Chapter 1: Science - Class 8
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A. Fill in the blanks.
1. All living organisms are made up of cells.
2. The cell wall provides rigidity and protection to the plant cell.
3. All cellular activities are controlled by the nucleus.
4. The mitochondria is known as the powerhouse of the cell.
5. Tissue is a group of cells performing a specific function.
6. It was Robert Hooke who observed cells for the first time.
B. Match the Following
| Column A | Column B |
|---|---|
| Golgi Complex | Packaging centre |
| Ribosomes | Protein synthesis |
| Chromosomes | Genes |
| Dead cells | Cork |
| Photosynthesis | Chloroplasts |
C. Tick (✓) the correct option
1. The cell organelle, which acts as a storage bag for a cell, is known as the—
- chloroplast
- chromoplast
- mitochondria
- vacuole
- chloroplast
- chromoplast
- mitochondria
- vacuole
2. Hen's egg is—
- a cell organelle
- a tissue
- a single cell
- an organ
3. The nucleus is separated from the cytoplasm by the—
- cytoplasm
- cell membrane
- nuclear membrane
- protoplasm
4. Which of the following will not be found in an egg cell, human liver cell and an Amoeba?
- ribosomes
- cell membrane
- mitochondria
- cell wall
5. Which of the following represents the correct sequence?
- tissue → cell → organ → organ system
- organ → tissue → organ system → cell
- cell → organ → tissue → organ system
- cell → tissue → organ → organ system
6. Which, amongst the following pairs, can be found only in a plant cell but not in an animal cell?
- cell wall and plastids
- cell wall and cell membrane
- plastids and cilia
- plastids and mitochondria
D. Answer the following questions in brief
1. List the (main) factors that determine the shape of a cell.
ANSWER: The shape of a cell is mainly determined by its function and the presence or absence of a cell wall.
2. Distinguish between unicellular and multicellular organisms. Give two examples of each.
ANSWER:
Unicellular: Made up of one cell.
Examples: Amoeba, Paramecium
Multicellular: Made up of many cells.
Examples: Humans, Plants
3. Give reasons for the following:
ANSWER:
- The cell is called the structural and functional unit of life:
Because all living organisms are made of cells, and all life processes occur inside cells.
- Plant cells are more rigid than the animal cells:
Due to the presence of a thick cell wall in plant cells.
4. Which cell organelle is known as the 'Powerhouse of the cell'? Why is it so called?
ANSWER:
Mitochondria – because it produces energy by breaking down food.
5. Name the cell organelles responsible for imparting colour to the leaves and fruits of a plant.
ANSWER:
Chloroplasts (leaves) and chromoplasts (fruits).
6. What are cilia and flagella? Write one similarity and one dissimilarity between the two of them.
ANSWER:
Similarity: Both help in locomotion.
Dissimilarity: Cilia are short and many; flagella are long and few.
7. If onion peel cells and cheek cells are observed through a microscope, state the two major differences that the observer is likely to find.
ANSWER:
- Onion cells have a cell wall; cheek cells do not.
- Onion cells are rectangular; cheek cells are round or irregular.
8. Classify the following into cells, tissue, and organ:
ANSWER:
Cell: Fat cell, RBC
Tissue: Blood, Muscle
Organ: Skin, Ear
E. Answer the following questions
1. "All cells in an organism do not have the same shape." Justify the above statement by drawing at least three different cell types found in human beings.
ANSWER: Cells differ in shape depending on their function. In human beings, here are three examples:
Nerve cell: Long and branched to carry messages.Muscle cell: Spindle-shaped to allow movement and contraction.
Red Blood Cell (RBC): Round and biconcave to carry oxygen.
Diagram: [Insert diagram of three human cell types here]
Example image: View Diagram
2. Where, and how, are chromosomes formed? State their significance.
ANSWER: Chromosomes are formed inside the nucleus of a cell. When a cell prepares to divide, the long thread-like structures of DNA condense and coil to form visible chromosomes.
Significance: Chromosomes carry genes which determine the traits of an organism. They are essential for inheritance, cell function, and growth.
3. With the help of well-labelled diagrams, highlight three differences between a plant cell and an animal cell.
ANSWER:
| Plant Cell | Animal Cell |
|---|---|
| Has a cell wall | No cell wall |
| Has chloroplasts | No chloroplasts |
| Has a large central vacuole | Vacuoles are small or absent |
Diagram: [Insert diagram comparing plant and animal cell here]
Example image: View Diagram
4. Write the functions performed by the following cell organelles.
ANSWER:
Endoplasmic Reticulum (ER): Transports materials. Rough ER helps in protein synthesis; Smooth ER helps in fat synthesis.Golgi Complex: Modifies, packages, and transports materials. Acts like a packaging unit.
Nucleus: Controls all activities of the cell and contains hereditary material.
Chromoplasts: Provide colour to fruits and flowers to attract pollinators.
Vacuoles: Store water, nutrients, and waste. Help maintain pressure in plant cells.
Mitochondria: Called the "powerhouse of the cell", they generate energy from food.
5. The cell membrane is a very important component of a cell. How is damage to the cell membrane likely to impact the functions of the cell?
ANSWER: The cell membrane controls the entry and exit of substances. If damaged:
Essential substances can leak out.
Harmful substances can enter.
Cell balance and communication will be disturbed.
This may cause the cell to malfunction or die.
6. Define the term 'cell', for plants/animals. Name the different organelles that make a cell. Explain why none of these is called the structural and functional unit of life.
ANSWER:
Definition: A cell is the smallest structural and functional unit of life. It can perform all life processes.
Organelles: Nucleus, Mitochondria, Endoplasmic Reticulum, Golgi Complex, Ribosomes, Cytoplasm, Vacuoles, Cell membrane, Cell wall (in plants), Plastids (in plants)
Explanation: Organelles have specific roles but cannot work alone. The complete cell performs all life activities, so it is the structural and functional unit of life.