Cambridge O level Biology Questions & Answers

The book outlines seven fundamental characteristics that define living organisms: movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition. These are often remembered by the acronym MRS GREN. Movement refers to changing position, respiration is energy release, sensitivity is responding to stimuli, growth is increasing in size, reproduction is producing offspring, excretion is waste removal, and nutrition is obtaining food.

The book explains photosynthesis as the process by which green plants and some other organisms convert light energy into chemical energy, stored in glucose. It details that chlorophyll in chloroplasts absorbs light energy, which is then used to convert carbon dioxide and water into glucose and oxygen. The equation 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂ is central to its explanation, highlighting the raw materials and products.

According to the textbook, the primary function of enzymes is to act as biological catalysts, speeding up the rate of biochemical reactions within living organisms without being used up themselves. They are highly specific, meaning each enzyme typically catalyzes only one or a few specific reactions, often explained using the 'lock and key' hypothesis, where the substrate fits precisely into the enzyme's active site.

The book differentiates aerobic respiration as the process of releasing energy from glucose in the presence of oxygen, producing carbon dioxide and water, and a large amount of ATP. Anaerobic respiration, conversely, occurs without oxygen, yielding less ATP and producing different byproducts like lactic acid in animals or ethanol and carbon dioxide in yeast. It emphasizes that aerobic respiration is far more efficient in energy production.

The textbook describes xylem as a crucial vascular tissue in plants responsible for the transport of water and dissolved mineral salts from the roots up to the rest of the plant, including the leaves. It explains that xylem vessels are dead, hollow tubes with lignified walls, providing structural support while facilitating the continuous upward flow of water through transpiration pull, cohesion, and adhesion.

The book describes the human circulatory system as a double circulatory system, meaning blood passes through the heart twice for each complete circuit of the body. It details the roles of the heart (a muscular pump), blood vessels (arteries, veins, capillaries), and blood itself. It explains the pulmonary circulation (heart to lungs and back) and systemic circulation (heart to body and back), ensuring efficient oxygen and nutrient delivery.

The textbook outlines a balanced diet as one that provides adequate amounts of carbohydrates, fats, proteins, vitamins, minerals, dietary fibre, and water. It explains the specific roles of each component: carbohydrates and fats for energy, proteins for growth and repair, vitamins and minerals for metabolic processes, fibre for digestive health, and water for various bodily functions, emphasizing the importance of correct proportions.

The author explains mitosis as a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus. Its significance lies in growth, as it increases the number of cells in a multicellular organism; repair, replacing damaged or dead cells; and asexual reproduction, allowing organisms to produce genetically identical offspring.

The book explains a food chain as a linear sequence showing how energy is transferred from one living organism to another through feeding, starting with producers and moving through consumers. A food web is presented as a more realistic and complex representation, consisting of interconnected food chains, illustrating that most organisms have multiple food sources and are eaten by multiple predators, showing the intricate energy flow within an ecosystem.

The book highlights several key differences between plant and animal cells. Plant cells typically possess a rigid cell wall outside the cell membrane, chloroplasts for photosynthesis, and a large central vacuole for turgor and storage. Animal cells, in contrast, lack a cell wall and chloroplasts, and usually have smaller, temporary vacuoles or none at all. Both share a cell membrane, cytoplasm, and nucleus.

The textbook describes the kidney's crucial role in the human body as an excretory organ responsible for filtering waste products, particularly urea, from the blood to form urine. It also plays a vital role in osmoregulation, controlling the water and salt balance in the body, thereby maintaining a constant internal environment (homeostasis). It explains the nephron as the functional unit of the kidney.

The book describes gaseous exchange in the human lungs as occurring in the alveoli, tiny air sacs with thin, moist walls and a rich blood supply. Oxygen from inhaled air diffuses across the alveolar and capillary walls into the blood, where it binds to haemoglobin. Simultaneously, carbon dioxide, a waste product from the blood, diffuses from the capillaries into the alveoli to be exhaled. This process relies on concentration gradients.

Homeostasis, as explained in the book, refers to the maintenance of a constant internal environment in living organisms, despite changes in external conditions. This includes regulating factors like body temperature, blood glucose levels, water potential, and pH. It emphasizes that various physiological processes, often involving negative feedback mechanisms, work together to ensure optimal conditions for enzyme activity and cell function.

The author explains hormones as chemical messengers produced by endocrine glands and transported in the blood to target organs or cells, where they exert specific effects. Their general function is to coordinate and regulate various physiological processes, such as growth, metabolism, reproduction, and stress responses. They act more slowly but have longer-lasting effects compared to nervous coordination.

The book differentiates sexual reproduction as involving the fusion of two gametes (sex cells), usually from two parents, leading to offspring with genetic variation. Asexual reproduction, in contrast, involves only one parent and produces genetically identical offspring (clones) without the fusion of gametes. It highlights that sexual reproduction promotes adaptation, while asexual reproduction is faster and simpler.

The 'lock and key' hypothesis, as presented in the book, is a model used to explain enzyme specificity. It suggests that the active site of an enzyme has a specific shape (like a lock) that only a particular substrate molecule (like a key) can fit into. This precise fit allows the enzyme to catalyze a specific reaction, forming an enzyme-substrate complex, which then breaks down to release products and the unchanged enzyme.

The textbook discusses two main types of immunity: active and passive. Active immunity is developed when the body produces its own antibodies in response to an antigen, either through natural infection or vaccination, providing long-lasting protection. Passive immunity involves receiving pre-formed antibodies from another source, such as from mother to child via placenta/breast milk or through injection, offering immediate but short-term protection.

The book explains transpiration as the process by which water vapor is lost from the aerial parts of a plant, primarily through the stomata on the leaves. It details that water evaporates from the moist surfaces of mesophyll cells into the air spaces, then diffuses out through the stomata. This loss creates a transpiration pull, drawing water up from the roots through the xylem, which is essential for water and mineral transport.

The ecology section discusses various environmental impacts of human activities, including deforestation, pollution (air, water, land), global warming due to increased greenhouse gases, and depletion of natural resources. It explains how these activities disrupt ecosystems, reduce biodiversity, and contribute to climate change, emphasizing the need for sustainable practices and conservation efforts to mitigate these effects.