Respiration in Plants

What is Plant Respiration?

“Plant respiration is the chemical reaction by which plants cells stay alive.” The process of respiration is expressed as: 

Glucose + Oxygen → Carbon Dioxide + Water (+ Energy)

Do Plants Breathe?

The answer to this question is not direct. Yes, plants need oxygen for respiration but at the same time they also give out carbon dioxide. Thus, plants have proper system to ensure the availability of oxygen. Unlike animals, plants do not possess any specialized organs for exchange of gases but they have lenticels and stomata (present in stems and leaves respectively) that carry out the function of gaseous exchange.

Plants do not have any specialized organ to respire and exchange gases because each part of the plant takes care of the need of gases themselves. The parts of the plant do not display any great demand for exchange of gases. Added to this, stems, leaves and roots respire at very lower rate as compared to animals. But during the process of photosynthesis, large exchange of gases takes place and each part of the plant is well adapted to fulfill its need of gases. Availability of oxygen is not a problem during photosynthesis because the cells release oxygen within cells. It is important to note that each living cell in a plant is located quite close to the surface of the plant and in case of stems, the living cells are arranged in the form of thin layers beneath and inside the bark and have openings which are referred as lenticels. Thereby, the respiration and translocation takes place at every part of the plant.

The complete combustion of glucose produces H₂O and CO₂ as end products and release energy in the form of heat. In case, this energy is required by the cell, it will utilize accordingly. Following reaction explains the entire process:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

During the process of respiration, O₂ is utilized and carbon dioxide, energy and water are released as products. There is also a situation when then the oxygen is not available. For instance, the first cell on this planet must have carried out reaction in the absence of oxygen and even in the current living world we are aware of several living organisms adapted to anaerobic conditions. Some of these organisms are facultative and some are obligate. In any of these cases, all living organisms retain enzymatic machinery to partially oxidize glucose in the absence of oxygen. This process is also called as Glycolysis which includes breaking down of glucose to Pyruvic Acid. 
 

Respiration In Roots

The process of respiration in roots is carried out in the following manner: 

• Air occurs in several interspaces of soil. The hairs of the roots are in direct contact with them.

• Oxygen of the soil gets diffused via root hairs and reaches all internal cells of the root for respiration.

• Carbon dioxide produced during the diffusion is released in the opposite direction.

• In the condition of water logging, air gets deficient in soil and in this case, metabolic activity of the roots declines.
   

Respiration In Stems

• The stems of herbaceous plants possess stomata and the air gets diffused via it and reaches the cells for respiration.

• The carbon dioxide produced during the process gets diffused in the air via stomata.

• When the stems are woody, this gaseous exchange is carried out by lenticels.
   

Respiration In Leaves

• Leaves of the plants have tiny pores which are referred as stomata. The exchange of gases takes place by the process of diffusion via stomata. The stomata are present in large number on lower surface of leaves of plant. Each stoma is surrounded and controlled by Guard Cells (two kidney shaped cells). Then the stoma, open gaseous exchange takes place between Atmosphere and Interior of Leaves.

Types of Respiration

Respiration is of two types: 

• Aerobic Respiration: In this type of respiration, the food substances are completely oxidized into H₂O and CO₂ with the release of energy. It requires atmospheric oxygen and all higher organisms respire aerobically. Following figure shows the steps included in Aerobic Respiration.

• Anaerobic Respiration: In this type of respiration, partial oxidation of food takes place and energy is released in the absence of oxygen. This type of respiration occurs in prokaryotic organisms like bacteria and yeast. Ethyl alcohol and carbon dioxide are formed in this process.

Glycolysis

The term glycolysis is derived from two Greek words, i.e. Glycos which means sugar and lysis means splitting. The scheme of glycolysis was given by Otto Meyerhof, J. Parnas and Gustav. In case of anaerobic respiration, respiration is carried out via glycolysis which occurs in cytoplasm of the cells. In it, partial oxidation of glucose is carried out resulting in two molecules of pyruvic acid. Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate via enzyme hexokinase. This phosphorylated glucose is isomerized to produce fructose-6-phosphate. 

The several steps of Glycolysis are depicted in the figure below. In this process, chain of ten reactions takes place under the control of various enzymes and the outcome is pyruvate. ATP is utilized at two steps:

• During the conversion of glucose into glucose-6-phosphate.

• During the conversion of fructose-6-phosphate in fructose 1 and 6-diphosphate.

The fructose 1, 6 diphosphate is broken into

(i). Dihydroxyacetone Phosphate and

(ii). 3-Phosphoglyceraldehyde (PGAL). 

Fermentation

In this process, the incomplete oxidation of glucose is carried out under anaerobic conditions via set of reactions where pyruvic acid is converted into ethanol and carbon dioxide. These reactions are catalyzed by two enzymes, i.e. achohol dehydrogenase and acid-decarboxylase. Other organisms such as bacteria produce lactic acid from pyruvic acid. The detailed steps are depicted in the figure below. In animal cells as well, during muscle exercise, in case of inadequacy of oxygen for cellular respiration, pyruvic acid is reduced to lactic acid by lactate dehydrogenase. NADH⁺H⁺ are the reducing agent which is oxidized to NAD⁺ in the process.

In both alcohol and lactic acid fermentation, very less energy is released. Both these process are hazardous because alcohol or acid is produced during the process. Fermentation process is used in our daily life such as in the formation of curd, vinegar, bread and alcoholic drinks.

Aerobic Respiration

For aerobic respiration to take place in mitochondria, pyruvate is transported into mitochondria from cytoplasm. The most important events in this respiration are: 

• The hydrogen atoms, that leaves 3 molecules of CO₂.
• Passing on of electrons removed as a part of hydrogen atoms to molecular oxygen with simultaneous synthesis of Adenosine Triphosphate (ATP).

Pyruvate, formed during glycolytic catabolism of carbohydrates in cytosol, enters the matrix of mitochondria and it undergoes oxidative decarboxylation by the complex set of reaction. This entire process is catalyzed by pyruvic dehydrogenase and this reaction requires involvement of several coenzymes such as Coenzyme A and NAD⁺.

During this entire process, 2 molecules of NADH are produced from the metabolism of 2 molecules of pyruvic acid. The acetyl CoA enters into a cyclic pathway called as Kreb’s cycle or tricarboxylic acid. The name Krebs Cycle is mentioned after the name of scientist Hans Kreb who first elucidated this cycle.
 

Tricarboxylic Acid Cycle

It is the second stage of cellular respiration. It plays an integral role in catabolism of breaking down of organic fuel molecule i.e. glucose, sugar, fatty acid and amino acids. The cycle starts with the condensation of acetyl group with oxaloacetic acid and water to release citric acid. The overall reaction of Krebs cycle is – 

2 acetyl CoA + 6NAD⁺ + 2FAD + 2ADP + 2P_i → 4 CO₂ + 6 NADH + 6H⁺ + 2 FADH₂ + 2ATP

This reaction is catalyzed by citrate synthase enzyme and a molecule of CoA is released. This citrate is then isomerized to isocitrate followed by decarboxylation that results in the formation of α-ketoglutaric acid and succinyl-CoA. Then succinyl-CoA is oxidized to OAA allowing the cycle to continue. During this conversion of succinyl-CoA to succinic acid one molecule of GTP is synthesized. In a coupled reaction GTP is converted to GDP along with the synthesis of ATP from ADP. Added to this, at three places in the entire cycle, NAD⁺ is reduced to NADH + H⁺ and at one point FAD⁺ is reduced to FADH₂. The entire cycle is shown in the figure below :