Class 9 Biology Chapter 7 Bioenergetics Notes

Class 9 Biology Chapter 7 Bioenergetics Notes

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Class 9 Biology Chapter 7 Bioenergetics Notes
Class 9 Biology Chapter 7 Bioenergetics Notes

Chapter No.7


Q1: Define Bioenergetics, Metabolism, and type of Metabolism and also write the importance of Bioenergetics?

Ans: Bioenergetics;


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



The word metabolism is derived from a Greek word meaning “change”.


Concept of metabolism was first of all given by Ibn-e-Nafees.


The sum of all Chemical reactions that takes place in living organisms is called metabolism.

Types of Metabolic reactions:

There are two types of metabolic reaction.

i)          Endergonic reaction (anabolism)

ii)         Exergonic reaction (catabolism)

  1. Endergonic reaction:


The reaction which required energy is called endergonic reaction.  (OR)

The reaction in which energy is stored is called endergonic reaction.



Importance of Bioenergetics:

  • Sun is the ultimate source of energy for all organisms.
  • Energy is transforming from one form to another form within living organisms.
  • Living organisms use energy for various life activities.
  • The green plants capture solar energy and convert it into store chemical energy in the process of photosynthesis.
  • This energy is then transformed to mechanical and heat energy during Respiration.

Q2. What is oxidation Reduction reaction? Write its importance.

Ans; Redox reaction:


A chemical reaction is which both oxidation and reduction occurs called Redox reaction.



Addition of oxygen or loss of hydrogen or electron is called oxidation.



The gaining or electron or hydrogen is called reduction.

Oxidation – reduction reaction:

Those reactions in which oxidation and reduction occur simultaneously is called oxidation-reduction or redox reaction.

Photosynthesis and respiration processes are basically oxidation-reduction processes. During photosynthesis energy of sunlight is absorbed by plants and use CO2 and H2O to make food molecule which involve oxidation-reduction reactions.

 Respiration is also oxidation-reduction reaction during which food molecule are broken down into CO2 and H2O and energy is released in the form of ATP which are used by the cell for life activities.


Due to redox reaction photosynthesis and respiration occur that produce energy which sustains life on earth.

Q3. What is ATP Molecule? Describe the structure of ATP Molecule?

Ans: ATP Molecule:-

The major energy currency for all cells is a nucleotide called ATP (Adenosine Tri Phosphate).

Discovery of ATP:

ATP was discovered in 1929 by Karl Lohmann.


The structure of ATP molecule consists of three components.

i.          Adenine (Double ring nitrogenous base).

ii.         Ribose sugar (5 carbon carbohydrates).

iii.        Phosphate groups (three molecule of phosphoric acid).

In ATP Molecule the adenine is covalently bonded to ribose sugar to form a Molecule called Adenosine to form ATP Molecule.

Bonds in ATP Molecules;

There are two types of phosphate bonds in ATP Molecules.

I. High energy bond:

These bonds are represented by symbol (~) and these bonds yield more energy (7.3 on hydrolysis.

ii. Low energy phosphate bond:

These bonds are represented by straight line (——). And they yield less energy on hydrolysis.

Q4. Write note on the synthesis, breaking and recycling of ATP?

Ans: Adenine is a nitrogenous base when covalently bonded to five carbons Ribose making a Molecule Adenosine.

Adenine + Ribose  Adenosine

i)          When Phosphate Molecule bonded with Adenosine forming a nucleotide called AMP (Adenosine mono phosphate)

ii)         By addition of phosphate with AMP molecules called ADP (Adenosine Di Phosphate)

AMP + Pi + energy – ADP

iii) By the addition of another phosphate with ADP form ATP.

ADP + Pi + energy – ATP

Breaking of ATP:

ATP + H2O —- ADP + Pi + energy (7.3 Kcal)

AMP cannot hydrolyze further.


ATP Molecules are constantly hydrolyzed by the cell into ADP and inorganic phosphate and energy is obtained for cellular functions.

At the same time ATP Molecules are constantly regenerated from ADP and phosphate using energy released from the breakdown of glucose Molecule in the process of Respiration. In this way a constant cycle of ATP broken down and reformation goes on in the living organism.

Q.5. Define Photosynthesis? Write its importance.

Ans: Photosynthesis:

Terminology: –

The word photosynthesis is derived from two Greek words.

i)          Photo mean light

ii)         Synthesis means manufacture


The process by which green plants prepare their own food (carbohydrates) from carbon dioxide and water in the presence of sunlight and chlorophyll and releasing oxygen as by product is called Photosynthesis.

Chemical equation:

  • Photosynthesis is energy storing process.
  • Due to photosynthesis green plants prepare their own food hence they are called Autotrophic organisms.
  • Heterotrophic organisms depend for their energy requirement on green plants.
  • Without photosynthesis life is impossible.

Q.6. Write the Role of Chlorophyll and Sunlight in Photosynthesis?

Ans: Chlorophyll:

Definition: –

The organic Molecules which enable plants to capture light energy and convert it into chemical energy for the formation of glucose (C6H6O6) are called chlorophyll.


In plants and algae chlorophyll is present in chloroplast while in photosynthetic prokaryotes it is present in the cell membrane.


Photosynthetic Pigments are organized in the form of clusters in the thylakoid membranes of chloroplast called photosynthesis.

Types of chlorophyll:

There are many types of chlorophyll.

Chlorophyll a, b, c, d, e and Bacteriochlorophyll

Chlorophyll a,

It is the main pigment found in all green plants and Algae except bacteria.

Chlorophyll b,

It is a photosynthetic pigment found in all higher plants and green Algae.

Chlorophyll (c, d, e):

It is found in various groups of Algae.

Absorption of light,

Chlorophyll (a) absorb mainly blue and red portion of sunlight. The green portion is mainly reflected therefore chlorophyll appear green. Chlorophyll absorb blue light have wavelength 390-430nm and red light have wavelength 670-700nm.

Role of light:

Sun is the main source of energy for all living organisms. Only a small amount of the total sunlight that strikes the green plants is used in the process of photosynthesis. This small portion of sun light sustains all life forms on earth.

Class 9 Biology Chapter 7 Bioenergetics Notes Free Download in PDF Format


Q.7. Describe the structure of chloroplast?

Ans: Chloroplast consist of three components

i. Outer membrane

ii.         Grana

iii.        Stroma

i. Outer membrane:

It is the double membrane covering that enclose the grana and stromal.

ii. Grana (sing. granum):

The granum is seen just like a pile of coins which is composed of stack of thylakoids.


(Gr: thylakoid mean sac or pouch)


Light reaction of photosynthesis occurs in grana of chloroplast.

iii. Stromal:

It is central large space in chloroplast contain enzymes and gel like solution called matrix.


Dark reaction of photosynthesis occurs in stromal of chloroplast.

Q8. Write the process of intake of carbon dioxide and water?

Ans: Intake of CO2:

CO2 is present in air (0.03 %) and is one of the raw materials of photosynthesis. CO2 is diffuse from outside air into the intercellular spaces of leaf through stomata. Stomata are small opening scattered particularly in the lower epidermis of leaf. Each stoma is guarded by two kidney shaped guards cell. Opening and closing of stomata regulates the diffusion of CO2, water vapours, and O2 between the intercellular spaces of leaf mesophyll and the external air. After diffusing into the intercellular spaces, CO2 attaches to the wet surface of mesophyll cells. then it diffuses into the green cells and enters the stromal of chloroplasts. In the dark reaction of photosynthesis, CO2 molecules are reduced to form glucose molecule.

Intake of water:

Water is also an essential requirement of photosynthesis. Water is absorbed by the roots of plants from the soil. It is then transported upward to their stem and leaves through xylem vessels. Most of this water is transpired from the leaves while a small portion is used in photosynthesis.

Q.9. Enlist the main events in mechanism of photosynthesis?

Ans: Mechanism of Photosynthesis:

The process of Photosynthesis is completed in two main steps.

i.          Light reaction (light dependent reaction).

ii.         Dark reaction (light independent reaction).

i.          Light reaction ( light dependent reaction)

  • The reaction is also called light dependent reaction.
  • Light reaction takes place in the grana of chloroplast.
  • Chlorophyll molecules absorb light energy and converted into chemical energy.
  • During light reaction photolysis take place.
  • In photolysis water molecules are broken down into hydrogen and oxygen in the presence of specific enzymes.
  • Oxygen is released while hydrogen atoms are used to reduce NADP into NADPH.
  • In this process various kinds of energy producing compounds such as energy source.


2H2O + 2NADP++ 3ADP + 3PiO2 = 2NADPH = 3ATP

ii.         Dark reaction ( Calvin Cycle):

  • Dark reactions were discovered by Melvin Calvin. That way this reaction is also called Calvin Cycle.
  • This is also called light independent reaction.
  • Dark reaction takes place in the stromal of chloroplast.
  • Chemical energy from the light reaction is used to reduce carbon dioxide for the synthesis of carbohydrates.


6CO2+ 24H + Chemical energy  C6h6O12 + 6H2O

Q.10. what is limiting factor? Write the concept of limiting factors in photosynthesis.

Ans: Limiting factor:

The concept of limiting factor was stated by Liebig’s law of limiting factors.

This law states that,

“When a chemical process is controlled by more than one factor then the rate of the chemical process will be limited by the factor which is present in minimum quantity in relation to other”.

Simple definition:

The absence or deficiency of any environmental factor which can decrease the rate of a metabolic reaction is called limiting factor.

Limiting factor for photosynthesis:

The process of photosynthesis is affected by many factors such as light, chlorophyll, Co2, water and optimum temperature but the rate of photosynthesis is limited by the factor which is in minimum value.


CO2 is one of the requirements of photosynthesis. When all the other requirements for photosynthesis are optimum and only CO2 is low. Then the low amount of CO2 will lower the rate of photosynthesis and thus act as a limiting factor.

i. Effect of CO2 Concentration:

CO2 comes from the air there are 0.03% CO2 atmosphere. Concentration of CO2 almost remains constant in air because it is produced in respiration in the bodies of living organisms. When the stomata are closed, the concentration of CO2 almost remains constant in air because it is produced in respiration in the bodies of living organisms. When the stomata are closed, the concentration of CO2 falls down in the Mesophyll tissues. This lower down the rate of photosynthesis. Without CO2 photosynthesis does not occur.

iii. Effect of temperature:

The optimum temperature is necessary for normal photosynthesis. Generally, temperature ranging between 20Co to 30Co is most suitable temperature. When temperature exceeds 30Co, the rate of photosynthesis lower down and below 20Cothe rate of photosynthesis decreases while on freezing temperature photosynthesis completely stops and at 45Cothe photosynthesis becomes stop.

Q11: Define respiration and its types.

Ans; Respiration:

Oxidation reduction process by which organic food is broken down to carbon dioxide and water and release energy inside the cell is called respiration.

Glucose is the most common food used in this process. The purpose of respiration is to release energy for the living processes. Respiration is common to all living organisms because all of them need energy for life activates.


There are two main types of respiration.

i. Anaerobic respiration;

The breakdown of organic food molecules to release energy in the absence of oxygen is called anaerobic respiration.

It is also called fermentation. It involves incomplete breakdown of organic food molecules and only a small amount of energy is released. It occurs in the cytoplasm of the cell.

ii. Aerobic respiration:

The breakdown of organic food molecules to release energy in the presence of oxygen is called aerobic respiration.

i.          Glycolysis

ii.         Krebs cycle

iii.        Electron transport Chain.

Q.12: Compare lactic acid fermentation with alcoholic fermentation?

Ans: Lactic Acid Fermentation:

It is called lactic Acid fermentation because the end product is Lactic Acid. It occurs in microorganisms just like bacteria during the fermentation of milk. It also occurs in muscle cells when the energy demand is high.


It is completed in two steps.

i. In this step glucose molecule are broken down into pyruvic Acid by the use of 2ATP Molecules and produce 4ATP Molecules So the net ATP produce in Glycolysis is 2ATP.

Glucose + 2ATP  4ATP + 2 pyruvic Acid.

ii. In the second step pyruvic Acids is reduced by NADH+H (co-enzyme) and convert into lactic acid.

Pyruvic Acid + NADH  Lactic Acid + Co2.

Alcoholic fermentation:

In alcoholic fermentation the end product is alcohol. It occurs in yeast and in some bacteria. i. The first stop is Glycolysis

Glucose + 2ATP  4 ATP + pyruvic Acid

ii. In the second step pyruvic Acid is reduced by NADH+H to ethyl Alcohol (ethanol). In alcoholic fermentation also two ATP molecules are Pyruvic Acid + NADH Alcohol+ CO2.

Q.13: Describe various steps of Aerobic Respiration.

Ans: Aerobic respiration:

The breakdown of organic food molecules to release energy in the presence of oxygen is called aerobic respiration.


C6H6O12 + 6O2 6CO2 6H2O + 36 ATP.

Steps of aerobic respiration:

Aerobic respiration completes in three steps.

  1. Glycolysis II. Krebs cycle

II. Electron transport chain.

I. Glycolysis:


  • Glycol mean glucose.
  • Lysis mean splitting.


The stepwise enzymatic breakdown of glucose up to the formation of Pyruvic Acid is called glycolysis.


It takes place in cytoplasm.


In this step a glucose molecule (6-c) is broken into two molecules of pyruvic acid (3-C). In this process two ATP molecules are used and four are produced. So, the net gain of two ATP molecules.

ii.         Krebs cycle:


The cyclic process in which high energy pyruvic acids are completely oxidized into CO2 and H2O is called Krebs cycle.


It occurs in mitochondria because inside mitochondrial necessary enzymes are present for reaction.


This step was first discovered by sir Hans Krebs therefore named after his name Krebs cycle.


 Before entering to kreb cycle, pyruvic acid is changed into a 2-Carbon compound called acetyl-Coenzyme A. It then goes through a series of reaction in which it is completely oxidized. These reactions produced CO2 along with ATP, NAD and FAD, NAD and FAD are also reduced into NADH and FADH respectively.

iii.        Electron transport chain:


The final step of aerobic respiration in which the oxidation of reduced Coenzyme NADH and FADH take place that produce in glycolysis and kreb cycle.


It is the last step of aerobic respiration that occurs in mitochondria.


During this step two electrons are released from NADH and FADH. These electrons are than pass a series of electron carrier called cytochrome. During the transport from one cytochrome to the other electron loss energy. This energy is used to form ATP from ADP and Pi. it the end, oxygen molecule accept electron to form water.

Q.14: How much energy is produced during respiration?

Ans: The complete oxidation of one molecule of glucose into CO2 and water generate 38 ATP. During glycolysis, two ATP molecules are also used. So, there is net gain of 36 ATP from one glucose molecule. These ATP are generated step wise some during glycolysis, some during crab cycle and many are generated during electron transport chain. Each NADH generates 3 ATP molecules and each FADH molecule produce 2 ATP molecule.

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