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 gradient  NET 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 direction  Diffusion can occur in open systems or across membranes

THE RATE OF DIFFUSION  Is DIRECTLY proportional to the  SURFACE AREA of the membrane  CONCENTRATION gradient  TEMPERATURE  Is INVERSELY proportional to the  DISTANCE (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 muscles  Oxygen diffusing from air spaces in the soil into root hair cells  Carbon dioxide diffusing from the air around leaves into the leaf for photosynthesis  Oxygen produced by photosynthesis moving from the leaf into the air  Carbon 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 concentration of the blood comes in the section about homeostasis and the kidney)