2.1 Introduction to cells

Define magnification

Magnification is the ratio between the size of an image (drawing or photograph) and the actual size of the object being viewed. It shows how many times larger the image is compared to the real specimen.

Define resolution

Resolution is the ability of a microscope or eye to distinguish two points as separate objects. Higher resolution means finer detail can be seen, and it is limited by the wavelength of light in light microscopy.

What are the main parts of a light microscope and their functions?

• Eyepiece lens: magnifies image (usually ×10)

• Objective lenses: provide different magnifications (×4, ×10, ×40)

• Stage: holds slide

• Coarse focus: brings image roughly into focus

• Fine focus: sharpens image

• Condenser lens: focuses light onto specimen

What are the typical total magnifications of a light microscope?

• Low power: ×40 (×4 objective ×10 eyepiece)

• Medium power: ×100 (×10 ×10)

• High power: ×400 (×40 ×10)

Why is a condenser lens important?

It focuses and concentrates light onto the specimen, improving image brightness and clarity for better viewing.

What limits the resolution of a light microscope?

The resolution is limited by the wavelength of visible light (400–700 nm), meaning structures smaller than about 200 nm cannot be distinguished as separate objects.

Why can’t cell membranes be seen with a light microscope?

Cell membranes are too thin (about 7–10 nm), which is far below the 200 nm resolution limit of light microscopes.

Difference between magnification and resolution

Magnification increases the size of the image, while resolution determines how clearly fine detail can be seen. A high magnification without good resolution results in a blurry image.

How do you prepare a temporary plant slide?

1. Place thin plant tissue (e.g. Elodea leaf) on a slide in water

2. Add iodine stain

3. Carefully place a coverslip to avoid air bubbles

4. Blot excess liquid

5. Observe under low power, then increase magnification

How do you prepare an animal cell slide?

1. Place sample (e.g. liver tissue) on slide

2. Add methylene blue stain

3. Add coverslip

4. Observe under low power, then increase magnification

Rules for biological drawings

• Use sharp pencil (no pen)

• Draw clear, continuous lines

• No shading or colouring

• Show correct proportions

• Label clearly

• Only draw what is seen, not what is expected

Why must biological drawings show proportions accurately?

To ensure the drawing correctly represents the actual structure of the cells, allowing valid comparison and analysis.

Differences between plant and animal cells (light microscope)

• Plant cells: have cell wall, chloroplasts, large permanent vacuole, regular shape

• Animal cells: no cell wall, no chloroplasts, small or no vacuole, irregular shape

Similarities between plant and animal cells

Both contain nucleus, cytoplasm, and cell membrane, and are similar in basic eukaryotic structure.

Approximate size difference between plant and animal cells

Plant cells are generally larger (~40 µm) than animal cells (~20 µm).

Why are cell membranes labelled in diagrams even if not visible?

Cell membranes are too thin to be seen with a light microscope, but they are included in diagrams to show the theoretical boundary of the cell.

Why is iodine used for plant cells?

Iodine stains starch and improves contrast, making structures like chloroplasts and cell walls easier to see.

Why is methylene blue used for animal cells?

It stains acidic structures such as the nucleus, increasing contrast and making cell components easier to observe.

How do you focus a microscope properly?

Start with low power, use coarse focus to find image, then switch to medium/high power and use fine focus to sharpen the image.

Why must you start with low power on a microscope?

Low power gives a wider field of view, making it easier to locate the specimen before increasing magnification.

Difference between plant and animal cells (summary exam answer)

Plant cells have a cell wall, chloroplasts, and a large vacuole giving a fixed shape, while animal cells lack these features and are more irregular in shape.