2.1.5 Biological membranes

Question 1

What are the roles of membranes within cells?

Answer 1

• compartmentalisation between organelles and cytoplasm;
  lysosome = hydrolytic enzymes need to be kept separate from other organelles;
  controls what enters and leaves the organelle
• allows for conc gradients to be established and maintained;
• embedding of enzymes into the membrane

Question 2

What are the roles of membranes at the surface of cells?

Answer 2

• act as partially permeable barriers and control what enters and leaves the cell e.g water, ions, gases
• site of chemical reactions
• cell signalling

Question 3

What is cell signalling?

Answer 3

communication between cells;
cell recognition;
cells working together;
to trigger a response inside the cell;

Question 4

Why do phospholipids arrange themselves in a bilayer at the cell surface?

Answer 4

the hydrophilic heads orientate themselves towards water and the hydrophobic tails orientate themselves away from water;
there is water outside the cell; (extracellular fluid)
and there is water inside the cell (cytoplasm)

Question 5

Why is the membrane called ‘fluid’?

Answer 5

• components in membrane move around + flexible

Question 6

Why is the membrane called ‘mosaic’?

Answer 6

• lots of proteins embedded

Question 7

What is the function of glycoprotein and glycolipids?

Answer 7

cell recognition;
cell signalling;
act as antigens;
act as antigens;
cell adhesion;

Question 8

What do channel proteins transport?

Answer 8

large and polar substances

Question 9

What do carrier proteins transport?

Answer 9

polar substances

Question 10

What is the role of cholesterol?

Answer 10

• found between the tails of the phospholipids
• regulates fluidity
• more cholesterol = less fluid the membrane is

Question 11

What 3 factors affect the permeability of cell membranes?

Answer 11

1) heat
2) ethanol
3) detergents

Question 12

How does heat effect the permeability of cell membranes?

Answer 12

as temp increases, the phospholipids gain kinetic energy;
this causes gaps to appear between the phospholipids;
the membrane becomes more permeable;
at high temp the proteins denature

Question 13

What occurs to cell membrane when put in cold temperature/freezing?

Answer 13

when water freezes it expands;
forms ice crystals which pierce and damage the cell membrane;

Question 14

How does ethanol effect the membranes permeability?

Answer 14

ethanol dissolves the phospholipid bilayer;
high conc of ethanol make the membrane more permeable;
same with detergent

Question 15

PAG; What should be controlled in the beetroot expt?

Answer 15

length of beetroot;
same species;
same volume of water;

Question 16

PAG; Why do you dry beetroot samples when you have cut them?

Answer 16

to remove any excess pigment caused by damaging the membranes in the cutting process

Question 17

What is diffusion?

Answer 17

the net movement of molecules from an area of high concentration to an area of low concentration

Question 18

What is facilitated diffusion?

Answer 18

diffusion via proteins

channel: ions
carrier: large (glucose, amino acids)

Question 19

What is active transport?

Answer 19

movement of molecules against the concentration gradient (low to high);
use of carrier proteins;
ATP required;

Question 20

Describe the process of exocytosis?

Answer 20

cytoskeleton contracts;
to move the vesicle to the cell plasma membrane;
the membrane and the vesicle fuse;
releasing it’s contents

endocytosis the opposite

= both require ATP

Question 21

What is osmosis?

Answer 21

the net movement of water molecules;
from a region of higher water potential to an area of lower water potential;
through a partially permeable membrane;

Question 22

What is meant by hypotonic?

Answer 22

water moves inside of a cell as the external solution has a higher water potential

Question 23

What is meant by hypertonic?

Answer 23

water moves out of the cell as the external solution has a lower water potential

Question 24

What happens if red onion cells are placed in a hypotonic solution?

Answer 24

water moves in down the water potential gradient;
by osmosis;
water enters the vacuole;
vacuole pushes out and puts turgid pressure on the cell wall;
cells become turgid

Question 25

What happens if a red onion cell is placed in a hypertonic solution?

Answer 25

water leaves the cells via osmosis; down the water potential gradient;
cells become flaccid;
eventually plasmolysed