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Gaseous Exchange and Respiration

13 min read

Respiration releases energy

Respiration is the process in every living cell that releases energy from food. It is not the same as breathing. The energy is used for movement, growth, keeping warm and making new substances.

Aerobic respiration (with oxygen) in mitochondria.
Aerobic respirationAnaerobic respiration
Oxygenneedednot needed
Energy releaseda lotlittle
Products in musclesCO₂ + waterlactic acid
Products in yeastCO₂ + wateralcohol + CO₂

Breathing and gaseous exchange

Breathing moves air in and out of the lungs. Gaseous exchange happens in the tiny air sacs called alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out. Alveoli are good for gas exchange because they have a large surface area, thin walls (one cell thick) and a rich blood supply.

The human breathing systemThe nose and mouth lead to the trachea or windpipe, which splits into two bronchi, one to each lung. Below the lungs a sheet of muscle called the diaphragm separates the chest from the abdomen.nose and mouthtrachea (windpipe)bronchuslungdiaphragm
Gas exchange in an alveolusAn alveolus, a tiny air sac in the lung, surrounded by a capillary. Oxygen passes from the air in the alveolus into the blood, and carbon dioxide passes from the blood into the alveolus to be breathed out. The walls are one cell thick and moist.air(oxygen in)oxygen → blood← carbon dioxidealveolus wall: one cell thickcapillary with blood
Oxygen passes into the blood and carbon dioxide passes out, across a wall that is only one cell thick.
Blue dots are oxygen. Green dots are carbon dioxide.
Air we breathe in and out
GasInhaled airExhaled air
Nitrogen78%78%
Oxygen21%16%
Carbon dioxide0.04%4%
Water vapourvariablemore

Gaseous exchange in plants

Plants also exchange gases. Tiny pores called stomata, mostly on the underside of a leaf, open and close between two guard cells. In daylight carbon dioxide enters for photosynthesis and oxygen leaves. Plants respire all the time, taking in a little oxygen and giving out carbon dioxide, but in bright light photosynthesis is faster, so there is a net release of oxygen.

Inside a leafA cross-section of a leaf showing layers from the top: upper epidermis, palisade layer packed with chloroplasts, spongy layer with air spaces, and lower epidermis with a stoma between two guard cells where gases enter and leave.sunlightcarbon dioxide inoxygen outupper epidermispalisade layer: most chloroplastsspongy layer: air spacesstoma between two guard cellsvein (xylem and phloem)
Try itBreathing at rest and in exercise

The volume of air breathed each minute (minute ventilation) is the volume of one breath multiplied by the number of breaths per minute. Exercise increases both.

Worked exampleMinute ventilation

A girl breathes 12 times a minute. Each breath moves 500 cm³ of air. How much air does she breathe in 5 minutes?

  1. Per minute.
  2. In 5 minutes.

Answer: She breathes 30 litres in 5 minutes.

Check yourself Where does gaseous exchange take place in the lungs?
Check yourself Which is a product of anaerobic respiration in muscle?

Key terms

Respiration
Release of energy from food in cells.
Alveolus
A tiny air sac in the lung where gases are exchanged.
Aerobic
Using oxygen.
Anaerobic
Without oxygen.
Stoma
A tiny pore in a leaf, opened and closed by two guard cells, where gases enter and leave.

Summary

  • Respiration releases energy from glucose in every cell; aerobic uses oxygen.
  • Gaseous exchange occurs by diffusion in the alveoli.
  • Breathing in: diaphragm flattens, ribs rise, volume increases.
  • Minute ventilation = volume per breath × breaths per minute.
  • In plants gases cross through stomata between guard cells.
Transport of Materials in Living Things