Transport across cell membrane

Question 1

What is the cell-surface membrane made up of and what are their functions?

Answer 1

Phospholipids - made of a fatty acid tail and phosphate head - used for simple diffusion

Intrinsic proteins - fluid filled pore needed for active transport

Extrinsic proteins - found on the outside of the membrane and needed for facilitated diffusion and active transport

Glycolipids - made of carbohydrates and lipids and used as cell surface markers for recognition and communication

Glycoproteins - made from carbohydrates and proteins

Cholesterol - needed for structure - gives membrane rigidity however is still flexible

Question 2

Why is the term ‘fluid mosaic’ used to describe the arrangement of the cell membrane?

Answer 2

It has fluidity and can constantly move and change shape

Is made up of 7 different components arranged in a mosaic looking pattern

Question 3

Define diffusion

Answer 3

The net movement of molecules from high to low concentration with no energy input

Question 4

How does simple diffusion occur?

Answer 4

Molecules can move through the membrane when the phospholipids move and have fluidity, which creates gaps for the molecules to move through. The molecules must be small and non-polar (neutral)

Question 5

How does facilitated diffusion occur?

Answer 5

FD requires carrier and channel proteins with specific receptors for the molecule being transported. The proteins will carry the molecule through the membrane. Molecules are larger and can be polar. Occurs down a concentration gradient.

Question 6

What are the similarities and differences between simple diffusion and facilitated diffusion?

Answer 6

• Both high to low concentration
• Both require non-charged molecules
• Both passive processes
• FD requires a carrier protein whereas SD does not
• FD moves larger molecules whereas SD moves smaller molecules

Question 7

Why does FD level off after time?

Answer 7

Limited carrier proteins means that FD will level off because at some point they will all be occupied. SD will always be linear

Question 8

Define osmosis

Answer 8

The net movement of water particles from high to low water potential through a partially permeable membrane

Question 9

If solute is added to a cell, what is the effect on the water potential?

Answer 9

KPa value would be more negative

Question 10

Define active transport

Answer 10

The net movement of molecules from low to high concentration using energy from the hydrolysis of ATP using carrier proteins

Question 11

What are the steps in co-transport?

Answer 11

• using energy from ATP, the sodium-potassium pump actively transports sodium ions from the epithilial cells into the bloodstream, so there is a low concentration of sodium ions in epithilial cells (potassium ions are also transported into epithilial cells)
• the lumen of the ileum contains a high concentration of sodium ions, leading to a concentration gradient
• sodium-glucose cotransporter diffuses sodium ions down their concentration gradient, as they are transported, they carry glucose molecules which are also transported against their concentration gradient by active transport
• glucose moves into the bloodstream by facilitated diffusion

Question 12

How are the phospholipids within a cell surface membrane organised?

Answer 12

• hydrophilic heads which point to the outside of the membrane attracted by water on both sides
• hydrophobic tails point into the centre of the membrane, repelled by water on both sides

Question 13

What is the function of phospholipids in the cell surface membrane?

Answer 13

• allow lipid soluble substances to enter and leave the cell
• prevent water soluble substances entering and leaving the cell
• make the membrane flexible and self-sealing

Question 14

In which 2 ways are proteins embedded into the surface membrane?

Answer 14

• some proteins occur in the surface of the bilayer (glycoproteins), and either mechanical support, or act as cell receptors for molecules such as hormones
• other proteins act as protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane. Others are carrier proteins that bind to ions like glucose and amino acids, then change shape to move these molecules across the membrane

Question 15

What are the functions of protein in the surface membrane?

Answer 15

• structural support
• allow active transport through carrier proteins
• act as channels supporting water-soluble substances across the membrane
• form receptors to identify cells

Question 16

Why is it important that cholesterol in the cell membrane is hydrophobic?

Answer 16

• they are very hydrophobic which prevents water loss and loss of dissolved ions

Question 17

What is the function of cholesterol in the membrane?

Answer 17

• reduces lateral movement of substances including phospholipids
• makes the membrane less fluid at high temperatures
• prevents leakage of water and dissolved ions from the cell

Question 18

What are the functions of glycolipids in the membrane?

Answer 18

• act as recognition sites
• maintain stability of membrane
• help cells attach to each other to form tissues

Question 19

What are the functions of glycoproteins in the membrane?

Answer 19

• act as recognition sites
• help cells attach to each other to form tissues
• allow cells to recognise each other eg. lymphocytes can recognise an organisms own cells

Question 20

Why do most molecules not freely diffuse into the membrane?

Answer 20

• not soluble in lipids so cannot pass through the phospholipid bilayer
• too large to fit through protein channels
• of the same charge as the protein channels, so repelled
• electrically charged so cannot pass through the non-polar hydrophobic tails

Question 21

How are epithelial cells specialised for rapid co-transport of glucose?

Answer 21

• large number of mitochondria - provide ATP for sodium potassium pump
• many folds called microvilli - large surface area with more carrier proteins
• steep concentration gradient because glucose is rapidly transported away in the bloodstream