Chapter 5 - Plasma Membrane

Question 1

What is compartmentalisation and why is it necessary?

Answer 1

The formation of separate membrane bound areas in a cell.

Vital because metabolism has different and incompatible reactions so separate parts allow specific conditions to be maintained.

Question 2

What is the plasma membrane and outline its structure.

Answer 2

Plasma membrane= the cell surface membrane that separates a cell from its external environment.
It is formed from a phospholipid bilayer:
- hydrophobic tails form a hydrophobic core inside the membrane.
- hydrophilic phosphate heads form the inner and outer surface of the membrane. Can interact with water.

Question 3

What is the fluid mosaic model and why is it given this name?

Answer 3

It is a model of the structure of a plasma membrane.
Called fluid mosaic because:
- phospholipids are free to move within the layer (are fluid), giving the membrane flexibility.
-proteins embedded in the bilayer vary in shape/size (like tiles of a mosaic).

Question 4

What are intrinsic proteins?

Answer 4

They are transmembrane proteins embedded through both layers of a membrane.
- contain amino acids with hydrophobic R groups on their external surfaces which interact with the hydrophobic core and keep them in place.

Question 5

What are channel and carrier proteins?

Answer 5

Intrinsic proteins involved in transport.

1. Channel proteins:
   - provide a hydrophilic channel that allows the passive movement (diffusion) of polar molecules and ions down a concentration gradient through membranes.
2. Carrier proteins:
   - have an important role in both passive and active transport into cells. This often involves the shape of proteins changing.

Question 6

What are glycoproteins?

Answer 6

(Intrinsic)
-proteins with a carbohydrate attached.
- they are embedded in the cell surface membrane.
- play a role in cell adhesion and as receptors for chemical signals.
The chemical binds to the receptor, it elicits a response which may causes a direct response inside the cell. = called cell signalling.
Eg. They act as receptors for neurotransmitters and hormones (insulin)

Question 7

What are glycolipids?

Answer 7

Lipids with attached carbohydrate chains.

• act as cell recognition sites (detect pathogens).
• are cell markers/ antigens. Recognised by immune system as self/ non-self.

Question 8

What are extrinsic proteins?

Answer 8

• present in one side of the bilayer.
• have hydrophilic R groups on outer surfaces and interact with polar heads of phospholipids/ intrinsic proteins.
• can be present in either layer, some move between layers.

Question 9

What is cholesterol?

Answer 9

• a lipid which regulates fluidity of membranes. Has a hydrophilic end and hydrophobic end.
• positioned between phospholipids in a bilayer.
• binds to the hydrophobic tails of lipids which causes them to pack closely= Therefore adds stability and some rigidity.
• stops membranes being solid by preventing phospholipid molecules getting close and crystallising.

Question 10

How does temperature affect membrane structure?

Answer 10

• An increase in temp=
• phospholipids have more kinetic energy and move more.
• membrane becomes more fluid and loses its structure.
• cell eventually breaks down completely.
• loss of structure= increases permeability so more particles cross.
• carrier/ channel proteins denature= membrane permeability is affected.

Question 11

How do solvents affect membrane structure?

Answer 11

• water is essential for the formation of the bilayer because phosphate heads interact with it to keep the bilayer in tact.
  -organic solvents dissolve membranes= disrupts cells. (alcohols used in antiseptic as they dissolve bacteria membranes and reduce infection).
• non-polar alcohol molecules enter the cell membrane and disrupt it:
  When the membrane is disrupted it becomes more fluid/permeable.
  This makes specific functions hard such as transmission of nerve impulses.

Question 12

Explain diffusion.

Answer 12

The net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.
- passive process which continues until there is a concentration equilibrium.
- particles move at high speeds and constantly collide which slows down the overall movement.
Diffusion rate slows down as diffusion distance increases (more collisions happen).

Question 13

What factors affect the rate of diffusion?

Answer 13

1. Temperature:
   - higher the temp, higher the rate of diffusion as particles have more kinetic energy (move faster).
2. Concentration difference:
   - greater the difference in conc, faster the rate of diffusion. Overall movement from hgh to low is larger.

Question 14

Explain diffusion across membranes.

Answer 14

Involves particles passing through the phospholipid bilayer.
-only happens if the membrane is permeable to the particles. Non-polar molecules diffuse freely.
-hydrophobic interior repels ions so they can’t pass through easily.
-polar molecules diffuse at very slow rates due to partial charges.
-small polar molecules diffuse more easily.
Therefore membranes = partially permeable.

Question 15

What factors affect the rate of diffusion across membranes?

Answer 15

1. Surface area:
   - larger the area of an exchange surface, higher the rate of diffusion.
2. Thickness of membrane:
   - the thinner the exchange surface, higher the rate of diffusion.

Question 16

Explain facilitated diffusion.

Answer 16

=Diffusion across a membrane through protein channels. (Down a concentration gradient)

• membranes with protein channels are selectively permeable. Most channels are specific to 1 ion/molecule.
• can involve carrier proteins which change shape when binding to a specific molecule.
• no external energy required.
• rate of diffusion affected by all other factors but also by the number of channel proteins present. (The more channel proteins, higher the rate of diffusion overall).

Question 17

What is active transport?

Answer 17

The movement of molecules/ions into or out of a cell from a region of lower conc to a region of higher conc.

• requires energy + carrier proteins.
• particles moved up a concentration gradient.
• metabolic energy supplied by ATP.

Question 18

Explain the 6 stages of active transport.

Answer 18

1. Molecule/ion binds to receptors in the channel of the carrier protein on the outside of the cell.
2. Inside the cell, ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
3. The phosphate molecule binds to the carrier protein so it changes shape and opens up towards the inside of the cell.
4. Molecule/ion is released to the inside of the cell.
5. Phosphate molecule is released and recombines with ADP = ATP.
6. Carrier protein returns to its original shape.

Question 19

What is bulk transport?

Answer 19

A form of active transport where large molecules (such as enzymes ) or whole bacteria cells are moved into and out of the cell by bulk transport.

They are too large to move through channel and carrier proteins.

Question 20

Explain endocytosis.

Answer 20

Bulk transport of materials into cells.

• phagocytosis= solids.
• pinocytosis= liquids.

1. Cell surface membrane invaginates (bends inwards) when it comes in contact with the material.
2. Membrane enfolds the material, until the membrane eventually fuses, forming a vesicle.
3. Vesicle pinches off and moves to the cytoplasm so material can be processed further.

(Vesicles with bacteria= pushed to lysosomes= bacteria digested by enzymes)

Question 21

Explain exocytosis.

Answer 21

Reverse of endocytosis.

1. Vesicles (made from golgi) move towards the cell membrane.
2. Vesicles fuse with the cell surface membrane.
3. Content of vesicles released outside the cell.

Question 22

Why is ATP needed in endo/exocytosis?

Answer 22

Energy (ATP) is needed for:

• movement of vesicles along the cytoskeleton.
• changing shape of cells to engulf materials.
• fusion of cell membranes as vesicles form/meet it.

Question 23

What is osmosis?

Answer 23

The diffusion of water through a partially permeable membrane from a region of high water potential to a region of low water potential.

• passive process.
• no energy required.

Question 24

Explain water potential.

Answer 24

The pressure exerted by water molecules as they collide with a membrane/container.

• measured in Pa or kPa
• pure water at RTP = 0kPa (highest possible value for water potential).

The more concentrated a solution, the more negative the water potential.

Diffusion of water causes increase in volume and in a closed system, this can increase pressure (hydrostatic pressure).

Question 25

Osmosis in animal cells.

Answer 25

In a solution with higher water potential than the cytoplasm: -water moves into the cell. Hydrostatic pressure increases. Cell surface membrane can’t stretch so breaks and the cell bursts (cytolysis). If placed in solution with lower water potential than the cytoplasm: - lose water by osmosis. Reduction in volume of the cell. Cell surface membrane will crenate. (Becomes shrivelled) Isotonic: -water constantly enters and leaves but at an equal rate. Multicellular animals have control mechanisms to ensure cells are surrounded by solutions with equal water potential. (Eg. Blood plasma)

Question 26

Osmosis in plant cells.

Answer 26

-plant cells have strong cellulose walls surrounding the cell surface membrane. When water enters by osmosis: - increased hydrostatic pressure pushes membrane against rigid walls (turgor). - turgor pressure increases and resists entry of more water. Therefore the cell=rigid. When placed in a solution of lower water potential: - water is lost. - reduction in volume of cytoplasm. - cell is plasmolysed as membrane is pulled away from the cell wall.