Outline for revision

DIFFUSION, OSMOSIS, ACTIVE

TRANSPORT

SIMPLE DIFFUSION

SIMPLE DIFFUSION (or just diffusion) is a PASSIVE PROCESS requires no outside energy, using KINETIC ENERGY in SOLUTES

Solutes move from a region of HIGH concentration to a region of LOW concentration (along a concentration gradient) A difference in the concentration of a substance between two places is

called a concentration gradient or in this case a diffusion gradientNET movement of solutes continues until equilibrium is reached

Once molecules have distributed evenly the system reaches equilibrium and diffusion stops

Molecules still move but equal numbers move in each directionDiffusion can occur in open systems or across membranes

THE RATE OF DIFFUSION

Is DIRECTLY proportional to theSURFACE AREA of the membraneCONCENTRATION gradientTEMPERATURE

Is INVERSELY proportional to theDISTANCE (e.g. thickness of the exchange surface)

EXAMPLES OF DIFFUSION:

Oxygen diffusing from the alveoli in the lungs into the red blood cells in the capillaries

Oxygen diffusing from the red blood cells into the musclesOxygen diffusing from air spaces in the soil into root hair cellsCarbon dioxide diffusing from the air around leaves into the

leaf for photosynthesisOxygen produced by photosynthesis moving from the leaf into

the airCarbon dioxide produced by respiration diffusing from the

muscles into the red blood cells

ACTIVE TRANSPORT

ACTIVE TRANSPORT (or just diffusion) is a ACTIVE PROCESS that requires energy from respiration

Solutes move from a region of LOW concentration to a region of HIGH concentration (i.e. against a concentration gradient)

EXAMPLES OF ACTIVE TRANSPORT:

Mineral ions are absorbed from soil water, where they are at low

concentrations, into root hair cells where there are at high

concentration.

Oxygen in the soil is needed to the cells do aerobic respiration to

provide energy for this process.

Active transport is also needed in the kidney to reabsorb glucose

into the blood

OSMOSIS

OSMOSIS (or just diffusion) is a PASSIVE PROCESS that requires no outside energy, using only KINETIC ENERGY in WATER

Is the net movement of water from a region of higher water potential to lower water potential across a semipermeable membrane. High water potential: where a solution is dilute, in other words is

relatively little solute. Low water potential: resolution is concentrated, in other words as a

large amount of solute.

TONICITY

Hypertonic = lower water potential, more solute

Isotonic = same water potential, same solute concentration

Hypotonic = higher water potential, less solute

EXAMPLES OF OSMOSIS:

Root hair cells have a higher concentration of minerals than the soil water around

them. In other words, they are hypertonic to the soil water the soil is hypotonic (higher water potential) so water moves from the soil into the root hair cell.

Vacuoles in plant cells, for example leaf palisade cells, contain cell sap, which contains a high solute concentration. Water fills the cell by osmosis to maintain turgor – making the cells stiff

enough to support the leaf

TURGOR & PLASMOLYSIS

TURGOR The hypertonic vacuole fills with

water, pressing the cytoplasm against the cell wall.

FLACCIDITY When the plant cells receive too little water, the surroundings do not have as much

water potential as they did. the vacuoles are no longer hypertonic to their surroundings. The vacuoles do not absorb water and the cells lose their turgor. the cells become soft – or flaccid – and no longer support the tissues of the plant, so it

wilts.

PLASMOLYSIS When the vacuoles shrink extremely, the cytoplasm pulls away from the cell wall,

showing gaps. The cell is now in a plasmolysed state.

OSMOREGULATION

In animals (including humans), if the blood is more dilute than the tissues (is hypotonic, i.e higher water potential) water moves into the tissues from the blood.

Keeping tissue and blood concentration normal is important - and is called osmoregulation

(The mechanism shown for controlling the concentrationof the blood comes in the section about homeostasis andthe kidney)