Cell Structure and Function — CCEA A-Level Biology Revision
This subtopic explores the fundamental structural components of cells, distinguishing between prokaryotic and eukaryotic organisation and contrasting the s
Topic Synopsis
This subtopic explores the fundamental structural components of cells, distinguishing between prokaryotic and eukaryotic organisation and contrasting the specialised features of animal and plant cells. A thorough grasp of these differences is essential for interpreting microscopic data, explaining cellular functions, and applying knowledge to areas such as disease diagnosis and biotechnology.
Key Concepts & Core Principles
- Eukaryotic vs. prokaryotic cells: Eukaryotes have a nucleus and membrane-bound organelles, while prokaryotes lack these and have a nucleoid region.
- Structure and function of organelles: e.g., mitochondria have a double membrane and cristae for ATP production; ribosomes are sites of protein synthesis (80S in eukaryotes, 70S in prokaryotes).
- Cell membrane structure: The fluid mosaic model, with phospholipid bilayer, integral and peripheral proteins, cholesterol, and glycoproteins, enabling selective permeability and cell signalling.
- Differences between plant and animal cells: Plant cells have a cell wall (cellulose), chloroplasts, and a large permanent vacuole; animal cells have centrioles and smaller, temporary vacuoles.
- Levels of organisation: Cells form tissues, tissues form organs, and organs form organ systems – understanding this hierarchy is essential for linking structure to function.
Exam Tips & Revision Strategies
- Use precise terminology: refer to 'nuclear envelope' rather than 'nuclear membrane', and 'cell surface membrane' to avoid confusion with internal membranes.
- When comparing cell types, always structure your answer to address both structural features and their functional significance, linking form to function explicitly.
- When comparing mitosis and meiosis, use a table to contrast chromosome number, genetic variation, and number of divisions, ensuring each point is clearly linked to the relevant process.
- In significance questions, explicitly connect mitosis to production of genetically identical cells for growth and repair, and meiosis to halving chromosome number and generating variation for sexual reproduction.
- Practice drawing and labelling diagrams of the stages, paying close attention to the arrangement of chromosomes and the spindle apparatus at each phase.
- When describing the fluid mosaic model, always refer to the 'fluid' nature due to phospholipid movement and the 'mosaic' pattern of proteins scattered throughout.
- Use precise terminology: 'phospholipid bilayer', 'hydrophilic heads', 'hydrophobic tails', 'integral proteins', 'peripheral proteins', etc.
- In transport questions, clearly state the type of transport, the energy requirement, and whether proteins are involved, linking back to the membrane structure.
Common Misconceptions & Mistakes to Avoid
- Confusing the size and sedimentation rate of ribosomes (70S in prokaryotes, chloroplasts and mitochondria; 80S in eukaryotic cytoplasm) and misattributing their presence.
- Stating that plant cells lack centrioles without noting that some lower plants do possess them, leading to overgeneralisation.
- Omitting membrane-bound organelles when describing eukaryotic cells, failing to recognise this as the key distinction from prokaryotes.
- Confusing the separation of sister chromatids in mitosis and meiosis II with the separation of homologous chromosomes in meiosis I.
- Incorrectly stating that mitosis produces haploid cells or that meiosis is involved in growth and repair.
- Omitting the importance of interphase and DNA replication prior to division, leading to misunderstanding of chromosome versus chromatid numbers.
Examiner Marking Points
- Award credit for accurately labelling and describing the function of key organelles (e.g., nucleus, mitochondria, ribosomes, chloroplasts, cell wall) in both animal and plant eukaryotic cells.
- Award credit for correctly identifying prokaryotic features (e.g., circular DNA, 70S ribosomes, peptidoglycan cell wall) and contrasting them with eukaryotic cell components.
- Award credit for constructing clear comparison tables or annotated diagrams that highlight similarities and differences between animal and plant cells, and between eukaryotic and prokaryotic cells.
- Award credit for accurately describing chromosome behaviour during each mitotic phase, including condensation, alignment at the metaphase plate, sister chromatid separation, and decondensation.
- Expect precise explanation of the role of spindle fibres and centrioles in chromosome movement, and the significance of cytokinesis in dividing the cytoplasm.
- Credit must be given for correctly comparing meiosis I and II, highlighting reduction division, crossing over, and independent assortment as sources of genetic variation.
- Award credit for correctly labeling and describing the components of the fluid mosaic model, including phospholipid bilayer, integral and peripheral proteins, cholesterol, glycoproteins, and glycolipids.
- Look for clear explanations of passive transport mechanisms (simple diffusion, facilitated diffusion, osmosis) with reference to concentration gradients and the role of channel and carrier proteins.