Cell membrane structure and function

Question 1

Tell me about the structure of the cell membrane

Answer 1

• It’s structure relies on a phospholipid bilayer
• Phospholipids are arranged as a double layer.
• This is about 7-10nm thick.
• Hydrophilic phosphate and glycerol heads are orientated outwards.
• Hydrophobic fatty acid chains orientate inwards.
• Made up almost entirely of phospholipids and proteins with a small amount of carbohydrates and cholesterol.

Question 2

Why do we use the terms fluid mosaic model?

Answer 2

• Fluid – proteins and other embedded molecules can move laterally (sideways).
• Mosaic – appears as such from surface view due to the close fitting arrangement of the phospholids and the integral proteins.

Question 3

What do channel proteins enable to happen?

Answer 3

Create pores through which water and water-soluble chemicals can diffuse through

Question 4

What are carrier proteins involved in?

Answer 4

Involved in facilitated diffusion and active transport

Question 5

What is a glycoprotein and what does it do?

Answer 5

A protein attached to polysaccharide chain

Helps cells to recognise each other

Question 6

What does cholesterol do in the cell membrane?

Answer 6

It stabilises it

Question 7

What is simple diffusion?

Answer 7

The movement of particles within a gas or liquid from a region of high concentration to a region of lower concentration until an equilibrium is reached.

Question 8

What is a concentration gradient?

Answer 8

The difference in concentration between two areas is the concentration gradient.

Question 9

Explain how diffusion happens (5)

Answer 9

All particles in liquids and gases are in constant random motion
If there is a difference in concentration between parts of a gas or liquid, these random movements carry particles from the area of high concentration to the area of lower concentration.
The difference in concentration between two areas is the concentration gradient.
The particles move down this gradient.
This continues until the particles become evenly dispersed – dynamic equilibrium.

Question 10

How does size and nature of particle affect diffusion into a cell? (5)

Answer 10

Fat-soluble molecules can pass through lipid bilayer eg alcohol and steroids
Small molecules such as oxygen and carbon dioxide can diffuse through small pores between the phospholipids
Larger water-soluble molecules such as glucose and amino acids must pass through protein pores.
Very large molecules cannot diffuse into cells at all.

Question 11

How does surface area affect the rate of diffusion?

Answer 11

The greater the surface area the greater the rate of diffusion.
eg Microvilli increase the surface area in some cells

Question 12

How is diffusion affected by the thickness of the membrane?

Answer 12

The greater the distance the slower the rate of diffusion.

Question 13

How does concentration gradient affect the rate of diffusion? How can it be maintained?

Answer 13

The greater the concentration gradient the greater the rate of diffusion.
Diffusion is more efficient if the gradient can be maintained.
This is done by transporting the substance away once diffused or combining with other chemicals so it cannot diffuse back.

Question 14

What is Fick’s Law?

You don’t have to know this, I just wondered if you did!!!

Answer 14

Rate of Diffusion is proportional to:

Surface area x concentration difference
_____________________________________
distance

Question 15

What does facilitated diffusion need?

Answer 15

Some substances that may not normally diffuse across a membrane are facilitated (helped) by proteins in the membrane.
Like diffusion this process is down a concentration gradient and is passive – there is no energy input.

Question 16

Tell me about Channel and carrier proteins (6)

Answer 16

1. Carrier proteins:
   • The molecule binds to protein
   • Protein changes shape
   • Molecule ends up facing the other side of the membrane and is released.
2. Channel proteins:
   • These are proteins that cross the membrane with a channel running throughout them.
   • In some cases, protein channels may open or close to signals:
   • Eg Voltage gated channels in neurones.
   • Used in transmission of nerve impulses.

• In both cases, these proteins are specific to the molecule that passes through them.
• This means that cells can be selective in terms of the type of molecule that passes through their cell membrane.

Question 17

What is osmosis?

Answer 17

Osmosis is the net movement of water molecules from a region of higher (less negative) water potential to a region of lower (more negative) water potential, across a partially permeable membrane.

Question 18

What is the water potential of pure water?

Answer 18

Pure water has a water potential of zero.( ψ= 0)

Question 19

In terms of water potential how does water move by osmosis?

Answer 19

Water diffuses from less negative ψ to more negative ψ

Eg. from -100kPa to –200kPa

Question 20

What happens if we put animal cells in water or dilute solutions?

Answer 20

In very dilute solutions, water enters animal cells.

They swell up and burst – this is called cell lysis.

Question 21

What happens if we put an animal cell in a concentrated solution?

Answer 21

In concentrated solutions, water leaves the cell by osmosis, and the cell shrinks.

Question 22

What do animal cells need if they are to stay the same osmotically?

Answer 22

Animal cells must always be bathed in an isotonic solution - a solution with the same solute concentration as the cytoplasm

Question 23

What do we call it if a plant cell shrinks by osmosis?

Answer 23

Plasmolysed

Question 24

When do plant cells become turgid?

Answer 24

Plant cells become “turgid” when they are put in dilute solutions.
Turgid means swollen and hard.
The pressure inside the cell rises.

Question 25

What is active transport and what does it need?

Answer 25

The movement of substances across a membrane against the concentration gradient.
Requires energy (ATP)
Uses specialised carrier proteins in the membrane.
Cells and tissues carrying out active transport are characterised by:
Presence of numerous mitochondria.
High concentration of ATP
High respiratory rate

Question 26

How does the membrane regulates the movement of substances into and out of cells? (10)

Answer 26

Non-polar/lipid soluble molecules move through phospholipid bilayer;
Small molecules/water/gases move through phospholipid layer/bilayer;
Ions/water soluble substances move through channels in proteins;
Some proteins are gated;
Reference to diffusion;
Carriers identified as proteins;
Carriers associated with facilitated diffusion;
Carriers associated with active transport/transport with ATP/pumps;
Different cells have different proteins;
Correct reference to cytosis;

Question 27

How are proteins are arranged in a plasma membrane and what is their role in transport? (6)

Answer 27

1 Some proteins pass right through membrane;
2 Some proteins associated with one layer;
3 Involved in facilitated diffusion;
4 Involved in active transport;
5 Proteins act as carriers;
6 Carrier changes shape / position;
7 Proteins form channels / pores;
8 Protein allows passage of water soluble molecules /
charged particles / correct named example;

Question 28

How is a plasma membrane is adapted for its functions? (6)

Answer 28

Phospholipid bilayer (as a barrier);
Forms a barrier to water soluble but allows non-polar substances to pass so maintains a different environment on each side (compartmentalisation)
Bilayer is fluid: can bend to take up different shapes for phagocytosis and form vesicles
Channel proteins (intrinsic): let water soluble/substances through (facilitated diffusion)
Carrier proteins (intrinsic): allow facilitated diffusion and active transport
Extrinsic proteins: act in cell recognition, act as antigens or receptors;
Cholesterol: regulates fluidity / increases stability;

Question 29

Describing the fluid-mosaic structure of a membrane

Answer 29

Phospholipids and proteins;
Phospholipid bilayer: Arrangement of phospholipid molecules ‘Tails to tails’;
Molecules can move in membrane;
Intrinsic proteins extend through bilayer: Channel and carrier proteins
Extrinsic proteins in outer layer only: Act as antigens, receptors
Glycoproteins and glycolipids form glycocalyx
Presence of cholesterol to help regulate fluidity