Biology A Level Revision (AQA)
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
Organisms Exchanging Substances
Surface Area to Volume Ratio
● Surface Area
- Is the total surface that substances and heat can be exchanged.
- Cannot influence the rate of exchange alone.
- Surface Area is more important relative to the Volume.
● Volume
- Volume determines the amount of substances needed to be taken in or transported
out.
- As volume increases, the number of cells increases,
meaning more material is needed for metabolism.
● SA:V
- As volume increases, surface area decreases as there are
more cells decreasing the area materials can be
exchanged.
- Smaller organisms have a higher SA:V than larger
organisms.
● Adaptations to increase rate of movement.
- A larger surface area allows more substances to diffuse
at the same time.
- Having a thinner membrane shortens the diffusion pathway.
- Larger organisms have a transport system to move substances around. Transport
systems allow a steeper concentration gradient meaning diffusion occurs faster
while also bringing the substances to cells that need them.
- Some animals have physical adaptations to allow heat to be transferred more
efficiently. (Elephants have large, vascularised ears to allow heat to be transferred
rapidly.)
Singled-Celled Organisms
- Exchange directly with their external environment.
● Why do they need to exchange?
- Single-Celled Organisms need substances from their environment to carry out
processes within the cell (mitochondria needs oxygen for respiration to release ATP
molecules).
- Waste also needs to be removed like Carbon Dioxide to avoid bringing harm to it.
● How do they exchange
- They exchange gases and other substances using their cell membranes.
- Cells with a larger surface area to volume ratio will be able to exchange substances
faster. Ways they may do this is by making themself wide or having folds.
● Rate of diffusion
- Diffusion rate is very quick as substances only need to travel through one cell-surface
membrane.
Page 1
, Biology Revision A Level AQA
Insects - Smaller Multicellular Organism
● Tracheal System
- Insects use the Tracheal System as a gas exchange surface consisting of their trachea,
a network of tubes called tracheoles which carry oxygen around their body.
- Insects also have Spiracles on their exoskeleton which are small holes along the
thorax. These allow oxygen to flow into the tracheoles and CO2 to flow out.
- This system is the most efficient gas exchange system as rate is maximised.
● Reduce water loss
- Terrestrial insects can minimise
water loss without reducing gas
exchange.
- They can close their spiracles to
prevent water from being lost.
- Some also have hairs around the
spiracle to lower the water potential
gradient between the trachea and
the environment which decreases
the rate of diffusion.
- Terrestrial insects are also
surrounded with a waxy cuticle giving them a waterproof body preventing water
from leaving them.
Fish - Larger Multicellular Organism
● Gills
- Gills obtain oxygen from water.
- Gills are filaments of thin tissue that
fold and are highly branched to create a
large surface area. They are also
surrounded with lamellae to further
increase SA.
- Oxygen from water diffuses into the
bloodstream of fish quickly when it
flows through the gills due to the
Counter-Current System.
- The counter-current system
allows blood to flow through the
lamellae in the opposite
direction to water providing a
steep concentration gradient
between the water and the
blood.
Page 2
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
Organisms Exchanging Substances
Surface Area to Volume Ratio
● Surface Area
- Is the total surface that substances and heat can be exchanged.
- Cannot influence the rate of exchange alone.
- Surface Area is more important relative to the Volume.
● Volume
- Volume determines the amount of substances needed to be taken in or transported
out.
- As volume increases, the number of cells increases,
meaning more material is needed for metabolism.
● SA:V
- As volume increases, surface area decreases as there are
more cells decreasing the area materials can be
exchanged.
- Smaller organisms have a higher SA:V than larger
organisms.
● Adaptations to increase rate of movement.
- A larger surface area allows more substances to diffuse
at the same time.
- Having a thinner membrane shortens the diffusion pathway.
- Larger organisms have a transport system to move substances around. Transport
systems allow a steeper concentration gradient meaning diffusion occurs faster
while also bringing the substances to cells that need them.
- Some animals have physical adaptations to allow heat to be transferred more
efficiently. (Elephants have large, vascularised ears to allow heat to be transferred
rapidly.)
Singled-Celled Organisms
- Exchange directly with their external environment.
● Why do they need to exchange?
- Single-Celled Organisms need substances from their environment to carry out
processes within the cell (mitochondria needs oxygen for respiration to release ATP
molecules).
- Waste also needs to be removed like Carbon Dioxide to avoid bringing harm to it.
● How do they exchange
- They exchange gases and other substances using their cell membranes.
- Cells with a larger surface area to volume ratio will be able to exchange substances
faster. Ways they may do this is by making themself wide or having folds.
● Rate of diffusion
- Diffusion rate is very quick as substances only need to travel through one cell-surface
membrane.
Page 1
, Biology Revision A Level AQA
Insects - Smaller Multicellular Organism
● Tracheal System
- Insects use the Tracheal System as a gas exchange surface consisting of their trachea,
a network of tubes called tracheoles which carry oxygen around their body.
- Insects also have Spiracles on their exoskeleton which are small holes along the
thorax. These allow oxygen to flow into the tracheoles and CO2 to flow out.
- This system is the most efficient gas exchange system as rate is maximised.
● Reduce water loss
- Terrestrial insects can minimise
water loss without reducing gas
exchange.
- They can close their spiracles to
prevent water from being lost.
- Some also have hairs around the
spiracle to lower the water potential
gradient between the trachea and
the environment which decreases
the rate of diffusion.
- Terrestrial insects are also
surrounded with a waxy cuticle giving them a waterproof body preventing water
from leaving them.
Fish - Larger Multicellular Organism
● Gills
- Gills obtain oxygen from water.
- Gills are filaments of thin tissue that
fold and are highly branched to create a
large surface area. They are also
surrounded with lamellae to further
increase SA.
- Oxygen from water diffuses into the
bloodstream of fish quickly when it
flows through the gills due to the
Counter-Current System.
- The counter-current system
allows blood to flow through the
lamellae in the opposite
direction to water providing a
steep concentration gradient
between the water and the
blood.
Page 2
