Cell Structures and Microscopy
Eukaryotic and Prokaryotic Cells
All living organisms are made of cells. There are two fundamental cell types you need to know: eukaryotic cells and prokaryotic cells.
Eukaryotic cells include animal and plant cells. They have a nucleus that encloses their genetic material within a nuclear membrane. They also contain membrane-bound organelles such as mitochondria and ribosomes. Prokaryotic cells — such as bacteria — are much smaller and simpler. They lack a true nucleus; instead, their genetic material is a single loop of DNA floating freely in the cytoplasm. Prokaryotes may also contain small rings of extra DNA called plasmids.
Eukaryotic cells (animal and plant) have a nucleus that encloses their DNA. Prokaryotic cells (bacteria) have no true nucleus — their DNA is a single loop in the cytoplasm, and they may also contain plasmids.
Sub-cellular Structures and Their Functions
Both animal and plant cells are eukaryotic, but they contain different sub-cellular structures. Your exam requires you to explain how each structure's design relates to its function. This means you need to go beyond naming the structure — you must say why its shape, composition or position allows it to carry out its role efficiently.
Nucleus — contains the cell's genetic material (chromosomes made of DNA) and controls the cell's activities by determining which proteins are made. The nuclear membrane keeps the DNA separate from the cytoplasm, protecting the genetic code.
Cytoplasm — a jelly-like substance where most chemical reactions take place. Enzymes in the cytoplasm control these metabolic reactions. It also contains structural proteins that give the cell its shape.
Cell membrane — a thin, selectively permeable layer that controls which substances enter and leave the cell. It contains receptor molecules that allow the cell to respond to chemical signals.
Mitochondria — the site of aerobic respiration. They contain the enzymes needed for cellular respiration and transfer energy for the cell's processes. Cells with high energy demands (such as muscle cells) contain more mitochondria.
Ribosomes — tiny structures in the cytoplasm where protein synthesis occurs. They are found in both eukaryotic and prokaryotic cells, though they are smaller in prokaryotes.
Plant cells have all the structures listed above, plus three additional ones that reflect their roles in photosynthesis and structural support:
Chloroplasts — contain the green pigment chlorophyll and are the site of photosynthesis. They absorb light energy to convert carbon dioxide and water into glucose. Chloroplasts are found mainly in leaf cells and cells near the surface of stems, where light is available.
Permanent vacuole — a large, fluid-filled space containing cell sap (a solution of sugars and salts). It helps maintain the cell's rigidity by pressing the cytoplasm against the cell wall, keeping the cell turgid.
Cell wall — made of cellulose and surrounds the cell membrane. It strengthens and supports the cell, preventing it from bursting when water enters by osmosis. Unlike the cell membrane, the cell wall is fully permeable.
Plant cells have a cell wall (cellulose), chloroplasts (photosynthesis) and a permanent vacuole (cell sap) in addition to the organelles shared with animal cells. Bacterial cells have a cell wall, but no true nucleus, no mitochondria and no chloroplasts.
Microscopy — Seeing Cells
Microscopes allow us to see cells and sub-cellular structures invisible to the naked eye. You need to know about light microscopes and electron microscopes and how to calculate magnification.
Light microscopes use glass lenses and visible light. They have a useful magnification of about ×1500 and a resolution of around 200 nm. To prepare a slide, place a thin sample on a glass slide, add a drop of stain (such as iodine or methylene blue) to make colourless structures visible, then lower a coverslip at an angle to avoid air bubbles.
Practical work with light microscopes is a core assessed skill. You should be able to describe how to prepare slides, use staining techniques, and calculate magnification. Key steps:
1. Place a thin sample (e.g. onion epidermis or cheek cells) on a clean glass slide. 2. Add a drop of stain — iodine stains plant cell walls and nuclei yellow-brown, while methylene blue stains animal cell nuclei blue. 3. Gently lower a coverslip at 45° to avoid trapping air bubbles. 4. Start on the lowest magnification objective lens, focus, then switch to higher magnifications.
To calculate the total magnification: \(\text{total magnification} = \text{eyepiece lens} \times \text{objective lens}\). For example, a ×10 eyepiece with a ×40 objective gives ×400 magnification.
Electron microscopes fire a beam of electrons at the specimen. A transmission electron microscope (TEM) has much higher resolution — down to about 0.2 nm — and a magnification of up to ×2,000,000. This allowed biologists to see sub-cellular structures such as ribosomes and the internal folds of mitochondria (cristae) in fine detail, greatly advancing our understanding of how cells work.
The development of electron microscopy was a turning point in biology. Before electron microscopes, scientists could see cells and nuclei with light microscopes, but could not resolve the detailed internal structure of organelles. Electron microscopy revealed that mitochondria have folded inner membranes (cristae) that increase the surface area for respiration enzymes — explaining why mitochondria are so efficient at energy transfer.
The magnification formula is: \(\text{magnification} = \dfrac{\text{image size}}{\text{real size}}\). Rearranged: \(\text{real size} = \dfrac{\text{image size}}{\text{magnification}}\).
Example: A cell appears 30 mm long in a diagram. The magnification stated is ×1000. What is the real size of the cell?
Real size = 30 mm ÷ 1000 = 0.03 mm = 30 μm.
Always convert to the same units before calculating. 1 mm = 1000 μm; 1 μm = 1000 nm.
Higher Tier: If a mitochondrion appears 8 mm long in an electron micrograph at ×40,000 magnification, its real length = 8 mm ÷ 40,000 = 0.0002 mm = 0.2 μm = 200 nm. An electron microscope resolves detail down to about 0.2 nm, so a 200 nm object is easy for it to see. A light microscope only resolves to about 200 nm, so it could not show this detail.
The magnification formula: \(\text{magnification} = \dfrac{\text{image size}}{\text{real size}}\). Always ensure both measurements are in the same units before calculating. Electron microscopes have much higher resolution (~0.2 nm) than light microscopes (~200 nm), allowing sub-cellular detail to be resolved.