Unit 8: Unit 8: Bioenergetics — Long Questions

Definition
'Bioenergetics is the study of how living organisms acquire, convert, store, and utilize energy to fuel their life processes'. Or 'Bioenergetics is the study of energy transformations in living organisms'.
Explanation
Organisms obtain energy primarily from their surroundings. Plants capture sunlight through photosynthesis, while animals and other organisms consume food. This energy is then converted into usable chemical energy, stored in molecules like ATP (adenosine triphosphate). ATP acts as a ready source of energy that cells can access whenever they need it for processes such as growth, movement, repair, and reproduction.

Introduction
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.
Role
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 a nucleotide called adenosine triphosphate (ATP).
Need of Energy
When cells store energy, they make ATP. When cells need energy, they break ATP.
Structure
ATP molecule has three subunits i.e. adenine, (a nitrogen containing base); ribose (a five-carbon sugar) and three phosphate groups.
ATP-ADP cycle
a) ATP into ADP
In the molecule of ATP, the covalent bonds between two phosphates are high-energy bonds. When one of these bonds is broken, inorganic phosphate (Pi) separates and energy is released.
Formation of ATP
The breaking of one phosphate bond releases about 7.3 kcal (7,300 calories) per mole of ATP.
ATP + H₂O → ADP + Pi + energy (7.3 kcal/mole)
In common energy reactions only the outer P-P high-energy bond breaks. When this happens, ATP becomes ADP (adenosine diphosphate) and one Pi is released.
b) ADP into AMP
In some cases, ADP is further broken down to AMP (adenosine monophosphate) and Pi:
ADP + H₂O → AMP + Pi + energy (7.3 kcal/mole)
Cells get energy from the oxidation of food. They store this energy by combining ADP with Pi to form ATP.
Summary
ATP is made during energy-releasing processes and is broken down during energy-consuming processes. In this way ATP transfers energy between metabolic reactions.

Introduction
Photosynthesis is the synthesis of glucose from carbon dioxide and water in the presence of sunlight (and chlorophyll), with oxygen as a by-product.
Chemical Equation
It is the most important metabolic reaction and all life depends on it. A simple general equation for photosynthesis is as follows:
6 CO₂ + 12 H₂O + photons → C₆H₁₂O₆ + 6 O₂ + 6 H₂O
Carbon dioxide + water + light energy → glucose + oxygen + water
Explanation
Autotrophic organisms (plants, algae, some bacteria) use inorganic raw materials to prepare their organic food. The organic food is in the form of carbohydrates. The carbohydrates are used for getting energy and are also converted to other molecules like proteins, lipids etc.

Introduction
Photosynthesis occurs in two phases i.e. light reactions and dark reactions. Light reactions take place on the thylakoid membranes of chloroplasts. Dark reactions take place in the stroma of the chloroplasts.
1. Light reactions
Definition
During light reaction, light energy is used to make high-energy molecules (ATP and NADPH).
Key Events of Light Reactions
Following are the key events of light reactions:
a) Emission of Electrons
When chlorophyll absorbs light, reactions start in it. High energy electrons are released from chlorophyll.
b) Production of ATP
The high energy electrons are passed to an electron transport chain. In this chain, electrons pass from higher to lower energy level. They release energy which is used to produce ATP.
c) Breakage of Water Molecule (Photolysis)
Light also breaks water molecule. Oxygen is released while hydrogen atoms give electrons to chlorophyll and become hydrogen ions.
d) Reaction of NADP into NADPH
The electrons of chlorophyll, after the production of ATP, and the hydrogen ions of water are used to reduce a NADP into NADPH.
e) Nicotinamide Adenine Dinucleotide (NAD)
It is a coenzyme. One form of this coenzyme also carries phosphate. It is called NADP.
2. Dark Reactions (Calvin Cycle)
Definition
During this phase, carbon dioxide is reduced to make glucose. The details of dark reactions were discovered by Melvin Calvin.
Summary of Dark Reactions
a) Splitting of 6-carbon Compounds
CO₂ molecules combine with 5-carbon compounds to form 6-carbon compounds. Each 6-carbon compound splits into two 3-carbon compounds.
b) Reduction of 3-Carbon Compounds
The 3-carbon compounds are reduced to 3-carbon carbohydrates by using ATP and hydrogen from NADPH. The 3-carbon carbohydrates are used to make glucose.
c) Regeneration of Original 5-Carbon Compounds
The 3-carbon carbohydrates are also used to regenerate the original 5- carbon compounds. This step also utilizes ATP.

The thylakoid membranes of chloroplasts contain pigments. Chlorophyll-a is the main pigment. Others· are called accessory pigments and include chlorophyll-b and carotenoids.
Absorption of Light by Pigments
Pigments absorb sunlight and convert it into chemical energy for photosynthesis. Only about 01% of the light falling on the leaf surface is absorbed, the rest is reflected or transmitted. The blue and red lights carry out photosynthesis. Different pigments absorb. different wavelength of light. Chlorophyll-a absorbs light of blue and red wavelengths. The wavelengths which are not absorbed by chlorophyll-a are absorbed by accessory pigments.
Pigments
Pigments are the substances that absorb visible light. Different pigments absorb light of different wavelengths (colours).
Formation of ATP
When a pigment absorbs light, reactions occur in it and its electrons are released. The high energy electrons pass through electron transport chain and their energy is used for the formation of ATP and for reducing NADP to NADPH.

Organisms get energy by breaking the C-H bonds of food. For this purpose, they carry out the oxidation of food inside cells. This oxidation of food is called cellular respiration. The most common food used by cells to get energy is glucose.
Types of Respiration
The two main types of respiration are:
(1) Aerobic Respiration
(2) Anaerobic Respiration
1. Aerobic Respiration
Definition: 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.
2. Anaerobic Respiration (Fermentation)
Definition: 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.
iii. In the second phase, pyruvic acid may be treated in two ways:
A. Alcoholic Fermentation
During anaerobic respiration in bacteria and yeast etc. pyruvic acid is further broken down into alcohol (C₂H₅OH) and CO₂. This type of anaerobic respiration is called alcoholic fermentation.
2(C₃H₆O₃) → 2(C₂H₅OH) + 2CO₂
Pyruvic acid Ethyl Alcohol
Yeast and bacteria can ferment sugars of berries to alcohol. Birds eating these berries can become quite drunk, as is obvious from their flight pattern.
B. Lactic Acid Fermentation
During anaerobic respiration in the skeletal muscles of humans and other animals, pyruvic acid is converted into lactic acid (C₃H₆O₃). This type of anaerobic respiration is called lactic acid fermentation.
2(C₃H₆O₃) + 4H → 2(C₃H₆O₃)
Pyruvic acid Lactic acid
Importance of Fermentation
i. Early Life and Anaerobes
The environment of Earth did not have free oxygen (O₂) in the early phases of life. The early organisms respired anaerobically and got energy for their life. Even today, some organisms including some bacteria and some fungi get energy from anaerobic respiration and are called anaerobes.
ii. Anaerobic Respiration in Skeletal Muscles
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.
iii. Use of Fermentation
Scientists have used fermentation in fungi and bacteria for making useful products for mankind.
Examples
a. The fermentation in bacteria is used for making cheese and yogurt.
b. Fermentation in yeasts is used in brewing and baking industries.
c. Similarly, the soy sauce is made through the fermentation by a fungus.

Introduction
For the study of all the reactions of cellular respiration, mechanism of aerobic respiration is studied. There are three main steps of aerobic respiration.
1. Glycolysis
In the first step, the glucose (6C) molecule is broken. It results in two molecules of pyruvic acid (3-C) with 2 ATPs and 2 molecules NADH. This process is called glycolysis and it occurs in cytoplasm. Oxygen is not required for glycolysis. That is why, it also occurs in anaerobic respiration.
2. Krebs Cycle
When oxygen is available, the molecules of pyruvic acid move from cytoplasm to the matrix of mitochondria. Here, a series of reaction called Krebs cycle (discovered by a British Scientist Sir Hans Krebs) occurs. Before Krebs cycle, each pyruvic acid is converted into acetyl coenzyme-A, carbon dioxide and NADH.
In Krebs cycle, the acetyl Coenzyme-A is completely oxidized to carbon dioxide. It results in the formation of ATP and energy-rich compounds i.e. NADH and FADH₂ (Flavin Adenine Dinucleotide - reduced).
3. Electron Transport Chain
This step occurs on the membranes of mitochondria. During it, NADH and FADH₂ change back to NAD and FAD by releasing electrons and hydrogen ions. The released electrons pass through an electron transport chain and release energy. This energy is used to make ATP. At the end of chain, electrons and hydrogen ions combine with oxygen and form water.

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

Difference between aerobic and anaerobic respiration

Presence of Oxygen | Aerobic Respiration: Yes | Anaerobic Respiration: No

Number of ATP as net profit | Aerobic Respiration: 36 | Anaerobic Respiration: 02

Final products | Aerobic Respiration CO₂, H₂O | Anaerobic Respiration: Lactic acid or Ethanol + CO₂

Site of Occurrence | Aerobic Respiration: Cytoplasm and Mitochondria | Anaerobic Respiration: Cytoplasm

Importance | Aerobic Respiration Major source of energy for most organisms | Anaerobic Respiration:

• Source of energy for anaerobic organisms.
• Source of energy for aerobic organisms in short supply of O₂
• Source of useful products (ethanol cheese etc.)

Difference between photosynthesis and respiration

Characteristics | Photosynthesis | Respiration

Type of metabolism | Anabolic process | Catabolic process

Energy investment / production | Energy is stored in the form of bond energy | Bond energy of food is transformed into ATP

Organisms capable of performing this process: | Some bacteria, all algae and all plants | All organisms

Site of Occurrence | Chloroplasts | In cytoplasm and mitochondria in all cells

Time of occurrence | In daytime only in the presence of light | All the time

Intake of water and CO₂ for photosynthesis
Water and CO₂ are the raw materials of photosynthesis. The plants have mechanisms for the intake and transport of these materials.
i. Intake of water
Water present in soil is absorbed by roots and root hairs through osmosis. This water is eventually transported to leaves through xylem vessels.(After the entry of water in the inner cells of the root, it reaches xylem vessels).
ii. Intake of Carbon dioxide (CO₂)
The air that enters the leaf through tiny pores (stomata) reaches into the air spaces present around mesophyll cells. Stomata cover only 1-2% of leaf surface but they allow much air to pass through them.
This air carries CO₂ which gets absorbed in the thin layer of water surrounding the mesophyll cells. From here, the CO₂ diffuses into the mesophyll cells.