Download Chapter 2

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Diffusion is the movement of
molecules
- from an area of high
concentration
- to an area of low
concentration
This continues until the
concentration is equal
The difference between 2 areas
is called the concentration
gradient
Movement is always from high
to low concentration
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Single celled organisms use up oxygen &
food
& produce CO2 & waste
There is generally a gradient between the
inside & outside of the cell
The cell membrane allows oxygen, CO2 food
across freely (freely permeable)
Oxygen/food will diffuse into the cell
CO2/waste will diffuse out of the cell
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Single celled animals (e.g.
Amoeba) engulfs food by
phagocytosis
Food becomes engulfed in a
food vacuole
Food is digested by enzymes
(inside lysosomes)
Inside the vacuole there is a
high conc. of food products, a
low conc. in the cytoplasm
The food diffuses from high
conc. to low conc.
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In animals
Diffusion is important for exchange
of gases
Blood returning to lungs
- high conc. of CO2, low conc. of
oxygen
CO2 diffuses out of blood into air
sacs
Oxygen diffuses from air sacs into
blood
Diffusion also important for
dissolved food & wastes
In plants
- CO2 diffuses from the air into the
leaves
- water diffuses from the soil into the
roots
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Cell membrane is
semi-permeable
Allows small
molecules (e.g.
water, oxygen) to
pass across freely
Won’t allow large
molecules (e.g.
starch) to pass across
Membranes have
tiny pores or gaps
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A solution with the higher water conc (HWC)
is said to be HYPOTONIC
The solution with the lower water conc (LWC)
is HYPERTONIC
If the 2 solutions are of equal conc, they are
ISOTONIC
Molarity (M) is the conc of solute (e.g.
sugar/salt) dissolved in water
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If a solution outside a cell is
hypotonic, water will diffuse in
The cell will expand
Animal cells will eventually
burst
Plant cells will swell and
become hard - TURGID
If a solution outside the cell is
hypertonic, water will diffuse
out
The cell will shrink & shrivel
up
Plant cells are said to be
PLASMOLYSED
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In plants, useful for:
- Soil to roots (water into
plant)
- Xylem to stem (for
stem support)
- xylem to green leaves
(for photsynthesis)
In single celled
organisms:
e.g Paramecium
contractile vacuoles
control water levels
(OSMOREGULATION)