Answer any three questions from this section. Write your answers in the spaces provided within this question paper from page 8 to 19. All answers must be in complete sentences and in paragraphs.

Here are the answers to the essay questions:

Question 1:

a) Describe the movement of water from the soil to the leaves in a plant.
Water movement from the soil to the leaves begins with root hair cells absorbing water from the soil by osmosis, as the soil's water potential is higher. This water then moves across the root cortex cells, primarily through the apoplast (cell walls and intercellular spaces) and symplast (cytoplasm via plasmodesmata) pathways, following a water potential gradient. The Casparian strip in the endodermis forces water into the symplast, regulating its entry into the vascular cylinder. Once in the root xylem, water forms a continuous column due to cohesion (attraction between water molecules) and adhesion (attraction to xylem walls). The primary driving force for this upward movement is the transpiration pull, a negative pressure created by the evaporation of water from the leaves. This tension pulls the entire water column upwards through the xylem vessels to the leaves.

b) Explain the process of transpiration.
Transpiration is the process by which plants lose water vapor from their aerial parts, mainly through small pores called stomata located on the leaf surface. Water inside the leaf evaporates from the moist surfaces of the mesophyll cells into the air spaces within the leaf. This water vapor then diffuses out of the leaf through the open stomata into the drier atmosphere. This movement is driven by a water potential gradient between the humid air spaces inside the leaf and the drier external air. The rate of transpiration is regulated by the opening and closing of the stomata, which is controlled by specialized guard cells.

Question 2:

a) Describe the digestion of proteins in the alimentary canal of human beings.
Protein digestion begins in the stomach. The stomach lining secretes hydrochloric acid (HCl) and pepsinogen. HCl denatures proteins, unfolding their complex structures, and activates pepsinogen into its active form, pepsin. Pepsin is an endopeptidase that breaks down large protein molecules into smaller polypeptides. The acidic chyme then moves into the small intestine. The pancreas releases bicarbonate to neutralize the stomach acid and pancreatic enzymes like trypsinogen and chymotrypsinogen. Enterokinase, an enzyme from the intestinal wall, activates trypsinogen to trypsin, which then activates chymotrypsinogen to chymotrypsin. Both trypsin and chymotrypsin further break down polypeptides into even smaller peptides. Finally, the brush border of the small intestine produces peptidases (e.g., aminopeptidases, dipeptidases) that break down these small peptides into individual amino acids, which are the final digestible units of protein.

b) Explain the following processes that take place in the human body:

(i) Absorption
Absorption is the process where digested food molecules pass from the lumen of the alimentary canal into the bloodstream or lymphatic system. This primarily occurs in the small intestine, which is highly adapted for this function with its long length, folds, villi, and microvilli, significantly increasing the surface area. Nutrients like amino acids and monosaccharides are absorbed into the capillaries of the villi, while fatty acids and glycerol are re-esterified into triglycerides, packaged into chylomicrons, and absorbed into the lacteals (lymphatic capillaries). Absorption occurs through various mechanisms, including diffusion (simple and facilitated), active transport, and osmosis for water.

(ii) Assimilation
Assimilation is the process by which absorbed nutrients are transported to the body's cells and then utilized for various metabolic processes, growth, and repair. For instance, glucose absorbed into the bloodstream is transported to cells and used in cellular respiration to produce energy (ATP). Amino acids are transported to cells and used as building blocks to synthesize new proteins, such as enzymes, hormones, and structural components, essential for growth and tissue repair. Fats are assimilated for energy storage, insulation, and the formation of cell membranes. The liver plays a central role in converting and storing many assimilated nutrients.

Question 5:

a) Describe the carbon cycle.
The carbon cycle describes the movement of carbon atoms between the atmosphere, oceans, land, and living organisms. Carbon exists in the atmosphere primarily as carbon dioxide ($\text{CO}_2$). Plants (producers) absorb atmospheric $\text{CO}_2$ during photosynthesis to synthesize organic compounds. Animals (consumers) obtain carbon by feeding on plants or other animals. Both plants and animals release $\text{CO}_2$ back into the atmosphere through respiration, breaking down organic compounds for energy. When organisms die, decomposers (bacteria and fungi) break down their organic matter, releasing $\text{CO}_2$ through respiration and returning carbon to the soil. Additionally, the burning of fossil fuels (coal, oil, natural gas), formed from ancient organic matter, and the combustion of biomass release large quantities of $\text{CO}_2$ into the atmosphere. Oceans also play a significant role by absorbing and releasing $\text{CO}_2$, and carbon is stored in marine organisms and sediments.

b) Explain the importance of conserving plant and animal species.
Conserving plant and animal species is crucial for several reasons. Firstly, it maintains biodiversity, which is essential for healthy and stable ecosystems. Diverse ecosystems are more resilient to environmental changes and disturbances. Secondly, species provide vital ecosystem services, such as oxygen production by plants, water purification, soil formation, nutrient cycling, and the pollination of crops, all of which are fundamental for human survival. Thirdly, many species have significant economic value, serving as sources of food, medicine, raw materials (e.g., timber, fibers), and supporting industries like tourism. Losing species means losing potential resources and future discoveries. Fourthly, wild species represent a vast genetic library that can be invaluable for developing new crops, medicines, or adapting to future environmental challenges. Finally, there are aesthetic and ethical reasons; many people value the beauty of nature and believe that all species have an intrinsic right to exist, contributing to human well-being and cultural heritage.