Q1. a) Outline the factors that influence the shape of a cell. (4mks) b) Distinguish between Gram positive and Gram negative bacterial cell walls. (6mks) Q 2 a) Compare and contrast the passive and active transport systems through cell membranes. (4mks) b) Discuss the physiological characteristics of biological membranes. (6mks) Q3 a) Briefly describe the structure and function of animal cytoskeleton. (6mks) (4mks) b) Explain the TWO key functions of the nucleus.

Q1. a) The factors that influence the shape of a cell include:<br /><br />1. **Cell Wall Composition**: The presence and composition of a cell wall can significantly influence cell shape. For example, plant cells have rigid cell walls made of cellulose, which maintain a fixed shape, while bacterial cells have peptidoglycan in their cell walls, influencing their shape.<br /><br />2. **Cytoskeleton Components**: The cytoskeleton, composed of microtubules, microfilaments, and intermediate filaments, provides structural support and shape to the cell. These components help in maintaining the cell's shape and facilitating movement and division.<br /><br />3. **Internal Pressure (Turgor Pressure)**: In plant cells, turgor pressure, which is the pressure of the cell contents against the cell wall, helps maintain the cell's shape. In animal cells, the internal pressure is balanced by the cell membrane, and the shape is more flexible.<br /><br />4Membrane Lipids and Proteins**: The composition of lipids and proteins in the cell membrane can influence the shape of the cell. Certain proteins and lipids can cause the membrane to curve or form specific shapes.<br /><br />b) **Distinguish between Gram-positive and Gram-negative bacterial cell walls**:<br /><br />1. **Gram Staining**: Gram-positive bacteria retain the crystal violet stain and appear purple under a microscope, while Gram-negative bacteria do not and appear pink or red.<br /><br />2. **Cell Wall Structure**:<br /> - **Gram-positive**: Have a thick peptidoglycan layer (20-80 nm) that is located in the cell wall and contains teichoic acids. The outer surface of the peptidoglycan layer is exposed.<br /> - **Gram-negative**: Have a thin peptidoglycan layer (2-7 nm) located within an outer membrane. The peptidoglycan layer is covered by an outer membrane composed of a lipopolysaccharide (LPS) layer, which contains endotoxins.<br /><br />3. **Periplasmic Space**: In Gram-negative bacteria, there is a larger periplasmic space between the outer membrane and the peptidoglycan layer, which is not present in Gram-positive bacteria.<br /><br />4. **Sensitivity to Antibiotics**: Gram-positive bacteria are generally more susceptible to antibiotics that target the peptidoglycan layer, while Gram-negative bacteria are more resistant due to the protective outer membrane.<br /><br />Q2. a) **Compare and contrast the passive and active transport systems through cell membranes**:<br /><br />1. **Energy Requirement**:<br /> - **Passive Transport**: Does not require energy (ATP) and relies on the natural movement of molecules from areas of higher concentration to lower concentration (down the concentration gradient).<br /> - **Active Transport**: Requires energy (ATP) to move molecules against their concentration gradient, from areas of lower concentration to higher concentration.<br /><br />2. **Transport Mechanisms**:<br /> - **Passive Transport**: Includes diffusion (simple and facilitated) and osmosis. Facilitated diffusion uses transport proteins like channel and carrier proteins.<br /> - **Active Transport**: Uses transport proteins like pumps (e.g., the sodium-potassium pump) to move molecules against their gradient.<br /><br />3. **Examples**:<br /> - **Passive Transport**: Oxygen and carbon dioxide diffusion through the cell membrane.<br /> - **Active Transport**: Sodium-potassium pump, proton pump, and calcium pumps.<br /><br />4. **Regulation**:<br /> - **Passive Transport**: Not regulated; it occurs naturally based on the concentration gradient.<br /> - **Active Transport**: Highly regulated; the cell controls when and how much to transport.<br /><br />b) **Discuss the physiological characteristics of biological membranes**:<br /><br />1. **Selective Permeability**: Biological membranes are selectively permeable, allowing certain molecules to pass through while restricting others. This is crucial for maintaining homeostasis and regulating the internal environment of the cell.<br /><br />2. **Fluidity**: Membranes have a fluid nature due to the movement of phospholipids and proteins. This fluidity is essential for membrane functions such as endocytosis, exocytosis, and cell signaling.<br /><br />3. **Dynamic Structure**: The phospholipid bilayer is dynamic, with phospholipids constantly moving and interacting with each other. This fluidity allows the membrane to self-heal after damage and to form various structures and vesicles.<br /><br />4. **Membrane Proteins**: Proteins embedded in the membrane play various roles, including transport (channels and carriers), signal reception (receptors), and structural support (integral and peripheral proteins).<br /><br />5. **Membrane Potential**: Biological membranes have a voltage difference across them, known as the membrane potential. This potential is crucial for nerve impulse transmission, muscle contraction, and other cellular processes.<br /><br />6. **Endocytosis and Exocytosis**: The membrane is involved in the processes of endocytosis (ingestion of materials) and exocytosis (expulsion of materials), which