AQA GCSE Combined Science: Trilogy

Eukaryotic and prokaryotic cells

Plant and animal cells are eukaryotic: their genetic material is enclosed in a nucleus, along with a cell membrane and cytoplasm. Bacteria are prokaryotic. A bacterial cell is much smaller and has no nucleus; its DNA is a single loop lying free in the cytoplasm, and it may also carry one or more small rings of DNA called plasmids. Outside the membrane it has a cell wall.

Sub-cellular structures and what they do

Each part of a cell is there because the cell needs a job doing, and the structure fits the job.

• The nucleus holds the chromosomes, so it carries the instructions that control what the cell does.
• The cytoplasm is a watery gel in which most chemical reactions of the cell take place, and it contains the enzymes that control them.
• The cell membrane controls which substances go into and out of the cell.
• Mitochondria are where aerobic respiration takes place, releasing the energy the cell uses. Cells that use a lot of energy, such as muscle cells, have many.
• Ribosomes are where proteins are made, by joining amino acids in the order the genes specify.
• Chloroplasts contain the green pigment chlorophyll, which absorbs light for photosynthesis, so they are found in the parts of plants exposed to light.
• The permanent vacuole is filled with cell sap, a solution of sugars and salts, and its pressure helps keep the cell firm.
• The cell wall is made of cellulose in plants and algae. It is a strong, rigid layer that supports the cell and stops it bursting when water enters.

Specialised cells

A specialised cell has a structure suited to one function. The examples to know are these.

Cell differentiation

The fertilised egg is a single cell, and every specialised cell in the body has developed from it. Differentiation is the process by which a cell changes and gains the structures it needs for its particular job, and once it has done so it is a specialised cell. It matters because it allows an organism to be built from many different kinds of cell working together, rather than from one general kind that does everything poorly.

In animals, most cells differentiate at an early stage of development. Once differentiated, a cell cannot easily become another type, so in a mature animal cell division is mostly for repair and replacement, such as healing a cut or replacing worn skin cells. Many plant cells keep the ability to differentiate throughout life, which is why a cutting can grow roots.

Microscopes

The light microscope was the first tool to show cells, and it is still widely used, but it can only show detail down to a certain size. Later, the electron microscope was developed. It uses a beam of electrons in place of light and has two advantages over the light microscope.

• Magnification is how many times bigger the image is than the object. An electron microscope can magnify much more.
• Resolving power, or resolution, is the ability to distinguish two points that are close together as separate. Electron microscopes resolve far finer detail.

Because of this, electron microscopes have shown biologists sub-cellular structures too small to see with a light microscope. The internal structure of mitochondria and chloroplasts, and the detail of ribosomes and plasmids, were learned in this way, and it has led to a much better understanding of how cells work.

Magnification calculations

Magnification, image size and real size are linked by one equation.

The image size and the real size must be in the same units before you divide. Rearranged, the real size is the image size divided by the magnification.

A drawing of a cell is 45 mm long and the cell is 0.15 mm long in real life.

A micrograph of a plant cell has a magnification of $\times 400$ and the cell measures 20 mm in the picture. Its real length is

Very small sizes are usually given in standard form or with prefixes. There are 1000 micrometres ($\mu$m) in a millimetre and 1000 nanometres (nm) in a micrometre, so $1\ \mu\text{m} = 1 \times 10^{-6}\ \text{m}$ and $1\ \text{nm} = 1 \times 10^{-9}\ \text{m}$. A typical animal cell is about $1 \times 10^{-5}$ m across, which is 10 $\mu$m, while a bacterial cell is often 1 to 2 $\mu$m long, so it is roughly a tenth of the size.

Required practical: observing cells with a light microscope

The aim is to look at plant and animal cells, draw them and include a magnification scale. Onion epidermis is a common plant specimen and cheek cells a common animal one.

1. Place a thin piece of the specimen on a clean slide. For onion, peel a single layer of the inner surface of a scale leaf and lay it flat in a drop of water.
2. Add a drop of stain: iodine solution stains onion cell contents, and methylene blue stains cheek cells. The stain makes structures such as the nucleus easier to see.
3. Lower a coverslip onto the specimen at an angle with a mounted needle, so that air bubbles are not trapped.
4. Put the slide on the stage and start with the lowest-power objective lens. Turn the coarse focus wheel until the specimen is roughly in focus, then use the fine focus wheel to sharpen it.
5. Move to a higher-power objective and refocus with the fine focus only.
6. Draw what you see with a sharp pencil, using clear continuous lines and no shading, filling most of the space. Label the parts with straight lines that do not cross.
7. Work out the total magnification by multiplying the eyepiece lens by the objective lens, so a $\times 10$ eyepiece with a $\times 40$ objective gives $\times 400$. Alternatively, measure the drawing and use the magnification equation with the known real size.

The lowest power is used first because it gives the widest field of view and the greatest distance between the lens and the slide, which makes it easy to find the specimen and unlikely that the lens hits the slide.