TOPIC 2
GAS 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
GAS 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
