Biological membranes

Question 1

Function of membranes at the surface of cells?

Answer 1

• Control what enters and leaves the cell- they are partially permeable- they let some molecules through but not others.
• They allow recognition by other cells, e.g. the cells in the immune system
• They allow cell communication (cell signalling)

Question 2

Function of membranes within cells?

Answer 2

• Divide the cell into different compartments
• They can form vesicles to transport substances between different areas of the cell
• They control which substances control and leave the organelle
• Membranes within organelles can be the site of chemical reactions e.g. the inner membrane of a mitochondria contains enzymes for respiration

Question 3

What is the model called to describe the membrane?

Answer 3

Fluid mosaic model

Question 4

Phospholipids: What is the tail and what does this mean?

Answer 4

Hydrophobic- it repels water

Question 5

Phospholipids: What is the head and what does this mean?

Answer 5

Hydrophilic- it attracts water

Question 6

Phospholipids: What do the phospholipids arrange themselves into and which way do they face?

Answer 6

Arrange themselves into a bilayer, the heads face out towards the water on either side of the membrane.

Question 7

Phospholipids: The centre of the bilayer is hydrophobic, what does this mean?

Answer 7

The membrane does not allow any water-soluble substances (like ions) through it.

Question 8

Cholestrol: Where is it and what does it do?

Answer 8

Cholestrol molecules fit between the phospholipids. They bind to the hydrophobic tails, causing them to pack in tightly together. This makes the membrane more rigid and less fluid.

Question 9

Cholestrol: What happens at lower temperatures?

Answer 9

Cholestrol prevents phospholipids from packing too close together and so increases membrane fluidity.

Question 10

Proteins: What do they do?

Answer 10

Control what enters and leaves the cell.

Question 11

Proteins: What do channel proteins do?

Answer 11

Allow small molecules or charged particles through .

Question 12

Proteins: What do carrier proteins do?

Answer 12

Transport molecules and ions across the membrane by active transport and facilitated diffusion.

Question 13

Proteins: Role in cell signalling?

Answer 13

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 14

Glycolipids and glycoproteins: What do they do?

Answer 14

Stabilise the membrane by forming hydrogen bonds with surrounding water molecules:
- Also the site where drugs, hormones and antibodies bind.
- They act as receptors for cell signalling
- They’re also antigens.

Question 15

What is cell signalling?

Answer 15

1) one cell releases a messenger molecule (e.g. a hormone).
2) This molecule travels (in the blood) to another cell.
3) The messenger molecule is detected by the cell because it binds to the receptor on it cell membrane.

Question 16

Membrane permeability: Effect of temperature below 0 degrees?

Answer 16

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 deform, increasing the chance of permeability of the membrane.

Question 17

Membrane permeability: Effect of temperature between 0 and 45 degrees ?

Answer 17

The phospholipids can move around and aren’t packed tightly together- the membrane is partially permable. As temperature increases the phospholipids move more because they have more energy- this increases the permeability of the membrane.

Question 18

Membrane permeability: Effect of temperature above 45 degrees?

Answer 18

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 deform so they can’t control what enters and leaves the cell- this increase the permeability of the membrane.

Question 19

What does surrounding cells in solvents do?

Answer 19

Increases the permeability of their cell membranes. This is because solvents dissolve the lipids in the cell membrane, so the cell membrane loses its structure.

Question 20

What can diffuse through the cell membrane?

Answer 20

Small, non-polar molecules such as oxygen, and carbon dioxide are able to easily diffuse through spaces between phospholipids.

Question 21

Can water diffuse through the cell membrane?

Answer 21

Water is small enough to fit between phospholipids, so its able to diffuse across plasma membranes even though it’s polar. This diffusion of water molecules is called osmosis.

Question 22

Rate of diffusion: How does concentration gradient effect it?

Answer 22

The higher it is, the faster the rate of diffusion

Question 23

Rate of diffusion: How does the thickness of the exchange surface effect it?

Answer 23

The thinner the excahnge surface (i.e. the shorter distance the particles have to travel), the faster the rate of diffusion is

Question 24

Rate of diffusion: How does surface area effect it?

Answer 24

The larger the surface area, the faster the rate of diffusion

Question 25

Rate of diffusion: How does temperature effect it?

Answer 25

The warmer it is, the faster the rate of diffusion because the particles have more kinetic energy and so move faster.

Question 26

What is facilitated diffusion?

Answer 26

When larger molecules, ions and polar molecules diffuse through carrier proteins or channel proteins rather than directly through the phospholipid bilayer.

Question 27

Facilitated diffusion: What do carrier proteins do?

Answer 27

Move larger molecules into and out of the cell, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.
1) First a large molecule attaches to the carrier protein in the membrane
2) Then the protein changes shape
3) This releases the molecule on the opposite side of the membrane.

Question 28

Facilitated diffusion: What do channel proteins do?

Answer 28

Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different channel proteins facilitate the diffusion of different charged particles.

Question 29

What is active transport?

Answer 29

Using energy to move molecules and ions across the plasma membranes, against their concentration gradient. This process involves carrier proteins.

Question 30

Active transport: What do carrier proteins do?

Answer 30

1) A molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side.
2) Energy is used (from ATP) to move the solute agaisnt its concentration gradient.

Question 31

How can endocytosis be used to take substances into cells?

Answer 31

• Some molecules are way to 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- this is endocytosis.
• This process requires ATP.

Question 32

How can cells use exocytosis to secrete substances?

Answer 32

• Vesicles containing substances pinch off from the sacs of the Golgi apparatus and move towards the plasma membrane.
• The vesicle fuses with the plasma membrane and releases the contents outside the cell.
• This process used ATP

Question 33

What is osmosis?

Answer 33

The diffusion of water molecules across a partially permable membrane down a water potential gradient. This means water molecules move from an area of high water potential (i.e. higher conc of water molecules) to an area of lower water potential (i.e. lower conc of water molecules).

Question 34

What has the highest water potential?

Answer 34

Pure water. All solutions have a lower water potential than pure water.

Question 35

Animal cells affected by the water potential of the surrounding solution: What happens when the solution has a higher water potential than the cell?

Answer 35

This is called a hypotonic solution.
Net movement of water is into the cell. Cell bursts

Question 36

Animal cells affected by the water potential of the surrounding solution: What happens when the solution has a same water potential as the cell?

Answer 36

This is called a isotonic solution.
Water molecules pass into and out of the cell in equal amounts. The cell stays the same size.

Question 37

Animal cells affected by the water potential of the surrounding solution: What happens when the solution has a lower water potential than the cell?

Answer 37

This is called a hypertonic solution.
Net movement of water is out of the cell. The cell shrinks

Question 38

Plant cells affected by the water potential of the surrounding solution: Hypotonic solution?

Answer 38

Net movement of water is into cell. The vacuole swells. The vacuole and cytoplasm push against the cell wall. The cell becomes turgid.

Question 39

Plant cells affected by the water potential of the surrounding solution: Isotonic solution?

Answer 39

Water molecules move into and out of the cell in equal amounts. The cell stays the same.

Question 40

Plant cells affected by the water potential of the surrounding solution: Hypertonic solution?

Answer 40

Net movement of water is out of the cell. The cell becomes flaccid (limp). The cytoplasm and the cell membrane pull away from the cell wall. This is called plasmolysis.