NECTA ACSEE Advanced Biology 2024 Past Paper Questions and Marking Scheme Answers

Question 1

(a) Briefly explain how each of the following features of a mitochondrion helps in breaking down food to release energy:
(i) Folded inner membrane
(ii) Matrix
(iii) Circular DNA
(iv) Ribosomes

(b) State two importance of each of the following organelles:
(i) Microtubules
(ii) Golgi body
(iii) Lysosomes

Accepted Answer

(a) Features of a mitochondrion and their roles in cellular respiration:
(i) Folded inner membrane (Cristae): Provides a vast surface area for the attachment of electron transport chain (ETC) protein complexes, electron carriers (cytochromes), and stalked particles (ATP synthase/F0-F1 ATPases), maximizing oxidative phosphorylation and ATP generation.
(ii) Matrix: Contains a concentrated aqueous mixture of soluble respiratory enzymes (e.g., citrate synthase, dehydrogenase) required for the Krebs cycle (TCA cycle) and beta-oxidation of fatty acids, as well as NAD+ and FAD coenzymes to accept hydrogen atoms.
(iii) Circular DNA (mtDNA): Encodes essential mitochondrial respiratory proteins and enzyme subunits (including components of the electron transport chain and ATP synthase), enabling the mitochondrion to synthesize its own respiratory machinery semi-autonomously and replicate when metabolic demands increase.
(iv) Ribosomes (70S Ribosomes): Translate mitochondrial mRNA into polypeptide chains and structural enzymes within the organelle, allowing direct local protein synthesis without total reliance on the cell's nucleus and cytoplasm.

(b) Importance of the specified organelles (two points each):
(i) Microtubules:
1. Spindle Apparatus Formation: Form the spindle fibers during mitosis and meiosis that attach to kinetochores and ensure precise segregation of sister chromatids and homologous chromosomes.
2. Cytoskeletal Support and Motility: Maintain eukaryotic cell shape, act as intracellular tracks for organelle and vesicle transport (via motor proteins kinesin and dynein), and form the 9+2 structural core (axoneme) of cilia and flagella for cellular movement.
(ii) Golgi body:
1. Modification and Sorting: Chemically modifies proteins and lipids received from the rough and smooth endoplasmic reticulum (e.g., glycosylation to form glycoproteins and glycolipids) and packages them for targeted delivery.
2. Secretion and Lysosome Formation: Packages secretory products into vesicles for exocytosis across the cell membrane, synthesizes plant cell wall pectins, and forms primary lysosomes packed with acid hydrolases.
(iii) Lysosomes:
1. Heterophagy and Autophagy: Fuse with phagocytic vacuoles to digest engulfed pathogens (e.g., bacteria in macrophages) and degrade worn-out intracellular organelles to recycle vital macromolecular constituents.
2. Autolysis (Programmed Cell Death): Release hydrolytic enzymes to break down the entire cell during metamorphosis, developmental tissue resorption (such as tadpole tail regression), or removal of damaged/senescent cells. Explanation: This question evaluates understanding of mitochondrial ultrastructure in relation to biochemical energetics, alongside the physiological roles of microtubules, Golgi apparatus, and lysosomes.

Question 2

(a) Analyse five differences between yeast cells and bacterium cells. (5 Marks)

(b) Using two points, justify the need for a plant cell to have a vacuole. (5 Marks)

Accepted Answer

(a) Five differences between yeast cells and bacterium cells:
1. Nuclear Organization: Yeast is a eukaryote with a true membrane-bound nucleus containing linear chromosomes with histone proteins and a nucleolus; a bacterium is a prokaryote lacking a nuclear membrane, possessing a single naked circular DNA molecule in an open nucleoid.
2. Organelles: Yeast cells possess membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus; bacterium cells lack all membrane-bound organelles (cellular respiration occurs at the plasma membrane/mesosomes).
3. Ribosome Size: Yeast cells have larger 80S cytosolic ribosomes; bacterium cells have smaller 70S ribosomes.
4. Cell Wall Composition: The yeast cell wall is composed primarily of chitin and beta-glucans; the bacterial cell wall is composed of peptidoglycan (murein).
5. Cell Size: Yeast cells are significantly larger, typically 5 to 10 micrometers in diameter; bacterial cells are much smaller, typically 0.5 to 5 micrometers.

(b) Justification for the need of a vacuole in a plant cell (two points):
1. Maintenance of Turgor Pressure and Mechanical Support: The central vacuole contains cell sap with a high concentration of dissolved mineral ions, organic acids, and sugars. This generates a low (more negative) osmotic potential, causing water to enter by osmosis and creating high internal hydrostatic turgor pressure against the rigid cellulose cell wall. This turgidity provides mechanical support and structural firmness to non-woody plant organs such as stems, leaves, and petals.
2. Dynamic Storage and Waste Sequestration: The vacuole serves as a primary storage reservoir for water, metabolic nutrients (sugars, amino acids), and secondary metabolites including anthocyanin pigments (which color flowers to attract pollinators). Additionally, it securely isolates potentially harmful metabolic wastes and lytic enzymes away from the cytoplasm, protecting the plant cell's metabolic machinery. Explanation: The candidate must accurately contrast eukaryotic (yeast) vs prokaryotic (bacterium) cytology and provide rigorous biological rationale for plant vacuole turgor regulation and storage.

Question 3

(a) Study the structure of the molecule shown in the given figure and answer the questions that follow:

[Chemical Structure: Adenine base bonded to Ribose sugar linked to a chain of three phosphate groups (Triphosphate)]

(i) Identify the molecule. (1 Mark)
(ii) Explain three ways in which the molecule is important in human body. (3 Marks)

(b) Analyse four features that are shared by gills and lungs. (6 Marks)

Accepted Answer

(a)(i) Identification of the molecule:
Adenosine Triphosphate (ATP).

(a)(ii) Three ways ATP is important in the human body:
1. Active Transport Across Membranes: Hydrolysis of ATP directly powers primary active transport pumps, most crucially the Sodium-Potassium ATPase (Na+/K+ pump), which maintains the resting membrane potential in nerve and muscle cells and powers secondary glucose/amino acid absorption.
2. Mechanical Work / Muscle Contraction: ATP binds to the myosin head, allowing cross-bridge detachment and subsequent hydrolysis to cock the myosin head, driving actin-myosin filament sliding during skeletal, cardiac, and smooth muscle contractions.
3. Anabolic Synthesis of Biological Macromolecules: Supplies the chemical free energy required to drive endergonic condensation reactions, including DNA replication, RNA transcription, polypeptide protein synthesis from amino acids, and glycogen formation.

(b) Four shared features between gills and lungs (Respiratory Surfaces):
1. Large Surface Area to Volume Ratio: Both possess extensively folded and branched structures (gill filaments and microscopic secondary lamellae in gills; millions of tiny spherical alveoli in lungs) providing an immense surface area for diffusion of respiratory gases relative to body volume.
2. Extremely Thin Diffusion Barrier: Both feature an exceptionally thin epithelial layer—a single layer of flattened squamous epithelial cells—which minimizes the diffusion distance for oxygen and carbon dioxide molecules between the respiratory medium and blood.
3. Dense Capillary Network (Rich Vascularization): Both surfaces are intimately lined by an extensive, high-density bed of blood capillaries, ensuring continuous rapid transport of dissolved gases and maintaining steep partial pressure concentration gradients.
4. Moist Respiratory Surface: Both surfaces are continuously covered by an aqueous fluid film (water in aquatic gills; pulmonary surfactant and moisture in alveolar lungs) in which oxygen and carbon dioxide must dissolve before diffusing across cellular membranes. Explanation: Candidates must recognize the biochemical formula of ATP (adenine + ribose + triphosphate) and describe universal physical adaptations of specialized respiratory surfaces (Fick's Law of Diffusion).

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