The Need for Gas Exchange Surface

Membranes and Transport

Define gas exchange.

The exchange of respiratory gases, mainly oxygen and carbon dioxide, between an organism and its environment.

How are oxygen and carbon dioxide exchanged across gas exchange surfaces?

By simple diffusion.

Why do organisms need oxygen?

Oxygen is required for aerobic respiration to release energy for metabolic processes.

Why must carbon dioxide be removed from organisms?

Carbon dioxide is a waste product of respiration and must be removed to prevent it accumulating.

In which direction do substances move during diffusion?

From an area of relatively high concentration to an area of relatively low concentration, down a concentration gradient.

What is surface area to volume ratio?

The amount of surface area an organism has relative to its volume.

How do you calculate surface area to volume ratio?

Surface area ÷ volume.

What happens to surface area to volume ratio as an organism gets larger?

It decreases.

Why do small organisms have a large surface area to volume ratio?

They have a large amount of surface area compared with their relatively small volume.

Why is a large surface area to volume ratio useful for gas exchange?

It provides a large surface across which gases can diffuse relative to the organism's requirements.

How can very small organisms exchange gases?

They can exchange gases directly across their body surface by diffusion.

Why do many small organisms not need specialised gas exchange systems?

Their large surface area to volume ratio and short diffusion distances allow enough oxygen and carbon dioxide to diffuse directly across their body surface.

Why can every cell of some small organisms obtain oxygen by simple diffusion?

The cells are close to the external environment, so the diffusion distance is short.

Why do large multicellular organisms need specialised gas exchange surfaces?

They have a small surface area to volume ratio, long diffusion distances and high metabolic demands, so diffusion across the body surface alone is too slow.

Why is a small surface area to volume ratio a problem for large organisms?

There is not enough body surface area relative to the volume of metabolically active tissue to exchange substances rapidly enough.

Why is diffusion distance greater in large multicellular organisms?

Many cells are located deep inside the body and are far from the external environment.

Why can oxygen not simply diffuse from the outside of a large animal to all its cells?

The diffusion distance is too great, so diffusion would be too slow to meet the cells' oxygen requirements.

Why can nutrients not reach all cells of a large organism by simple diffusion from the environment?

Many cells are too far from the body surface and diffusion over these distances would be too slow.

Why do large organisms generally have higher demands for gas exchange?

They contain many metabolically active cells that require oxygen and produce carbon dioxide.

How does a high metabolic rate affect the need for gas exchange?

A high metabolic rate increases oxygen demand and carbon dioxide production, so gases must be exchanged more rapidly.

Why do mammals and birds have particularly high demands for gas exchange?

They have high metabolic rates and maintain a relatively constant body temperature, requiring large amounts of energy from respiration.

What is the specialised gas exchange surface in humans and many other large land animals?

The lungs.

What is the specialised gas exchange surface in fish?

The gills.

What is the specialised gas exchange system in insects?

The tracheal system.

Where does gas exchange occur in plants?

Mainly in the leaves.

What gases are normally exchanged at respiratory gas exchange surfaces?

Oxygen enters the organism and carbon dioxide leaves.

What are the three main properties that increase the rate of diffusion across a gas exchange surface?

A large surface area, a steep concentration gradient and a thin exchange surface.

How does a large surface area increase the rate of diffusion?

More particles can cross the exchange surface at the same time.

How does concentration gradient affect the rate of diffusion?

The steeper the concentration gradient, the faster the rate of diffusion.

Why does a steep concentration gradient increase diffusion rate?

There is a greater difference in concentration between the two sides of the exchange surface, increasing net movement of particles.

How can a steep concentration gradient be maintained at a gas exchange surface?

By continually bringing fresh gas to the surface and/or transporting exchanged substances away from the surface.

How does removing oxygen from a gas exchange surface help maintain diffusion?

It keeps the oxygen concentration low on one side of the surface, maintaining a steep concentration gradient.

How does a thin exchange surface increase the rate of diffusion?

It creates a short diffusion distance, so particles cross the surface more quickly.

What is diffusion distance?

The distance particles must travel across an exchange surface.

What is Fick's Law of Diffusion?

A relationship describing how surface area, concentration difference and thickness of the exchange surface affect the rate of diffusion.

State Fick's Law of Diffusion.

Rate of diffusion = (surface area × concentration difference) ÷ thickness of exchange membrane or barrier.

According to Fick's Law, what happens to diffusion rate if surface area increases?

The rate of diffusion increases.

According to Fick's Law, what happens to diffusion rate if concentration difference increases?

The rate of diffusion increases.

According to Fick's Law, what happens to diffusion rate if the exchange surface becomes thicker?

The rate of diffusion decreases.

According to Fick's Law, what happens if the diffusion distance decreases?

The rate of diffusion increases.

Which factors in Fick's Law are directly proportional to diffusion rate?

Surface area and concentration difference.

Which factor in Fick's Law is inversely proportional to diffusion rate?

The thickness of the exchange membrane or barrier.

How can the rate of diffusion be maximised according to Fick's Law?

By having a large surface area, a large concentration difference and a thin exchange surface.

Why are specialised gas exchange surfaces usually highly folded?

Folding increases their surface area and therefore increases the rate of diffusion.

Why are gas exchange surfaces usually very thin?

A thin surface reduces diffusion distance and increases the rate of diffusion.

Why is a good blood supply useful at a gas exchange surface?

It rapidly transports gases to and from the surface, helping maintain steep concentration gradients.

How does blood flow maintain an oxygen concentration gradient at a gas exchange surface?

Blood carries absorbed oxygen away, keeping the oxygen concentration on the blood side relatively low.

How does ventilation help maintain concentration gradients?

It continually replaces air or water at the gas exchange surface, maintaining differences in oxygen and carbon dioxide concentration.

Why do lungs contain thousands of tiny air sacs?

The large number of air sacs provides a very large surface area for gas exchange.

What are the tiny air sacs in the lungs called?

Alveoli.

How are alveoli adapted for rapid gas exchange?

They provide a large surface area, have very thin walls and are surrounded by capillaries that maintain steep concentration gradients.

Approximately what surface area do the lungs provide according to the textbook?

Approximately 50 m².

Why do alveoli and surrounding blood vessels have very thin walls?

They create a short diffusion distance between the air and the blood.

How does oxygen move during gas exchange in the lungs?

Oxygen diffuses from the air in the alveoli, where its concentration is higher, into the blood, where its concentration is lower.

How does carbon dioxide move during gas exchange in the lungs?

Carbon dioxide diffuses from the blood, where its concentration is higher, into the alveoli, where its concentration is lower.

Why are specialised gas exchange systems necessary in large organisms but often unnecessary in very small organisms?

Large organisms have smaller surface area to volume ratios, greater diffusion distances and greater metabolic demands, whereas small organisms have large surface area to volume ratios and short diffusion distances.