Membranes

Question 1

Describe the structure of a cell membrane. (5)

Answer 1

membrane is a 7 nm thick phospholipid bilayer with protein molecules
spanning the bilayer or within one or other layer. Phospholipids and some proteins move within the
layers. Hence the structure is described as a fluid mosaic – molecules flow freely, there are many types of molecules

Question 2

Phospholipid bilayers are …

Answer 2

a barrier to most water soluble substances because the interior of the membrane is hydrophobic

Question 3

Cholesterol is needed for …

Answer 3

membrane fluidity and stability

Question 4

What do transport proteins do?

Answer 4

they transport molecules or ions across the membrane. They may be either channel proteins or carrier proteins. Channel proteins have a fixed shape; carrier proteins change shape

Question 5

Example of Proteins acting as enzymes ?

Answer 5

In the cell surface membranes of microvilli in the gut

Question 6

What 2 things do Glycolipids and glycoproteins do?

Answer 6

• form receptors – for example, for hormones or neurotransmitters
• form antigens, which are cell recognition markers

Question 7

The cell surface membrane …

Answer 7

controls exchange between the cell and its environment.

Question 8

Where do some chemical reactions take place on membranes?

Answer 8

Inside cell organelles, as in photosynthesis and respiration.

Question 9

Diffusion is …

Answer 9

the net movement of molecules or ions from a region of their higher concentration to one
of lower concentration. Oxygen, carbon dioxide and water cross membranes by diffusion through the
phospholipid bilayer.

Question 10

What is facilitated diffusion?

Answer 10

Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins, down the conc. gradient

Question 11

Describe how substances move across cell-surface membrane via facilitated diffusion

Answer 11

• Carrier/Channel proteins
• Passive bc it doesn’t require ATP
• Proteins complimentary to substance
• Substances move down conc. gradient

Question 12

Osmosis is …

Answer 12

Water moves from regions of higher water potential to regions of lower water potential. When this takes place through a partially permeable membrane such as the cell surface membrane.

Question 13

Water Potential = ψ = ψp + pψs

Answer 13

Water Potential (ψ) = Pressure Potential (ψp) + Solute Potential (ψs)

Question 14

What is the process of Plasmolysis

Answer 14

In dilute solutions, animal cells burst as water moves into the cytoplasm from the solution. In dilute
solutions, a plant cell does not burst, because the cell wall provides resistance to prevent it expanding.
The pressure that builds up is the pressure potential. A plant cell in this state is turgid. In concentrated
solutions, animal cells shrink, while in plant cells the protoplast shrinks away from the cell wall.

Question 15

Active Transport is where …

Answer 15

• Some ions and molecules move across membranes
• Against the concentration gradient
• only involves CARRIER PROTEINS
• ATP to provide energy

Question 16

Exocytosis and Endocytosis involve (occurs due to availability of mem to fuse and pinch off) …

Answer 16

the formation of vacuoles to move larger quantities of materials respectively out of, or into, cells by bulk transport. There are two types of endocytosis (in), namely phagocytosis (cell eating) and pinocytosis (cell drinking).

Question 17

process of Mitochondria

Answer 17

• Molecule binds to complimentary site on carrier proteins
• ATP is Hydrolyzed to ADP & Carrier proteins changes tertiary structure
• Molecules transported across mem
• One way flow as Molecule can only fit into carrier proteins on one side of membrane

Question 18

Why measure rate of uptake in /ug per million cells per hour?

Answer 18

• Allows direct comparison
• Easy to manipulate
• Diff no. of cells

Question 19

What happens to carrier proteins in Facilitated diffusion?

Answer 19

Changes the tertiary structure, so the binding of the substrate e.g. sugar, changes the tertiary structure of carrier protein allowing molecules to enter the cell

Question 20

Water Potential

Answer 20

Pure water has a water potential (ψ) of zero. Adding solute reduces the water potential by an amount known as the solute potential (ψs), which has a negative value. Adding pressure to a solution
increases the water potential by an amount known as the pressure potential (ψp), which has a positive
value.

Question 21

What is the movement of Glucose in the blood?

Answer 21

• Glucose moves in the blood via carrier proteins using facilitated diffusion
• Na+/k+ ATPase pump (embedded in the basal mem - bottom of mem facing towards capillaries not lumen) pumps out 3Na+ ions in exchange for 2K+ ions (Active trans)
• Maintains low conc. grad of Na+ inside the cell

Question 22

What happens in Exocytosis

Answer 22

vesicles move towards cell surf mem + fuse + release content outside the cell

Question 23

Scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase.
Describe competitive and non-competitive inhibition of an enzyme. (5)

Answer 23

1. Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;
   (Competitive inhibition),
2. Inhibitor similar shape (idea) to substrate;
3. (binds) in to active site (of enzyme);
4. (Inhibition) can be overcome by more substrate;
   (Non-competitive inhibition),
5. Inhibitor binds to site on enzyme other than active site;
6. Prevents formation of active site / changes (shape of) active site;
7. Cannot be overcome by adding more substrate;

Question 24

Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some
of these cells have membranes with a carrier protein called NHE3.
NHE3 actively transports one sodium ion into the cell in exchange for one proton
(hydrogen ion) out of the cell.
Use your knowledge of transport across cell membranes to suggest how NHE3
does this.
[3 marks]

Answer 24

Co-transport, uses hydrolysis of ATP. Sodium ions move/diffuse into the cell ALONG their concentration gradient (i.e. from high to low concentration) and in doing so drive H+ ions out of the cell into the lumen AGAINST their concentration gradient. Sodium ion and proton bind to the protein. The NHE3 transporter is a carrier protein, not a channel. Carrier proteins transport ions across membranes by changing their shape.

Question 25

facilitated diffusion from Lumen of Small Intestine, Epithelial Cell, Blood Capillary.

Answer 25

• G & Na+ ions are co - transported from the lumen to the epithelium sodium to lower his conc. grad. Glucose moves against its conc. grad
• Na+ ions actively transported from epithelium through carrier proteins
• K+ ions pumped into Epithelium from blood to conc. grad of Na+ is maintained therefore G is absorbed into the blood.
• G transported from epithelium into blood via facilitated diffusion via channel proteins. G= transported to various bodily tissues for respiration

Question 26

Explain why using a stain makes the appearance of the cell - surface membrane as two dark lines

Answer 26

• Membrane has a phospholipid bilayer
• The stain binds the phospholipid head
• On inside and outside of membrane

Question 27

Water Potential of Potato tissue

Answer 27

• Water potential of solution is more negative then that of the potato tissue
• Tissue looses water by osmosis

Question 28

Describe how you would use the student’s processed results to find the water potential of the potato tissue

Answer 28

• Plot a graph with conc. on the x-axis and % change in mass on y-axis
• Find conc. where curve crosses the x-axis and where the % change is 0
• Use external resource to find Water Potential of sucrose conc. where curve crosses x-axis

Question 29

How to calculate the mean rate of uptake ?

Answer 29

Rate = change / time
! Read units properly !