Gas exchange

Eduqas Biology Alevel

What is gas exchange?

The process by which oxygen reaches cells and carbon dioxide is removes

Why is gas exchange needed?

-obtaining minerals like CO2 and O2 form environment

-removing waste from cells

ficks law

rate of diffusion∝ (surface area x concentration difference)/ diffusion distance

What is needed on a respiratory surface for maximum rate of diffusion?

-large surface area relative to volume

-thin

-permeable

-moist to allow a medium where gases dissolve before diffusion

-maintain a concentration gradient

diffusion in single-celled organism (amoeba)

-large surface area: volume

-lower metabolic needs so less oxygen needed and less waste product

-cell membrane is thin and moist

-so small that distance between organelles is small so diffusion is sufficient as a transport method

-oxygen enters by passive diffusion as oxygen content in water is less then inside cell

Simple multicellular organisms?

-decrease SA:V ratio so slower diffusion

-gases can diffuse through skin surface, don’t need gas exchange organs

-lower oxygen requirements

Flat worms (aquatic)

-take oxygen from water

-exchange gas over entire body surface

-larger species are flattened to increase SA:V and decrease distance over which gases have to diffuse

-slower so more modest oxygen requirements

Earth worms

-Elongated shape provides larger SA:V

-slower so more modest oxygen requirements

-damp environment for moistness + keep moist by secreting mucus

-O2 diffuses into capillaries of circulatory system

-haemoglobin

-diffusion gradient maintained as O2 is carried to other parts of the body

Larger Multiucellular

-SA:V too small to supply their needs as they have a higher metabolic rate

Amphibians

-babies use gills and skin for gas exchange (SA and capillary supply, short diffuson distance)

-adults at rest use thin, permeable skin and capillary network near skin

-when active use lungs which cut down on water loss and improve surface area for gas exchange

Fish problems

-Small SA:V

-very active so high O2 demand

-water is more dense than air so has a lower O2 content

How gills work in fish

Mouth (buccal cavity) opens and the floor (bottom of mouth) loweres. Volume of cavity increases so pressure inside the head decreases FORCING water to come in

-at the same time the Opercular valve is closed and the opercular cavity containing the gills expand. This increases the volume, decreases the pressure and water form the mouth moves into the opercular cavity (gills)

-Mouth closes causing a high pressure in buccal cavity and low in opercular cavity so water continues to move over gills. The opercular valve opens and lets water out

-sides of opercular cavity force inward so increasing pressure and floor of buccal cavity steadily moves up maintaining flow over gills.

Structure of Gills

Fish adaptations

-gills increase surface area

-ventilation causes a continuous movement of water over the gills

what is countercurrent flow

blood in gill plates / lamellae flows in the opposite direction to the water (bony fish)

What is good about countercurrent flow

More efficient for oxygen intake as: the oxygen concentration in the water will always be higher than in the blood where they meet throughout the whole lamellae, meaning the concentration gradient is always upheld

 

What is Parallel flow?

When the blood in the lamellae and water outside flow in the same direction (cartilaginous fish)

Why is parallel flow bad?

Eventually the concentration of oxygen inside and outside of the lamellae will reach equilibrium and there will be no more net diffusion after that point.