Maarg Manthan · Topic 4.6

Cellular Respiration in Plants

Science & Technology › Plant Biology and Nutrition · Topic 4.6

Cellular respiration in plants is the breakdown of food, mainly sugars made in photosynthesis, to release energy as ATP in all living cells, day and night, in roots, stems, leaves, flowers and seeds. It follows the same steps as in animals: glycolysis in the cytoplasm, the Krebs cycle in the mitochondria and the electron transport chain, using oxygen and releasing carbon dioxide and water. Plants lack a lung or heart, so gases move in by diffusion through stomata, lenticels and the surface of roots. Plant respiration is slower than in animals. When oxygen is short, as in waterlogged roots or inside fruits and seeds, plants can ferment. The respiratory quotient, the ratio of carbon dioxide released to oxygen used, shows which fuel is being respired.

5 min read · UPSC Prelims · Science & Technology · Plant Biology and Nutrition

At a Glance

Occurs In all living plant cells, day and night
Gas exchange Stomata, lenticels, root surface
RQ of carbohydrate 1
RQ of fat About 0.7
Fruit ripening gas Ethylene
Exam link UPSC Prelims, Science & Technology: Plant Biology and Nutrition

Where Does It Fit?

This is the sixth topic of the macro on plant biology and nutrition. It links the general topic on cellular respiration to plants and to practical issues such as ripening and storage. Previous topic: Animal Nutrition. Next topic: Economic Zoology – Beneficial Animals: Corals, Earthworms (Vermiculture), Prawns, Lobsters, Crabs, Pearl Oysters, Fishes.

How Does Respiration Work in Plants?

Aspect	Detail
Stages	Glycolysis, Krebs cycle and electron transport, as in Cellular Respiration and Energy Metabolism (ATP Production)
Fuel	Mainly sucrose and starch, and sometimes fats, as in oil seeds, and proteins
Gas exchange	Oxygen enters and carbon dioxide leaves by diffusion through stomata in leaves, lenticels in woody stems and the surface of roots
Rate	Lower than in animals and highest in actively growing tissue, germinating seeds and flowers
Fermentation	In roots of waterlogged soil and in storage organs with little oxygen, giving ethanol and carbon dioxide
Alternative pathway	Plant mitochondria have an alternative oxidase that releases heat instead of making ATP

What Is the Respiratory Quotient?

Definition: the volume of carbon dioxide released divided by the volume of oxygen used in respiration.
Carbohydrates: the quotient is 1, because oxygen used equals carbon dioxide released.
Fats and proteins: the quotient is less than 1, about 0.7 for fats, since fats need more oxygen to burn.
Organic acids: the quotient is more than 1, as in succulents during some conditions.
Anaerobic respiration: the quotient is very high or undefined, since carbon dioxide is released without oxygen.
Use: shows the type of fuel being used by seeds and tissues.

What Else Should You Remember?

Respiration and photosynthesis: photosynthesis occurs only in cells with chloroplasts and in light, stores energy and releases oxygen, while respiration occurs in all living cells all the time, releases energy and uses oxygen, as in Photosynthesis: Light Reaction and Dark Reaction.
Net gas exchange: in the day, photosynthesis usually exceeds respiration, so plants release oxygen. At night only respiration continues.
Climacteric fruits: fruits such as banana, mango, apple and tomato show a sharp rise in respiration and ethylene production during ripening, while non-climacteric fruits such as grapes, citrus and strawberry do not.
Ethylene: a gaseous hormone that ripens fruit, and calcium carbide use for ripening mangoes is banned in India because of health risks.
Storage of grain and fruit: low moisture, low temperature and low oxygen reduce respiration, and controlled atmosphere storage and cold storage extend shelf life.
Heat from respiration: the flowers of Arum lily and skunk cabbage warm up by thermogenesis to attract insects, and moist stored grain can heat up.
Temperature effect: respiration rises with temperature up to a limit, roughly doubling for every 10 degrees Celsius rise, and enzymes are damaged at high temperature.
Waterlogging: roots lack oxygen, which causes damage in crops like wheat, and mangroves have pneumatophores, as in Respiratory System: External and Internal Respiration in Humans, Animals, Plants and Insects.
Photorespiration: a wasteful process in C3 plants in which RuBisCO takes up oxygen in light, and it is not a source of ATP.
Respiration in seeds: a dry seed respires very slowly, and when it takes in water during germination, respiration rises sharply and releases heat.
Respiration in roots: root cells depend on oxygen in the soil air spaces, so loose and well-drained soil supports better growth.
Compensation between processes: in a dense forest, respiration of the whole community nearly balances photosynthesis, which is why mature forests store large amounts of carbon but add little net oxygen.

Why Does It Matter for the Exam?

Comparison questions: differences between respiration and photosynthesis, and between respiration and photorespiration.
Quotient questions: the respiratory quotient values for different fuels.
Agriculture link: ripening, post-harvest loss and storage technology.
Food safety link: bans on artificial ripening agents and the role of FSSAI.
Common errors: believing that plants respire only at night.

As an analytical point, post-harvest losses of fruits and vegetables in India are high. Managing the respiration of produce with cooling and storage is one of the cheapest ways to cut waste.

Why Is This Topic Important?

Facts RQ values asked
Compare Respiration and photosynthesis
Link Ripening and storage

What Came Next?

The next topic covers economic zoology and beneficial animals. Next topic: Economic Zoology – Beneficial Animals: Corals, Earthworms (Vermiculture), Prawns, Lobsters, Crabs, Pearl Oysters, Fishes.

Key People and Terms

Respiratory quotient CO2 released divided by O2 used
Climacteric Rise in respiration during ripening
Ethylene Ripening hormone
Thermogenesis Heat production
Lenticel Pore in woody stem
Photorespiration Oxygen uptake by RuBisCO in light

Exam Corner

Points to Remember

RQ of carbohydrate 1; fat about 0.7.
Plants respire day and night.
Ethylene ripens fruits.
Banana and mango are climacteric.
Gas exchange via stomata and lenticels.

Do Not Confuse With

Respiration and photosynthesis Releases versus stores energy
Climacteric and non-climacteric fruit Respiratory rise versus none
Respiration and photorespiration ATP making versus wasteful oxygen uptake

Memory Hook

RQ one for sugar, point-seven for fat; plants breathe day and night; ethylene ripens banana, mango right.

Mains Angle

Questions link plant respiration with post-harvest management. These points add depth.

Process Stages and gas exchange
RQ Fuel indicator
Use Storage and ripening

A Question You May Face

An original practice question, not a past paper question.

“Compare respiration and photosynthesis in plants and explain the importance of controlling respiration in the storage of fruits and grains.”

How to Answer

Process Same stages.
RQ Fuel.
Compare Photosynthesis.
Storage Cold, low oxygen.

GS Relevance

Prelims: respiration in plants versus photosynthesis, respiratory quotient, fruit ripening, storage. GS Paper 3: science and technology, agriculture.

Frequently Asked Questions

Do plants respire at night as well as in the day?

Yes, plants respire all the time, in the day and at night, in all their living cells. In the day, photosynthesis is usually faster than respiration, so there is a net release of oxygen, while at night only respiration continues, and plants take in oxygen and release carbon dioxide.

What is the respiratory quotient?

The respiratory quotient is the ratio of the volume of carbon dioxide released to the volume of oxygen used in respiration. It is 1 when carbohydrates are used, about 0.7 for fats and less than 1 for proteins, and it helps to show which fuel a tissue, such as a germinating seed, is respiring.

What are climacteric fruits?

Climacteric fruits, such as banana, mango, apple and tomato, show a sudden rise in respiration and in ethylene production as they ripen, and can ripen after being picked. Non-climacteric fruits, such as grapes, citrus and strawberry, do not show this rise and ripen only on the plant.

How can the respiration of stored produce be reduced?

Respiration of stored grains, fruits and vegetables can be slowed by lowering the temperature, keeping the moisture low, and reducing the oxygen and raising the carbon dioxide in storage, which is called controlled atmosphere storage. This reduces loss of food and delays ripening and spoilage.

How does respiration differ from photosynthesis?

Photosynthesis occurs only in green cells in light, uses carbon dioxide and water, makes sugar and releases oxygen and stores energy. Respiration occurs in all living cells all the time, uses sugar and oxygen, releases carbon dioxide and water, and releases energy as ATP, so the two are almost opposite.

PYQ Practice — Statement Analysis

1 The respiratory quotient of carbohydrates is 1.

True

Oxygen used equals carbon dioxide released.

2 Plants respire only during the night.

False

They respire day and night.

3 Ethylene promotes ripening of fruits.