Exercise Solutions (From Screenshot Page 74)

Q1: Fill in the blanks.

1. The cell is the fundamental structural and functional unit of life.
2. The cell was discovered by Robert Hooke in 1665.
3. Cells lacking a true nucleus are called prokaryotic cells.
4. Cells possessing a true nucleus are called eukaryotic cells.
5. The outermost rigid layer of a plant cell is the cell wall.
6. The selectively permeable membrane is the cell membrane (plasma membrane).
7. The jelly-like substance filling the cell is called cytoplasm.
8. The control center of the cell is the nucleus.
9. The 'powerhouse' of the cell is mitochondria.
10. Plastids containing chlorophyll are called chloroplasts.
11. Lysosomes are also known as suicidal bags of the cell.
12. Centrosomes are found only in animal cells.

Q2: Match the pairs.

(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)

• Robert Hooke - Discovery of cell
• Prokaryotic cell - Bacteria
• Eukaryotic cell - Plant cell
• Cell wall - Protection and support
• Cell membrane - Selectively permeable
• Nucleus - Control center
• Mitochondria - Powerhouse of the cell
• Chloroplasts - Photosynthesis
• Lysosomes - Suicidal bags
• Centrosome - Cell division (in animal cells)

Q3: Give scientific reasons.

1. Mitochondria are called the 'powerhouse' of the cell.
   Reason: Mitochondria are called the 'powerhouse' of the cell because they are the primary sites where cellular respiration takes place. During cellular respiration, glucose and oxygen are used to produce a large amount of energy in the form of ATP (Adenosine Triphosphate), which is the main energy currency required for various life processes of the cell.
2. Lysosomes are called 'suicidal bags' of the cell.
   Reason: Lysosomes are called 'suicidal bags' because they contain powerful digestive enzymes capable of breaking down and digesting various organic materials, including worn-out cell organelles, foreign invaders like bacteria, and even the entire cell itself if their membrane gets damaged or bursts. In such cases, the enzymes would digest the cell, leading to its death.
3. Plant cells have a cell wall, but animal cells do not.
   Reason: Plant cells have a rigid cell wall, primarily made of cellulose, located outside the cell membrane. This cell wall provides structural support, protection, and maintains the fixed shape of the plant cell, helping it withstand internal turgor pressure. Animal cells lack a cell wall because they need flexibility for movement and changes in shape, and they do not experience the same osmotic pressures as plant cells in their typical environments.
4. Chloroplasts are found in plant cells but not in animal cells.
   Reason: Chloroplasts are organelles that contain the green pigment chlorophyll and are the sites of photosynthesis. Plants are autotrophs, meaning they produce their own food using sunlight, carbon dioxide, and water through photosynthesis. Animal cells, being heterotrophs, obtain their food by consuming other organisms and therefore do not perform photosynthesis, hence they do not have chloroplasts.

Q4: Answer the following questions.

1. Explain the structure and function of the nucleus.
   Answer: Structure: The nucleus is a large, spherical organelle typically located in the center of animal cells and towards the periphery in mature plant cells. It is enclosed by a double-layered nuclear membrane, which has pores. Inside, it contains chromatin (a network of DNA and proteins) and a small, dense structure called the nucleolus. Function: The nucleus is the 'control center' of the cell. It controls all cellular activities, including cell growth, metabolism, protein synthesis, and cell division. It stores the genetic material (DNA) in the form of chromosomes, which carry hereditary information from one generation to the next. The nucleolus within the nucleus is involved in ribosome synthesis.
2. Explain the structure and function of mitochondria.
   Answer: Structure: Mitochondria are rod-shaped or oval-shaped organelles found in the cytoplasm of eukaryotic cells. They are double-membraned structures. The outer membrane is smooth, while the inner membrane is highly folded into finger-like projections called cristae. The inner compartment is filled with a jelly-like matrix. Function: Mitochondria are known as the 'powerhouse' of the cell. They are the sites of cellular respiration, where glucose is broken down in the presence of oxygen to release a large amount of energy. This energy is stored in the form of ATP (Adenosine Triphosphate) molecules, which the cell then uses to perform all its vital activities. The folds (cristae) increase the surface area for energy production.
3. Explain the structure and function of chloroplasts.
   Answer: Structure: Chloroplasts are disk-shaped organelles found only in plant cells and some algae. They are double-membraned structures. Inside, they contain flattened sacs called thylakoids, which are stacked to form grana. The fluid-filled space surrounding the grana is called the stroma. Chloroplasts contain the green pigment chlorophyll. Function: Chloroplasts are the sites of photosynthesis. Chlorophyll within the chloroplasts traps light energy from the sun. This energy is then used to convert carbon dioxide and water into glucose (food) and oxygen. Thus, chloroplasts are essential for the survival of plants and, indirectly, for most life on Earth as they produce organic food and oxygen.
4. Explain the structure and function of vacuoles.
   Answer: Structure: Vacuoles are membrane-bound sacs found in the cytoplasm of cells. Their size and number vary significantly between plant and animal cells. Plant cells typically have a single, large central vacuole that can occupy up to 90% of the cell's volume, pushing other organelles to the periphery. Animal cells usually have several small, temporary vacuoles or sometimes none. Function:
   • Storage: Vacuoles store water, nutrients (sugars, amino acids), ions, and waste products.
   • Turgidity and Rigidity (in Plant Cells): The large central vacuole in plant cells absorbs water, swells, and pushes against the cell wall, creating turgor pressure. This pressure provides rigidity and structural support to the plant cell and, consequently, to the plant itself.
   • Waste Removal: In some cells, vacuoles help in isolating and removing unwanted substances.

Q5: Differentiate between.

1. Prokaryotic Cell and Eukaryotic Cell
   

   Feature	Prokaryotic Cell	Eukaryotic Cell
   Nucleus	No true nucleus (genetic material in nucleoid region)	True nucleus present (genetic material enclosed by nuclear membrane)
   Membrane-bound Organelles	Absent (e.g., mitochondria, ER, Golgi)	Present (e.g., mitochondria, ER, Golgi, chloroplasts)
   Size	Generally smaller (1-10 µm)	Generally larger (10-100 µm)
   Examples	Bacteria, Blue-green algae	Plant cells, Animal cells, Fungi, Protists

2. Plant Cell and Animal Cell
   

   Feature	Plant Cell	Animal Cell
   Cell Wall	Present (outermost rigid layer)	Absent
   Chloroplasts	Present (for photosynthesis)	Absent
   Vacuole	Large, single, central vacuole	Small, numerous, or absent
   Centrosome	Absent (or present in lower plants)	Present (helps in cell division)
   Shape	Generally rectangular/fixed shape	Generally irregular/round shape