Cell structure — OCR A-Level Biology

Microscopy lets you observe and investigate cell structure across many eukaryotic organisms. Light microscopes use visible light and give lower magnification and resolution but allow living specimens and colour; electron microscopes use a beam of electrons, giving far higher resolution and revealing organelles such as mitochondria and ribosomes, but require dead, prepared specimens. To investigate, you select a specimen, prepare it (for example a thin section or a stained slide), choose the correct magnification, and calculate actual size using magnification = image size ÷ actual size. Comparing cells from different eukaryotes, such as plant, animal and fungal cells, shows shared features (nucleus, membrane-bound organelles) and differences (cell walls, chloroplasts, vacuoles).

This specification point covers both making a temporary slide and using a light microscope to examine it. Preparation means selecting suitable material, placing it on a clean slide, adding a mounting medium such as water, lowering a coverslip carefully to avoid air bubbles, and where needed staining or squashing the specimen. Examination means setting the microscope up correctly: placing the slide on the stage and securing it with clips, selecting the lowest-power objective first, adjusting the coarse focus to find the image, then switching to a higher-power objective and using the fine focus. Learners should also know how to calculate total magnification by multiplying eyepiece magnification by objective magnification, and how to measure specimen size using an eyepiece graticule calibrated with a stage micrometer.

This point requires learners to understand why stains are used in light microscopy and to know named examples. Staining increases contrast because many biological specimens are transparent and have little natural colour, so without a stain cell structures are hard to distinguish. Different stains bind to different components: iodine solution stains starch granules blue-black and is used to make plant cell contents visible; methylene blue stains nuclei and other acidic components blue in animal cells; eosin can be used as a counterstain. Learners should be able to select the appropriate stain for a named structure and explain the resulting colour change. MCQ items may ask why a stain is needed, which stain is suitable for a particular specimen, or what colour a structure appears after staining.

This criterion asks you to show that you can turn what a light microscope reveals into accurate biological communication. Practise producing two complementary representations. First, a whole-cell drawing: choose a cell from a prepared slide or micrograph, draw it large with a sharp pencil, keep line quality clean and continuous, and preserve the true proportions and relative sizes of the structures you can actually see. Second, an annotated diagram of cells in a section of tissue: show several neighbouring cells sharing walls or membranes, indicate the plane of section, and label features such as the nucleus, cytoplasm, cell wall, vacuole or chloroplasts where these are genuinely visible. Annotations must be concise, correctly spelled and linked by straight label lines that touch the structure.

You must be able to use and rearrange the magnification relationship: magnification = image size ÷ actual size. Learn to identify which two quantities are given and which is required, then rearrange before substituting. If magnification and actual size are known, image size = magnification × actual size. If image size and magnification are known, actual size = image size ÷ magnification. Keep units consistent: convert micrometres to millimetres (1 mm = 1000 µm) or to metres (1 m = 10⁶ µm) before calculating, and state the unit in the answer. For example, a structure 20 µm across drawn at ×500 has an image size of 500 × 20 µm = 10 000 µm = 10 mm. Worked in reverse, an image of 8 mm at ×400 represents an actual size of 8 mm ÷ 400 = 0.02 mm = 20 µm. Always sanity-check that the answer is plausible for a cell or organelle.

Magnification and resolution are separate properties of a microscope image. Magnification is how many times larger the image is than the object, calculated as image size divided by actual size; a magnification of ×400 means the image is 400 times the object's linear dimension. Resolution is the smallest distance at which two points can still be distinguished as separate, so it sets the detail visible regardless of how much you enlarge. Enlarging a low-resolution image adds no new detail: it simply makes blur larger. An electron microscope has far greater resolution than a light microscope because electrons have a much shorter wavelength than visible light, so it reveals organelles such as mitochondria and ribosomes in fine detail. Learn to state each definition precisely and to explain why high magnification without high resolution is unhelpful.

Ultrastructure means the fine internal detail of a eukaryotic cell as revealed by an electron microscope. You must know each component and its function. The nucleus contains chromatin and a nucleolus and controls the cell via transcription of DNA. Mitochondria have a double membrane with folded cristae and carry out aerobic respiration, releasing ATP. Rough endoplasmic reticulum, studded with ribosomes, synthesises and transports proteins; smooth ER synthesises lipids. The Golgi apparatus modifies proteins and packages them into vesicles. Ribosomes are the site of protein synthesis. Lysosomes contain hydrolytic enzymes that digest material. The cell surface membrane controls exchange, and the cytoskeleton maintains shape and moves components. Chloroplasts in plant cells carry out photosynthesis, and the cellulose cell wall gives support.

Photomicrographs are images taken through a microscope, and interpreting them is a core practical skill. You must recognise cellular components across a range of eukaryotic cells, not just one example. Learn to identify the nucleus (often large, with a darker nucleolus), mitochondria (oval with folded internal membranes), chloroplasts (green, stacked thylakoids), the cell surface membrane, a large permanent vacuole in plant cells, the cellulose cell wall, rough and smooth endoplasmic reticulum, Golgi apparatus and ribosomes. Use scale and magnification to judge real size, and compare animal, plant and fungal cells. Distinguish structures by shape, position, membrane pattern and staining, and note that resolution limits what can be seen.

Protein secretion is a coordinated pathway. A gene in the nucleus is transcribed to mRNA, which leaves through a nuclear pore and binds to a ribosome. On the rough endoplasmic reticulum, the ribosome translates mRNA into a polypeptide, which enters the ER lumen and is folded. The ER packages the protein into vesicles that fuse with the Golgi apparatus, where it is modified, for example by adding carbohydrate. The Golgi packages the protein into secretory vesicles that move to the cell surface membrane and fuse with it, releasing the protein by exocytosis. Mitochondria supply the ATP for these transport and fusion events. Learn the order and the role of each organelle.

The cytoskeleton is a network of protein fibres running through the cytoplasm of eukaryotic cells. It is not a single structure but several systems working together. Microfilaments (actin) are thin and allow movement and shape change. Microtubules are hollow tubes that form tracks for organelles and make up spindle fibres and cilia/flagella. Intermediate filaments give mechanical strength. Its importance lies in four linked roles: maintaining cell shape and mechanical support; holding organelles in position within the cytoplasm; enabling movement of materials inside the cell, for example vesicles travelling along microtubules; and producing whole-cell movement or movement of cell extensions such as cilia. It also moves chromosomes during cell division by forming the spindle.

Prokaryotic and eukaryotic cells share some features but differ in structure and ultrastructure. Both have a cell surface membrane, cytoplasm, ribosomes and genetic material (DNA). Differences: eukaryotic cells have a nucleus enclosing DNA, membrane-bound organelles such as mitochondria and endoplasmic reticulum, and are generally larger. Prokaryotic cells have no nucleus; DNA is a circular loop in the cytoplasm plus small circular plasmids. They lack membrane-bound organelles but have a cell wall (not cellulose), a capsule sometimes, and may have flagella or pili. Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S). A strong answer compares like with like, stating a feature and how it differs, rather than listing features of one cell only.