13. Respiration and Energy Transfer

-Created By-

Shri. Deshmukh A. B.

Asst. Teacher,

Agasti Arts, Commerce & Dadasaheb Rupwate Science College, Akole

Can you recall?

• 1. Which nutrients are used for energy production?
• 2. Which is most preferred nutrient among carbohydrate, protein and fat for energy production? Why?
• 3. Why do organisms take up oxygen and release carbon dioxide?
• 4. What is aerobic and anaerobic respiration?
• 5. Which steps are involved in aerobic respiration?

Always Remember

• 1. Maintenance of life requires continuous supply of energy.
• 2. Respiration fulfils the continuous need of energy.

13.1 Formation of ATP :

• Formation of ATP is called as phosphorylation.
• In nature, phosphorylation occurs in three different ways as –
• photophosphorylation,
• substrate level phosphorylation and
• oxidative phosphorylation.
• Photophosphorylation is synthesis of ATP by addition of inorganic phosphate to ADP in presence of light .
• Substrate-level phosphorylation is a direct phosphorylation of ADP by transfer of a phosphate group from any suitable substrate.
• It occurs in cytoplasm of the cells and matrix of mitochondria.

• Oxidative phosphorylation:
• It is phosphorylation of ADP at the cost of energy released during oxidation of substrates like NADH+H⁺ and FADH₂ .
• This occurs on the inner mitochondrial membrane only.
• When energy is required for any metabolic process, ATP is hydrolysed.
• ATP hydrolysis releases the energy which is used for the metabolic activities.

• Respiration is a catabolic process wherein complex organic substrate is oxidized to simple components to generate biological energy.
• Cellular respiration occurs in two different ways as
• Anaerobic respiration
• Aerobic respiration.

13.2 Anaerobic respiration :

• Anaerobic respiration is the cellular respiration that does not involve the oxygen at all.
• It is also called as fermentation.
• It is completed through steps like
• Glycolysis and
• Conversion of glycolytic product to any suitable product like lactic acid, ethanol, etc.

Glycolysis :

• This involves the breakdown of glucose molecule into two pyruvic acid molecules.
• Hence known as glycolysis.
• This is a common step in anaerobic as well as aerobic respiration.
• It occurs in cytoplasm of cell.
• It is completed in two phases as
• Preparatory phase
• Pay-off phase.

• Overall process of glycolysis is completed through ten steps.
• First five steps constitute the preparatory phase.
• Glucose is phosphorylated twice at the cost of two ATP molecules and a molecule of fructose 1, 6-bisphosphate is formed.
• This molecule is split to form
• A molecule of glyceraldehyde 3-phosphate (3PGAL)
• A molecule of dihydroxyacetone phosphate (DHAP)
• Both of these molecules are 3-carbon carbohydrates (trioses) and are isomers of each other.
• DHAP is isomerised to second molecule of 3PGAL.
• Thus, two molecules of glyceraldehyde-3- phosphate are formed and here, first phase i.e. preparatory phase of glycolysis ends.

• In the pay-off phase, both molecules of glyceraldehyde-3-phosphate are converted to two molecules of 1, 3-bisphoglycerate by oxidation and phosphorylation.
• Here, phosphorylation is brought about with the help of inorganic phosphate and not ATP.
• Overall reaction of glycolysis:
• Glucose + 2ATP + 2iP + 4ADP + 2NAD⁺ -----------------> 2Pyruvate + 2ADP + 4ATP + 2NADH+H⁺ +2H₂O

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• Both molecules of 1, 3-bisphosphoglycerate are converted into two molecules of pyruvic acid through series of reactions accompanied with release of energy.
• This released energy is used to produce ATP (4 molecules) by substrate-level phosphorylation.

• In muscles, the NADH+H⁺ produced during glycolysis is reoxidized to NAD⁺ by donating one proton and two electrons to pyruvic acid which yields lactic acid.
• Skeletal muscles usually derive their energy by anaerobic respiration.
• After vigorous exercise lactic acid accumulates, leading to muscle fatigue.
• During rest, however, the lactic acid is reconverted to pyruvic acid and is channeled back into the aerobic respiration pathway.

• In yeast, the pyruvate is decarboxylated to acetaldehyde.
• The acetaldehyde is then reduced by NADH+H⁺ to ethanol.
• Carbon dioxide is also produced in this process.
• This type of anaerobic respiration is termed alcoholic fermentation.
• Accumulation of ethanol by fermentation in a culture of yeast may stop further multiplication and lead to the death of cells.
• In the presence of oxygen however, yeast can respire aerobically.

• Glycolysis is under tight control.
• Its rate depends upon the requirement of ATP and many other factors.
• Glycolytic rate control is achieved by complex interplay between ATP consumption, NADH2 regeneration and regulation of various glycolytic enzymes like hexokinase, PFK-1, pyruvate kinase, etc.
• Besides, it is also controlled by hormones like glucagon, epinephrine and insulin.

Use your brain power:

• 1. What is role of Mg⁺⁺, Zn⁺⁺ in various steps of glycolysis?
• 2. Why some reactions of glycolysis are reversible and some irreversible?
• 3. Why is glycolysis considered as biochemical proof of evolution?
• 4. Why do athletes like sprinters have higher proportion of white muscle fibers?

Do you know?

• 1. Glycolysis is only source of energy production in erythrocytes, renal medulla, brain and sperm.
• 2. Some plant tissues which are modified to store starch (like potato) mainly depend upon glycolysis for energy production.
• 3. In chapter 3, Biomolecules, you have read about the oxygen storing and transporting pigment myoglobin of skeletal muscles.
• Red (dark) muscles are richer in myoglobin than the white (pale) muscles.

• Therefore, red fibers can utilize the oxygen stored in myoglobin to continue energy production over prolonged period by aerobic oxidation of glucose.
• This enables them to perform sustained work over a long period.
• On the contrary, white fibers produce the energy needed for very fast and severe work by glycolysis as sufficient oxygen is not immediately available to them for such work.
• But white muscles accumulate lactic acid and get fatigued in a short time.
• Thus athletes with a higher proportion of red fibers in their muscles are physiologically better adapted for sustained events like marathon or swimming over long distances.

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