1.3 - Cell Membranes and Transport

Question 1

(a)

Fluid-Mosaic model

Answer 1

Fluid – because the phospholipid molecules within a layer can move relative to each other.
Mosaic – because the proteins within the phospholipid layer are of different sizes and shapes and form different patterns.

Question 2

(a)

Extrinsic proteins

Answer 2

Found on either outer surface of the bilayer. Those with sugars attached
(glycoproteins) form the glycocalyx layer of the membrane which has a role in cell-to-cell recognition or hormone receptor sites.

Question 3

(a)

Intrinsic proteins

Answer 3

These proteins span the whole phospholipid bilayer and can form channel proteins and carrier proteins

Question 4

(a)

Channel proteins

Answer 4

Pores lined with polar (hydrophilic) groups that allow charged ions through, e.g. Na+.

Question 5

(a)

Carrier proteins

Answer 5

Allow larger polar molecules through,
such as water-soluble sugars and amino acids. Binding of the molecules changes the shape of the protein moving the substance into or out of the cell.

Question 6

(a)

Phospholipid bilayer

Answer 6

The hydrophilic phosphate heads of the phospholipids form the outer and inner surface of the cell membrane. The hydrophobic fatty acid tails of the phospholipids point towards each other in the centre of the bilayer.

Question 7

(a)

Cholesterol

Answer 7

Found between the
phospholipids making it
more rigid and stable.

Question 8

(b)

Factors affecting permeability of the plasma membrane

Answer 8

Permeability can be increased by:
Temperature
Organic solvents
pH

Question 9

(b)

Temperature effect on permeability of the plasma membrane

Answer 9

As temperature increases, the phospholipids have more kinetic energy and move more, increasing the fluidity and permeability of the plasma membrane.

Question 10

(b)

Temperature effect on permeability of the plasma membrane (2)

Answer 10

If the temperature becomes too high, channel and carrier proteins will become denatured, affecting membrane permability. The cell will eventually break down completely.

Question 11

(b)

Organic solvents

Answer 11

Organic solvents dissolve membranes, disrupting cells. This increases the fluidity and permeability of the plasma membrane.

Question 12

(c)

Diffusion

Answer 12

Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient. It is a passive process and so requires no energy from ATP. Simple diffusion occurs through the
phospholipid bilayer.

Question 13

(c)

Factors affecting the rate of
diffusion

Answer 13

Diffusion rate is increased by:
* higher concentration gradient
* thinner membrane/shorter diffusion
distance
* larger surface area
* smaller molecules
* being non-polar or fat soluble
* increased temperature.

Question 14

(c)

Facilitated diffusion

Answer 14

This is the process of diffusion but for polar molecules or ions that cannot pass directly though the phospholipid bilayer. Protein channels or carriers are used.

Question 15

(c)

Active transport

Answer 15

The movement of substances aganist a concentration gradient (from a region of low concentration to a region of high concentration) across a membrane

Question 16

(c)

Active transport and the influence of cyanide

Answer 16

Cyanide is a respiratory inhibitor which will prevent aerobic respiration and the production of ATP in the mitochondria. Without ATP, active transport cannot occur, so cyanide reduces active transport.

Question 17

(c)

Osmosis

Answer 17

The net diffusion of water molecules from a region of high water potential to low water potential across a selectively permeable membrane.

Question 18

(c)

Water potential

Answer 18

Water potential (ψ) is the tendency of water molecules to move. The solute potential is the osmotic strength of the solution.

Question 19

(c)

Water potential equation

Answer 19

ψ = ψp + ψs
ψ = water potential
ψp = solute potential
ψs = pressure potential

Question 20

(c) Osmosis

Hypertonic

Answer 20

means higher concentration of solute and therefore lower water potential.

Question 21

(c) Osmosis

Hypotonic

Answer 21

means lower concentration of solute and therefore higher water potential.

Question 22

(c) Osmosis

Isotonic

Answer 22

means the same concentration of
solute and the same water potential, so no net movement of water between the two solutions.

Question 23

(c) Osmosis

Turgid

Answer 23

Turgid (firm) cells – in a hypotonic (less concentrated solution), cells take up water by osmosis. The pressure potential of the cell increases as the cytoplasm pushes on the cell wall.

Question 24

(c) Osmosis

Incipient plasmolysis

Answer 24

A cell in this state has lost enough water for the cell membrane to start being drawn away from the cell wall. This lowers the pressure potential to 0.

Question 25

(c) Osmosis

Plasmolysed

Answer 25

cells in hypertonic (more
concentrated) solutions become flaccid (floppy).

Question 26

(c) Osmosis

Animal cells

Answer 26

It is important animal cells are in an isotonic solution (same concentration of dissolved solutes inside and
outside cell) as they lack a cell wall. Cells can burst in hypotonic and shrink in hypertonic solutions due to osmosis.

Question 27

(c) Bulk transport

Endocytosis

Answer 27

Endocytosis (two main types):
* Phagocytosis – solids enter the cell.
* Pinocytosis – liquids enter the cell.

Question 28

(c) Bulk Transport

Endocytosis process

Answer 28

1. Plasma membrane folds inwards.
2. Plasma membrane engulfs the
   material.
3. Vesicle formed from plasma
   membrane enters the cell.

Question 29

(c) Bulk transport

Exocytosis process

Answer 29

1. Vesicle formed from the golgi moves towards the plasma membrane.
2. Vesicle fuses with plasma
   membrane.
3. Vesicle contents empty out of cell.
   ATP is required to move the vesicles so this is an active process.