Diffusion & Osmosis 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

Diffusion (Single celled organisms)

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

Food diffusion -single celled organisms

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.

Multicellular organisms 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

Role of Cell Membrane

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

Water Concentrations

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

Effect of osmosis on cells 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

Osmosis & Cells

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)