2d: Movement of substances in and out of cells

movement of molecules from an area of higher concentration to an area of lower concentration

diffusion

does diffusion require energy?

no, it is passive

which molecules can diffuse?

they have to be small: oxygen, glucose, amino acids and water
larger molecules such as starch and protein can't

diffusion in uni-cellular organisms

use diffusion to transport molecules into their body from the air

uni-cellular organisms' diffusion adaptation

large surface area to volume ratio: low metabolic demands mean diffusion across the surface is sufficient

diffusion in multi-cellular organisms

small surface area to volume ratio so only diffusion cannot be relied upon. these then have other adaptations.

factors affecting rate of diffusion

- concentration gradient
- temperature
- surface area to volume ratio
- distance

concentration gradient

greater difference concentration (steep) = faster rate of diffusion

in diffusion, particles move randomly ____ the gradient rather than _______ it.

down, against

temperature

greater temperature = greater movement of particles

increased temperature results in more __________ and a faster rate of diffusion

collisions

surface area to volume ratio

greater surface area = more space for particles to move through

distance

shorter distance = faster diffusion

osmosis

movement of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane

does osmosis require energy?

no, it is passive

dilute solution of sugar

A high concentration of water and a low concentration of sugar

concentrated solution of sugar

A low concentration of water and a high concentration of sugar

high concentration of water

high water potential

low concentration of water

low water potential

movement of water from a dilute solution to concentrated

moves from an area of high water potential to low water potential, down the concentration gradient

isotonic solution

a solution whose solute concentration is equal to the solute concentration inside a cell

hypertonic solution

a solution whose solute concentration is higher than the solute concentration inside a cell

hypotonic solution

a solution whose solute concentration is lower than the solute concentration inside a cell

active transport

movement of particles from an area of low concentration to an area of high concentration (against the concentration gradient)

does active transport require energy?

yes, in the form of ATP

examples of active transport

- root hair cells
- gut

root hair cells

- take up mineral and water ions
- mineral ions are in higher concentration so diffusion cannot take place
- this requires ATP

gut

- substances such as glucose and amino acids have to move from gut into bloodstream
- sometimes there is a lower concentration of sugar molecules in the gut so diffusion cannot occur
- this then requires active transport

practical

investigating diffusion and osmosis

investigating the effect of temperature on diffusion using beetroot

heat the beetroot above 45 so that its dark purple-red pigment leaks out as the cell membrane is damaged

what does the leakage of the pigment tell us about the effect of temperature

the speed at which this pigment leaks out of the cell tells us about the rate of diffusion

investigating the effect of temperature on diffusion - apparatus

- beetroot
- knife
- cutting board
- ruler
- test tubes
- water baths
- stopwatch

investigating the effect of temperature on diffusion - method

- cut 2 equally sized cubes of beetroot
- rinse
- put 5cm water into 2 test tubes labelled A and B
- keep test tube A at rtp
- move test tube B to a hot water bath at 90C
- leave test tube for 2 minutes
- add beetroot into each
- observe colour of liquid after 10 minutes

why should the beetroot be equally sized

dimensions affect rate at which pigment leaks out

why do we rinse the beetroot

to remove pigment released during cutting

investigating the effect of temperature on diffusion - results

at a higher temperature more pigment leaks out

investigating the effect of temperature on diffusion - analysis

- the cell membrane has been damaged for more pigment to leak out
- particles have more kinetic energy

investigating the effect of temperature on diffusion - limitations

- identical size and shape of beetroot
- some parts could have more pigment than others
- results would be more reliable at many different temperatures
- observing the colour is subjective

investigating the effect of temperature on diffusion - overcoming limitations

- repeat and find a mean
- use many temperatures
- use a calorimeter

investigating the effect of temperature on diffusion - CORMMS

C - temperature
O - beetroot cubes
R - repeat for reliability
M - observe colour change
M - 10 mins
S - control volume of water used and beetroot dimensions

investigating the effect of concentration on osmosis using potatoes

place cylinders of potato into distilled water and sucrose solutions of increasing concentration

investigating the effect of concentration on osmosis - apparatus

- potatoes
- cork borer
- knife
- sucrose solutions
- balance
- test tubes
- paper towels
- ruler
- test tube rack

investigating the effect of concentration on osmosis - method

- prepare a range of sucrose solutions from 0 to 1 mol/dm3
- set up 6 tubes with 10cm of each sucrose solution
- cut 6 equally sized cylinders of potato
- blot each one and weigh
- put 1 piece into each concentration of sucrose solution
- remove after 4 hours, blot and reweigh

investigating the effect of concentration on osmosis - results

calculate percentage change in mass

calculating percentage change in mass

(final mass - initial mass) / initial mass x 100

investigating the effect of concentration on osmosis - CORMMS

C - concentration of sucrose solutions
O - potato cylinders
R - repeat for reliability
M - change in mass
M - 4 hours
S - control volume of sucrose solution used and potato dimensions