1. Explain the structural model of cell membrane and give the roles of cell membrane.
Answer: Structural Model of Cell Membrane (Fluid Mosaic Model): The cell membrane is described by the fluid mosaic model, proposed by Singer and Nicolson in 1972. According to this model:
Phospholipid Bilayer: The basic structure is a double layer of phospholipids. Each phospholipid has a hydrophilic (water- loving) head and two hydrophobic (water-fearing) tails. The heads face outward towards the aqueous environment, and the tails face inward, creating a barrier. Proteins: Proteins are embedded within or attached to the lipid bilayer. Integral proteins span the membrane, while peripheral proteins are on the surface. These proteins form a mosaic pattern. Cholesterol: Cholesterol molecules are interspersed among the phospholipids, regulating membrane fluidity. Carbohydrates: Short carbohydrate chains are attached to lipids (glycolipids) or proteins (glycoproteins) on the outer surface, involved in cell recognition. Fluidity: The membrane is fluid, meaning lipids and proteins can move laterally.
Roles of Cell Membrane:
1. Selective Permeability: Controls the entry and exit of substances, maintaining homeostasis.
2. Cell Signaling: Contains receptors that bind to hormones and neurotransmitters.
3. Cell Adhesion: Forms junctions with other cells, holding tissues together.
4. Transport: Facilitates diffusion, osmosis, active transport, endocytosis, and exocytosis.
5. Cell Recognition: Glycoproteins act as identification tags.
6. Protection: Acts as a barrier against external threats.
In summary, the fluid mosaic model describes the dynamic structure of the cell membrane, which is essential for its diverse functions.
2. How cell wall is important in the lifestyle of plants?
Answer:
The cell wall is a rigid, semi-permeable layer outside the plasma membrane of plant cells. It is primarily composed of cellulose and is crucial for the plant's lifestyle.
Importance of Cell Wall in Plants:
1. Structural Support: The rigid cell wall provides mechanical strength and support, allowing plants to stand upright against gravity. It gives shape to the cell and the plant.
2. Protection: It protects the cell from mechanical damage and pathogens (bacteria, fungi).
3. Prevents Osmotic Lysis: When water enters the cell by osmosis, the cell wall prevents the cell from bursting (cytolysis) by exerting turgor pressure.
4. Regulates Growth: The cell wall is flexible in young cells, allowing growth. It becomes rigid in mature cells, limiting further expansion.
5. Transport: The cell wall is porous, allowing water and minerals to move freely through the apoplast pathway.
6. Cell-to-Cell Communication: Plasmodesmata (channels through the cell wall) connect adjacent cells, allowing communication and transport.
7. Support for Xylem: The lignified cell walls of xylem vessels provide support and allow water transport.
In summary, the cell wall is essential for the structural integrity, protection, and survival of plant cells and the plant as a whole.
3. If a cell is rich in SER, list the roles in which this cell will be more efficient.
Answer: The smooth endoplasmic reticulum (SER) is involved in lipid synthesis, detoxification, and calcium storage. If a cell is rich in SER, it will be more efficient in the following roles:
1. Lipid Synthesis:
Synthesis of phospholipids and cholesterol for cell membranes. Synthesis of steroid hormones (e.g., in adrenal cortex, gonads).
Synthesis of oils and waxes (in plants).
2. Detoxification:
Detoxification of drugs, alcohol, and metabolic waste products (especially in liver cells). The SER contains enzymes (e.g., cytochrome P450) that modify toxic substances to make them water-soluble for excretion.
3. Calcium Storage:
In muscle cells, the SER (sarcoplasmic reticulum) stores calcium ions. Release of calcium triggers muscle contraction.
4. Carbohydrate Metabolism:
In liver cells, the SER is involved in glycogen breakdown (glycogenolysis).
Examples of Cells Rich in SER:
Liver cells: For detoxification and glycogen metabolism. Adrenal cortex cells: For steroid hormone synthesis. Muscle cells: For calcium storage.
In summary, a cell rich in SER would be highly efficient in lipid synthesis, detoxification, and calcium regulation, depending on its specific function.
4. Give the significance of muscles in the life of animals.
Answer: Muscles are specialized tissues that enable movement in animals. They are essential for a wide range of functions.
Significance of Muscles:
1. Movement: Muscles enable locomotion, allowing animals to find food, escape predators, and migrate.
2. Posture and Support: Muscles maintain posture and support the body against gravity.
3. Circulation: Cardiac muscle pumps blood through the circulatory system.
4. Digestion: Smooth muscles in the digestive tract move food through peristalsis.
5. Respiration: Muscles of the diaphragm and rib cage enable breathing.
6. Reproduction: Muscles are involved in mating and childbirth.
7. Heat Production: Muscle contractions generate heat, helping to maintain body temperature.
8. Communication: Muscles enable facial expressions, vocalization, and body language.
In summary, muscles are essential for movement, support, circulation, digestion, respiration, reproduction, thermoregulation, and communication. They are vital for the survival and functioning of animals.
5. Give the types of plastids and enlist the roles of each type.
Answer: Plastids are membrane-bound organelles found in plant cells and some algae. They are classified into three main types based on their pigments and functions:
1. Chloroplasts:
Pigment: Chlorophyll (green). Function: Site of photosynthesis. They convert light energy into chemical energy (glucose). Structure: Double membrane, thylakoids (grana), stroma. Location: Green parts of plants (leaves, stems).
2. Chromoplasts:
Pigment: Carotenoids (red, orange, yellow). Function: Provide color to flowers, fruits, and other plant parts. Attract pollinators and seed dispersers. Location: Flowers, fruits, roots (e.g., carrot).
3. Leucoplasts:
Pigment: None (colorless). Function: Store food (starch, lipids, proteins). Types:
Amyloplasts: Store starch (e.g., potato). Elaioplasts: Store lipids. Aleuroplasts: Store proteins.
Location: Non-photosynthetic parts (roots, seeds).
In summary, plastids are essential for photosynthesis, pigmentation, and food storage in plants.
6. Describe the structure and functions of animal cell. How it is different from plant cell?
Answer: Structure of Animal Cell:
Cell Membrane: Outer boundary, selectively permeable. Cytoplasm: Jelly-like substance containing organelles. Nucleus: Contains DNA, controls cell activities. Mitochondria: Site of cellular respiration, produces ATP. Endoplasmic Reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis). Golgi Apparatus: Modifies, sorts, and packages proteins.
Lysosomes: Digestive enzymes, break down waste. Ribosomes: Protein synthesis. Centrioles: Help in cell division. Cytoskeleton: Provides shape and support.
Functions of Animal Cell:
Carry out metabolism, growth, reproduction, response to stimuli. Specialized functions based on cell type (e.g., muscle contraction, nerve impulse transmission).
Differences from Plant Cell:
| Feature | Animal Cell | Plant Cell |
| Cell Wall | Absent | Present (cellulose) |
| Chloroplasts | Absent | Present |
| Vacuole | Small, many | Large, central |
| Centrioles | Present | Absent (mostly) |
| Shape | Irregular/round | Fixed, rectangular |
| Lysosomes | Present | Rare |
| | |
Cilia/Flagella Often present Rare
In summary, animal cells are eukaryotic cells that lack a cell wall and chloroplasts, and have a more flexible shape compared to plant cells.
7. Justify how the cells of leaf have a variety of specialized structure and function.
Answer: Leaf cells are highly specialized for photosynthesis and gas exchange. Different cell types have specific structures and functions:
1. Epidermal Cells:
Structure: Flattened, tightly packed, covered by a waxy cuticle. Function: Protection, reduce water loss.
2. Guard Cells:
Structure: Bean-shaped, contain chloroplasts, thick inner walls. Function: Regulate stomatal opening and closing for gas exchange and transpiration.
3. Palisade Mesophyll Cells:
Structure: Columnar, tightly packed, many chloroplasts. Function: Main site of photosynthesis.
4. Spongy Mesophyll Cells:
Structure: Irregular, loosely packed, few chloroplasts, large air spaces.
Function: Gas exchange, some photosynthesis.
5. Xylem Vessels:
Structure: Dead, hollow tubes, lignified walls. Function: Transport water and minerals.
6. Phloem Sieve Tubes:
Structure: Living cells, sieve plates, companion cells. Function: Transport sugars.
In summary, leaf cells are specialized to maximize photosynthesis, gas exchange, and transport, demonstrating the relationship between structure and function.
8. State the relationship between structure and function of mesophyll cells, epidermal cells, neurons, muscles, red blood cells and liver cells.
Answer:
| Cell Type | Structure | Function |
| Mesophyll Cells | Columnar/spongy, many chloroplasts. | Photosynthesis, gas exchange. |
| Epidermal Cells | Flattened, tightly packed, cuticle. | Protection, reduce water loss. |
| Neurons | Long extensions (axon, dendrites). | Transmit electrical impulses. |
| | |
| Muscle Cells | Elongated, contractile proteins. | Contraction, movement. |
| Red Blood Cells | Biconcave, no nucleus, hemoglobin. | Oxygen transport. |
| Liver Cells | Many mitochondria, RER, SER. | Detoxification, metabolism, protein synthesis. |
In summary, the structure of each cell type is highly adapted to its specific function.
9. Describe the role of the cell membrane in maintaining equilibrium while exchanging matter?
Answer: The cell membrane plays a crucial role in maintaining equilibrium by regulating the exchange of matter.
1. Selective Permeability:
Allows some substances to pass through while blocking others. Small, nonpolar molecules (O₂, CO₂) pass freely. Polar molecules and ions require transport proteins.
2. Passive Transport:
Diffusion: Movement of molecules from high to low concentration. Osmosis: Movement of water across a semi-permeable membrane. Facilitated Diffusion: Movement through channel or carrier proteins.
3. Active Transport:
Movement of molecules against the concentration gradient using ATP. Example: Sodium-potassium pump.
4. Endocytosis and Exocytosis:
Bulk transport of materials into and out of the cell.
5. Maintaining Equilibrium:
The membrane maintains a stable internal environment (homeostasis) by balancing the intake of nutrients and the removal of waste. It regulates ion concentrations, pH, and water balance.
In summary, the cell membrane is essential for maintaining equilibrium by controlling the exchange of matter and energy between the cell and its environment.
