Cell biology — AQA GCSE Study Guide

 ## Overview Welcome to Cell Biology! This topic is the absolute foundation of your Biology GCSE. Understanding cells—the fundamental units of all living organisms—is crucial because every other topic, from human anatomy to plant physiology, builds upon this knowledge. In this section, we will explore the intricate structures that make up different types of cells, how cells replicate to allow growth, and the vital mechanisms they use to transport substances in and out. Examiners frequently test this topic across all papers. You can expect a mix of short factual recall questions (like naming organelles), mathematical challenges (calculating magnification), and longer explanatory questions (comparing transport processes). Pay special attention to the command words and ensure you are using precise biological terminology.  ## Key Concepts ### Concept 1: Eukaryotic and Prokaryotic Cells All cells can be divided into two main categories: eukaryotic and prokaryotic. Eukaryotic cells are complex and include all animal and plant cells. Their defining feature is that their genetic material (DNA) is enclosed within a true nucleus. Prokaryotic cells, such as bacteria, are much smaller and simpler. Crucially, they **do not** have a nucleus. Instead, their genetic material floats freely in the cytoplasm as a single circular loop of DNA. They may also contain small rings of extra DNA called plasmids. **Examiner Tip**: Candidates frequently lose marks by stating that bacteria have a nucleus. Always remember: Prokaryotes = No true nucleus.  ### Concept 2: Sub-cellular Structures (Organelles) You must know the function of key organelles. - **Nucleus**: Contains genetic material and controls the activities of the cell. - **Mitochondria**: The site of aerobic respiration, where energy is released for the cell to use. (Note: Do not say 'makes energy' - energy is *released*). - **Ribosomes**: Where protein synthesis occurs. - **Cell Membrane**: Controls the passage of substances into and out of the cell. - **Cytoplasm**: A jelly-like substance where most chemical reactions take place. Plant cells have additional structures: - **Chloroplasts**: Contain chlorophyll, which absorbs light for photosynthesis. - **Permanent Vacuole**: Filled with cell sap to help keep the cell turgid (firm). - **Cell Wall**: Made of cellulose, it strengthens the cell and provides support. ### Concept 3: Cell Division and the Cell Cycle Cells divide to allow organisms to grow, replace dead cells, and repair tissues. The type of cell division you need to know here is **mitosis**. Before a cell can divide, it must grow and replicate its DNA to form two copies of each chromosome. It also increases the number of sub-cellular structures like mitochondria and ribosomes. During mitosis, one set of chromosomes is pulled to each end of the cell, and the nucleus divides. Finally, the cytoplasm and cell membranes divide to form two identical daughter cells. **Example**: If a human skin cell with 46 chromosomes undergoes mitosis, it will produce two daughter cells, each containing exactly 46 chromosomes. ### Concept 4: Stem Cells A stem cell is an undifferentiated cell capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation. - **Embryonic stem cells**: Can differentiate into *any* type of human cell. - **Adult stem cells**: Found in bone marrow, they can only differentiate into certain types of cells, like blood cells. - **Plant meristems**: Found in the tips of roots and shoots, these can differentiate into any type of plant cell throughout the life of the plant. ### Concept 5: Transport in Cells Substances move into and out of cells via three main processes: 1. **Diffusion**: The spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration. It is a passive process (requires no energy). Example: Oxygen diffusing into cells for respiration. 2. **Osmosis**: The diffusion of *water* from a dilute solution to a concentrated solution through a partially permeable membrane. 3. **Active Transport**: The movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration. Example: Root hair cells absorbing mineral ions from the soil.  ## Mathematical/Scientific Relationships ### Magnification Formula **Formula**: Magnification = Size of Image / Size of Real Object $$ M = rac{I}{A} $$ - **I (Image Size)**: How big the object appears in the diagram or photo. - **A (Actual Size)**: The real-life size of the object. - **M (Magnification)**: How many times larger the image is compared to the real object. *Must memorise!* Examiners will expect you to recall this formula. Always ensure your units for Image Size and Actual Size are the same before calculating. ## Practical Applications ### Required Practical: Microscopy You must know how to use a light microscope to observe, draw, and label a selection of plant and animal cells (e.g., onion epidermal cells or human cheek cells). - **Key step**: Use a stain (like iodine for onion cells) to make the sub-cellular structures visible. - **Common error**: Drawing thick, sketchy lines. Examiners want clear, continuous, unbroken lines with no shading when you draw what you see. - **Safety**: Take care with glass slides and coverslips. ### Required Practical: Osmosis Investigating the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue (usually potato cylinders). - **Method**: Cut equal-sized potato cylinders, weigh them, place them in different concentrations of sugar solution, leave for a set time, dry them, and re-weigh. - **Expected results**: Cylinders in dilute solutions will gain mass (water moves in by osmosis). Cylinders in concentrated solutions will lose mass (water moves out). - **Calculation**: Always calculate the *percentage change in mass* to allow for valid comparisons, as the starting masses won't be exactly identical.