Cell: The Building Block of Life

The cell is the basic unit of life: its parts and their jobs, plant and animal cells, how water moves in and out, and how cells divide.

Key ideas

All living things are made of cells, and new cells come only from existing cells.
Prokaryotic cells (bacteria) have no nuclear membrane; eukaryotic cells have a true nucleus and membrane-bound organelles.
Plant cells have a cell wall, chloroplasts and a large central vacuole; animal cells do not.
Diffusion moves substances from high to low concentration; osmosis is the movement of water across a selectively permeable membrane.
In a hypotonic (more dilute) solution a cell gains water and swells; in a hypertonic one it loses water and shrinks.
Mitosis gives two identical cells for growth and repair; meiosis gives four cells with half the chromosomes, for gametes.

Remember

Nucleus	controls the cell; holds the chromosomes
Mitochondria / chloroplasts	release energy (ATP) / make food by photosynthesis
Ribosomes / Golgi / lysosomes	make proteins / package and send / digest waste
Microscope magnification	eyepiece × objective; real size = image size ÷ magnification

Watch out

Saying osmosis moves salt or sugar: it moves water.
Saying the cell wall is selectively permeable (the membrane is; the wall is fully permeable).
Adding the eyepiece and objective powers instead of multiplying them.

Worked example

An animal cell with 8% sugar in its sap is placed in a 20% sugar solution. Which way does water move? Is the outside solution hypotonic or hypertonic? What happens to the cell?

Working: water moves from the dilute to the concentrated side

Answer: Water moves out of the cell; the solution is hypertonic; the cell shrinks.