In 1665 Robert Hooke sliced a piece of cork as thinly as he could and looked at it under his microscope. He saw rows of tiny empty boxes and called them "cells", after small rooms. He didn't know what they were for. A few years later the Dutch draper Antonie van Leeuwenhoek, grinding his own lenses, saw living "animalcules" swimming in pond water. He was the first person known to see bacteria. Topic 1 is everything we've learned about cells since.
Two kinds of cell
- Eukaryotic cells (animals and plants) keep their genetic material in a nucleus.
- Prokaryotic cells (bacteria) are much smaller, with no nucleus. Their DNA is a single loop in the cytoplasm, sometimes with extra small rings called plasmids.
Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells have all of these, plus a cellulose cell wall, often chloroplasts, and a permanent vacuole.
Seeing cells
Light microscopes let you see living cells; electron microscopes have much higher magnification and resolution, enough to see ribosomes. You'll need:
Magnification = size of image ÷ size of real object (same units first).
Specialised cells and stem cells
Cells differentiate to do particular jobs: sperm cells have tails, nerve cells are long, root hair cells have a big surface area. Stem cells haven't specialised yet. Embryonic stem cells can become almost any cell; adult stem cells in bone marrow make mainly blood cells; plant meristems keep making new cells all through a plant's life.
Dividing
In the cell cycle, a cell grows, copies its DNA, then divides by mitosis into two genetically identical cells. That's how you grow and heal.
Moving things in and out
- Diffusion: particles spread from high to low concentration.
- Osmosis: water diffuses from a dilute solution to a more concentrated one, across a partially permeable membrane.
- Active transport: moving substances against the gradient, using energy from respiration.
Required practicals: using a light microscope, and investigating osmosis in plant tissue (often potato pieces in sugar or salt solutions).
Common exam mistakes
- Saying bacteria have a nucleus. They don't; that's what makes them prokaryotes.
- Forgetting plant cells have mitochondria. They respire too.
- Mixing units in magnification. Convert both sizes to the same unit first.
- Calling active transport "diffusion that needs energy". It goes against the gradient.
Try it
Say each answer before you flip. Then think like Hooke: if you'd just seen empty boxes in cork, what would you have tested next?
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