Respiration at a glance: overview of glycolysis, krebs cycle, ETC, pentose phosphate pathway, energetics of respiration, fate of pyruvate

1. Theory
Introduction to crop physiology and its importance in Agriculture; Plant cell: an Overview; Diffusion and osmosis; Absorption of water,
transpiration and Stomatal Physiology; Mineral nutrition of Plants: Functions and deficiency symptoms of nutrients, nutrient uptake mechanisms;
Photosynthesis: Light and Dark reactions, C3, C4 and CAM plants; Respiration: Glycolysis, TCA cycle and electron transport chain; Fat
Metabolism: Fatty acid synthesis and Breakdown; Plant growth regulators: Physiological roles and agricultural uses, Physiological aspects of
growth and development of major crops: Growth analysis, Role of Physiological growth parameters in crop productivity.
Practical
Study of plant cells, structure and distribution of stomata, imbibitions, osmosis, plasmolysis, measurement of root pressure, rate of transpiration,
Separation of photosynthetic pigments through paper chromatography, Rate of transpiration, photosynthesis, respiration, tissue test for mineral
nutrients, estimation of relative water content, Measurement of photosynthetic CO2 assimilation by Infra Red Gas Analyser (IRGA).
References
1. Taiz, L. and zeiger,E. 2010. Plant Physiology 5th edition, Sinauer Associates, Sunderland, MA, USA. 2. Gardner, F.P., Pearce, R.B., and
Mitchell, R.L. 1985. Physiology of Crop Plants, Scientific Publishers, Jodhpur. 3. Noggle, G.R. and Fritz, G.J., 1983. Introductory Plant Physiology.
2nd Edition. Prentice Hall Publishers, New Jersey, USA
Fundamentals of Crop Physiology [ASPH1201]
Pradipta Banerjee, Ph.D.
Dept. of Biochemistry & Crop Physiology
CUTM, Paralakhemundi, Odisha

2. 1. What are Respiratory substrates
2. Aerobic Respiration (mitochondria)
3. Anaerobic respiration (cytosol)
4. Oxidation of carbohydrates in 3 phases: Glycolysis, TCA cycle, ETS or Terminal respiration
5. Why TCA cycle is link between carbohydrate and protein metabolism?
6. TCA is an amphibolic pathway. Explain why?
7. Oxidative and substrate level phosphorylation
8. Significance of Pentose Phosphate Pathway
9. What is Pasteur Effect?
10. What is Crabtree Effect?
11. What is Extinction Point?
12. What is Bioluminescence
13. What is Respiratory Quotient?
14. Factors affecting Respiration in plants
FIND OUT…

3. The minimum amount of oxygen, at which aerobic respiration takes place & anaerobic respiration become extinct
is called as extinction point.
The ratio of the volume of CO2 released to the volume of O2 taken in respiration is called RQ. Value of RQ
depends upon the type of respiratory substrate used & measured by Ganong's respirometer.

4.  The Crabtree Effect (named after the English biochemist Herbert Grace Crabtree) occurs in metabolically
adapted cell lines that are grown in hypoxic/anaerobic conditions with high glucose.
 These cells have a reduced need for oxidative phosphorylation by the TCA cycle and depend on glycolysis as
their major source of energy instead of mitochondria; therefore these cells also have a reduced susceptibility
to mitochondrial toxicants.
 The addition of galactose circumvents the Crabtree Effect and increases the reliance of cells on
mitochondrial oxidative phosphorylation; these cells are more sensitive to mitochondrial toxicity.
Crabtree Effect
When the concentration of oxygen is lowered, respiration of mitochondria decreases, and aerobically
produced CO2 declines with an increase in fermentation, which is related to the low-oxygen consequences and
a rise in overall CO2 production causing glycolysis increase, known as the Pasteur effect
Pasteur Effect

5. Outline of Respiratory Metabolism
1. Glycolysis or EMP (Embden-Meyerhof-Parnas) Pathway or Hexose Diphosphate
Pathway
2. Pentose Phosphate Pathway
3. TCA Cycle
4. ETS

6. General Outline of Carbohydrate metabolism

7. In animals, excess glucose is converted to its storage form, glycogen (plant – starch), by glycogenesis. When glucose
is needed as a source of energy or as a precursor molecule in biosynthetic processes, glycogen is degraded by
glycogenolysis. Glucose can be converted to ribose-5-phosphate (a component of nucleotides) and NADPH (a
powerful reducing agent) by means of the pentose phosphate pathway. Glucose is oxidized by glycolysis, an energy-
generating pathway that converts it to pyruvate. In the absence of oxygen, pyruvate is converted to lactate. When
oxygen is present, pyruvate is further degraded to form acetyl-CoA. Significant amounts of energy in the form of
ATP can be extracted from acetyl-CoA by the citric acid cycle and the electron transport system. Note that
carbohydrate metabolism is inextricably linked to the metabolism of other nutrients. For example, acetyl-CoA is
also generated from the breakdown of fatty acids and certain amino acids. When acetyl-CoA is present in excess, a
different pathway converts it into fatty acids.
General Outline of Carbohydrate metabolism

8. GLYCOLYSIS

9. GLYCOLYSIS
Each phosphoryl group, represented here as P, has two negative charges (--PO3
2-).

14. FATE OF PYRUVATE GENERATED IN GLYCOLYTIC PATHWAY

16.  The first phase of the
pentose phosphate pathway
consists of two oxidations
that convert glucose 6-
phosphate to ribulose 5-
phosphate and reduce
NADP+ to NADPH. The
second phase comprises
non-oxidative steps that
convert pentose phosphates
to glucose 6-phosphate,
which begins the cycle
again.
 Entry of glucose 6-
phosphate either into
glycolysis or into the
pentose phosphate pathway
is largely determined by the
relative concentrations of
NADP and NADPH.
PENTOSE PHOSPAHTE PATHWAY

17. General scheme of the pentose phosphate pathway. NADPH formed in the oxidative phase is used to reduce glutathione, GSSG and
to support reductive biosynthesis. The other product of the oxidative phase is ribose 5-phosphate, which serves as precursor for
nucleotides, coenzymes, and nucleic acids. In cells that are not using ribose 5-phosphate for biosynthesis, the non-oxidative phase
recycles six molecules of the pentose into five molecules of the hexose glucose 6 phosphate, allowing continued production of NADPH
and converting glucose 6-phosphate (in six cycles) to CO2.
Non-oxidative phase
PENTOSE PHOSPAHTE PATHWAY

18. Significance of Pentose Phosphate Pathway in plants

19. Differences between Glycolysis and Kreb’s Cycle

22. KREB’S CYCLE
1. Condensation
2. Dehydration
3. Hydration
4. Oxidative decarboxylation
5. Oxidative decarboxylation
6. Substrate level phosphorylation
7. Dehydrogenation
8. Hydration
9. Dehydrogenation

23. The citric acid cycle is amphibolic, serving in both
catabolism and anabolism; cycle intermediates
can be drawn off and used as the starting material
for a variety of biosynthetic products.
The citric acid cycle (Krebs cycle, TCA cycle) is a
nearly universal central catabolic pathway in
which compounds derived from the breakdown of
carbohydrates, fats, and proteins are oxidized to
CO2, with most of the energy of oxidation
temporarily held in the electron carriers FADH2
and NADH. During aerobic metabolism, these
electrons are transferred to O2 and the energy of
electron flow is trapped as ATP.

25. Significance of Kreb’s Cycle

26. Substrate Level Phosphorylation
 Substrate level phosphorylation involves the
enzyme catalyzed transfer of inorganic
phosphate from a molecule to ADP to form ATP.
 The synthesis of ATP via ETS with oxygen as
terminal electron acceptor, is known as
oxidative phosphorylation and it takes place
in mitochondria.

27. Organization of the
electron transport chain
and ATP synthesis in the
inner membrane of plant
mitochondria.
In addition to the five
standard protein complexes
found in nearly all other
mitochondria, the electron
transport chain of plant
mitochondria contains five
additional enzymes marked
in green. None of these
additional enzymes pumps
protons. Specific inhibitors,
rotenone for complex I,
antimycin for complex III,
cyanide for complex IV, and
salicylhydroxamic acid
(SHAM) for the alternative
oxidase, are important tools
to investigate the electron
transport chain of plant
mitochondria.
Electron Transport Chain

28.  ATP is the energy carrier used by cells to drive living processes, and chemical energy conserved during the
citric acid cycle in the form of NADH and FADH2 (redox equivalents with high-energy electrons) must be
converted to ATP to perform useful work in the cell. This O2-dependent process, called oxidative
phosphorylation, occurs in the inner mitochondrial membrane.
 The Electron Transport Chain catalyses a flow of electrons from NADH to O2,the final electron acceptor
of the respiratory process. For the oxidation of NADH, the overall two-electron transfer can be written as
follows: NADH + H+ + 1⁄2 O2 → NAD+ + H2O
 For each molecule of sucrose oxidized through glycolysis and the citric acid cycle pathways, 4 molecules of
NADH are generated in the cytosol and 16 molecules of NADH plus 4 molecules of FADH2 (associated with
succinate dehydrogenase) are generated in the mitochondrial matrix. These reduced compounds must be
re-oxidized or the entire respiratory process will come to a halt.
Electron Transport Chain – Key Points

29.  Each step of ETS is characterized by decrease in energy level
 The carriers presumably operate in order of an increasing tendency to undergo
reduction (reducing potential becomes increasingly positive from NADH through
cyctochrome a3
Electron Transport Chain – Key Points

30. Complex I (NADH dehydrogenase). Electrons from NADH generated in the mitochondrial matrix during the citric
acid cycle are oxidized by complex I (an NADH dehydrogenase). The electron carriers in complex I include a tightly
bound cofactor (flavin mononucleotide [FMN], which is chemically similar to FAD; and several iron–sulfur centers.
Complex I then transfers these electrons to ubiquinone. Four protons are pumped from the matrix to the
intermembrane space for every electron pair passing through the complex.
Ubiquinone, a small lipid-soluble electron and proton carrier, is located within the inner membrane. It is not
tightly associated with any protein, and it can diffuse within the hydrophobic core of the membrane bilayer.
Complex II (succinate dehydrogenase). Oxidation of succinate in the citric acid cycle is catalyzed by this complex,
and the reducing equivalents are transferred via the FADH2 and a group of iron–sulfur proteins into the ubiquinone
pool. This complex does not pump protons.
Complex III (cytochrome bc1 complex). This complex oxidizes reduced ubiquinone (ubiquinol) and transfers the
electrons via an iron–sulfur center, two b-type cytochromes (b565 and b560), and a membrane-bound cytochrome
c1 to cytochrome c. Four protons per electron pair are pumped by complex III.
Cytochrome c is a small protein loosely attached to the outer surface of the inner membrane and serves as a mobile
carrier to transfer electrons between complexes III and IV.
Components of Electron Transport Chain

31. Complex IV (cytochrome c oxidase). This complex contains two copper centers (CuA and CuB) and cytochromes a
and a3. Complex IV is the terminal oxidase and brings about the four-electron reduction of O2 to two molecules of
H2O. Two protons are pumped per electron pair.
Both structurally and functionally, ubiquinone and the cytochrome bc1 complex are very similar to plastoquinone and
the cytochrome b6 f complex, respectively, in the photosynthetic electron transport chain.
The FoF1-ATP synthase (also called complex V) consists of two major components, F1 and Fo. F1 is a peripheral
membrane protein complex that is composed of at least five different subunits and contains the catalytic site for
converting ADP and Pi to ATP. This complex is attached to the matrix side of the inner membrane. Fo is an integral
membrane protein complex that consists of at least three different polypeptides that form the channel through which
protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic cycle of the
F1 component of the ATP synthase, allowing the ongoing synthesis of ATP. For each ATP synthesized, 3 H+ pass
through the Fo from the intermembrane space to the matrix down the electrochemical proton gradient.
…Components of Electron Transport Chain

32. 1st -5th reactions = Glycolysis; 6th reaction = Oxidative decarboxylation of Pyruvate; 7th -11th reactions = TCA Cycle