Section 5: Biological Membranes

Question 1

What is the function of a membrane.

Answer 1

Cells (and many of the organelles inside them) are surrounded by membranes that have a wide range of functions.

Question 2

What is the function of membranes at the cell surface (plasma membranes)?

Answer 2

Plasma membranes are a barrier between the cells and its environment, controlling which substances enter and leave the cell. They’re partially permeable - they let some molecules through but not others.
Substances can move across the plasma membrane diffusion, osmosis or active transport. Plasma membranes also allow recognition by other cells (e.g. the cells of the immune system) and cell communication (sometimes called cell signalling).

Question 3

What is the function of membranes within cells and give examples.

Answer 3

The membranes around organelles divide the cell into different compartments - they act as a barrier between the organelle and the cytoplasm. This makes different functions more efficient. For example, the substances needed for respiration (like enzymes) are kept together inside a mitochondrion by the mitochondrion‘s outer membrane.
Membranes can form vesicles to transport substances between different areas of the cell. For example, proteins are transported in vesicles from the rough endoplasmic reticulum to the Golgi apparatus during protein synthesis.
Membranes within cells are also partially permeable so they can control which substances enter and leave the organelle. For example, RNA leaves the nucleus via the nuclear membrane (nuclear envelope). DNA is too large to pass through the partially permeable membrane, so it remains in the nucleus.
You can also get membranes within organelles - these act as barriers between the membrane contents and the rest of the organelle. For example, thylakoids membranes in chloroplasts keep the components needed for the light-dependent reactions of photosynthesis together.
Membranes within cells can be the site of chemical reactions. The membranes of some organelles are folded increasing their surface area and making chemical reactions more efficient. For example, the inner membrane of a mitochondrion contains enzymes needed for respiration. It has a large surface area, which increases the number of enzymes present and makes respiration more efficient.

Question 4

What is the structure of membranes?

Answer 4

The structure of all membranes is basically the same. They’re all composed of lipids (mainly a type called phospholipids), proteins and carbohydrates (usually attached to proteins or lipids).

Question 5

What is the fluid mosaic model?

Answer 5

In 1972, the fluid mosaic model was suggested to describe the arrangement of molecules in the membrane. In the model, phospholipid molecules form a continuous, double layer (called a bilayer). The bilayer is ‘fluid’ because the phospholipids are constantly moving. Protein molecules are scattered through the bilayer, like tiles in a mosaic. Some proteins have a carbohydrates attached - these are called glycoproteins. Some lipids also have a carbohydrate attached - these are called glycolipids. Cholesterol molecules are also present within the bilayer.

Question 6

What are the five main membrane components?

Answer 6

Phospholipids, cholesterol, proteins, glycolipids and glycoproteins.

Question 7

What are phospholipids and what do they do?

Answer 7

Phospholipid molecules form a barrier to dissolved (water-soluble) substances. Phospholipids have a ‘head’ and a ‘tail’. The head is hydrophilic - it attracts water. The tail is hydrophobic - it repels water. The molecules automatically arrange themselves into a bilayer - the heads face out towards the water on either side of the membrane.
The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances (like ions and polar molecules) to diffuse through it - it acts as a barrier to these dissolved substances. Fat-soluble substances, e.g. fat-soluble vitamins, dissolve in the bilayer and pass directly through the membrane.

Question 8

What is cholesterol, where is it found and what do they do?

Answer 8

Cholesterol gives the membrane stability. It is a type of lipid that’s present in all cell membranes (except bacterial cell membranes). Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This makes the membrane less fluid and mor rigid. Cholesterol also has hydrophobic regions, so it’s able to create a further barrier to polar substances moving through the membrane.

Question 9

What are proteins, where are they found and what do they do?

Answer 9

Proteins control what enters and leaves the cell. Some proteins form channels in the membrane - these allow small, charged particles through. Other proteins (called carrier proteins) transport larger molecules and charged particles across the membrane by active transport and facilitated diffusion. Proteins also act as receptors for molecules (e.g. hormones) in cell signalling. When a molecule binds to the protein, a chemical reaction is triggered inside the cell.

Question 10

What are glycolipids and glycoproteins, where are the found and what do they do?

Answer 10

Glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules. They act as receptors for messenger molecules in cell signalling and are sites where drugs, hormones and antibodies bind. They’re also antigens - cell surface molecules involved in self-recognition and the immune response.

Question 11

What factors affect membrane permeability?

Answer 11

Solvents and temperature.

Question 12

How do solvents affect membrane permeability?

Answer 12

The permeability of cell membranes depends on the solvent surrounding them. This is because some solvents (such as ethanol) dissolve the lipids in a cell membrane, so the membrane loses its structure. Some solvents increase membrane permeability more than others, e.g. ethanol increases membrane permeability more than methanol. Increasing the concentration of the solvent will also increase membrane permeability.

Question 13

How does the temperature affect membrane permeability?

Answer 13

Cell membranes are affected by temperature - it affects how much the phospholipids in the bilayer can move, which affects membrane structure and permeability.
Temperatures below 0 degreesC - the phospholipids don’t have much energy, so they can’t move very much. They’re packed closely together and the membrane is rigid. But channel proteins and carrier proteins in the membrane denature (lose structure and function), increasing the permeability of the membrane. Ice crystals may form and pierce the membrane, making it highly permeable when it thaws.
Temperatures between 0 and 45 degreesC - The phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable. As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane.
Temperatures above 45 degreeC - The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins in the membrane denature so they can’t control what enters or leaves the cell - this increases the permeability of the membrane

Question 14

How do you investigate cell membrane permeability?

Answer 14

You can investigate how different variables (e.g. solvent concentration and temperature) affect cell membrane permeability by doing experiments using beatroot. Beatroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane, the more pigment leaks out of the cell.

Question 15

What is cell signalling?

Answer 15

Cells need to communicate with each other to control processes inside the body and to respond to changes in the environment. Cells communicate with each other by cell signalling, which uses messenger molecules.
Cell signalling starts when one cell releases a messenger molecule (e.g. a hormone). This molecule travels to another cell (e.g. in the blood). The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane. The binding then triggers a change in the cell, e.g. a series of chemical signals is set off

Question 16

How are membrane receptors significant?

Answer 16

The cell membrane is important in the signalling process. Proteins in the cell membrane act as receptors for messenger molecules. These receptor proteins are called membrane-bound receptors. Receptor proteins have specific shapes – only messenger molecules with a complimentary shape can bind to them. Different cells have different types of receptors – they respond to different messenger molecules. A cell that response to a particular messenger molecule is called a target cell.

Question 17

How do hormones act as messenger molecules and give examples.

Answer 17

Many messenger molecules are hormones. Hormones work by binding to receptors on cell membranes and triggering response in the cell.
For example Glucagon is a hormone that is released when there isn’t enough glucose in the blood. It binds to receptor is on liver cells, causing the liver cells to break down stores of glycogen to glucose. Another example is FSH, a hormone that is released by the pituitary gland during the menstrual cycle. It binds to receptors on cells in the ovaries, causing an egg to mature ready for ovulation.

Question 18

What is the role of drugs in cell signalling and give examples.

Answer 18

Many drugs work by binding to receptors in cell membranes. They either trigger a response in the cell, or block the receptor and prevent it from working. Understanding how cells communicate using membrane-bound receptors is important in the development of medicinal drugs – the receptors can be used as sites for targeted action.
For example, the body produces chemicals called endorphins, to relieve pain. Endorphins bind to opioid receptors in the brain and reduce the transmission of pain signals. Morphine is a drug use to relieve pain. It works by binding to the same opioid receptors as endorphins, also triggering a reduction in pain signals.
Another example is that cell damage causes the release of a chemical called histamine. Histamine binds to receptors on the surface of other cells and causes inflammation. Antihistamines work by blocking histamine receptors on the cell surfaces. This prevents histamine from binding to the cell and stops inflammation.

Question 19

What is diffusion and give examples.

Answer 19

Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration. Molecules will diffuse both ways, but the net movement will be to the area of lower concentration. This continues until particles are evenly distributed throughout the liquid or gas. The concentration gradient is the path from an area of higher concentration to an area of lower concentration. Particles diffuse down a concentration= gradient. Diffusion is a passive process – no energy is required for it to happen.Particles can diffuse across plasma membranes, as long as they can move freely through the membrane.
For example, small non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through the spaces between phospholipids. Also, water is small enough to fit between phospholipids, so it’s able to diffuse across plasma membranes even though it’s polar. The diffusion of water molecules like this is called osmosis.

Question 20

What are the factors that affect the rate of diffusion?

Answer 20

There are four main factors that affect the rate of diffusion:
The concentration gradient – the higher it is, the faster the rate of diffusion.
The thickness of the exchange surface – the thinner the exchange surface (i.e. the shorter the distance the particles travel), the faster the rate.
The surface area– The larger the surface area (e.g. of the plasma membrane), the faster the rate of diffusion.
The temperature – the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster.

Question 21

What is osmosis and give an example.

Answer 21

Osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient. This means water molecules move from an area of high water potential (i.e. higher concentration of water molecules) to an area of lower water potential (i.e. lower concentration of water molecules). Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution. Pure water has the water potential of zero. Adding solutes to pure water lowers its water potential – so the water potential of any solution is always negative. The more negative the water potential, the stronger the concentration of solutes in the solution.
For example glass A contains pure water – it’s got a water potential of zero. Glass B contains a solution of orange squash. The orange squash molecules are a solute. They lower the concentration of the water molecules. This means that the water potential of the orange squash is lower than the water potential of the pure water.

Question 22

How does the water potential affect cells?

Answer 22

Cells are affected by the water potential of the surrounding solution. Water moves in or out of the cell by osmosis. How much moves in or out depends on the water potential of the surrounding solution compared to that of the cell. Animal and plant cells behave differently in different solutions.

Question 23

What is an isotonic solution?

Answer 23

If two solutions have the same water potential that said to be isotonic. Cells in an isotonic solution won’t lose or gain any water – there is no net movement of water molecules because there is no difference in water potential between the cell and the surrounding solution. Both plant and animal cells will stay the same when placed in an isotonic solution.

Question 24

What is a hypotonic solution?

Answer 24

If a cell is placed in a solution that has a higher water potential, water will move into the cell by osmosis. Solutions with a higher water potential compared with the inside of the cell are called hypotonic. An animal cell in hypotonic solution will swell and could eventually burst. If a plant cell is placed in a hypotonic solution, the vacuole will swell and the contents of the vacuole and cytoplasm will push against the cell wall. This causes the cell to become turgid (swollen). The cell won’t burst because the inelastic cell wall is able to with stand the increase in pressure.Therefore the overall net movement of water is into the cell.

Question 25

What is a hypertonic solution?

Answer 25

If a cell is placed in a solution that has a lower water potential, water will move out of the cell by osmosis. Solutions with a lower water potential than the cell are called hypertonic. If an animal cell is placed in a hypertonic solution it will shrink. If a plant cells is placed in a hypertonic solution it will become flaccid (limp). The cytoplasm and plasma membrane eventually pull away from the cell wall. This is called plasmolysis. Therefore the overall net movement of water is out of the cell.

Question 26

What is active transport?

Answer 26

Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient. This process involves carrier proteins and is pretty similar to facilitated diffusion. A molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across a membrane, releasing it on the other side. The only difference is that energy is used (from ATP - a common source of energy used in the cell), to move the solute against its concentration gradient.

Question 27

What is endocytosis?

Answer 27

Some molecules are way too large to be taken into a cell by carrier proteins, e.g. proteins, lipids and some carbohydrates. Instead a cell can surround a substance with a section of its plasma membrane. The membrane then pinches off to form a vesicle inside the cell containing the ingested substance - the substance has been taken in by endocytosis.
Some cells also take in much larger objects by endocytosis - for example, some white blood cells (mainly phagocytes), use endocytosis to take in things like microorganisms and dead cells so that they can destroy them. Like active transport, this process uses ATP fo energy.

Question 28

What is exocytosis?

Answer 28

Some substances produced by the cell (e.g. digestive enzymes, hormones, lipids) need to be released from the cell - this is done by exocytosis. Vesicles containing these substances pinch off from the sacs of the Golgi apparatus and move towards the plasma membrane. The vesicles fuse with the plasma membrane and release their contents outside the cell. Some substances (like membrane proteins) aren’t released outside the cell - instead they are inserted straight into the plasma membrane. Exocytosis also uses ATP as an energy source.

Question 29

What is facilitated diffusion?

Answer 29

Some larger molecules (e.g. amino acids, glucose) would diffuse extremely slowly through the phospholipid bilayer because they’re so big. Charged particles, e.g. ions and polar molecules, would also diffuse slowly - that’s because they’re water soluble, and the centre of the bilayer is hydrophobic. So to speed things up, large or charged particles diffuse through carrier proteins or channel proteins in the cell membrane instead - this is called facilitated diffusion.
Like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration. It’s also a passive process - it doesn’t use energy. But unlike diffusion, there are two types of membrane protein involved - carrier proteins and channel proteins.

Question 30

What are carrier proteins and give an example?

Answer 30

Carrier proteins move large molecules (including polar molecules and ions) into or out of the cell, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.
An example is GLUT1 which is a carrier protein found in almost all animal cell. It specifically helps to transport glucose across the plasma membrane.

Question 31

How do carrier proteins work?

Answer 31

First, a large molecule attaches to a carrier protein in the membrane. Then, the protein changes shape. This releases the molecule on the opposite side of the membrane.

Question 32

What are channel proteins?

Answer 32

Channel proteins form pores in the membrane for smaller ions and polar molecules to diffuse through, down their concentration gradient. Different channel proteins facilitate the diffusion of different charged particles.