Why do we respire?
Food stores energy. To use it, every cell breaks down food, mainly glucose, to release energy. This process is cellular respiration, and it happens in the cells of every living thing, all the time.
Aerobic and anaerobic respiration
Aerobic respiration | Anaerobic respiration | |
|---|---|---|
Oxygen | used | not used |
Products | carbon dioxide + water + a lot of energy | alcohol + carbon dioxide (yeast), or lactic acid (muscles); less energy |
Where | most plant and animal cells | yeast, some bacteria, our muscles during hard exercise |
Aerobic: glucose + oxygen → carbon dioxide + water + energy
Anaerobic in yeast: glucose → alcohol + carbon dioxide + energy
Yeast is a single-celled fungus that respires anaerobically. This is used to make bread rise (the carbon dioxide puffs up the dough) and to make alcohol.
Anaerobic in muscles: during heavy exercise, such as sprinting, the muscles need energy faster than oxygen can be supplied. They respire anaerobically for a short time:
glucose → lactic acid + energy
The build-up of lactic acid causes muscle cramps. A hot bath or a massage improves blood circulation, bringing more oxygen to break the lactic acid down.
Breathing
Breathing is taking in oxygen-rich air (inhalation) and giving out air rich in carbon dioxide (exhalation). It is the first step that supplies oxygen for respiration.
Air enters through the nostrils, travels down the windpipe and reaches the two lungs in the chest cavity.
The chest cavity is enclosed by the ribs and closed at the bottom by a muscular sheet, the diaphragm.
Inhalation | Exhalation | |
|---|---|---|
Ribs | move up and outward | move down and inward |
Diaphragm | moves down (flattens) | moves up (relaxes) |
Chest cavity | gets larger | gets smaller |
Air | rushes into the lungs | is pushed out |
An adult at rest breathes about 15 to 18 times a minute. During exercise the breathing rate goes up to supply more oxygen.
Exhaled air contains more carbon dioxide than inhaled air. Blowing into lime water through a straw turns it milky, which shows the carbon dioxide.
Hairs and mucus in the nose trap dust and germs. When an irritant gets in, we sneeze to expel it.
How other animals breathe
Animal | Breathing organ |
|---|---|
Cockroach and other insects | small openings on the sides of the body called spiracles, leading to air tubes called tracheae |
Earthworm | moist, slimy skin |
Frog | lungs, and also moist skin |
Fish | gills, which take oxygen dissolved in water |
Respiration in plants
Plants respire all the time, day and night, taking in oxygen and releasing carbon dioxide.
Leaves exchange gases through stomata.
Roots take in air from the spaces between soil particles. That is why overwatering a pot, which fills those spaces with water, can kill a plant.
Stems of woody plants exchange gases through small openings in the bark.
During the day photosynthesis is faster than respiration, so a plant gives out more oxygen than it uses.
Summary
Cellular respiration releases energy from food in every cell.
Aerobic respiration uses oxygen and gives more energy; anaerobic respiration does not use oxygen.
Breathing involves the ribs and diaphragm; we breathe 15 to 18 times a minute at rest.
Insects use spiracles, fish use gills, earthworms use skin; plants respire through stomata, roots and stems.