Cell biology — AQA GCSE Biology

Cell biology is the study of the structure and function of cells, the basic units of life. In AQA GCSE Biology, this topic covers the differences between eukaryotic and prokaryotic cells, the roles of various organelles, and how cells divide by mitosis. Understanding cell biology is fundamental because it underpins all other topics in biology, from genetics to human physiology. You'll learn how cells are adapted for specific functions, such as sperm cells for reproduction or root hair cells for absorption, and how microscopes have revealed the intricate details of cellular structure.

This topic also introduces key processes like diffusion, osmosis, and active transport, which explain how substances move in and out of cells. These concepts are crucial for understanding how organisms obtain nutrients, exchange gases, and maintain homeostasis. Cell biology is not just about memorising parts of a cell; it's about appreciating how cells work together to form tissues, organs, and entire organisms. Mastery of this topic will give you a solid foundation for more advanced topics like photosynthesis, respiration, and the nervous system.

In your exams, cell biology typically appears in multiple-choice questions, short-answer questions, and extended writing tasks. You may be asked to label diagrams, compare cell types, or explain how a specific adaptation helps a cell carry out its function. Practical skills are also assessed, such as using a microscope to observe cells and calculating magnification. By the end of this topic, you should be able to confidently describe the structure of animal, plant, and bacterial cells, and explain how substances move across cell membranes.

• →Eukaryotic vs prokaryotic cells: Eukaryotic cells (animal, plant, fungal) have a nucleus and membrane-bound organelles, while prokaryotic cells (bacteria) lack a nucleus and have a single circular DNA molecule.
• →Cell organelles and their functions: Nucleus (contains genetic material), mitochondria (aerobic respiration), ribosomes (protein synthesis), chloroplasts (photosynthesis in plants), and cell wall (provides structural support).
• →Cell specialisation and differentiation: Cells become specialised to perform specific functions, e.g., sperm cells have a tail for swimming, red blood cells have no nucleus to carry more oxygen.
• →Mitosis and the cell cycle: Mitosis produces two identical daughter cells for growth and repair; the cell cycle includes interphase (DNA replication) and mitosis (prophase, metaphase, anaphase, telophase).
• →Transport across membranes: Diffusion (passive movement down concentration gradient), osmosis (diffusion of water through a partially permeable membrane), and active transport (movement against concentration gradient requiring energy).

• Identification of sub-cellular structures (nucleus, mitochondria, ribosomes, chloroplasts, cell wall, vacuole) and their specific functions.
• Distinction between eukaryotic (plant/animal) and prokaryotic (bacterial) cells, including the presence of plasmids and DNA loops.
• Explanation of the cell cycle and mitosis, resulting in two genetically identical daughter cells.
• Calculation of magnification, real size, and image size using the formula: magnification = size of image / size of real object.
• Explanation of how surface area to volume ratio affects the efficiency of transport in single-celled vs multicellular organisms.
• Description of diffusion, osmosis, and active transport, including the requirement for energy in active transport.
• Explanation of stem cell function in embryos, adults, and plant meristems, including therapeutic cloning.

• 💡Always show your working out for magnification and size calculations to gain method marks.
• 💡Use standard form when dealing with very small or large numbers, as required by the specification.
• 💡When describing transport processes, clearly state the concentration gradient involved.
• 💡Ensure you can label diagrams of plant and animal cells accurately.
• 💡Practice interpreting microscopy images and identifying sub-cellular structures.
• 💡Always use the correct terminology: For example, say 'partially permeable membrane' not 'semi-permeable' (though both are accepted, 'partially permeable' is more precise in AQA).
• 💡When comparing cells, use a table to clearly show similarities and differences. For example, compare animal and plant cells: both have a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes; plant cells also have a cell wall, chloroplasts, and a permanent vacuole.
• 💡For calculation questions on magnification, remember the formula: magnification = image size / actual size. Always include units and convert to the same unit (e.g., mm to μm) before calculating.

• Confusing the function of mitochondria (respiration) with chloroplasts (photosynthesis).
• Incorrectly stating that bacteria have a nucleus.
• Failing to include units or using incorrect units when calculating magnification or cell size.
• Confusing the direction of movement in osmosis (dilute to concentrated) versus diffusion (high to low concentration).
• Forgetting that active transport requires energy from respiration.
• Misinterpreting the cell cycle stages, specifically failing to mention DNA replication before mitosis.
• Misconception: All cells have a nucleus. Correction: Only eukaryotic cells have a nucleus; prokaryotic cells (bacteria) have a single circular chromosome in the cytoplasm, not enclosed in a nucleus.
• Misconception: Osmosis is the movement of any substance. Correction: Osmosis specifically refers to the movement of water molecules across a partially permeable membrane from a dilute to a concentrated solution.
• Misconception: Active transport is the same as diffusion. Correction: Active transport requires energy (ATP) to move substances against their concentration gradient, while diffusion is passive and moves substances down the gradient.