This document is a revision booklet for unit 5 biology. This covers ALL topics in unit 5 biology, which is an examined unit in applied science. This booklet helped e to achieve a distinction in my mock exams, and will help you achieve a distinction in your exams too.
Explain the “Fluid mosaic model’ of membrane structure.
Link concentration gradients to a biological process
List key factors affecting the rate of diffusion
Define what facilitated diffusion is
Model of membrane structure
Lipids provide fluidity
Proteins give a patchwork (mosaic) appearance
Molecules move about
Question: Explain why the model for membrane structure is known as the fluid mosaic model (3
marks)
The membrane structure is known as the fluid mosaic model as the phospholipid molecules can
move freely and lipids provide fluidity. The proteins are distributed throughout the membrane in
evenly and in a mosaic pattern. The agreed structure is based upon experimental and chemical
evidence and so is classed as a model.
Channel Proteins-
Allows movement of some large, hydrophilic molecules directly across the membrane
The movement will be down a concentration gradient by diffusion.
Pores- allows a specific ion through
May be ‘gated’ open or closed
Carrier proteins-
Actively move substances across membrane (example-plant roots- use energy from ATP)
Specific shape to allow specific molecules to fit in
Changes shape to allow molecules across membrane
Diffusion-
Diffusion is a movement of molecules from a region of high concentration to a region of lower
concentration down a concentration gradient.
Diffusion occurs because the particles are free to move, so once they move due to kinetic
energy; The particles collide and start to spread out.
There is no energy from the cell needed.
, “Passive”.
Tendency to spread out away from high concentration if they are not contained.
Increasing the rate of diffusion-
1. Higher temperature for more kinetic energy)
2. Bigger concentration gradient for more molecules on one side of membrane)
3. Mixing (stirring) for more movement of molecules
4. Greater surface area – Red blood cells are biconcave, alveoli in lungs, microvilli in small
intestine
5. Shorter distance/ thickness
6. Smaller molecules
Particles that can diffuse
Lipid based molecules (Fat soluble, can diffuse through phospholipids including steroid
hormones)
Very small molecules and ions (such as CO2, O2 which fit between phospholipids and some
water will diffuse even though they are polar/charged).
Why do we have cholesterol in the membrane?
We have cholesterol as it reduces the permeability in the cell membrane.
More cholesterol means less movement of the phospholipid bilayer.
Provides some eukaryotic cell membrane stability- fits between fatty acid tails
Completes the hydrophobic barrier
Prevents water and ion movement
Learning objectives-
- Compare active transport with facilitated diffusion
- Draw Endocytosis and Exocytosis
- Recap Osmosis and apply new definition using Water potential
Active transport
Active transport is a movement of molecules or ions across membranes, using ATP to drive protein
‘pumps’- If substance is moved in opposite direction than it would move by diffusion or more quicker
than by diffusion, so for example- moving magnesium ion from soil against concentration gradient
by roots for chlorophyll manufacture.
Osmosis is a process which uses a spontaneous network of solvent molecules through a partially
permeable membrane from a less concentration solution to a more concentrated solution.
Solute is a component of a solution present in smaller amount than the solvent.
A solvent is a substance that dissolves a solute, resulting in a solution.
A solution is a mixture dissolved in another substance
‘Free’ water molecules
, - Water molecules can only diffuse if they are ‘free’
- When solutes dissolve water molecules cluster around solute molecules so they can’t
diffuse
- Concentration of solute affects diffusion of water
Water potential
Water moves from high water potential to low water potential
- High water potential has a high concentration of ‘free’ water molecules
- Mole solute in a solution.
- Less ‘free’ water molecules
- Lower water potential
Cells in high water potential solutions
- Water potential gradient from outside to inside
- Water moves by osmosis into cell
- Cell swells
Animal cells may burst
Plant cell; Cytoplasm pushes against cell wall
- Slows osmosis, even if there is still a water potential gradient
Cells in low water potential solutions
- Water moves by osmosis out of cells
- - Cell shrinks
Stress, alcohol, and drugs can affect the hearts performance
Has 4 valves
Valves
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