Biochemical Pathways: A Comparative Analysis of Energy Transfer in Photosynthesis and Cellular Respiration
An essay comparing the biochemical processes of photosynthesis and cellular respiration in energy transfer.
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Biochemical Pathways: A Comparative Analysis of Energy Transfer in
Photosynthesis and Cellular Respiration
Essay Questions:
1. Define ΔG and describe how it relates to changes in entropy and enthalpy. How ΔG is
related to the equilibrium constant of a reversible reaction and explain why cells maintain
product/substrate concentrations below the equilibrium constant in metabolic pathways?
Answer: ΔG (Gibbs free energy) represents the energy available to do work in a
system at constant temperature and pressure. It relates to changes in entropy (ΔS)
and enthalpy (ΔH) by the equation:
• ΔH is the change in enthalpy (heat content),
• T is the temperature in Kelvin,
• ΔS is the change in entropy (disorder).
If ΔG is negative, the reaction is spontaneous. If ΔG is positive, the reaction is non-
spontaneous.
Equilibrium and ΔG: At equilibrium, ΔG = 0. The relationship between ΔG and the
equilibrium constant (K) is given by:
where R is the gas constant and T is the temperature. A large K (favorable reaction)
means a negative ΔG, and a small K means a positive ΔG.
Cells maintain product/substrate concentrations below equilibrium to ensure
reactions proceed in the direction that supplies energy (negative ΔG), preventing
equilibrium from being reached and sustaining metabolic processes.
Photosynthesis and Cellular Respiration
Essay Questions:
1. Define ΔG and describe how it relates to changes in entropy and enthalpy. How ΔG is
related to the equilibrium constant of a reversible reaction and explain why cells maintain
product/substrate concentrations below the equilibrium constant in metabolic pathways?
Answer: ΔG (Gibbs free energy) represents the energy available to do work in a
system at constant temperature and pressure. It relates to changes in entropy (ΔS)
and enthalpy (ΔH) by the equation:
• ΔH is the change in enthalpy (heat content),
• T is the temperature in Kelvin,
• ΔS is the change in entropy (disorder).
If ΔG is negative, the reaction is spontaneous. If ΔG is positive, the reaction is non-
spontaneous.
Equilibrium and ΔG: At equilibrium, ΔG = 0. The relationship between ΔG and the
equilibrium constant (K) is given by:
where R is the gas constant and T is the temperature. A large K (favorable reaction)
means a negative ΔG, and a small K means a positive ΔG.
Cells maintain product/substrate concentrations below equilibrium to ensure
reactions proceed in the direction that supplies energy (negative ΔG), preventing
equilibrium from being reached and sustaining metabolic processes.
2. Why ATP is considered as an energy currency of the cells? By giving examples
describe how ATP provides energy for driving endergonic reactions. Giving an example
show how ATP is produced by substrate level phosphorylation in glycolysis.
Answer: ATP is considered the energy currency of the cell because it stores and
transfers energy for biochemical reactions. It is a high-energy molecule due to the
phosphate bonds, which, when broken (hydrolyzed), release energy that can be
used by the cell.
ATP and Endergonic Reactions:
Endergonic reactions require an input of energy. ATP provides this energy through
coupling. When ATP is hydrolyzed (ATP → ADP + Pi), it releases energy that is used to
drive the endergonic reaction. For example:
• Muscle contraction: ATP provides energy to myosin for muscle movement.
• Active transport: ATP powers pumps like the sodium-potassium pump to move
ions against their concentration gradient.
ATP Production by Substrate-Level Phosphorylation:
In glycolysis, ATP is produced by substrate-level phosphorylation, where a phosphate
group is directly transferred from a substrate to ADP to form ATP.
Example: During glycolysis, 1,3-bisphosphoglycerate transfers a phosphate group to
ADP, forming ATP and 3-phosphoglycerate.
The reaction:
This process occurs twice in glycolysis, producing 2 ATP molecules per glucose
molecule.
describe how ATP provides energy for driving endergonic reactions. Giving an example
show how ATP is produced by substrate level phosphorylation in glycolysis.
Answer: ATP is considered the energy currency of the cell because it stores and
transfers energy for biochemical reactions. It is a high-energy molecule due to the
phosphate bonds, which, when broken (hydrolyzed), release energy that can be
used by the cell.
ATP and Endergonic Reactions:
Endergonic reactions require an input of energy. ATP provides this energy through
coupling. When ATP is hydrolyzed (ATP → ADP + Pi), it releases energy that is used to
drive the endergonic reaction. For example:
• Muscle contraction: ATP provides energy to myosin for muscle movement.
• Active transport: ATP powers pumps like the sodium-potassium pump to move
ions against their concentration gradient.
ATP Production by Substrate-Level Phosphorylation:
In glycolysis, ATP is produced by substrate-level phosphorylation, where a phosphate
group is directly transferred from a substrate to ADP to form ATP.
Example: During glycolysis, 1,3-bisphosphoglycerate transfers a phosphate group to
ADP, forming ATP and 3-phosphoglycerate.
The reaction:
This process occurs twice in glycolysis, producing 2 ATP molecules per glucose
molecule.
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