Every organism has to exchange things with its environment
1) Cells need to take in things like oxygen + glucose for aerobic respiration + other
metabolic reactions
2) They also need to excrete waste products from these reactions - like CO2 and urea
Smaller animals have a higher surface area : volume ratios
Multicellular organisms need exchange surfaces:
Single-celled - substances can diffuse directly into/out of the cell across cell surface
membrane - diffusion rate is quicker because of small distances substances have to travel
Multicellular animals - diffusion across the outer membrane is too slow, for several reasons:
● Some cells are deep within the body - big distance between them and outside
environment
● Larger animals have low surface area to volume ratio - difficult to exchange enough
substances to supply a large volume of animal through a relatively smaller outer
surface
● Multicellular organisms have a higher metabolic rate than single-celled organisms, so
they use up oxygen + glucose faster
Exchange surfaces have special features to improve their efficiency:
Most exchange surfaces have a large surface area:
Example - root hair cells - cells on plant roots grow into long hairs that stick out into the soil -
each branch of a root is covered in these hairs - gives them a large surface area, helps to
increase rate of absorption of water (by osmosis) + mineral ions (by active transport) from
the soil
They are thin:
Example - the alveoli - the gas exchange surface in the lungs - each alveolus is made from a
single layer of thin, flat cells called alveolar epithelium. Oxygen diffuses out of alveolar space
into the blood. CO2 is the opposite. Thin alveolar epithelium helps to decrease the distance
over which oxygen and CO2 diffusion takes place - increases rate of diffusion
Good blood supply and/or ventilation:
Example 1 - alveoli - surrounded by large capillary network, giving each alveolus its own
blood supply - blood constantly takes oxygen away from alveoli + brings more CO2 - lungs
are also ventilated so the air in each alveolus is constantly replaced - this helps maintain
conc gradient
, Example 2 - fish gills - gills are gas exchange surface in fish - Oxygen and CO2 are
exchanged between the fish’s blood + surrounding water
Fish gills contain a large network of capillaries - keeps them well-supplied with blood - also
well-ventilated - freshwater constantly passes over them - this helps maintain the conc
gradient of oxygen - increasing the rate at which oxygen diffuses into the blood
The gaseous exchange system in mammals
Lungs are exchange organs:
● As you breathe in, air enters the trachea (windpipe)
● Trachea splits into 2 bronchi - one bronchus leading to each lung
● Each bronchus then branches off into smaller tubes called bronchioles
● Bronchioles end in small ‘air sacs’ called alveoli where gases are exchanged
● Ribcage, intercostal muscles + diaphragm all work together to move air in and out
Structures in the Gaseous exchange system have different functions
● Goblet cells (lining the airways) secrete mucus - mucus traps microorganisms + dust
particles in inhaled air, stopping them from reaching alveoli
● Cilia (on the surface of cells lining the airways) beat mucus - moves it upwards away
from alveoli towards the throat, where it’s swallowed - helps prevent lung infections
● Elastic fibres in walls of the trachea, bronchi, bronchioles + alveoli help the process
of breathing out - on breathing in, lungs inflate + elastic fibres are stretched. Then the
fibres recoil to help push the air out when exhaling
● Smooth muscle in walls of trachea, bronchi + bronchioles allow their diameter to be
controlled - during exercise muscle relaxes, making tubes wider - less resistance to
airflow + air can move in/out of lungs more easily
● Rings of cartilage in walls of trachea + bronchi provide support - strong but flexible -
stops trachea + bronchi collapsing when you breathe in and pressure drops
1) Cells need to take in things like oxygen + glucose for aerobic respiration + other
metabolic reactions
2) They also need to excrete waste products from these reactions - like CO2 and urea
Smaller animals have a higher surface area : volume ratios
Multicellular organisms need exchange surfaces:
Single-celled - substances can diffuse directly into/out of the cell across cell surface
membrane - diffusion rate is quicker because of small distances substances have to travel
Multicellular animals - diffusion across the outer membrane is too slow, for several reasons:
● Some cells are deep within the body - big distance between them and outside
environment
● Larger animals have low surface area to volume ratio - difficult to exchange enough
substances to supply a large volume of animal through a relatively smaller outer
surface
● Multicellular organisms have a higher metabolic rate than single-celled organisms, so
they use up oxygen + glucose faster
Exchange surfaces have special features to improve their efficiency:
Most exchange surfaces have a large surface area:
Example - root hair cells - cells on plant roots grow into long hairs that stick out into the soil -
each branch of a root is covered in these hairs - gives them a large surface area, helps to
increase rate of absorption of water (by osmosis) + mineral ions (by active transport) from
the soil
They are thin:
Example - the alveoli - the gas exchange surface in the lungs - each alveolus is made from a
single layer of thin, flat cells called alveolar epithelium. Oxygen diffuses out of alveolar space
into the blood. CO2 is the opposite. Thin alveolar epithelium helps to decrease the distance
over which oxygen and CO2 diffusion takes place - increases rate of diffusion
Good blood supply and/or ventilation:
Example 1 - alveoli - surrounded by large capillary network, giving each alveolus its own
blood supply - blood constantly takes oxygen away from alveoli + brings more CO2 - lungs
are also ventilated so the air in each alveolus is constantly replaced - this helps maintain
conc gradient
, Example 2 - fish gills - gills are gas exchange surface in fish - Oxygen and CO2 are
exchanged between the fish’s blood + surrounding water
Fish gills contain a large network of capillaries - keeps them well-supplied with blood - also
well-ventilated - freshwater constantly passes over them - this helps maintain the conc
gradient of oxygen - increasing the rate at which oxygen diffuses into the blood
The gaseous exchange system in mammals
Lungs are exchange organs:
● As you breathe in, air enters the trachea (windpipe)
● Trachea splits into 2 bronchi - one bronchus leading to each lung
● Each bronchus then branches off into smaller tubes called bronchioles
● Bronchioles end in small ‘air sacs’ called alveoli where gases are exchanged
● Ribcage, intercostal muscles + diaphragm all work together to move air in and out
Structures in the Gaseous exchange system have different functions
● Goblet cells (lining the airways) secrete mucus - mucus traps microorganisms + dust
particles in inhaled air, stopping them from reaching alveoli
● Cilia (on the surface of cells lining the airways) beat mucus - moves it upwards away
from alveoli towards the throat, where it’s swallowed - helps prevent lung infections
● Elastic fibres in walls of the trachea, bronchi, bronchioles + alveoli help the process
of breathing out - on breathing in, lungs inflate + elastic fibres are stretched. Then the
fibres recoil to help push the air out when exhaling
● Smooth muscle in walls of trachea, bronchi + bronchioles allow their diameter to be
controlled - during exercise muscle relaxes, making tubes wider - less resistance to
airflow + air can move in/out of lungs more easily
● Rings of cartilage in walls of trachea + bronchi provide support - strong but flexible -
stops trachea + bronchi collapsing when you breathe in and pressure drops
