1.3 - Cell Membranes and Transport

Question 1

What can phospholipids do?

Answer 1

Form bilayers, with sheets opposite eachother.

Question 2

What is the inner layer of a phospholipid bilayer?

Answer 2

Hydrophilic heads pointing towards the cell
Interacts with water in the cytoplasm

Question 3

What is the outer layer of a phospholipid bilayer?

Answer 3

Outer layer has hydrophilic heads pointing outwards
Interacts with water outside the cell

Question 4

What do the tails do?

Answer 4

Hydrophobic tails point towards each other, in centre.
Allows lipid soluable molecules across, not water soluable.

Question 5

What are hydrophilic heads made of?

Answer 5

Polar phosphates

Question 6

What are hydrophobic tails made of?

Answer 6

Non polar fatty acids.

Question 7

What are the 2 types of proteins found in a phospholipid bilayer?

Answer 7

Extrinsic and intrinsic.

Question 8

What are extrinsic proteins?

Answer 8

Either side of the bilayer
Provide structure and form recognition sites

Question 9

What are intrinsic proteins?

Answer 9

Extends across both ends of bilayer
Can use for transport, active transport of ions by forming channels

Question 10

Who was the fluid mosaic model proposed by?

Answer 10

Singer and NIcolson in 1972

Question 11

What is the meaning behind the fluid mosaic model?

Answer 11

Fluid - the individual phosphate molecules can move within a layer relative to another
Mosaic - the proteins vary in shape, size and pattern

Question 12

What does cholestral do in the bilayer?

Answer 12

Makes the membrane rigid and stable

Question 13

What do glycolipids/glycoproteins do in the bilayer?

Answer 13

Transmission of signals and act as hormone receptors

Question 14

Difference between water and lipid soluable molecules across a membrane?

Answer 14

Lipid soluable substances dissolve in phospholipids and can move through the cell membrane easily
Water soluable substances require intrinsic proteins to cross
Proof the bilayer is selectively permeable.

Question 15

What is the definition of diffusion?

Answer 15

The passive movement of a molecule or ion down a concentration gradient, from a region of high concentration to a region of low concentration, until equilibrium is met.

Question 16

What is meant by ‘passive’?

Answer 16

Not requiring any energy

Question 17

What 3 things affect diffusion rates?

Answer 17

• Steepness of the concentration gradient
• Thickness of exchange surface and distance travelled
• Surface area of the membrane

Question 18

What is the equation for the rate of diffusion?

Answer 18

rate of diffusion = surface area x difference in concentration/ length of diffusion path

Question 19

What secondary factors may affect diffusion?

Answer 19

• size of the diffusing molecule
• nature of the diffusing molecule
• temperature and kinetic energy

Question 20

What is the definition of facilitated diffusion?

Answer 20

The passive transfer of molecules or ions down a concentration gradient across a membrane by protein carrier molecules in the membrane.

Question 21

What are channel proteins?

Answer 21

molecules with pores lined with polar groups
channels are hydrophilic, so ions can pass through
the channel opens and closes depending on the needs of the cell

Question 22

What are carrier proteins?

Answer 22

Diffusion of larger polar molecules such as sugars and amino acids
a molecule attaches to a binding site, the carrier protein changes shape and releases the molecule on the other side of the membrane

Question 23

What is active transport?

Answer 23

ions and molecules are moved from a lower to a higher concentration, against the concentration gradient
the process requires energy from ATP
occurs through intrinsic proteins across the membrane

Question 24

How is active transport limited?

Answer 24

The number of carrier proteins available for use.

Question 25

What are the 6 steps of active transport?

Answer 25

1) molecule combines with carrier protein 2) ATP transfers a phosphate group to the carrier protein 3) the carrier protein changes shape 4) the molecule is released into the cytoplasm 5) phosphate group combines with ADP to form ATP again 6) the carrier protein returns to its original shape

Question 26

What is the definition of osmosis?

Answer 26

The passive net diffusion of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential

Question 27

What is water potential?

Answer 27

Psy or trident sign The tendancy for water to move into a system, water moves from a solution with higher water potential )less negative) to one with a lower water potential (more negative)

Question 28

How is water potential decreased?

Answer 28

Decreased by the addition of solute

Question 29

What water potential does pure water have?

Answer 29

Zero

Question 30

What happens to plant cells in hypertonic solutions?

Answer 30

A plant cell will have its cytoplasm diffuse out, becoming flaccid. The cytoplasm shrinks and moves away from the cell wall (plasmolysis)

Question 31

What happens to plant cells in hypotonic solutions?

Answer 31

As osmosis occurs, it will start to take up water and swell. At this stage it can be called turgid. The cell wall stops it from bursting. The hydrostatic pressure rises until internal and external pressures are equal.

Question 32

What happens to a plant cell in an isotonic solution?

Answer 32

Incipient plasmolysis is when a cell has lost enough water for the cell membrane to start moving away from the cell wall. This lowers the pressure potential to zero.

Question 33

What happens to an animal cell in a hypertonic solution?

Answer 33

Water diffuses out of the cell due to osmosis and the cells shrink. Animal cells must be surrounded by an isotonic solution, which in humans, is the kidney (controls water and salt in blood)

Question 34

What happens to an animal cell in a hypotonic solution?

Answer 34

Animal cells swell up and explode, and they do not have a cell wall like a plant does. Instead, when a cell is in danger, organelles called contractive vacuoles will pump water out of the cell.

Question 35

What does it mean if a cell is 'crenated'?

Answer 35

The formation of abnormal notched surfaces on cells as a result of water loss through osmosis. Occurs in hypertonic solutions, the cell shrivels.

Question 36

What is endocytosis?

Answer 36

Phagocytosis - uptake of solid material too large to be transported any other way Pinocytosis - uptake of liquids

Question 37

What happens to membranes in endocytosis?

Answer 37

Decreases the surface area

Question 38

What is exocytosis?

Answer 38

A vesicle containing a substance is transported through the cell via the cytoskeleton The vesicle fuses with the cell membrane and releases the product

Question 39

What happens to the membrane in exocytosis?

Answer 39

Increase in overall surface area of the cell membrane

Question 40

What 2 things does endo and exocytosis require?

Answer 40

- require energy for cell membranes to change shape - require fluidity of the membrane