describe and interpret photomicrographs, electron micrographs and drawings of typical plant and animal cells

Objective

Describe and interpret photomicrographs, electron micrographs and schematic drawings of typical plant, animal and prokaryotic cells, and relate the observed structures to their functions and to cellular energy use (ATP).

1. The Microscope in Cell Studies

1.1 Preparing a Temporary (wet‑mount) Slide

1. Place a clean glass slide on a flat surface.
2. Put a drop (≈ 0.5 mL) of distilled water or an appropriate mounting medium in the centre.
3. Transfer a small fragment of the specimen (e.g. onion epidermis, cheek cells) onto the drop using fine forceps or a needle.
4. Lower a cover‑slip at an angle to avoid air bubbles; let it settle gently.
5. If the specimen is thick, remove excess liquid with filter paper.
6. Label the slide with specimen name, date and the magnification you intend to use.

1.2 Drawing Cells from the Microscope

• Start at low power (×4 or ×10) to locate an area of interest.
• Switch to a higher power (×40, ×100) and note the ocular graticule – usually 0.1 mm per division.
• Calculate total magnification:
  

  Total magnification = ocular magnification × objective magnification.
  

  Example: Ocular = 10×, objective = 40× → total = 400×.

• Derive the size represented by one graticule division:
  

  Size per division = 0.1 mm ÷ total magnification.
  

  Using the example above: 0.1 mm ÷ 400 = 0.25 µm per division.

• Sketch the outline first, then add organelles, indicating relative size and position.
• Label each structure clearly and add a scale bar based on the calculation above.

1.3 Worked Example – Converting a Measured Length

Scenario (light microscope): On a photomicrograph taken at 400× total magnification, a nucleus measures 12 graticule divisions.

1. Size per division = 0.1 mm ÷ 400 = 0.25 µm.
2. Actual size = 12 divisions × 0.25 µm = 3 µm.

Scenario (TEM): A TEM image is printed with a scale bar of 200 nm. You measure the length of a mitochondrial crista from the image and obtain 35 mm on the printed page. The printed scale bar (200 nm) measures 10 mm on the page.

1. Determine the image scale: 200 nm ÷ 10 mm = 20 nm mm⁻¹.
2. Actual length = 35 mm × 20 nm mm⁻¹ = 700 nm = 0.7 µm.

1.4 Key Definitions

2. Types of Microscopy

3. Typical Plant Cell – Main Features

• Cell wall – rigid cellulose layer; gives shape and support.
• Plasma membrane – lies just inside the wall; controls movement of substances.
• Large central vacuole – stores water, ions and metabolites; maintains turgor pressure.
• Chloroplasts – contain thylakoid stacks (grana) and stroma; site of photosynthesis.
• Nucleus – usually peripheral because of the vacuole; contains nucleolus.
• Mitochondria, rough & smooth ER, Golgi apparatus, ribosomes – as in animal cells.
• Cytoskeleton – actin filaments and microtubules in the peripheral cytoplasm.
• Plasmodesmata – microscopic channels linking adjacent plant cells.

4. Typical Animal Cell – Main Features

• Plasma membrane – flexible phospholipid bilayer; no rigid wall.
• Nucleus – central, with nucleolus and double membrane (nuclear envelope).
• Centrosome with a pair of centrioles – organiser of the mitotic spindle.
• Mitochondria – numerous; site of aerobic respiration (ATP production).
• Endoplasmic reticulum (rough & smooth) – protein synthesis and lipid metabolism.
• Golgi apparatus – modifies, sorts and packages proteins.
• Lysosomes & peroxisomes – digestive and oxidative functions.
• Ribosomes – free in cytoplasm or bound to rough ER.
• Cytoskeleton – actin filaments, intermediate filaments, microtubules; gives shape and aids movement.
• Cell junctions – tight junctions, desmosomes, gap junctions (visible in high‑power photomicrographs).

5. Prokaryotic Cells and Viruses (Syllabus Requirement 1.2)

5.1 Typical Prokaryotic Cell (Bacterium)

• Size: 0.5–5 µm (generally smaller than eukaryotic cells).
• Cell envelope – plasma membrane plus a rigid peptidoglycan wall (Gram‑positive) or an outer membrane (Gram‑negative).
• Nucleoid region – irregularly shaped circular DNA, not enclosed by a membrane.
• Ribosomes – 70 S (smaller than the 80 S eukaryotic ribosomes).
• Often a single, simple flagellum for motility.
• No membrane‑bound organelles (no nucleus, mitochondria, chloroplasts, etc.).

5.2 Viruses

• Non‑cellular infectious particles.
• Core: nucleic acid (DNA or RNA) surrounded by a protein capsid; some have a lipid envelope derived from the host cell.
• Obligate intracellular parasites – cannot carry out metabolism or reproduce outside a host cell.
• Size range: 20–300 nm (visible only with electron microscopy).

6. Interpreting Photomicrographs (Light Microscopy)

When analysing a photomicrograph, ask the following questions and compare the observations with the table.

7. Interpreting Electron Micrographs

8. Comparative Summary of Cell Features

9. Energy (ATP) Link

All active processes shown in the diagrams – active transport across the plasma membrane, synthesis of macromolecules in the ER/Golgi, movement of cytoskeletal elements, and the operation of flagella – require ATP. In eukaryotes the majority of ATP is generated by mitochondria; in prokaryotes ATP is produced mainly by substrate‑level phosphorylation in the cytoplasm (and, in aerobic bacteria, by oxidative phosphorylation across the plasma membrane).

10. Practical Tips for Exam Questions

1. Identify the microscopy technique from clues such as “electron‑dense” (TEM) or “surface texture” (SEM).
2. Spot hallmark organelles: chloroplasts → plant; centrioles → animal; cell wall → plant; peptidoglycan layer → prokaryote; capsid symmetry → virus.
3. Use size and location to differentiate: large central vacuole = plant; numerous mitochondria = animal; absence of nucleus = prokaryote.
4. Check labels in drawings against typical positions (e.g., nucleus near centre of animal cell, peripheral in many plant cells, nucleoid central in bacteria).
5. Apply the SA:V concept to explain why cells are small and why large plant cells develop a large vacuole to reduce cytoplasmic volume.
6. Remember ATP dependence for any process that involves transport, biosynthesis or movement; cite mitochondria (or bacterial cytoplasm) as the source.
7. Calculate magnification and scale bars when required: total = ocular × objective; size per division = 0.1 mm ÷ total magnification.
8. Convert measured lengths using the worked examples in section 1.3.