1 - Cell Membrane Overview

An amphipathic molecule refers to a molecule that is both hydrophobic and hydrophilic. Since there are both of these properties on a single phospholipid, it is considered to be amphipathic.

A phospholipid is considered to be an amphipathic molecule. Why?

Carrier proteins are able to go against the concentration gradient. So, if your body needs a lot of Cl-, but there is already Cl- in our cells, then the carrier protein is able to go against the concentration gradient and bring in Cl- into the cell. Since carrier proteins are able to go against the concentration gradient, then energy would be required for that. But if they do not go against the concentration gradient, no energy is needed.

Do carrier proteins require energy? How can this relate to the concentration gradient?

No - no energy or ATP is needed. Channel proteins will always go down the concentration gradient. So, if there is too much Na+ outside of the cell, the channel protein will allow these ions to flow into the cell. Notice it will not "pump" ions b/c no energy is needed.

Do channel proteins require energy? How can this relate to the concentration gradient?

Saturated fatty acids make the fatty acid tails of the phospholipids straighter and easier to fit close to one another. So, saturated fatty acids tails make the phopholipids closer. This decreases membrane fluidity.

How do saturated fatty acids affect membrane fluidity?

Unsaturated fatty acids make the fatty acid tails of the phospholipids bulkier and harder to fit close to one another. So, unsaturated fatty acids tails make the phopholipids farther apart. This increases membrane fluidity.

How do unsaturated fatty acids affect membrane fluidity?

Cholesterol will insert itself between phospholipids. They are already farther apart to one another due to the temperature, but the cholesterol decreases the distance between the phospholipids. They decrease the distance because the phospholipids will notice the cholesterol and want to attach to the cholesterol so they will pull closer together. This will cause membrane fluidity to decrease.

How does cholesterol affect membrane fluidity in high temperature?

Cholesterol will insert itself between phospholipids. They are already close to one another due to the temperature, but the cholesterol increases the distance between the phospholipids, causing membrane fluidity to increase.

How does cholesterol affect membrane fluidity in low temperature?

Temperature affects membrane fluidity by impacting how close phospholipids are to one another. In low temperature the phospholipids will be very close to one another because they do not have a lot of energy to move. This decreases the membrane fluidity. In high temperature the phospholipids will be farther apart from one another because they do have a lot of energy to move. This increases the membrane fluidity.

How does low or high temperature affect membrane fluidity?

The cell membrane is in charge of protecting the cell. It acts as a sort of shield and allows or does not allow items to enter the cell.

If something wants to enter a cell, what part of the cell allows or does not allow it to enter?

When there is too much or too little of something channel proteins can allow things into or out of the cell - depending on the need of the cell. Since there is too much Na+ in this example, a channel protein would be able to allow Na+ out of the cell.

Lets say there is a cell that has too much sodium (Na+), what could a channel protein do to help?

The cell membrane will generally allow small, non-polar molecules into the cell.

Since the cell membrane is semi-permeable, what is generally allowed to enter the cell?

The portion labeled "polar head" is the phosphate head. The glycerol backbone cannot be pointed out in this photo. It is the portion that actually holds the fatty acid tails and the phosphate head together - literally acting as a "backbone." The thicker gray "legs" are the two fatty acid tails.

Using the photo, point out the three key parts of a phospholipid.

They are phospholipids, cholesterols, and proteins.

What are the main 3 things that make up a cell membrane?

The three factors are temperature, cholesterol, and the presence of unsaturated or saturated fatty acids.

What are three main factors that affect membrane fluidity?

Saturated fatty acids have a single bond, while unsaturated fatty acids have a double bond. The double bonds create a kink, making them not as easy to pack.

What causes unsaturated fatty acids to have a kink in the chain?

If the membrane is in a cold environment then it is a very rigid membrane. It is not very fluid, so it may break. The cholesterol would help in this case.

What could happen to a membrane that is found in a very cold environment without cholesterol?

If the membrane is in a hot environment then it is a very fluid membrane. Since it is very fluid, it may not hold its shape. The cholesterol would help in this case.

What could happen to a membrane that is found in a very hot environment without cholesterol?

This would be referred to as the fluid mosaic model.

What did scientist decide to name this?

Polarity refers to the concentration of electrons. If a molecule is polar then it means the electrons are not evenly distributed - which makes one side of the molecule more positive or more negative. If a molecule is nonpolar then it means the electrons are evenly distributed - which creates an evenly charged molecule.

What does it mean when something is polar or nonpolar?

Proteins are typically found throughout or on top of the lipid bilayer. Thus proteins that are found within the lipid bilayer are rare - they are called lipid bound proteins.

What is a rare type of protein found in the cell membrane?

That is a carbohydrate molecule that is called glyco for short. Since it is bound to a lipid it is a glycolipid. These can also be bound to proteins and would be called glycoproteins.

What is in the red box? Where else can that object be found?

They play a large role in communication/signalling. It allows a cell to recognize another cell in our body. This is why they are positioned the way they are - to be good at communication.

What is the function of a glycoprotein or glycolipid?

Cholesterol is found between phospholipids in the cell membrane. Cholesterol acts as a buffer by maintaining the fluidity of the cell membrane. As the temperature gets lower, cholesterol helps to increase the fluidity. As the temperature gets higher, cholesterol helps to reduce the fluidity.

What is the importance of cholesterol in the cell membrane?

They can act as receptors - they can "tell" the cell what is going on outside the cell. They can also help transport molecules in and out of the cell.

What is the importance of proteins in the cell membrane?

A cell is semi-permeable when it can allow some things to enter, but will also not allow some things to enter.

What makes a cell semi-permeable?

It is composed of a phosphate head, a glycerol backbone, and two fatty acid tails.

What makes up a single phospholipid?

The phosphate head is the hydrophilic portion of the phospholipid. Hydrophilic means it is water loving - it will always want to be by water.

What portion of a phospholipid is hydrophilic?

The two fatty acid tails are the hydrophobic portion of the phospholipid. Hydrophobic means it is water fearing - it will always want to get away from water.

What portion of a phospholipid is hydrophobic?

The two general types are uncatalyzed movement and catalyzed movement. Uncatalyzed movement does not require a catalyst, while catalyzed does require a catalyst.

What two general types of movements are there for phospholipids?

Flippase is a catalyzed movement. It is when a phospholipid moves from the extracellular border to the intracellular border only. But it will require a catalyst (like a protein, for example). It will require ATP and is a fast movement.

What type of movement is flippase? Is it fast or slow?

Floppase is a catalyzed movement. It is when a phospholipid moves from the intracellular border to the extracellular border of the membrane only. But it will require a catalyst (like a protein, for example). It will require ATP and is a fast movement.

What type of movement is floppase? Is it fast or slow?

Lateral diffusion is an uncatalyzed movement. It is when a phospholipid moves around but does not move from the extracellular border to the intracellular border of the membrane, or vice versa. It is a fast movement.

What type of movement is lateral diffusion? Is it fast or slow?

Scramblase is a catalyzed movement. It is when a phospholipid moves from the intracellular border to the extracellular border, or vice versa. But it will require a catalyst (like a protein, for example). It is a fast movement and does not require ATP.

What type of movement is scramblase? Is it fast or slow?

Transbilayer diffusion is an uncatalyzed movement. It is when a phospholipid moves from the extracellular border to the intracellular border of the membrane, or vice versa. It is a very slow movement.

What type of movement is transbilayer diffusion? Is it fast or slow?

This is an example of a carrier protein. Carrier proteins can allow things into or out of the cell (like channel proteins), but they protect the molecule as they are bringing it in or out.

What type of protein is this an example of? What does it do?

This is an example of a channel protein. Channel proteins allow things in or out of the cell.

What type of protein is this an example of? What does it do?

A and B are both examples of integral proteins because they both go through the entire phospholipid bilayer. D and E are both examples of peripheral proteins because they do not go through any of the phospholipid bilayer, rather they just sit on top of the membrane.

What type of proteins are A/B and D/E?

Cis-unsaturated fats is harder to pack.

Which is more difficult to pack, cis-unsaturated fats or trans-unsaturated fats?

Phospholipids are the "building blocks" to a cell membrane. They are important because they are vital to building a cell membrane.

Why are phospholipids important to a cell membrane?

Since the fatty acid tails are hydrophobic, this is formed so they avoid water. The phosphate heads will be touching the water.

Why does a phospholipid bilayer form?

Since they are small, it is easier to squeeze through. The fatty acid tails are non-polar, so they would allow a non-polar molecule in much easier. It is a fast process.

Why is it easier for small, nonpolar molecules to pass through the cell membrane?

The term "fluid" refers to the fact that the things (like proteins, glycolipids, etc.) in the cell membrane can move around. They are not all set into one location.

Why is the term "fluid" in the fluid mosaic model?

It can pass through but it is much more difficult because they are polar. The fatty acid tails are non-polar so they do not like polar molecules. It is a slow process.

Would a cell membrane allow small, polar molecules through?

Yes - it is possible but the size makes it more difficult. It is a slow process.

Would a large, nonpolar molecule be able to pass through a cell membrane?

No - the size and charge would not be able to pass through. It would need help for it to pass through.

Would a large, polar molecule be able to pass through a cell membrane?

No - proteins act as receptors and help transport molecules so if they were located in the lipid bilayer they would not do their job effectively. So it is more common (and better) for proteins to not be positioned that way.

Would a protein that is completely within the lipid bilayer be common? Why or why not?

Yes - integral proteins are found throughout the lipid bilayer while peripheral proteins are found on the top of the lipid bilayer. So it is easier to remove. Peripheral proteins are known to attach to a cell membrane to do a task and then remove themselves.

Would peripheral proteins be easier to remove than integral? Why or why not?