Structure of eukaryotic cells — AQA A-Level Biology

The ultrastructure of eukaryotic cells encompasses the specialised organelle networks that enable compartmentalised biochemical reactions in animals, plants, algae, and fungi. Understanding the distinct architectural roles of the nucleus, endomembrane system, mitochondria, and chloroplasts is fundamental to explaining how complex cells maintain metabolic efficiency.

This topic forms the core foundation for subsequent AQA units including cellular transport, immunology, bioenergetics (photosynthesis and respiration), and protein synthesis. A strong command of organelle structure and organelle isolation techniques such as cell fractionation enables students to link micro-scale structures with macro-scale biological functions.

• →Compartmentalisation: Membrane-bound organelles provide isolated microenvironments with specialised enzymes and pH conditions, preventing interfering metabolic reactions.
• →The Endomembrane System: The continuous trafficking network involving the nuclear envelope, rough and smooth endoplasmic reticulum, transport vesicles, Golgi apparatus, and lysosomes.
• →Energy Transducing Organelles: Mitochondria and chloroplasts contain double membranes, internal fluid matrices, and folded internal membranes (cristae and thylakoids) designed to maximise surface area for ATP synthesis.
• →Structural Diversity Across Taxa: Plant and algal cells contain cellulose or glycoprotein walls, vacuoles, and chloroplasts; fungal cells feature chitin cell walls and lack chloroplasts; animal cells lack cell walls and large permanent vacuoles.

• identifying a membrane-bound organelle as evidence that a cell is eukaryotic, such as a mitochondrion, nucleus, endoplasmic reticulum, Golgi apparatus, lysosome or vesicle
• describing the nucleus as having a double membrane or nuclear envelope with pores, and containing chromosomes of DNA bound to histones plus a nucleolus
• stating the Golgi apparatus modifies, packages or transports proteins or lipids, or forms vesicles and lysosomes
• identifying that lysosomes contain hydrolytic enzymes that are released to digest material
• linking mitochondria to the release of ATP and ribosomes to the production of protein
• stating that chloroplasts are found in plants and algae
• Describe how differential gene expression leads to specialisation by altering cell shape and organelle proportions without changing the genome of somatic cells.
• Identify the specific organelle that is abundant or modified in a named specialised cell, such as extensive rough endoplasmic reticulum in pancreatic cells.
• Link the abundance of a specific organelle directly to the cell's function, for example, mitochondria releasing ATP for active transport or movement.
• Explain structural losses as functional adaptations, such as the absence of a nucleus in a mature red blood cell to accommodate more haemoglobin.
• Award credit for defining a tissue as a group of similar or specialised cells with a shared function.
• Award credit for defining an organ as different tissues working together to perform a particular function.
• Award credit for defining an organ system as a group of organs working together.
• Award credit for a correct worked example running through all the levels, such as epithelial cell, epithelium, ileum, digestive system.
• Award credit for linking the level of organisation to body size and a small surface area to volume ratio, so that diffusion alone is insufficient.
• one mark for naming the feature seen in the cell rather than restating the data given
• one mark for saying there are many of the organelle, or that the membrane is folded, where number or surface area is the adaptation
• one mark for the process the feature carries out, such as mitochondria releasing ATP in aerobic respiration
• one mark for linking that process to the overall function of the cell, such as the co-transport of glucose into the cell from the lumen
• one mark for a second, genuinely different adaptation where the question asks for more than one

• 💡Answer structure and function in matched pairs; a list of organelle names without functions rarely provides a complete explanation.
• 💡Read exclusions carefully, as in 'other than the Golgi apparatus, name one structure', because an excluded answer cannot be credited.
• 💡When identifying chloroplasts, state their occurrence in plants and algae to match the specification clause.
• 💡When describing cellular adaptations, explicitly link the structural feature to the process it performs and its ultimate function for the cell.
• 💡If presented with an unfamiliar cell in a table or micrograph, identify which organelles are unusually abundant to deduce the cell's primary function.
• 💡Give one worked example at every level in the same answer; examiners credit the chain, not the definitions alone.
• 💡Use plant examples as readily as animal ones, since leaf and root questions are common alongside gut questions.
• 💡If asked why large organisms need organ systems, bring in surface area to volume ratio explicitly.
• 💡Count the marks and give that many separate feature-and-function pairs; three marks means three distinct adaptations.
• 💡Where transport proteins are involved, say which kind of transport they carry out, since channel, carrier and co-transport proteins function differently.
• 💡Answer 'describe and explain' in two halves for every point: what the feature is, then why it helps.
• 💡Always specify 80S ribosomes when discussing eukaryotic cytoplasm, and 70S ribosomes when describing the interior of mitochondria or chloroplasts.
• 💡When asked to describe organelle adaptations, explicitly link structure to function: do not just state 'folded cristae', specify 'folded cristae provide a large surface area for enzymes and electron transport chain proteins involved in ATP synthesis'.
• 💡Ensure units match before calculating magnification: always convert measurements into the units required by the question (e.g. mm to um) before dividing by magnification.

• naming the nucleolus or the cell-surface membrane as evidence that a cell is eukaryotic; prokaryotes also have a cell-surface membrane, and the nucleolus is a region inside the nucleus rather than a membrane-bound organelle
• writing 'mitochondria for ATP' without specifying that they release ATP, or stating that they 'produce energy'
• saying the Golgi apparatus makes proteins, when it modifies, packages and transports them
• describing ribosomes as membrane-bound structures, whereas they lack a membrane
• stating that chloroplasts are found in all eukaryotic cells; correct by specifying that they occur in plants and algae
• Stating that all cells in a multicellular organism contain identical DNA; correct by noting that mature mammalian red blood cells lack a nucleus and DNA, while gametes are haploid.
• Writing that sperm cells or muscle cells 'produce energy'; correct by specifying that their abundant mitochondria release ATP during aerobic respiration.
• Claiming a red blood cell 'has no organelles' when asked about a specific missing structure; correct by explicitly naming the absent structure, such as the nucleus.
• Confusing differentiation with mutation; correct by explaining that specialisation usually involves changes in gene expression via transcription factors, not changes to the DNA base sequence.
• Calling blood, or xylem, an organ when each is a tissue.
• Defining a tissue as 'lots of cells' without saying that they are similar and share a function.
• Assigning organs to only one system, for example placing the pancreas in the endocrine system alone when it is also a digestive organ.
• Stating that an organ is made of one type of tissue.
• Mixing plant and animal examples within the same chain, so the hierarchy no longer holds together.
• listing the organelles visible in a figure without linking any of them to the function of the cell
• omitting 'many', since it is the number rather than the presence of an organelle that is being credited
• writing that mitochondria 'give energy' rather than release ATP in aerobic respiration
• giving two versions of the same adaptation, such as microvilli and folded membrane, when two different features are required
• conflating channel proteins, which only perform facilitated diffusion, with carrier proteins that can also perform active transport and co-transport
• Believing plant cells contain chloroplasts instead of mitochondria. In reality, plant cells possess both; mitochondria are vital for ATP production in darkness and non-photosynthetic tissues such as roots.
• Confusing the function of smooth endoplasmic reticulum (SER) with rough endoplasmic reticulum (RER). SER synthesises, stores, and transports lipids and carbohydrates, whereas RER processes proteins synthesised by surface-bound 80S ribosomes.
• Assuming that eukaryotic cell walls act as selective barriers to solutes. Cell walls are fully permeable to small dissolved substances; the underlying phospholipid bilayer of the cell surface membrane regulates permeability.

1. 1Day 1-2: Review and sketch the ultrastructure of animal, plant, fungal, and algal cells, labelling all organelles and noting structural differences.
2. 2Day 3-4: Construct a detailed flow diagram tracking the pathway of protein synthesis, post-translational modification, and secretion through the endomembrane system.
3. 3Day 5: Practice magnification and scale calculations using the formula A = I / M, converting accurately between mm, um, and nm.
4. 4Day 6-7: Complete past exam questions focused on cell fractionation, organelle identification from electron micrographs, and comparison tables of eukaryotic taxa.

• 📋Organelle Identification and Function: Label diagrams or identify organelles from transmission electron micrographs and state their specific functions.
• 📋Magnification and Unit Conversion Calculations: Calculate actual size, image size, or magnification level from micrographs, requiring unit conversions.
• 📋Comparative Essay and 6-Mark Questions: Compare the ultrastructure of different eukaryotic kingdoms or explain the coordinated journey of a secreted protein.