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Section E: Movement across membranes (diffusion (the bigger the difference…
Section E: Movement across membranes
key words
diffusion = the movement of substances from an area of
high
concentration to an area of
low
concentration down a concentration gradient
osmosis = the movement of
water molecules
through a
semi-permeable membrane
from an area of high
water
concentration to an area of low
water
concentration
active transport = the movement of particles
against
a concentration gradient/movement of particles from an area of high concentration using
energy
released during
respiration
passive transport = the movement of particles
without
energy
osmosis
diffusion
the bigger the difference in concentration, the faster the diffusion rate
diffusion is really important in living organisms
spreading out
takes place in both liquids & gases - because the particles in these substances are free to move about randomly
cell membranes - hold cell together, control what comes in & out by processes such as diffusion, osmosis, active transport
only small molecules can diffuse through cell membranes such as glucose, amino acids, water & oxygen. Big molecules i.e. starch & proteins can't fit through
random movement
active transport
plants use active transport to get minerals from the soil (lower mineral concentration) into the root hair cells (higher mineral concentration)
some seaweeds contain iodine at concentrations a million times higher than in sea water - takes up iodine from sea water against the conc gradient by active transport
the cell membrane of the seaweed cell is selectively permeable
sometimes, as in the seaweed, cells must move particles against the concentration gradient
respiration provides a cell with energy
the active uptake of molecules against a concentration gradient using ATP (adenosine triphosphate) - or just using energy
against a concentration gradient means from an area of low concentration to an area of high concentration (opposite of diffusion/osmosis)
carrier proteins transport the molecules from one side of the cell membrane to the other side
occurs in the root hair cells when they actively uptake mineral ions from the soil even though there is a greater concentration in the root hair cells
it is also used when humans absorb nutrients from the gut into the blood (such as glucose into the villi)
factors that affect the rate of movement of substances into and out of cells
factors the effect the rate of diffusion
• 1. concentration gradient
The higher the concentration gradient the faster the rate of diffusion.
• 2. Temperature
• 3. surface area to volume ratio
As size increases the surface area to volume ratio decreases.
As surface area to volume ratio decreases the rate of diffusion decreases.
Therefore, as size increases the rate of diffusion decreases.
experiments investigating diffusion and osmosis in living and non-living systems
Living system - potato cylinders
cut up a potato into identical cylinders and get some beakers with different sugar solutions
One should be water and another very concentrated.
measure the length of each cylinder and leave a few in each beaker for a couple of hours
take them out and measure the length again
If the Cylinders have drawn in Water by osmosis they'll be a bit longer. if water has been drawn out they'll have shrunk.
Non-Living systems - Visking tubing
tie a piece of wire around the end of some Visking Tubing and put a glass tube in the other end. fix the tubing around it with some wire. Then pour some sugar solution down the glass tube into the Visking tubing
Put the Visking tubing in the beaker of pure water, measure where the sugar solution comes up to on the glass tube.
Leave the tubing overnight, then measure where the liquid is in the glass tube. Water should be drawn from the Visking tubing by osmosis and this will force the liquid up the glass tube.
