Respiration in plants and humans

Breathing vs respiration

We can live without food for several days, but not without breathing for even a short while, because our cells need oxygen all the time.

How plants exchange gases

• Plants have no special organs for respiration. Each part takes in oxygen and gives out carbon dioxide by diffusion.
• Roots: oxygen from the air trapped in the soil diffuses into the root hairs, and carbon dioxide diffuses out.
• Leaves: gases pass through the stomata. When the guard cells fill with water they swell (become turgid) and curve apart, so the pore opens. When they lose water (flaccid), it closes.
• Woody stems and old roots: tiny openings in the bark, the lenticels, let oxygen reach the inner living cells.
• Like all living cells, germinating seeds respire and give out carbon dioxide.

Path of air in the human respiratory system

1. 1
   Air enters through the nostrils (external nares) into the nasal cavities inside the nose.
2. 2
   It passes through the internal nostrils into the pharynx (throat).
3. 3
   From the pharynx, air enters the trachea (windpipe) through an opening, the glottis. The larynx (voice box) sits at the top of the trachea.
4. 4
   Rings of cartilage support the trachea, so it does not collapse even when there is little air in it.
5. 5
   The trachea divides into two bronchi (singular: bronchus), one going to each lung.
6. 6
   Inside the lungs, each bronchus branches into fine bronchioles. These end in tiny, thin-walled air sacs, the alveoli.
7. 7★
   Gas exchange: oxygen diffuses from the alveoli into the blood capillaries around them. Carbon dioxide diffuses from the blood into the alveoli and is breathed out.
8. 8
   In the tissues, blood gives oxygen to the cells and picks up carbon dioxide. Veins carry it back to the heart and lungs.

★ key step

Inhalation vs exhalation

Activity: carbon dioxide is given out in respiration (practical)

1. 1
   Soak about 25 g of gram or moong seeds overnight. Wrap them in a wet cloth for one or two days, until they sprout (germinate).
2. 2
   Put two or three layers of the sprouted seeds in a dry conical flask.
3. 3
   Hang a small test tube holding a few pellets of potassium hydroxide (KOH) inside the flask with a thread. The KOH must not touch the seeds.
4. 4
   Close the flask with an airtight one-holed cork carrying a bent glass tube. Dip the other end in a beaker of coloured water, and mark the level.
5. 5
   Check the water level in the tube every half hour.
6. 6
   Observe: the coloured water rises in the tube.
7. 7★
   Reason: the seeds respire and give out CO₂. KOH absorbs it, so the pressure in the flask falls and the outside air pushes water up the tube.

   Germinating seeds respire fast; dry or boiled seeds would not work

★ key step

How to draw the human respiratory system

• Draw the nostrils and the nasal cavity at the top, with the mouth cavity below it.
• Draw the pharynx leading down to the larynx (voice box) and a straight tube, the trachea. Show rings of cartilage as short curved lines across it.
• Split the trachea into two bronchi, one entering each lung.
• In one lung, draw branching bronchioles ending in grape-like alveoli; add an enlarged circle showing an alveolar sac wrapped in blood capillaries.
• Draw the ribs around the lungs and the dome-shaped diaphragm below them, separating the chest from the abdomen. Label every part.