1
Differentiate between prokaryotic and eukaryotic cells on the basis of the presence of a nucleus and membrane-bound organelles. 2marks
2
Explain the concept of osmosis with the help of an activity. What happens when a plant cell is placed in a hypertonic solution? 3marks
3
Describe the structure and function of the Endoplasmic Reticulum (ER). Distinguish between Rough ER and Smooth ER. 3marks
4
Robert Hooke observed cells in a thin slice of cork. Explain how microscopes help us study cells that are too small to be seen with the naked eye. 2marks
5
A researcher observes that a cell swells when placed in a certain solution and shrinks when placed in another.(i) What type of solutions might these be? [1 mark](ii) Explain the movement of water molecules in each case. 2marks
6
What are chromosomes and where are they located in a eukaryotic cell? Explain their role in inheritance. 3marks
7
Explain the function of lysosomes in a cell. How does their absence or malfunction affect the cell? 2marks
8
Describe the process of photosynthesis in plants, highlighting the role of chloroplasts. 3marks
9
A bacterial cell and a plant cell are observed under a microscope. List three structural differences between them. 3marks
10
What is the cell wall made of? Explain its significance in plant cells. 2marks
11
Explain the Fluid Mosaic model of the cell membrane. 3marks
12
Why are mitochondria called the "powerhouses of the cell"? Explain their structure. 3marks
13
Discuss the importance of cell division for living organisms, mentioning the two main types of cell division. 3marks
14
Two students were observing a sample of cheek cells and onion peel cells under a microscope. They noticed that onion peel cells were rectangular and regularly arranged, while cheek cells were irregularly shaped. Explain the reason for this difference. 2marks
15
What is the role of ribosomes in a cell? 1mark
16
Discuss the functions of plastids in plant cells, including chloroplasts, chromoplasts, and leucoplasts. 3marks
17
Explain how the cell membrane regulates the passage of substances into and out of the cell. 2marks
18
If a cell is kept in pure water, what phenomenon will occur and what will be the consequence for the cell? Justify your answer. 2marks
19
Compare and contrast mitosis and meiosis in terms of their occurrence, number of daughter cells produced, and chromosome number in the daughter cells. 3marks
20
A student observes that mature red blood cells in humans lack a nucleus. What is the functional significance of this observation? 2marks
Answers
1
Prokaryotic Cells:
- Lack a well-defined nucleus; genetic material is in the nucleoid region.
- Lack membrane-bound organelles (e.g., mitochondria, Golgi apparatus, ER).
Eukaryotic Cells: - Possess a well-defined nucleus enclosed by a nuclear membrane.
- Contain various membrane-bound organelles.
2
Osmosis: Osmosis is the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through a selectively permeable membrane.In a hypertonic solution: When a plant cell is placed in a hypertonic solution (higher solute concentration outside), water moves out of the cell sap and the cytoplasm into the external solution by osmosis. The cell membrane and cytoplasm shrink, pulling away from the cell wall. This phenomenon is called plasmolysis. The cell wall, however, maintains the overall shape of the cell.
3
Endoplasmic Reticulum (ER):Structure: A large network of interconnected tubules and sacs (cisternae) spread throughout the cytoplasm. It is continuous with the outer membrane of the nuclear envelope.Function: Plays a key role in the synthesis and transport of proteins, fats (lipids), and some hormones. It also helps in detoxification.Distinction between RER and SER:
- Rough ER (RER): Has ribosomes attached to its surface, giving it a rough appearance. Primarily involved in protein synthesis and secretion.
- Smooth ER (SER): Lacks ribosomes. Involved in the synthesis and storage of fats (lipids) and hormones.
4
Microscopes are essential tools for studying cells because most cells are too small to be seen with the unaided eye. They use lenses to magnify small objects, allowing us to observe their fine structures. Light microscopes use light to magnify objects up to 1000 times, while electron microscopes use a beam of electrons to achieve much higher magnifications (up to millions of times), revealing details at the nanometre scale. This magnification and improved resolution allow biologists to see and study the structure and function of cells.
5
(i) The solution in which the cell swells is likely a hypotonic solution (pure water or a very dilute solution). The solution in which the cell shrinks is likely a hypertonic solution (concentrated salt solution). 1mark(ii) In hypotonic solution: Water molecules move from the surrounding pure water (higher water concentration) into the cell (lower water concentration) through the cell membrane by osmosis. This causes the cell to swell. [1 mark]In hypertonic solution: Water molecules move out of the cell (higher water concentration inside) into the surrounding concentrated salt solution (lower water concentration outside) through the cell membrane by osmosis. This causes the cell to shrink. [1 mark]
6
Chromosomes: Chromosomes are thread-like structures found within the nucleus of eukaryotic cells. They are composed of DNA and proteins.Location: They are located inside the nucleus of eukaryotic cells. In prokaryotic cells, the genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane.Role in Inheritance: Chromosomes carry genetic information in the form of DNA. Segments of DNA called genes contain instructions for the inheritance of characters from parents to their offspring. This ensures the continuity of traits across generations.
7
Function of Lysosomes: Lysosomes are membrane-bound sacs containing powerful digestive enzymes. They act as the cell's "clean-up crew" by breaking down:
- Waste materials produced during cellular metabolism.
- Damaged or worn-out organelles.
- Foreign particles like bacteria or viruses that enter the cell.
Consequences of absence/malfunction: If lysosomes are absent or their enzymes are not functional, waste materials and cellular debris would accumulate inside the cell, making it unhealthy and potentially leading to cell death. In some cases, this can lead to genetic disorders.
8
Photosynthesis: Photosynthesis is the process by which green plants use sunlight, carbon dioxide, and water to synthesize food (glucose) and release oxygen.Role of Chloroplasts: Chloroplasts are the organelles where photosynthesis occurs. They contain chlorophyll, a green pigment that absorbs light energy from the sun. This light energy is used to convert carbon dioxide and water into glucose. The sugars synthesized are then stored in the stroma of the chloroplast.
9
Structural Differences between Bacterial and Animal Cells:
| Feature | Bacterial Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present | Absent |
| Nucleus | Absent (Nucleoid) | Present (Well-defined) |
| Membrane-bound Organelles | Absent | Present |
10
Cell Wall Composition and Significance:Composition: The plant cell wall is primarily made of cellulose, a complex carbohydrate.Significance:
- Provides structural support and rigidity to the plant cell, helping it maintain its shape.
- Protects the plant cell from mechanical stress and osmotic lysis (bursting due to excessive water uptake).
- Allows water and dissolved minerals to pass through, facilitating nutrient absorption from the soil.
- Helps leaves and flowers remain firm and upright.
11
Fluid Mosaic Model of Cell Membrane: This model describes the cell membrane as a fluid structure in which a mosaic of proteins is embedded.
- Lipid Bilayer: The basic structure is a double layer of phospholipid molecules. The hydrophobic (water-repelling) tails point inwards, and the hydrophilic (water-attracting) heads point outwards.
- Proteins: Various proteins are embedded within or attached to the lipid bilayer. These proteins have diverse functions, acting as channels, carriers, enzymes, or receptors.
- Fluidity: The lipid molecules and most proteins can move laterally within the plane of the membrane, giving it a fluid character. This allows for membrane flexibility, cell movement, and fusion of cells.
12
Mitochondria as "Powerhouses of the Cell": Mitochondria are called the powerhouses of the cell because they are the primary sites of cellular respiration, where energy stored in food molecules (like glucose) is released and converted into a usable form of energy called Adenosine Triphosphate (ATP). ATP serves as the energy currency for most cellular activities.Structure: Each mitochondrion is enclosed by two membranes: an outer smooth membrane and an inner folded membrane. The folds of the inner membrane are called cristae, which significantly increase the surface area for ATP synthesis.
13
Importance of Cell Division:
- Growth: Cell division is essential for the growth of multicellular organisms from a single cell to a complex organism.
- Repair: It helps in repairing damaged tissues by replacing old, dead, or injured cells.
- Reproduction: It is the basis of reproduction, allowing organisms to produce offspring.
Types of Cell Division: - Mitosis: Produces two genetically identical daughter cells, important for growth, repair, and asexual reproduction.
- Meiosis: Produces four genetically diverse gametes, essential for sexual reproduction.
14
Reasons for Difference in Cell Shapes:
- Onion Peel Cells (Plant Cells): These cells have a rigid cell wall made of cellulose outside the cell membrane. This cell wall provides structural support and a fixed shape, making them appear rectangular and regularly arranged.
- Cheek Cells (Animal Cells): Animal cells lack a cell wall and have only a flexible cell membrane. This flexibility allows them to change their shape, enabling them to perform specific functions like movement and engulfing particles, resulting in their irregular shape.
15
Role of Ribosomes: Ribosomes are tiny organelles responsible for protein synthesis in the cell. They read the genetic code carried by messenger RNA (mRNA) and assemble amino acids into proteins.
16
Functions of Plastids in Plant Cells:
- Chloroplasts: Contain chlorophyll and are the sites of photosynthesis, where light energy is converted into chemical energy in the form of glucose.
- Chromoplasts: Contain pigments other than chlorophyll (e.g., yellow, orange, red) and are responsible for the colours of flowers, fruits, and some leaves, attracting pollinators and seed-dispersing animals.
- Leucoplasts: Colourless plastids that store food materials like starch, oils, and proteins in various parts of the plant, such as roots and seeds.
17
Regulation of Substance Passage by Cell Membrane: The cell membrane is selectively permeable. This means it controls which substances can pass into or out of the cell.
- Small molecules like oxygen () and carbon dioxide () can diffuse freely across the membrane.
- Water moves across the membrane by osmosis.
- Larger molecules or ions are transported across the membrane with the help of transport proteins embedded in the membrane, often requiring energy (active transport).
18
Phenomenon in Pure Water: If a cell is placed in pure water (a hypotonic solution), the concentration of water outside the cell is higher than inside the cell. By osmosis, water will move into the cell through the selectively permeable cell membrane.Consequence: The cell will swell. If it is an animal cell, it will burst (cytolysis) because it lacks a rigid cell wall. A plant cell will become turgid, but it will not burst due to the presence of the rigid cell wall.
19
Comparison of Mitosis and Meiosis:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Occurrence | Somatic (body) cells | Reproductive cells (in gonads) |
| Number of Divisions | One | Two |
| Daughter Cells | Two | Four |
| Chromosome Number | Same as parent cell (diploid, 2n) | Half of parent cell (haploid, n) |
| Genetic Identity | Genetically identical to parent | Genetically different from parent (due to crossing over/R.O.) |
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gamete formation), genetic diversity |
20
Significance of Nucleus Absence in Mature Red Blood Cells: Mature red blood cells (RBCs) in humans lack a nucleus and most other organelles. The absence of a nucleus provides more space within the cell for haemoglobin. This allows RBCs to carry a larger amount of oxygen, maximizing their efficiency in oxygen transport to all tissues and organs of the body. However, without a nucleus, RBCs cannot divide or repair themselves, limiting their lifespan to about 120 days.
