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 Oxygen-

Haemoglobin

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.