生物体的组成

Every living thing is made of cells 细胞. A cell is the smallest part that can carry out the life processes. In this topic you compare a plant 植物 cell, an animal 动物 cell and the cell of a bacterium 细菌. All new cells are made by the division 分裂 of cells that already exist.

Which structures are in each cell?

So plant and animal cells share five structures: cell membrane, cytoplasm, ribosomes, nucleus and mitochondria. A plant cell has three extra structures: a cell wall, a large vacuole and (in green parts) chloroplasts.

What each structure does

• Cell membrane — controls which substances enter and leave the cell.
• Cytoplasm — a jelly-like liquid where many chemical reactions happen.
• Nucleus — controls the cell's activities and holds the genetic material 遗传物质.
• Ribosomes — where proteins 蛋白质 are made.
• Mitochondria — where respiration 呼吸作用 happens to release energy 能量.
• Cell wall — made of cellulose 纤维素; it gives a plant cell strength and a fixed shape.
• Chloroplasts — contain chlorophyll 叶绿素, a green substance that traps light for photosynthesis 光合作用.
• Large vacuole — filled with cell sap 细胞液; it helps keep the plant cell firm.

Bacterial cells

A bacterium is a prokaryote 原核生物 — its cell has no nucleus. Instead its DNA is a single loop (a ring), loose in the cytoplasm. Bacteria also have small extra rings of DNA called plasmids 质粒. A bacterial cell has a cell wall and cell membrane, cytoplasm and ribosomes, but no mitochondria and no chloroplasts.

Most cells are specialised cells 特化细胞 — their shape and parts suit one special job.

Sperm and egg cells are the sex cells, called gametes 配子.

An organism 生物体 made of many cells is built up in levels, from small to large:

• cell — the smallest unit of life (for example a red blood cell).
• tissue 组织 — a group of similar cells that work together (for example muscle 肌肉).
• organ 器官 — several different tissues that work together to do a job (for example the heart, or a leaf).
• organ system 器官系统 — several organs that work together (for example the digestive system 消化系统, which breaks down food).
• organism — all the organ systems together make one whole living thing.

Cells are tiny, so you look at them under a microscope 显微镜, which makes them appear much larger. How many times larger the image is, is called the magnification 放大倍数.

• image size = the size in the drawing or photo.
• actual size = the real size of the specimen 标本.

Magnification has no unit — it is just a number, for example ×250. You can rearrange the formula:

Worked example. A structure is shown at a magnification of 250. Its image size is 5.00 mm. So the actual size = 5.00 ÷ 250 = 0.02 mm. Always put both sizes in the same unit before you divide.

Changing units (Supplement)

Cells are often measured in micrometres 微米 (μm), which are smaller than millimetres (mm):

So 0.02 mm = 20 μm. To change mm into μm, multiply by 1000. To change μm into mm, divide by 1000.

Worked example. A cell is drawn 60 mm wide, and its real width is 30 μm. Find the magnification. First make the units match: image size = 60 mm = 60 × 1000 = 60,000 μm. Then magnification = image ÷ actual = 60,000 ÷ 30 = ×2000. If you forget to convert and divide 60 ÷ 30, you get ×2 — a thousand times too small. This is the most common mistake in the whole topic.

Common mistakes.

• Saying mitochondria "make energy". Energy cannot be made; respiration in the mitochondria releases energy from glucose.
• Saying all plant cells have chloroplasts. Root cells are underground and have no chloroplasts, though they still have a cell wall and a vacuole.
• Mixing up the cell wall and the cell membrane. The cell membrane controls what enters and leaves; the cell wall (plants and bacteria) gives strength and shape.
• Putting units on a magnification. Magnification is just a number, such as ×250.
• Measuring the image in mm but the actual size in μm. Change both to the same unit before you divide.
• Calling the heart a tissue, or muscle an organ. A tissue is one kind of similar cell; an organ is several tissues working together.

• A plant cell has a cell wall, a large vacuole and chloroplasts; an animal cell has none of these. Both have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
• A cell with no nucleus but with plasmids and a loop of DNA is a bacterium.
• For magnification, cover the quantity you want: magnification = image ÷ actual; actual = image ÷ magnification.
• Always convert to the same unit before you calculate. Remember 1 mm = 1000 μm.
• For a specialised cell, link its job to the one feature that suits it (for example a red blood cell has no nucleus, so it can hold more oxygen).