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1. Cell Structure

Syllabus
9700–2028–2029
Section
1
Level
AS

1.1 The Microscope in Cell Studies

Syllabus
9700–2028–2029
Topic
1.1
Level
AS

A temporary slide keeps the specimen thin, visible and still

A temporary preparation lets fresh cellular material be viewed with a light microscope without making a permanent slide.

  1. Put a liquid sample on a clean slide, or cut a solid specimen thin enough for light to pass through.
  2. Add a suitable stain when contrast is needed.
  3. Lower the coverslip from one edge to reduce trapped air bubbles.
  4. Remove excess liquid and view the preparation promptly so it stays in place and does not dry out.

The coverslip protects the objective lens and limits drying. Iodine can increase contrast in plant tissue, while methylene blue can make cheek-cell structures easier to distinguish.

A stain improves contrast but does not create detail that the microscope cannot resolve. The preparation must remain thin and representative of the material being investigated.

A biological drawing records what the slide actually shows

A biological drawing is a large, accurate line record of the structures that can actually be seen in a microscope slide or photomicrograph—not a reconstruction of the textbook cell.

  1. Inspect the image first and decide which structures are genuinely visible.
  2. Set the drawing size and relative proportions from the image; include a title and magnification or scale bar when supplied.
  3. Draw the observed outline and internal features with clear, single lines and no shading.
  4. Label only visible structures with straight, separate label lines that point precisely to the feature.

A cell drawing records individual cells and their internal features, usually at higher magnification. A plan drawing records the distribution of tissues at lower magnification, so it should not invent cell-level detail.

Neatness cannot justify an unseen organelle. If a feature is not resolved in the image, leaving it out is more accurate than drawing the expected textbook version.

Magnification converts an object size into an image size

Magnification is the ratio of image size to actual size: it tells how many times larger the image is than the specimen.

M=IAsoI=M×AextandA=IMM=\frac{I}{A}\quad\text{so}\quad I=M\times A\quad ext{and}\quad A=\frac{I}{M}

  1. Identify the unknown quantity: magnification (M), image size (I) or actual size (A).
  2. Rearrange the relationship if needed.
  3. Convert image and actual sizes into the same unit before dividing or multiplying.
  4. Substitute, calculate, then report the size with its unit and a sensible precision.
  5. Sense-check: a magnified image should be larger than the actual specimen.

A 1 µm bacterium viewed at ×50 000 forms an image equivalent to 50 000 µm, or 50 mm. The unit conversion is part of the calculation, not an optional presentation step.

Magnification has no unit; image size and actual size do. Do not compare or combine lengths until their units match, and do not treat higher magnification as proof of higher resolution.

Calibration gives eyepiece divisions a real length

An eyepiece graticule is an arbitrary scale in the microscope view. A stage micrometer calibrates how much real distance one graticule division represents at the current magnification.

\text{calibration factor}=\frac{\text{real stage distance}}{\text{graticule divisions}},\qquad \text{specimen size}=\text{specimen divisions}\times\text{calibration factor}.

  1. Superimpose the graticule and stage-micrometer scales.
  2. Choose a matching interval and record its real stage-micrometer distance.
  3. Divide that real distance by the number of graticule divisions.
  4. Count specimen divisions and multiply by the calibration factor.
  5. Recalibrate whenever the objective lens changes.

1\text{ mm}=1000\text{ µm},\qquad 1\text{ µm}=1000\text{ nm}.

If 20 graticule divisions coincide with 0.20 mm on the stage micrometer, one division represents 0.0100.010 mm =10=10 µm. A plant cell spanning 4 divisions is therefore 40 µm at that objective setting.

The eyepiece scale is not automatically in micrometres. Changing the objective changes the real length represented by each division, so the previous calibration cannot be reused.

Resolution is the detail limit; magnification is only enlargement

Resolution is the smallest separation at which two points can still be distinguished as separate. Magnification makes an image larger; resolution determines whether that enlargement reveals new detail.

Feature Magnification Resolution
Meaning Image size ÷ actual size Smallest separation that can still be distinguished
What increasing it does Enlarges the image Improves the ability to separate close points
Limiting idea Can enlarge existing blur Depends on the instrument and wavelength; enlargement alone cannot recover lost detail
  • Light microscope: visible light has a longer wavelength, so closely spaced structures have a coarser resolution limit.
  • Electron microscope: electrons have a much shorter effective wavelength, so finer ultrastructure can be resolved.

Cristae in a mitochondrion may be present in an image but appear merged under a light microscope. Higher magnification alone would enlarge the blur; higher resolution is what separates the membranes.

A larger image is not automatically a more informative image. State magnification and resolution separately when explaining why an electron microscope is needed.

1.2 Cells as Basic Units of Living Organisms

Syllabus
9700–2028–2029
Topic
1.2
Level
AS

Eukaryotic organelles divide work between specialised compartments

Eukaryotic cells divide their cytoplasm into membrane-bound organelles. Each organelle has a structure that supports a particular job, while ribosomes carry out protein synthesis but are not membrane-bound.

  • Cell surface membrane: partially controls exchange between the cell and its surroundings.
  • Nucleus, nuclear envelope and nucleolus: the nucleus contains chromosomes; pores in the envelope regulate traffic; the nucleolus makes ribosomal components.
  • Rough and smooth endoplasmic reticulum: rough ER has ribosomes and processes proteins; smooth ER lacks ribosomes and makes lipids.
  • Golgi body and vesicles: the Golgi modifies and packages cell products; vesicles transport or store them.
  • Mitochondria: folded inner membranes and matrix enzymes support aerobic respiration; mitochondria also contain small circular DNA and 70S ribosomes.
  • Ribosomes: 80S ribosomes occur in the eukaryotic cytoplasm; 70S ribosomes occur in mitochondria and chloroplasts; both are sites of translation.
  • Lysosomes: hydrolytic enzymes digest worn-out structures, engulfed material and cell debris.
  • Centrioles and microtubules: organise microtubules and support cell shape, movement and chromosome movement during division.
  • Cilia: move substances across the cell surface.
  • Microvilli: increase cell-surface area for absorption.
  • Cell wall: provides support outside the cell-surface membrane; plant walls contain cellulose.
  • Chloroplasts: thylakoid membranes and grana absorb light for photosynthesis; chloroplasts contain small circular DNA and ribosomes.
  • Plasmodesmata: cytoplasmic bridges that allow transfer between neighbouring plant cells.
  • Large permanent vacuole and tonoplast: the vacuole stores cell sap and supports the plant cell; the tonoplast is its partially permeable membrane.

A structure–function link is not the same as an exclusive identity: not every eukaryotic cell contains every organelle, and centrioles are not found in flowering plants and fungi. Do not describe a ribosome as membrane-bound or treat a cell wall as the membrane controlling exchange.

Read a cell image from visible evidence, not from a memorised diagram

Interpreting a plant or animal cell image means using visible evidence to identify plausible structures. A photomicrograph or electron micrograph records a specimen, while a drawing selects and simplifies what is shown.

  1. Identify the source: photomicrograph, electron micrograph or drawing, and note any scale bar or magnification.
  2. Find the cell boundary and describe observations such as relative size, shape, position, contrast and repeated patterns.
  3. Match several clues to a structure: for example, a large dark region may be a nucleus, a folded internal membrane may be a mitochondrion, and a large clear compartment in a plant cell may be a vacuole.
  4. Use the whole pattern to distinguish a plant cell from an animal cell, including a wall, chloroplasts or a large vacuole when visible.
  5. State the identification as an inference from the evidence; do not add structures that are hidden, unresolved or absent from the drawing.

Several polygonal cells with a regular outer boundary and large clear internal spaces support the inference of plant cells with cell walls and vacuoles. The conclusion is stronger because multiple features agree, not because one shape was memorised.

A mitochondrion is not always cylindrical: section angle and specimen condition can change its appearance. Image resolution, staining, contrast and the drawing convention can hide structures, so distinguish what is visible from what is inferred.

Plant and animal cells share a core plan but specialise differently

Plant and animal cells are both eukaryotic, so they share a core plan of membrane, cytoplasm, nucleus, mitochondria, endoplasmic reticulum, Golgi bodies, vesicles or lysosomes, ribosomes and microtubules. Their differences reflect how each cell gains support, materials and energy.

Feature Plant cells Animal cells
Cell surface membrane and cytoplasm Present; membrane controls exchange Present; membrane controls exchange
Nucleus, mitochondria, ER, Golgi, vesicles/lysosomes, ribosomes and microtubules Shared eukaryotic structures Shared eukaryotic structures
Cellulose cell wall Present; provides rigid support outside the membrane Absent
Large permanent vacuole Typical; stores cell sap and supports the cell Not typical; animal vacuoles, when present, are small and temporary
Chloroplasts Present in photosynthetic cells; absorb light for photosynthesis Absent
Plasmodesmata Connect neighbouring plant-cell cytoplasm Absent
Centrioles and microvilli Not typical of flowering-plant cells Typical comparison features; centrioles organise division machinery and microvilli increase absorptive surface area

The wall and vacuole help a plant cell keep its shape, chloroplasts capture light energy, and plasmodesmata permit transfer between neighbouring plant cells. Animal cells rely more on a flexible surface membrane and external support, while specialised animal cells may use centrioles or microvilli for particular jobs.

Use the table as a comparison of typical structures, not as a rule for every specialised cell: a leaf cell may contain chloroplasts, while a mature red blood cell lacks a nucleus. Do not infer cell type from one feature or from shape alone.

ATP transfers usable energy immediately where a cell needs it

ATP is the immediate energy-transfer molecule used by cells. When ATP is hydrolysed to ADP and inorganic phosphate (Pi), energy is released and can be coupled to an energy-requiring process.

Respiration continually supplies energy to rebuild ATP from ADP + Pi. ATP then carries that immediately usable energy to cellular machinery; hydrolysis releases it at the site of work, and the ADP and Pi can be recycled into ATP. This links energy release in respiration to work elsewhere in the cell without treating glucose as the direct fuel for each process.

ATP from respiration is used for energy-requiring processes such as:

  • building larger molecules in anabolic reactions;
  • moving substances across membranes by active transport or within the cell;
  • muscle contraction and nerve impulse transmission in animals.

Glucose is broken down in respiration to help make ATP, but ATP—not glucose—is the immediate energy source used directly by cellular processes. ATP is a short-term transfer molecule, not the cell’s main long-term energy store.

A prokaryote keeps its genome compact without a nucleus

A prokaryote is a unicellular cell whose genetic material is not enclosed in a nucleus and whose cytoplasm has no double-membrane organelles.

The syllabus feature hierarchy is:

  • Size: typically 1–5 µm in diameter.
  • Cell wall: made of peptidoglycan.
  • DNA: a circular DNA molecule lies in the cytoplasm rather than inside a nucleus.
  • Ribosomes: 70S ribosomes are present for protein synthesis.
  • Organelles: double-membrane organelles such as a nucleus or mitochondrion are absent.

This compact organisation distinguishes a bacterial cell from a eukaryotic cell: the DNA is not separated into a nuclear compartment, and respiration or other processes are not housed in double-membrane organelles. Size supports recognition, but the structural features provide the stronger identification.

No nucleus does not mean no DNA, and a cell wall does not by itself prove that a cell is prokaryotic. Do not add a capsule, flagellum or plasmid to every bacterium: those features are not universal requirements of this objective.

The prokaryote–eukaryote contrast is about organisation, not just size

Prokaryotes and eukaryotes both have a cell-surface membrane, cytoplasm, DNA and ribosomes. The key structural contrast is that eukaryotes organise DNA and many cell processes inside membrane-bound organelles, whereas prokaryotes do not.

Structural feature Prokaryotes Eukaryotes
Cell organisation Usually unicellular; no membrane-bound organelles May be unicellular or multicellular; membrane-bound organelles present
Genetic material Circular DNA in the cytoplasm, not enclosed by a nucleus Linear chromosomes inside a nucleus
Ribosomes 70S 80S in the cytoplasm; 70S in mitochondria and chloroplasts
Typical size Usually about 1–5 µm in diameter Often larger; plant cells may be about 10–100 µm across
Cell wall, when present Peptidoglycan Cellulose in plants or chitin in fungi

The eukaryotic nucleus and other compartments keep different reactions in controlled local conditions. A prokaryote’s simpler organisation leaves its circular DNA in the cytoplasm and has no double-membrane organelles. These are structural patterns to compare together, not a single size test.

Both cell types contain DNA, ribosomes and a membrane. A large bacterium can still be prokaryotic, and a unicellular yeast cell can still be eukaryotic; identify the cell type from DNA location, ribosome type and organelle organisation rather than apparent size alone.

All viruses have a nucleic-acid core and protein capsid

A virus is a non-cellular structure, not a prokaryotic or eukaryotic cell. Every virus has a nucleic-acid core containing either DNA or RNA, surrounded by a protein coat called a capsid.

Viral component Present in which viruses? Material
nucleic-acid core all viruses DNA or RNA
capsid all viruses protein
outer envelope some viruses only phospholipids

The core carries genetic information and the capsid surrounds it. When an outer envelope is present, it lies outside the capsid. Because a virus is non-cellular, it does not have the cellular organisation used to classify bacteria, plant cells or animal cells.

A virus contains either DNA or RNA, not both as its genetic core, and an envelope is optional rather than universal. ‘Non-cellular’ is the required structural boundary; it does not make a virus a type of prokaryotic cell.