Respiration in Plants

Respiration in Plants

We all need to breathe in order to stay alive. Do plants also breathe in order to stay alive? If so, how? What is the process of respiration in plants? Do plants also possess any specialized organs for the exchange of gases like animals? What is plant respiration all about? Let us find answers to all these questions within this article about plant respiration.

What is Plant Respiration?

The process by which cells obtain chemical energy by the consumption of oxygen and the release of carbon dioxide is known as respiration. In order to carry on respiration, plant cells need oxygen and a means of disposing of carbon dioxide just as animal cells do. In plants, each part such as root, stem performs respiration because plants do not possess any specialized organs like animals for the exchange of gases.

The process of respiration is expressed as:

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

We can conclude same from the above equation as well that respiration uses oxygen and produces carbon dioxide.

Do Plants Breathe?

This is the major question when we think about plant respiration. Since plants do not possess any specialized organs like lungs so we can say plants do not breathe rather they respire. Plants respire through lenticels and stomata (present in stems and leaves respectively) which carry out the function of the gaseous exchange.

Breathing refers to the process of taking air into and expelling it from the lungs which simply means that plants do not breathe, they respire.

Respiration In Roots

In plants respiration also takes place through roots. In soil oxygenated air is present in air spaces between soil particles. This oxygen is absorbed into the roots by root hair present on the roots. The hairs of the roots are in direct contact with them. In fact, a root hair is a lateral tubular outgrowth of the outer epidermal cells of a root. The oxygen present in between the soil particles diffuse into the root hairs. From root hairs, oxygen is supplied to all the parts of roots for respiration. During respiration, oxygen is converted into carbon dioxide gas which is diffused in opposite direction i.e. out of the roots by the same root hairs which complete the process of respiration in roots.

Respiration In Stems

In the plants having herbaceous stem exchange of gases takes place through stomata and the carbon dioxide produced during the process gets diffused into the air via stomata only. While in the plants having hard and woody stems the exchange of gases takes place through lenticels. Lenticels are actually loosely packed dead cells which are present as small pores on the bark of woody plants or old roots. These enable oxygen to reach the intercellular spaces of the interior tissues and carbon dioxide to be released into the atmosphere by the process of diffusion which completes the process of respiration in stems.

Respiration In Leaves

In leaves, the exchange of respiratory gases takes place through very tiny pores called stomata. The stomata are present in large number on the lower surface of leaves of the plant. Each stoma has a pore at its centre which is surrounded and controlled by two kidney-shaped cells called guard cells. When the stoma opens the exchange of gases takes place between the atmosphere and interior of the leaf by the process of diffusion which completes the process of respiration in leaves.

Types of Respiration

There are two main types of respiration:

(i) Aerobic Respiration

This type of respiration leads to a complete oxidation of stored food in the presence of oxygen and releases carbon dioxide, water and a large amount of energy present in the respiratory substrate. All higher organisms respire aerobically.

The process of aerobic respiration is expressed as:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

(ii) Anaerobic Respiration

This type of respiration occurs in complete absence of oxygen. In this type of respiration, partial oxidation of food takes place and energy is released in the absence of oxygen. Such respiration generally occurs in lower prokaryotic organisms like bacteria, yeast and fungi. This type of respiration results in incomplete oxidation of stored food and formation of carbon dioxide,  ethyl alcohol and sometimes various organic acids such as malic acid, citric acid, oxalic acid, tartaric acid etc.

The process of aerobic respiration is expressed as:

C6H12O6  → 2C2H5OH + 2CO2 + Energy

Differences Between Respiration and Photosynthesis

Photosynthesis is simply a process that is used by plants and certain kinds of bacteria to create energy from sunlight. Chlorophyll, as we know, is the green pigment in plants that is the reason behind this conversion process. All other living things rely on the process of respiration to remain alive. On the contrary, respiration is basically the concept of taking oxygen from the air and putting it through the lungs, which provides more oxygen to blood that can be used in the body. The carbon dioxide waste that is produced is expelled out of the lungs. Cellular respiration ideally uses glucose, or sugars, from food molecules and converts them into water, carbon dioxide, and ATP, a nucleotide that is essential to the body.

Let’s take a look at the primary differences between the both.

Photosynthesis Respiration
Photosynthesis takes place only in chlorophyll-containing cells of plants. Respiration happens in all animal and plant cell.
It Synthesizes foods It Oxidizes foods
It Stores energy It releases energy
Photosynthesis is an anabolic process Respiration is a catabolic process
It is an endothermal process It is an exothermal process
It comprises products like sugar, oxygen and water as products. It comprises products like hydrogen and carbon-dioxide.
During this process, radiant energy is converted into potential energy. During this process potential energy is converted into kinetic energy.
It happens only in the presence of light It happens continually both in light and in the dark

Mechanism of Respiration

There are two major steps in the mechanism of respiration:

(i) Glycolysis: During the process of respiration, carbohydrates are converted into pyruvic acid through a series of enzymatic reactions. This series of reactions is known as glycolysis which takes place in the cytosol.

(ii) Krebs cycle:  After glycolysis pyruvic acid enters mitochondria where several enzymes catalyse the reactions and pyruvic acid is finally converted into COand water. This series of enzymatic reactions is known as Krebs cycle.

Let us discuss the whole mechanism of plant respiration in more detail.

Glycolysis

The term glycolysis is derived from two Greek words, i.e. Glycos, which denotes sugar and lysis, means splitting. It is a term used to describe the sequential series of reactions present in a wide variety of tissues that starts with a hexose sugar (usually glucose) and ends with pyruvic acid. The scheme of glycolysis was given by Otto Meyerhof, J. Parnas and Gustav Embden and therefore referred as EMP pathway.

Glycolysis is the first stage in the breakdown of glucose. Glycolysis is common to both aerobic and anaerobic modes of respiration. Glycolysis occurs in the cytoplasm of cells. During this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.

In plants, glucose is derived from sucrose which is the end product of photosynthetic carbon reactions.

Sucrose is converted into glucose and fructose by the enzyme invertase. Now, these two monosaccharides ( glucose and fructose) enter glycolysis or EMP pathway.

Main Steps of EMP Pathway

  • Phosphorylation of Sugar: Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate and fructose-6-phosphate respectively, by the activity of enzyme hexokinase, in presence of ATR. The phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. After this isomerisation takes place with the help of enzyme phosphohexose isomerase.
  • Phosphorylation of Fructose-6-Phosphate: Now, fructose-6-phosphate is phosphorylated and fructose-1, 6-bisphosphate is produced by the action of enzyme phosphofructokinase in the presence of ATP.
  • Splitting: Now, fructose- 1, 6-bisphosphate splits into two molecules of triose phosphate i.e. 3-phosphoglyceraldehyde (PGAL) and dihydroxyacetone phosphate ( Di HAP) which are interconvertible.
  •  Oxidative Dehydrogenation: After formation of 3-phosphoglyceraldehyde (PGAL), the glycolytic pathway enters the energy conserving phase. Here, it is oxidized to a carboxylic acid i.e. 1,3-bisphosphoglycerate and NAD is reduced to NADH.
  •  Formation of ATP: In next step of glycolysis, 3-phosphoglycerate is formed from 1, 3-bisphosphoglycerate by the enzymatic activity of phosphoglycerate kinase.  ATP is generated during this process. Direct synthesis of ATP from intermediate metabolites is called substrate-level phosphorylation.
  •  Isomerisation: In next step, 3-phosphoglycerate converts into its isomer 2-phosphoglycerate by the catalytic activity of enzyme phosphoglyceromutase.
  • Dehydration: In next step, 2-phosphoglycerate converts into phosphoenolpyruvate (PEP) in the presence of enzyme pyruvate kinase and releases ATP.
  • Generation of ATP during Glycolysis: By direct transfer of phosphate to ATP and by oxidation of NADH produced during glycolytic pathway to NAD+.

 All steps can be summarized as:

Oxidative Decarboxylation of Pyruvic Acid

The pyruvic acid generated in the cytoplasm through glycolysis is transferred to mitochondria. This is the initiation of the second phase of respiration. As soon as, pyruvic acid enters the mitochondria, one of the three carbon atoms of pyruvic acid is oxidised to carbon dioxide in a reaction called oxidative decarboxylation. Here, pyruvate is first decarboxylated and then oxidised by enzyme pyruvate dehydrogenase. This enzyme is made up of a decarboxylase, lipoic acid, TPP, transacetylase and Mg+2.

The reaction involved is:

Krebs Cycle

It is the second stage of cellular plant respiration.  It is also known as a tricarboxylic acid cycle (i.e. TCA cycle). It discusses the role of pyruvate in the conversion of glucose hydrogens into fumarate. This cycle involves two decarboxylations and four dehydrogenations. It plays an integral role in the 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→ 4 CO2 + 6 NADH + 6H+ + 2 FADH2 + 2ATP

The starting point of Krebs cycle is the entrance of acetyl Co-A into a reaction to form citric acid. In the first reaction of Krebs cycle, one molecule of acetyl Co-A combines with 4-carbon oxaloacetic acid (OAA); with the result 6-carbon citric acid is produced, and Co-A is released. This reaction is catalysed by enzyme citrate synthase. Now, citrate (citric acid) is isomerised to isocitrate (isocitric acid). Then, Cis-aconitic acid is converted into isocitric acid with the addition of water in the presence of iron-containing enzyme aconitase.

During citric acid cycle (Krebs cycle) 3 molecules of NAD+ and one molecule of the FAD (Flavin Adenine Dinucleotide) are reduced to produce NADH and FADHrespectively. Along with that NADH and FADH are produced. Now, they are linked with electron transport system (ETS) and produce ATP by oxidative phosphorylation. At the end of Krebs cycle, glucose molecule is completely oxidised. From one glucose molecule, two pyruvic acid molecules are formed. After oxidation of one pyruvic acid molecule, three CO2 molecules are released. Thus, in total 6 molecules of CO2 are released.

The entire process can be summarized as:

Significance of Krebs Cycle

  • During Krebs cycle, carbon skeletons are obtained for use in growth and maintenance of the cell.
  • It is a cyclic pathway.
  • Many intermediate compounds are formed which are used in the synthesis of other biomolecules, such as amino acids, nucleotides, chlorophyll, cytochromes and fats.
  • It generates 6 NADH2, 2FADH2 and 2 ATP molecules on the breakdown of 2 acetyl–CoA molecules.
  • During this pathway, amino acids are synthesised from α-ketoglutaric acid, pyruvic acid and oxaloacetic acid.
  • Here succinyl Co-A acts as starting molecule for the synthesis of chlorophyll.
  • Krebs cycle is a major pathway for generation of ATP molecules, which make energy currency of the cell.
  • Energy is released from glucose and is used in various biochemical reactions.
  • Phenol, anthocyanin, etc., are produced from acetyl Co-A, whereas fatty acids are formed from glycerol.
  • Glutamic acid is formed from α-ketoglutaric acid; aspartic acid from oxaloacetic acid, and alanine from aspartic acid.
  • Amino acids are used in the synthesis of proteins, nucleic acids, purines and pyrimidines.
  • Succinyl Co-A carries synthesis of pyrrole compounds of chlorophyll, cytochrome and phytochrome.
  • Krebs cycle is directly related to nitrogen metabolism, α-ketoglutaric acid, an intermediate of Krebs cycle is first acceptor molecule of NH3 forming an amino acid, the glutamic acid. From glutamic acid, various transamination reactions begin to form different amino acids which ultimately condense to form proteins.
  • Krebs cycle is also intimately related to fat metabolism. Dihydroxyacetone phosphate produced in glycolysis may be converted into glycerol via glycerol-3-phosphate and vice versa. After β-oxidation, fatty acids give rise to active 2-C units, the acetyl Co-A which enters the Krebs cycle.

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