TOPIC 4
4.2 Cell transport mechanisms
i Know the structure of the cell surface membrane with reference to the fluid mosaic model.
Consists of phospholipids with protein molecules. Fluid due to composition of cholesterol that allows
the membrane to maintain fluidity. Controls the movement of substances into and out of the cell,
contains receptors for other molecules such as hormones, and enables adjacent cells to stick
together.
ii Understand how passive transport is brought about by:
diffusion
facilitated diffusion (through carrier proteins and protein channels)
osmosis.
Diffusion – passive movement of small, non-polar, lipid soluble molecules down the concentration
gradient directly through phospholipid bilayer. Rate increases with increased SA, small diffusion
distance and large gradient.
Facilitated diffusion – protein channels needed to transport polar, charged, water-soluble molecules
across membrane
Osmosis – movement of water molecules down concentration gradient through a partially
permeable membrane
iii Understand how the properties of molecules affects how they are transported, including solubility,
size and charge.
Property Transport mechanism
Polar molecules Endocytosis, facilitated diffusion or active
transport
Soluble molecules Passive diffusion
Insoluble molecules Facilitated diffusion
iv Know that large molecules can be transported into and out of cells through the formation of
vesicles, in the processes of endocytosis and exocytosis.
,Endocytosis/exocytosis – movement of large particles into and out of cells in vesicles. In endocytosis,
vesicles are made from cell surface membrane. In exocytosis, vesicles fuse with cell surface
membrane and transported out of cell
CORE PRACTICAL 5: Investigate the effect of temperature on beetroot membrane permeability.
CORE PRACTICAL 6: Determine the water potential of plant cells.
Water potential = turgor pressure + osmotic potential
ψ=P+π
4.3 Gas Exchange
i Understand how insects, fish and mammals are adapted for gas exchange.
Gas exchange in fish
1. Mouth opens
2. Operculum closes
3. Buccal cavity floor lowers
4. Contracted muscles of operculum cause it to bulge outwards
5. Volume of opercular cavity
increases
6. Pressure decreases
7. Mouth closes
8. Floor of buccal cavity raises
9. Volume of opercular cavity
decreases
10. Pressure increases
11. Operculum opens due to high
pressure
12. Water flows out of cavity
Gas exchange in insects
Spiracles – hair on surface of insect that reduce water loss;
prevent dust and parasites entering. Open and close in response
to CO2 levels and determine rate of respiration through muscles
contracting and relaxing
Trachea – kept open by rings of chitin; end at respiring tissue; have
fluid in them when insect is less active to reduce dehydration
Air sacs – reservoir of air in larger insects; increase respiratory
efficiency by providing a large surface area for gas exchange; expand
and contract using movements of muscles
Nymphs Adult mayfly
Large SA Large SA
, Small diffusion distance Small diffusion distance
Contain feathery gills Have trachea
External gas exchange system Internal gas exchange system
Mammalian gas exchange
Breathing
1. Diaphragm contracts and flattens
2. External intercostal muscles contract and internal
intercostal muscles relax
3. Ribs move upwards and outwards
4. Volume of thoracic cavity increases
5. Pressure decreases
6. Air moves down the pressure gradient passively from
the external atmosphere into the lungs
Chemoreceptors
There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to
changes in carbon dioxide levels in the blood.
If the level is too high (the pH would drop, enzyme action would be affected with serious results),
impulses are sent from these cells to the inspiratory part of the centre so that breathing rate
increases.
This means that carbon dioxide is got out of the body as quickly as possible and more oxygen comes
in.
CORE PRACTICAL 7: Dissect an insect to show the structure of the gas exchange system, taking into
account the safe and ethical use of organisms.
ii Understand gas exchange in flowering plants, including the role of stomata, gas exchange surfaces
in the leaf and lenticels.
4.2 Cell transport mechanisms
i Know the structure of the cell surface membrane with reference to the fluid mosaic model.
Consists of phospholipids with protein molecules. Fluid due to composition of cholesterol that allows
the membrane to maintain fluidity. Controls the movement of substances into and out of the cell,
contains receptors for other molecules such as hormones, and enables adjacent cells to stick
together.
ii Understand how passive transport is brought about by:
diffusion
facilitated diffusion (through carrier proteins and protein channels)
osmosis.
Diffusion – passive movement of small, non-polar, lipid soluble molecules down the concentration
gradient directly through phospholipid bilayer. Rate increases with increased SA, small diffusion
distance and large gradient.
Facilitated diffusion – protein channels needed to transport polar, charged, water-soluble molecules
across membrane
Osmosis – movement of water molecules down concentration gradient through a partially
permeable membrane
iii Understand how the properties of molecules affects how they are transported, including solubility,
size and charge.
Property Transport mechanism
Polar molecules Endocytosis, facilitated diffusion or active
transport
Soluble molecules Passive diffusion
Insoluble molecules Facilitated diffusion
iv Know that large molecules can be transported into and out of cells through the formation of
vesicles, in the processes of endocytosis and exocytosis.
,Endocytosis/exocytosis – movement of large particles into and out of cells in vesicles. In endocytosis,
vesicles are made from cell surface membrane. In exocytosis, vesicles fuse with cell surface
membrane and transported out of cell
CORE PRACTICAL 5: Investigate the effect of temperature on beetroot membrane permeability.
CORE PRACTICAL 6: Determine the water potential of plant cells.
Water potential = turgor pressure + osmotic potential
ψ=P+π
4.3 Gas Exchange
i Understand how insects, fish and mammals are adapted for gas exchange.
Gas exchange in fish
1. Mouth opens
2. Operculum closes
3. Buccal cavity floor lowers
4. Contracted muscles of operculum cause it to bulge outwards
5. Volume of opercular cavity
increases
6. Pressure decreases
7. Mouth closes
8. Floor of buccal cavity raises
9. Volume of opercular cavity
decreases
10. Pressure increases
11. Operculum opens due to high
pressure
12. Water flows out of cavity
Gas exchange in insects
Spiracles – hair on surface of insect that reduce water loss;
prevent dust and parasites entering. Open and close in response
to CO2 levels and determine rate of respiration through muscles
contracting and relaxing
Trachea – kept open by rings of chitin; end at respiring tissue; have
fluid in them when insect is less active to reduce dehydration
Air sacs – reservoir of air in larger insects; increase respiratory
efficiency by providing a large surface area for gas exchange; expand
and contract using movements of muscles
Nymphs Adult mayfly
Large SA Large SA
, Small diffusion distance Small diffusion distance
Contain feathery gills Have trachea
External gas exchange system Internal gas exchange system
Mammalian gas exchange
Breathing
1. Diaphragm contracts and flattens
2. External intercostal muscles contract and internal
intercostal muscles relax
3. Ribs move upwards and outwards
4. Volume of thoracic cavity increases
5. Pressure decreases
6. Air moves down the pressure gradient passively from
the external atmosphere into the lungs
Chemoreceptors
There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to
changes in carbon dioxide levels in the blood.
If the level is too high (the pH would drop, enzyme action would be affected with serious results),
impulses are sent from these cells to the inspiratory part of the centre so that breathing rate
increases.
This means that carbon dioxide is got out of the body as quickly as possible and more oxygen comes
in.
CORE PRACTICAL 7: Dissect an insect to show the structure of the gas exchange system, taking into
account the safe and ethical use of organisms.
ii Understand gas exchange in flowering plants, including the role of stomata, gas exchange surfaces
in the leaf and lenticels.
