Diffusion

Question 1

What is the definition of diffusion?

Answer 1

the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed

Question 2

Describe simple diffusion through the phospholipid gaps

Answer 2

Small non-polar and lipid-soluble molecules can pass between the phospholipid molecules in the cell membrane

the fluid movements f the phospholipids create gaps in the membrane and the molecules can slip between

this happens without any energy being available from the cell, by simple diffusion

Question 3

What can and what cannot pass through the bilayer by simple diffusion (name the examples)

Answer 3

CAN: non-polar and small molecules (such as oxygen, CO2 water, urea and steroid hormones)

CAN’T: ions, glucose and amino acids

Question 4

What is facilitated diffusion?

Answer 4

Facilitated diffusion describes the process of passive transport (doesn’t require energy expenditure) of molecules across a membrane, with the help of carrier proteins.

Each substance has one specific carrier protein.

Facilitated diffusion usually occurs when a molecule cannot pass through the plasma membrane because of their polarity or charge.

Question 5

Why is facilitated diffusion a passive process?

Answer 5

because it relies only on the inbuilt motion of the diffusing molecules.

there is no external input of ATP from respiration

Question 6

What does it mean to say that a channel is gated and why might some channels be gated?

Answer 6

it opens and closes = regulates when molecules can enter and leave

Question 7

How do Carrier proteins help in facilitated diffusion?

Answer 7

Moves large molecules down the concentration gradient

Proteins that aid in facilitated diffusion shield these molecules from the hydrophobic core of the membrane, allowing molecules to cross the membrane through a pore or channel.

Question 8

What is the definition of osmosis?

Answer 8

the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane, down a concentration gradient

Question 9

Describe how you could use your graph to find out the water potential of the leaf cells

Answer 9

Draw a graph with percentage change on the y-axis and concentration on the x-axis, then a curve of best fit.

Where the line meets the x-axis is the water potential of the cell because that is where the leaf cells have not yet gained or lost length = when the solution is isotonic

Question 10

What is a hypotonic solution

Answer 10

when the solution has a higher water potential (lower conc) than the cell

Question 11

What is a hypertonic solution

Answer 11

When the solution has a lower water potential (higher conc) than the cell

Question 12

Describe what happens to cells when they are placed in a hypotonic solution

Answer 12

Water potential is lower in the cell than the solution

so water moves from the solution, from a low concentration to a high concentration,

down a concentration gradient through a selectively permeable membrane and into the cell via osmosis,

causing the cell to burst.

Question 13

Describe what happens to cells when they are placed in a hypertonic solution

Answer 13

Water potential is higher in the cell than in the solution

so water moves from the cell, from a low concentration to a high concentration

, down the concentration gradient,

through a selectively permeable membrane,

into the solution by osmosis and the cell shrinks

Question 14

Describe what happens to PLANT cells when they are placed in a hypertonic solution

Answer 14

because the cell has a higher water potential than the solution,

the water molecules move down the concentration gradient from the cell to the solution,

through a selectively permeable membrane

causing the cells to become flaccid and then, if it loses so much water, plasmolysed

Question 15

What does plasmolysed mean

Answer 15

The cytoplasm of the plant cell removes from the cell wall

Question 16

What does flaccid mean

Answer 16

when the volume of the vacuole decreases

Question 17

Describe what happens to PLANT cells when they are placed in a hypotonic solution

Answer 17

it takes up water by osmosis and starts to swell, but the cell wall prevents it from bursting = turgid

Question 18

Young carrots store sugars in their tissues but, in older carrots, some of this is converted to starch. How would using cylinders of tissue from older carrots affect the results obtained for a sucrose solution of 0.6moldm–3? Give a reason for your answer.

Answer 18

Greater decrease/length smaller;

More water removed;

Greater difference in water potential/cell with higher/less negative water potential;

Starch is insoluble/has no effect on osmosis

Question 19

What is the definition of active transport?

Answer 19

the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

Question 20

Describe the process of active transport

Answer 20

1. Molecules/ions bind to receptors on the carrier protein
2. On the inside of the cell: ATP binds to carrier protein = causing it to split into ADP and phosphate molecule

Causes carrier protein molecule to change shape, releasing molecules/ions to the other side of the membrane

Phosphate molecule is released from carrier protein & recombines with ADP to form ATP during respiration

Causes carrier protein to revert to its original shape

Question 21

When is active transport used?

Answer 21

• when substances need to move into a cell against the concentration gradient
• when a cell needs to move substances in quantities that diffusion would not allow

Question 22

What are adaptations for the small intestine/ ileum for the absorption for glucose

Answer 22

folded to form Villi (large surface area)

cells lining SI have Microvilli (large surface area)

wall of SI is thin (short diffusion distance)

rich blood supply (maintains concentration gradient)

cells lining SI have transport proteins, enzymes (maltase, lactase, sucrase, didpeptidase) and many mitochondri

Question 23

Describe the co-transport of sodium ions and glucose

Answer 23

sodium ions are actively transported from the epithelial cells of the SI into the blood via a sodium-potassium pump

lowers the sodium ion concentration in the cell than in the lumen

therefore sodium ions move from the lumen of the SI into the cell, down the concentration gradient trough a co-transport protein

this pulls in glucose and amino acids

therefore glucose and amino acids builds up in the cell and moves into the blood by facilitated diffusion using a carrier protein

Question 24

Why does glucose need to be co-transported in the small intestine

Answer 24

because there is already a high conc of glucose in the epithelial cell