Unit 6: Unit 6: Bioenergetics — Short Questions

Absorption Spectrum
A graph showing different wavelength absorbed by pigments is called absorption spectrum.

Action Spectrum
A graph showing effectiveness of different wavelength on the rate of process of photosynthesis is called action spectrum.

As CO₂ photosynthesis is redox (reduction-oxidation) reaction where CO₂ is reduced to form glucose and water is oxidized to release oxygen.

It is carbon dioxide that contributes oxygen to glucose while oxygen of water is released into atmosphere.

CO₂ acts as source of carbon for making sugars during photosynthesis. Sugar is formed by the reduction of CO₂ by using ATP and NADPH. In this way, CO₂ acts as the source of carbon for making sugars.

An electron transport chain is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions.

Glycolysis is breakdown of glucose molecule into two molecules of pyruvic acid.

Chlorophyll-a has CH₃ at second pyrrole ring while chlorophyll-b has CHO.

A cell can synthesize ATP through substrate level phosphorylation with the help of enzyme.

Pyruvic acid cannot enter Krebs cycle as such. It is first converted into acetyl CoA.

In C3-plants CO₂ reacts with ribulose bis PO4 molecule and results in a 3-C stable molecule 3 phosphoglyceric acid while in C4-plant, CO₂ combine with phosphoenol pyruvic acid (3-C molecule) and results in a 4-Carbon stable compound oxaloacetic acid.

Bioenergetics is the study of how energy flows through living systems. It explores the processes through which cells store and expend energy.

Nearly all the energy used by living organisms on Earth comes from photosynthesis. Plants, algae, and certain bacteria capture sunlight and convert it into chemical energy, forming the base of the food chain.

Cells use a special energy currency for their reactions. This currency is actually a nucleotide called adenosine triphosphate (ATP) When cells store energy, they make ATP. When cells need energy, they break ATP A molecule of ATP has three subunits i.e., adenine, (a nitrogen containing base); ribose (a five-carbon sugar) and three phosphate groups.

Photosynthesis involves the use of light energy that is absorbed and converted into chemical energy by photosynthetic pigments. Photosynthesis in plants can be summarized as:
6CO₂ + 12H₂O+Light —→ C₆H₁₂O₆ + 6O₂ + 6H₂O

At dawn and dusk, when light intensity is low, the rate of photosynthesis and respiration may be equal for a short time. Thus, the oxygen released from photosynthesis is just equal to the amount required for cellular respiration. Also, the carbon dioxide released by respiration is just equal to the amount required by photosynthesizing cells. At this moment there is no net gas exchange between leaves and atmosphere. This is termed as compensation point.

Plants convert only about 1-2% of the solar energy they receive into chemical energy during photosynthesis. Despite this seemingly low efficiency, this conversion is enough to sustain almost all life on Earth.

The different wavelengths are also differently effective in photosynthesis. The effectiveness of different wavelengths of light is determined in terms of action spectrum. For getting action spectrum of light, a plant is illuminated with different colours of light one by one. While providing each colour, the rate of photosynthesis is measured by measuring the amount of oxygen emitted from leaves. The data is plotted in a graph called action spectrum.

Light plays a crucial role in photosynthesis, providing the energy required to drive the chemical reactions that transform simple molecules into complex organic compounds. Light energy is absorbed by chlorophyll. The absorbed light energy is converted into chemical energy, which is in turn stored in organic compounds in the form of C-H bond energy.

Oxygen released during photosynthesis comes from water, and so water is an important source of atmospheric oxygen which most organisms need for aerobic respiration and thus for obtaining energy to live.

A molecule of chlorophyll consists of two parts.
1. Hydrophilic head: It is made of porphyrin ring made up of four pyrrole rings with magnesium atom in center.
2. Hydrophobic tail: Tail is made of long hydrocarbon change. It anchors the molecule in the thylakoid membrane.

About 10% of total photosynthesis is carried out be terrestrial plants, the rest occurs in oceans, lakes and ponds. Aquatic photosynthetic organisms use dissolved CO₂, bicarbonates and soluble carbonates as carbon source. Land photosynthesis organisms use atmospheric CO₂ as carbon source.

Photosynthetic pigments are present in thylakoid membranes. These pigments capture light energy necessary for photosynthesis. Some of the pigments are chlorophyll a, chlorophyll-b, xanthophylls, carotenes. Different pigments absorb light of different wavelengths (colours).

Each photosystem consists of two parts
1. Antenna complex: It has many pigment molecules reaction center.
2. Reaction center: The reaches center has one or more molecules of chlorophyll-a which pass the high energy electrons to primary electron acceptor.

The reaction centre has one or more molecules of chlorophyll-a which pass the light energy electron to a primary electron acceptor. This passes them on the series of electron carriers called electron transport chain.

Carotenoids, such as beta-carotene, play dual role in photosynthesis. They capture light energy in the blue and green regions of the spectrum and protect the photosynthetic apparatus from damage by excess light.

Accessory pigments include all the pigments, other than chlorophyll-a, which can gather light for photosynthesis. Chlorophyll b is an accessory pigment and others are carotenoids (carotenes and xanthophylls) and phycobilins. Chlorophyll b and carotenoids are found in plants while phycobilins are found in the red algae and cyanobacteria.
When accessory pigments absorb light, they pass on the energy towards chlorophyll a. It is generally believed that the order of transfer of energy in plants is;
Carotenoids → Chlorophyll b → Chlorophyll-a

Some wavelengths are not absorbed by chlorophyll-a are very effectively absorbed by chlorophyll-b and vice-versa. Such differences increase the range of light absorbed by both chlorophylls.

A graph showing different wavelengths absorbed by a pigment is called absorption spectrum of the pigment. Absorption peaks of carotenoids are different from those of chlorophylls. Action spectrum of photosynthesis also shows that blue and red parts lights are the most effective. This means that the action spectrum of photosynthesis coincides with the absorption spectrum of photosynthetic pigments.

During light dependent (light) reaction, light energy is absorbed and converted into chemical energy, which is in the form of reducing and assimilating powers i.e., NADPH and ATP.

Light independent (dark) reactions use NADPH and ATP for the reduction of CO₂ and thus store chemical energy in the form of C-H bond energy.

It is a way of production of ATPs during light dependent reactions. In this both photosystems i.e., PS-I and PS-II participate and two electron chains are involved. During this ATP and NADPH are produced alongwith evolution of O₂.

When water molecule reacts with oxidized chlorophyll in PS-II, it breaks into two hydrogen ions, an oxygen atom and two electrons. This water splitting step of photosynthesis is called photolysis.

The zigzag path taken by electrons through PS-II and PS-I and electron transport chains is called z-scheme.

Chemiomosis is the mechanism in which thylakoid membranes couple the redox reactions with the synthesis of ATP

These are called dark reactions because these can occur in the absence as well as in the present of light, as long as ATP and NADPH are available.

Since the product of initial carbon fixation is a three-carbon compound, the Calvin cycle is also known as C-3 pathway.

Rubisco (Ribulose bis Phosphate Carboxylase and oxygenase) mediates the first reaction of Calvin cycle.

During calcium cycle one molecule of glyceraldehyde 3PO₄ is produced which combines with another G3P and make Glucose during Calvin cycle.

The oxygen produced during photolysis of water is the main source of atmospheric oxygen.

1. Anaerobic respiration (fermentation) takes place in the absence of oxygen.
2. Aerobic respiration takes place in the presence of oxygen.

The exchange of CO₂ and O₂ between the organism and its environment is called external respiration or breathing. Cellular respiration is the process by which energy is made available to cells in a step-by-step oxidation of food in the cells.

In anaerobic respiration, glucose is not completely oxidized. This type of respiration yields relatively small amount of energy from glucose molecule. As a result of anaerobic respiration, one glucose molecule yields only two ATPs (only about 2% of the energy present in glucose). The energy in two ATPs is equivalent to 14.6 kcal.

a. Alcoholic Fermentation: In primitive prokaryotic cells (bacteria) and in some eukaryotic cells such as yeast, pyruvic acid is further broken down by alcoholic fermentation into alcohol (C₂H₅OH) and CO₂.

2(C₃H₆O₃) —4H→ 2(C₂H₅O₃)
Pyruvic acid Ethyl alcohol

b. Lactic Acid Fermentation: In lactic acid fermentation, each pyruvic acid molecule is converted into lactic acid C₃H₆O₃ in the absence of oxygen gas.

2(C₃H₆O₃) ——→ 2(C₃H₆ OH) + 2CO₂
Pyruvic acid Ethyl alcohol

Glucose enters cells from the tissue fluid by passive transport using a specific glucose carrier. This carrier can be controlled (gated) by hormones such as insulin.

Pyruvic acid can also be turned back into glucose by reversing glycolysis. This is called gluconeogenesis.

1. Glycolysis
2. Pyruvic acid oxidation
3. Krebs Cycle
4. Electron transport chain

In this process, an enzyme transfers a phosphate group from an organic substrate molecule to ADP. The products are a new organic molecule and a molecule of ATP. For example; during the last step of glycolysis, an enzyme transfers phosphate group from phosphoenol pyruvic (PEP) acid to ADP. As a result, ADP becomes ATP and PEP is changed into pyruvic acid. Substrate-level phosphorylation accounts for only a small percentage of the ATP that a cell generates. This reaction can be shown as:

In aerobic oxidation of glucose 38 molecules of ATPs are obtained. However, 2 ATP molecules are consumed in NADPH transfer therefore net profit of aerobic oxidation of glucose is 36 ATP.

Free glucose molecules are not common in out diet. Rather, we consume sucrose and other disaccharides, starch, fats and proteins. Proteins may also be used as fuel but they must be digested to their constituent amino acids. Typically, a cell uses most of the amino acids to make its own proteins.
Lipids are excellent cellular fuel because they contain many carbon-hydrogen bonds. They are first hydrolysed into glycerol and fatty acids. Glycerol is converted to glyceraldehyde 3-phosphate, an intermediate in glycolysis, while the fatty acids are changed into acetyl CoA. In this way both the fatty acids and the glycerol enter cellular respiration.

In C-4 photosynthesis, the energy cost for making a glucose molecule is almost double. However, in hot climates, in which photorespiration would otherwise remove more than half of the carbon fixed, it is best compromise available.

It is respiratory activity that occurs in green cells in the presence of light resulting in release of CO₂.

When relative concentration of CO₂ decreases and there is more oxygen in leaf cells, rubisco adds O₂ in RuBP instead of CO₂. It results in the break down of RuBP into two molecules i.e., one phosphoglycerate and one phosphoglycolate (2-C).

Rubisco is an enzyme, which is most abundant protein on earth.
It combines 3 molecules of CO2 with 3 molecules of five carbon sugar named ribulose bisphosphate to form six molecules of a three carbon compound called 3-phosphoglyceric acid.

Following these organelles are involved in photorespiration.
1. Chloroplast 2. Peroxisome
3. Mitochondria

1. C-4 plants: corn, sugarcane and sorghum.
2. CAM-plants: Cacti, pineapple, etc.

Since the product of initial carbon fixation is a three-carbon compound, the calvin cycle is also known as C-3 pathway.

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Because in cellular respiration there in complete breakdown of glucose into CO₂ and H₂O while in fermentation there is incomplete breakdown of glucose.

The conversion of glucose into ATP during cellular respiration considered a more efficient use of energy than burning glucose directly, because cellular respiration is an enzyme-controlled process. So, most of energy released in not wasted but converted into ATPs for use by the cell.

During photosynthesis CO₂ is used and oxygen released while during respiration oxygen is used and CO₂ released so both processes play role in maintaining the CO₂ and O₂ levels in atmosphere. Any disruption in their processes can cause imbalance of their concentration in atmosphere. Hence resulting in environmental pollution.