Review Questions

Bioenergetics:

Bioenergetics is the study of energy relationships and energy transformations (conversions) in living organisms.

Bioenergetics and the Role of ATP:

Organisms obtain energy by metabolizing the food they eat or prepare. The food contains potential energy in its bonds. When these bonds are broken down, a large amount of kinetic energy is usually released. Some of this energy is stored in the form of potential energy in the bonds of ATP molecules while the rest escapes as heat. The potential energy stored in ATP is again transformed into kinetic energy to carry out life activities.

Bioenergetics and Oxidation-Reduction Reactions:

For all life processes, oxidation-reduction reactions are the direct source of energy Oxidation-reduction reactions (Redox reactions) involve the exchange of electrons between atoms.

Oxidation:

The loss of electrons is called oxidation.

Reduction:

The gain of electrons is called reduction.

Role of Electrons in Bioenergetics:

Electrons can be an energy source. It depends upon their location and arrangement in atoms. For example; when they are present in oxygen, they make a stable association with oxygen atoms and are not a good energy source. But if electrons are dragged away from oxygen and attached to some other atom e.g. carbon or hydrogen, they make an unstable association. They try to move back to oxygen and when this happens, energy is released.

Role of Redox Reactions:

In living, organisms’ redox reactions involve the loss and gain of hydrogen atoms. We know that a hydrogen atom contains one proton and one electron. It means that when a molecule loses a hydrogen atom, it actually loses an electron, and similarly, when a molecule gains a hydrogen atom, it gains an electron.

ATP (adenosine triphosphate):

The major energy currency of all cells is a nucleotide called adenosine triphosphate (ATP), ATP was discovered in 1929 by Karl Lohmann, and was proposed to be the main energy-transfer molecule in the cell by the Nobel prize winner, Fritz Lipmann in 1941.

ATP: The Cell's Energy Currency:

ATP is the main energy source for the majority of the cellular functions like the synthesis of macromolecules (DNA, RNA, and proteins), movement, the transmission of nerve impulses, active transport, exocytosis, endocytosis, etc.

The ability of ATP to store and release energy is due to its molecular structure.

Sub-Units of ATP:

Each ATP molecule has three subunits:

(a) adenine -a double-ringed nitrogenous base;

(b) a ribose - a five-carbon sugar, and

(c) three phosphate groups in a linear chain.

ATP Transfers Energy Between Metabolic Reactions:

The covalent bond connecting two phosphates is indicated by the "tilde" (~) and it is a high-energy bond. The energy in this bond is released as it breaks and inorganic phosphate (Pi) gets separated from ATP.

Energy Released by breaking off one phosphate bond: The breaking of one phosphate bond releases about 7.3 kcal (7,300 calories) per mole of ATP as follows:

ATP + H₂O → ADP + P₁ + energy (7.3 kcal/mole)

The energy from ATP is sufficient to drive most of the cell's energy-requiring reactions. In common energy, I reactions only the outermost of the two high-energy bonds break. When this happens, ATP becomes ADP (adenosine diphosphate) and one P_i is released. In some cases, ADP is further broken down to AMP (adenosine monophosphate) and P: as follows:

ADP + H₂O → AMP + P₁ + energy (7.3 kcal/mole)

Recycling of ADP:

Cells constantly recycle ADP by recombining it with Pi to form ATP the synthesis of ATP from ADP and P (requires the expenditure of 7.3 kcal of energy per mole. This energy is obtained from the oxidation of foodstuff. So, we can summarize that ATP is generated by energy-releasing processes and is broken down by energy-consuming processes. In this way, ATP transfers energy between metabolic reactions.

Role of Light in photosynthesis:

Sunlight energy is absorbed by chlorophyll. It is then converted into chemical energy, which drives the photosynthetic process. Not all the light falling on the leaves is absorbed. Only about one percent of the light falling on the leaf surface is absorbed, the rest is reflected or transmitted.

The light rays of different wavelengths are not only differently absorbed by photosynthetic pigments but are also differently effective in photosynthesis. The blue and red lights carry out more photosynthesis.

The function of Chlorophyll:

When chlorophyll absorbs light, its electrons are excited and they leave a chlorophyll molecule. The excited electrons are passed through the electron transport chain and their energy is captured for the formation of ATP and for reducing NADP to NADPH.

Role of Chlorophyll in photosynthesis:

The photosynthetic pigments are organized in the form of clusters, called photosystems, in thylakoid membranes of chloroplasts. Chlorophyll-a is the main photosynthetic pigment.

Others are called accessory pigments and include Chlorophyll-b and carotenoids. Chlorophylls absorb mainly blue and red lights. Some wavelengths not absorbed by chlorophyll 'a' are very effectively absorbed by accessory pigments and vice-versa.

Photosynthesis: Photosynthesis is an anabolic (building) process and is an important component of bioenergetics in living systems.

Preparation of Organic Food: Autotrophic organisms use inorganic raw materials such as carbon dioxide and water for the preparation of organic food, which primarily comprises carbohydrates. The organic food thus prepared may be used for getting energy or maybe converted to 10 other forms such as proteins, lipids, etc.

All Life Forms are Dependent on Photosynthesis: Photosynthesis is the synthesis of glucose from carbon dioxide and water in the presence of sunlight (and chlorophyll), with oxygen as a by-product. It is the most important biochemical pathway and nearly all life depends on it. It comprises many coordinated biochemical reactions that occur in plants, some protists (algae), and some bacteria.

General Equation for Photosynthesis: A simple general equation for photosynthesis is as follows:

6CO₂ + 12H₂O + photons → C₆H₁₂O₆ + 6O₂ + 6H₂O

Carbon dioxide + Water + Light Energy → Glucose + Oxygen + water

Mechanism of Photosynthesis:

Photosynthesis occurs in two phases.

First Phase of Photosynthesis (Light Reactions):

During the first phase, light energy is captured and is used to make high-energy molecules (ATP and NADPH). These reactions, which are known as light reactions, take place on the thylakoid membranes of chloroplasts.

Second Phase of Photosynthesis (Dark Reactions):

During the second phase, carbon dioxide is reduced to make glucose. The energy in the form of ATP is utilized in this process and is then stored in the bonds of glucose. Since these reactions do not use light directly, they are known as dark reactions. The dark reactions take place in the stroma of the chloroplasts.

Intake of Water and Carbon dioxide:

Water and carbon dioxide are the raw materials of photosynthesis. The plants have elaborate mechanisms for the intake and transport of these raw materials.

Intake of Water by root hairs:

Water and salts are absorbed by the roots and roots hairs which provide the larger surface areas for this absorption. The sap present in the root hairs is more concentrated (contains more salts) as compared to the dilute solution (containing fewer salts) present in the surrounding soil. Thus, water enters from the soil into the root hairs by osmosis thus diluting their sap as compared to the inner cells of the root. In this way, water enters the inner cells of the root and eventually reaches xylem vessels. The xylem vessels transport water to the leaves using the force of transpiration pull.

Intake of Carbon dioxide into the mesophyll cells:

The air that enters the leaf through tiny pores (stomata) diffuses into the air spaces present around mesophyll cells. This air carries CO₂, which gets absorbed in the thin layer of water surrounding the mesophyll cells. From here the carbon dioxide diffuses into the mesophyll cells.

Effect of varying light intensity:

The rate of photosynthesis varies with light intensity. It decreases as the light intensity decreases and increases as the intensity increases. However, at a much higher light intensity, the rate of photosynthesis becomes constant.

Effect of varying temperature:

The rate of photosynthesis decreases with a decrease in temperature. It increases as the temperature is increased over a limited range. But if the light intensity is low, increasing the temperature has little influence on the rate of photosynthesis.

Effect of varying carbon dioxide concentration:

As carbon dioxide concentration rises, the rate of photosynthesis goes on increasing until limited by other factors. The initial enzyme of dark reactions, which combines carbon dioxide with a 5-carbon compound, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, the enzyme will capture carbon dioxide. However, if the oxygen concentration is high, it will bind oxygen instead of carbon dioxide and there would be no photosynthesis. An increase in carbon dioxide concentration beyond a threshold level causes the closure of stomata and decreases the rate of photosynthesis.

Anaerobic Respiration (Fermentation):

In the absence of oxygen, glucose is incompletely oxidized with less amount of energy released. In anaerobic respiration, the first phase is exactly similar to that of aerobic respiration. A molecule of glucose is broken down into two molecules of pyruvic acid. But in the second phase, Pyruvic acid is not completely oxidized (due to the absence of oxygen). It is transformed into ethyl alcohol or lactic acid. In this way, many of the C-H bonds are left unbroken in the products. Anaerobic respiration is further classified as:

A- Alcoholic fermentation:

It occurs in bacteria, yeast, etc. In this type of anaerobic respiration, pyruvic acid is further broken down into alcohol (C₂H₅OH) and CO₂.

2(C₃H₄O₃) + 4H → 2(C₂H₅OH) + 2CO₂

Pyruvic acid Hydrogen → Ethyl Alcohol + Carbon dioxide

B- Lactic acid fermentation:

It occurs in the skeletal muscles of humans and other animals during extreme physical activities when oxygen cannot be transported to the cells as rapidly as it is needed. This also happens in the bacteria present in milk. In this type of anaerobic respiration, each pyruvic acid molecule is converted into lactic acid (C₂H₆O₃).

2(C₃H₄O₃) + 4H = 2(C₂H₆O₃)  

Pyruvic acid Hydrogen - Lactic acid

Mechanism of Respiration:

Aerobic respiration is a continuous process, but for convenience, we can divide it into three main stages;

a- glycolysis,          b-Krebs cycle, and         c-electron transport chain.

Glycolysis occurs in the cytoplasm and oxygen is not involved at this stage. That is why it occurs in both types of respiration i.e. anaerobic and aerobic. The other two stages occur within mitochondria where the presence of oxygen is essential.

Glycolysis:

In glycolysis, the glucose molecule is broken into two molecules of pyruvic acid. Following is the summary of glycolysis.

1. Two ATPs add phosphates to glucose molecules to make them reactive.
2. The reactive compound breaks into two molecules of the 3-carbon compound
3. Each 3-carbon compound is oxidized and its hydrogen atoms are transferred to NAD⁺. During this step, one more phosphate is added to each compound.
4. Each 3-carbon compound gives up phosphates to form 2 ATPs and then it is converted to pyruvic acid.

Krebs Cycle:

In the Krebs cycle, the pyruvic acid molecules are completely oxidized into CO₂ and H₂O. Before entering in Krebs cycle, pyruvic acid is changed into a 2-carbon compound called acetyl-CoA. One molecule of CO2 and one NADH are also produced during this reaction.

After the formation of acetyl, Co-A, the reactions of the Krebs cycle start:

1. The Co-A reacts with a 4-carbon compound and produces a 6-carbon compound.
2. The 6-carbon compound releases CO₂ in two steps and is changed into a 4-carbon compound. Two NADH and one ATP are also produced in these steps.

3. The 4-carbon compound goes through several reactions and is finally converted to the original 4. carbon compound of the first step. One FADH2 and one NADH are produced during these reactions.

Note:

A British biochemist, Sir Hans Krebs discovered this series of reactions that is why it is called the Krebs cycle.

Difference between aerobic and anaerobic respiration 

The Energy Budget of Respiration:

1. Each NADH molecule is generated in the link step (between glycolysis and Krebs cycle) and the Krebs cycle produces three ATP molecules in the electron transport chain. 

2. While each NADH generated in glycolysis gives a profit of two ATP molecules because it has to be transported across the mitochondrial membrane and it costs one ATP
3. Each FADH2 molecule produces two ATP molecules.
4. Note that during anaerobic oxidation of a glucose molecule only 2 ATP molecules are gained as the net profit. It is because there is no Krebs cycle and electron transport chain in anaerobic respiration. In this way, the respiratory energy is used in the body of organisms

Difference between photosynthesis and respiration:

Adaptations in Leaf structure for photosynthesis:

The external and internal structure of the leaf is well adapted to its function i.e. photosynthesis. The leaves on the branches of plants are arranged in such a manner that they get maximum sunlight for photosynthesis. Similarly, the majority of the leaves are flat with a maximum surface area for maximum absorption of the fallen light.

The function of the Cuticle:

The upper surface of the leaves called the epidermis is thin and is composed of a single-cell layer. It is covered by a layer of cuticle, which lessens the evaporation of water from the surface.

The function of Stomata:

The epidermis contains tiny pores known as stomata through which the exchange of gases between leaf cells and the environment occurs.

The function of Mesophyll Cells:

Inside the thin epidermis, specialized photosynthetic cells of leaves, called mesophyll cells, are present. The upper layers of mesophyll cells are compact and are called palisade mesophyll while the lower layer, called spongy mesophyll, contains many air spaces among its cells.

Passage of Air through Leaf:

Air enters the leaf through stomata and diffuses into the air spaces in the leaf. The carbon dioxide present in the air first gets dissolved in the thin watery layer over the mesophyll cells and then diffuses into the cells. Mesophyll cells contain chloroplasts which are the chief organelles concerned with photosynthesis. In higher plants, leaves also contain vascular (xylem and phloem) supply. Xylem vessels transport water and salts to the leaves while phloem vessels transport food from leaves to other parts.

All Life Forms are Dependent on Photosynthesis:

Photosynthesis is the synthesis of glucose from carbon dioxide and water in the presence of sunlight (and chlorophyll), with oxygen as a by-product. It is the most important biochemical pathway and nearly all life depends on it. It comprises many coordinated biochemical reactions that occur in plants, some protists (algae), and some bacteria.

Intake of Water and Carbon dioxide:

Phenomena of photosynthesis and osmosis are involved in the intake of carbon dioxide and water by plants.

Intake of Water by root hairs:

The xylem vessels transport water to the leaves using the force of the transpirational pull.

Intake of Carbon dioxide into the mesophyll cells:

The air that enters the leaf through tiny pores (stomata) diffuses into the air spaces present around mesophyll cells. This air carries CO₂, which gets absorbed in the thin layer of water surrounding the mesophyll cells. From here, the carbon dioxide diffuses into the mesophyll cells.

Bond energy transformed into chemical energy of ATP.

Importance of anaerobic respiration:

1. Source of energy for anaerobic organisms.
2. Source of energy for aerobic organisms in short supply of O₂.
3. Source of many products (ethanol, cheese etc).