ICSE Class 9 Biology Chapter 07 Respiration in Plants

Respiration In Plants

Syllabus: Respiration in plants: outline of the process, gaseous exchange.

A brief outline of the process mentioning the terms Glycolysis and Krebs cycle and their significance. A reference to be made to aerobic and anaerobic respiration with chemical equations in each case. Experiments on gaseous exchange and on heat production.

Like all other organisms, plants too require energy for carrying out body activities. In this chapter you will learn about the two kinds of respiration (aerobic and anaerobic), the manner in which diffusion of respiratory gases occurs, as well as a number of interesting experiments to demonstrate respiration in plants.

When energy in the form of ATP is used, the ATP is converted to ADP (adenosine diphosphate) and again when more energy is available by further breakdown of glucose, the ADP is reconverted to ATP and so it goes on and on (Fig. 7.1). One mole of glucose on complete oxidation yields 38 molecules of ATP.

What Is Respiration?

All living cells in a plant, as well as those in animals, require energy for various body activities. For example, building up proteins from amino acids, making starch from glucose, absorbing minerals from the soil, or the laying down of cell walls by the plant cell, are all such activities that require energy. This energy is made available by the breakdown of glucose a simple carbohydrate. This chemical breakdown occurs by utilizing oxygen and is represented by the following overall reaction:

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}\]

There are three important characteristics of respiration in this equation.

1. The breakdown of glucose (\(C_6H_{12}O_6\)) to carbon dioxide and water does not occur in a single step, but in a series of chemical steps. These steps occur in two major phases - (i) glycolysis (glucose - pyruvate) occurring in cytoplasm and (ii) Krebs cycle (pyruvate - \(CO_2 + 6H_2O + ATP\)) occurring in mitochondria.

2. Each breakdown step is due to a particular enzyme.

3. The energy liberated in the breakdown of the glucose molecule is not all in the form of heat, but a large part of it is converted into chemical energy in the form of ATP - a chemical substance called adenosine triphosphate.

ATP - The energy currency of the cell. All activities inside a living cell need energy which is available in the form of ATP as the immediate source. Hence, this chemical compound ATP is described as "The energy currency of the cell".

Respiration is a catabolic process of releasing energy from simple sugar glucose for carrying out life processes.

Living organisms show two types of metabolic activities:

(i) anabolic (constructive or biosynthetic processes), it consumes energy.

(ii) catabolic (destructive or breaking down processes), it gives out energy for use by the organism.

Teacher's Note

Respiration is like a battery charger in cells - just as a charger converts electricity to stored energy in a battery, respiration converts glucose into ATP energy that powers all cellular activities throughout the day.

Respiration vs. Burning (Combustion)

Sometimes, respiration is compared with burning such as the burning of coal. Both liberate energy, and both give the end products \(CO_2\) and water. But this comparison is only superficial. The differences between the two are as follows:

Respiration is a breaking down process by which a living cell oxidises organic substances (glucose) and releases carbon dioxide, water and energy.

Teacher's Note

Unlike burning coal which explodes with heat all at once, respiration is like a slow, controlled burn where cells carefully release energy bit by bit, allowing the body to use it efficiently for work and growth.

The Entire Plant Respires

Every part of a plant such as the leaves, stem, roots and even the deepest placed cell in any region respires. Oxygen is obtained from the atmosphere through three inlets:

- stomata in leaves

- lenticels in stem

- general surface of the roots

Ploughing or tilling of the soil creates tiny air spaces around soil particles and provides the source of oxygen for the roots. Water-logged and compact soil does not have air spaces which affect respiration of the roots.

During daytime, due to photosynthesis, the leaves produce oxygen, some of which is used in respiration and the rest is diffused out. The carbon dioxide produced during respiration in the leaves serves as a raw material for photosynthesis.

At night, even the leaves obtain oxygen from the atmosphere and give out carbon dioxide. Hence, there appears to be some truth in the belief that one should not sleep under a tree at night (also see the box at the end of this chapter page 62). (But actually, there is more carbon dioxide inside our houses than under a tree at night). Sleeping under a tree during hot mid-day is definitely good as one gets both oxygen due to photosynthesis and coolness due to transpiration.

Progress Check:

1. What are the three important aspects about the overall chemical equation of respiration, pertaining to the following?

(i) Single or several steps

(ii) Direct or enzyme-catalysed

(iii) Forms of energy liberated

2. List three ways in which respiration is different from burning:

(i)

(ii)

(iii)

3. Name the three inlets of oxygen for respiration in plants.

(i)

(ii)

(iii)

Teacher's Note

Think of the roots as little breathing mouths and the soil as lungs - healthy soil with air pockets lets roots breathe properly, just like how tight clothing restricts our breathing.

Two Kinds Of Respiration - Aerobic And Anaerobic

A. Aerobic Respiration

Normally, free oxygen is used in respiration and there is complete oxidation of glucose with the formation of carbon dioxide and water as end-products. This is clearly represented by the following overall reaction (actually there are numerous steps in the entire process):

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 38 \text{ ATP (energy/heat)}\]

Respiration proceeds only when oxygen (a constituent of air) is available and is therefore called aerobic (or oxybiotic) respiration.

B. Anaerobic Respiration

Sometimes certain parts of the plants (including fruit and seeds) may temporarily respire even in the absence of oxygen. In this type of respiration, the glucose molecule is incompletely broken down into ethanol (ethyl alcohol) and carbon dioxide with the release of a small quantity of energy. This chemical reaction can be represented in the following manner:

\[C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + 2 \text{ ATP}\]

This type of respiration is called anaerobic respiration (or anoxybiotics) as it proceeds even without oxygen. Anaerobic respiration in any part of a plant cannot continue in a plant for more than a few days and the part ultimately dies. But there are certain microscopic organisms such as certain bacteria and fungi which normally respire only anaerobically. Even the germinating seeds when deprived of air respire anaerobically as described in Experiment No. 5 Fig 7.7(p. 54).

A Proper Experiment Must Have Two Components: EXPERIMENTAL and CONTROL. An experiment consists of two set-ups - an experimental set-up and a control set-up. Both are identical in which every condition is the same except one. The set-up in which the condition under study is missing is called the control.

Teacher's Note

Aerobic respiration is like a well-ventilated room where things work smoothly and efficiently, while anaerobic respiration is like a stuffy, sealed room where things work poorly and can only last a short time.

Experiments On Respiration In Plants

1. Experiment to prove that oxygen is used up in respiration.

An apparatus is arranged as shown in Fig. 8.2. Flask (A) contains germinating bean seeds and Flask (B) has dead (boiled) seeds together with some antiseptic to avoid bacterial decay. Wet cotton provides water to both samples. A small tube containing soda lime (a mixture of sodium hydroxide and slaked lime) is suspended in each flask for absorbing any carbon dioxide released by the seeds. After a few days, the delivery tube connected with flask (A) will show a greater rise in water level. When a burning paper is introduced into each flask after removing their corks, the flame is immediately put off in flask (A), but it continues for a short while in flask (B). This proves that oxygen was absent in flask (A) showing thereby that it was used up by the germinating seeds and the volume of oxygen so used up was indicated by the rise of water level in the delivery tube. There are two simple questions which surely you can answer. Why is there a slight increase in the level of water in the delivery tube of flask (B)? Which of the two flasks, (A) or (B), is a control?

2. Experiment to prove that carbon dioxide is produced during respiration in germinating seeds.

Take two flasks A and B. Place some wet cotton-wool at the bottom of each flask. Soaked seeds (such as pea or bean) are placed in flask (A) and an equal number of boiled (dead) seeds are placed in flask (B) (Fig. 7.3).

A little antiseptic (such as carbolic acid) is added to flask (B) to prevent bacterial growth on dead seeds, which would otherwise respire and release carbon dioxide. The flasks are securely corked and left in similar conditions of light and temperature. A few days later, the seeds in flask (A) will be found to have clearly germinated and those in flask (B) showing no signs of any germination (as they are dead). The gases in each flask are then tested by removing the cork and tilting the flask over a test-tube containing limewater and then shaking up the test-tube. The expected gas carbon dioxide being heavier than air would "flow down" into the test-tube. The gas from flask (A) would turn the lime- water milky, showing the presence of carbon dioxide in it, while the gas in flask (B) will show no effect. Therefore, the conclusion is that the germinating (respiring) seeds give out carbon dioxide.

An alternative method for the same experiment

A similar more directly observable experiment to show that carbon dioxide is produced during respiration in germinating seeds (pea or grain seeds) is arranged as shown in Fig. 7.4. The air drawn in conical flask A is cleared of any \(CO_2\) present in it. The clear limewater in flask B confirms that the air entering flask C is \(CO_2\) free. The limewater in flask D turning milky doubtless proves that the source of \(CO_2\) was only the germinating seeds.

3. Experiment to prove that carbon dioxide is produced by green plants during respiration.

Set up an apparatus as shown in (Fig. 7.5) using a small potted plant such as Geranium. The bell-jar should be placed on a glass sheet and its rim as well as all other connections should be vaselines to make them air-tight. The outside air is drawn into the apparatus with the help of an air pump. This air passes through the soda-lime which absorbs any carbon dioxide present in the incoming air, and the limewater (Flask A) through which it passes will not turn milky. As the air leaves the bell-jar, and passes through another sample of limewater (Flask B), the carbon dioxide present in it would turn the limewater milky. It is (for example, in plants the process produces little heat as compared to animals.

necessary that this experiment is carried out in the dark, or the bell-jar is completely covered by a piece of black cloth to prevent photosynthesis (so that the carbon dioxide liberated in respiration is not used up in the synthesis of starch).

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