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 fromglucose 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 exchangedirectly 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 exchangedacross 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 whichcan exchange with both –eels, crabs, frogs.• Oxygen in air (21%) is more abundant than in water.• Warm or salty water hasless oxygen than fresh, cold water; water also requires more energy to move it becauseit 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: usuallythe result of the elasticrecoil of the chest cavityas it returns to it’s relaxed state, not an activeprocess. The diaphragmrelaxes and moves up, the ribcage 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 inthe airways after each breath which isdrawn back into thelungs on the next Inhalation. Thus lungscan never be filled withcompletely 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, carbondioxide and water virtually stops, Small quantities of gas can pass directlythrough 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.