Unit 8: Bioenergetics — Short Questions

Oxidation-reduction (redox) reactions are fundamental to the metabolism of organisms. In these reactions, electrons are transferred between molecules. In oxidation, molecule loses electrons and in reduction, it gains electrons. This electron flow is essential for generating energy in the form of ATP during processes like cellular respiration and photosynthesis.

ATP
Cells use a special energy currency for their reactions. This currency is nucleotide called adenosine triphosphate (ATP). ATP is the main energy transfer molecule in the cell.

ADP
In common energy reactions, when outer P-P high-energy bond of ATP breaks, ATP become ADP· (adenosine diphosphate) and one Pi is released.
Cells get energy from the oxidation of food. They store this energy by combining ADP with Pi to form ATP.

6 CO₂ + 12 H₂O + photons → C₆H₁₂O₆ + 6 O₂ + 6 H₂O
Carbon dioxide + water + light energy → glucose + oxygen + water

Sunlight is absorbed by chlorophyll. It is then converted into chemical energy which drives the photosynthetic process.

During light reaction of photosynthesis, light breaks water molecule. Oxygen is released while hydrogen atoms give electrons to chlorophyll and become hydrogen ions.

i. Autotrophic organisms (plants, algae, some bacteria) use inorganic raw materials to prepare their organic food in the form of carbohydrates during photosynthesis.
ii. Chloroplast are the sites of photosynthesis in eukaryotes. They contain pigments which absorb sunlight and convert it into chemical energy for photosynthesis.

In cellular respiration, food is oxidized to CO₂, H₂O and energy is released. This respiratory energy (ATP) produced during cellular respiration is used in various ways:
1. Muscle contractions and movement
2. Active transport of substances
3. Synthesis of biomolecules
4. Replication of DNA and mitosis etc.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6 H₂O + Energy
Glucose oxygen carbon water dioxide

Cellular respiration occurring in the presence of oxygen is called aerobic respiration. Oxygen helps in the complete oxidation of glucose with maximum release of energy.

Aerobic respiration
Cellular respiration occurring in the presence of oxygen is called aerobic respiration.
Explanation
i. It is the complete oxidation of glucose with maximum release of energy.
ii. It is first phase, a molecule of glucose (6-C) is broken down into two molecules of pyruvic acid (3-C).
iii. In the second phase, the molecules of pyruvic acid are completely oxidized (all C-H bonds are broken) and all energy is released.
Anaerobic Respiration (Fermentation)
Cellular respiration that occurs in the absence of oxygen is called anaerobic respiration.
Explanation
i. In anaerobic respiration, glucose is incompletely oxidized with less amount of energy released.
ii. Its first phase is exactly similar to that of aerobic respiration. A molecule of glucose is broken down into two molecules of pyruvic acid.

The end product of anaerobic respiration in animals is lactic acid and in yeast is ethyl alcohol.

When skeletal muscles of humans work hard (during exercise etc.) but oxygen supply is not sufficient to fulfil the demand, the skeletal muscles carry out anaerobic respiration to get energy.

Respiratory energy produced during cellular respiration is used in various ways:
1. Muscle contractions and movement
2. Active transport of substances
3. Synthesis of biomolecules
4. Replication of DNA and mitosis
5. Transmission of nerve impulse
6. Maintenance of body temperature
7. Break down and elimination of toxins from the body

Structure of ATP Molecule
ATP molecule consists of three components:
1. Adenine (double ringed nitrogen base)
2. Ribose (Five carbon sugar)
3. Three Phosphate Groups (PO₄) linked in a chain called a triphosphate group Adenine nitrogen base binds to ribose sugar and become adenosine.

Adenine + Ribose Sugar → Adenosine

AMP stands for adenosine monophosphate. ADP is broken down to AMP and Pi is released.

Once the light reactions produce ATP and NADPH, a photosynthetic cell can fix carbon dioxide to synthesize sugar molecules. These reactions does not depend directly on light that is why it is called dark reactions.

(a) Photosynthesis
6CO₂ +12H₂O → C₆H₁₂O₆ +6H₂O+ 6O₂.
Light
Chlorophyll
Carbon dioxide Water Glucose Water Oxygen

(b) Fermentation
C₆H₁₂O₆ →C₂H₅OH / C₃H₆O₃+ CO₂
Glucose Ethyl alcohol/Lactic acid Carbon dioxide

(c) Aerobic Respiration
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O+ ATP
Glucose Oxygen Carbon dioxide Water Energy

Both are interlinked in the following ways:
(i) Both take place in living organisms.
(ii) Energy conversion take place in both process.
(iii) The exchange of carbon dioxide and oxygen during photosynthesis and cellular respiration worldwide helps to keep atmospheric oxygen and carbon dioxide at stable levels.
(iv) Photosynthesis requires the product of aerobic respiration (CO₂ and H₂O) while aerobic respiration requires the products of photosynthesis (glucose and oxygen).

lactic acid (C₃H₆O₃).

Phases of aerobic respiration are:
(i) Glycolysis (ii) Krebs cycle
(iii) Electron transport chain

No it cannot enter in Krebs cycle directly. Each pyruvic acid molecule is oxidized to a two-carbon acetyl group which also combines with coenzyme A to form acetyl CoA. Carbon dioxide is removed and NADH is produced. The acetyl CoA enters mitochondrion where Krebs cycle will occur.

Before entering in Krebs cycle, pyruvic acid combines with coenzyme A and changed into a 2- carbon compound called acetyl Co-A.

A type of respiration in which complete oxidation of glucose occur with maximum release of energy in the presences of oxygen.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O +Energy

In this type of anaerobic respiration, pyruvic acid is further broken down into alcohol (C₂H₅OH) and carbon dioxide (CO₂).
Pyruvic Acid → Ethyl alcohol + Carbon dioxide

Some organisms oxidize their food incompletely without using any molecular oxygen called anaerobic respiration. Glucose is incompletely oxidized with less amount of energy released. It is of two types:
(i) Lactic acid fermentation (ii) Alcoholic fermentation.

It is like the cell's battery because it stores and releases energy. It has three phosphate groups that are connected by bonds that are easy to break. When these bonds break during hydrolysis, ATP releases energy that cells use for various activities and when bond forms energy is restored again. Its ability to regenerate quickly makes it an efficient energy carrier.