Photosynthesis: Provider of Food for All

ICSE Class 10 Biology • Chapter 06 • Detailed Master Notes

Photosynthesis is the most significant life process. It provides food for all animal life (including humans) and the life-supporting free oxygen gas in the atmosphere for breathing.

6.1 What is Photosynthesis?

Photosynthesis is the process by which living plant cells, containing chlorophyll, produce food substances (glucose and starch), from carbon dioxide and water, by using light energy. Plants release oxygen as a by-product during this process.

Importance of Photosynthesis:

Provides food for all: Plants are the ultimate source of food and energy for all living organisms (directly for herbivores, indirectly for carnivores).
Provides Oxygen: It is the only biological process that releases free oxygen into the atmosphere, which is essential for respiration and combustion.

6.2 Chlorophyll — The Vital Plant Pigment

Chlorophyll is the green pigment found in plants. It is contained in microscopic cell organelles called Chloroplasts.

Chloroplasts are minute oval bodies bounded by a double membrane.
Their interior contains closely packed flattened sacs (thylakoids) arranged in piles called grana.
The colourless ground substance is called the stroma.
Chlorophyll is contained directly in the walls of the thylakoids. It is a highly complex substance, composed of carbon, hydrogen, oxygen, nitrogen, and magnesium.

Fig 6.3 & 6.4: Structure of a Chloroplast

Fig. 6.3 & 6.4: Three dimensional and highly magnified diagrammatic view of a chloroplast showing Grana (thylakoids) and Stroma. (Very Important for ICSE exams)

Interesting Facts about Chlorophyll:
There are nine types of chlorophyll. The two most prominent are Chlorophyll-a and Chlorophyll-b. Chlorophyll absorbs light at both ends of the visible spectrum (blue and red light) and reflects the green light. That is why leaves appear green!
Too much light destroys chlorophyll. However, it is continuously reformed by the plant (provided there is light).

Location of Chloroplasts

Chloroplasts are mainly contained in the mesophyll cells located between the upper and lower epidermis of leaves. These are of two types: palisade cells and spongy cells. They are also found in the guard cells of stomata and in the outer layers of young green stems.

6.3 Regulation of Stomatal Opening for Letting in $CO_2$

Stomata are minute openings occurring in large numbers on the lower surface of a dicot leaf. The main function of stomata is to let in $CO_2$ from the atmosphere for photosynthesis. When stomata are open, water is lost by transpiration.

The closing and opening of stomata are controlled by the Guard cells. Two main theories explain this mechanism:

1. Potassium Ion ($K^+$) Concentration Theory (Recent & Accepted)

According to recent researchers, the stomatal opening depends on the generation of potassium ion ($K^+$) gradient.

During the day: The chloroplasts in the guard cells photosynthesize, leading to the production of ATP. This ATP is used to actively pump Potassium ions ($K^+$) from the adjacent subsidiary cells into the guard cells. This increased $K^+$ concentration makes the guard cells hypertonic, drawing in water by endosmosis. The guard cells become turgid and bulge outwards, opening the stoma.
During the night: The $K^+$ ions leak out of the guard cells, reducing their concentration. Water leaves the guard cells by exosmosis, making them flaccid, and the stoma closes.

Fig 6.5 & 6.6: Opening and closing of stoma based on Potassium Ion theory

Fig. 6.5 & 6.6: Diagrammatic representation of opening and closing of stoma based on the Potassium ion ($K^+$) concentration theory. (Highly important ICSE diagram)

2. Sugar Concentration Theory (Old)

During the daytime, guard cells photosynthesize and produce sugar (glucose). This increases the osmotic pressure of the cell sap, causing endosmosis. The guard cells become turgid and the stoma opens. At night, the sugar is converted into insoluble starch, lowering the osmotic pressure, causing exosmosis and closing the stoma.

6.4 The Process of Photosynthesis

Balanced Chemical Equation:

$$ 6CO_2 + 12H_2O \xrightarrow{\text{light energy, chlorophyll}} C_6H_{12}O_6 + 6H_2O + 6O_2 \uparrow $$

(Note: It is important to write $12H_2O$ on the reactant side and $6H_2O$ on the product side to accurately represent the photolysis of water).

Phases of Photosynthesis

Photosynthesis occurs in two main phases:

A. Light-Dependent Phase (Photochemical Phase)

This phase takes place in the Thylakoids (Grana) of the chloroplasts because it requires light energy.

Activation of Chlorophyll: Chlorophyll molecules absorb photons of light and become activated.
Photolysis of water: The absorbed energy is used to split water molecules into hydrogen ions ($H^+$) and oxygen.
$$ 2H_2O \xrightarrow{\text{light energy}} 4H^+ + 4e^- + O_2 \uparrow $$ The oxygen ($O_2$) is released into the atmosphere.
Production of reducing agent (NADPH): The $H^+$ ions are picked up by a compound called NADP (Nicotinamide Adenine Dinucleotide Phosphate) to form NADPH.
$$ NADP + e^- + H^+ \xrightarrow{\text{enzyme}} NADPH $$
Photophosphorylation: Electrons are used in converting ADP (Adenosine Diphosphate) into energy-rich ATP (Adenosine Triphosphate) by adding one inorganic phosphate ($P_i$).
$$ ADP + P_i \rightarrow ATP $$

B. Light-Independent Phase (Biosynthetic / Dark Phase)

This phase takes place in the Stroma of the chloroplasts. It does not require light directly but depends on the products of the light phase (ATP and NADPH).

The hydrogen from NADPH is used to reduce Carbon dioxide ($CO_2$) to produce Glucose ($C_6H_{12}O_6$).
This process utilizes the energy stored in the ATP produced during the light phase.
Several glucose molecules are transformed to produce one molecule of Starch (polymerisation).

6.5 Adaptations in Leaf for Photosynthesis

Leaves are perfectly adapted to maximize photosynthesis:

Large surface area: To absorb maximum light.
Leaf arrangement: At right angles to the light source to obtain maximum light.
Cuticle and upper epidermis: Transparent and waterproof to allow light to enter freely.
Numerous stomata: Present on the lower surface to allow rapid exchange of gases ($CO_2$ in, $O_2$ out).
Thinness of leaf: Reduces the distance for rapid transport of materials.
Chloroplast concentration: More in the upper layers (palisade mesophyll) to trap more light.
Extensive vein system: For rapid transport of water to the mesophyll cells and transport of manufactured food away from them.

6.6 Factors Affecting Photosynthesis

External Factors

Light Intensity: Rate increases with light intensity up to a certain point. Beyond that, the plant gets "light saturated" and the rate stabilizes. Very high light intensity destroys chlorophyll.
Carbon dioxide ($CO_2$): An increase in $CO_2$ concentration increases the rate of photosynthesis (but very high concentrations become toxic).
Temperature: Rate increases with temperature up to an optimum of $35^\circ C$. Beyond $40^\circ C$, the rate drops sharply because the enzymes involved get destroyed (denatured).
Water Content: Less water reduces photosynthesis because stomata close, stopping $CO_2$ intake.

Internal Factors

Chlorophyll: Nutritional deficiencies can lead to loss of chlorophyll, reducing photosynthesis.
Protoplasm: Dehydration or accumulation of carbohydrates reduces the rate.
Structure of Leaf: Cuticle thickness, stomata distribution, and leaf size directly affect the process.

6.7 Experiments on Photosynthesis

(Very important for ICSE Practical/Section B questions)

1. Destarching the Plant

Before any experiment, the plant must be destarched by keeping it in complete darkness for 24-48 hours. This ensures that any starch present in the leaves is consumed or transferred, and any new starch found after the experiment was formed during the experiment.

2. Testing a Leaf for Starch (Iodine Test)

Dip the leaf in boiling water for a minute to kill the cells and burst the starch grains.
Boil the leaf in methylated spirit over a water bath until it becomes pale-white. (Reason: To extract chlorophyll so the colour change is visible. We use a water bath because alcohol is highly flammable).
Wash the leaf in warm water to soften it.
Place the leaf in a petri dish and add a few drops of Iodine solution.
Result: If starch is present, it turns blue-black. If absent, it remains brown/yellow.

3. Experiment to show Chlorophyll is necessary

Use a plant with variegated leaves (leaves with green and non-green patches, e.g., Coleus, Geranium). Destarch the plant. Keep it in sunlight for a few hours. Pluck a leaf, trace its outline on paper marking green/non-green areas, and test for starch.
Observation: Only the previously green parts turn blue-black.

4. Experiment to show Light is necessary

Destarch a potted plant. Attach a piece of black paper with a cut-out design (e.g., a star) on one of the leaves. Keep the plant in sunlight for a few hours. Test the leaf for starch.
Observation: Only the exposed part (the star shape) turns blue-black.

5. Experiment to show $CO_2$ is necessary (Moll's Half-leaf Experiment)

Destarch a plant. Insert half of a leaf into a conical flask containing Potassium Hydroxide (KOH).
Reason: KOH absorbs all the $CO_2$ inside the flask.
Keep the setup in sunlight. After a few hours, test the leaf for starch.
Observation: The half of the leaf outside the flask turns blue-black (received $CO_2$), while the half inside the flask remains brown (did not receive $CO_2$).

Fig 6.13: Experiment to prove that Carbon dioxide is necessary for photosynthesis

Fig. 6.13: Experiment to prove that $CO_2$ is necessary (Moll's Half-leaf Experiment). Note the split-cork and the KOH in the flask.

6. Experiment to show Oxygen is produced

Place an aquatic plant (like Hydrilla or Elodea) in a beaker containing water. Invert a short-stemmed funnel over it. Invert a test tube full of water over the stem of the funnel. Add a pinch of sodium bicarbonate to the water (to provide $CO_2$). Keep the setup in sunlight.
Observation: Bubbles of gas rise from the plant and collect in the test tube. Testing the gas with a glowing splinter causes it to burst into flame, proving it is Oxygen.

Fig. 6.14: Experiment to show that oxygen is evolved during photosynthesis using a Hydrilla plant.

6.8 The Carbon Cycle

The carbon cycle is the series of chemical reactions in which carbon as $CO_2$ is removed from the atmosphere, used by living organisms in their body processes, and is finally returned to the atmosphere.

Steps in Carbon Cycle:

Photosynthesis: Green plants use atmospheric $CO_2$ to manufacture food (carbon compounds).
Food Chains: The carbon compounds are passed from plants to herbivores, and then to carnivores.
Respiration: All plants and animals respire, breaking down glucose and returning $CO_2$ to the atmosphere.
Decay: Bacteria and fungi decompose dead bodies, releasing $CO_2$.
Combustion: Burning of fossil fuels (coal, petroleum) and wood releases huge amounts of $CO_2$.

Exam Practice Questions (ICSE PYQ Trends)

Frequently Asked (1 Mark)

NAME THE FOLLOWING The splitting of water molecules into hydrogen and oxygen in the presence of light.
Ans: Photolysis
NAME THE FOLLOWING The chemical used to absorb carbon dioxide during photosynthesis experiments.
Ans: Potassium Hydroxide (KOH)
NAME THE FOLLOWING The energy currency of the cell produced during the light reaction.
Ans: ATP (Adenosine Triphosphate)
NAME THE FOLLOWING An aquatic plant commonly used in experiments to demonstrate the release of oxygen.
Ans: Hydrilla / Elodea

Reasoning

REASONING Answer the following:

Why is the leaf boiled in methylated spirit over a water bath during the starch test?
Ans: The leaf is boiled in methylated spirit to remove the green pigment chlorophyll so the colour change from the iodine test can be clearly observed. A water bath is used because alcohol is highly flammable.
Why is it necessary to destarch the plant before performing any experiment on photosynthesis?
Ans: To ensure that any starch present in the leaves is consumed, and any new starch found after the experiment was synthesized strictly *during* the experimental conditions.
Why does the rate of photosynthesis drop sharply at temperatures above $40^\circ C$?
Ans: Photosynthesis is an enzyme-driven process. At high temperatures (above $40^\circ C$), the enzymes get destroyed (denatured), halting the process.

Differentiate

DIFFERENCES Differentiate between the following pairs:

Light-dependent phase and Light-independent phase: The light-dependent phase occurs in the thylakoids (grana) and requires light to produce ATP and NADPH. The light-independent phase occurs in the stroma and uses the ATP/NADPH to reduce $CO_2$ to glucose (does not directly require light).
Stroma and Grana: Grana are piles of flattened sacs (thylakoids) containing chlorophyll where the light reaction occurs. Stroma is the colourless ground substance of the chloroplast where the dark reaction occurs.