Photosynthesis Class 10 ICSE — Selina Chapter 6

Photosynthesis class 10 ICSE — in Selina Concise Biology Chapter 6 — explains how green plants make their own food. This ICSE chapter covers the chemical equation, the two phases, the pigments, stomata, the experiments and the factors that limit the rate. Each section below follows the book’s own order and numbering, with the formulas explained and the Selina chapter PDF for revision. Read it with your textbook open and every detail will connect.

Download Photosynthesis Class 10 ICSE Biology Chapter 6 PDF

Download the Photosynthesis Class 10 ICSE Biology chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 12 pages, 12 sections, 16 questions.

What is inside Chapter 6

12 pages · 12 sections · 16 exercise questions

• PROGRESS CHECK: 1 questions
• PROGRESS CHECK: 7 questions
• PROGRESS CHECK: 3 questions
• PROGRESS CHECK: 5 questions

6.1 PLANTS — SELF FOOD PRODUCERS

Every living organism needs food to stay alive. Animals obtain food from plants, either directly or through other animals. Plants cannot move to find food. So they prepare it for themselves. They do this by photosynthesis. This makes plants the starting point of all food on Earth.

Photosynthesis class 10 ICSE begins here, with this simple fact. The chapter builds every later idea on it. The leaf is the plant’s food factory. The raw materials arrive through roots and stomata. The product feeds every other living thing.

6.2 WHAT IS PHOTOSYNTHESIS ?

Photosynthesis is the process by which living plant cells containing chlorophyll produce food substances. The food substances are glucose and starch. The raw materials are carbon dioxide and water. The energy comes from light. Oxygen is released as a waste product. The same essential chemical steps occur in all green plants.

Why does photosynthesis matter? Two reasons stand out. First, food for all. Plants make food directly. Animals and humans get it indirectly by eating plants or plant-eaters. Second, oxygen to breathe. Photosynthesis is the only biological process that releases oxygen into the atmosphere. That oxygen supports all life on Earth.

6.3 CHLOROPHYLL – THE VITAL PLANT PIGMENT

Chlorophyll is the green colouring matter in plants. The name comes from Greek: chloro means green and phyll means leaf. It sits inside small organelles called chloroplasts.

A chloroplast is a minute oval body with a double membrane. Inside are flattened sacs called thylakoids, arranged in piles called grana. The grana lie in a colourless ground substance called stroma. A cell may hold 40 to 50 chloroplasts. The pigment sits in the walls of the thylakoids.

Chlorophyll is a complex substance. It contains carbon, hydrogen, oxygen, nitrogen and magnesium. There are nine types. Chlorophyll-a and chlorophyll-b are the most abundant. Chlorophyll absorbs blue and red light and reflects green light. That is why leaves appear green.

Where is chlorophyll found? In the mesophyll cells of leaves, in guard cells, and in young green stems. Too much light destroys chlorophyll. Grass under a stone turns yellowish because no new chlorophyll forms in the dark.

6.4 REGULATION OF STOMATAL OPENING FOR LETTING IN CO₂

Stomata are minute openings on the lower surface of a leaf. They let carbon dioxide in from the atmosphere for photosynthesis. When it is dark, they close. This minimises water loss through transpiration. After sunrise, they reopen to let carbon dioxide diffuse in.

Transpiration happens along with photosynthesis. The book says transpiration is the price the plant pays for photosynthesis. Read more about this in the transpiration chapter of this book.

Stomata open and close because of water movement in guard cells. Each guard cell has a thick inner wall and a thin outer wall. Its cytoplasm contains chloroplasts. Two theories explain the movement.

• Sugar concentration theory (old): during the day, guard cells photosynthesise. The glucose produced raises osmotic pressure. Water enters by endosmosis. The cells become turgid and bulge outwards, widening the stoma. If water is short, exosmosis makes them flaccid and the stoma closes.
• K⁺ ion concentration theory (recent): ATP from photosynthesis pumps potassium ions into the guard cells. The cells become hypertonic. Water enters and the cells become turgid. At night the ions leak out, turgor falls, and the pore closes.

6.5 PROCESS OF PHOTOSYNTHESIS

Mesophyll cells — both palisade and spongy — are the principal centres of photosynthesis. Light energy is trapped by the chlorophyll in the upper mesophyll layers, especially the palisade cells.

Carbon dioxide enters the leaf by diffusion down a concentration gradient through the stomata. The air outside holds more \text{CO}_2 than the leaf inside. Water moves from the soil into the roots, up the stem, and into the leaves.

The balanced chemical equation for the process is:

6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow[katex display="true"]\text{chlorophyll}{\text{light energy}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{H}_2\text{O} + 6\text{O}_2 \uparrow[/katex]

Glucose is a simple sugar, readily soluble in water. The six water molecules on the product side are re-formed during a chain of reactions. They are not the original water molecules.

Worked example: a leaf fixes 12 molecules of \text{CO}_2. It splits 24 molecules of water. It releases 12 molecules of oxygen and forms two molecules of glucose. Every \text{CO}_2 needs two water molecules, and every two water molecules yield one oxygen molecule. This is the equation the ICSE Class 10 exam expects.

6.6 TWO MAIN PHASES OF PHOTOSYNTHESIS — (A) LIGHT-DEPENDENT PHASE AND (B) LIGHT-INDEPENDENT (DARK) PHASE

Light plays the key role in the light-dependent phase, also called the photochemical phase. It occurs in the thylakoids of the chloroplast, which contain the chlorophyll. The reactions happen in two main steps.

Step I — Activation of chlorophyll. The chlorophyll absorbs photons, the smallest units of light energy, and becomes activated.

Step II — Splitting of water. The absorbed energy splits each water molecule into hydrogen ions, electrons and oxygen. This is photolysis, meaning splitting by light.

2\text{H}_2\text{O} \xrightarrow{\text{energy of 4 photons}} 4\text{H}^+ + 4\text{e}^- + \text{O}_2

The products of photolysis have three fates:

• Hydrogen ions join NADP to form NADPH: \text{NADP}^+ + \text{e}^- + \text{H}^+ \xrightarrow{\text{enzyme}} \text{NADPH}.
• Oxygen atoms pair up to form molecular oxygen: 2\text{O} \rightarrow \text{O}_2.
• Electrons convert ADP to ATP by phosphorylation: \text{ADP} + \text{Phosphate} \rightarrow \text{ATP}.

The light-independent phase is also called the biosynthetic phase. It occurs in the stroma of the chloroplast. It runs within one-thousandth of a second of the light reaction. The old term “dark phase” never meant night. It only meant the reactions do not need light. The book marks the RuBP, Rubisco and Calvin cycle details as “Not in syllabus” for ICSE Class 10.

Here is how the two phases compare:

6.7 ADAPTATIONS IN LEAF FOR PHOTO-SYNTHESIS

Leaves show seven structural adaptations for photosynthesis:

1. Large surface area for maximum light absorption.
2. Leaves arranged at the right angle to the light source.
3. Transparent, waterproof cuticle and upper epidermis.
4. Numerous stomata for rapid gas exchange.
5. Thinness reduces the distance between cells.
6. Chloroplasts concentrated in the upper leaf layers.
7. Extensive vein system for rapid transport.

Some leaves narrow to conserve water, as in oleander and pine. Each adaptation improves light capture or gas exchange. Together they make the leaf an efficient photosynthetic organ.

6.8 END RESULT OF THE PRODUCTS OF PHOTOSYNTHESIS

Three end products come out of photosynthesis: glucose, water and oxygen. Glucose is the simple sugar \text{C}_6\text{H}_{12}\text{O}_6.

Glucose is used in four ways:

• Consumed immediately by plant cells.
• Stored as insoluble starch.
• Converted into sucrose.
• Used to synthesise fats and proteins.

Water may be re-utilised in photosynthesis. Oxygen is partly used in photorespiration. Most of it diffuses out through the stomata.

Glucose forms rapidly and cannot be transported that fast. So it is converted into insoluble starch for temporary storage in the leaf. At night, the starch becomes soluble sugar again. It is transported through the phloem to other parts. Potato tubers store it as starch again.

6.9 FACTORS AFFECTING PHOTOSYNTHESIS

Four external and three internal factors affect photosynthesis. The external factors are light intensity, carbon dioxide concentration, temperature and water content. The internal factors are chlorophyll, protoplasm and leaf structure. These are the ones the Class 10 ICSE syllabus expects you to name.

The rate rises with light intensity up to a limit. It stabilises at S'(0.02\% \text{CO}_2). If the carbon dioxide concentration is raised, the rate rises again. It stabilises at S'' = (0.05\% \text{CO}_2) for the two factors together.

Temperature has an optimum of 35°C. Above that the rate falls and stops above 40°C because the enzymes are destroyed. A rise of 10°C up to the optimum doubles the rate. For example, 20°C to 30°C, 22°C to 32°C, or 25°C to 35°C.

Water scarcity closes the stomata and reduces \text{CO}_2 intake. Only 1% of the water absorbed by the roots is used in photosynthesis.

Internal factors also matter. Chlorophyll loss cuts energy trapping. Dehydrated protoplasm slows the rate. Leaf structure — cuticle thickness, stomata distribution and leaf size — affects light and \text{CO}_2 entry.

The rate rises from dawn to midday and declines towards dusk. The book rejects the belief that the peepal tree gives out oxygen at night.

6.10 EXPERIMENTS ON PHOTOSYNTHESIS

A plant must be destarched before experiments. It is kept in the dark for 24 to 48 hours. All the starch moves to the storage organs, so the leaves become free from starch.

The iodine test checks a leaf for starch:

1. Boil the leaf in water for a minute to kill the cells.
2. Boil in methylated spirit over a water bath to remove chlorophyll.
3. Soften the leaf in hot water.
4. Add iodine solution. Blue-black means starch. Brown means none.

Four classic experiments prove the requirements for photosynthesis.

Experiment 1 shows chlorophyll is necessary. Use a variegated leaf such as Coleus, Geranium or Croton. Destarch it, expose it to sun, then test. Only the green parts turn blue-black.

Experiment 2 shows sunlight is necessary. Cover a leaf with black paper cut with a design. Test the leaf. Only the parts that got light turn blue-black.

Experiment 3 shows carbon dioxide is necessary. Put half a leaf into a flask with potassium hydroxide, which absorbs \text{CO}_2. The exposed half acts as the control. Only the exposed half shows starch.

Experiment 4 shows oxygen is produced. Place Hydrilla or Elodea in pond water under a funnel. Bubbles collect in an inverted test tube. A glowing splinter bursts into flame, proving the gas is oxygen.

This is the most exam-weighted section in this ICSE chapter. Board questions often ask for the iodine test steps or a diagram of one experiment. See the full set of Class 10 ICSE Biology chapters for related material.

6.11 IMPORTANCE OF PHOTOSYNTHESIS

If there were no green plants, all life on Earth would end. Photosynthesis provides food. Every food chain starts with a plant, the producer. Examples include corn → rat → snake → hawk and grass → grasshopper → lizard → kite. Fungi and bacteria feed on decaying matter that ultimately came from photosynthesis.

Photosynthesis provides oxygen. Free oxygen exists in the atmosphere only because of photosynthesis. About 2 billion years ago, there was no free oxygen. Green plants appeared and set the stage for animals.

6.12 CARBON CYCLE (Fig. 6.10)

The carbon cycle removes carbon as \text{CO}_2 from the air, passes it through living organisms, and returns it to the air. It has six steps:

1. Photosynthesis removes \text{CO}_2 to make carbohydrates.
2. Food chains pass carbon from plants to animals.
3. Respiration oxidises carbohydrates and returns \text{CO}_2.
4. Decay by bacteria and fungi releases \text{CO}_2.
5. Combustion of wood and fossil fuels releases \text{CO}_2.
6. Heating limestone also releases \text{CO}_2.

The starting point is photosynthesis, not respiration. This is a common exam trap. Trace the loop from photosynthesis onwards and you will remember it.

What to practise from Photosynthesis Class 10 ICSE

This chapter contains four PROGRESS CHECK blocks placed after key sections. Each one tests what you have just read.

• One question after section 6.4 on stomatal regulation.
• Seven questions after section 6.6 on the two phases.
• Three questions after section 6.9 on factors affecting photosynthesis.
• Five questions after section 6.11 on importance and experiments.

Attempt each check immediately after revising its section. Board questions on this ICSE chapter most often target section 6.10 experiments and section 6.6 phases, so practise those twice. The book also gives a large bank of multiple-choice, short-answer and structured questions at the end of the chapter for the Class 10 ICSE books set.

Photosynthesis Class 10 ICSE frequently asked questions

The answers below come from the chapter text. For the official Class 10 Biology syllabus, check the CISCE website.

Why is the dark reaction called dark in photosynthesis class 10 ICSE?

The old name “dark phase” meant the reactions do not depend on light. It never meant they happen at night. The book now calls it the light-independent phase. It runs within one-thousandth of a second of the light reaction, even in bright sunlight.

Does the oxygen released in photosynthesis come from carbon dioxide or from water?

The oxygen comes from water. Photolysis splits water into hydrogen ions, electrons and oxygen. The oxygen atoms pair up to form molecular oxygen. Carbon dioxide does not supply the released oxygen.

Why is a plant kept in the dark before an experiment on photosynthesis?

To destarch the leaves. Keeping the plant in the dark for 24 to 48 hours removes all starch from the leaves. Then any starch found after the experiment must have been made during it, so the result is reliable.

What is the difference between chlorophyll and chloroplast?

Chlorophyll is the green pigment. Chloroplast is the organelle that contains it. Chlorophyll sits in the walls of the thylakoids inside the chloroplast. A common exam mistake is to treat the two words as the same thing.

Why does photosynthesis stop when the temperature rises above 40 degrees Celsius?

The enzymes are destroyed above 40°C. The optimum temperature is 35°C. Below that, a 10°C rise doubles the rate. Above 40°C the rate stops because the enzymes cannot work.

Reference: ICSE Class 10 Biology textbook material, chapter 6: Photosynthesis — CISCE-aligned.

More from this book

• Chapter 2: Structure of Chromosomes Cell Cycle and Cell Division
• Chapter 3: Genetics Some Basic Fundamentals
• Chapter 4: Absorption By Roots the Processes Involved
• Chapter 5: Transpiration
• Chapter 7: The Circulatory System
• Chapter 8: The Excretory System
• Chapter 9: The Nervous System
• Chapter 10: Endocrine Glands
• Chapter 11: The Reproductive System
• Chapter 12: Population
• Chapter 13: Aids to Health
• Chapter 14: Health Organisations
• Chapter 15: Pollution