2.5- Biological Membranes

Question 1

What is the fluid mosaic model?

Answer 1

Theory of cell membrane structure with proteins embedded in a sea of phospholipids.

Question 2

Define glycolipid, and define its functions

Answer 2

Lipid/phospholipid with a chain of carbohydrate molecules.
Play a role in communication between cells and in cell to cell recognition
Can also act as antigens

Question 3

What is a glycoprotein? What are its functions

Answer 3

Protein with a chain of carbohydrate molecules attached.
Function as surface antigens enabling cells to distinguish from self and non self
Act as receptors to hormones

Question 4

What is the plasma membrane?

Answer 4

Cell surface membrane.

Question 5

What does permeability refer to in the context of cell membranes?

Answer 5

The ability to let substances pass through.

Question 6

How do very small molecules pass through the cell membrane?

Answer 6

They simply diffuse through the cell membrane, in between its structural molecules.

Question 7

What allows some substances to pass through the cell membrane?

Answer 7

Some substances dissolve in the lipid layer and pass through; (eg steroid hormones small molecules eg O2) others pass through special protein channels (water soluble molecules/ ions) or are carried by carrier proteins.

Question 8

What does it mean for membranes to be described as partially permeable barriers?

Answer 8

They do not let all types of molecule pass through them.

Question 9

List some roles of the plasma membrane.

Answer 9

• Separates the cell’s components from its external environment
• Regulates transport of materials into and out of the cell
• May contain enzymes involved in specific metabolic pathways
• Has antigens for immune system recognition
• May release chemicals that signal to other cells

Question 10

What is the role of inner membranes, such as cristae, in cells?

Answer 10

They provide a large surface area for reactions of aerobic respiration and house enzymes needed for respiration.

Question 11

What do thylakoid membranes contain, and what is their function?

Answer 11

They house chlorophyll and are the site of photosynthesis reactions.

Question 12

What is the significance of digestive enzymes on the plasma membrane of epithelial cells?

Answer 12

They catalyse some of the final stages in the breakdown of certain types of sugars.

Question 13

What does the fluid mosaic model explain about cell membranes?

Answer 13

It allows passage of molecules through the membrane and describes how cell membranes can be dynamic and interact with the environment.

Question 14

What is the structure of the lipid bilayer in the fluid mosaic model?

Answer 14

Made up of phospholipid molecules with hydrophilic heads facing watery environments and hydrophobic tails in the center.

Question 15

True or False: The hydrophobic tail regions of phospholipids are in contact with water.

Answer 15

False.

Question 16

Explain why phospholipids form a bilayer in plasma membranes

Answer 16

Phospholipids have a polar phosphate group which is hydrophilic and faces the aqueous environments of the cells exterior and interior (tissue fluid, cytoplasm)
The fatty acid tails are non polar and hydrophobic and face away from the aqueous environments
This forms a bilayer with the hydrophobic tails facing inward and the phosphate groups facing outwards, interacting with the aqueous environments

Question 17

What are channel proteins and what are their functions?

Answer 17

Channel proteins are intrinsic proteins that have a channel lined with hydrophilic amino acid molecules and is filled with water molecules
They allow water soluble molecules and ions to diffuse through the membrane

Question 18

What are intrinsic and extrinsic proteins

Answer 18

Intrinsic- extend across the phospholipid bilayer
Extrinsic- are partially embedded into the bilayer

Question 19

Functions of extrinsic proteins

Answer 19

Structural roles
Enzymes
Receptors for other molecules eg hormones

Question 20

What is simple diffusion?

Answer 20

The net movement of ions/ molecules down the concentration gradient until equilibrium is reached and there is no net diffusion.
Can diffuse through the lipid bilayer or through channel proteins

Question 21

What molecules can transport across cell membranes via simple diffusion?

Answer 21

Small molecules eg N2 O2 CO2
Lipid soluble molecules of any size as they are non polar and dissolve in the lipid bilayer
H2O- although polar and insoluble in lipids, it can diffuse as it is present in such large concentrations

Question 22

How is concentration gradient maintained? Give examples

Answer 22

By molecules entering the cells and then passing into organelles
E.g O2 diffuse into cytoplasm and then into mitochondria for aerobic respiration

Question 23

Factors affecting rate of simple diffusion

Answer 23

1) temperature
2) diffusion distance
3) surface area
4) size of diffusing molecule
5) concentration gradient