Topic 6 – Plant structures and their functions — Edexcel GCSE Biology

Topic 6 – Plant structures and their functions explores how plants are adapted to obtain resources, transport them, and respond to their environment. You'll study the structure of leaves, roots, and stems, and how these organs work together to support photosynthesis, gas exchange, and water transport. This topic also covers the mechanisms of transpiration and translocation, as well as plant hormones and their role in growth responses. Understanding these processes is essential for explaining how plants survive and thrive in diverse habitats.

This topic builds on your knowledge of cells and photosynthesis from earlier in the course. It connects to ecology (how plants interact with their environment) and to practical skills like using a potometer to measure transpiration rates. Mastering this content will help you answer questions about adaptations, transport systems, and plant responses – all of which are common in exams. It also provides a foundation for A-level Biology, where you'll explore these processes in greater depth.

• →Photosynthesis occurs in the palisade mesophyll cells of leaves; these cells are packed with chloroplasts and arranged near the upper surface to maximise light absorption.
• →Transpiration is the loss of water vapour from leaves via stomata; it creates a transpiration pull that moves water up the xylem from roots to leaves.
• →Translocation is the movement of sucrose and amino acids through phloem sieve tubes from sources (e.g., leaves) to sinks (e.g., roots, fruits).
• →Plant hormones like auxin control tropisms: phototropism (growth towards light) and gravitropism (growth in response to gravity). Auxin accumulates on the shaded side of a shoot, causing cells to elongate and bend the shoot towards light.
• →Root hair cells are adapted for efficient water and mineral ion uptake: they have a large surface area, thin walls, and many mitochondria for active transport of ions.

• Definition of health as physical, mental and social well-being
• Distinction between communicable and non-communicable diseases
• Pathogens include viruses, bacteria, fungi and protists
• Mechanisms of pathogen spread and prevention
• Physical and chemical human body defences
• Specific immune system response (antigens, antibodies, memory lymphocytes)
• Antibiotics only treat bacterial infections
• Stages of medicine development (discovery, development, testing)
• Production and use of monoclonal antibodies
• Lifestyle factors affecting non-communicable diseases (BMI, alcohol, smoking)

• 💡Ensure you can distinguish between the lytic and lysogenic pathways of viruses
• 💡Be prepared to calculate cross-sectional areas of bacterial cultures using pi*r^2
• 💡Understand the ethical and practical implications of using monoclonal antibodies
• 💡Know the specific physical and chemical barriers of the human body
• 💡Be able to evaluate treatments for cardiovascular disease
• 💡When describing transpiration, always mention the role of stomata, guard cells, and the transpiration pull. Use key terms like 'cohesion' and 'adhesion' to explain how water moves up the xylem – this shows deeper understanding and gains higher marks.
• 💡For plant hormone questions, remember that auxin distribution changes in response to light or gravity. In shoots, auxin promotes cell elongation; in roots, high auxin concentrations inhibit elongation. Be precise about the direction of bending.
• 💡In practical questions about measuring transpiration (e.g., using a potometer), state that you would measure the distance moved by an air bubble over time. Control variables like temperature, humidity, and wind speed to ensure fair testing.

• Confusing communicable and non-communicable diseases
• Assuming antibiotics can kill viruses
• Misunderstanding the role of memory lymphocytes in secondary immune response
• Incorrectly calculating BMI or waist:hip ratios
• Failing to describe aseptic techniques correctly in microbial culture investigations
• Misconception: Water moves up the xylem by 'suction' from the leaves. Correction: Water is pulled up by transpiration pull – evaporation from leaves creates a negative pressure that draws water molecules up the xylem due to cohesion (water molecules sticking together) and adhesion (water sticking to xylem walls).
• Misconception: Phloem transports water and minerals. Correction: Phloem transports sucrose and amino acids (food), not water. Xylem transports water and minerals.
• Misconception: Stomata are always open to allow gas exchange. Correction: Stomata open during the day for CO₂ uptake but close at night or when the plant is water-stressed to reduce water loss. Guard cells control opening and closing.