Exchange between organisms and their environement

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  • Created by: LilyIM
  • Created on: 02-05-16 11:49

Surface area to volume ratio

Gas Exchange in small organisms

They're suface area compared to volume is large enough to allow gas exchange on their surface. As surface area increasing, volume increases faster. This would mean it would take too long for substances to reach the middle of the organism is diffusion was only used.

Small organisms can adapt to have;

Flattened shape

specialised exchange surface

A specialised exchange surface has

Large surface area:volume    Very thin (diffusion distance is short)  Selectively permeable    Movement of environmental medium (to keep the con gradient e.g air)    Transport system (blood)

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More gas exchange basics

Ficks law

diffusion = surface area x diffusion in concentration


                Length of diffusion path

Metabolic rate

The higher the metabolic rate, the more a organism respires, and more gas exchange there is.

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Gas exchange in organisms

Single celled organisms

Can exchange gas across their cell-surface membrane. They need no adaptions as their cells are in touch with their environment. Oxygen and Carbon dioxide enter/exit by diffusion.

Gas exchange in insects

They are multicellular, has a exoskeleton, coated with a waxy substance so gas exchange cannot occur on the surface. So instead it has openings called spiracles, that lead to tracheae and tracheoles.

Tracheae have rigid rings made of Chitin that hold them open. They branch into Tracheoles that go into the muscle fibres.

Larger insects ventilate the tracheal system by closing and opening the spiracles and contracting adbdominal muscles which squeeze the tracheal system. This pumps the air.

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Gas exchange in large insects detail

Insects at rest

Gas moves along the concentration gradient, cells respire so at the end of the Tracheoles the concentration falls. Creates diffusion gradient causing oxygen to enter the spiracles. Cells making carbon dioxide creates a concentration gradient the other way. Air diffuses faster then water, so exchange is quick.

Contraction of muscles causing mass movement, this further speeds up gas exchange.


Anaerobic respiration occurs in cells, lactate is made increasing solute, water potential in the muscle decreases, water enters by osmosis,air is drawn into tracheoles.

Oxygen and carbon dioxide diffuses faster in gases then liquids.

There is increased respiration.

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Gas exchange in fish

Fish obtain oxygen from water they swim in. water is forced over the gills and then out through an opening on each side of the body.

The gill arch is bony and supports two stacks of thin filaments, filaments have rows of thin lamellae on them (which stick up vertically). The surface of them consist of thin flattened cells. Underneath is a layer of capillaries.

The blood vessels are arranged so counter-currents can occur.

Counter-current exchange system

Blood loaded with it's lots of oxygen meets water with it's maximum oxygen capacity, diffusion occurs.

Blood with little oxygen meets water which has had most it's oxygen removed. Diffusion from water to blood takes place.

This maintains the concentration gradient.

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Gas exchange from Dicotyledonous plants

Plants need carbon dioxide and water for photosynthesis. Mesophyll cells contain chloroplasts need sunlight- so leaves and broad and thin.

But to obtain carbon dioxide they also need a large exchange surface.

Spongy mesophyll cells are mainly exposed to air spaces in the leaf. Palisade cells also have small gaps so they are in contact to air.

Adaptions for rapid diffusion

Stomata, no cells is far away so diffusion pathway is short.

Lots of interconnecting air space, gases can readily come in contact with mesophyll cells.

Large surface area of mesophyll cells for rapid diffusion.


Surrounded by guard cells. They control the rate of gasous evapouration.

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Terrestrial insects and Xerophytic plant water los

Limiting water loss in insects

Exoskeleton that is rigid and coated with a waxy substance, making it waterproof.

Spiracles have valves that can be closed. (not all insects) Conflicts with oxygen, so occurs at rest.

Hairs surrounding the spiracles trap water and reduce the water potential gradient between the Tracheae and outside air.


They can't have a small surface area because of photosynthesis.

Waterproof cuticle

Stomata that can be closed

Xerophytes adapt to reduce water lost in transpiration.

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Xerophytes adaptions to reduce Transpiration

A thick cuticle

rolling up of leaves (most stomata lower epidermis. traps region warm air, saturated with water vapour, no water potential in/out leaf no water loss)

Hairy leaves (traps moist air reduces water potential gradient)

Stomata in pits or grooves (trap moist air again)

Reduced surface area to volume ratio (Smaller surface area, slower diffusion rate)

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Structure of human gas exchange system

Mammalian lungs

Protected by the ribcage, the lungs are ventilated by a tidal stream of air. The lungs consist of;


Lobed structures made of bronchioles and aveoli.


Flexiable airway supported by rings of cartilage. The trachea walls are made up of muscle, lined with ciliated epithelial cells and goblet cells.


Two divisions of the Trachea. Like the Trachea they produce mucus to trap dirt particles and have cilia to move the mucus towards the throat. The large bronchi is supported by cartilage, but the cartilage gets smaller as the bronchi do.

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Rest of human gas exchange structure


Branching subdivisions of the Brochi. Walls are muscle lined with epithelial cells. Muscles constrict to control air flow.


Minute air sacs. Between them are collagen and elastic fibres. They are lined with epithelium. It is the gas exchange surface.

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Features of aveolar epithelium

Diffusion between the blood and aveoli is rapid because...

Red blood cells are slowed in the capillaries

There is a minimum distance between air and blood

Thin walls of aveoli and capillaries

They are a large surface area

Breathing action keeps the concentration gradient high

Blood flow maintains the concentration gradient

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ventilation in the lungs

Ventilation- the movement of air in/out of the lungs.


breathing in

Diaphragm contracts making it flatter. External intercostal muscles between the ribs contract, Intercostal muscles relax. pulling the ribs up, and out. Volume of chest capacity increases, decreasing pressure. Air enters.


Breathing out

Diaphragm relaxes, moving back into its orginal dome shape. External intercostal muscles relax too, internal contract. The ribs fall down and in. Decreases chest capacity, pressure increases. Air leaves.

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