Welcome to the foundation of biology! Every living organism is built from tiny blocks called cells. To understand how complex systems like your heart, brain, or immune system work, you first need to look closely at the individual structures operating inside these microscopic factories.

What you'll learn

• How to compare the structures of animal, plant, and bacterial cells.
• How specialised cells are adapted to perform highly specific roles in reproduction and transport.
• How microscope technology has evolved to reveal the inner workings of cells.
• How to convert between different scientific units of scale and calculate magnification.

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Eukaryotic vs Prokaryotic Cells

All living organisms can be divided into two main categories based on the structure of their cells: eukaryotes and prokaryotes.

To help you visualise how these differ, look at this side-by-side comparison of animal, plant, and bacterial cells:

Animal Cells (Eukaryotic)

Animal cells are eukaryotic and contain several key sub-cellular structures (organelles) that you must be able to identify and explain:

• Nucleus: Controls the activities of the cell and contains the genetic material (DNA), which is organized into chromosomes.
• Cell membrane: A selective barrier that controls which substances enter and leave the cell (e.g., oxygen, carbon dioxide, glucose).
• Mitochondria (singular: mitochondrion): The site of aerobic respiration, where glucose and oxygen react to release energy for the cell.
• Ribosomes: Tiny structures where protein synthesis occurs.

Plant Cells (Eukaryotic)

Plant cells contain all the sub-cellular structures found in animal cells, plus three extra structures that provide support and allow them to make food:

• Cell wall: Made of cellulose, this tough, outer layer surrounds the cell membrane, providing support and keeping the cell's shape stable.
• Chloroplasts: Organelles containing a green pigment called chlorophyll, which absorbs light energy for photosynthesis.
• Permanent vacuole: A large space filled with cell sap (a dilute solution of sugar and salts) that helps keep the cell firm and rigid.

Bacterial Cells (Prokaryotic)

Bacteria are unicellular (single-celled) organisms. Because they are prokaryotes, they have a completely different architecture to plant and animal cells:

• Chromosomal DNA: Since bacteria do not have a nucleus, their genetic material is a single, circular strand of DNA that floats freely in the cytoplasm.
• Plasmid DNA: Small, loops of extra DNA that are separate from the main chromosomal DNA. Plasmids contain genes that can be passed between bacteria, such as antibiotic resistance.
• Cell wall: A protective outer barrier. Note that a bacterial cell wall is not made of cellulose, unlike a plant cell wall.
• Ribosomes: Bacterial cells contain ribosomes for protein synthesis, but they are smaller than eukaryotic ribosomes.
• Flagella (singular: flagellum): Tail-like structures that rotate to allow the bacterium to move through liquids.

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Specialised Cells

As multicellular organisms grow, their cells undergo differentiation. This is the process where cells change shape and structure to become specialised to perform specific jobs. You need to know the specific adaptations of three key cell types:

1. Sperm Cell (Male Gamete)

Sperm cells are designed to carry the male DNA to the female egg cell for fertilisation.

• Haploid nucleus: Contains only half the normal number of chromosomes (23 in humans instead of 46) so that when fertilisation occurs, the resulting embryo has the correct diploid number (46).
• Acrosome: A specialized sac at the very tip of the head containing digestive enzymes. These break down the jelly coat of the egg cell so the sperm can penetrate it.
• Mitochondria: Packed tightly into the middle piece of the sperm, these release the large amount of energy needed for the tail to swim.
• Tail: A long flagellum that lashes back and forth to propel the sperm toward the egg.

2. Egg Cell (Female Gamete)

Egg cells are adapted to nourish a developing embryo and ensure only one sperm can fertilise them.

• Haploid nucleus: Contains 23 chromosomes to combine with the sperm's genetic material.
• Nutrients in cytoplasm: The cytoplasm is rich in nutrients to support the early growth and division of the embryo before it implants in the uterus.
• Cell membrane changes: Immediately after a single sperm penetrates, the egg's cell membrane chemically changes its structure to become impenetrable, preventing any more sperm from entering.

3. Ciliated Epithelial Cells

These cells line surfaces such as your airways (trachea) and the oviducts.

• Cilia: Tiny, hair-like structures on the top surface of the cell.
• Coordinated movement: The cilia beat rhythmically in unison to sweep substances along the surface. In the airways, they sweep mucus (which traps dust and bacteria) up and away from the lungs to the throat to be swallowed.

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Scale, Standard Form, and Units

Cells are extremely small, meaning scientists have to work with tiny quantities. To master microscopy calculations, you must be comfortable with the units of measurement and how to convert between them.

Each unit in biology is 1,000 times smaller than the previous one:

• Millimetre (mm) = $10^{-3}$ metres
• Micrometre ($\mu$m) = $10^{-6}$ metres
• Nanometre (nm) = $10^{-9}$ metres
• Picometre (pm) = $10^{-12}$ metres

To convert from a larger unit to a smaller unit, you multiply by 1,000. To go the other way, you divide by 1,000.

Using Standard Form (Higher Tier Only)

In the exam, you may be asked to calculate or write cell sizes using standard form. Standard form is written as:

$$A\times 10^n$$

where $A$ is a number between 1 and 10, and $n$ is an integer (positive or negative).

For example:

• A human red blood cell is roughly $0.000008m$ in diameter. In standard form, this is $8\times 10^{-6}m$.
• A bacterial cell is roughly $0.0000015m$ long. In standard form, this is $1.5\times 10^{-6}m$ (or $1.5\mu m$).

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Microscopy Calculations

To study cells, we must magnify them. You must know the formula linking Image size, Actual size, and Magnification:

$$Image\; size=Actual\; size\times Magnification$$

Or, using the triangle acronym I = AM:

$$Magnification=\frac{Image\; size}{Actual\; size}$$ $$Actual\; size=\frac{Image\; size}{Magnification}$$

Let's look at how to approach this systematically in a calculation.

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Evolution of Microscopy

The discovery of sub-cellular structures is closely linked to developments in technology.

Light Microscopes

Developed in the late 16th century, these use a beam of light passing through lenses to magnify an image.

• They allow us to see whole cells and large structures inside them, like the nucleus and cell membrane.
• However, their magnification and resolution are limited because light has a relatively long wavelength.

Electron Microscopes

Developed in the 1930s, these use a focused beam of electrons instead of light.

• Magnification: Electron microscopes can magnify up to $\times 1,000,000$, compared to just $\times 1,500$ for light microscopes.
• Resolution: Electrons have a much smaller wavelength than light, providing a vastly higher resolution.

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Core Practical: Investigating Biological Specimens

You must be able to describe how to prepare slides and use a light microscope safely to observe specimens (like onion epidermis or cheek cells).

1. Slide Preparation (Onion Cells)

1. Use a pipette to add a drop of water to the centre of a clean slide.
2. Peel off a thin, transparent layer of onion epidermis using tweezers.
3. Place the onion skin flat on the water drop.
4. Add a drop of iodine stain. Stains are used to add colour to cell structures, making them visible under the microscope.
5. Carefully lower a coverslip onto the slide at an angle using a mounted needle. This prevents air bubbles from trapping under the slip, which would obscure your view.

2. Using the Light Microscope

1. Clip your prepared slide onto the stage.
2. Select the lowest-power objective lens (usually $\times 4$).
3. Turn the coarse adjustment knob to move the stage up to just below the lens. Look from the side while doing this so you do not smash the slide into the lens.
4. Look through the eyepiece lens and turn the coarse adjustment knob to move the stage downwards until the cells come into focus.
5. Turn the fine adjustment knob to adjust the focus until you get a clear, sharp image.
6. If you need greater magnification, switch to a higher-power objective lens and refocus using only the fine adjustment knob.

3. Producing Scientific Drawings

When drawing what you observe under the microscope, follow these strict rules:

• Use a sharp HB pencil and draw clean, unbroken, single lines. No shading or sketching.
• Draw structures in the correct proportion.
• Label structures using straight, horizontal ruler lines that touch the target structure precisely (no arrowheads).
• Always include the magnification and a scale bar.

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