AQA A LEVEL BIOLOGY: Osmosis

osmosis

the passage of water from a region where it has a higher water pot. to a region where it has a lower water pot. through a selectively permeable membrane.

water potential

the likelihood of water molecules to diffuse out of or into a solution. the water potential of any solution is always negative - the more negative, the stronger the concentration.

isotonic

if two solutions have the same water potential.

hypotonic

solution has higher water potential than inside the cell.

hypertonic

solution has lower water than inside the cell.

factors affecting rate of osmosis

water potential gradient, thickness of exchange surface, surface area of exchange surface.

factors affecting rate of osmosis: water potential gradient

the higher the water potential gradient, that faster the rate of osmosis. rate levels off overtime.

factors affecting rate of osmosis: thickness of exchange surface

the thinner the exchange surface, the faster the rate of osmosis.

factors affecting rate of osmosis: surface area of exchange surface

the larger the surface area, the faster the rate of osmosis.

for animal cells : water pot. of external solution compared to cell solution being higher (less negative)
1) Net movement of water ?
2) State of cell ?

1)enters cell
2) swells and bursts

for animal cells : water pot. of external solution compared to cell solution being equal
1) Net movement of water ?
2) State of cell ?

1) neither leaves nor enters
2) no change

for animal cells : water pot. of external solution compared to cell solution being lower (more negative)
1) Net movement of water ?
2) State of cell ?

1) leaves cell
2) shrinks

for plant cells : water pot. of external solution compared to cell solution being higher (less negative)
1) Net movement of water ?
2) protoplast (outer cell-surface membrane, nucleus, cytoplasm and the inner vacuole membrane)?
3) State of cell

1) enters cell
2) swells
3) turgid - protoplast pushed against cell wall

for plant cells : water pot. of external solution compared to cell solution being equal
1) Net movement of water ?
2) protoplast (outer cell-surface membrane, nucleus, cytoplasm and the inner vacuole membrane)?
3) State of cell

1) neither enters nor leaves
2) no change
3) incipient plasmolysis

for plant cells : water pot. of external solution compared to cell solution being lower (more negative)
1) Net movement of water ?
2) protoplast (outer cell-surface membrane, nucleus, cytoplasm and the inner vacuole membrane)?
3) State of cell

1) leaves cell
2) shrinks
3) plasmolysed - protoplast completely pulled away from cell wall

Syggest how the water potential of a sweet potato tissue is likely to differ from a white potatoe tissue. Explain. [2]

the sweet potate is likely to contain a higher concentration of sucrose, having a lower water potential than white potato tissue.

Give 2 similarities between osmosis and diffusion. [2]

- Botrh passive, requier no enerfy from respiration
- both move substances down the concentration gradient

Give 2 reasons why potato cylinders left in a solution overnight must be blotted dry before weighing. [2]

- To remoive excess water from cylinders that could yield inaccurate mass results
- each cylinder of portatoe would have different amounts of water on them

What is the advantage of calculating teh % change in mass rather than the change in mass in potato practical. Explain the advantage of this. [2]

-Allows for a better comparison
- Shows proportional change

A student carried out several repeats at each concentration of sodium chloride solution. Explain why the repeats are important. [2]

-Increases reliability, results not due to chance.
- reduces variation/anomalies