Science Learning | 7th Std

1. Summary

The source text discusses the critical topic of nutrition in plants, focusing on how they obtain and utilize food for growth, development, and energy. Unlike animals, plants are autotrophs, capable of synthesizing their own food through photosynthesis. This process occurs in chlorophyll-containing cells, primarily in leaves, utilizing sunlight, carbon dioxide, and water to produce carbohydrates and release oxygen.

The source details the essential role of each component in photosynthesis: chlorophyll captures light energy, carbon dioxide enters through leaf pores called stomata, and water is absorbed by roots and transported through vessels. It highlights the importance of sunlight as the ultimate energy source for almost all life on Earth.

However, not all plants rely solely on photosynthesis. Some, like Cuscuta (Amarbel), are parasitic, obtaining food from a host plant. Others, like fungi, are saprotrophs, deriving nutrients from dead and decaying matter. The source also touches upon insectivorous plants, such as the pitcher plant, which supplement their nutrient intake by trapping and digesting insects.

The text emphasizes the importance of replenishing soil nutrients, which are depleted as plants absorb them. It explains how farmers utilize fertilizers and manures to enrich the soil and how certain bacteria like Rhizobium convert atmospheric nitrogen into a usable form for plants, forming a symbiotic relationship with legumes.

2. Key Points 

• Imagine plants having their own kitchens called leaves where they cook food! This cooking process is called photosynthesis.
• Like we need ingredients, plants need sunlight, air (carbon dioxide), water, and a green ingredient called chlorophyll to cook their food.
• Just like we eat different foods, some plants get their food from other plants or even dead things! These plants are like sneaky eaters!
• Plants are super important because they are the main source of food for all living things on Earth!

1. Why do organisms take food? Organisms need food to obtain energy. This energy is used for growth, development, and to carry out essential life processes.

2. Distinguish between a parasite and a saprotroph.

• Parasite: A parasite depends on a living organism (host) for its nutrition, harming the host in the process. Example: Cuscuta.
• Saprotroph: A saprotroph feeds on dead and decaying matter, breaking it down and obtaining nutrients. Example: Fungi.

3. How would you test the presence of starch in leaves? The presence of starch in leaves can be tested using the iodine test. If a leaf containing starch comes into contact with iodine, it turns a blue-black color.

4. Give a brief description of the process of synthesis of food in green plants. Green plants synthesize their food through photosynthesis. In this process, chlorophyll in leaves captures light energy. This energy is used to combine carbon dioxide from the air and water from the soil to produce carbohydrates (sugars). Oxygen is released as a byproduct.

5. Show with the help of a sketch that plants are the ultimate source of food. (You would need to draw a simple food chain diagram. For example: Sun → Grass → Deer → Lion. This demonstrates that the sun's energy is captured by plants and passed on to animals through the food chain.)

6. Fill in the blanks:

   • (a) Green plants are called autotrophs since they synthesize their own food.
   • (b) The food synthesized by plants is stored as starch.
   • (c) In photosynthesis, solar energy is absorbed by the pigment called chlorophyll.
   • (d) During photosynthesis, plants take in carbon dioxide and release oxygen gas.

7. Name the following:

   • (i) A parasitic plant with a yellow, slender and branched stem: Cuscuta (Amarbel).
   • (ii) A plant that is partially autotrophic: Pitcher plant (It performs photosynthesis but also consumes insects).
   • (iii) The pores through which leaves exchange gases: Stomata.

8. Tick the correct answer:

   • (a) Cuscuta is an example of: (ii) parasite.
   • (b) The plant which traps and feeds on insects is: (iii) pitcher plant.

9. Match the items given in Column I with those in Column II:

   • Chlorophyll → Leaf
   • Nitrogen → Rhizobium
   • Cuscuta → Parasite
   • Animals → Heterotrophs
   • Insects → Pitcher plant

10. Mark ‘T’ if the statement is true and ‘F’ if it is false:

    • (i) Carbon dioxide is released during photosynthesis. (F) - Oxygen is released.
    • (ii) Plants which synthesise their food are called saprotrophs. (F) - They are called autotrophs.
    • (iii) The product of photosynthesis is not a protein. (T) - It is primarily carbohydrates.
    • (iv) Solar energy is converted into chemical energy during photosynthesis. (T) - Light energy is stored as chemical energy in the form of sugars.

11. Choose the correct option from the following: Which part of the plant takes in carbon dioxide from the air for photosynthesis? (ii) Stomata

12. Choose the correct option from the following: Plants take carbon dioxide from the atmosphere mainly through their: (iv) leaves

13. Why do farmers grow many fruits and vegetable crops inside large greenhouses? What are the advantages to the farmers?

Farmers use greenhouses to create a controlled environment for their crops.

• Advantages:
  • Protection from harsh weather: Greenhouses protect crops from extreme temperatures, strong winds, heavy rain, and hail.
  • Pest and disease control: Enclosed structures help control pests and diseases.
  • Extended growing season: Farmers can grow crops year-round, even in cold climates.
  • Optimized growing conditions: Light, temperature, and humidity can be controlled for optimal growth and higher yields.

Long Answer Questions:

Q1: What are the different modes of nutrition found in plants and how do they differ from one another?

A: Plants exhibit diverse modes of nutrition:

• Autotrophic Nutrition: The majority of plants are autotrophs, meaning they produce their own food through photosynthesis. They utilize sunlight, carbon dioxide, and water in this process.
• Parasitic Nutrition: Parasitic plants, like Cuscuta, lack chlorophyll and cannot produce their own food. They obtain nutrients by attaching themselves to a host plant and drawing food from it.
• Saprotrophic Nutrition: Saprotrophs, like fungi, obtain nutrients by breaking down dead and decaying organic matter. They release enzymes onto the decaying material, digest it externally, and then absorb the nutrients.
• Insectivorous Nutrition: Some plants, like the pitcher plant, are insectivorous. While they can photosynthesize, they grow in nutrient-poor soils and supplement their diet by trapping and digesting insects for additional nutrients.

Q2: How do plants obtain and utilize essential nutrients for growth and development?

A: Plants obtain essential nutrients through various mechanisms:

• Root Absorption: Plants absorb water and essential minerals from the soil through their roots. Root hairs increase the surface area for absorption.
• Photosynthesis: Photosynthesis is crucial for producing carbohydrates, which serve as the primary energy source and building blocks for other organic compounds.
• Nitrogen Fixation: Plants need nitrogen for protein synthesis. While atmospheric nitrogen is abundant, plants cannot use it directly. Some bacteria, like Rhizobium, convert nitrogen into a usable form through nitrogen fixation, forming a symbiotic relationship with certain plants.
• Nutrient Transport: After absorption by roots, water and minerals travel upwards through specialized tissues called xylem vessels to reach all plant parts.

Q3: What are the environmental factors necessary for photosynthesis and how do they impact plant life?

A: Photosynthesis relies on several environmental factors:

• Sunlight: It's the primary energy source driving photosynthesis. Light intensity, duration, and quality influence photosynthetic rates.
• Carbon Dioxide: CO2 is a crucial ingredient used to create sugars. Its concentration in the atmosphere affects the rate of photosynthesis.
• Water: Water is essential for photosynthesis, transporting nutrients, and maintaining cell turgidity. Water scarcity directly limits plant growth and photosynthetic activity.
• Temperature: Photosynthesis is an enzymatic process influenced by temperature. Each plant species has an optimal temperature range for photosynthesis. Extremely high or low temperatures can negatively affect enzyme activity and reduce photosynthetic rates.

These factors collectively influence plant growth, distribution, and survival. Any imbalance can lead to stress, reduced productivity, and even death.