Cells: the building blocks of life

Seeing cells for the first time

• Cells are far too small to see with your eyes alone, so nobody knew they existed until microscopes were invented.
• In 1665 the English scientist Robert Hooke published drawings of a thin slice of cork seen through his microscope. It was full of tiny empty boxes, which he named cells because they reminded him of small rooms.
• In the 1670s the Dutch lens-maker Antonie van Leeuwenhoek built simple microscopes with a single tiny lens that magnified over 200 times.
• He was the first to see living single-celled organisms, which he called animalcules, swimming in pond water and in scrapings from his own teeth, as well as blood cells.
• Later scientists, with better microscopes, showed that every living thing is made of cells and that new cells come only from existing cells.

Using a light microscope

• A school light microscope shines light up through a thin specimen on a glass slide.
• You look through the eyepiece lens, and an objective lens close to the slide makes the image bigger.
• Total magnification = eyepiece magnification × objective magnification.
• Always start with the lowest-power objective, focus with the coarse focus knob, then switch to a higher power and use only the fine focus knob.

Try it: change the objective lens

Inside a cell

• A cell is the smallest unit that can carry out all the processes of life. It contains tiny structures called organelles, each with a job.
• Every animal and plant cell has a nucleus, cytoplasm, a cell membrane and mitochondria.
• Plant cells also have a cell wall, chloroplasts and usually a large vacuole.

Try it: animal or plant cell

Specialised cells

• In a living thing made of many cells (a multicellular organism), cells divide the work between them.
• A specialised cell has a shape and set of parts that suit one job.
• Some cells are built for movement: a sperm cell swims with its tail, and muscle cells shorten to move bones.
• Some cells are built for storage: potato cells are packed with grains of starch, and fat cells store energy as oil.
• The shape of a cell is often the biggest clue to its job.

From cells to systems

• Similar cells working together form a tissue, such as muscle tissue.
• Several tissues working together form an organ, such as the heart, which has muscle, nerve and lining tissues.
• Organs working together form an organ system, such as the circulatory system (heart, blood vessels and blood).
• Organ systems working together make up a whole organism.
• Plants are organised the same way: leaf cells, leaf tissues, the leaf as an organ, and the shoot system.

Try it: cell, tissue, organ or system?

Life in a single cell

• Microorganisms (microbes) are living things too small to see without a microscope. Many are made of just one cell, which does every job of life on its own.
• Bacteria are single cells with no nucleus. Some cause disease, but many are helpful, such as the bacteria that turn milk into yoghurt.
• Yeast is a single-celled fungus. Bakers use it to make bread rise, because it gives off carbon dioxide gas.
• Some algae are single cells, floating in ponds and the sea. The tiny algae in the ocean around New Zealand make food by photosynthesis and feed huge food webs.
• Not all fungi and algae are single-celled: a mushroom and a kelp forest are made of many cells.