Chapter 5: The Fundamental Unit of Life ( Questions and Answer )

 

1. Who discovered cells, and how?

• Robert Hooke discovered cells in 1665.
• He observed a thin slice of cork under a self-designed microscope.
• He noticed small box-like structures resembling a honeycomb.
• He called these structures "cells", which means "little rooms" in Latin.

2. Why is the cell called the structural and functional unit of life?

• Structural Unit:

  • All living organisms are made up of cells.
  • Cells form tissues, tissues form organs, and organs form organ systems in multicellular organisms.

• Functional Unit:

  • Cells carry out essential life processes like respiration, digestion, and excretion.
  • Each cell has organelles that perform specialized functions (e.g., mitochondria for energy production).
  • Unicellular organisms (like Amoeba) perform all life functions with just one cell.

Thus, the cell is the basic building block of life and is responsible for all biological activities.

Structural Organization of a Cell

1. How do substances like CO₂ and water move in and out of the cell?

• Carbon dioxide (CO₂) and water move in and out of the cell by the processes of diffusion and osmosis, respectively.

  • CO₂: CO₂ is a waste product of cellular processes and needs to be expelled from the cell. The concentration of CO₂ is higher inside the cell and lower outside. Through diffusion, CO₂ moves from the region of higher concentration (inside the cell) to the region of lower concentration (outside the cell), until equilibrium is achieved.

  • Water: Water moves across the cell membrane through osmosis, a type of diffusion specific to water. Water molecules move from an area of high water concentration (outside the cell) to an area of low water concentration (inside the cell) through the selectively permeable membrane. This helps in maintaining the cell's water balance and the movement of substances in and out of the cell.

2. Why is the plasma membrane called a selectively permeable membrane?

The plasma membrane is called a selectively permeable membrane because it allows only certain substances to pass through while blocking others.

• Selective Permeability: The plasma membrane has the ability to control what enters and exits the cell based on factors like size, charge, and solubility.
• Transport Mechanisms: Some small molecules like oxygen (O₂) and carbon dioxide (CO₂) can pass through the membrane easily by diffusion, while larger or charged molecules require specific channels or carriers to move across the membrane.
• Regulation: The selective permeability allows the cell to maintain homeostasis by regulating the exchange of materials necessary for growth, energy production, and waste removal, while keeping harmful substances out.

Feature of the Cell

1) Differences Between Prokaryotic and Eukaryotic Cells

Prokaryotic Cell	Eukaryotic Cell
1. Size: Generally small (1-10 μm)	1. Size: Generally large (5-100 μm)
2. Nuclear region: Not well defined and known as nucleoid	2. Nuclear region: Well-defined and surrounded by a nuclear membrane
3. Chromosome: Single chromosome	3. Chromosome: More than one chromosome
4. Membrane-bound cell organelles: Absent	4. Membrane-bound cell organelles: Present (e.g., mitochondria, endoplasmic reticulum)

Cell Organelles

1. Can you name the two organelles we have studied that contain their own genetic material?

• Mitochondria and plastids (specifically chloroplasts) contain their own genetic material (DNA).

2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

• If the cell's organization is destroyed, it will disrupt normal cellular functions. This can lead to:
  • Damage to organelles and their inability to perform their functions.
  • Cell death or disintegration due to the loss of structural integrity.
  • Disruption of metabolic processes, causing the cell to lose its ability to maintain homeostasis or produce essential compounds.

3. Why are lysosomes known as suicide bags?

• Lysosomes are called "suicide bags" because they contain powerful digestive enzymes. If the cell is damaged or the metabolism is disturbed, lysosomes may burst, releasing these enzymes that can digest the cell’s own components, leading to self-destruction (a process known as autolysis).

4. Where are proteins synthesised inside the cell?

• Proteins are synthesized in the ribosomes, which are located on the rough endoplasmic reticulum (RER) and also free in the cytoplasm. The ribosomes use messenger RNA (mRNA) as a template to build proteins through the process of translation.

Excercises 

1. Make a comparison and write down ways in which plant cells are different from animal cells.

Answer:

• Shape: Plant cells have a rectangular shape while animal cells have an irregular shape.
• Cell Wall: Plant cells have a cell wall made of cellulose for structural support; animal cells lack a cell wall.
• Chloroplasts: Plant cells contain chloroplasts for photosynthesis; animal cells do not have chloroplasts.
• Vacuoles: Plant cells have a large central vacuole for storage and maintaining turgidity; animal cells have smaller vacuoles.
• Centrioles: Animal cells have centrioles involved in cell division, while plant cells typically do not.

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2. How is a prokaryotic cell different from a eukaryotic cell?

Answer:

• Nucleus: Prokaryotic cells lack a defined nucleus (have a nucleoid region), while eukaryotic cells have a defined nucleus enclosed by a nuclear membrane.
• Organelles: Eukaryotic cells contain membrane-bound organelles like the endoplasmic reticulum, Golgi apparatus, and mitochondria, whereas prokaryotic cells lack these organelles.
• Size: Prokaryotic cells are typically smaller and simpler than eukaryotic cells.
• Chromosomes: Prokaryotic cells have a single circular chromosome, while eukaryotic cells have linear chromosomes stored in the nucleus.

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3. What would happen if the plasma membrane ruptures or breaks down?

Answer:

• If the plasma membrane ruptures or breaks down, the cell contents would leak out, and homeostasis would be lost. The cell would no longer be able to regulate the movement of substances in and out, leading to cell death due to loss of structure and function.

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4. What would happen to the life of a cell if there was no Golgi apparatus?

Answer:

• Without the Golgi apparatus, the cell would be unable to modify, package, and transport proteins and lipids to their correct destinations. This would disrupt cellular communication, the formation of lysosomes, and the processing of important cellular products, which may lead to cell dysfunction.

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5. Which organelle is known as the powerhouse of the cell? Why?

Answer:

• The mitochondrion is known as the powerhouse of the cell because it produces ATP (adenosine triphosphate), the energy currency of the cell. ATP is required for various chemical reactions and mechanical work in the cell.

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6. Where do the lipids and proteins constituting the cell membrane get synthesized?

Answer:

• Lipids and proteins constituting the cell membrane are synthesized in the endoplasmic reticulum (ER). Rough ER produces proteins, while smooth ER synthesizes lipids.

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7. How does an Amoeba obtain its food?

Answer:

• Amoeba obtains its food through a process called Endocytosis, where it engulfs food particles by extending its pseudopodia around the food and forming a food vacuole, which then digests the food with enzymes.

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8. What is osmosis?

Answer:

• Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration through a semi-permeable membrane. It is a type of passive transport that does not require energy.

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9. Carry out the following osmosis experiment:

Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now:

(a) Keep cup A empty
(b) Put one teaspoon of sugar in cup B
(c) Put one teaspoon of salt in cup C
(d) Put one teaspoon of sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following:

(i) Explain why water gathers in the hollowed portion of B and C.
(ii) Why is potato A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed-out portions of A and D.

Answer: 

• (i) Explanation of water gathering in cups B and C:
  Water enters the potato cups B (sugar) and C (salt) because osmosis occurs as water moves from the surrounding water (low solute concentration) into the cups where there is a higher solute concentration (sugar or salt).

• (ii) Necessity of potato A:
  Potato A serves as a control to show that water will only move into the potato cells when the cell is intact and not boiled, which would destroy its cellular structure.

• (iii) Why water does not gather in A and D:
  In cup A, there is no solute, so no water movement occurs. In cup D, the boiled potato’s cells are damaged, causing them to lose their ability to retain water, preventing osmosis.

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10. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?

Answer:

• Mitosis is required for growth and repair of body cells.
• Meiosis is involved in the formation of gametes (sperm and egg cells).