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

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.