BIOENERGETICS

 BIOENERGETICS

Q.1: What is Photosynthesis?

Ans: Photosynthesis is a process in which plants manufacture their food in the presence of chlorophyll and sunlight by the combination of carbon dioxide and water to form simple carbohydrates. In this process, oxygen is liberated. It is important to note that the amount of carbon dioxide consumed in this process is equal to the amount of oxygen liberated.

In photosynthesis, not only simple carbohydrates are formed but also a considerable amount of energy, which is initially obtained from sunlight as radiant energy, is transferred by green cells into chemical energy.

$$6CO_2 + 12H_2O \xrightarrow{\text{Light, Chlorophyll}} C_6H_{12}O_6 + 6H_2O + 6O_2 \uparrow$$

In terms of energy, photosynthesis can be defined as the metabolic process during which light energy is converted into chemical or food energy in the presence of chlorophyll and electron carriers.

Q.2: What are the reactants and products of Photosynthesis?

Ans: Reactants: In photosynthesis, $CO_2$ and water are the raw materials. The water is absorbed from the soil by the help of roots, and $CO_2$ enters the body through stomata of leaf from the atmosphere. In aquatic plants, the dissolved $CO_2$ enters the plant along with water.

Photosynthetic products: In photosynthesis, simple carbohydrate glucose is formed by the combination of $CO_2$ and hydrogen of water. Oxygen is produced by the hydrolysis of water, which is released out as a byproduct. The important product, simple sugar, forms other complex compounds like starch and cellulose. A part of sugar is changed into oil and another part produces nitrogen, sulfur, and phosphorus, which help to form proteins and other compounds.

Q.3: What is the role of Chlorophyll and other Pigments in Photosynthesis?

Ans: Role of Chlorophyll and Other Pigments:
Chloroplasts are the chlorophyll-containing organelles. Each photosynthetic cell of leaf has about 20-100 chloroplasts. Each chloroplast consists of three parts:

• A surrounding double membrane.
• A liquid called stroma that is surrounded by a double membrane. It has important enzymes required for the formation of carbohydrates.
• Thylakoids:
  These are sets of flattened sac-like structures, which are embedded in the stroma. The thylakoid membrane contains an inner fluid-filled cavity or lumen. This cavity (lumen) is separated from the stroma by the thylakoid membrane. The columns of thylakoid are called grana.

Chlorophyll and other photosynthetic pigments are present in the thylakoid membranes and show the green color of plants. Electron acceptors of photosynthesis are also found in these membranes, so thylakoid membranes are involved in the formation of ATP compounds.

Chlorophyll and other pigments absorb sunlight.
This light energy is converted into chemical energy of ATP and NADPH. These are used to prepare glucose in the stroma region of the chloroplast. The pigments of plants are Carotenes (orange), Phaeophytin (grey), Xanthophyll (yellow), Chlorophyll-a (blue-green), and Chlorophyll-b (yellow-green). Chlorophylls are of different types, a, b, c, d.

The formula of chlorophyll a and b is given below:
Chlorophyll a - $C_{55}H_{72}O_5N_4Mg$
Chlorophyll b - $C_{55}H_{70}O_6N_4Mg$

Q.4: What is the Photosystem in Plants?

Ans: Photosynthesis takes place by the help of sunlight energy, which is absorbed by photosynthetic pigments. The photosynthetic pigments are present in groups, called photosystems. The photosystem consists of two parts: a light-gathering part called the antenna complex and a reaction center. The antenna complex part contains many molecules of chlorophyll-a, chlorophyll-b, and carotenoid molecules.

When light is received, the light energy is transferred from one pigment molecule to another pigment molecule until it reaches the reaction center, where chemical reactions of the photosystem occur, and light energy is converted into chemical energy.

Types of Photosystem or Pigment System:
There are two types of photosystems or pigment systems.

• Pigment System I or Photosystem I
• Pigment System II or Photosystem II

Pigment System I or Photosystem I (PS-I)
It consists of chlorophyll-a molecules which absorb maximum light of 700 nm, called $P_{700}$. FRS (Ferredoxin reducing substance), ferredoxin molecule. PS-I controls the process of reducing NADP into NADPH + H.

Pigment System II or Photosystem II (PS-II)
It consists of chlorophyll-b ($P_{680}$) a primary electron acceptor $Q$, a plastoquinone, cytochrome-$b_{559}$, cytochrome-$b_6$, plastocyanin, Mn molecules bound with protein and chloride.

PS-II is concerned with the formation of oxidant and reductant with the release of oxygen.

Q.5: What is the role of light in Photosynthesis?

Ans:
Role of Light:
The light comes in packets, called quanta. A single quantum of light is known as a photon. The light travels in waves which have different lengths. The light of longer wavelengths has less energy, and light of short wavelengths contains more energy. The human eye can only see the light of a certain wavelength. The visible light has a wavelength between 400 nm (violet) and 700 nm (red). The light is composed of seven different colors: violet, blue, green, yellow, orange, and red.

The molecules that absorb light are called pigments. Chlorophyll a and b absorb violet, blue, and red regions of light, while yellow and orange light is absorbed very slightly. Green light is reflected, so plants are shown green in color. When light is captured in the antenna complex, it is rapidly transferred to the chlorophyll molecules present in the photosynthetic reaction center. The energy of light photons results in the activation of an electron from the ground state to an excited state. This energy is used to start chemical reactions and all other steps of photosynthesis.

Q.6: What is the role of water and $CO_2$ in Photosynthesis?

Ans:
Role of Water:
Water has a very important role in photosynthesis. Photosynthesis is a redox (reduction-oxidation) process. It needs $H^+$ and electrons. For this purpose, photolysis of water takes place. $H_2O$ splits into oxygen and hydrogen. Hydrogen ions ($H^+$) are used in dark reactions to produce sugar by the combination of $CO_2$. The oxygen is released from the body of the plant.

Role of $CO_2$:
$CO_2$ plays a basic role in photosynthesis. In the dark reaction, it takes part in chemical processes and involves in the formation of simple carbohydrates. The rate of photosynthesis is increased by the increase of $CO_2$ in the atmosphere.

Q.7: Describe the light reaction of Photosynthesis?

Ans:
The light reaction consists of two steps.

• Electron transport
• Phosphorylation (Formation of ATP)

Electron Transport:
The light reaction of photosynthesis is started from the reaction center of PS-II ($P_{680}$). The steps of this process are as follows:

• The Chlorophyll b of Pigment System II (P.S.II) absorbs sunlight and becomes activated. It releases an electron. The electron becomes excited to a high energy level. The excited electron produced within P is transferred to the primary electron acceptor Phaeophytin.
• From primary electron acceptor (Phaeophytin), it is transferred to the plastoquinone (PQ), which is associated with Fe ions.
• From the plastoquinone, the electron is passed to the cytochrome complex.
• The chlorophyll $P_{680}$ gets electron from water by its splitting into two hydrogen ions and an oxygen. This process is called photolysis of water.
• From cytochrome complex, the electron is moved to the plastocyanin (PC). Plastocyanin is reduced, it is present in the lumen. Plastocyanin is a copper-containing protein.

(Diagram titled "Non-cyclic electron flow during the light reaction generates ATP and NADPH (Z-Scheme)")    

• The plastocyanin (PS) moves along the membrane to PS-I, where the chlorophyll-a ($P_{700}$) accepts the electron. This chlorophyll also absorbs light energy.
• From the chlorophyll-a $P_{700}$, the electron is transferred to Ferredoxin reducing substrate (FRS). It is the primary acceptor of Photosystem-I.
• From Ferredoxin reducing substrate (FRS), the electron is transferred to the Ferredoxin (Fd).
• From Ferredoxin, the electron is transferred to NADP. The NADP is reduced to NADP + $H^+$.

Photophosphorylation (Formation of ATP): (Non-cyclic)
During the electron transport, when the electron moves through the chain, its energy is used by thylakoid membrane to produce ATP; this is called photophosphorylation. The electron does not come back to its original position, but it takes part in the reduction of NADP to form NADPH + $H^+$. In this way, a cycle is not completed, so this process is called non-cyclic photo-phosphorylation.

Q.8: Describe the light-independent reaction (Dark reaction) of Photosynthesis?

Ans:
Light Independent Reaction: (Dark Reaction)
(Calvin-Benson Cycle)
The dark reaction does not require light and may take place both in light and dark during the night.

In the dark reaction, the hydrogen separated from water combines with Carbon of $CO_2$ and forms simple carbohydrates with the help of the energy-rich compound ATP. Melvin Calvin worked on the pathway of carbon to carbohydrates. The cycle of chemical reactions in the dark reaction is known as Calvin-Benson cycle. It is also called $C_3$ Photosynthetic carbon reaction cycle.

(Diagram of Benson Calvin Cycle showing Carbon-Fixation Phase-I, Reduction Phase-II, and Regeneration of Ribulose biphosphate (RuBP) in Phase-III)

The Calvin cycle actually consists of 13 main reactions which are catalyzed by 11 enzymes, but it can be divided into three distinct phases:

1. Carboxylation:
   In which $CO_2$ combines with organic molecules, it is also called carbon fixation.

2. Reduction:
   In which organic molecules are reduced, and phosphoglyceraldehyde (PGAI) compound is formed.

3. Regeneration:
   In which reduced carbon is used to regenerate the carbon acceptor molecules or used for metabolism.

Q.9: Describe $C_4$ Photosynthesis mechanism in plants?

Ans:
$C_4$ Photosynthesis:
It is also called Hatch and Stack cycle or dicarboxylic acid cycle. In this cycle, the $CO_2$ acceptor molecule is Phosphoenol pyruvic acid (PEP).

The steps of $C_4$-Photosynthesis are as follows:

• In the chloroplasts of mesophyll cells, $CO_2$ is accepted by phosphoenol pyruvate to form oxaloacetate. It is a 4-carbon compound, so the process is called $C_4$ mechanism.
• Oxaloacetate is converted into malate. Malate is transferred to the chloroplasts of bundle sheath cells.
• Malate combines with NADP to produce pyruvate and $CO_2$. $CO_2$ is used in the Calvin cycle to form sugar. This process is common in sugar-cane, maize of grass family.

(Diagram showing $C_4$-Photosynthesis cycle)

Q.10: Describe the CAM Cycle of Photosynthesis in Plants?

Ans:
CAM Cycle (Crassulacean Acid Cycle):
This process of Photosynthesis occurs in Cacti plants, Pineapple, and many other plants. In such plants, stomata are closed during the day and remain open during the night, so the loss of water is prevented. These plants take up $CO_2$ and use them to produce organic acids, so the process is called crassulacean acid cycle (CAM). These organic acids are stored in their vacuoles. During the day, by the help of ATP and NADPH + H, $CO_2$ is released from organic acids and takes part in the Calvin cycle to form sugar molecules.

Step to CAM Cycle:

• In the night $CO_2$ is accepted by phosphoenol pyruvate to form oxaloacetate.
• Oxaloacetate is converted into malate (Malic acid organic acid).
• In daytime malate is converted into pyruvate and $CO_2$.
• $CO_2$ enters the Calvin cycle to take part in the formation of sugars.

(Diagram showing CAM Cycle of Photosynthesis)

Q.11: What is oxidative phosphorylation?

Ans: Oxidative Phosphorylation:
Respiration is an oxidation-reduction or redox process, in which electrons are donated or accepted in different reactions. The glucose loses hydrogen atoms when it is converted into $CO_2$, and when molecular oxygen is converted into water, it gains hydrogen atoms. Each hydrogen atom contains one electron and one proton, so the transfer of hydrogen atoms is the transfer of electrons and protons.

In respiration, oxidation of glucose takes place with the loss of electrons, and reduction of oxygen occurs by the gain of electrons. During redox reaction, the energy of electrons is used in the formation of ATP from ADP and inorganic phosphate (Pi). The synthesis of ATP is known as oxidative phosphorylation.

Q.12: What is aerobic respiration?

Ans: Aerobic Respiration:
This type of respiration occurs in the presence of $O_2$. The glucose is converted into $CO_2$ and $H_2O$. In this process, a sufficient amount of energy is released. Most organisms show aerobic respiration; such organisms are called aerobes.

Q.13: What is anaerobic respiration?

Ans: Anaerobic Respiration:
This type of respiration takes place in the absence of oxygen. In this process, glucose is converted into $CO_2$ and ethyl alcohol or lactic acid. In this mechanism, very little energy is released.

The organisms which show anaerobic respiration are called anaerobes. These are bacteria, yeast, some species of annelids which live in mud containing less oxygen, parasites in the intestine such as Tapeworms, roots of plants growing in water-logging conditions.

$$C_6H_{12}O_6 \rightarrow C_2H_5OH + CO_2 + 28 \text{ K. cal energy}$$

There are two types of anaerobes:

• Obligate Anaerobes: These organisms show respiration without oxygen, i.e., they never need oxygen.
• Facultative Anaerobes: These organisms show aerobic respiration in the presence of oxygen, but in the absence of oxygen, they start anaerobic respiration.

Q.14: What is Fermentation?

Ans: Fermentation:
Anaerobic respiration (without $O_2$) is also called fermentation. When ethyl alcohol is formed in this process, it is called anaerobic respiration. When lactic acid is formed in the process, it is called lactic acid fermentation.

Economic Importance Of Fermentation:
The economic importance of fermentation is as follows:

• By this process many chemical products are prepared, such as ethyl alcohol, lactic acid, propionic acid, and butanol.
• It is used in the brewing industry.
• It is used in the dairy industry, in the formation of curd, cheese, butter, etc.
• It is used in the preparation of wines and beers. Wines are produced from fruits like grapes. Beers are produced from malted cereals, e.g., Barley.
• Yeast is used in bakeries to prepare many items, such as bread.
• Lactic acid is used to produce flavor for yogurt and cheese. Lactic acid prevents the spoilage of dairy products.
• Lactic acid is also used to produce flavor in pickles.
• Fermentation is also used in the preparation of acetone and other industrial solvents.

Q.15: Describe briefly the process of glycolysis in plants?

Ans: Glycolysis:
The initial stage of respiration is called glycolysis in which $O_2$ is not used. It is common both in aerobic and anaerobic respiration. In this process, carbohydrates are changed into a 3-carbon compound, pyruvic acid. The reactions of glycolysis are as follows:

1. Glycolysis starts with glucose. The glucose is converted into glucose-6-phosphate by the utilization of an ATP compound (an energy-rich compound).
2. Glucose-6-phosphate is changed into fructose 1-6 biphosphate by the arrangement of carbon atoms.
3. Fructose-6-phosphate is converted into fructose 1-6 biphosphate by the use of an ATP compound.
4. Fructose 1-6 diphosphate is divided into two 3-carbon compounds, 3-phosphoglycer-aldehyde and bihydroxy acetone-phosphate.
5. 3-Phospho-glycer-aldehyde is changed into 1-3-biphospho glyceric acid.
6. 1-3-diphospho glyceric acid is converted into 3-phospho-glyceric acid. It reacts with ADP (adenosine diphosphate), which forms ATP (adenosine-triphosphate).
7. 3-phosphoglyceric acid is changed into 2-phospho-glyceric acid.
8. 2-phospho-glyceric acid is changed into phosphoenol pyruvate, which finally produces pyruvate (end product).

Q.16: Describe briefly the Kreb Cycle (Tricarboxylic Cycle)?

Ans: In the presence of sufficient amount of oxygen, the pyruvic acid is oxidized. In the presence of coenzyme A, it releases $CO_2$ and changes into acetyl coenzyme A. In this process, two hydrogens are also removed which are accepted by NAD molecules to form $NADH_2$. The acetyl coenzyme A enters the Krebs cycle or Tricarboxylic acid cycle or citric acid cycle.

Steps Of Krebs Cycle:

• In the first reaction of Krebs cycle, the acetyl-CoA combines with oxalo-acetic acid, a 4-carbon compound, and changes into citric acid. In this reaction, coenzyme A is released, and one molecule of water is used.
• Citric acid is converted into cis-aconitic acid in the presence of enzyme by dehydration in which a water molecule is released.
• Cis-aconitic acid is converted into isocitric acid. In this reaction, a molecule of water is used.
• Isocitric acid produces oxalo-succinic acid by the liberation of hydrogen. It combines with NAD to form $NADH_2$.

Q.17: What is electron transport chain?

Ans: ELECTRON TRANSPORT CHAIN:
The last stage of aerobic respiration is the electron transport chain. In glycolysis and Krebs cycle, hydrogen is transferred to different carriers such as NAD, NADP, and FAD. These compounds are then oxidized through the electron transport system, and the energy released in this process is used in the formation of ATP.

The electron transport system takes place in the mitochondria where hydrogen acceptors are arranged in an order. The system is controlled by enzymes. The series of carriers is called the electron transport chain. The hydrogen atoms are removed in pairs at various stages during glycolysis and Krebs cycle. A pair of hydrogen atoms is separated into a pair of electrons and a pair of protons.

Reaction:
$2H \rightarrow 2H^+ + 2e^-$

The electrons are accepted by NAD and FAD. They are passed along a series of electron carriers, such as cytochrome "b," cytochrome "c," cytochrome "a," and cytochrome "a3." In the electron transport system, a pair of electrons forms 3 ATP. One ATP is produced when NAD transfers its electron to FAD. One ATP is formed when one electron is migrated from cyt.b to cyt.c. The third ATP is produced when electron is transferred from cyto.a to cyto.a3. The formation of ATP is called oxidative phosphorylation.

Q.18: What are the trophic levels?

Ans: TROPHIC LEVELS:
Food is very important for all living organisms because it provides energy. In an ecosystem, the flow of energy occurs through a chain, for example, plants are eaten by herbivores and the herbivores are eaten by carnivores, thus the food manufactured by plants travel from producers to primary consumers, i.e., herbivores and then to secondary consumers, i.e., carnivores. "This stepwise process through which food energy moves, with repeated stages of eating and being eaten, is known as the food chain."
e.g.,
Grass → Sheep → Man

Food chain represents various levels of nourishment. These levels are called trophic levels. The green plants occupy the first trophic level. It is the primary producer level. The herbivores form the second level or primary consumer level.

These trophic levels are arranged in a systematic manner:
Plants → Primary consumers → Secondary consumers → Tertiary consumers → Bacteria (decomposers).
In each step, the number and mass of organisms is limited by the amount of energy available. Because some energy is lost in the form of heat, thus the steps become progressively smaller near the top. These trophic levels are shown graphically by means of pyramids, called ecological pyramids. In the pyramid, the producer level constitutes the base of the pyramid and tertiary consumers or decomposers level make the apex.

Q.19: What is the Pyramid of Energy?

Ans: PYRAMID OF ENERGY:
This pyramid shows the rate of energy flow or productivity at successive trophic levels. This pyramid is always upright and it gives the best picture of the overall nature of the ecosystem. It indicates the amount of energy available for successively higher trophic levels. In most cases, there is always a gradual decrease in the energy content at successive trophic levels from the producers to various consumers.

Q.20: What is Productivity (Energy Flow)?

Ans: PRODUCTIVITY (ENERGY FLOW):

• The most approximate rate of product obtained from photosynthesis during one year in an ecosystem is known as productivity.
• The productivity cannot be estimated from the crop of a field, because they obtain productivity is different. In an ecosystem, the productivity depends upon the sunlight...

Q.21: Distinguish between the following:

Respiration & Photosynthesis:

Respiration	Photosynthesis
It is a catabolic process, i.e., compounds are broken down.	It is an anabolic process, i.e., compounds are formed in this process.
In this process, carbohydrates are broken down into simpler compounds.	In this process, carbohydrates are manufactured.
This process starts with carbohydrates (glucose) and O₂.	This process starts with CO₂ and H₂O.
The end-products of this process are CO₂ and H₂O.	The end-products of this process are simple carbohydrates.
In respiration, O₂ enters the plant body and CO₂ is released.	In photosynthesis, CO₂ enters the plant and O₂ is given out.
It occurs during both day and night. Light is not necessary for this process.	It occurs during the daytime only. Light is necessary for this process.
It takes place inside mitochondria (Krebs cycle) and cytoplasm (glycolysis).	It takes place within mitochondria.
In respiration, energy is released from food material.	It is an energy-consuming process, but energy is stored in the form of food material.

 In this process, the dry weight of the plant body decreases.

In this process, dry weight of plant body increases.

During the breakdown of one glucose molecule, 38 ATP molecules are formed.
During the formation of one glucose molecule, 18 ATP molecules are utilized.

Light Dependent and Independent Phase of Photosynthesis:

Light Dependent Phase of Photosynthesis (Light Reaction)	Light Independent Phase of Photosynthesis (Dark Reaction)
This phase requires light.	This phase does not require light.
In this system, light is used in the photolysis of water. The water splits into Hydrogen & Oxygen.	In this system there is no photolysis of water.
In this phase, Hydrogen is attached with NADP compound, called reduction of NADP.	In this phase, Hydrogen of NADP takes part in the chemical process to form glucose.
In this process, CO₂ does not play any role.	In this phase, CO₂ enters the process and helps in the formation of glucose.
Glucose is not produced in this phase.	Glucose is produced in this phase.
In this phase, chlorophyll absorbs sunlight.	In this phase, there is no role of chlorophyll.
In this phase, Pigment System I (P.S.I) and Pigment System II (P.S.II) are involved.	P.S.I and P.S.II are not involved in this phase.

PS-I and PS-II (Pigment System I and Pigment System II)

PS-I (Pigment System I)	PS-II (Pigment System II)
Pigment System I (P.S-I) contains chlorophyll a, which can absorb maximum light of 700 nm, called P₇₀₀.	Pigment System II (P.S-II) consists of chlorophyll b which can absorb maximum light of 680 nm, called P₆₈₀.
It contains Ferredoxin reducing substrate (FRS) and Ferridoxin.	It contains plastoquinone, plastocyanin, and cytochromes.
In this system, Mn and chloride are not involved.	In this system, Mn and chloride are also involved.
In this system, reduction of NADP with Hydrogen takes place.	In this system, there is no reduction of NADP compound.

Photophosphorylation & Oxidative Phosphorylation:

Photophosphorylation	Oxidative Phosphorylation
In this process, formation of ATP compounds takes place in the presence of sunlight.	In this process, ATP compounds are not produced in the presence of sunlight. It is by the process of oxidation.
It occurs during the light reaction of photosynthesis.	It occurs in the process of respiration.
In this process, chlorophyll is necessary to absorb sunlight.	In this process, there is no role of chlorophyll.
In this process, light energy is used in the electron transport.	In this process, energy of glucose is used by oxidation in the electron transport.

Aerobic Respiration & Anaerobic Respiration:

Aerobic Respiration	Anaerobic Respiration
In this process O₂ is required.	In this process O₂ is not required.
In this process glucose is converted into CO₂ and H₂O.	In this process glucose is converted into CO₂ and Ethyl alcohol (or Lactic acid in animals).
In this process huge amount of energy from glucose molecule (673 k cal) is released.	In this process very less amount of energy is produced i.e. 28 k cal.
It occurs in all higher plants and animals.	It occurs in Bacteria and fungi, mostly which are decomposers.