Respiration in Plants

• Respiration (cellular respiration) is a process of oxidation of food within the cell to extract energy and to convert this energy into ATP (energy currency of the cell). 
• Respiration is an Exergonic, Multistep, Amphibolic process (involves both catabolic and anabolic processes)

1.0What is Respiration in Plants?

• Respiration in plants is a biological process by which glucose molecules are broken down with other organic molecules to release energy in the form of ATP. This forms a basis for maintaining cellular activities in the form of growth, reproduction, and repair in plants.
• All living organisms including plants and animals respire to obtain energy.The mechanism of breakdown of food materials within the cell to release energy, and the trapping of this energy for synthesis of ATP is called respiration or the breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called respiration.

2.0Do Plant Breath ?

• Plants do not have an efficient gaseous transport system and also don't have respiratory organs like lungs. But, yet they can survive easily. 
• Why do plants not require an efficient gaseous transport system like a circulatory system?

 Reasons: 

• Each plant takes care of its own gas-exchange needs and there is very little transport of gasses from one plant to another.
• The distance that gasses must diffuse even in large, bulky plants is not great. Each living cell in a plant is located quite close to the surface of the plant. 

3.0Why Do Plants not require Respiratory Organs like Lungs? 

 Reasons:

• Plants do not present great demands for gas exchange.
• Roots, stems and leaves respire at rates far lower than animals do.
• Only during photosynthesis are large volumes of gaseous exchange and each leaf is well adapted to take care of its own needs during these periods.
• When cells photosynthesize, the availability of O₂ is not a problem in these cells since O₂ is released within the cell. 
• This is also facilitated by the loose packing of parenchyma cells in leaves, stems and roots, which provide an interconnected network of air spaces.

4.0Difference Between Respiration in Plants and Animals

5.0Respiratory Substrate

• The compounds that are oxidized during the process of cellular respiration are called respiratory substrates 
• These may be either carbohydrates, fats, proteins & organic acids (in some plants).
• In the absence or less availability of carbohydrates, the respiratory substrates can be fats or proteins. 

6.0Types of Respiration

• On the basis of type of respiratory substrates :

1. Floating respiration :

•  When carbohydrates or fats are oxidized inside the cell. 
• Carbohydrates and fats are floating inclusions of cells thus, this is called floating respirations.

 2. Protoplasmic respiration : 

• When protein is oxidized inside the cell. This occurs in starved cells.
• Protein is a constituent of protoplasm thus, this is called protoplasmic respiration.
•  On the basis of the presence or absence of O₂ :

 1. Aerobic respiration 

 2. Anaerobic respiration / Fermentation

Process of Respiration in Plants

• Respiration in plants is explained as the biochemical process by which plants transform stored energy in carbohydrates—like glucose—to form ATP with the release of carbon dioxide and water as side products. It happens inside the mitochondria of plant cells, producing energy for many cellular activities.
• The steps involved in respiration in plants are Glycolysis, Krebs Cycle (Citric Acid Cycle) and Electron Transport Chain.
• This entire process is needed to produce energy for maintaining several activities within the plant, such as nutrient uptake, growth, and overall development.

Glycolysis/Embden Meyerhof Parnas (EMP) Pathway:

• The glycolysis is a common phase between aerobic & anaerobic respiration.
• It takes place in the cytoplasm of the cell. 
• This process does not involve the consumption of oxygen & also does not liberate CO₂.
• In all living organisms whether it is aerobic or anaerobic, the first step in cellular respiration is partial breakdown or oxidation of glucose into two molecules of pyruvic acid and it is called glycolysis. 
• It involves a series of ten biochemical enzymatic reactions

7.0Fate of Pyruvic Acid

• Depends on the cellular needs 
• There are 3 major ways in which different cells handle pyruvic acid produced by glycolysis.
• These are- 

1. Lactic acid fermentation 

2. Alcoholic fermentation

3. Aerobic respiration 

8.0Aerobic Respiration

• For aerobic respiration to take place within the mitochondria, the final product of glycolysis i.e. pyruvic acid is transported from the cytoplasm into the mitochondria.
• The crucial events in aerobic respiration are: 

1. Link Reaction 

2. Krebs Cycle 

3. Electron Transport System (ETS) & Oxidative Phosphorylation

Link Reaction/Oxidative Decarboxylation/Transition Reaction

• This process takes place in the mitochondrial matrix.
• In this step, pyruvic acid is converted into acetyl Co-A by the activity of enzyme pyruvate dehydrogenase complex and this involves decarboxylation and dehydrogenation (Oxidative decarboxylation) in a single step process.
• This reaction requires five cofactors (Mg++, LA (Lipoic Acid), TPP(Thiamine pyrophosphate), NAD₊ , Coenzyme-A (Co-A).
• Acetyl Co-A thus formed in this process is now ready to enter into Krebs cycle.
• This process connects Glycolysis and Kreb's cycle, so it is called Link reaction or Gateway reaction.
• Acetyl Co-A is a connecting link between glycolysis

Krebs Cycle/TCA (Tricarboxylic Acid) Cycle/CA (Citric Acid) Cycle

• This cycle was discovered by H.A. Krebs. (Nobel Prize). 
•  Krebs cycle is also called the Citric acid (CA) cycle because the first compound is Citric acid (6C).
• Citric acid contains three COOH groups, so the process is also called the Tricarboxylic Acid (TCA) cycle. 
• In the Krebs cycle, oxaloacetic acid (OAA) is the first member and it also acts as the first acceptor of Acetyl Co-A and at the end of this cycle, OAA regenerates. 
• CA cycle occurs in the mitochondrial matrix

Mechanism of Krebs Cycle: 

Electron Transport System (ETS) or Respiratory Chain & Oxidative Phosphorylation

• ETS is the chain of some hydrogen and electron carriers present in the inner mitochondrial membrane. 
• In this system, all the reduced hydrogen acceptors like NADH⁺ + H⁺ and FADH₂ move to the ETS where they release their hydrogen and get re-oxidised to NAD⁺ & FAD⁺, so that they can again enter into the respiration process.
• Hence, this process is also called terminal oxidation of NADH + H⁺ and FADH₂.
• During this process, ATP and H₂O (metabolic water) are produced.
• CoQ/UQ is a mobile H(e^– + H⁺ ) carrier and Cyt-C is a mobile electron carrier. 
• Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system.
• Oxygen acts as the final hydrogen acceptor.
• During each step of mitochondrial ETS, redox reaction occurs and energy is released which is utilized in creation of proton gradient for the synthesis of ATP in presence of oxygen (Oxidative phosphorylation).

Component of Electron Transport System (ETS):

9.0Anaerobic Respiration/Fermentation 

• Anaerobic respiration in plants takes place under anaerobic conditions in many prokaryotes, unicellular eukaryotes and in germinating seeds.
• The products obtained by anaerobic respiration in plants are ethanol and carbon dioxide.
• During anaerobic respiration, glucose undergoes partial oxidation to create two molecules of ethanol and two molecules of carbon dioxide along with two ATPs.
• It is of different types-

1. Alcoholic fermentation: Occurs in yeast.

•  Alcoholic fermentation is used in the formation of beverages (alcoholic drinks) and Bread.
• Bread becomes puffed or spongy due to the release of CO₂ during the process.
• Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.

2. Lactic acid fermentation:

• In human muscles (during exercise when oxygen is inadequate). 
• Lactic acid fermentation is also performed by the bacteria Lactobacillus. 
• It is used in curd and other dairy products.
• Curds become sour due to excess lactic acid fermentation.
• In both lactic acid and alcohol fermentation not much energy is released; less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATPs