Cell Surface Membrane

Question 1

What are cell surface membranes

Answer 1

Surround cells. They are a barrier between the cell and its environment, controlling which substances enter and leave the cell.
They’re partially permeable- they let some through but not others.

Question 2

How do substances move through the cell surface membrane

Answer 2

Through diffusion, osmosis or active transport

Question 3

What is a cell surface membrane sometimes called

Answer 3

Plasma membrane

Question 4

What do the membranes around organelles do give examples

Answer 4

Divide the cell into different compartments - they act as a barrier between the organelle and the cytoplasm.
E.g substances needed for respiration (like enzymes) are kept together inside a mitochondrion by the membrane surrounding the mitochondrion.

They are partially permeable and control what substances enter and leave the organelle
E.g RNA leaves the nucleus via the nuclear membrane (also called nuclear envelope) . DNA is too large to pass through the partially permeable membrane so remains in the nucleus.

Question 5

What are the three functions of membranes

Answer 5

1) membranes around organelles divide the cell up into different compartments to make the different functions more efficient - eg substances needed for respiration like enzymes are kept together inside the mitochondria.
2) membranes control which substances enter and leave a cell or organelle.
3) membranes recognise specific chemical substances and other cells.

Question 6

What is the structure of a membrane

Answer 6

The structure of all membranes is the same. They are composed of lipids (mainly phospholipids) , proteins and carbohydrates (usually attached to proteins or lipids).

Question 7

What is the fluid mosaic model

Answer 7

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 (bilayer). This bilayer is ‘fluid’ because the phospholipids are constantly moving.

Question 8

What are the proteins like in the fluid mosaic model

Answer 8

Proteins are scattered through the bilayer, like tiles in a mosaic. These include channel proteins and carrier proteins, which allow large molecules and ions to pass through the membrane.
Receptor proteins on the cell surface membrane allow the cell to detect chemicals released from other cells. The chemical signal to the cell to respond in some way e.g the hormone insulin binds to receptor proteins on liver cells, which tell the cell to absorb glucose

Question 9

What can some proteins do

Answer 9

Some can move sideways through the bilayer while others are in a fixed position

Question 10

What are glycoproteins

Answer 10

Proteins have a carbohydrate attached to them. They’re found in the outer layer.
Are proteins with attached polysaccharides of short, branched chains of monosaccharides. Have a variety of specific shapes due to the different branching patterns of the monosaccharides. These allow different cells to recognise each other. E.g some glycoproteins are antigens- they’re recognised by white blood cells which starts an immune response.

Question 11

What are glycolipids

Answer 11

Lipids that have a carbohydrate attached to them

Lipids combined with polysaccharides, they’re found in the outer layer of cell membranes. Involved in cell recognition.

Question 12

Is cholesterol present in the bilayer

Answer 12

Belongs to a group of lipids called steroids. It’s present in all cell membranes except those of bacteria. It can make up to 25% of the lipids in animal cell membranes but isn’t found so much in plant cell membranes.

Having cholesterol molecules between phospholipid molecules makes the membrane less fluid and more stable.

Cholesterol the hydrophobic tails of the phospholipids causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
Cholesterol helps to maintain the shape of animal cells which don’t have cell walls. This is particularly important for cells that aren’t supported by other cells e.g red blood cells which float free in the blood.
Cholesterol also has hydrophobic regions so it’s able to create a further barrier to polar substances moving through the membrane.

Question 13

What are intrinsic proteins for

Answer 13

Completely span the membrane from inside to outside
For transport
Intrinsic channel proteins form a tiny gap in the membrane to allow water soluble molecules and ions through by diffusion.
Intrinsic carrier proteins carry water soluble proteins and ions through the membrane by active transport and facilitated diffusion.

Question 14

What are extrinsic proteins for

Answer 14

Only partly span the membrane they’re stuck in either the outer phospholipid layer or the inner phospholipid layer
They’re receptors
Recognise and bind on to specific molecules e.g hormones which trigger other events.
Enzymes can be embedded in the inner membrane of a cell or organelle e.g ATPase in the inner membrane of mitochondria
Can also be involved in cell recognition and cell signalling or enzymes such as Maltase in the small intestine .

Question 15

What can these proteins do

Answer 15

Slide around the membrane quickly and collide with each other, but never flip from one side to another.

In the inside surface of cell membranes are often attached to the cytoskeleton and are involved in maintaining the cell’s shape, or in cell motility. They may also be enzymes catalysing reactions in the cytoplasm.

Help strengthen the membrane there are hydrogen bonds between the proteins and the hydrophilic heads of the phospholipids

The proteins have hydrophilic amino acids in contact with the water on the outside of membranes and hydrophobic amino acids in contact with fatty chains inside the membrane.

Question 16

What do carbohydrates do

Answer 16

Found on the outer surface of the eukaryotic cell membrane and attached to the membrane proteins and lipids e.g glycoproteins and glycolipids
The carbs are short polysaccharides composed of different monosaccharides and form a cell coat or glycocalyx outside the cell membrane.
The glycocalyx is involved in protection and cell recognition and antigens such as ABO antigens on blood cells are usually cell surface glycoproteins.

Question 17

What is a membrane composed of

Answer 17

Not just lipid bilayer but lipid protein and carbohydrate parts.

Question 18

What do the phospholipids do

Answer 18

Form a barrier to dissolved (water soluble) substances.
They have a head and a tail. The head / glycerol head is hydrophilic it attracts water and the tail / fatty acid tails 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 hydrophobic tails pack together inside the layer away from the water. This is also in aqueous solutions.

Phospholipids consist of a glycerol molecule two molecules of fatty acid and a phosphate group

Question 19

What is the centre of the bilayer like

Answer 19

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.
Small non polar substances e.g carbon dioxide and water can diffuse through the membrane.
Water soluble molecules e.g glucose can’t pass through the fatty hydrophobic interior of the membrane.

Question 20

Why can water diffuse through the membrane if it’s polar

Answer 20

It can diffuse through osmosis

Question 21

What is a polar molecule

Answer 21

Has one end with a slightly positive charge and one end with a slightly negative charge. They’re not as strong as charged on ions. Non polar substances have no charges.

Question 22

What is the definition of simple diffusion (lipid diffusion)

Answer 22

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.

Question 23

Does simple diffusion need energy

Answer 23

Is a passive process no energy is needed for it to happen. Particles can diffuse across cell membranes as long as they can move freely through the membrane. When molecules diffuse directly through a cell membrane also known as simple diffusion.

Question 24

Give an example of simple diffusion

Answer 24

Oxygen and carbon dioxide can diffuse easily through cell membranes because they’re small, so they can pass through spaces between the phospholipids. They’re also non polar which makes them soluble in lipids so they can dissolve in the hydrophobic bilayer.

Question 25

What few substances can diffuse directly through the lipid bilayer

Answer 25

Only substances that can do this are lipid soluble (uncharged) molecules such as steroids or very small molecules such as H2O, O2, and CO2. For these molecules the membrane is no barrier at all. Charged molecules such as ions cannot go through phospholipids.

Question 26

Factors affecting the rate of diffusion

Answer 26

Rate of diffusion across both external and internal cell membranes can vary. Some specialised cells are adapted for rapid transport across their membranes

Question 27

Rate of diffusion depends on

Answer 27

Concentration gradient - the higher it is the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium (ie concentration on both sides is equal) this means that diffusion slows down over time.

The thickness of the exchange surface- the thinner the exchange surface I.e the shorter the distance the particles have to travel. The faster the rate of the diffusion. Short diffusion path .

The surface area- the larger the surface area e.g of the cell surface membrane ) the faster the rate of diffusion

Question 28

Give an example of something with a large surface area

Answer 28

Epithelial cells in the small intestine have microvilli

Question 29

What molecules diffuse in facilitated diffusion

Answer 29

Some larger molecules e.g amino acids and glucose would diffuse extremely slowly through the phospholipid bilayer because they’re so large. 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 facilitated diffusion.

Question 30

How is facilitated diffusion similar to simple diffusion

Answer 30

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.

Question 31

Carrier proteins in f diffusion

Answer 31

Move large molecules across membranes down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.

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.

Have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane . The substance will bind on the side where it’s a high concentration and causes a conformational shape change. and be released where it is at a lower concentration.

Question 32

Channel proteins in f diffusion

Answer 32

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.

Form a water filled pores or channel in the membrane. This allows charged substances (usually ions) to diffuse across the membrane. Most channels can be gated (opened or closed) allowing the cell to control the entry and exit of ions.

Question 33

Factors rate of f diffusion

Answer 33

Depends on concentration gradient - the concentration gradient the faster the rate of f diffusion as equilibrium is reached the rate of f diffusion will level off.

The number of channel or carrier proteins - once all the proteins in a membrane are in use, f diffusion can’t happen any faster even if you increase the concentration gradient.

Question 34

What is osmosis

Answer 34

Diffusion of water molecules across a partially permeable membrane from an area of higher water potential higher concentration of water molecules to an area of lower water potential I.e lower concentration of water. Water potential is the potential the likelihood of water molecules to diffuse out of or into a solution.
Water molecules are small and can diffuse easily through the cell membrane but large solute molecules can’t.

Question 35

What’s the water potential of pure water

Answer 35

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.

Question 36

What is an isotonic solution

Answer 36

If two solutions have the same water potential
Cells in an isotonic solution won’t lose or gain any water there’s no net movement of water molecules because there’s no difference in water potential between the cell and the surrounding solution.

Question 37

What is hypotonic

Answer 37

Solutions with a higher water potential compared with the inside of the cell are hypotonic it will swell.

Question 38

Hypertonic

Answer 38

Cell is placed in a solution that has a lower water potential it may shrink as water moves out by osmosis.