Gas Exchange
Exchanging Gases with the
Environment: Animals and Plants
Learning Goals
Describe how oxygen and carbon dioxide cross the plasma
membrane
Explain why cells need oxygen but must remove carbon
dioxide
Contrast gas exchange in lungs and gills
Describe the structure and function of the human
respiratory system
Explain how carbon dioxide and oxygen are transported in
the blood
Describe gas exchange in plants including the day/night
pattern and the role of stomata
Why do organisms exchange
gases with their environment?
In order for a cell to obtain energy, the cell must
take in oxygen as oxygen is a reactant of cellular
respiration. Without oxygen this process will not
occur and the cell will not receive the energy it
requires.
Why do organisms exchange
gases with their environment?
One of the waste products of cellular respiration is
carbon dioxide. If this waste product builds up in
the cell then the inside of the cell will become
acidic. Therefore carbon dioxide must be removed
from the cell.
How do gases cross the plasma
membrane?
At the cellular level, gases move
into and out of a cell across the
plasma membrane via diffusion
along a concentration gradient.
Carbon dioxide and oxygen
molecules are small enough to
move straight through the
membrane.
Conditions for Efficient Gas
Exchange
While many gases can easily cross the plasma membrane, certain
conditions are need for the most efficient exchange to occur:
1) The environment be moist, as the gases dissolve in the water and
diffuse from one side of the membrane to the other
2) The membrane must be thin and permeable, so the gas
molecules can move across it easily and quickly
3) There must be a large surface area in relation to the volume of
the organism so as to adequately provide the gaseous requirements
4) There must be a greater concentration of required gas on one
side of the membrane than the other so that a concentration
gradient is maintained. Therefore gases must be readily supplied
and removed.
Exchanging with Air or Water
Organisms will exchange gases with the environment
they live in: air or water.
Some organisms can exchange gases with both e.g.
frogs
Water holds a lot less dissolved oxygen than air and
warm water is able to hold even less dissolved oxygen
than cold water. Aquatic animals therefore have adapted
ways of obtaining as much oxygen as possible from
their environment. See Gas Exchange in Aquatic
Animals.
Gas Exchange in Unicellular Organisms
and Very Small Multi-cellular Organisms
For unicellular organisms, the entire organism is in
contact with the external environment and, due to a
high surface area to volume ratio, gas exchange across
the plasma membrane is sufficient.
For many tiny multicellular organisms (no more than
1mm thick), exchange of gases over the body surface is
adequate as the gases will diffuse to internal cells.
Gas Exchange in Multi-cellular
Organisms
Multi-cellular organisms face a challenge when
considering the need of every cell within the body to
exchange gases. Some cells are just too far away from
the external environment to obtain gases by diffusion.
Multi-cellular organisms over-come this challenge by
increasing the surface area available for gas exchange
and linking this to a transport system that connects with
every cell.
Gas Exchange in Multi-cellular
Organisms
Animals have specialised structures that allow for
efficient gas exchange
The complexity of these structures depends on the
size, behaviours and activity levels of the organism.
In small organisms these structure can be very
simple.
Gas Exchange in Multi-cellular
Organisms: Air Breathers
Air breathers have the advantage that oxygen is much
more readily available in air
However, as gas exchange occurs across a moist
surface air breathers will continually loose water to
their environment. Respiratory surfaces are a major
site for water loss.
To overcome this problem, larger animals have
developed internal respiratory organs. However, as
the surface is internal there needs to be a system for
efficient ventilation of these organs.
Gas Exchange in Multi-cellular
Organisms: Invertebrate Air Breathers
Many invertebrates have small holes in their
abdomen known as spiracles. Air enters the spiracles
and is distributed through the body of the organism
via tracheae and tracheoles that come into close
contact with the organisms cells.
Some invertebrates also have air sacs that can be
pumped to move air through the system.
Gas Exchange in Invertebrates
Gas Exchange in Multi-cellular
Organisms: Air Breathers
Many vertebrates have internal respiratory organs
known as lungs. Ventilation of the lungs can occur in
two ways:
– Air is forced into the lungs under pressure
– Air is drawn into the lungs under negative pressure (suction)
Frogs are an example of an animal that ventilates their
lungs under pressure.
Gas Exchange in Multi-cellular
Organisms: Air Breathers
In air breathers, oxygen is readily available. On the
other hand carbon dioxide diffuses slowly in air and
can accumulate in body fluids during exercise.
Air breathers therefore are more sensitive to changes
in carbon dioxide concentration and this drives
ventilation. Receptors that are sensitive to carbon
dioxide and blood pH will indicate when ventilation
needs to be modified.
The Human Respiratory System
Air is drawn through
the nose and enters the
pharynx (throat)
Air then passes into the
trachea and the paired
bronchi. Here the dust
and bacteria are
trapped by mucus and
swept up to the throat
by cilia. The trachea is
supported by cartilage
rings that prevent its
collapse.
Air passes into the
bronchioles and from
here into the alveolus.
It is here that gas
exchange takes place.
The Human Respiratory System
The alveoli are designed for extremely efficient gas
exchange
The alveoli provide a large area for gas exchange
(equivalent to the size of a tennis court)
They are lined with a very thin layer of flat cells that
is in direct contact with a network of capillaries
These cells are also lined with a surfactant, a
lipoprotein, that prevents the alveoli from collapsing.
The Human Respiratory System:
The Alveolus
The Human Respiratory System:
The Alveolus
The Human Respiratory System:
Lung Ventilation
The lungs are kept expanded due to pressure
differences in the thorax (chest cavity)
This negative pressure keeps the lungs inflated
At the base of the lungs is a diaphragm- the largest
muscle in the body.
When the diaphragm contracts (active process) it pulls
downward expanding the chest cavity and the ribs and
causing the lungs expand.
The Human Respiratory System:
Lung Ventilation
This expansion draws air into the lungs.
When the diaphragm relaxes (passive process) the
thorax returns to its resting position forcing air out of
the lungs.
Lung Ventilation: Tidal Volume
and Vital Capacity
Tidal volume represents the amount of air that is
moved in and out during each breath.
Tidal volume varies according to oxygen demand.
Vital capacity represents the maximum amount of
air that we can move into and out of the lungs in
one breath.
Lung Ventilation: Residual
Volume
One-way ventilation (in and back out the same
pathway) is not the most efficient way to exchange gas.
We can never exhale all of the air from our lungs and
so “stale air” is drawn back into the lungs in the next
breath.
The volume of air left in the lungs after exhalation is
referred to as the residual volume.
Residual volume has a benefit as this air prevents the
lungs from collapsing.
Transporting Gases:
Haemoglobin
Oxygen is transported around the body in the blood by
respiratory pigments such as haemoglobin that
combine reversibly with oxygen and increase the
oxygen carry capacity of blood.
Haemoglobin is found in red blood cells. Four oxygen
molecules can bind with one haemoglobin molecule.
When oxygen is bound to haemoglobin they form a
complex known as oxyhaemoglobin. In this state the
haemoglobin turns red.
Haemoglobin
Oxyhaemoglobin
Affinity for OxygenHaemoglobin
Hb4 is abbreviation for Haemoglobin
Affinity of HB4 for oxygen is:
Hb4 < Hb4O2 < HB4O4 < (Hb4O8).
Transporting Gases:
Haemoglobin
Muscles require a ready supply of oxygen to fuel cells
during activity. They have the ability to store oxygen
bound to a form of haemoglobin known as myoglobin.
Any depleted stores of oxygen will be replaced as soon
as possible.
Myoglobin has a higher affinity for oxygen than
haemoglobin and so myoglobin can take oxygen from
haemoglobin.
Transporting Gases: Carbon
Dioxide
Carbon dioxide forms an acid when it combines with
water the therefore only a limited amount can be
carried in the blood (7%)
Some carbon dioxide combines with haemoglobin to
form carbaminohaemoglobin (23%).
The rest (70%) is converted by red blood cells into
hydrogen carbonate ions. As soon as the hydrogen
carbonate reaches the lungs it returns to the red blood
cells and is turned back into carbon dioxide for release.
Controlling Ventilation
In air breathers rate of ventilation is in response to levels
of carbon dioxide and not oxygen as in aquatic animals.
When the levels of carbon dioxide in the blood are high,
receptors in the arteries send a message to the brain. A
message is then sent to the diaphragm and rate of
ventilation increases to remove the excess carbon dioxide
from the blood.
Levels of oxygen to a lesser extent control ventilation.
Gas Exchange in
Aquatic Animals
Gills are outward projections of the body surface
(increasing the surface area to volume ratio)
These projections will have a ready supply of blood
vessels to allow for transport of gases to and from body
cells
Gas Exchange in Aquatic
Animals
Gills rely on the buoyancy of water to keep them from
collapsing. Therefore a fish will die when out of water
due to the collapse of the gills.
Gills also require water to be moved over their surface
There are two ways that this may be achieved: either
the gills is moved through the water or the animal is
able to move water over the gill. The ability to move
water over the gills is beneficial to larger organisms.
Gas Exchange in Aquatic
Animals
Some animals use cilia to move water over their gills
Larger fish will take water in through their mouth and
then close their mouth forcing the water over their gills
and out via the operculum that protects their gills. See
next slide.
Larger fish are very efficient at obtaining oxygen from
water using countercurrent flow
Blood flows through the gills in the opposite direction
to the water allowing up to 90% of the oxygen in the
water to be extracted. See next slide.
Gas Exchange in Aquatic
Animals
Ventilation (breathing) is regulated by receptors that
sense the levels of oxygen in the blood
When these receptors detect low oxygen levels,
ventilation is increased.
Carbon dioxide is readily lost to water as it dissolves
easily and so ventilation is controlled by oxygen
levels alone.
Gas Exchange in Plants
During the day plants will produce
more oxygen via photosynthesis than
they consume during respiration.
Therefore there is net production of
oxygen and net consumption of
carbon dioxide.
At night however when plants are not
photosynthesising there will be a net
production of carbon dioxide and
consumption of oxygen.
Gas Exchange in Plants
Plants do not have specialised structures for
gas exchange.
In small plants such as mosses, leaves and
other structures are very thin and gases are
able to move in and out via diffusion.
In more complex plants oxygen and carbon
dioxide is exchanged through the stomata
on leaves, stems and roots.
Gas Exchange in Plants: Stomata
Stomata are able to regulate gas exchange by
controlling when they are open and when they are
closed.
Stomata refer to the actual pore (hole) in the cell
Stomata are most abundant on the leaves of the plant
This pore is bordered by two cells known as guard
cells which control when the stomata is opened and
closed.
Gas Exchange in Plants: Guard
Cells
The opening and closing of stomata can be in response
to water moving in or out of the cell
When water moves into the cell, the turgor of the cell
increases causing them to lengthen and open the
stomata.
Opening and closing of the stomata can also be in
response to light and low internal carbon dioxide
levels.
Gas Exchange in Plants
The cells of plant structures are
loosely packed meaning that gas
can diffuse through the spaces
between cells and there is no need
for a plant to have a gas transport
system
Gas Exchange in Plants: Aquatic
Plants
Water plants will have special adaptations that allow
them to exchange gases within their moist
environment.
Some plants such as lilypads have leaves that float
giving them ready access to air.
Mangroves have pneumatophores also known as aerial
roots that grow above the water’s surface.
Submerged aquatic plants are able to exchange gases
with water across their epidermis.