cell biology ; unit 3. cell transport

diffusion definition

(the spreading out of particles resulting in a) net movement from an area of higher concentration to and area of lower concentration (down the concentration gradient)

passive process - no Ê required

example of O2 and CO2 diff. into and out of cells

cells need O2 to release Ê through aerobic resp by mitochondria

cells are surrrounded by a high concentration of oxygen (from the blood - from the lungs)

higher conc. out → lower conc. in (by diff.)

now CO2 is produced and there is a higher conc of it inside the cell than outside, so CO2 diff. out

urea : inside cells to outside

main - factors affecting the rate of diff.

conc. grad. : the greater difference between concentration the more molecules can diffuse in a given time (due to the difference) therefore an increased rate in diff.

higher temp : greater rate of diff. because particles have more Kin Ê and therefore are moving faster

SA of the membrane - a greater SA membrane means that molecules can diffuse at a greater rate due to the bigger space

periphery factors affecting the rate of diff.

path - shorter diff. path means that molecules can travel faster and a greater rate of diff.

membrane structure - the permeability - larger pores = faster

type of molecule - smaller molecules = faster more soluble in the barrier therefore faster (lower relative molecular mass means that they are also lighter)

SA : V in unicellular organisms

unicellular organisms have a very high SA : V , meaning that for their volume they have a really large SA

this means that they can rely on diff to transport molecules into and out of the cell e.g. all the O2 and CO2

trend of SA : V as organisms get larger

and how mult. animals have adapted to this problem

as organisms get larger their SA:V falls sharply - big problem for multicellular organisms - their SA is not large enough for their volume for diffusion of particles

cells on the surface can get enough O2 by diff however not enough O2 can diffuse into the cells into the center of the organism as it is too far away from the surface (length from Surface to center inc.)

they have specialised exchange structures for gas exchange w a v. high SA

they have a transport system to carry gases around the body

how gills bring O2 into the fish

O2 aq water passes into the mouth, where it then flows over the gills, where the O2 is transported into the bloodstream

deo2 bl passes into the fillament where O2 diffuses from the water into the blood. O2 bl returns into the fish

gills adaptations for an efficient exchange surfaces

gills are covered in very fine folded fillaments meaning that they have a really large SA (inc. rate of diff)

fillaments have a thin membrane so the diff path is short so greater rate

dense capillary network that rapidly takes away exchanged molecules ensuring that the conc. grad. is always high

SA:V experiment with agar cubes

agar cube has a similar consistency to cytoplasm and has lots of water, in the center there is a block of some random chemical that is purple and disappears when reacted w acid

there are 3 agar cubes w an equal sized cube of the chemical inside but different side lengths

we put cubes into the beaker and submerge w acid

record how long it takes for a colour change

explanation of the Agar jelly cube SA:V experiment

the larger the cube, the lower SA:V

meaning that the rate of diffusion in proportion of the length to the center of the cube (where the chemical is) decreases,

therefore it takes longer for the acid to diffuse through to reach the center

this is the same w organisms as the larger the organism the lower sa - molecules can only diffuse into the outer cells

Osmosis definition

Overall (net) movement of water from an area of higher water potential (dilute solution) to an area of lower WP (conc. sol.) [down the conc grad] through a partially permeable membrane

Osmosis explanation

partially permeable means that it is decisive on what it will let through e.g. water is small so it is permeable through thr membrane but not sugar - this means that only water can pass to neutralise solution conc.

water moves both ways but moves net from the area of high conc. to the area of low conc. down the CG

will occur when there is a conc. grad. between the WPs of the solutions. (always specify what is moving, from where and by what process)

osmosis in tissue fluid transmission

Body cells are surrounded by tissue fluid (glucose and other substances dissolved), formed from squeezed plasma out of capillaries

transport between molecules between cell. c.plasm and this fluid

it is important that the TF and c.plasm have the same WP so osmosis doesn’t occur and cells do not shrink or swell up and burst

and also so that only the substances needed e.g. O2 diffuse

possibilites of osmosis in animal cells

where the cell is placed in a solution w a lower WP, water moves out by osmosis net down the CG, causing it to shrink and be crenated

when placed in a solution w a higher WP, water moves in by osmosis net, causing it to swell ; it may burst as the CM may not withstand the force (lysis)

possibilites of osmosis in plant cells

where the cell is placed in a solution w a lower WP, water moves out by osmosis net down the CG, causing it to shrink and be plasmolysed - CW wont shrink

when placed in a solution w a higher WP, water moves in by osmosis net, causing it to swell (turgid) it does not burst as the membrane pushes onto the CW

osmosis potato experiment set up

take a corkscrew borer through a potato and cut the drill piece into 5 equal slices of length (no skin), and weigh the mass of each

place one slice in distilled water, and the other four in distilled concs. of sugar solution, and leave for a day

then pat dry v. gently the surface moisture and measure again

calc % change and plot the graph

factors to control in the osmosis experiment

temperature - affects rate of diff

skin and length (why we use a corkscrew borer) - controlling sa and v

vol of solution

time left in solution

same potato to ensure same WP inside the potato

explanation of the osmosis potato practical

the one w distilled water has a higher WP and the potato so water will move net in by osmosis (mass gain)

one w a more concentrated one than the potato has a lower WP and therefore water moves out net by osmosis (mass lost)

(the difference in WP between the potato and sol will determine how much mass is lost or gained as part osmosis water)

on the graph of mass change against conc. x int is the approx. conc. of the potato

eval. of the osm potato experiment

more data and in between concentrations makes the LOBF more accurate as more points / pattern more clear and accurate

reruns and mean for mass change makes the data more reliable

an error could be squeezing osmosis water out of the potato when drying or not enough surface moisture removed causing result innacuracies

Active transport

The movement of particles from an area of lower to higher concentration against the conc. grad. {caused by the cell} will take place in the cell by protein pumps and involes Ê from resp. (m.chon)

it is caused because the villi, roots, or kidneys need to take in as many molecules as possible

benefits and general adaptations of cells for AT

in RHCs, if more mineral aq+ are taken into the cell, this makes the WP of the cell c.plasm lower therefore causing osmosis to occur into the cell

lots of well developed m.chon to release energy for AT by resp

also have a thin, high SA membrane

Osmosis, AT, DIFF differences

Diff and Osm are passive processes, where AT is not

Diff and Osm happen down the CG where as AT draws against

Osm can only happen when the membrane is par perm in favour of water only (or else any other substance can move to equalise conc, not js water alone)

Osm is only water where Diff and AT are any