I have published a compilation of study notes on every topic of the CIE IGCSE Biology course. Using these study notes, I got an A* in Biology (well actually a 9 out of 9) and I found that without them, this would have been impossible. Each document is worth 5$ which is a very reasonable price, as m...
DIFFUSION → the net movement of molecules and ions from a region of high concentration to a region of lower concentration down a CONCENTRATION
GRADIENT, as a result of their random movement.
OSMOSIS → the diffusion of water molecules from a region of high-water potential (dilute) to a region of lower water potential (concentrated), through a
PARTIALLY PERMEABLE MEMBRANE.
ACTIVE TRANSPORT → the movement of particles and ions through a cell membrane from a region of lower concentration to a region of higher
concentration using energy from respiration
3. 1. D I F F U SI O N
DIFFUSION AND LIVING ORGANISMS
DIFFUSION → Helps living organisms obtain many of their requirements
→ Helps living organisms get rid of their waste products
→ Important: In gas exchange for respiration in plants and animals
CELL MEMBRANES: freely permeable to oxygen and carbon dioxide, so they can
diffuse in and out of cells
E.G. PLANTS → they need carbon dioxide for photosynthesis
→ diffuses from the air into the leaves (there is a
lower concentration of carbon dioxide inside the leaf than in the air)
BROWNIAN MOTION
EXAMPLES OF DIFFUSION IN LIVING ORGANISMS
• All particles move randomly at all times
• This is known as Brownian motion
• The energy for diffusion comes from the kinetic energy of this
random movement of molecules and ions
FACTORS AFFECTING RATE OF DIFFUSION
Distance
• The smaller the distance molecules have to travel the faster transport
will occur
• This is why blood capillaries and alveoli have walls which are only one cell
thick, ensure the rate of diffusion across them is as fast as possible
Temperature
• The higher the temperature, the faster molecules move as they have
more energy
• This results in more collisions against the cell membrane and therefore a
faster rate of movement across them
Concentration Gradient
• The greater the difference in concentration either side of the membrane,
the faster movement across it will occur
• This is because on the side with the higher concentration, more random
collisions against the membrane will occur
3. 2. O SM O SI S
Water → a solvent for many different substances
The difference in concentrations between substances creates a CONCENTRATION
GRADIENT
CELL MEMBRANES
• The cell membrane is partially permeable which means it allows small molecules (like
water) through but not larger molecules (like solute molecules)
• They separate two different solutions, the cytoplasm and the solution around the cell.
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