2.2 Adaptations for Gas Exchange

Material Needs...• What sort of things do living things need to

obtain from the environment?

• What sort of things do they need to remove from their cells?

• Organisms are differently adapted so gas exchange can take place whether it be in water or on land.

• In particular oxygen is needed to convert organic molecules into energy through the process of respiration.

• NB: Gas exchange is the process by which oxygen reaches cells and waste products are removed, don’t confuse with respiration which is energy production in cells.

• By what process does gas exchange take place?

Diffusion and problem of size• Diffusion occurs according to Fick’s Law:

Rate of DiffusionSurface Area x concentration difference

distance

Think about this for a moment... What does it mean?

An organism’s requirements may be proportional to volume

...however diffusion is proportional to surface area

organism length SA (m²) vol (m³) SA/vol (m-1)

bacterium 1 μm(10-6 m)

6 x 10-12 10-18 6,000,000

amoeba 100 μm(10-4 m)

6 x 10-8 10-12 60,000

fly 10 mm(10-2 m)

6 x 10-4 10-6 600

dog 1 m(100 m)

6 x 100 100 6

whale 100 m(102 m)

6 x 104 106 0.06

• As organisms get larger their volume to surface area increases meaning they cannot rely on diffusion alone as the diffusion path would be too long.

Exchange in Protozoa• Protozoa –

single celled, hetrotroph, which does not have a cell wall.

E.g. paramecium, amoeba (eukaryote)

• In small, unicellular organisms the surface area to volume ratio is so large that diffusion through the body surface is sufficient to supply their needs.

• An example of this is an amoeba where the cell membrane acts as the exchange surface, It is thin and moist so is efficient at its job

• Multicellular organisms are an aggregation of cells. • Cells aggregate together to increase their size but

decreases their surface area to volume ratio • Diffusion path increases • Materials are needed to be exchanged between

different organs as well as the organism and the environment.

• So diffusion is no longer a viable process of exchange.

• Larger organisms therefore possess special surfaces for gaseous exchange, gills for aquatic environments, lungs for terrestrial environments.

• Gas exchange surfaces such as the gills of a fish,

the alveoli in the lungs of a mammal, the trachae of an insect and the spongy mesophyll cells in the leaves of a plant are effective exchange surfaces.

Exchange surfaces• a large surface area• a thin permeable surface • Most are moist to allow a medium for gasses

dissolve in before diffusion • a mechanism to maximise the diffusion gradient

by replenishing the source and/or sink.

Systems that increase rate of Exchangesystem large surface area small distance high concentration gradient

human lungs 600 million alveoli with a total area of 100m² each alveolus is only one cell thick constant ventilation replaces the air

Fish gills feathery filaments with lamellae lamellae are two cells thick water pumped over gills in countercurrent to blood

Leaves surface area of leaves of 1 tree is 200m², surface area of spongy cells inside leaves

of 1 tree is 6000m².

gases diffuse straight into leaf cells wind replaces air round leaves, and photosynthesis counteracts

respiration

The large moist area for gaseous exchange is a region of potential water loss

Worms• Simple multicellular animals such as worms have a

low oxygen requirement • They have an extremely low metabolic rate as they

move very slowly. • Oxygen and carbon dioxide are able to diffuse

across the skin surface• No specialised gas exchange surface is required

Earthworm• Elongated shape ensures a

larger surface area to volume ratio enabling diffusion to take place.

• Low metabolic rate so does not require much oxygen.

• Mucus is secreted to keep the skin moist

• They possess a simple circulatory system: a closed blood system containing blood within vessels .

• The blood contains a respiratory pigment that transports oxygen.

• Oxygen is carried to the cells whilst carbon dioxide is transported in the opposite direction, thus maintaining the diffusion gradient at the respiratory surface.

Flatworms• Flattened shape

increases the SA: V and ensures a short diffusion path for exchange.

• Lives in aquatic environments

Fish• Fish are more active• In fish gaseous exchange happens at the gill.• At the gill a specialised pumping mechanism pumps

a one way current of water over the surface.

• The density of the water keeps the gills from collapsing in on themselves which would reduce surface area.

• Gills are made up of many folds providing a large surface area.

• Fish can be further classified into two groups according to the material that makes up their skeleton:– Cartilaginous fish– Bony Fish

Cartilaginous fish• Skeletons are made entirely of cartilage.• Normally live in the sea e.g. shark• Characterised by five gill clefts that open at five gill slits.• Gas exchange involves parallel flow.• Therefore blood travels through the capillaries in the same

direction as the sea water.• This is relatively inefficient as a diffusion gradient is not

maintained.

Bony Fish• Internal skeleton made of bone• Gills are covered by the operculum• Gas exchange involves Counter current flow.• Blood in the capillaries flow in the opposite direction

to the water flowing over the gill surface.• This is more efficient as the gradient is maintained and

exchange is able to happen over the whole gill surface

• FLASH

• Water is a dense medium with a low oxygen content

• To increase efficiency, it needs to be forced over the gill filaments by pressure differences

• This maintains a continuous, unidirectional flow of water

• So how does a fish do this?

• A lower pressure is maintained in the opercula cavity than in the bucco-pharynx.• The operculum acts as a valve• It also acts as a pump drawing water over the gill filaments.

The ventilation mechanism for forcing water over the gill is:• mouth• Opens• operculum• Closes • Floor of buccal cavity• lowered• volume• increases• pressure• decreases• Water flows in

• Compared with parallel flow, counter current flow increases efficiency because the diffusion gradient between the adjacent flows is maintained over the whole surface.

• Blood flows between the gill plates under pressure in the opposite direction the blood in the capillaries.

• The blood always meets water with higher oxygen content than itself.

• It removes 80% of the oxygen from the water.• This high level of extraction is important as there is 25%

less oxygen in water than in air.

Terrestrial Vertebrates• What classes does this include?• Amphibians, reptiles, birds and mammals

• Terrestrial vertebrates have adapted for exchange with air, a less dense medium, instead of water, so have internal lungs.

• Internal lungs minimise loss of water and heat.

Amphibians• This was likely the first vertebrate group to

colonise the land. • They have lungs for use on land but in water gas

exchange occurs through diffusion as they have permeable moist skin which acts as a respiratory surface.

• Tadpoles have gills for use in water.

• How then do frogs sit at the bottom of a pond for hours on end?

• Can use its skin as a respiratory surface but ONLY when it is inactive

Reptiles and Birds• Reptiles and birds have more efficient lungs than

amphibians.

Reptiles:• Ribs protect the inner organs and provide ventilation

to the lungs.• Tissues in lungs provide a greater surface area.

Birds:• Large volumes of oxygen are needed for flight.• ventilation of the lungs are assisted by air sacs• the action of flight muscles ventilate the lungs.

Human Respiratory system

Ventilation• What is ventilation?– Ventilation is movement of air into and outside

the body

• Why do we need it?

So what is the respiratory system????

• The respiratory system includes everything we use to breath and supply our bodies with oxygen.We breath air into

Our lungs

Oxygen is then transferred into the blood

Blood then takes the oxygen around the body supplying vital organs and limbs with oxygen

Alveoli

Single layer of flattened epithelial cells

Capillaries run close by (1 cell thick)

Layer of elastic tissue holds them together

Secrete surfactant to stop alveoli collapsing

SIMPLE DIFFUSION – oxygenated air in alveoli; deoxygenated blood in capillaries

Gaseous exchange in the alveoli

• CO2 moves from the blood into the alveoli (alveolar air)

• O2 moves from oxygen-containing air, across to the red blood cells (tidal air)

Ficks Law

Rate of Diffusion Surface Area x concentration difference

distance

How does this apply to alveoli?

Large surface area – ‘grape’ like structure of alveoli

Short diffusion distance – alveoli are 1 cell thick

Concentration gradient – continuous blood flow maintains gradient on capillary side; air in the alveoli is constantly refreshed

Breathing…

INHALING

Diaphragm contracts and lowers

Chest cavity volume increases

Pressure in the lungs decreases

Air is drawn in

Breathing…

EXHALING

Diaphragm relaxes and moves up

Chest cavity volume decreases

Pressure in the lungs increases

Air is forced out

Different types of lung capacity• Tidal volume:

• Is the amount of air you breath in or out with each breath• Inspiratory capacity:

• is the most air you can breath in after breathing out normally.• Expiratory reserve volume:

• Is the most air you could force out after breathing out normally • Vital Capacity:

• Is the most air you could possible breath in or out in one go.• Residual Volume

• Is the amount of air left in your lungs after you’ve breathed out as much as possible.

A SPIROMETER is used to measure these

Insects• Insects have a relatively inefficient, open

circulatory system with no vessels to carry oxygen to different parts of their body.

• A centralised respiratory system, such as lungs, would not meet the respiratory demands of the insect's cells.

• Insects have evolved a very simple tracheal system that relies on a network of small tubes that channel O2 directly to the different parts of the body

• The tracheal system is composed of chitin-ringed tubes called tracheae

• these connect directly to the air through openings in the body wall called spiracles.

• The tracheae are reinforced with rings of chitin, the same material that makes up the exoskeleton

• Small inactive insects rely solely on diffusion• Bigger active ones may ventilate the tracheae

Plants

Xeromorphic adaptations in flowering plants

• Xeromorphic adaptations reduce water loss from aerial parts

• Thicker Cuticles

Xeromorphic adaptations in flowering plants

• Xeromorphic adaptations reduce water loss from aerial parts

• Thicker Cuticles• Reduction in leaf size• Curling or rolling of leaves

Xeromorphic adaptations in flowering plants

• Xeromorphic adaptations reduce water loss from aerial parts

• Thicker Cuticles• Reduction in leaf size• Curling or rolling of leaves• Number of stomata• Epidermal hairs

Hydrophytes• Grow where water freely available:– in, around or on ponds lakes and streams

• Some live completely submerged:

Canadian Pond weed (Elodea sp.)Water Milfoil (Myriophyllum sp.)

No Cuticle

No Stomata

Reduced vascular / supporting tissue

Small leaves / dissected leaves

Air spaces

Reduced root systems

• Some Hydrophytes like water lilies are rooted to bottom, leaves float on surface

• These leaves have special adaptations:– Stomata on upper surface– No stomata on lower surface– Elongated lignified cells (sclerids) stop leaves

rolling up– Thick palisade mesophyll

Stomata• Opening: • Hydrogen ions (or products) leave the

guard cells.• Potassium ions enter the guard cells.• Chloride ions subsequently enter the

guard cells.• The osmotic pressure of the cell

increases (water potential decreases).• Water enters the guard cells.• The guard cells become turgid.• Because of the construction of the

cells, the stomata open.

• Closing: • Potassium ions leave the guard

cells.• Chloride ions also leave the

guard cells.• The osmotic pressure

decreases (water potential increases).

• Water leaves the guard cells.• The guard cells become flaccid.• Because of the construction of

the cells, the stomata close.