Chapter B4: Using food and controlling growth — OCR GCSE Combined Science

Chapter B4: Using food and controlling growth explores how organisms use nutrients for energy, growth, and repair, and how growth is regulated. In Combined Science (OCR GCSE), this topic covers the processes of digestion, absorption, and assimilation of food, as well as the role of enzymes and hormones in controlling growth. Understanding these mechanisms is crucial for explaining how the body maintains health and responds to changes in diet and environment.

This chapter builds on earlier work about cells and tissues, linking the structure of the digestive system to its function. You'll learn how carbohydrates, proteins, and lipids are broken down by specific enzymes, how the products are absorbed into the bloodstream, and how they are used for cellular respiration or building new cells. Additionally, the role of plant growth hormones (auxins) in phototropism and gravitropism is covered, showing how growth responses are controlled in plants.

Mastering B4 is essential for understanding homeostasis, metabolism, and the interdependence of organ systems. It also provides a foundation for later topics on genetics, evolution, and biotechnology. By the end of this chapter, you should be able to describe the stages of digestion, explain the lock-and-key model of enzyme action, and outline how plant hormones coordinate growth responses.

• →Digestion: The breakdown of large insoluble food molecules into small soluble ones by mechanical and chemical processes, involving enzymes like amylase, protease, and lipase.
• →Absorption: The movement of digested nutrients (e.g., glucose, amino acids, fatty acids) from the small intestine into the bloodstream via villi.
• →Enzyme action: Enzymes are biological catalysts that speed up reactions; they have an active site that fits specific substrates (lock-and-key model). Factors like temperature and pH affect enzyme activity.
• →Plant growth hormones: Auxins control phototropism (growth towards light) and gravitropism (growth in response to gravity) by causing unequal cell elongation.
• →Metabolic rate: The rate at which chemical reactions occur in the body, influenced by factors like age, gender, and activity level.

• Comparison of aerobic and anaerobic respiration (conditions, inputs, outputs, ATP yield)
• Explanation of cellular respiration as an exothermic process
• Role of mitochondria in eukaryotic cells
• Description of the cell cycle (interphase and mitosis)
• Explanation of meiosis in gamete production
• Differentiation of stem cells and the role of meristems in plants
• Ethical and practical considerations of stem cell therapy

• 💡Ensure you can clearly distinguish between aerobic and anaerobic respiration in terms of ATP yield
• 💡Be prepared to interpret data or graphs related to respiration rates in yeast
• 💡Use precise biological terminology when describing the cell cycle and mitosis
• 💡When discussing stem cells, ensure you address both the potential benefits and the ethical concerns to gain full marks in extended response questions
• 💡When describing enzyme action, always mention the 'lock and key' or 'induced fit' model, and state that high temperatures denature the enzyme by changing the shape of the active site.
• 💡For digestion questions, be specific about where each enzyme is produced and where it acts (e.g., amylase is made in salivary glands and pancreas, works in mouth and small intestine).
• 💡In plant growth questions, use the terms 'positive phototropism' (shoots grow towards light) and 'positive gravitropism' (roots grow downwards) to show precise understanding.

• Confusing the products of anaerobic respiration in animals (lactic acid) versus plants/yeast (ethanol and CO2)
• Failing to distinguish between the roles of embryonic and adult stem cells
• Misunderstanding the role of the cell cycle in growth versus gamete production
• Incorrectly describing the process of differentiation as a permanent loss of genetic information
• Misconception: Enzymes are 'used up' in reactions. Correction: Enzymes are catalysts and remain unchanged after the reaction; they can be reused.
• Misconception: All digestion happens in the stomach. Correction: Most chemical digestion occurs in the small intestine, where pancreatic enzymes and bile break down fats, proteins, and carbohydrates.
• Misconception: Plants grow towards light because they 'need' light. Correction: Auxins cause cells on the shaded side to elongate more, bending the plant towards light; it's a hormonal response, not a conscious need.