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Summary Biology Edexcel A full notes (SNAB) topics 1-8
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Summary of 9 pages for the course Topic 2 at PEARSON (All notes)
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TOPIC 2GAS EXCHANGE SYSTEM
Gas exchange surfaces may be adapted for efficient diffusion because have:
- A large surface are (SA:V) ratio
- A short diffusion path because they are thin
- A rick blood supply i.e. a network of capillaries to create a steep
concentration gradient
The larger the object, the smaller the SA:V ratio and therefore the
slower the rate of gas exchange
The lung is adapted for efficient gas exchange
- Lots of alveoli provides a large surface area
- The capillary endothelium and alveolar epithelium are one cell
thick which provides a short diffusion pathway
- The alveoli have a rich blood supply from the continuous flow of blood through the capillaries which take away
oxygen and bring more carbon dioxide to maintain the concentration gradient
- Breathing in and out refreshes the air in the alveoli which keeps the concentration gradients high
Gas exchange in the lungs
- Oxygen diffuses out of the alveoli, across the alveolar epithelium and the capillary endothelium and into the blood
- Carbon dioxide diffuses into the alveoli from the blood and is breathed out
area of diffusion surface × difference∈concentration
FIC K ' S LAW :rate of diffusion ∝
thickness of diffusion surface
- The rate of diffusion will double if the surface area of the difference in concentration doubles or the thickness of
the surface halves
(C 1−C2 )
- rate=P× A ×
T
CELL MEMBRANES
Cell membranes are composed of lipids, proteins and carbohydrates
Phospholipid molecules
- Phosphate head which is polar, hence its hydrophilic
- Hydrophobic tail because it is non polar and made of 2 fatty acids
- This makes the molecules arrange themselves into a bilayer so the phosphate
heads face out and the centre is hydrophobic, hence it doesn’t allow water – soluble substances like ions through it
The fluid mosaic model was suggested in 1972 to describe the arrangement of molecules in the membrane
Fluid mosaic model
- Phospholipid molecules make up the bilayer
- The bilayer is fluid because the phospholipids are constantly
moving
- Protein molecules are scattered through the bilayer
- Glycoproteins are proteins which have a polysaccharide
(carbohydrate chain) attached
- Glycolipids are lipids which have a polysaccharide chain attached
- Cholesterol is between the phospholipids, forming bonds with them which makes the membrane more rigid
The membrane is partially permeable
- Small molecules can move through gaps between the phospholipids
- Large molecules and ions can only pass through special membrane proteins called channel proteins and carrier
proteins
Scientific investigation
- Initially believed cell membranes were a phospholipid bilayer between 2 continuous layers of proteins because EM
images showed 3 layers
- Improved EM techniques and new methods for analysing proteins showed the proteins were randomly distributed
and not in a continuous layer (freeze fracture electron microscopy studies)
- Fused a mouse cell with a human cell and found the mouse and human membrane proteins intermixed throughout
the cell membrane which proves that that the membrane is fluid because the proteins diffused through the
membranes
, The greater the ratio of phospholipids containing unsaturated fatty acids to fatty acids, the more fluid the membrane
will be
- The kinks in the hydrocarbon tails prevent the unsaturated from packing as closely together so more movement is
possible
TRANSPORT ACROSS THE CELL MEMBRANE
Osmosis
- Diffusion of water molecules across a partially permeable membrane
from an area of higher concentration of water molecules to an area of
lower concentration of water molecules
- Water molecules diffuse both ways across the membrane but the net
movement is to the side with the lower concentration of water
molecules
- The more solute molecules, the more hydrogen bonds that are formed
between them and the water molecules so there is less movement of
the water molecules
- Continues until each side of solution is isotonic
- Passive
Diffusion
- The net movement of molecules from an area of high concentration to one of lower concentration
- Moves down the concentration gradient
- Continues until equilibrium is reached
- Small molecules diffused across the cell membrane
Facilitated diffusion
- The net movement of molecules from an area of high concentration to one of
lower concentration
- Moves down the concentration gradient
- Passive
- Carrier proteins move large molecules
1. A large molecule attaches to the binding site of a specific carrier protein the
membrane
2. The protein changes shape
3. The molecule is released on the opposite side of the membrane
- Specific channel proteins form pores in the membrane for specific charged particles to
diffuse through
- Gated channels open or close depending on the presence of a signal
Active transport
- The net movement of molecules from an area of low concentration to one of a higher concentration gradient
- Against the concentration gradient
- Active – requires energy in the form of ATP
- Involves carrier proteins in the same way as facilitated diffusion
- ATP is produced in respirate and acts as the immediate source of energy in the cell
- When ATP is hydrolysed in the cell, energy is released and it is used to move the molecule again the concentration
gradient
- Active transport proteins may be referred to as pumps
Endocytosis
- Bulk transport of substances into the cell
- Some molecules are too lathe to be taken into a cell by a carrier protein so they can be
taken in by endocytosis
- The cell surrounds the substance with a section of the cell membrane
- The membrane pinches off to form a vesicle inside the cell containing the ingested
substance
- This method is used in phagocytosis
- Active – used ATP for energy
Exocytosis
- Bulk transport of substances out of the cell
- Vesicles containing the substances pinch off from the sacs of the golgi apparatus and move towards the cell
membrane
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