Respiration in Plants | ICSE Class 9 Biology Notes

This note covers the ICSE Class 9 Biology topic Respiration in Plants: what respiration is, how plants exchange gases, aerobic and anaerobic respiration with their chemical equations, a brief outline of glycolysis and the Krebs cycle and their significance, how energy is trapped as ATP, and the experiments that show gaseous exchange and heat production in respiring plant parts.

What is respiration, and why do plants need it?

All living organisms need energy for their life activities, such as absorption, transport, movement and reproduction. This energy is obtained by the oxidation of food.

Green plants make food by photosynthesis, storing light energy as chemical energy in carbohydrates such as glucose, sucrose and starch.

Only cells with chloroplasts photosynthesise, so food has to be carried to all the non-green parts of the plant, which also need it for energy.

Definition: Respiration is the breaking of the C-C bonds of complex compounds through oxidation within the cells, releasing a considerable amount of energy. The compounds oxidised are called respiratory substrates.

Carbohydrates are the usual respiratory substrates, and glucose is the favoured one. Proteins, fats and even organic acids can be used in some plants under certain conditions. In eukaryotic cells, respiration takes place partly in the cytoplasm and partly in the mitochondria.

The energy in food is not released all at once. It is released in a series of slow, step-wise reactions controlled by enzymes and trapped as chemical energy in ATP (adenosine triphosphate).

ATP is broken down whenever and wherever energy is needed, so it is called the energy currency of the cell. The carbon skeletons produced during respiration are also used to build other molecules in the cell.

How do plants exchange gases without breathing organs?

Plants need oxygen for respiration and give out carbon dioxide, but unlike animals they have no specialised organs for gaseous exchange.

Gases pass in and out by diffusion through stomata in leaves and young stems, and through lenticels in the bark of woody stems. Roots take in oxygen from the air spaces in the soil.

Plants can manage without respiratory organs for three reasons:

1. Each part meets its own needs. Every plant part takes care of its own gas exchange, and very little gas is transported from one part to another.
2. The demand is low. Roots, stems and leaves respire at rates far lower than animals do. Large volumes of gas are exchanged only during photosynthesis, and then the oxygen released inside leaf cells is available at once.
3. The distances are short. Most living cells lie close to the surface. In woody stems the living cells form thin layers beneath the bark, which has lenticels, and the inner cells are dead. Loosely packed parenchyma cells in leaves, stems and roots provide a connected network of air spaces.

Respiration goes on day and night in every living cell. During the day, in bright light, photosynthesis in green parts is faster than respiration, so the leaves as a whole take in carbon dioxide and give out oxygen.

At night, with no photosynthesis, the leaves take in oxygen and give out carbon dioxide, which shows the gas exchange of respiration on its own.

What are aerobic and anaerobic respiration?

Aerobic respiration is the complete oxidation of food in the presence of oxygen. It releases carbon dioxide, water and a large amount of energy, and it is the most common type in higher organisms:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (38 ATP)

Anaerobic respiration is the incomplete breakdown of food without oxygen. In yeast and in plant tissues short of oxygen, such as germinating seeds or roots in waterlogged soil, glucose is converted to ethyl alcohol and carbon dioxide. This is alcoholic fermentation:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy (2 ATP)

In human muscles during vigorous exercise, when oxygen is inadequate, glucose is broken down to lactic acid instead:

C₆H₁₂O₆ → 2C₃H₆O₃ (lactic acid) + energy (2 ATP)

Anaerobic respiration in plants and in man

In plants and yeast, anaerobic respiration gives ethyl alcohol and carbon dioxide. In human muscles it gives lactic acid and no carbon dioxide.

Both processes are hazardous to the cell because either alcohol or acid is produced; yeasts poison themselves to death when the alcohol concentration reaches about 13 per cent.

What happens in glycolysis, and why is it significant?

Glycolysis (from the Greek glycos, sugar, and lysis, splitting) is the breakdown of glucose to pyruvic acid. Its scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas, so it is also called the EMP pathway. It takes place in the cytoplasm and is present in all living organisms.

In plants the glucose comes from sucrose, the end product of photosynthesis, or from stored carbohydrates. Glycolysis is a chain of ten reactions, each controlled by an enzyme. In outline:

1. Glucose (6C) is phosphorylated to glucose-6-phosphate, using one ATP, and then changed to fructose-6-phosphate.
2. A second ATP is used to form fructose 1,6-bisphosphate (6C).
3. This splits into two three-carbon triose phosphates, 3-phosphoglyceraldehyde (PGAL) and dihydroxyacetone phosphate, which are interconvertible.
4. Each PGAL is oxidised to 1,3-bisphosphoglyceric acid, and the hydrogen removed is passed to NAD⁺ to form NADH + H⁺.
5. Further steps through 3-phosphoglyceric acid and phosphoenolpyruvate make ATP, and two molecules of pyruvic acid (3C) are formed from each glucose.

Since two ATP are used and four are formed directly, glycolysis gives a net gain of 2 ATP for each glucose, along with NADH + H⁺.

A vertical flow chart from glucose (6C) at the top to 2 × pyruvic acid (3C) at the bottom.

ATP is shown being used, turning into ADP, at the steps to glucose-6-phosphate and to fructose 1,6-bisphosphate. Fructose 1,6-bisphosphate splits into two triose phosphates (3C), shown with a double arrow between them.

NAD⁺ becomes NADH + H⁺ at the step to 1,3-bisphosphoglyceric acid, ATP is formed from ADP at the next step and at the last step to pyruvic acid, and water is removed in forming phosphoenolpyruvate.

See Fig. 12.1 in your NCERT textbook

Significance of glycolysis

• It is the first stage of both aerobic and anaerobic respiration, and it occurs in all living organisms.
• In anaerobic organisms it is the only process in respiration.
• It needs no oxygen, so it can release some energy even when oxygen is scarce.
• It produces pyruvic acid, which can go on to the Krebs cycle or to fermentation, depending on the cell's needs.

What happens to pyruvic acid when oxygen is not available?

Pyruvic acid is the key product of glycolysis, and its fate depends on the cell's need and on the supply of oxygen. There are three major routes: lactic acid fermentation, alcoholic fermentation and aerobic respiration.

Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes, and in germinating seeds.

• Alcoholic fermentation, as in yeast: pyruvic acid is converted to carbon dioxide and ethanol, by the enzymes pyruvic acid decarboxylase and alcohol dehydrogenase.
• Lactic acid fermentation, as in some bacteria and in muscle cells short of oxygen: pyruvic acid is reduced to lactic acid by lactate dehydrogenase.

In both, NADH + H⁺ is used as the reducing agent and is re-oxidised to NAD⁺, which allows glycolysis to continue. Not much energy is released: less than seven per cent of the energy in glucose, and not all of it is trapped as ATP.

Glucose passes down to glyceraldehyde 3-phosphate, where NAD⁺ becomes NADH + H⁺, then to 3-phosphoglyceric acid, phosphoenolpyruvic acid and pyruvic acid.

From pyruvic acid one branch goes up to lactic acid and another down to ethanol + CO₂; on each branch NADH + H⁺ is turned back into NAD⁺.

See Fig. 12.2 in your NCERT textbook

Fermentation is also useful to people. Yeast fermentation is used in making bread, where the carbon dioxide makes the dough rise, and in making alcoholic drinks.

Plants can survive short periods without oxygen, as in waterlogged soil, but not for long, because little energy is released and alcohol collects in the tissues.

What happens in the Krebs cycle, and why is it significant?

For aerobic respiration, pyruvic acid is transported from the cytoplasm into the mitochondria. There, in the mitochondrial matrix, it is oxidised with the release of carbon dioxide to a two-carbon compound, acetyl coenzyme A (acetyl CoA), and NADH is formed.

The acetyl CoA enters a cyclic pathway called the tricarboxylic acid (TCA) cycle, or Krebs cycle, after Hans Krebs, who first worked it out.

1. The acetyl group (2C) combines with oxaloacetic acid (4C) and water to form citric acid (6C), the first product of the cycle.
2. Citric acid is changed to isocitrate and then loses carbon dioxide twice, forming α-ketoglutaric acid (5C) and then succinyl-CoA.
3. Succinyl-CoA is converted to succinic acid (4C), and one GTP is formed, which gives one ATP.
4. Succinic acid is oxidised through malic acid (4C) back to oxaloacetic acid, which picks up the next acetyl group, so the cycle continues.
5. In each turn of the cycle, NAD⁺ is reduced to NADH + H⁺ at three points and FAD⁺ is reduced to FADH₂ at one point. One glucose gives two pyruvic acid molecules, so the cycle turns twice (6 NADH + H⁺ and 2 FADH₂), and the link reaction adds 2 NADH, which gives the total of eight NADH + H⁺ counted below.

At the top, pyruvate (3C) joins CoA and becomes acetyl coenzyme A (2C), with NAD⁺ turning to NADH + H⁺ and CO₂ released.

Acetyl CoA enters a circle labelled "citric acid cycle", joining oxaloacetic acid (4C) to form citric acid (6C). Going round the circle: α-ketoglutaric acid (5C) and succinic acid (4C), with CO₂ released twice and NADH + H⁺ formed;

GDP turning to GTP; FAD⁺ turning to FADH₂ between succinic acid and malic acid (4C); and NAD⁺ turning to NADH + H⁺ on the way back to oxaloacetic acid.

See Fig. 12.3 in your NCERT textbook

Significance of the Krebs cycle

• It completes the oxidation of glucose: all its carbon leaves as carbon dioxide.
• It produces NADH + H⁺ and FADH₂, which carry the energy that is later used to make most of the ATP.
• Its intermediates are also used to build other substances such as fatty acids and amino acids, so the respiratory pathway works in both breakdown and synthesis. It is therefore called an amphibolic pathway.

How is ATP made, and how much energy does respiration yield?

By the end of the Krebs cycle, glucose has been broken down to carbon dioxide, and eight molecules of NADH + H⁺ and two of FADH₂ have been formed in the mitochondria, besides just two ATP.

Oxygen has not yet been used, and most of the ATP has not yet been made.

The energy stored in NADH + H⁺ and FADH₂ is released in the electron transport system (ETS) on the inner membrane of the mitochondria.

Electrons pass from one carrier to another, and the energy released is used by ATP synthase to make ATP. Oxidation of one NADH gives 3 ATP, and one FADH₂ gives 2 ATP.

Oxygen is needed only at the last stage, but it is vital: it acts as the final hydrogen (electron) acceptor and is reduced to water. Without oxygen to remove hydrogen, the whole process stops. Because the energy of oxidation-reduction is used to make ATP, this is called oxidative phosphorylation.

In theory there can be a net gain of 38 ATP for each glucose in aerobic respiration, against only 2 ATP in fermentation. This figure assumes an orderly sequence of pathways with only glucose being respired.

In a living cell the pathways run together and intermediates are drawn off for other uses, so it is a useful estimate, not an exact count.

How can you show that germinating seeds give out carbon dioxide and take in oxygen?

Germinating seeds respire actively, so they are used to show gaseous exchange in plants. Two simple experiments follow.

Using lime water

1. Place some germinating seeds, such as soaked gram or pea seeds, in a conical flask, and close it with a cork carrying a delivery tube.
2. Set up an identical flask with boiled (dead) seeds as the control.
3. After a few hours, push the air out of each flask (for example by pouring water in through a funnel fitted in a second hole of the cork) so that it bubbles through clear lime water in a test tube.
4. The lime water from the flask with germinating seeds turns milky; that from the control stays clear.
5. Conclusion: germinating seeds give out carbon dioxide in respiration, since carbon dioxide turns lime water milky.

Using potassium hydroxide

1. Place germinating seeds in a flask and hang a small tube of potassium hydroxide (KOH) solution inside it. KOH absorbs carbon dioxide. KOH solution is corrosive, so the teacher should handle it and students should wear gloves and goggles.
2. Close the flask with a cork carrying a bent glass tube, and dip the free end of the tube in a beaker of coloured water.
3. After some hours, the water rises in the tube. A control flask with boiled seeds shows no rise.
4. Conclusion: the seeds take in oxygen, and the carbon dioxide they give out is absorbed by the KOH, so the volume of gas in the flask falls and water is drawn up. This shows that germinating seeds use oxygen.

Note: In every such experiment the control, with boiled seeds, shows that the result is caused by the living, respiring seeds and not by anything else in the set-up.

How can you show that respiration produces heat, and demonstrate other gas exchange?

Heat production by germinating seeds

1. Take two vacuum (thermos) flasks, A and B. Place soaked, germinating seeds in A and boiled (dead) seeds in B.
2. Under the teacher's supervision, wash both lots of seeds briefly in a dilute antiseptic such as formalin (toxic, so handle with care), then rinse them, so that microbes on them do not decay and produce heat.
3. Insert a thermometer into each flask through a cotton-wool plug, with its bulb among the seeds, and note the temperatures.
4. After 24 hours or so, the temperature in flask A has risen, while that in flask B shows no rise.
5. Conclusion: germinating seeds release heat in respiration. The vacuum flask prevents this heat from escaping.

Anaerobic respiration by yeast

1. Dissolve some sugar in boiled and cooled water (boiling drives out dissolved oxygen), and add yeast.
2. Pour a layer of oil on top to keep air out, and connect the flask by a delivery tube to a test tube of lime water.
3. After some time the lime water turns milky, and the mixture smells of alcohol.
4. Conclusion: yeast respires anaerobically, producing carbon dioxide and ethyl alcohol.

Gas exchange by a whole plant in the dark

Stand a potted plant on a glass plate, cover it with a bell jar sealed to the plate with vaseline, put a small dish of lime water beside the pot inside the jar, and keep it in the dark so that there is no photosynthesis. After some hours the lime water turns milky.

A control bell jar holding a pot of soil but no plant shows little change. This shows that the living plant gives out carbon dioxide in respiration, which is hidden in daylight by photosynthesis.

Glossary

• Respiration — The oxidation of food within living cells, breaking C-C bonds step by step to release energy that is trapped as ATP.
• Respiratory substrate — A compound oxidised in respiration; glucose is the favoured one.
• ATP — Adenosine triphosphate, the molecule that stores the energy of respiration; the energy currency of the cell.
• Lenticels — Openings in the bark of woody stems through which gases are exchanged.
• Glycolysis — The breakdown of glucose to two molecules of pyruvic acid in the cytoplasm, with a net gain of 2 ATP.
• Fermentation — Anaerobic breakdown of pyruvic acid to ethanol and carbon dioxide, or to lactic acid.
• Acetyl CoA — The two-carbon compound formed from pyruvic acid in the mitochondria, which enters the Krebs cycle.
• Krebs cycle — The cycle in the mitochondrial matrix that oxidises acetyl CoA completely, releasing carbon dioxide and forming NADH and FADH₂.
• Oxidative phosphorylation — Formation of ATP using the energy released as electrons pass along the electron transport system to oxygen.
• Amphibolic pathway — A pathway used in both breakdown and synthesis, as the respiratory pathway is.

Common errors and misconceptions

• Misconception: Plants respire only at night. Correct: Plants respire all the time. In daylight, photosynthesis is faster, so the leaves' net exchange hides respiration.
• Misconception: Respiration is the same as breathing. Correct: Respiration is the oxidation of food in cells. Plants have no breathing organs; gases diffuse through stomata and lenticels.
• Misconception: Glycolysis takes place in the mitochondria. Correct: Glycolysis takes place in the cytoplasm; the Krebs cycle and the electron transport system are in the mitochondria.
• Misconception: Anaerobic respiration in plants produces lactic acid. Correct: In plants and yeast it produces ethyl alcohol and carbon dioxide; lactic acid is formed in muscles.
• Misconception: Oxygen is used at the start of aerobic respiration. Correct: Oxygen is used only at the end of the electron transport system, as the final hydrogen acceptor.
• Misconception: The boiled seeds in the heat experiment are there to produce less heat. Correct: They are the control, to show that the heat comes from living, respiring seeds.

Exam-style questions with model answers

Q1. Write the chemical equation for (a) aerobic respiration, (b) anaerobic respiration in yeast. [2 marks]

1. (a) Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy, with a net gain of 38 ATP for each glucose.
2. (b) Anaerobic respiration in yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy, with a net gain of only 2 ATP.

Q2. Name the site in the cell of (a) glycolysis, (b) the Krebs cycle. [2 marks]

1. (a) Glycolysis takes place in the cytoplasm of the cell, where glucose is broken down to two molecules of pyruvic acid.
2. (b) The Krebs cycle takes place in the matrix of the mitochondria, where acetyl CoA is completely oxidised to carbon dioxide.

Q3. Give three differences between aerobic and anaerobic respiration. [3 marks]

1. Aerobic respiration needs oxygen; anaerobic respiration does not.
2. In aerobic respiration glucose is completely broken down to carbon dioxide and water; in anaerobic respiration it is only partly broken down, to ethyl alcohol and carbon dioxide in plants and yeast, or to lactic acid in muscles.
3. Aerobic respiration gives a net gain of 38 ATP per glucose and takes place in the cytoplasm and mitochondria; anaerobic respiration gives only 2 ATP and takes place in the cytoplasm alone.

Q4. Why can plants manage without special organs for breathing? [3 marks]

1. Each plant part takes care of its own gas exchange, so very little gas has to be transported from one part to another.
2. Roots, stems and leaves respire at far lower rates than animals, so the demand for gas exchange is low; during photosynthesis the oxygen released inside leaf cells is available at once.
3. Most living cells lie close to the surface, and loosely packed parenchyma cells form a network of air spaces, so gases diffuse in and out through stomata and lenticels over short distances.

Q5. State the significance of glycolysis and of the Krebs cycle. [4 marks]

1. Glycolysis is the first stage of both aerobic and anaerobic respiration and occurs in all living organisms; in anaerobic organisms it is the only process of respiration.
2. It needs no oxygen, gives a net gain of 2 ATP, and produces pyruvic acid for either the Krebs cycle or fermentation.
3. The Krebs cycle completes the oxidation of glucose, releasing all its carbon as carbon dioxide, and produces NADH and FADH₂, which yield most of the ATP.
4. Its intermediates are also used to make fatty acids and amino acids, which is why the respiratory pathway is called amphibolic.

Q6. Describe an experiment to show that heat is produced during respiration in germinating seeds. [5 marks]

1. Take two vacuum flasks, A and B. Put soaked, germinating seeds in flask A and boiled (dead) seeds in flask B, which acts as the control.
2. Wash both lots of seeds in a dilute antiseptic such as formalin, so that microbes do not decay the seeds and produce heat.
3. Place a thermometer in each flask through a cotton-wool plug, with the bulb among the seeds, and record the starting temperatures.
4. After about 24 hours, the thermometer in flask A shows a rise in temperature, while that in flask B shows no rise.
5. Conclusion: germinating seeds release heat as a result of respiration. The vacuum flask prevents the heat from escaping, and the control shows that the heat comes from living seeds.

Key takeaways

• Respiration is the step-wise, enzyme-controlled oxidation of food in cells, with the energy trapped as ATP, the energy currency of the cell.
• Plants have no breathing organs; gases diffuse through stomata and lenticels, and each part meets its own low demand.
• Aerobic respiration, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy, gives 38 ATP; anaerobic respiration gives only 2 ATP.
• Anaerobic respiration forms ethyl alcohol and carbon dioxide in plants and yeast, but lactic acid in human muscles.
• Glycolysis in the cytoplasm splits glucose into two pyruvic acid molecules, with a net gain of 2 ATP, and needs no oxygen.
• In the mitochondria, pyruvic acid becomes acetyl CoA, and the Krebs cycle oxidises it fully, forming carbon dioxide, NADH and FADH₂.
• Most ATP is made in the electron transport system, where oxygen is the final hydrogen acceptor and forms water.
• Germinating seeds turn lime water milky and warm a vacuum flask, showing carbon dioxide release and heat production in respiration.

Test yourself

Why is ATP called the energy currency of the cell?

Energy from respiration is stored in ATP, which is broken down whenever and wherever energy is needed for cell activities.

Through which openings do woody stems exchange gases?

Woody stems exchange gases through lenticels, openings in the bark, while leaves use stomata.

What is the final product of glycolysis?

Glycolysis ends with two molecules of pyruvic acid, a three-carbon compound, from each molecule of glucose.

What is the first product of the Krebs cycle?

Citric acid, a six-carbon compound formed when acetyl CoA combines with oxaloacetic acid.

What is the role of oxygen in aerobic respiration?

Oxygen is the final hydrogen acceptor at the end of the electron transport system, where it is reduced to water.

Why are the seeds in the heat experiment washed with an antiseptic?

To kill microbes on the seeds, so that heat from their decay does not affect the result.

At what alcohol concentration do yeasts poison themselves?

Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.

Why does lime water turn milky in the germinating seed experiment?

The germinating seeds give out carbon dioxide during respiration, and carbon dioxide turns lime water milky.