Edexcel IAL Biology Topic 2 – Membranes, Proteins, DNA and Gene Expression
Introduction to Exchange and size
In order to survive, organisms need to exchange substances (e.g. oxygen and nutrients) with
the external environment. Removal of wastes to prevent, which will become toxic if allowed
to build up, is also necessary for survival.
The size of an organism affects whether a specialised exchange surface is required:
Single-Celled Organisms:
● Can exchange directly over their body surface so do not require specialised
exchange systems.
● Are entirely exposed to the external environment so diffusion is sufficient for
exchanging substances with it
● Oxygen diffuses in through the cell-surface membrane, and carbon dioxide from
respiration diffuses out.
Small Multicellular Organisms:
● Can exchange substances directly over their body surface
● Cells are within acceptable diffusion distances from the external environment
-diffusion is therefore sufficient to supply cells with nutrients and oxygen
Larger Multicellular Organisms:
● Require specialised exchange surfaces and transport mechanisms
● Cells within the organism are too far from the external environment -diffusion alone
is too slow to supply the innermost cells as the diffusion pathway is longer.
Surface Area to Volume Ratio and Exchange
As an organism gets larger its surface area to volume ratio (SA:V) decreases
SA:V is a useful measure because some organisms can be relatively large but have a low
enough ratio to avoid needing an exchange surface.
● An organism with a smaller SA:V will
have a greater need for a specialised
exchange surface, as diffusion distance to
the cells in the centre of the organism is
longer
● A useful general rule is that small
organisms have large SA:V and large
organisms have small SA:V. there are some
exceptions to this rule; some plants have
large roots which increases their SA:V
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,Edexcel IAL Biology Topic 2 – Membranes, Proteins, DNA and Gene Expression
Fick’s Law
The rate of diffusion is also dependent on the above features. This is shown by Fick’s law
that states that the rate of diffusion is proportional to the surface area multiplied by the
difference in concentration, divided by the length of the diffusion pathway.
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑥 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
𝑅𝑎𝑡𝑒 𝑜𝑓 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
Any factor which makes the top number large -such as large surface area or high
concentration gradient- or makes the bottom number small -such as thickness of exchange
membrane- will increase the rate of diffusion
Metabolic and Physical Activity
An additional factor that affects an animal's need for a gas exchange surface is its level of
metabolic activity. Metabolic reactions require energy and oxygen to release additional
energy (through aerobic respiration) some of which is used for movement.
● Organisms with lower metabolic rate require fewer nutrients and produce less waste,
therefore may not require a specialised exchange surface.
● Organisms with a higher metabolic rate require more nutrients and produce more
waste, therefore require a specialised exchange surface.
Mammals which regulate their own internal temperatures require high metabolic rates as
heat is released in aerobic respiration. Their need for energy, thus an exchange surface is
therefore high.
Specialised Exchange Surfaces
Different organisms have different exchange surfaces. All exchange surfaces attempt to
maximise the efficiency of exchange and all good exchange surfaces have certain features
in common that help them maximise their efficiency, such as large surface area, thin barriers
and associated transport systems.
Surface area
● A large surface area will provide more space for substances to pass through
● Exchange surfaces can maximise their surface area through folding and hairs
● For example, the cilia of ciliated epithelial cells increases their surface area
Thin Barriers
● Molecules have to travel across the exchange surface to get from the external
environment to the internal environment
● A thin exchange surface will reduce the diffusion distance necessary for molecules
to travel. A shorter diffusion distance will result in faster exchange.
● For example, alveoli have very thin walls which are only one cell thick.
Good blood supply
● Exchange surfaces are often closely associated with a good blood supply
● The blood supply can take important substances away from the exchange surface
in the body and provide the exchange surface with waste substances to be
removed from the body.
● Having good blood supply and transport system close to the exchange surface
helps maintain a strong diffusion gradient
● The same principle applies to a good ventilation system. For example, lungs
ventilate to refresh the air surrounding the alveoli exchange surface. As with a
transport system this helps maintain a strong diffusion gradient.
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,Edexcel IAL Biology Topic 2 – Membranes, Proteins, DNA and Gene Expression
Introduction to the lungs
Mammals are large organisms that require a large amount of oxygen, whilst also producing a
large amount of carbon dioxide. They have a high metabolic rate and high level of activity
but a low surface area to volume ratio. In order to deal with these constraints and maximise
gas exchange, mammals have lungs, which along with associated airways form the
gaseous exchange system of mammals.
● Air is inhaled during ventilation and
travels from the oral nasal cavities down
the trachea (windpipe)
● The trachea then divides into the right
and left bronchi
● The bronchi then divide into many
bronchioles
● The bronchioles lead to many millions of
alveoli which is where gas exchange
takes place.
When we breathe, air enters the mouth, passes into the trachea which splits into 2 b
ronchi;
one on the left side and one on the right side of the body. Each bronchus branches out into
smaller bronchioles which end intiny air sacs called alveoli, where gas exchangetakes
place. Deoxygenated bloodflows into the alveoli, where carbon dioxide diffuses out of the
capillary, through the alveolar membrane and into the surrounding air, down a concentration
gradient. Oxygen moves in the opposite direction from the surrounding air and into the
bloodstream, making the blood o xygenated.
The Alveoli
The lungs consist of millions of alveoli. This is where gas exchange takes place.
● The alveoli are tiny air-filled sacs that carry out
the process of gas exchange with the blood
● Oxygen diffuses from the air in the alveoli into
the blood within the capillaries. Carbon dioxide
diffuses in the opposite direction
● Alveoli consists of squamous epithelial tissue
and elastic fibres. The elastic fibres recoil
during expiration, reducing the size of alveoli
and helping to expel air from them.
● They have a large surface area, which is
increased by the folding of alveoli walls.
● Alveoli have a good blood supply which maintains a steep concentration gradient
so gaseous exchange is maintained.
● Alveolus and capillary walls are only one cell thick. Therefore, the diffusion
distance for gases to travel is short.
● The capillaries are in close contact with the alveoli walls. The capillaries are
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, Edexcel IAL Biology Topic 2 – Membranes, Proteins, DNA and Gene Expression
narrow, so red blood cells have to squeeze through, allowing maximum time for
gas exchange
● The lungs produce surfactant which is a thin layer of moisture lining the alveoli to
prevent alveoli collapse
Ventilation
Ventilation is the process of breathing in (inspiration) and out (expiration). The lungs need
ventilation in order to maintain gas exchange in the alveoli. Ventilation provides/removes
gases and maintains a strong gradient for gas exchange and diffusion.
The process of ventilation requires additional structures in the lungs: the diaphragm and the
external and internal intercostal muscles.
● The intercostal muscles lie between the ribs:
○ Contraction of the external intercostal muscles causes inspiration
○ Contraction of the internal intercostal muscles causes expiration
● The diaphragm is a sheet of muscle that separates the thorax from the abdomen
Inspiration and Expiration
Inspiration Expiration
Inspiring (inhaling) is an active process that Expiration (exhaling) is a mostly passive
requires ATP. process that requires much less ATP than
1. The external intercostal muscles inhaling.
contract and the internal intercostal 1. The external intercostal muscles
muscles relax relax and the internal intercostal
2. The ribs are pulled up and out and muscles contract
the rib cage expands 2. The ribs move down and inwards
3. The diaphragm contracts and 3. The diaphragm relaxes and is
flattens pushed up by the contents of the
4. The volume of the thorax increases, abdomen
decreasing the pressure within it. 4. The volume of the thorax is
5. Atmospheric pressure is greater decreased, increasing the pressure
than pulmonary pressure. within it
6. Air is forced into the lungs 5. Pulmonary pressure is greater than
atmospheric pressure
6. Air is forced out of the lungs
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