M2 Plasma Membranes

Question 1

What is the role of plasma membranes?

Answer 1

• Compartmentalisation (separates cells)
• Controlling exchange and transport
• Communication between cells (cell signalling)
• Site of chemical reactions
• Anchorage for the cytoskeleton and the extracellular matrix
• Cell to cell joining, eg. tissue formation

Question 2

Describe membrane structure

Answer 2

• All membranes in a cell have the same basic structure. The cell surface membrane which separates the cell from its external environment is the plasma membrane.
• Membranes are formed in a phospholipid bilayer. The hydrophilic (soluble) phosphate heads form the inner and outer surface of a membrane, sandwiching the hydrophobic (insoluble) fatty acid tail to form a hydrophobic core inside the membrane.
• Cells normally exist in aqueous environments. The inside of cells and organelles are also usually aqueous environments. Phospholipid bilayers are perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.

Question 3

Describe the fluid-mosaic model

Answer 3

In 1972 Singer and Nicholson proposed a model, building on an earlier lipid-bilayer model, in which proteins occupy various positions in the membrane. (Fluid-mosaic model - as phospholipids are free to move within the layer relative to each other, giving the membrane flexibility because the proteins embedded in the bilayer vary in shape, size and position)

Question 4

Describe the formation of a phospholipid

Answer 4

• Two hydrophobic fatty acid tails are joined to a glycerol by water bonds
• One phosphate group is joined to the same glycerol by ester bonds
• Weak hydrophobic interactions between the tails hold the bilayer together

Question 5

Describe the structure of cell membrane components

Answer 5

• Phospholipid hydrophobic, non-polar tails point inwards, while hydrophilic, polar heads point outwards
• Integral/intrinsic proteins (channel protein and carrier proteins) are between phospholipids, going all the way through the membrane
• Peripheral proteins (extrinsic) are in between phospholipids, but are only at one surface
• There is a cholesterol molecule in between fatty acids, with a glycoprotein (that acts as a receptor) attached

Question 6

How can phospholipids be chemically modified to act as signalling molecules?

Answer 6

• Moving within the bilayer to activate other molecules
• Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

Question 7

Describe intrinsic proteins

Answer 7

• Intrinsic/integral proteins are transmembrane proteins that are permanently embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.
• Examples include channel proteins and carrier proteins - both involved in transport across a membrane

Question 8

Describe channel proteins

Answer 8

• Channel proteins are intrinsic proteins that provide a hydrophilic channel that allows the passive movement (only diffusion) of polar molecules and ions down a concentration gradient through membranes.
• They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.
• The protein shape does not change.
• Channel proteins are ion-selective and most regulate the passage of ions in response to a certain stimuli.
• They have a much faster rate of transport than carrier proteins.

Question 9

Describe carrier proteins

Answer 9

• Carrier proteins have an important role in the passive and active transport into cells.
• They are instrinsic glycoproteins which bind to a solute and undergo a conformational change to translocate the solute across the membrane.
• Carrier proteins only bind to a specific molecule.
• Carrier proteins may move molecules against concentration gradients in the presence of ATP (as they are used in active transport)
• They have a much slower rate of transport than channel proteins

Question 10

Describe the structure of glycoproteins

Answer 10

• Glycoproteins are intrinsic proteins. They are embedded in the cell-surface membrane of it’s attached carbohydrate (sugar) chains of varying lengths and shapes.
• They have a role in cell adhesion and act as receptors.

Question 11

Describe functions of glycoproteins

Answer 11

Cell signalling:
- Receptors (binding site for a chemical signal) for neurotransmitters at nerve cell synapses. The binding of neurotransmitters triggers or prevents an impulse in the next neurone. And receptors for peptide hormones, including insulin and gulcagon, which affect the uptake and storage of glucose by cells.
- Recognition of cells/antigens as it is a surface to recognise self vs not self.
- Adhesion (holds cells together in a tissue)

Question 12

Describe the structure and function of glycolipids

Answer 12

• Glycolipids are lipids with attached carbohydrate (sugar) chains by glycosidic bonds.
• They act as cell markers or antigens bans can be recognised by immune cells.
• Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connections that allow cells to connect to one another to form tissues.

Question 13

Describe extrinsic proteins

Answer 13

• Extrinsic/peripheral proteins are temporarily present on one side of the bilayer.
• They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins.
• They can be present in either layer and some move between layers.

Question 14

Describe the structure and function of cholesterol

Answer 14

• Cholesterol is a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid.
• Cholesterol maintains mechanical stability and fluidity of membranes.
• Cholesterol molecules are positioned between phospholipids in a membrane bilayer with the hydrophilic end interacting with the heads, and the hydrophobic end interacting with the tails, pulling them together.
• In this way cholesterol adds stability to membranes without making them too rigid.
• The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules grouping too closely and crystallising.

Question 15

How does cholesterol affect fluidity of the membrane?

Answer 15

• Cholesterol binds to phospholipid fatty acid tails, increasing the packaging of the membrane, therefore reducing fluidity of the membrane.
• Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity. It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross. It functions to separate phospholipid tails to prevent crystallisation of the membrane. It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein.

Question 16

How do proteins affect sites of chemical reactions?

Answer 16

• Proteins in the membranes forming organelles, or within organelles have to be in particular positions for chemical reactions to take place.

Question 17

How does temperature affect cell membrane permeability?

Answer 17

• Proteins and lipids (the major components in cell membranes) are both affected by temperature.
• As temperature increases, lipids become more fluid, this increased fluidity reduces the effectiveness of the cell membrane as a barrier to polar molecules, meaning polar molecules can pass through.
• At higher temperatures, any diffusion taking place through the cell membrane will occur at a higher speed (due to increased kinetic energy), changes in membrane fluidity are reversible, if temperatures decrease, lipids will return to their normal levels of fluidity.
• At certain temperatures many proteins begin to denature. This disrupts the membrane structure, meaning it no longer forms an effective barrier. As a result, substances can pass freely through the disrupted membrane, this process is irreversible.

Question 18

How do solvents affect membrane structure?

Answer 18

• Water (a polar solvent) is essential in the formation of the phospholipid bilayer. The non-polar tails of the phospholipids are orientated away from the water, forming a bilayer with a hydrophobic core. The charged phosphate heads interact with water, helping to keep the bilayer intact.
• Many organic solvents are less polar than water. Organic solvents are less polar than water, they will dissolve membranes, disrupting cells - alcohols in antiseptic wiped dissolve the bacteria membranes killing them.
• Very strong alcohol solutions are toxic as they destroy cells in the body. Less concentrated solutions of alcohol will not dissolve membranes but still cause damage and the presence of these molecules between phospholipids disrupts the membrane.
• When the membrane is disrupted it becomes more fluid and more permeable. Some cells need intact cell membranes for specific functions eg. transmission of nerve impulses by neurones.

Question 19

Define diffusion

Answer 19

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
It is a passive process and will continue until there is a concentration equilibrium between the two areas.

Question 20

Why does diffusion occur?

Answer 20

• Particles in a gas or liquid have kinetic energy. This movement is random and an unequal distribution of particles will eventually become an equal distribution.
• Particles move at high speeds and are constantly colliding, which slows down their overall movement. Over a short distance diffusion is fast, but as diffusion distance increases the rate of diffusion slows as more collisions have taken place.
• Therefore cells are microscopic, the movement of particles within cells depend on diffusion and a large cell would lead to slow rates of diffusion.

Question 21

What factors affect the rate of diffusion?

Answer 21

1. Temperature - the higher the temperature the higher the rate of diffusion as particles have more kinetic energy and move at greater speeds.
2. Concentration difference - the greater the difference in concentration between two regions the faster the rate of diffusion as the overall movement from the higher concentration to the lower concentration will be larger.

These factors only affect diffusion across membranes:
3. Surface area:volume ratio - increasing the surface area to volume ratio gives more space for diffusion to occur.
4. Membrane thickness - decreasing membrane thickness increases the rate of diffusion as particles have a shorter distance to cover.

Question 22

What molecules are able to diffuse across membranes?

Answer 22

• Diffusion across membranes involves particles passing through the phospholipid bilayer. It can only happen if the membrane is permeable to the particles (non-polar molecules such as oxygen diffuse through freely, down a concentration gradient)
• The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions) so they cannot pass through. Polar molecules (such as water) with partial positive and negative charges can diffuse through membranes, but only at a slow rate.
• Small polar molecules pass though more easily than larger ones. Membranes therefore are described as partially permeable.

Question 23

Define facilitated diffusion

Answer 23

• Facilitated diffusion is the passive movement of molecules across a cell membrane via the aid of membrane proteins.
• Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion.
• Facilitated diffusion can also involve carrier proteins which can change shape when a specific molecule binds.
• The rate of facilitated diffusion is dependant on the temperature, concentration gradient, membrane surface area and thickness, but also the number of channel proteins present.

Question 24

Define active transport

Answer 24

• Active transport is the movement of molecules or ions out of a cell from a region of lower concentration to a region of higher concentration. This process requires energy and carrier proteins.
• Energy is needed as particles are being moved up a concentration gradient. Metabolic energy is supplied by ATP.
• Carrier proteins span the membranes and act as ‘pumps’.

Question 25

Describe the process of active transport

Answer 25

1. The molecule or ion binds to receptors in the channel of the carrier protein on the outside of the cell.
2. On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
3. Binding of the phosphate molecule to the carrier protein causes the protein to change shape, opening up to the inside of the cell.
4. The molecule or ion is released to the inside of the cell.
5. The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP.
6. The carrier protein returns to its original shape.

Question 26

Define bulk transport

Answer 26

• Bulk transport is a form of active transport where large molecules such as enzymes, molecules and whole cells like bacteria are too large to move through a channel or carrier proteins, so they are moved into and out of the cell by bulk transport.
• Energy in the form of ATP is required for the movement of vesicles along The cytoskeleton, changing the shape of cells to engulf materials and the fusion of cell membranes as vesicles form or as they meet the cell surface-membrane.

Question 27

What is endocytosis?

Answer 27

• Endocytosis is the bulk transport of material into cells.
• Phagocytosis is for solids, and pinocytosis for liquids.
  1. The cell-surface membrane bends inwards when it comes into contact with the material to be transported.
  2. The membrane enfolds the material until eventually the membrane fuses, forming a vesicle.
  3. The vesicle pinches off and moved into the cytoplasm to transfer the material for further processing within the cell.

Question 28

What is exocytosis?

Answer 28

• Exocytosis is the bulk transport of materials out of the cell.
  1. Vesicles (usually formed by the Golgi apparatus) move inwards and fuse with the cell surface membrane.
  2. The contents of the vesicle are then released outside of the cell.

Question 29

Define water potiental

Answer 29

• Water potiental is the pressure exerted by water molecules as they collide with a membrane or container (ψ)
• It is measured in kPa.
• Pure water has a water potiental of 0kPa (at standard temperature and atmospheric pressure) this is the highest possible value for water potiental, the presence of solute in water lowers the water potential below 0. All solutions have negative water potentials, the more concentrated the solute the more negative the water potential.

Question 30

Define osmosis

Answer 30

Osmosis is the net movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane. This will continue until the water potential is equal on both sides of the membrane.

Question 31

What is hydrostatic pressure?

Answer 31

• The diffusion if water into a solution leads to an increase in volume of this solution. If the solution is in a closed system this results in an increase in pressure, called hydrostatic pressure (kPa).
• At cellular level this pressure is large and potentially damaging.

Question 32

Describe the effects of osmosis on animal cells

Answer 32

• If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, water will move into the cell by osmosis.
• This increases the hydrostatic pressure inside the cell.
• The cell-surface membrane cannot stretch much and cannot withstand much pressure. It will break and the cell will burst - cytolysis.
• If an animal cell is placed in a solution that has a lower water porcentual than the cytoplasm it will loose water to the solution by osmosis down the water potential gradient.
• This causes a reduction in volume of the cell and the cell-surface membrane to ‘pucker’ (crenation).

Question 33

Describe the effects of osmosis on plant cells

Answer 33

• Plants have strong cellulose walls surrounding the cell-surface membrane.
• When water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell walls.
• This pressure against the cell wall is turgor pressure. As turgor pressure increases it resists the entry of further water and the cell is said to be turgid.
• When plant cells are placed in a solution with a lower water potential than their own, water is lost from the cells by osmosis.
• This eventually leads to a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall - the cell is said to be plasmolysed.