1.3 Membranes and Transport

Question 1

What are all cells surrounded by?

Answer 1

A cell membrane

May also be called the cell surface membrane or the plasma membrane

Question 2

How does the cell membrane appear under the electron microscope?

Answer 2

As a double line

Question 3

What is the usual distance across the cell membrane under the electron microscope?

Answer 3

7-8nm

Question 4

What are the principal biochemical constituents of the cell membrane?

Answer 4

Protein and phospholipids

Question 5

How are the phospholipid molecules arranged in the cell membrane?

Answer 5

As a bilayer

Hydrophilic heads and hydrophobic tails (tails on the inside of the membrane)

Question 6

What proteins can be found in the membrane?

Answer 6

Channel proteins (like a tube straight through the membrane. The inside of the tube is polar.)
Extrinsic proteins (partially crosses bilayer)
Intrinsic (fully cross the bilayer)
Surface proteins (Some have a carbohydrate chain used for cell recognition = glycoproteins)
Carrier proteins (Some require ATP. Could be enzymes, electron carrier/proton pump. 'Pick up' a molecule, turn so they face the inside of the cell and deposit the molecule inside.)

Question 7

What is the fluid-mosaic model?

Answer 7

The components are free to move with respect to each other

Question 8

Draw a simple diagram to illustrate the fluid mosaic model.

Answer 8

Label:
phospholipid bilayer
channel, surface, extrinsic, intrinsic & carrier proteins
glycoproteins

Question 9

What are the major functions of the cell membrane?

Answer 9

Taking up nutrients & other requirements of the cell
Secreting chemicals
Cell recognition

Question 10

What is the cell membrane selectively permeable to?

Answer 10

Water and some solutes

Question 11

What affects the permeability of a cell membrane?

Answer 11

Diffusion is affected by:
Steepness of the concentration gradient - If there is a greater difference in the concentrations of a solute inside and outside the cell, then it will travel faster down the concentration gradient.
Distance - shorter distance = shorter time eg alvioli
Warmer temperatures = more kinetic energy for the particles so they move faster.
Size of the molecule - smaller = better because it can diffuse through the phospholipid bilayer (eg O2 and CO2) rather than through a channel protein. Also have more kinetic energy at any given temperature.

Question 12

How do lipid-soluble substances and water-soluble substances enter and leave a cell?

Answer 12

Lipid-soluble substances can dissolve in the phospholipid bilayer and diffuse through.
Water-soluble substances use temporary protein channels.

Question 13

What is diffusion?

Answer 13

The movement of molecules or ions from a region of high concentration to one of low concentration.
It is passive (doesn’t require energy).

Question 14

What is osmosis?

Answer 14

A particular form of diffusion in which water molecules move down a water concentration gradient through a selectively permeable membrane.

Question 15

What is water potential?

Answer 15

The potential for water to move out of a solution by osmosis.
It has the symbol psi (like a little trident).
Pure water has the highest w/p, given the value 0psi.
All solutions have a lower potential than water bc they have a lower proportion of water molecules. Therefore psi(of a solution) always has a nevative value

Question 16

In plant cells, what is water potential the sum of?

Answer 16

The solute potential (psi little s) (the effect of solutes lowering the w/p of the cell sap)
and psi little p (opposite pressure provided by the cell wall. Usually positive)

Question 17

What happens to a cell when it becomes plasmolysed?

Answer 17

The cytoplasm of a plant cell draws away from the cell wall, pulling the cell membrane with it.
Animal cells simply shrink.
This is due to a lack of water inside the cell.

Question 18

What happens if a cell has an increased volume of water in it?

Answer 18

Animal cells will burst.
Plant cells continue to take in water until the opposing wall pressure prevents it. At this point the cell is referred to as turgid.

Question 19

What is facilitated diffusion?

Answer 19

It allows rapid exchange due to substances being helped across the membrane by carrier proteins. Does not require ATP.

Question 20

What is phagocytosis?

Answer 20

Where a large particle may enter the cell, become enclosed by a membrane to form a vesicle and be transported through the cytoplasm.

Question 21

Draw a diagram to illustrate phagocytosis.

Answer 21

.

Question 22

What is secretion?

Answer 22

Also called exocytosis.
Substances leaving the cell after being transported through the cytoplasm in a vesicle.
This and phagocytosis mean that the membrane is continually having portions removed or added to it.

Question 23

What is pinocytosis?

Answer 23

The entry of liquid into a cell by the same mechanism as phagocytosis.

Question 24

What is active transport?

Answer 24

Requires ATP from respiration
Movement of solutes against a concentration gradient
Will not take place in the presence of a respiratory inhibitor eg cyanide