LIWA

Respiration in Plants

ICSE Class 9 · Biology · Chapter 7

What this chapter covers

Key definitions

Respiration is the process by which living cells break down food, usually glucose, step by step with the help of enzymes to release the energy stored in it, with carbon dioxide and water (or another end product) given out.

Cellular respiration is the name used for respiration when it is described as it happens inside the cell, to stress that it is a chemical process of the cell and not a movement of air.

ATP (adenosine triphosphate) is the energy currency of the cell, a molecule that stores energy released during respiration and gives it up wherever the cell needs energy for its work.

Breathing is the physical process of taking in air rich in oxygen and giving out air rich in carbon dioxide, by the movement of a body part such as the chest.

Gaseous exchange is the movement of oxygen into a living organism and of carbon dioxide out of it (or the reverse), across a moist surface by diffusion.

Diffusion is the movement of molecules of a substance from a region where they are more concentrated to a region where they are less concentrated, until they are evenly spread.

Stomata are tiny pores, mostly on the lower surface of leaves and also on young green stems, each surrounded by a pair of bean-shaped guard cells that open and close the pore.

Lenticels are small, raised, lens-shaped openings in the bark of woody stems and on some roots and fruits, made of loosely packed cells with air spaces between them.

Root hairs are thin, tube-like outgrowths of the outer cells of a young root that, besides absorbing water, take in oxygen from the air present between the particles of soil.

Mitochondria are rod-shaped or oval cell organelles, bounded by a double membrane with the inner membrane folded into finger-like ridges called cristae, in which the later stages of aerobic respiration take place.

Glycolysis is the first stage of respiration, in which one molecule of glucose, containing six carbon atoms, is broken down in the cytoplasm into two molecules of pyruvic acid, each containing three carbon atoms, without the use of oxygen.

Pyruvic acid is the three-carbon compound formed at the end of glycolysis, which is the branching point of respiration: it is broken down further either aerobically in the mitochondria or anaerobically in the cytoplasm.

Key points

Questions answered

Define respiration. Why is it described as a process of every living cell rather than of a particular organ?
Respiration is the process by which living cells break down food, usually glucose, step by step with the help of enzymes to release the energy stored in it. Every living cell needs energy for its own work and cannot receive ready energy from another cell, so each cell must carry out respiration for itself.
Give two differences between breathing and respiration.
Breathing is a physical process of moving air in and out and releases no energy; respiration is a chemical process that breaks down food and releases energy. Breathing takes place in breathing organs; respiration takes place inside every living cell.
Why is it incorrect to say that "plants respire only at night"?
Plants respire all the time. During the day the carbon dioxide from respiration is used at once by photosynthesis, and more oxygen is produced than respiration uses, so respiration is masked rather than absent.
Name three structures through which gaseous exchange takes place in plants, and state where each is found.
Stomata, in the epidermis of leaves and young green stems; lenticels, in the bark of woody stems and on some fruits and roots; root hairs, on young roots in the soil.
Define glycolysis. Where in the cell does it occur, and what is its net ATP gain?
Glycolysis is the first stage of respiration in which one molecule of glucose is broken down into two molecules of pyruvic acid without the use of oxygen. It occurs in the cytoplasm, and its net gain is 2 ATP per glucose molecule.
State two points of significance of the Krebs cycle.
It completes the breakdown of pyruvic acid to carbon dioxide and water, and together with the hydrogen carrier chain it produces most of the ATP of aerobic respiration (about 36 of 38 ATP). It also supplies raw materials for making other substances such as some amino acids.
Why does the Krebs cycle stop when oxygen is not available?
Oxygen is the final acceptor of the hydrogen removed during the cycle; it combines with the hydrogen to form water. Without oxygen the hydrogen cannot be removed, so the carriers stay loaded and the reactions of the cycle cannot continue.
Write the balanced chemical equation for (a) aerobic respiration and (b) anaerobic respiration in plants.
(a) C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (38 ATP). (b) C6H12O6 → 2C2H5OH + 2CO2 + energy (2 ATP).