We respire to obtain energy which is used to do work. During respiration, food is oxidised in our cells and energy is released. But what about plants? Plants lead a sedentary life and spend their entire lives fixed to a specific location, do you think they need energy? Of course they do, even though their energy requirements are much less compared to that of animals. So, needless to say plant cells also carry out respiration.
In plants, photosynthesis and respiration, both occur during the day time. Through photosynthesis, food is synthesised from inorganic substances and through respiration, these food molecules get oxidised to produce energy.
Let’s discuss respiration in plants in more detail.
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All living beings require energy to survive and the primary source of energy for all living beings is the Sun. Plants help to fix this energy during photosynthesis and convert it into the chemical energy stored in the food they synthesise. This energy is transferred to all other forms of life through the food chain. The energy stored in the food we consume is released by oxidising it through a process known as respiration. Most organisms require oxygen for respiration, which is obtained from the atmosphere, and in the process produce carbon dioxide as a waste product, which is released into the atmosphere. Thus, respiration involves two processes - a physical process called breathing during which oxygen is taken in and carbon dioxide is given out, and a chemical process which involves chemically breaking down or oxidising food materials to release energy. The latter is known as cellular respiration.
All cells break down organic molecules, mainly carbohydrates, to release energy. If this energy is released at once, there might be a brief blast of light and heat leading to death of the cell. Living things cannot withstand this sudden release of energy, so energy rich molecules are synthesised in the cells to release energy in a stepwise process. The process of oxidative breakdown of food materials within the cell to release energy and trap it in the form of ATP is called cellular respiration.
The process of breathing in plants is much different compared to that in animals. Plants, unlike humans and other animals, don't have an elaborate respiratory system and specialised respiratory organs for the exchange of gases, but they do have specialised structures like stomata (found in leaves) and lenticels (found in stems) that play a part in it. This is because the energy requirements in plants is much less and the rate of respiration in plant parts is much slower than that in animals. Thus, each plant part can take care of its own requirement for gaseous exchange.
Respiratory gases cannot diffuse over very far distances and in the absence of a respiratory system they cannot be transported over long distances either. But, plants have multiple mechanisms to overcome this shortcoming. In plants the process of photosynthesis and respiration occur simultaneously during daytime. The oxygen released as a result of photosynthesis can be directly used up for respiration by the leaf cells. The living plant cells are also situated close to the plant surface so that respiratory gases can directly be exchanged through the tiny apertures (lenticels and stomata) present on the surfaces of leaves, stems and roots.
The mature and woody roots of plants have openings called lenticels through which exchange of gases can occur. The root hair cells can also directly exchange gases with the air present in the soil and can help in taking in oxygen for respiration and giving out carbon dioxide produced during respiration.
Young green stems have stomata in their epidermis which are minute pores that open during the day to allow the exchange of gases for respiration and photosynthesis. Woody stems have permanent openings called lenticels in their impervious bark which allow the oxygen to diffuse in for respiration and carbon dioxide to diffuse out. The living cells are present just beneath the bark and are thus in contact with the air moving in and out of the lenticels.
The epidermis of leaves consist of tiny pores known as stomata which help in exchange of gases in leaves. The stomata remain open during the daytime and closed during night. This opening and closing of stomata is controlled by the guard cells which surround them. During daytime, the leaf mesophyll cells carry out respiration and photosynthesis simultaneously and the rate of photosynthesis is usually greater than that of respiration. So the oxygen produced due to photosynthesis is directly used up by the leaf cells for respiration and the excess is released into the atmosphere. The carbon dioxide produced by the cells as a result of respiration is directly used up by them for photosynthesis and the excess required is taken in from the atmosphere through the stomata.
At night, photosynthesis stops but respiration continues. Thus the leaf cells produce carbon dioxide and require oxygen for respiration. Although the stomata close during the night, the respiratory gases continue to diffuse in and out through the tiny gap between the guard cells
On the basis of respiratory substrate, respiration is of two types:
Fats or carbohydrates are used as respiratory substrates and broken down during floating respiration. This is the type of respiration that takes place in a living cell most frequently. As no harmful chemicals are produced during floating respiration, floating respiration keeps the cell in good health.
Protein serves as the respiratory substrate in protoplasmic respiration. When the body is starving and lacks fat or carbohydrates, it uses protein instead. This is a unique type of respiration that doesn't happen frequently during life. Byproducts of protoplasmic respiration are generally toxic.
There are two types of respiration:
This kind of respiration requires oxygen for its occurrence. High-energy organic substances like glucose are completely broken down into low-energy substances like water and carbon dioxide with the assistance of oxygen. Plants normally
Aerobic respiration releases a significant amount of energy most of which is in the form of heat and the rest is stored as chemical energy in the high energy bonds of the ATP (adenosine triphosphate) molecule. A single glucose molecule undergoes complete oxidation in this process to yield 38 ATP molecules.
Aerobic respiration involves -
Plants are never completely devoid of oxygen but they do suffer from low availability of oxygen under flood conditions or water-logging in the soil. At such times, the root cells undergo anaerobic metabolism to break down glucose via the glycolytic pathway in the cytoplasm to generate energy. As the oxygen availability is negligible, the rest of the processes that are carried out in the mitochondria during aerobic respiration cannot be carried out during anaerobic respiration. The amount of energy released during anaerobic metabolism of glucose is much less, that is only two molecules of ATP are generated. The pyruvate produced can further be fermented to form ethyl alcohol or lactic acid by alcoholic fermentation or lactic acid fermentation. Let’s see how.
Firstly, pyruvic acid/pyruvate undergoes decarboxylation and reduction to form acetaldehyde and carbon dioxide. Acetaldehyde is further reduced to ethyl alcohol.
Fig: Ethyl alcohol fermentation
Pyruvate can also be converted to lactic acid in presence of the lactate dehydrogenase enzyme. No carbon dioxide is generated in this type of fermentation.
Fig: Lactic acid fermentation
Q1. During daytime, which gas do the leaves take in?
C. Carbon dioxide
Solution: During daytime, the leaf mesophyll cells carry out respiration and photosynthesis simultaneously and the rate of photosynthesis is usually greater than that of respiration. So the oxygen produced due to photosynthesis is directly used up by the leaf cells for respiration and the excess is released into the atmosphere. The carbon dioxide produced by the cells as a result of respiration is directly used up by them for photosynthesis and the excess required is taken in from the atmosphere through the stomata. Hence, the correct option is c.
Q2. Differentiate between aerobic and anaerobic respiration.
Answer: The differences are -
Requires O2 for its occurrence
Does not require O2 for its occurrence
Glucose is completely oxidised to yield CO2 and H2O.
Glucose is partially oxidised to form ethanol and CO2 or lactic acid.
38 ATP molecules are generated
2 ATP molecules are generated.
Q3. Plants respire through the _________ on their young stems and leaves.
Solution: The little pores known as stomata are found on the epidermis of leaves and young green stems. Stomatal pores allow respiratory gases to diffuse in and out along their concentration gradient. Hence, the correct option is a.
Q4. Do plants respire only during night time?
Answer: Plants respire both during the day and at night. Both photosynthesis and respiration occur during the day. While photosynthesis predominates during the day, respiration is the only activity going on at night.
Q1. How respiration in plants differs from respiration in animals?
Answer: Plants manufacture their own sugars through photosynthesis, which is then oxidised for energy during respiration. However, animals obtain sugars by feeding on plants or other animals. These complex sugars are digested to simple sugars such as glucose, fructose, etc which can be oxidised via respiration to generate energy.
Q2. Why do germinating seeds show a higher rate of respiration?
Answer: To support the cell-building processes necessary to break and open the seed and create the initial root and stem structures, the rates of cellular respiration increase. In seeds that are germinating, oxygen consumption is high because the seeds are alive and require more oxygen to thrive.
Q3. Which types of leaves show maximum rate of respiration?
Answer: Young leaves show maximum rate of respiration as compared to mature and senescent leaves.
Q4. What is substrate level phosphorylation?
Answer: Substrate level phosphorylation is the kind of metabolic reaction via which high energy molecules such as ATP or GTP are produced by direct transfer of a phosphate group from a substrate to ADP or GDP.
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