Essay on Catabolism in Plants

Respiration:

i. It is a catabolic process

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ii. Plant respiration (Aerobic) is the biological process by which reduced organic compounds are mobillized and subsequently oxidised in the controlled manner.

iii. Main product of respiration is energy output.

iv. Free energy is released and incorporated into a form ATP that can be readily utilized for the maintenance and development of the plant.

C₆H₁₂O₆ ^oxidation → CO₂ ↑ + H₂O + ATP

v. Production of ATP is called phosphorylation (3types).

1. Photo phosphorylation: Light → ATP (light reaction)

2. Substrate level phosphorylation: Fructose 1, bis PO₄ → ATP → Glycolysis

3. Oxidative phosphorylation:

ADP → ATP → ETS.

Respiration Process:

1. Glycolysis → С₆Н₁₂О₆ → Pyruvate (Cytosol/cytoplasm)

2. AcetylcoA → Pyruvate → Acetyl COA (Mitochandrain Matrix inner wall)

3. TCA/Kreb cycle → Acetyl COA

→ ATP

FADH₂

NADH(Matrix of Mitochand rion)

4. Electron transport system →

FADH₂

NADH (Inner Mitochandrion Membrane)

Energy released during respiration can be calculated on the basis of stages of respiration

1. Glycolysis — 2 ATP

2. Kreb’s cycle — 2 ATP

3. ETS – 34 ATP (Total 38 ATP)

Glycolysis:

i. Greek word:

Glykos – Sugar

Lysis – Splitting

ii. The First stage of respiration, glucose (a six carbon molecule), is split into two 3 carbon molecules sugar are then oxidised and rearranged to yield 2 molecules of Pyruvate are partially oxidised and further oxidised through Link and Krebs cycle.

iii. Glycolysis occurs in all living organisms (Prokaryotes and Eukaryotes) .

iv. No oxygen is required to convert glucose to pyruvate and glycolytic mechanisms can become the primary mode of energy production in plant tissue when oxygen levels are low.

v. In animal cells the enzymes of glycolytic pathway do not exist independently in the cytosol but associated in a supermolocular complex that may loosely bound to the surface of outer mitochandrion membrane.

vi. This complex is thought to facilitate the conversion of substrate to products in the multistep glycolytic process.

vii. In this process phosphofructokinase is one of the control points in both plants and animals.

viii. This type of ATP synthesis is referred to as substrate level phosphorylation phosphat moiety from the substrate molecule to ADP to form ATP.

Alternative Glycolytic Pathway:

i. The synthesis of glucose through reversal of the glycolytic pathway known as gluconogenesis.

ii. It is not common in plants, but it does operate in seed of some plants (esp. oil seeds sunflower).

iii. Carbon stored in the form of oil (Triglycerols). After seed germinate, the oil, are converted into sugar by gluconogenesis.

Respiratory quotient:

The molar ratio of carbon dioxide evolved per mole oxygen consumed in respiration is termed as the respiratory ratio, respiratory coefficient or respiratory quotient (RQ). If respiration proceeds according to the general equation given in the nature of the process, RQ should be equal near to 1 (unity). However due to different environmental factors and variations in the make-up of the substrate RQ may be deviate from this value. Actually, the quotient can vary from infinity (no oxygen taken in) to zero (no carbon dioxide given off). Some values found experimentally are presented in Table.

Table: Respiratory quotients in different plants (After Thomas et al., 1960):

Plant material-RQ

Leaves rich in carbohydrates-1.0

Darkened shoots of prickly pear (Opuntia)-0.03

Germinating starchy seeds-1.0

Germinating linseed (high fat)-0.64

Germinating bucksheat (high protein)-0.50

Germinating Peas 1.5-2.4

Acetyle COA Formation:

i. Inside the mitochandiral matrix, pyruvate is oxidatively decarboxylated by the enzyme pyruvate dehydrogenate to produce NADH, CO₂ and acetic acid.

ii. The acetic acid is linked via thioester bond to a sulphur-containing cofactor, со enzyme.

TCA/Kreb/Citric Acid Cycle:

i. It occurs in mitochondrial Matrix.

ii. 36 ATP released in Eukaryotes produce NADH, CO₂ and acetic acid.

iii. Acetic acid is linked via thioester bond to a sulphur-containing cofactor, со enzyme.

Electron Transport System:

(ETS) (Oxidative phosphorylation)

i. Here the high energy electron capture during the TCA cycle (In the form of FADH₂ and NADH) can be converted into ATP.

ii. This O₂ dependent process occurs on the inner mitochondrial membrane and invites series of electron carriers known as electron transport chain.

iii. Two molecules of NADH from cytosol, 8 molecules of NADH and Two molecules of FADH₂ from TCA – appear in mitochandrial matrix.

iv. The individual electron transport proteins are organised into a series of four multiprotein complexes and localised in the inner membrane of mitochondrial matrix.

v. Complex I: TCA cycle NADH dehydrogenase, Flavin mono Nucleotide (FMN) and several sulphur proteins for the transfer of electrons.

vi. Complex II: TCA cycle enzyme succinyl dehydrogenase, electron derived from the oxidation of succinate are transfered via FADH₂.

vii. Complex III: Act as a ubiquinol: Cytochrome С Oxidoredutase, oxidising reduced ubiquinol and transfering electron through an iron-sulfur center, two b-type cytochrome and a membrane bound Cyt. C. Cytochrom С – is the only protein in ETC that is not an integral membrane properties, and it serves as a mobile carrier to transfer the electron from complex III to IV.

viii. Complex IV: Represents the cytochrom С oxidase. Which contain two copper centre (CUA and CUB) and Cyt a.

ix. Electron carriers within the mitochondrial inner-membrane allows for transfer of proton. The electron transport is associ­ated with a transfer of protons from the mitochondrial matrix to the inner- membrane space.

x. Complex V: This complex consist of two major components

F₁: Peripheral membrane protein complex contain atleast five different subunits and contains a catalytic centre for converting. ADP + Pi to ATP.

F₂: Integral membrane protein complex that consists of atleast three different polypeptides that form the channel through which protons are able to cross the inner membrane.

Uncouplers:

Dinitrophenol. PMA break the membrane leads to free flow H⁺ atoms.

So, no energy production.

Factors affecting Respiration:

External Factors —

1. Oxygen:

O₂ is a important factor because of its role as substrate in the over all process.

i. The diffusive movement of O2 represents a limitation on plant respiration.

2. Temperature:

i. In the physiological temperature range, the respiration is temperature dependent.

3. Injury:

Physical injury to plant tissues often stimulate

O₂ uptake because of increases in respiration and in mitochandria oxygen consuming enzyme activity as catalysed by lipoxygenase, polyphenol oxidase and peroxidase.

4. Water:

Proper hydration of respirating cell is essential for respiration.

5. Mechanical Effects:

Gentle rubbing or bending of leaf blade shows increased rate of respiration.

6. Light:

Indirect effect through synthesis of organic matter.

Internal Factors —

1. Protoplasmic Factor:

The amount of protoplasm in the cells and its state of activity influence rate of respiration.

(i) High rate of young meristamatic cells which divide actively require more energy.

(ii) Rate of respiration is undoubtedly affected by internal structural relationship of various cell organells and occurance and distribution of enzymes.

2. Concentration of Respiratory Factor or Substrate:

i. Favourable and increased concentrate respirable food material brings about an increase in the rate of respiration.

ii. Under starvation condition, leaves are etiolated and slow downs the respiration.