecules that have areas of whole or partial positive charge.

2. Which organelles use phospholipid membranes to perform their functions more efficiently?
A. Chloroplasts
B. Mitochondria
C. Nucleus
D. Endoplasmic Reticulum
E. All of the above

Answer to Question #2

E is correct. All of these organelles use phospholipid membranes to perform their functions.

3. Which of the following is most likely to be an amphipathic molecule?
A. A sugar
B. A DNA molecule
C. A membrane protein
D. None of the above

Answer to Question #3

C is correct. A membrane protein likely has to interact with both the nonpolar interior and the polar exterior of the phospholipid membrane. As such, it’s likely to have both polar and non-polar parts.

Amphipathic

Bahamut

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Beneath the cosmos, surrounded by water and mist, swims Bahamut, a fish of incomprehensible dimensions who carries the world on his back. No human eye can see Bahamut, but without him, all humans would be plunged into darkness.

What is Bahamut?

Bahamut (also called Behemoth) is a vast fish who serves as the supporter of the world in Arabic cosmography (the study of the cosmos’ organization). Alternatively, in Hebrew mythology, he is the largest land-dwelling creature ever to have been created. He is currently lurking in the underworld, but he will return during the chaos and destruction of the Day of Judgment.

Characteristics

Physical Description

In Arabic mythology, Bahamut is usually described as an unimaginably large fish. To paint a picture of his size, ancient mythology states that,

“all of the waters in the world, placed in one of his nostrils, would be like a mustard seed in a desert.”

Some myths describe Bahamut as having the head of a hippopotamus or an elephant. Occasionally, he is given a more monstrous form, appearing as a sea-serpent with limbs and fierce teeth.

Hebrew texts abandon Bahamut’s fish form altogether, and describe him as an enormous, river-dwelling creature with “strength in his loins, […] force in the navel of his belly, […] tail like a cedar, and […] bones like bars of iron.”

Special Abilities

Bahamut’s power lies in his massive size and strength. According to Arabic mythology, he supports the “seven stages of the earth,” which may refer to the seven astronomical bodies visible to the naked eye—Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon—or to some division of the heavens above the Earth.

On his back, Bahamut carries a bull, named Kujata. On Kujata’s back, there is a mountain made of ruby. On top of the ruby mountain, an angel holds the seven stages of the earth. Alternatively, a beach of sand lies on Bahamut’s back. Kujata is standing on the sand, and a rock on his back contains the waters in which the earth is floating. Beneath Bahamut is a dark, mysterious realm of swirling mist or water. Some accounts claim that, beneath the dark realm, there is a fiery world inhabited by a snake named Falak.

In addition to his brute strength, Bahamut also has the ability to baffle human vision. He is so large that even the mere sight of him would drive a man out of his senses.

A variation of Bahamut appears in Hebrew legend, under the name Behemoth. Behemoth usually takes the form of a hippopotamus, elephant, or bull. He dwells on land and is famous for his huge appetite. He is sometimes cast as a servant of Satan and said to preside over gluttonous banquets in Hell.

The terrible roar of the Hebrew Behemoth takes on special powers during the summer solstice. With one roar, the mighty Behemoth tames all of the wild predators on Earth, so that they are less ferocious during the rest of the year.

Weaknesses

The Bahamut of Arabic mythology has no known weaknesses, although he must answer to the commands of his creator. It’s possible that he could be consumed by Falak, the snake of the fiery underworld, if Falak wasn’t restrained by fear of that same creator.

The Hebrew Behemoth is less invincible. He too must obey his creator. On the Day of Judgment, he will be sentenced to battle Leviathan, a sea monster who God created as his counterpart. Both monsters will eventually be killed by their creator and served to worthy humans at a banquet that follows the Day of Judgment.

Related Creatures

Bahamut interacts with a variety of other mythological creatures. The most notable among them are Kujata, the bull who stands on top of his head; Falak, the snake who lives in the underworld beneath him; and Leviathan, the sea-creature with which he is to do battle on the Hebrew Day of Judgment.

Although Bahamut interacts with his fellow creatures, there are no other creatures in Arabic or Hebrew mythology that share his characteristics. According to Hebrew legend, Bahamut was purposefully made one-of-a-kind because his appetite was so big that his creator didn’t want him to reproduce; his offspring would have eaten the whole world.

Cultural Representation

Origin

Bahamut probably made his first appearance in Arabic cosmography. He appears in tomes of cosmography that date back as far as 1291. From there, his character was rapidly assimilated into Hebrew culture, but by the time he appeared in Hebrew writings, he had undergone a number of important transformations.

The word “Bahamut” in Arabic means “beast.” Bahamut was probably given this name because of his size and because he is sometimes given fearsome attributes, like sharp teeth and claws. “Behemoth” is the Hebrew translation of “Bahamut.”

Literature

Bahamut appears in many records of Arabic cosmography, most notably, in the works of the ancient Arabic historian, Ibn al-Wardi. The most famous references to Bahamut, however, appear in One Thousand and One Nights and in the Bible.

In One Thousand and One Nights, Bahamut is glimpsed by a man named Isa. Horror-stricken by Bahamut’s size, Isa loses consciousness. When he awakes, Allah (God) asks him if he has seen the enormous fish. Isa replies that he has only seen the bull on the fish’s head and that it was the length of three days’ journey. Allah then impresses Isa with the fact that he creates 40 fishes like Bahamut every day.

In the Bible, Bahamut (referred to as Behemoth) is described in the book of Job. The passage primarily focuses on the incredible might of Behemoth, as a way of glorifying God, who is able to create and control such an awesome creature.

Some Jewish writings, including the Book of Enoch and the Haggadah, expand upon Behemoth’s lore by describing the battle that will be waged between him and Leviathan on the Day of Judgment.

Visual Arts

Sci-fi movies, stretching all the way from the 1950s to the present day, have spotlighted the monstrosity of Bahamut (Behemoth). Few of them stay true to early mythological descriptions of Bahamut, but the creatures who take Bahamut’s name are always portrayed as gigantic.

Perhaps Bahamut’s biggest impact on modern culture is his role in the Final Fantasy video game series. Bahamut appears as a dragon capable of wielding deadly amounts of energy as a weapon. He is often the final and most dangerous villain who players face in the game.

Explanations of the Myth

While Bahamut himself is certainly larger than life, several real animals have been put forward as prototypes for “the beast.”

The passage in the Book of Job, which gives a lengthy physical description of “Behemoth,” has been scrutinized by zoologists for decades in the hope of determining which animal might have inspired the Behemoth legend. Most agree that Behemoth is probably based on a hippopotamus because he is described as feeding on grass like an ox, and lying under the lotuses and reeds of a marsh or river.

An alternate explanation of Behemoth has been popularized by young Earth creationists, who believe that the Bible contains a perfectly accurate account of the creation of the world. They claim that Behemoth represents a sauropod dinosaur.

Bahamut

Primary Succession

Primary Succession Definition

Primary succession is the orderly and predictable series of events through which a stable ecosystem forms in a previously uninhabited region. Primary succession occurs in regions characterized by the absence of soil and living organisms.

It begins with the appearance of pioneer species – lichen, mosses and fungi – that can grow on rocks and exposed land. These are small, simple organisms that can survive harsh conditions, fix inorganic carbon and nitrogen, and accelerate the process of weathering. As they die and decompose, their organic matter becomes the foundation for a thin layer of soil. Pioneer species pave the way for more complex communities of organisms, because the pioneers have altered the physical environment to make it more habitable. Once grasses and weeds begin to grow, soil formation is accelerated and more animal species begin to appear. The environment retains moisture, and ideal conditions are created for the growth of shrubs and small trees. This is followed by larger trees and animals, and the complex web of interactions between them.

Examples of Primary Succession

Primary succession can occur after a variety of events. These include:

• Volcanic eruptions


• Retreat of glaciers


• Flooding accompanied by severe soil erosion


• Landslides


• Nuclear explosions


• Oil spills


• Abandonment of a manmade structure, such as a paved parking lot

While some of these are natural events, some are anthropogenic, or manmade.

Example #1: Primary succession after a volcanic eruption

Lava from an erupting volcano incinerates everything in its path and forms new land that is made from inorganic material. While it is rich in minerals, the land cannot support a varied and complex ecosystem. Its capacity to sustain a stable ecosystem is limited. Pioneer species that colonize areas after volcanic eruptions include swordfern and green algae.

Carrizozo Lava Flow

A few small invertebrate animals may also venture into this territory, followed by crickets and spiders.

In the case of volcanic eruptions in the ocean, the atolls formed are isolated from other terrestrial ecosystems and have unique food chains and webs. Pioneer species often arise from spores carried through ocean currents.

Example #2: Primary succession in sand dunes

Seashores are harsh environments because of high wind speeds, moving sand and the minimal availability of fresh water and organic nutrients. Pioneer plants in such environments tend to have symbiotic bacteria in their root nodules to fix nitrogen. They also have root systems that can anchor them in shifting sand, have multiple adaptations to harvest fresh water, and reduce water loss through transpiration. Examples of pioneer species in sand dunes include sand couch grass and lyme grass.

These are followed by other grasses, and then by lichens that are deposited on the thin layer of organic matter created by the pioneer species. As the ecosystem develops, bracken, gorse, heather, hawthorn and brambles can be seen.

Eventually, a woodland will develop, containing organisms that can thrive in a high salt environment.

Example #3: Primary succession after a nuclear explosion

Some islands in French Polynesia were used for extensive testing of nuclear bombs in the 1960s and 70s. They were completely denuded of all plant, animal and microbial life. Scientists estimated that it would take centuries before life returned to these islands. However, surveys conducted over the course of 30 years show that primary succession has begun, and many islands have grasses, mosses and some plants. Some species of mollusks have also begun to live on these islands.

After the major accident at Chernobyl Nuclear Reactor in Ukraine (1986), the area was evacuated and has had minimal human habitation for the past three decades. The central reactor is still highly radioactive and is considered a completely ‘dead’ zone. However, robots sent into the heart of this reactor returned with black fungi that were using the radiation itself as an energy source.

While the high radiation levels limit the scope of research into these ecosystems, it will be of great interest to continue studying primary succession in these environments.

Differences Between Primary and Secondary Succession

Secondary succession occurs after an event that deeply disturbs an existing, stable ecosystem when most above-ground vegetation and living organisms disappear from the region. Though it appears as if the region is ‘dead’, the soil remains fertile and contains enough organic matter to support the reappearance of life. Grasses are among the first species to appear, quickly followed by shrubs and small trees.

The major difference between primary and secondary succession is the quality of the soil. Secondary succession does not require pedogenesis or soil formation. For example, primary succession would occur on barren land that was previously covered by a glacier, while secondary succession would occur on land after a forest fire. The forest fire may destroy all the plants and drive away the animals, but the ashes and decomposing organic matter can enrich the soil, and life restarts from sprouting roots and shoots and through the germination of seeds already present in the soil. In the case of the retreating glacier, however, the land has not supported life for hundreds of thousands of years and lacks any organic matter.

Related Biology Terms

• Abiotic Factors – The non-living, physical and chemical components of an ecosystem.


• Climax Species – Plants seen in stable and mature ecosystems that have reached a steady state. Example: white spruce trees.


• Pioneer Species – Species that first appear in an uninhabited area.


• Secondary Succession – The orderly sequence of events that occurs after most above-ground vegetation and all life forms are removed from a region.

Test Your Knowledge

1. Which of these events can trigger primary succession? Choose all that apply.
A. Deforestation
B. Volcanic eruption
C. Heavy rain
D. Nuclear explosions

Answer to Question #1

B & D are correct. Volcanic eruptions and nuclear explosions can both trigger primary succession. They destroy all living species in the vicinity and remove the capacity of the soil to support life. Deforestation and heavy rain do not always cause complete loss of soil fertility.

2. Which of these is a pioneer species?
A. Shrubs
B. Flowering plants
C. Small mammals
D. Lichens

Answer to Question #2

D is correct. Lichens are a pioneer species. They are among the first living beings to colonize a barren landscape. Shrubs occur after soil has formed, and flowering plants and mammals are usually climax species, appearing after the ecosystem has matured.

3. Primary succession can occur over decades or centuries.
A. True
B. False

Answer to Question #3

True. The rate at which primary succession proceeds depends on the nature of the event that preceded it and the ecosystems that surround the barren area. A place that has experienced a volcanic eruption will take longer to recover than an area that has experienced flooding. Similarly, a small patch of denuded landscape surrounded by lush green forests will regenerate more quickly than a large desert.

Primary Succession

Nondisjunction

Nondisjunction Definition

Nondisjunction occurs in cell division when chromosomes do not divide properly. Chromosomes contain all of a cell’s DNA, which it needs in order to function and reproduce. Normally, when a cell divides, the chromosomes line up in an orderly way and then separate from each other before cell division. When these chromosomes fail to separate properly, nondisjunction has occurred. The resulting daughter cells have an incorrect number of chromosomes; one may have too many, while another may have too few. This causes problems in cell function because a cell cannot function normally without the right amount of chromosomes.

Types of Nondisjunction

Nondisjunction can occur during mitosis, meiosis I, or meiosis II.

During Mitosis

Somatic cells, or cells of the body, divide through mitosis. From each original parent cell, two identical daughter cells are created. In the parent cell, each chromosome is composed of two identical sister chromatids. During the anaphase stage, these chromatids normally separate, and one chromatid goes into each daughter cell. However, when nondisjunction occurs, the chromatids do not separate. The result is that one cell receives both chromatids, while the other cell receives neither. Each daughter cell then has an abnormal number of chromosomes when mitosis is complete; one cell has an extra chromosome, while the other is missing one.

The diagram below shows nondisjunction taking place in mitosis:

Mitotic nondisjunction

During Meiosis I

Gametes (eggs and sperm) are made through meiosis. One cell divides into four daughter cells through the combined processes of meiosis I and meiosis II. Meiosis I is similar to mitosis, but in meiosis I each pair of chromosomes lines up next to each other in preparation for making gametes, whereas in mitosis, the chromosomes are all in one line. In anaphase of meiosis I, nondisjunction happens when a pair of homologous chromosomes does not separate. In the resulting cells, one cell has two copies of a chromosome, while the other cell has no copies. When each of these cells goes on to divide into two cells during meiosis II, the four total cells produced will all have chromosomal abnormalities.

During Meiosis II

Even if a cell divides normally in meiosis I, nondisjunction can still occur in meiosis II. During meiosis II, each cell splits and goes from diploid (two pairs of each chromosome) to haploid (one of each chromosome) in preparation for fertilization. If a pair of sister chromatids fail to separate properly during anaphase of meiosis II, one daughter cell will have an extra chromosome and one daughter cell will be missing a chromosome. If the other daughter cell created in meiosis I splits properly, the other two of the four total daughter cells created during meiosis II will have the normal number of chromosomes.

Examples of Nondisjunction Disorders

Examples #1: Down Syndrome

Down syndrome occurs as a result of nondisjunction during meiosis I that produces an egg cell with an extra copy of chromosome 21. The fertilized egg has three copies of chromosome 21—two from the mother, and one from the father—which is called a trisomy. People with Down syndrome have three copies of chromosome 21 in all of their somatic cells.

The extra chromosome in the cells of those with Down syndrome is responsible for a host of characteristics, including delays in physical growth, certain facial features, and mild intellectual disability. Rates of nondisjunction increase with age, which is why older mothers have a higher chance of giving birth to a child with Down syndrome. According to Mayo Clinic, this chance drastically increases between the ages of 35 and 45, going from 1 in 350 at 35 years to 1 in 30 at 45 years.

Examples #2: Sex Chromosome Aneuploidy

Sex chromosome aneuploidy is the term for an abnormal number of sex chromosomes. Normally, females have two X chromosomes, while males have one X and one Y. Nondisjunction can cause individuals to be born female with one X (Turner syndrome), female with three X chromosomes (Trisomy X), male with XXY (Klinefelter syndrome), or male with XYY (XYY syndrome). Rarer combinations, such as having five X chromosomes, can also occur. Sometimes, sex chromosome aneuploidy goes unnoticed in individuals, but other times it may present as a recognizable syndrome with characteristics such as intellectual disability.

Examples #3: Other Types of Trisomy

Most cases of trisomy result in miscarriage during the first trimester of pregnancy because the fetus cannot survive the chromosomal abnormality. Trisomy 16 occurs in over 1% of pregnancies and is the most common trisomy, but most individuals with this trisomy do not survive unless some of their cells are normal.

The three most common types of trisomy that are survivable are Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome).

Related Biology Terms

• Mitosis – the process by which somatic cells divide into two identical daughter cells.


• Meiosis – the process by which four gamete (sex) cells are produced from one parent cell; it consists of meiosis I and meiosis II.


• Anaphase – the stage of mitosis and meiosis where chromosomes separate from one another before cell division.


• Trisomy – a cell has three copies of a chromosome instead of two.

Test Your Knowledge of Nondisjunction

1. Nondisjunction can occur in what type of cell division?
A. Mitosis
B. Meiosis I
C. Meiosis II
D. All of the above

Answer to Question #1

D is the correct answer. Nondisjunction can occur any time chromosomes have to separate, so it may occur during mitosis, meiosis I, and/or meiosis II.

2. In what phase in the process of cell division can nondisjunction occur?
A. Prophase
B. Metaphase
C. Anaphase
D. Telophase

Answer to Question #2

C is correct. During anaphase, the sister chromatids (in mitosis and/or meiosis II) or paired chromosomes (in meiosis I) separate from each other in order to prepare for cell division. When they do not separate correctly, nondisjunction occurs.

3. Which trisomy is associated with Down syndrome?
A. Trisomy 18
B. Trisomy 21
C. Trisomy 13
D. Trisomy 16

Answer to Question #3

B is correct. People with Down syndrome have three copies of chromosome 21 in each of their somatic cells. Down syndrome is also known as trisomy 21.

Nondisjunction

Passive Transport

Passive Transport Definition

Passive transport is a process by which an ion or molecule passes through a cell wall via a concentration gradient, or from an area of high concentration to an area of low concentration. It’s like moving from the train to the platform of a subway station, or stepping out of a crowded room. Basically, passive transport gives an ion or molecule “room to breathe.”

This term is best remembered when juxtaposed with its opposite, active transport. Like physical activity, active transport requires energy. Passive transport, on the other hand, needs no energy at all.

Examples of Passive Transport

Passive transport takes four forms:

• simple diffusion


• facilitated diffusion


• filtration


• osmosis

Example #1: BAC Goin’ Up

It’s the classic high school health lesson: once ethanol – the “alcohol” ingredient in beer, wine, and spirits – enters your body, it hits your bloodstream at lightning speed. This is why you can have a BAC without feeling drunk, and why some people become thoroughly intoxicated within minutes of taking a shot.

The reason this happens is because ethanol molecules enact simple diffusion, a type of passive transport, with expert ease. Their ultra-microscopic size allows them to pass through cell and tissue membranes without any help, and affect the body without consuming energy.

Ions perform simple diffusion

Example #2: Neurotransmission Impossible

…well, not quite. The fact that neurons – or brain cells – rely on passive transport to communicate is easy to miss, partly because of how complicated we make them out to be.

Crazily enough, the spindly web of synapses (brain activity) in our head relies on two ions, sodium (Na⁺) and potassium (K⁺), which work along a gradient. A neuron in resting potential (not firing) contains a concentration of K⁺ ions on the inside, and a concentration of Na+ ions on the outside. When the neuron fires (active potential), protein “pumps” on its outer membrane allow Na+ ions to enter the body an K⁺ ions to exit.

As you can see, Na⁺ and K⁺ ions move from an area of high concentration to an area of lower concentration like the ethanol molecules in Example 1. However, they need help to do so. Because they require a little assistance, they perform facilitated diffusion instead of simple diffusion.

Molecules undergo facilitated diffusion

Example #3: (Not) a Pile of Waste

Our intestines do a lot more than push excrement through our bodies. In fact, you could say that extracting nutrients from our food is actually their main job. Although vitamins and minerals tend to be much larger than ethanol and ions, our bodies nonetheless extract them using a form of passive transport.

Filtration, specifically, happens when we separate solids from liquids, and liquids from gasses, via a membrane. Returning to our example, we see that nutrients (liquid) separate from waste (solid) by passing through the intestinal membrane and into the bloodstream.

Example #4: Fresh Veggies

Soak a raisin in water, and you will get a grape. More than “re-juicing,” soaking raisins constitutes another instance of passive transport – this time, osmosis.

Different from other types of passive transport, it seeks equilibrium rather than simple movement along a concentration gradient. Water passes through the raisin’s membrane not only to reach a less-concentrated interior, but also to make the grape “equal” to its outside environment. This process can happen with other fruits and vegetables, as well, as long as the produce has undergone some form of dehydration.

Related Biology Terms

• Simple diffusion – A process of diffusion that occurs without the aid of an integral membrane protein. Allows substances to pass through cell membranes without any energy.


• Facilitated diffusion – A process that occurs when molecules or ions pass through a cell membrane with the assistance of an integral protein.


• Concentration gradient – The difference in concentration of a dissolved substance in a solution between a region of high density and one of lower density.


• Filtration – The removal of water from solid materials, usually for nutrients to be selectively absorbed into the body.


• Osmosis – The tendency of a fluid to pass through a membrane into a solution where the solvent concentration is higher, thus equalizing the concentrations of materials on either side of the membrane.


• Ion – An electrically-charged atom or group of atoms.


• Molecule – The smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound.

Test Your Knowledge

1. In passive transport, ions and molecules move from an area of _______ concentration to an area of ______ concentration.
A. medium, low
B. high, low
C. low, high
D. hard, soft

Answer to Question #1

D is correct. Ions and molecules pass from areas of high concentration to areas of low concentration. Rarely do we encounter areas of “medium” concentration, and “hard” and “soft” concentrations do not exist.

2. When the large intestine absorbs nutrients, it is performing:
A. filtration
B. active transport
C. osmosis
D. facilitated diffusion

Answer to Question #2

False. The large intestine performs filtration, because it removes vitamins and minerals (in liquid form) from solid waste. It cannot be active transport because it does not require energy. It is neither osmosis nor facilitated diffusion because it relies on neither equilibrium nor integral proteins.

3. Ethanol molecules can perform simple diffusion because:
A. …they don’t care about authority.
B. …they are larger than most membranes.
C. …they are smaller than most membranes.
D. …they’re really smart.

Answer to Question #3

True. Ethanol molecules can perform simple diffusion because they are smaller than most membranes. The process would take longer if they were larger, and would make the effects of alcohol far less intense.

Passive Transport

Aerobic Respiration

Aerobic Respiration Definition

Aerobic respiration is the process by which oxygen-breathing creatures turn fuel, such as fats and sugars, into energy.

Respiration is a process used by all cells to turn fuel, which contains stored energy, into a usable form. The product of respiration is a molecule called ATP, which can easily use the energy stored in its phosphate bonds to power chemical reactions the cell needs to survive.

Aerobic respiration is respiration that uses oxygen as a reactant. Aerobic respiration is much more efficient, and produces ATP much more quickly, than anaerobic respiration (respiration without oxygen). This is because oxygen is an excellent electron acceptor for the chemical reaction.

The complex process of aerobic respiration is illustrated in this graphic. You may wish to reference this image as you study the different parts of the process of cellular respiration.

Here, we will break down the process into simpler steps to illustrate how cellular respiration turns energy from glucose into a form that the cell can use to power its life functions.

Common Steps Between Aerobic Respiration and Anaerobic Respiration

Both aerobic respiration and anaerobic respiration use an electron transport chain to move energy from its long-term storage in sugars to a more usable form.

In respiration, the energy from sugar is moved into ATP, which can be used to power many chemical reactions necessary to a cell’s survival.

Both aerobic and anaerobic respiration start with the process of glycolysis. “Glycolysis,” which literally means “sugar splitting,” breaks a sugar molecule down into two smaller molecules.

In the process of glycolysis, two ATP molecules are consumed and four are produced. This results in a net gain of two ATP molecules produced for every sugar molecule broken down through glycolysis.

In cells that use oxygen, a sugar molecule is broken down into two molecules of pyruvate. In cells that do not have oxygen, the sugar molecule is broken down into lactate.

Although our cells normally use oxygen for respiration, which is much more efficient than anaerobic respiration, when we use ATP faster than we are getting oxygen molecules to our cells, our cells can perform anaerobic respiration to supply their needs for a few minutes.

Fun fact: Buildup of lactate from anaerobic respiration is one reason why muscles can feel sore after intense exercise!

Differences Between Aerobic Respiration and Anaerobic Respiration

After glycolysis, different respiration chemistries take a few different paths:

• Cells that are deprived of oxygen but are not made for anaerobic respiration, like our own muscle cells, may leave the end products of glycolysis sitting around, obtaining only two ATP per sugar molecule they split.


• Cells that are made for anaerobic respiration may continue the electron transfer chain to extract more energy from the end products of glycolysis.


• Cells using aerobic respiration continue their electron transfer chain in a highly efficient process that ends up yielding 38 molecules of ATP from every sugar molecule!

After glycolysis, cells that do not use oxygen may use a different electron acceptor, such as sulfate or nitrate, to drive their reaction forward.

These processes are called “fermentation.” Some types of fermentation reactions actually have alcohol as their end product. So now you know where alcoholic drinks come from: the respiration processes of yeasts splitting sugars to produce energy!

Aerobic respiration, on the other hand, sends the pyruvate left over from glycolysis down a very different chemical path.

Aerobic Respiration and Weight Loss

Aerobic respiration is the process by which many cells, including our own, produce energy using food and oxygen. It also gives rise to carbon dioxide, which our bodies must then get rid of.

This equation explains why we need both food and oxygen, as both are reacted together to produce the ATP that allows our cells to function.

This equation also explains why we breathe out carbon dioxide – and how we lose weight!

Hint: We breathe in O₂ and we breathe out the same number of molecules of CO₂. Where did the carbon atom come from? It comes from the food, such as sugar and fat, that you’ve eaten!

This is also why you breathe harder and faster while performing calorie-burning activities: your body is using both oxygen and food at a faster-than-normal rate, and is producing more ATP to power your cells, along with more CO₂ waste product, as a result.

Unfortunately, simply breathing faster doesn’t mean you’ll unload more carbon: to lose carbon faster, your cells need to be consuming energy at a faster-than-normal rate. So get out those running shoes!

Aerobic Respiration Equation

The equation for aerobic respiration describes the reactants and products of all of its steps, including glycolysis. That equation is:

1 glucose + 6O₂ → 6CO₂+ 6 H₂O + 38 ATP

The reactions of aerobic respiration can be broken down into four stages, described below:

Steps of Aerobic Respiration

1. Glycolysis. In aerobic cells, the equation for glycolysis is:

Glucose + 2 HPO₄^2- + 2 ADP^3- + 2 NAD⁺ → 2 Pyruvate^– + 2 ATP^4- + 2 NADH + 2 H⁺ + 2 H₂O

As discussed above, glycolysis in aerobic respiration refers to the splitting of a sugar molecule into two pyruvate molecules. This process creates two ATP molecules.

You will notice that this process also creates NADH from NAD⁺. This is important because later in the process of cellular respiration, NADH will power the formation of much more ATP through the mitochondria’s electron transport chain.

Pyruvate is then processed to turn it into fuel for the citric acid cycle, using the process of oxidative decarboxylation.

2. Oxidative decarboxylation of pyruvate

2 (Pyruvate^– + Coenzyme A + NAD⁺ → Acetyl CoA + CO₂ + NADH)

In this process, pyruvate is combined with coenzyme A to produce acetyl-CoA.

You will note that more NADH is created in this step. This means more fuel to create more ATP later in the process of cellular respiration!

This is important because acetyl-CoA is an ideal fuel for the citric acid cycle, which can in turn power the process of oxidative phosphorylation in the mitochondria, which produces huge amounts of ATP!


3. Citric acid cycle

2 (Acetyl CoA + 3 NAD⁺ + FAD + GDP^3- + HPO₄^2- + 2H₂O → 2 CO₂ + 3 NADH + FADH₂ + GTP^4- + 2H⁺ + Coenzyme A)

In the citric acid cycle, both NADH and FADH₂ – another carrier of electrons for the electron transport chain – are created. All the NADH and FADH₂ created in the preceding steps now come into play in the process of oxidative phosphorylation.

4. Oxidative phosphorylation

34 (ADP^3- + HPO₄^2- + NADH + 1/2 O₂ + 2H⁺ → ATP^4- + NAD⁺ + 2 H₂O)

Oxidative phosphorylation uses the folded membranes within the cell’s mitochondria to produce huge amounts of ATP.

In this process, NADH and FADH₂ donate the electrons they obtained from glucose during the previous steps of cellular respiration to the electron transport chain in the mitochondria’s membrane.

The electron transport chain consists of a number of complexes in the mitochondrial membrane, including complex I, Q, complex III, cytochrome C, and complex IV.

All of these ultimately serve to pass electrons from higher to lower energy levels, harvesting bits of their energy in the process. This energy is used to power proton pumps, which in turn power ATP formation.

Just like the sodium-potassium pump of the cell membrane, the proton pumps of the mitochondrial membrane are used to create a concentration gradient which can be used to power other processes.

In the case of the mitochondria’s proton gradient, the protons that are transported across the membrane using the energy harvested from NADH and FADH₂ “want” to pass through channel proteins from their area of high concentration to their area of low concentration.

These channel proteins are actually ATP synthase – the enzyme that makes ATP. When protons pass through ATP synthase, they drive the formation of ATP.

This process is why mitochondria are referred to as “the powerhouses of the cell.” The mitochondria’s electron transport chain makes nearly 90% of all the ATP produced by the cell from breaking down food.

This is also the process that requires oxygen. Without oxygen molecules to accept the depleted electrons at the end of the electron transport chain, the electrons would back up and the process of ATP creation would not be able to continue.

No wonder we need oxygen to live!

Function of Aerobic Respiration

Aerobic respiration produces ATP, which is then used to power other life-sustaining functions, such as the action of the sodium-potassium pump, which allows us to move, think, and perceive the world around us; the actions of many enzymes; and the actions of countless other proteins that sustain life!

Related Terms

• ATP – The cellular “fuel” which can be used to power countless cellular actions and reactions.


• Mitochondria – An important organelle in animal cells which efficiently extracts energy from sugars.


• Sodium-potassium pump – An important transport protein which uses about 20-25% of all ATP in the human body. It illustrates the importance of ATP because of what happens if this pump runs out of fuel.

Test Your Knowledge

1. All cells perform glycolysis.
A. True
B. False

Answer to Question #1

True! All cells split sugars to release some of the chemical energy stored in the sugar molecules.

Some cells stop there, while others go on to use processes of fermentation or aerobic respiration to obtain much more energy from the sugar fragments left over after glycolysis.

2. The process of aerobic respiration explains how we lose weight when we diet and exercise.
A. True
B. False

Answer to Question #2

True. The equation for aerobic respiration shows how we break down sugars and consume oxygen in order to produce energy. We then breathe out the carbon from the sugars we have broken down in the form of CO₂.

When the cell needs more energy, such as when we exercise, it turns oxygen into CO₂ faster in order to generate more ATP.

Likewise, if we take in less sugar than we need to meet our energy needs, our body breaks down fat molecules – ultimately into carbon, which we breathe out.

3. The only important product of the citric acid cycle is ATP.
A. True
B. False

Answer to Question #3

False! NADH and FADH₂ are also very important products of the citric acid cycle, because they are used to make much more ATP in the mitochondria later, during oxidative dephosphorylation.

Aerobic Respiration