Living things are built from cells. This topic covers what is inside a cell, how cells differ, and how they are organised into larger structures.
Cells are the basic structural and functional units of living organisms.
They are far too small to see with the naked eye, so a light microscope is used. Prepare a slide with a thin sample, add a stain such as iodine to make structures visible, and lower the coverslip at an angle to avoid trapping air bubbles. Start with the lowest-power objective and focus with the coarse wheel, then move to higher power and use the fine wheel.
Total magnification is the eyepiece magnification multiplied by the objective magnification. Drawings should be in pencil, with clear unbroken lines, no shading, and labels with straight ruled lines.
Each part of a cell has a job:
Both plant and animal cells have a nucleus, cytoplasm, a cell membrane and mitochondria.
Plant cells have three things animal cells do not: a cell wall, chloroplasts and a large permanent vacuole. Not every plant cell has chloroplasts. Root cells do not, because there is no light underground.
Diffusion is the net spreading of particles from a region of higher concentration to a region of lower concentration, because of their random motion.
Small molecules such as oxygen can enter cells by diffusion, and carbon dioxide can leave the same way, moving through the cell membrane. It needs no energy from the cell, because the particles are moving randomly anyway.
Some organisms consist of a single cell, which carries out all of the organism's life processes. Their structures are adapted accordingly.
A euglena has a flagellum, which it can use to move towards favourable light conditions, and chloroplasts for photosynthesis. An amoeba changes shape to move and to surround its food. Both have a large surface area compared with their volume, so diffusion alone is enough to supply them.
Cells are organised into ever larger structures: cells → tissues → organs → organ systems → organism.
A tissue is a group of similar cells working together, such as muscle tissue. An organ is several tissues working together, such as the stomach. An organ system is several organs working together, such as the digestive system.
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