Membranes 2

Question 1

Affect of temperature on membranes

Answer 1

Phospholipids in cell membrane are constantly moving. Temperature increase, phospholipids have more kinetic energy and will move more. Membrane becomes more fluid and eventually loses structure. Cell eventually breaks down.

-This loss of structure increases the permeability of the membrane, making it easier for particles to travel across.

-Carrier and channel proteins become denatured at high temperatures. As they denature, membrane permeability is affected.

Question 2

Affect of solvents on membrane structure

Answer 2

-Water, a polar solvent, is essential in the formation of the phospholipid bilayer. The non-polar tails of the phospholipids are oriented away from the water, forming a bilayer with a hydrophobic core. Charged phosphate heads interact with water, helping to keep the bilayer intact.

-Organic solvents e.g. alcohol aren’t as polar as water. Will dissolve membranes, disrupting cells.

-Less concentrated solutions of alcohols will not dissolve membranes but still cause damage. Non-polar alcohol molecules can enter cell membrane and the presence of these molecules between phospholipids disrupts membrane.

-When membrane is disrupted it becomes more fluid and permeable.

Question 3

Define diffusion

Answer 3

Diffusion is the net, or overall, movement of particles from a region of higher concentration to a region of lower concentration.

-passive process and will continue until there is a concentration equilibrium between the two areas.

Question 4

Why does diffusion happen

Answer 4

Diffusion happens because the particles in a gas or liquid have kinetic energy. This movement is random and an u equal distribution of particles will eventually become an equal distribution.

Question 5

Why does diffusion slow down over longer distances

Answer 5

Particles move at high speeds and are constantly colliding, slows down movement. Means that over short distances diffusion is fast, but as diffusion distance increases the rate of diffusion slows down because more collisions have taken place.

Question 6

How does temperature affect diffusion

Answer 6

Higher temperature , higher rate of diffusion, more kinetic energy- more speed.

Question 7

How does concentration affect diffusion

Answer 7

The greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger.

Question 8

How does diffusion across a membrane occur

Answer 8

Involves particles passing through the phospholipid bilayer. Can only happen if the membrane is permeable to the particles- Non-polar molecules such as oxygen diffuse freely down a concentration gradient.

Question 9

What does the hydrophobic interior of the membrane do during diffusion

Answer 9

It repels substances with a positive or negative charge ions, so they cannot easily pass through. Polar molecules such as water with a partial positive and negative charges can diffuse through membranes, but only at a very slow rate. Smaller polar molecules pass through more easily than larger ones. Membranes are therefore described as partially permeable.

Question 10

The rate at which molecules or ions diffuse across membranes is affected by:

Answer 10

-Surface area- the larger the surface area of an exchange surface, the higher the rate of diffusion.

-Thickness of membrane- the thinner the exchange surface, the higher the rate of diffusion.

Question 11

What’s facilitated diffusion

Answer 11

-Membranes contain channel proteins through which polar molecules and ions can pass. Diffusion across a membrane through protein channels is called facilitated diffusion.

-Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion.

-In facilitated diffusion, the movement of the molecules is down a concentration gradient and does not require external energy.

Question 12

What’s the rate of facilitated diffusion dependant on

Answer 12

Temp, concentration gradient, membrane surface area and thickness, but is also affected by the number of channel proteins present. The more protein channels, the higher the rates of diffusion overall.

Question 13

Why is active transport used

Answer 13

Many biological processes depend of the presence of a concentration gradient, to maintain this concentration gradient particles must be moved up it at a faster rate than the rate of diffusion.

Question 14

Define active transport

Answer 14

Movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration. Process requires energy(atp) and carrier proteins.

Question 15

Describe the process of active transport

Answer 15

Carrier proteins span the membranes and act as pumps.

1.The molecule or ion to be transported 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 molecules is released from the carrier protein and recombined with ADP to form ATP.

6.The carrier protein returns to its original shape.

The process is selective- specific substances are transported by specific carrier proteins.

Question 16

What’s bulk transport

Answer 16

Another form of active transport. Large molecules such as enzymes, hormones and cells like bacteria are too large to move through channel or carrier proteins, so they are moved by bulk transport.

Question 17

What is endocytosis

Answer 17

Bulk transport of materials into cells. Two types- phagocytosis for solids and pinocytosis for liquids-process is the same for both.

-Cell surface membrane first invaginates (bends inwards) when it comes into contact with the material to be transported. The membrane enfolds the material until eventually the membrane fuses, forming a vesicle. The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.

Question 18

What’s exocytosis

Answer 18

Reverse of endocytosis, vesicles, usually formed by the Golgi apparatus, move towards and fuse with the cell surface membrane. The contents of the vesicle are then released outside of the cell.

Question 19

What is ATP required for

Answer 19

Movement of vesicles along the cytoskeleton m, changing the shape of cells to engulf materials, and the fusion of cell membrane as vesicles form or as they meet the cell surface membrane.

Question 20

What is osmosis

Answer 20

Diffusion of water across a partially permeable membrane- passive process and energy is not required.

Question 21

What’s a solute

Answer 21

A substance dissolved in a solvent forming a solution.

Question 22

What’s concentration

Answer 22

The amount of solute in a certain volume of aqueous solution m

Question 23

What’s water potential

Answer 23

The pressure exerted by water molecules as they collide with a membrane or container. Measure in pressure pascals (Pa) or kilopascals (kPa). The symbol for water potential is the Greek letter psi.

Question 24

What’s the water potential of pure water

Answer 24

Pure water is defined as having a water potential of 0 kPa. This is the highest possible value as the presence of solute in water lowers water potential below 0.

Question 25

What happens when there are solutions of different concentrations separated by a partially permeable membrane

Answer 25

Water molecules can move between the solutions but the solutes usually cannot. There will be a net movement of water from solution with higher water potential (less concentrated) to a solution with a lower water potential. This will continue until the water potential is equal on both sides of the membrane.

Question 26

What’s hydrostatic pressure

Answer 26

The diffusion of water into a solution leads to an increase in volume of this solution. If the solution is in a closed system such as a cell, this results in a MM increase in pressure. This pressure is called hydrostatic pressure and has same units as water potential, kPa. At the cellular level this pressure is relatively large and potentially damaging.

Question 27

Affect of osmosis on animal cells

Answer 27

Increased hydrostatic pressure, all cells have thin cell surface membrane (7nm) which can’t stretch much and can’t withstand the increased pressure. It will break and the cell will burst, an event called cytolysis.

-Lower water potential than cytoplasm-reduced volume, cell surface membrane ‘puckers’ referred to as crenation.

-To prevent cytolysis or crenation, multicellular animals usually have control mechanisms to make sure cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic). In blood the aqueous solution is blood plasma.

Question 28

Affect of osmosis on plant cells

Answer 28

-Like animal cells, plant cells contain a variety of solutes, mainly dissolved in a large vacuole. However, unlike animals, plants are unable to control the water potential of the fluid around them, for example roots are usually surrounded by pure water.

-Plants have strong cellulose cell walls surrounding 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 called turgor. As the turgor pressure increases it resists the entry of further water and the cell is said to be turgid.

-When water leaves, volume of cytoplasm decreases, eventually pulls the cell surface membrane away from the cell wall- cell is said to be plasmolysed.