gas exchange

what is gas exchange

exchange of gases between an organism and its surroundings

via what process does gas exchange occur

diffusion

single celled organisms

can exchange oxygen and carbon dioxide directly with the atmosphere

Large multicellular organisms

need a system to exchange respiratory gases as most cells are not in direct contact with the

what are the factors that determine the rate of diffusion

- the size of the surface area available for gas exchange
- the difference in concentration (concentration gradient)
- the length of the diffusion path

surface area

the greater the surface area, the greater the rate of diffusion

concentration gradient

maintaining a lower concentration of oxygen in the cell that outside. The greater the concentration gradient, the greater the rate of diffusion

length of diffusion path

the shorter the diffusion path, the greater the rate of diffusion

specialised organs for gas exchange

skin of small organisms
gills of aquatic organisms
lungs of some larger terrestrial organism

characteristics of gas exchange surfaces

thin - to keep diffusion distances short
moist - to encourage gas diffusion
large surface area - maximum diffusion
permeable to respiratory gases (oxygen and carbon dioxide)

three ways concentration gradients are maintained

dense network of blood vessels: capillaries provide a large surface area for the diffusion of respiratory gases
continuous blood flow: maintains the difference in concentration of molecules between air and the blood by carrying oxygen away from the gas exchange surface and carbon dioxide to them
ventilation: brings oxygen to the gas exchange surface and removes carbon dioxide - air for lungs and water for gills

lungs organisation

trachea divides into two bronchi which divides into smaller bronchioles which end with alveoli
in the thoracic cavity which is closed off from the rest of the body

arrangement of the alveoli

arranged in clusters at the end of the bronchioles
bronchioles exist in a branched network throughout the lungs, which provide an even distribution of alveoli

structure

- a capillary system wraps around the cluster of alveoli, providing a large surface area for the diffusion of oxygen from the alveoli into the blood and carbon dioxide from the blood to the alveoli
- inner surface is lined by surfactant

surfactant

a thin phospholipid and protein filament
reduces surface tension of most inner surface and helps to prevent each alveolus from collapsing each time air is expired

air inspired into the alveoli

higher oxygen concentration and lower carbon dioxide concentration compared to the blood in the nearby capillary

thickness of the capillaries and alveoli

one cell thick - short diffusion path

muscles in the thoracic cavity

lungs are passive not muscular and are therefore not capable of purposeful movement
diaphragm
muscles of the abdomen
external and internal intercostal muscles of the ribs

boyle's law

increase in volume will lead to a decrease in pressure

where is the one opening of the lungs

through the trachea

diaphragm

large, dome shaped muscle that forms the bottom of the thoracic cavity
- contracted: flattens
- relaxed: curved

breathing in

the diaphragm contracts - flattens
increases the volume of the thoracic cavity
decrease in the pressure which draws air in the lungs
external intercostal muscles contract
internal intercostal muscles relax

breathing out

the diaphragm relaxes - curved
decreases the volume of the thoracic cavity
increase in the pressure which causes air to move out of the lungs
external intercostal muscles relax
internal intercostal muscles contract

spirometer

measures ventilation
consists of a plastic lid that moves up and down over a tank of water in the chamber
when someone breathes into a mouthpiece, the lid rises and fall as as the volume of air in the chamber changes

ways lung volume is measured

tidal volume, inspiratory reserve volume, expiratory reserve volume and vital capacity

tidal volume

volume of air breathed in and out during normal, relaxed, rhythmical breathing
- air flow in mammals in tidal as air enters and leaves by the same route - resulting in residual air

inspiratory reserve volume

the maximum volume of air that a person can breathe in
- measured from the max point of tidal volume

expiratory reserve volume

the maximum volume of air that a person can breathe out
- measured from the min point of tidal volume

vital capacity

the sum of inspiratory reserve volume, expiratory reserve volume and tidal volume

epidermis

outer layer of cells

palisade mesophyll

single layer of cells that are full of chloroplasts
- photosynthesis occurs here

spongy mesophyll

loosely packed, creating air spaces where air circulates providing a large surface area for gas exchange

xylem

vascular tissue that transports water

phloem

vascular tissue that transports food

stoma

pores in the epidermis of a leaf, surrounded by two guard cells

transpiration

process of the loss of water vapour through the leaves
- when the stomata is open for gas exchange, water vapour is lost from the cellulose cell walls in the leaf

what happens if the air outside the plant is less saturated (less humid) and the stomata are open

the water vapour will diffuse out in the drier air outside

why don't desert plants have much transpiration

need to conserve water

increased light on the rate of transpiration

increases
light stimulates guard cells to open stomata allowing the exchange of respiratory gases + rate of photosynthesis

increased temperature on the rate of transpiration

increases molecular movement including water evaporation

increased wind speed on the rate of transpiration

increases
wind removes water vapour at the entrance of the stomata, thereby increasing the water concentration gradient between the inside and outside of the leaf

increased humidity on the rate of transpiration

decreases
lessens the water concentration gradient between the inside and outside of the leaf

what is used to measure transpiration

potometer

stomatal density

the more stomata per unit, the more CO2 can be taken into the spongy mesophyll and the more water can be released by transpiration

how can stomatal density be observed

micrographs and leaf casts

stomatal density formula

mean number of stomata / area of field of view (mm^3)