Singapore-Cambridge O-Level Biology

Cells as the basic unit of life

Living things differ from non-living things in that they grow, reproduce, move and respond to changes around them. All of them are built from cells, and the activities of an organism can be explained by what its cells do. Cells are organised into tissues (groups of similar cells doing one job), tissues into organs, organs into organ systems, and the systems together make up the organism.

Most cells are far too small to see without a microscope. A light microscope shows the nucleus, cytoplasm, cell membrane and, in plants, the cell wall, vacuole and chloroplasts. Finer detail, such as the endoplasmic reticulum, Golgi body, mitochondria and ribosomes, is only visible in an electron micrograph, which uses a beam of electrons and has a much higher resolution.

Cell structures and their functions

Structures seen with a light microscope

The cell membrane surrounds the cytoplasm of every cell. It is partially permeable, so it controls which substances enter and leave the cell.

The cytoplasm is the jelly-like material inside the membrane where most chemical reactions of the cell take place, and in which the organelles are suspended.

The nucleus contains the cell's DNA in the form of chromosomes. It controls the activities of the cell, since the genes it holds carry the instructions for making the proteins (including enzymes) the cell uses.

The cell wall is found in plant cells, outside the cell membrane. It is made of cellulose, which is a strong and fully permeable material, so it supports the cell and stops it bursting when it takes in water.

A vacuole is a fluid-filled space bounded by a membrane. In a mature plant cell there is one large, permanent vacuole containing cell sap, a solution of sugars, salts and other solutes. It keeps the cell firm (turgid) and stores substances. Animal cells may have small, temporary vacuoles, such as those that hold food or waste for a short time.

Chloroplasts are found in the green parts of plants. They contain chlorophyll, which absorbs light energy for photosynthesis, so they make carbohydrates from carbon dioxide and water.

Structures seen with an electron microscope

Ribosomes are tiny structures, some free in the cytoplasm and some attached to the endoplasmic reticulum. They are where amino acids are joined together to make proteins.

The endoplasmic reticulum is a network of flattened membrane sacs and tubes running through the cytoplasm. Rough endoplasmic reticulum has ribosomes on its surface and transports the proteins made there through the cell. Smooth endoplasmic reticulum has no ribosomes and is involved in making and transporting lipids.

The Golgi body is a stack of flattened membrane sacs. It receives proteins from the endoplasmic reticulum, modifies them and packs them into small vesicles, which carry them to the cell membrane for secretion or to other parts of the cell.

Mitochondria are small, rod-shaped organelles with a folded inner membrane. They are the site of aerobic respiration, where energy is released from glucose, so cells with a high demand for energy contain many of them.

Comparing plant and animal cells

Both kinds of cell have a nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, endoplasmic reticulum and a Golgi body.

Observing cells under the light microscope

Cells are usually too transparent to see clearly, so a temporary stain is used to give the structures contrast. Iodine solution stains the nucleus and starch grains in plant cells, and methylene blue is often used for animal cells such as cheek cells.

To prepare a slide of onion epidermis, a small piece of the thin inner layer of a bulb scale is peeled off with forceps and laid flat on a drop of water on a clean slide. A drop of iodine solution is added, and a coverslip is lowered at an angle with a mounted needle so that no air bubbles are trapped. Air bubbles appear as dark-edged circles and hide the cells. The slide is examined under low power first, to find the cells, and then under higher power to see the detail.

Cheek cells are collected by gently scraping the inside of the cheek with a clean cotton bud or sterile spatula, smearing them in a drop of water on a slide, and staining with methylene blue before adding the coverslip.

Magnification and drawings

Total magnification is the magnification of the eyepiece lens multiplied by that of the objective lens, so a $\times 10$ eyepiece with a $\times 40$ objective gives $\times 400$.

The magnification of a drawing or a micrograph is found from

Both lengths must be in the same unit. A cell drawn as 48 mm long that is really $0.06$ mm long has a magnification of $\frac{48}{0.06} = 800$, so the drawing is $\times 800$. Rearranged, the real size is the image size divided by the magnification, so a structure measuring 30 mm on a $\times 600$ micrograph is $\frac{30}{600} = 0.05$ mm, which is 50 micrometres.

A good biological drawing uses a sharp pencil, has clear continuous lines with no shading, is large enough to show detail, and has label lines ruled to the structure without crossing each other. The magnification is written beside it.

Quantities, symbols and units

Measurements in biology use the following quantities and units.

Microscopic sizes are given in micrometres, and $1\ \mathrm{mm} = 1\,000\ \mu\mathrm{m}$. A volume of $1\ \mathrm{dm^3}$ is the same as $1\,000\ \mathrm{cm^3}$, so a solution of $5\ \mathrm{g\,dm^{-3}}$ has 5 g in every $1\,000\ \mathrm{cm^3}$, or $0.005\ \mathrm{g}$ in each $\mathrm{cm^3}$.