Biology // Module 2 // Biological Membranes

Question 1

What is a plasma membrane and its uses?

Answer 1

A phospholipid bilayer which surrounds the cells of living organisms.
Uses:
-Compartmentalise organelles
-Regulates transport of materials in and out of cells.
-Partially permeable
-Chemical signals to other cells for communication.

Question 2

What is the fluid mosaic model of the plasma membrane?

Answer 2

Describes how the plasma membrane is a fluid, mosaic-like structure composed of phospholipids arranged in a bilayer structure. Membrane consists of a phospholipid bilayer with proteins floating in it making it a mosaic. Proteins and lipids in it change places making it fluid.

Question 3

What is the structure of phospholipid bilayer?

Answer 3

A main component, the phospholipids, are types of lipids made from two fatty acid chain ‘tails’ attached to a phosphate group ‘head’. Phosphate group head is polar and hydrophilic while the fatty acid chains of the tail are non-polar and hydrophobic.

Question 4

What are the structures formed by phospholipids?

Answer 4

When exposed to water, phospholipids form one of two structures - a micelle or a bilayer.
Micelle- Tails shields from water, one layer of water-fearing tails. Helps body absorb lipid and fat-soluble vitamins.
Bilayer- Two layers of water fearing tails. Two fatty acid chains are too big and bulky to fit into interior of a micelle.

Question 5

What is the role of Cholesterol in phospholipid bilayer and where is it formed?

Answer 5

-Adds stability to phospholipid bilayer.
-Waxy and fat-like substance produced in the liver
-Important in controlling membrane fluidity

Question 6

What is embedded in the plasma membranes?

Answer 6

Intrinsic and Extrinsic proteins

Question 7

What are intrinsic proteins?

Answer 7

Span across the whole width of a membrane. They are embedded in it.

Question 8

What is a Channel protein?

Answer 8

An intrinsic protein which acts like a pore in the membrane. It allows substances in a single file into the cell through hydrophilic channel. Allows polar molecules across which are too big to diffuse.

Question 9

What is a carrier protein?

Answer 9

An intrinsic protein which carries substances from one side of the membrane to another. They are used in passive and active transport.

Question 10

What are glycoproteins?

Answer 10

An intrinsic protein which is used in cell adhesion. It acts as receptors for cell signalling. It is a carbohydrate attached to a protein.

Question 10

What are glycolipids?

Answer 10

An intrinsic protein which is the site where drugs, hormones and antigens bind forming antibodies. It can act as receptors for viruses and other pathogens. They are carbohydrates attached to a lipid. Key in cell signalling and are called cell markers.

Question 11

What are extrinsic proteins?

Answer 11

Confined to either the inner or outer surface of the membrane. May be free on the membrane surface, or bound to an integral protein.

Question 12

What are the functions of extrinsic proteins?

Answer 12

On extracellular side:
-They act as receptors for hormones and neurotransmitters.
-Are involved in cell recognition
- Can be either glycoproteins or glycolipids

On cytocystic side:
-Involved in cell signalling or chemical reactions
-Can dissociate from membrane and move into cytoplasm

Question 13

What is the process of Cell Communication?

Answer 13

One cell releases a messenger molecule, e.g hormone. This molecule travels in the blood to another cell. This messenger molecule is detected by the cell because it binds to a receptor on the plasma membrane.

Question 14

How do cells communicate?

Answer 14

Cells communicate with each other using receptors and messenger molecules

Question 15

What are the three ways you can affect membrane fluidity?

Answer 15

1) Increasing temperature
2) Decreasing temperature
3) Solvents and their concentration

Question 16

How does increasing temperature affect membrane fluidity?

Answer 16

-Increasing temperature means there is more kinetic energy so molecules move faster.
-The movement can make the membrane leaky forming gaps so it can allow substances in/out that may not do that normally (increasing permeability).
-High temperatures can denature the proteins embedded.
which can change their functions and their rates of reaction.
-Cholesterol helps the bilayer when it is too hot.

Question 17

How does decreasing temperature affect membrane fluidity?

Answer 17

-Can stiffen membrane
-Can crack membrane (ice crystals)
-Decrease overall fluidity which can decrease permeability and restrict entry of important molecules into the cell.
-Can slow cell growth.

Question 18

How can the plasma membrane protect against the effects of decreasing temperature?

Answer 18

-Saturated fats become compressed, but unsaturated fatty acid tail-kink push the phospholipids next to them away, maintaining fluidity.
-Proportion of saturated and unsaturated fatty acids in a cell determines the membranes fluidity at low temperatures.
-Cholesterol is long and pushes through phospholipids to prevent them crunching together and cracking.
-Pulls together the fatty acid tails of phospholipids making the membrane stronger and less fluid.

Question 19

How does alcohol affect the cell membrane?

Answer 19

The alcohol disrupts the membranes of bacterial cells by dissolving the fatty acids of the phospholipids. killing the bacteria.

Question 20

How do solvents affect membranes?

Answer 20

-Ethanol, benzene and acetone can dissolve lipids (meaning no homeostasis can happen).
-This reduces bilayer stability by increasing permeability.
-If ethanol concentration is too high, enough phospholipids will dissolve to cause the plasma membrane to disintegrate completely which will kill the cell.

Question 21

What is diffusion?

Answer 21

Passive process, net movement of molecules from a high concentration to low concentration down a concentration gradient.

Question 22

What factors affect simple diffusion?

Answer 22

Temperature, Diffusion distance, surface area, size of diffusing molecule, concentration gradient

Question 23

What molecules can pass through plasma membrane by simple diffusion?

Answer 23

Oxygen, carbon dioxide, steroid hormones, water

Question 24

Why can oxygen and carbon dioxide pass through plasma membrane by simple diffusion?

Answer 24

They are small and are used for metabolic reasons (aerobic respiration)

Question 25

Why can steroid hormones pass through plasma membrane by simple diffusion?

Answer 25

It is fat soluble, and dissolves in the lipid bilayer, e.g cortisol

Question 26

Why can water pass through plasma membrane by simple diffusion?

Answer 26

It is polar and insoluble in lipids, they are present in large concentrations. Could use aquaporins (channel proteins for water)

Question 27

How does temperature affect the rate of simple diffusion?

Answer 27

Increase causes more kinetic energy between molecules increasing rate of diffusion. Decrease in heat means diffusion rate will slow down.

Question 28

How does diffusion distance affect the rate of simple diffusion?

Answer 28

Thicker the membrane, the slower the rate of diffusion

Question 29

How does surface area affect the rate of simple diffusion?

Answer 29

Large surface area, more diffusion. E.g specialised cells like red blood cells.

Question 30

How does size of diffusing molecule affect the rate of simple diffusion?

Answer 30

Smaller molecules diffuse faster than larger molecules

Question 31

How does concentration gradient affect the rate of simple diffusion?

Answer 31

The steeper the gradient, the faster the diffusion to the side with less molecules.

Question 32

What is facilitated diffusion?

Answer 32

When polar or larger molecules cannot pass straight through the bilayer, carrier or channel proteins allow the molecules to pass through. This is still passive.

Question 33

Name three examples of molecules which must diffuse using facilitated diffusion?

Answer 33

-Water
-Glucose
-Amino acids

Question 34

What is osmosis?

Answer 34

-Diffusion but with water.
-Passive process
-Water molecules are transferred down their potential water gradient from an area of high water concentration to low water concentration across a partially permeable membrane.

Question 35

What is water potential?

Answer 35

Measure of the tendency of water molecules to diffuse from one area to another.
Measured in KPa
Measures pressure water molecules are exerting on membrane/container
Pure water has the highest water potential possible at 0KPa because pure water has the highest concentration of water molecules.

Question 36

How does osmosis effect plant and animal cells?
Situation- Pure water (0KPa)

Answer 36

Water potential in cells is lower than water potential of pure water.
Plant cells:
Water moves into plant cell. Cellulose cell stop plant from bursting, and it swells/becomes TURGID.

Animal cells:
Water moves into animal cell. Cell swells, bursts (due to no cellulose cell wall), plasma membrane breaks= CYTOLYSIS

Question 37

How does osmosis effect plant and animal cells?
Situation- Concentrated sucrose solution (-400KPa)

Answer 37

Very concentrated sugar/sucrose solution. Has very low water pressure. Inside cell has higher water potential than outside.
Plant cells:
Cytoplasm separates from membrane leaving gaps. Water flows out of cell to surrounding solution. Plant cell = PLASMOLYSED

Animal cells:
Water moves out of cell (higher water potential) to outside cell (lower water potential). Cell shrivels and shrinks = CRENATED
crenated= dehydrated, metabolism cannot continue