We know that all living beings, including plants, grow and require food for this growth. While animals obtain food by eating plants or other animals, plants have a unique way of getting the nutrients they need. This chapter explores the key life processes that enable plants to grow and survive.

Just like animals increase in size and weight and undergo visible changes as they grow, plants also show noticeable changes, such as increased height, thicker stems, and the emergence of new leaves and branches.

Food provides essential nutrients like carbohydrates, fats, proteins, vitamins, and minerals, along with water, which are crucial for the growth of all organisms. Let's delve into how plants acquire these necessary components.

10.1 How Do Plants Grow?

What factors influence the growth and development of a plant throughout its lifespan?

Activity 10.1: Let Us Test Some Explanations

To test some common ideas about what plants need to grow (like water or sunlight), we can set up an experiment using identical plant saplings in three pots:

• Pot A: Placed in direct sunlight and watered daily.
• Pot B: Placed in direct sunlight but not watered.
• Pot C: Placed in the dark but watered daily.

Observe and record changes in the plants over a couple of weeks, noting their height, number of leaves, and leaf colour.

Pots kept under different conditions	Availability of Sunlight	Availability of Water	Height of plant (Day 1)	Height of plant (After 2 weeks)	Number of leaves (Day 1)	Number of leaves (After 2 weeks)	Colour of leaves (Day 1)	Colour of leaves (After 2 weeks)
Pot A: Direct sunlight, with water	Yes	Yes	(Initial height)	(Maximum increase)	(Initial count)	(Increase in count)	Green	Green
Pot B: Direct sunlight, without water	Yes	No	(Initial height)	(Little or no increase, may die)	(Initial count)	(May decrease or drop)	Green (initially)	May turn yellow/brown
Pot C: Dark, with water	No	Yes	(Initial height)	(Little increase, may become weak)	(Initial count)	(May stay same or drop)	Green (initially)	May turn pale yellow/white

The plant in Pot A, receiving both sunlight and water, will show the best growth. The plant in Pot B, lacking water, will struggle or die, even with sunlight. The plant in Pot C, lacking sunlight, will also show poor growth and its leaves might turn pale, despite getting water.

This experiment demonstrates that both sunlight and water are essential for healthy plant growth.

Fascinating Fact: Ancient texts like the Vrikshayurveda document detailed observations and practices for promoting plant growth and crop health, emphasizing factors like soil preparation and organic manures, showing a long history of understanding plant needs.

10.2 How Do Plants Get Food For Their Growth?

Unlike animals, plants don't consume food in the way we do. They produce their own food.

10.2.1 Leaves: The ̒ Food Factories̓Of Plants

Plants produce and store food in the form of starch, a complex carbohydrate. This food production primarily happens in the leaves.

Leaves are generally broad and flat to efficiently capture sunlight. Their green colour is due to a pigment called chlorophyll, which is crucial for trapping sunlight energy.

Activity 10.2: Let Us Check (Demonstration Activity)

To confirm the presence of starch in leaves, perform the iodine test. Boil a leaf to soften it, then decolorize it in alcohol using a water bath (alcohol is flammable, so direct heating is dangerous). Once the leaf is pale, spread it out and add a few drops of iodine solution. A blue-black colour indicates the presence of starch.

Decolorizing the leaf removes the green chlorophyll, making it easier to see the blue-black colour change produced by the iodine reaction with starch.

Activity 10.3: Let Us Check

To understand the role of sunlight and the green pigment, chlorophyll, in starch production, compare starch presence in leaves from:

• A plant kept in sunlight.
• A plant kept in the dark.

Use leaves that have both green (chlorophyll present) and non-green patches (chlorophyll absent or insufficient). Perform the iodine test on both leaves after exposing one plant to sunlight and keeping the other in the dark.

S.No.	Light conditions for potted plant	Leaf Appearance (Green and non-green patches)	Observations after Iodine Test
1.	Plant kept in sunlight	Leaf with green and non-green patches	Green patches turn blue-black (Starch present). Non-green patches do not turn blue-black (Little or no starch).
2.	Plant kept in the dark (for 36 hours)	Leaf with green and non-green patches	Neither green nor non-green patches turn blue-black (No starch produced in the dark).

This experiment shows that starch is produced only in the green parts of a leaf that are exposed to sunlight. This indicates that both chlorophyll (found in green parts) and sunlight are essential for plants to make food (starch). Because leaves are the primary sites for this food production, they are often called the 'food factories' of plants.

Fascinating Fact: Leaves with red, violet, or brown patches still contain chlorophyll, but the green colour is masked by other pigments. An iodine test can confirm starch presence in these leaves, showing photosynthesis still occurs in the presence of chlorophyll and sunlight.

10.2.2 Role Of Air In The Preparation Of Food

Besides sunlight and chlorophyll, plants also need something from the air to make food.

Activity 10.4: Let Us Experiment (Demonstration Activity)

To determine the role of air (specifically carbon dioxide) in food production, take a de-starched potted plant (one kept in the dark for a few days to use up stored starch). Insert half of one leaf into a bottle containing caustic soda (sodium hydroxide) solution, which absorbs carbon dioxide from the air. Keep the other half of the leaf outside the bottle.

Place the entire setup in sunlight for a few hours, then perform the iodine test on the leaf.

Part of the leaf	Availability of Water	Availability of Sunlight	Availability of Chlorophyll	Availability of Carbon dioxide	Starch present (Yes/No)
Outside the bottle	Yes	Yes	Yes (if green part)	Yes (from air)	Yes
Inside the bottle	Yes	Yes	Yes (if green part)	No (absorbed by NaOH)	No

The part of the leaf outside the bottle will test positive for starch (turn blue-black), while the part inside the bottle (deprived of carbon dioxide) will not. This demonstrates that carbon dioxide from the air is essential for plants to produce starch.

Activity 10.5: Let Us Explore

Plants don't just take in substances; they also release something during photosynthesis. Observe a water plant (like Hydrilla) placed in a beaker of water, covered with an inverted funnel and a test tube filled with water (as shown in figure). If the setup is placed in sunlight, bubbles of gas will be seen emerging from the plant and collecting in the test tube.

If the setup is placed in the dark, fewer or no bubbles will be observed. Testing the collected gas with a lit matchstick (which will flare up intensely) confirms that the gas produced is rich in oxygen. This experiment shows that oxygen is released by plants during photosynthesis, and this process requires sunlight.

Based on these activities, we conclude that plants use sunlight, water, carbon dioxide, and chlorophyll to produce food (glucose/starch) and release oxygen.

10.2.3 Photosynthesis: In A Nutshell

The entire process by which green plants make their own food from carbon dioxide and water using sunlight energy, in the presence of chlorophyll, is called photosynthesis.

The simplified word equation for photosynthesis is:

$ \text{Carbon dioxide} + \text{Water} \xrightarrow{\text{Sunlight}} \text{Glucose} + \text{Oxygen} $

Glucose, a simple carbohydrate, is the initial food produced. It can be used immediately for energy or converted into starch for storage in leaves, stems, roots, or seeds.

KNOW A SCIENTIST: Scientists like Rustom Hormusji Dastur in India have significantly contributed to understanding photosynthesis by studying factors like the effect of water, temperature, and light colour on this process.

10.2.4 How Do Leaves Exchange Gases During Photosynthesis?

Plants need to take in carbon dioxide and release oxygen during photosynthesis. How do leaves facilitate this gas exchange?

Activity 10.6: Let Us Examine (Demonstration Activity)

Observing a thin peel from the surface of a leaf under a microscope reveals the presence of tiny pores called stomata.

These stomata, primarily located on the underside of leaves, are surrounded by guard cells that regulate their opening and closing. Stomata serve as the main points for the exchange of gases – carbon dioxide enters the leaf, and oxygen (produced during photosynthesis) and water vapour are released from the leaf through these pores.

10.3 Transport In Plants

Plants need to transport water and minerals from the soil to the leaves, and the food produced in the leaves to other parts of the plant. Specialized transport systems within the plant handle these movements.

10.3.1 Transport Of Water And Minerals

Plants absorb water and dissolved minerals from the soil through their roots. Minerals are essential nutrients for plant growth.

But how do water and minerals move upwards from the roots to the stem, branches, leaves, and even flowers?

Activity 10.7: Let Us Experiment

Place a plant twig with white flowers in coloured water (e.g., water with red ink). After some time (e.g., the next day), you will notice red colour appearing in the stem, leaves, and flowers. This indicates that the coloured water has moved upwards through the plant.

Cutting the stem and observing the cut end with a magnifying glass will show the red colour in specific thin tube-like structures. These structures are called xylem. Xylem vessels form a continuous network of tubes throughout the roots, stem, branches, and leaves, transporting water and dissolved minerals upwards from the roots to all parts of the plant.

10.3.2 Transport Of Food

Once food is produced through photosynthesis in the leaves, it needs to be transported to other parts of the plant (like roots, fruits, storage organs) for growth, energy, and storage. Another set of tube-like structures called phloem are responsible for transporting this prepared food from the leaves to all other parts of the plant.

So, xylem transports water and minerals upwards, and phloem transports food from the leaves throughout the plant.

10.4 Do Plants Respire?

Yes, like all living organisms, plants also undergo respiration to release energy from the food they produce or store.

Activity 10.8: Let Us Find Out (Demonstration Activity)

To show that plants respire and release carbon dioxide, set up an experiment with germinating moong bean seeds in a sealed flask over moist cotton. Connect this flask to a test tube containing lime water.

After some time (e.g., 24 hours in the dark), the lime water connected to the flask with seeds will turn milky. This indicates an increased concentration of carbon dioxide in the flask compared to normal air. The germinating seeds are respiring and producing this extra carbon dioxide.

During respiration, glucose (food) reacts with oxygen to release energy. This is represented by the word equation:

$ \text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon dioxide} + \text{Water} + \text{Energy} $

The energy released through respiration powers the plant's growth and development processes. Respiration occurs in all living cells of a plant, both in green and non-green parts, all the time (unlike photosynthesis which requires sunlight).

Therefore, plants have complex mechanisms for photosynthesis (producing food), transport (moving water, minerals, and food), and respiration (utilizing food for energy) – all working together to sustain life.

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In a Nutshell:

• Plants are autotrophs; they produce their own food.
• Photosynthesis is the process where green plants use sunlight, water, and carbon dioxide to create glucose (food) and oxygen, with the help of chlorophyll.
• Leaves are the main sites for photosynthesis, acting as 'food factories'.
• Chlorophyll is the green pigment in leaves that captures sunlight energy.
• Stomata are tiny pores on leaf surfaces that facilitate the exchange of gases (carbon dioxide intake, oxygen and water vapour release).
• Plants have transport systems: Xylem vessels transport water and minerals from roots upwards, and phloem vessels transport food (glucose/starch) from leaves to all parts of the plant.
• Plants, like all living beings, undergo respiration, using oxygen to break down glucose and release energy, producing carbon dioxide and water.
• Respiration occurs in all living plant cells, day and night, to provide energy for growth and other functions.
• Photosynthesis ($ \text{CO}_2 + \text{H}_2\text{O} + \text{Sunlight} \rightarrow \text{Glucose} + \text{O}_2 $) produces food and oxygen.
• Respiration ($ \text{Glucose} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{Energy} $) uses food and oxygen to release energy.

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