Membranes

Question 1

roles of membranes at the surface of cells

Answer 1

1- they are a barrier between the cell and the environment, controlling which substances can enter and leave the cell, they are partially permeable
2- play roles in cell signalling by acting as an interface for communication between cells

Question 2

roles of intracellular membranes

Answer 2

1- divide cell into compartments, such as organelles and vacuoles
2- can form vesicles to transport substances between different areas of the cell
3- control what substances enter and leave organelles, eg RNA leaving nucleus by nuclear membrane
4- membranes within cells can be site of chemical reactions eg inner membrane of mitochondria contains enzymes needed for respiration

Question 3

what does the fluid mosaic model consist of

Answer 3

• phospholipids
• cholesterol
• glycoproteins
• glycolipids

Question 4

phospholipids role in membrane

Answer 4

• they have hydrophobic tail and hydrophilic head so arrange themselves into a bilayer

Question 5

cholesterol role in membrane

Answer 5

it regulates the fluidity of membranes
- positioned between phospholipids in a membrane bilayer

Question 6

what are the two types of proteins in a cell membrane

Answer 6

intrinsic and extrinsic

Question 7

what is an intrinsic protein

Answer 7

they are proteins that are embedded through both layers of a membrane
- channel and carrier proteins are intrinsic and involved in transport across the membrane
- glycoproteins, protein with attached carbohydrate chain, they play a role in cell adhesion/anchorage

Question 8

what is an extrinsic protein

Answer 8

they are present in only one side of the bilayer

Question 9

explain cell signalling

Answer 9

1- one cell releases a messenger molecule eg a hormone
2- this molecule travels in the blood to another cell
3- messenger molecule is detected by the cell as it binds to a receptor on its cell membrane

Question 10

what are receptors

Answer 10

• they are proteins in the membrane
• membrane bound receptors have specific shapes- only messenger molecules with specific shape can bind to them
• a cell that responds to a particular messenger molecule is a target cell

Question 11

what are two factors that affect membrane structure

Answer 11

1- temperature
2- solvents

Question 12

explain how temperature affects membrane structure

Answer 12

• as temp increases the phospholipids will have more kinetic energy and will move more
• this makes the membrane more fluid and it begins to lose its structure
• the loss of structure increases the permeability of the membrane making it easier for particles to cross it
• carrier and channel proteins in membrane will be denatured at higher temperatures, so membrane permeability will be affected

Question 13

explain how solvents affect membrane structure

Answer 13

many organic solvents are less polar than water eg alcohols. organic solvents will dissolve membranes disrupting cells.
when a membrane is disrupted it becomes more fluid and more permeable
increasing the concentration will increase the membrane permeability

Question 14

describe how you would investigate membrane permeability using beetroot

Answer 14

1- cut five equal sized pieces of beetroot
2- place each piece into a test tube with 5cm3 of water
3- place each test tube in a water bath at different temperatures for the same length of time
4- remove the beetroot pieces, leaving just the coloured liquid
5- using a colorimeter measure the absorbance of each test tube.

you should find that higher the temp, the higher the permeability so more pigment is released and higher the absorbance

Question 15

name the different types of movement through a cell membrane

Answer 15

diffusion - passive
facilitated diffusion - passive
osmosis - passive
active transport - active

Question 16

define diffusion

Answer 16

is the net/overall movement of particles from a region of higher concentration to a region of lower concentration

Question 17

name the factors that affect diffusion

Answer 17

temperature- particles have more kinetic energy, so move at higher speeds, overall faster rate of diffusion
concentration difference- the greater the difference in concentration (concentration gradient )between two regions, the faster the rate of diffusion,

Question 18

explain how facilitated diffusion works

Answer 18

it is diffusion across a membrane through protein channels. membranes with protein channels are selectively permeable.
it is a passive process and occurs down a concentration gradient

Question 19

define active transport

Answer 19

is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration
against a concentration gradient
it is an active process, requires ATP
- it also used carrier proteins, acting as pumps

Question 20

what is bulk transport

Answer 20

larger molecules such as enzymes, hormones are too large to move through channel or carrier proteins so move into and out of cell by bulk transport

Question 21

define endocytosis

Answer 21

it is the bulk transport of material into cells
there are two types:
- phagocytosis for solids
-pinocytosis for liquids

Question 22

define exocytosis

Answer 22

vesicles formed by the golgi move towards and fuse with the cell surface membrane. the contents of the vesicle is then released outside the cell

Question 23

define osmosis

Answer 23

it is the net movement of water across a partially permeable membrane from a high water potential to a more negative water potential down a water potential gradient

Question 24

define water potential

Answer 24

The likelihood of water molecules diffusing is called the water potential. When there are lots of water molecules (in a dilute solution) on one side of a partially permeable membrane, but not many on the other side (in a concentrated solution), water will move from the dilute to the concentrated solution.