Transport Across membranes

Question 1

What are the functions of phospholipids in a phospholipid bilayer

Answer 1

• Allow lipid-soluble substances to enter and leave the cell
• Prevent water-soluble substances enter and leave the cell
• Make the membrane flexible and self-sealing

Question 2

Explain why phospholipids form a bilayer in plasma membranes (4)

Answer 2

• Phospholipids have a charged phosphate group which is part of the hydrophilic polar head
• Fatty acid tails (hydrocarbon chains) are not charged/ not polar and are hydrophobic
• Both tissue fluid and cytoplasm are aqueous solutions
• Phospholipids form two layers called a bilayer, with the hydrophobic fatty acid tails facing inward towards each other
• And hydrophilic phosphate groups facing outward interacting with the aqueous environment

Question 3

Explain why a cell membrane may be described as a fluid-mosaic model?

Answer 3

• Phospholipid molecules can move freely with respect to one another, this makes the membrane fluid
• Proteins are distributed throughout the membrane unevenly and in a mosaic pattern when viewed from above
• The agreed structure is based upon experimental/ chemical evidence and so is classed as a model

Question 4

What are the functions of carbohydrates in the cell membrane? (Glycoproteins, glycolipids)

Answer 4

Involved in cell signalling and cell to cell recognition

Question 5

What is the function of cholesterol in cell membrane?

Answer 5

Controls membrane fluidity

Question 6

What is a glycolipid?

Answer 6

A phospholipid with a carbohydrate attached that faces out of the membrane

Question 7

How does a intrinsic protein differ from an extrinsic protein (1)

Answer 7

Intrinsic proteins span the phospholipid bilayer but extrinsic proteins do not span the membrane (extrinsic proteins are embedded in one side only or are surface proteins)

Question 8

Which part of plasma membrane is involved in cell-cell recognition?

Answer 8

Some glycolipids or glycoproteins

Question 9

Give three functions of the cell membrane

Answer 9

Controls what goes in and out the cell:
- Exocytosis/ secretion of certain molecules
- Taking in certain molecules into the cell
- Allow O2 into the cell + CO2 out of the cell

Question 10

Describe diffusion across membranes

Answer 10

Diffusion is the net movement of molecules from a higher concentration to a lower concentration across a partially permeable membrane

Question 11

What factors affect rate of diffusion

Answer 11

• Temperature (increase in KE therefore faster rate)
• Surface Area (more “space” for molecules to pass through therefore increases rate)
• Concentration gradient (as concentration difference increases, rate of diffusion increases)
  -Diffusion distance (shorter distance faster movement)

Question 12

What is Facilitated diffusion?

Answer 12

Diffusion of polar, hydrophilic molecules that pass through the membrane via the help of channel or carrier proteins.

Question 13

Name 2 factors which affect the rate of diffusion

Answer 13

Any two from:
Temperate, Diffusion distance, Concentration gradient, Surface area

Question 14

Describe osmosis

Answer 14

Osmosis is the net movement of water molecules from higher water potential to a solution with lower water potential through a partially permeable membrane.

Question 15

Describe Active Transport

Answer 15

Net movement of molecules from a low concentration to high concentration with the help of carrier proteins and energy supplied by ATP
(Against concentration gradient)

Question 16

What is Exocytosis

Answer 16

Method of transport that uses vesicles to move very large molecules (like enzymes/ and other proteins) from inside the cell to outside the cell

(ATP is required)

Question 17

What is Fick’s Law?

Answer 17

Rate of diffusion is proportional to =
(Surface Area x Concentration Gradient) / Diffusion Distance

Question 18

What is Endocytosis

Answer 18

The cell surface membrane is “pulled” inwards, to create a vesicle and any molecule next to it will be enclosed within the vesicle.

(Requires ATP)

Question 19

Contrast the processes of facilitated diffusion and active transport (3)

Answer 19

Facilitated diffusion can involve both channel proteins and carrier proteins BUT active transport involves only carrier proteins

Facilitated diffusion is down a concentration gradient BUT active transport is against a concentration gradient

Facilitated diffusion is passive (no ATP) needed BUT active transport requires ATP hydrolysis