B4 Movement of substances in and out of cells T6 Flashcards
Diffusion
net movement of particles from an area of higher concentration to a lower concentration
diffusion is a ________ process
How does it work
passive
uses kinetic energy of molecules so energy from respiration not needed
where can diffusion take place
- cells
e.g. oxygen andCO2 - non living systems
e.g. perfume diffusing across a room
[5 marks]
Explain the factors affecting diffusion
1) Distance - diffusion is faster, the shorter the diffusion distance
2) temperature - diffusion is faster at hgiher temperatures because as temperature increases, particles gain kinetic energy therefore faster diffusion
3) concentration gradient - faster diffusion when there is a steeper concentration gradient (greater difference in concentrations)
4) surface area - larger SA, faster diffusion
5) molecular size - the smaller the mass of the molecule, the faster the diffusion, as the molecules can move faster
How to answer diffusion questions
Why will oxygen diffuse into the cell?
There is a higher concentration of oxygen outside the cell than inside, so the molecules will move into the cell by diffusion
Osmosis
movement of water molecules across a semi permeable membrane from an area of higher concentration to lower concentration
osmosis in animals
external solution is more dilute (higher water potential), it will move into animal
cells causing them to burst.
If the external solution is more concentrated (lower water potential), excess water will
leave the cell causing it to become shrivelled.
osmosis in plant cells
If the external solution is more dilute, water will move into the cell and into the vacuole,
causing it to swell, resulting in pressure called turgor (essential in keeping the leaves
and stems of plants rigid).
If the external solution is less dilute, water will move out of the cell and they will become
soft. Eventually the cell membrane will move away from the cell wall (called
plasmolysis) and it will die
define active transport
movement of particles from area of low to gih concentration using energy from respiration and requires a membrane with a carrier protein
(active process)
example of active transport in living organisms
In root hair cells:
○ They take up water and mineral ions (for healthy growth) from the soil
○ Mineral ions are usually in higher concentrations in the cells, meaning diffusion cannot
take place
○ This requires energy from respiration to work
In the gut:
○ Substances such as glucose and amino acids from your food have to move from your
gut into your bloodstream
○ Sometimes there can be a lower concentration of sugar molecules in the gut than the
blood, meaning diffusion cannot take place
○ Active transport is required to move the sugar to the blood against its concentration gradient
Practical: investigate diffusion in non-living systems
(uses agar and sodium hydroxide indicator)
1) Cut a 1cm3 cube of agar made of sodium hydroxide and phenolphthalein indicator
2) Place cube in solution of hydrochloric acid
3) Remove the cube and wash with water to stop further reaction
4) Cut the cube in half and measure the distance that the acid has caused agar to become
colourless from outside inwards
5) Repeat the experiment two more times and calculate the mean
6) Repeat with different concentrations of hydrochloric acid
Practical: investigating osmosis in potatoes
1) Place different sucrose solutions including 0% for a control, in different boiling tubes
2) Dry potato strips on a paper towel and measure the masses = size is 5x1x1
3) Place each potato strip into each sucrose solution for 20 minutes, blot to remove excess solution and leaving dry mass and record how the mass changed
4) Repeat tests at each solution several times with potato strips of similar masses
Practical: investigating osmosis in non-living systems
Explain SA:V ratio
The smaller an object is, the larger the SA:V ratio.
The larger the SA:V ratio, the faster the rate of diffusion
surface area
SA =
area of each side added together
