Biological Membranes

Question 1

Describe the permeability of the cell surface membrane

Answer 1

Selectively permeable= controls what is allowed to enter and leave the cell

Question 2

What is the basic structure of cell membranes?

Answer 2

• Phospholipids
• Proteins
• Cholestrol

Question 3

What is a phospholipid? Refer to its structure, and arrangement in cells

Answer 3

• A glycerol molecule with one phosphate group= head (hydrophilic/ charged) and two fatty acid tails (hydrophobic/ uncharged)
• Forms a phospholipid bilayer in cell surface membranes because water based substances are present on both sides of the membrane

Question 4

What is the approximate width of the phospholipid bilayer?

Answer 4

7nm

Question 5

What substances can pass directly through the phospholipid bilayer?

Answer 5

1) Lipid soluble molecules:
- glycerol
- fatty acids
- monoglycerides
- steroid-based hormones

2) Small uncharged molecules
- oxygen
- carbon dioxide

3) Water (in small-medium quantities)
- osmosis occurs when water passes through a phospholipid bilayer

Question 6

Why are many molecules unable to pass through the phospholipid bilayer through such a direct method?

Answer 6

• Too large to fit through the small temporary gaps

- Polar so unable to pass through the non-polar region of the fatty acid tails

Question 7

What are the two types of proteins found in the phospholipid bilayer and what are their roles?

Answer 7

1) Intrinsic= pass through both phospholipid layers. Allow larger or more polar molecules to pass through the membrane
2) Extrinsic= embedded in only on of the phospholipid layer. Have functions other than to allow passage through the cell membrane

Question 8

What are the three types of intrinsic proteins found within the cell surface membrane?

Answer 8

• carrier proteins
• unrated channel proteins
• gated channel proteins

Question 9

What are carrier proteins functions/ how do they work?

Answer 9

• Used to carry larger molecules e.g glucose through the membrane
• has a binding site which is specific for a particular type of molecule (complementary shape). Once specific molecule has bound to the binding site the carrier protein changes shape in such a way the molecule is transported through the membrane

Question 10

What are ungated channel proteins?

Answer 10

-a permanently open pore which exists in the membrane to allow charged molecules e.g ions to pass through

Question 11

What are gated channel proteins?

Answer 11

• also allows ions to pass through but is only opened at particular times
• the action of nerve impulse travelling through the axon of a neurone (nerve cell) depend on the momentary opening and then closing of the gated ion channels

Question 12

What is an extrinsic protein with a carbohydrate attached to it called?

Answer 12

Glycoprotein

Question 13

What is the first function of extrinsic proteins?

Answer 13

• antigens for cell recognition
• proteins or glycoproteins are located on the outer surface of the cell. Cells within a multicellular organism have the same self-antigens which are recognised as ‘self’ by the immune system. An invading pathogen will then be recognised as ‘non-self’ and an immune response is initiated

Question 14

What is the second function of extrinsic proteins?

Answer 14

• receptors for cell signalling
• proteins or glycoproteins located on the outer surface of the cell= complementary in shape to molecule which binds to them and are important in cell to cell signalling.
• receptors can be binding sites for Hormones (on outer surface of target cells) or Neurotransmitter (on post synaptic neurones

Question 15

What is the third function of extrinsic proteins?

Answer 15

• enzymes (pure protein structures)
• usually found on cytoplasmic side of the membrane. Have an active site which is complementary in shape to their substrate and will speed up a chemical reaction within the metabolism of the cell

Question 16

What is the fourth function of extrinsic proteins?

Answer 16

-Increasing the stability of the membrane

Question 17

Explain why the cell surface membrane could be described as a fluid-mosaic model.

Answer 17

Fluid: Constantly moving phospholipids within the bilayer
Mosaic: The proteins float within the membrane and give it a mosaic appearance. Proteins can move within the membrane and not in a static, fixed position.

Question 18

What is cholesterol and what is it’s role in the membrane?

Answer 18

• it’s a form of lipid
• It adds stability and regulates fluidity in the membrane. Without it, the fluid nature of the membrane would make it unstable. When present it allows enough movement of the phospholipids for them to move passed one another but not so much movement that the membrane becomes unstable
• it can remain within the hydrophobic fatty acid tail region of the bilayer

Question 19

What is meant by cell signalling?

Answer 19

• A cell must stay in contact with its environment , with other cells in order to survive, and must be able to react to changes in their environment. A cell therefore needs to be able to pick up signals at its surface.
• signals arrive at the plasma membrane from outside the cell as particular substances e.g hormones and neurotransmitters. A receptor in the cells plasma membrane picks up these signals and brings about actions within the cell

Question 20

What are hormone receptors?

Answer 20

• Hormones are chemical messengers produced in specific tissues, travel in the blood and bind at the receptor of the target cell
• receptors are often extrinsic proteins or glycoproteins.
• a hormone receptor is where a hormone binds to a receptor on a target cell’s plasma membrane outer surface because the two have complementary shapes. The binding causes the target cells to respond in a certain way.

Question 21

What are insulin receptors?

Answer 21

• Insulin is released from the pancreas in response to increased blood glucose concentration. The insulin travels in the blood and then attaches to the insulin receptors on the outer plasma membranes of many cells including liver + muscle cells
• the attaching of insulin to it’s receptor triggers an internal response in the cell that leads to more glucose carrier proteins being present in the plasma membrane. This allows the cell to take up more glucose from the blood so blood glucose concentration is reduced

Question 22

What are the 2 potential implications cell signalling has for medicine?

Answer 22

• Some diseases are due to the failure of cell signalling. E.g Type 2 diabetes (liver cells don’t respond to insulin) and cancer (cells don’t respond to signals that should stop them dividing)
• Medicinal drugs: have been developed that are complementary to the shape of the receptor molecule

Question 23

How can temperature be investigated to see if it has a change on the permeability of cell membranes?

Answer 23

• increase of temperature= phospholipids gain kinetic energy and move quickly around one another
• as fluidity increases therefore, more frequent larger gaps appear between the heads at greater frequencies so too large molecules can pass through and molecules able to pass through can do so at a greater rate

-If temperature is increased too high (60 c +) then the proteins within the membrane gain so much kinetic energy that the molecules within the proteins vibrate to which the bonds maintaining the tertiary structure are broken. Shape of protein is therefore lost as it denatures,, allowing previously impermeable substances to now pass through.

Question 24

How can ethanol be investigated to see if it has a change on the permeability of cell membranes?

Answer 24

• ethanol is a solvent for lipids
• when cell membranes are exposed to ethanol the phospholipids dissolve in the solvent leaving holes in the membrane,, allowing molecules previously unable to pass through the membrane to do so in large quantities