EXCHANGING GASES

Chapter 8

Exchanging Gases with the Environment

• Organisms must exchange oxygen and carbon dioxide with the environment in order to carry out photosynthesis and cellular respiration.

• The rate at which oxygen

is exchanged limits the amount

of energy released from

glucose for cellular activities.• Carbon dioxide must be

removed efficiently because it is

a waste product from cellular

respiration and can build up in body fluids as a weak acid in solution with water.

• Single celled and very small organisms with a high SA:V ratio exchange gases directly with the environment by diffusion.

• Plants also exchange

directly with the

environment, but can

regulate the rate at which

this occurs.• Animals in water often

have huge surface areas

because oxygen is in short

supply.

• Larger animals with a high metabolic rate and therefore a higher need for gas exchange have well developed systems e.g. gills, lungs

• The movement of air or water across a gas exchange surface is called ventilation (breathing).

•

Diffusion

• Gases are always exchanged

across a moist membrane by

diffusion.• Oxygen and carbon dioxide are small

enough molecules to diffuse directly through• a phospholipid bilayer along• their concentration gradient,• not actively pumped across• like many nutrients.•

Exchanging with Air or Water

• Multicellular organisms usually exchange respiratory gases in the medium in which they live i.e. air or fresh or salt water.

• Some exceptions which

can exchange with both –

eels, crabs, frogs.• Oxygen in air (21%) is

more abundant than in water.• Warm or salty water has

less oxygen than fresh, cold

water; water also requires

more energy to move it because

it is more viscous than air.• Therefore most large, active organisms obtain oxygen from air

rather than water.

Gas exchange in Mammals• http://www.bmu.unimelb.edu.au/examp

les/gasxlung/

Lung Ventilation• The lungs are kept expanded by a small negative

pressure in the chest cavity.• Inhalation: the diaphragm contracts down, the rib cage is

raised, the lungs expand and air is drawn in through the airways. This is an active process which requires energy.

• Exhalation: usually

the result of the elastic

recoil of the chest cavity

as it returns to it’s

relaxed state, not an active

process. The diaphragm

relaxes and moves up, the rib

cage moves down.

• Tidal Volume – the volume of air moved out with each breath, varies according to the activity of the organism.

• Vital Capacity- the maximum amount of air which can be moved into and out of our lungs.

• There is always

some stale air left in

the airways after

each breath which is

drawn back into the

lungs on the next

Inhalation. Thus lungs

can never be filled with

completely fresh air.

• Lung diseases such as asthma, pneumonia or emphysema can seriously affect efficient gas exchange in the lungs.

a) Normal lung tissue – large air spaces and thin membranes for efficient gas exchange.

b) Lung tissue showing emphysema, membranes are thicker and the air sacs break down. Black dots = coal dust.

c) Pneumonia – where fluid and white blood cells fill large areas of the lung

Transporting Gases

• Large amounts of oxygen must be carried by the bloodstream but only a very small amount will dissolve in the blood (90% water)

Thus oxygen is carried by respiratory • pigments - molecules which • combine reversibly with • oxygen. In mammals this is • haemoglobin Hb, carried in

red blood cells. Haemoglobin

contains iron. Four oxygen

molecules will combine with one

haemoglobin molecule.

Haemoglobin

• Oxyhaemoglobin: formed when haemoglobin combines with oxygen at high oxygen concentrations e.g. blood vessels in the lungs. At low oxygen concentrations e.g. exercising muscles, oxygen is released from the haemoglobin.

• For any given oxygen concentration in the air, there will be a fixed proportion of oxyhaemoglobin formed.

• If we graph this amount of oxyhaemoglobin at a number of different oxygen concentrations, this produces a haemoglobin-oxygen

dissociation curve.

Haemoglobin-oxygen dissociation curve

Oxygen in the Tissues

• Myoglobin is another form of haemoglobin and is contained in muscles, giving them their red colour.

• It carries a reserve store of oxygen which can be used as an emergency supply if oxygen levels suddenly drop e.g. during strenuous exercise or if a blood vessel is temporarily blocked.

• When the blood supply (and therefore oxygen supply) is restored,

then the myoglobin oxygen store

is immediately refilled.

Carrying Carbon Dioxide• Carbon dioxide, produced from cellular respiration, is carried

in the blood in three different ways:

• The 23% which combines with haemoglobin is on a different site on the Hb molecule from the oxygen. It forms carbaminohaemoglobin.

• The hydrogen carbonate ions in the plasma move back into the red blood cells when the blood reaches the lungs. It is then converted back into carbon dioxide to be exhaled.

Gas Exchange in Plants

• Cellular respiration occurs continuously in plants, where oxygen is used and carbon dioxide is produced. During the day a plant will require more carbon dioxide than it can produce itself in order to carry out photosynthesis. Thus it will have to take carbon dioxide from the environment.

• Some of the oxygen produced during photosynthesis will then be used in respiration, the rest diffuses out of the leaf.

NET RESULT:

1. During the day – plants take up carbon dioxide and release oxygen. BOTH photosynthesis and cellular respiration occur.

2. At night – plants take in oxygen and release carbon dioxide. ONLY cellular respiration occurs NOT photosynthesis.

Gas exchange Organs - Stomata

• Simple plants with small, thin leaves do not have special organs for gas exchange – carbon dioxide and oxygen can diffuse directly in and out of a cell.

• Vascular plants have

special openings called

stomata, which allow

gas exchange. They can

be found anywhere on a

plant except the roots,

but are abundant on

leaves in the epidermis. • When stomata are closed,

the exchange of oxygen, carbon

dioxide and water virtually stops,

Small quantities of gas can pass directly

through the epidermis and cuticle.

• Plant cells in leaves, roots and stems are loosely packed which allows the rapid diffusion of gases through the intercellular spaces.

Guard Cells• When water enters guard cells they will become

turgid and expand to open the stoma.• Favourable conditions for this are – abundant water,

light and low carbon dioxide concentrations. • Stoma also close if it is very hot and dry which

prevents water loss but reduces the rate of photosynthesis.

Stems and Roots • In roots and woody stems the epidermis is replaced

by a layer of cork cells (dead tissue) through which air can freely pass.

• Roots exchange gases with the air in spaces in the soil e.g. oxygen diffusing into the roots. BUT if the soil is waterlogged, spaces become filled with water INSTEAD of air.

• Water only contains a small

amount of dissolved oxygen,

thus the roots may not get enough

oxygen for their needs and may die,

killing the plant.