Chapter 4: cell membranes and transport

Question 1

describe the ‘fluid mosaic model’

Answer 1

• both phospholipids and proteins can move about by diffusion
• the phospholipids move sideways, mainly in their own layers
• some protein molecules also move about within the phospholipid bilayer; others remain fixed to structures inside or outside the cell

Question 2

describe the features of the fluid mosaic model

Answer 2

• double layer membrane of phospholipid molecules

- phospholipid tails point inwards

Question 3

the more unsaturated the phospholipids, the more…the membrane

Answer 3

fluid. This is because the unsaturated fatty acid tails are bent, and therefore fit together more loosely

Question 4

fluidity is also affected by…

Answer 4

tail length. The longer the tail, the less fluid the membrane. As temperature decreases, membranes become less fluid

Question 5

proteins that are found within the membrane are called…

Answer 5

intrinsic proteins

Question 6

proteins that span the whole membrane are called…

Answer 6

transmembrane proteins

Question 7

‘extrinsic protein’ definition

Answer 7

proteins that are found on the inner or outer surface of the membrane

Question 8

role of phospholipids

Answer 8

• form the bilayer
• their tails are non-polar so polar molecules can’t pass through
• they act as a barrier to most water-soluble substances

Question 9

role of cholesterol

Answer 9

• they have hydrophilic heads and hydrophobic tails
• at low temperatures, cholesterol increases the fluidity of the membrane, preventing it from becoming to rigid
• the interaction of the phospholipid tails with the cholesterol molecules also helps to stabilise cells at higher temperatures when the membrane could become too fluid

Question 10

the carbohydrate chains form a sugary coating to the cell, known as the…

Answer 10

glycocalyx

Question 11

the carbohydrate chains perform what function?

Answer 11

they help the glycoproteins and glycolipids to act as receptor molecules, which bind with particular substances at the cell surgace

Question 12

examples of receptors

Answer 12

• ‘signalling receptors’- part of a signalling system that coordinates the activities of cells
• receptors involved in endocytosis. They bind to molecules that are parts of the structures to be engulfed by the cell surface membrane
• receptors involved in binding cells to other cells in tissues and organs of animals

Question 13

name the two types of transport proteins

Answer 13

• channel proteins

- carrier proteins

Question 14

name the three basic ways in which a receptor can change the activity of a cell

Answer 14

• opeining an ion channel
• acting as a membrane-bound enxyme
• acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane

Question 15

define diffusion

Answer 15

the net movement, as a result of random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration, down a concentration gradient

Question 16

name the three factors that affect the rate of diffusion:

Answer 16

• the ‘steepness’ of the concentration gradient
• the temperature
• the surface area across which diffusion is taking place
• the nature of the molecules or ions

Question 17

define ‘facilitated diffusion’

Answer 17

the diffusion of a substance through transport proteins in a cell membrane; the proteins provide hydrophilic areas that allow the molecules or ions to pass through the membrane which would otherwise be less permeable to them

Question 18

what is a partially permeable membrane?

Answer 18

a membrane that allows only certain molecules through

Question 19

what is water potential

Answer 19

the tendency of water to move out of a solution

Question 20

what is the solute potential?

Answer 20

the extent to which the solute molecules decrease the water potential of the solution

Question 21

what is the protoplast of a cell?

Answer 21

the living part of the cell inside the cell wall

Question 22

what is the role of the Na+-K+ pump?

Answer 22

to pump three sodium ions out of the cell at the same time as allowing two potassium ions into the cell for each ATP molecule used

Question 23

active transport definition

Answer 23

the movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP

Question 24

endocytosis definiton

Answer 24

the bulk movement of liquids or solids into a cell, by the infolding of the cell surface membrane to form vesicles containing the substance; endocytosis is an active process requiring ATP

Question 25

exocytosis definition

Answer 25

the bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane; exocytosis is an active process requiring ATP