module 2 - cell membranes and transport

Question 1

describe the roles of membranes within cells and at the surface of cells

Answer 1

• physical barrier : separates intracellular and extracellular environments from one another
• regulates exchange of substances : as they are partially permeable they can allow certain substances through
• compartmentalisation : they enclose and isolate organelles to maintain specific conditions for reactions

Question 2

describe the roles of membranes within cells and at the surface of cells (con.)

Answer 2

• supports the cytoskeleton
• sites for chemical reactions
• sites of cell communication —> this is another way of saying cell signalling

Question 3

FLUID MOSAIC - what is meant by the ‘fluid mosaic’ model of membrane structure?

Answer 3

• fluidity of membrane and the mosaic arrangement of the protein gives structure of membrane its name - fluid mosaic model
• includes components scattered within membrane such as phospholipids, cholesterol, proteins and carbohydrates

Question 4

what is a glycoprotein and its function in the membrane?

Answer 4

• WHAT IS IT : the carbohydrate portion of a protein which branches
• FUNCTION : is a recognition site for chemicals such as hormones

Question 5

what is the function of a glycolipid within a membrane?

Answer 5

• another recognition site, for example for toxins

Question 6

what is the function of cholesterol within a membrane?

Answer 6

• found within phospholipid bilayer
• adds strength to the structure, helps maintain fluidity of the membrane

Question 7

what is an extrinsic protein within the membrane and describe its function

Answer 7

• WHAT IS IT : a protein molecules which is partially embedded within the membrane
• could also be a protein molecule lying on the surface of the membrane
• FUNCTION : act as channels/pumps to allow specific molecules to pass through the membrane, or as receptors to allow the cell to respond to signals from other cells/environment

Question 8

what are hydrophobic tails within a membrane?

Answer 8

• point inwards, are the tails of phospholipid molecules

Question 9

what are hydrophilic heads within a membrane?

Answer 9

• point outwards, are the heads of phospholipid molecules

Question 10

what is the function of pores within a membrane?

Answer 10

• provide selective partial permeability to the membrane

Question 11

what is an intrinsic protein within a membrane and describe its function

Answer 11

• a protein molecule which spans the phospholipid layer
• FUNCTION : act as channels/pumps to allow specific molecules to pass through the membrane, or as receptors to allow the cell to respond to signals from other cells/environment

Question 12

describe the effect of temperature on membrane structure/permeability

Answer 12

• TEMPERATURE DECREASE : reduces fluidity and becomes rigid, this is because phospholipids move less due to having lower kinetic energy
• TEMPERATURE INCREASE : increases fluidity therefore also increases permeability (proteins become heavily denatured)
• however, above a certain temperature, the membrane will also start to lose its structure, break apart

Question 13

how does type of solvent affect membrane structure/permeability?

Answer 13

• organic, weakly polar solvents such as ethanol disrupt or dissolve the membrane, therefore make it more permeable —> lipids dissolve in alcohol, - phospholipids in cell membrane easily dissolve, - cell membrane becomes more fluid and permeable as it starts to break down
• non-polar solvents such as benzene have the same effect

Question 14

TRANSPORT ACROSS MEMBRANES - what are the ways molecules can move across membranes?

Answer 14

• diffusion, facilitated diffusion (passive)
• active transport : endocytosis, exocytosis

Question 15

define the term ‘diffusion’

Answer 15

• net movement of particles from a region of higher to lower concentration
• passive process not requiring energy

Question 16

how does diffusion through the bilayer occur?

Answer 16

• large lipid-soluble molecules, non-polar molecules and very small polar molecules can pass directly through the bilayer

Question 17

how does facilitated diffusion take place?

Answer 17

• ions and larger polar molecules pass through transmembrane proteins for example :
• channel proteins which act as gated pores and allow the diffusion of ions
• carrier proteins which have specific shapes allowing only the passage of specific molecules or ions

Question 18

list the factors that increase the rate of diffusion

Answer 18

• steeper concentration gradient : this means a bigger difference between high and low concentration
• shorter diffusion pathway
• greater surface area
• a higher concentration of carrier proteins in facilitated diffusion

Question 19

describe the process of active transport and what is required for it to take place?

Answer 19

• requires energy in the form of ATP (adenosine triphosphate)
• describes the net movement of particles being moved against the concentration gradient, from lower to higher concentration

Question 20

what are carrier protein pumps?

Answer 20

• carry out much of the same process as in facilitated diffusion, but this time they require ATP hydrolysis and therefore is not a passive process
• phosphate ions are a product of ATP hydrolysis

Question 21

define the term ‘bulk transport’

Answer 21

• the movement of large particles within vesicles in and out of cells

Question 22

describe the process of endocytosis and the two types

Answer 22

• form of bulk transport into cells
• vesicles are formed through plasma membrane closing off
• two forms : pinocytosis & phagocytosis
• pinocytosis : where the cell engulfs small particles and liquid
• phagocytosis : where the cell engulfs large solid material

Question 23

TRANSPORT OF WATER ACROSS MEMBRANES - what is osmosis?

Answer 23

• can be defined as the diffusion of water through a partially permeable membrane down a water potential gradient
• always operates from an area of high water potential to an area of lower water potential

Question 24

describe what is meant by ‘water potential’

Answer 24

• water potential decides the direction in which water may diffuse across a membrane, and this is measured in kilopascals (kPa)
• pure water has the highest possible water potential of 0
• the more solute dissolves in water, the lower the water potential will be
• dilute glucose solution has a high water potential of -20kPa
• concentrated glucose solution has a low water potential of -400kPa

Question 25

what are the effects that solution of different water potentials can have on plant and animal cells? (HIGH)

Answer 25

HIGH WATER POTENTIAL surrounding the cell
- the net movement of water is that it enters the cell
- an animal cell will undergo lysis, it will burst and swell
- a plant cell will swell and become turgid

Question 26

what are the effects that solutions of different water potentials can have on plant and animal cells? (EQUAL)

Answer 26

EQUAL WATER POTENTIAL surrounds the cell as inside the cell
- the net movement is equal regarding water leaving and entering the cell
- an animal cell will undergo no change
- a plant cell will undergo no change

Question 27

what are the effect that solutions of different water potentials can have on plant and animal cells? (LOW)

Answer 27

LOWER WATER POTENTIAL surrounds the cell than inside the cell
- the net movement of water is water leaving the cell
- an animal cell will undergo crenation, which is shrinkage
- a plant cell will undergo plasmolysis, which is where the membrane detaches from the cell wall