Science 10 · Cells
An open system the size of a dot
A cell is a bag that keeps an inside different from its outside, brings in what it needs and gets rid of what it does not. Every organelle is part of doing that, and every transport mechanism is about crossing the boundary.
- 1. The words, first
- 2. What each part does
- 3. Crossing the membrane
- 4. Photosynthesis and respiration
- 5. What costs marks
The words, first
The idea: Passive and active are the distinction to be clear about: one costs ATP and one does not.
| Word | What it means |
|---|---|
| Cell theory | All living things are made of cells; the cell is the basic unit of life; all cells come from existing cells. |
| Organelle | A structure inside a cell with a specific job. |
| Nucleus | Holds the DNA and controls the cell's activities. |
| Mitochondrion | Releases energy from glucose in cellular respiration. |
| Chloroplast | Carries out photosynthesis. Plants only — and plant cells have mitochondria as well. |
| Cell membrane | The selectively permeable boundary. Every cell has one. |
| Cell wall | A rigid outer layer in plant cells, outside the membrane. |
| Diffusion | Movement from higher to lower concentration. Passive — no ATP. |
| Osmosis | Diffusion of water across a selectively permeable membrane. |
| Active transport | Movement against a concentration gradient, using ATP. The only way to concentrate something. |
What each part does
The idea: Learn them as jobs rather than as a diagram to label, because the exam asks what they do.
| Organelle | Job | Where |
|---|---|---|
| Nucleus | Holds DNA, controls activities | Both |
| Mitochondrion | Cellular respiration — releases energy | Both |
| Ribosome | Builds proteins | Both |
| Cell membrane | Controls what enters and leaves | Both |
| Chloroplast | Photosynthesis | Plant only |
| Cell wall | Rigid support | Plant only |
| Large central vacuole | Storage, and turgor pressure | Plant only |
Prokaryotes — bacteria — have DNA but no nucleus and no membrane-bound organelles. They are cells, so the cell theory still holds.
Microscopes matter here. Cells were invisible until lenses were good enough, which is why the cell theory is only about 200 years old. Technology and theory advance together, and Alberta's Unit C makes that point explicitly.
Crossing the membrane
The idea: Three mechanisms. The question that sorts them is always: which way is the gradient, and is energy being spent?
- Diffusion. High to low concentration, passive. Oxygen entering a cell works this way.
- Osmosis. Water moving across a membrane towards the more concentrated solution. It is diffusion with water named.
- Active transport. Low to high — uphill — so ATP is required. This is the only way a cell can concentrate something it already has plenty of.
The test question. If a substance moves from lower to higher concentration, energy is being spent. If from higher to lower, it is not. That one sentence answers most transport questions in the course.
Why cells stay small. Double a cell's width and its surface grows four times while its volume grows eight. Past a certain size the membrane cannot supply the interior fast enough, so cells divide rather than grow. It is the same square-cube relationship as in the measurement unit.
Photosynthesis and respiration
The idea: Two equations that reverse each other, and one asymmetry that explains why the Sun still matters.
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light in)
Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy out as ATP)
Matter cycles; energy does not. The same carbon and oxygen atoms go round this pair indefinitely. The energy passes through once — in as light, out as heat — which is why the Sun has to keep supplying more.
The most common misconception. Plants respire too, constantly, day and night. In daylight photosynthesis usually outpaces respiration, which is why plants are net oxygen producers overall — but respiration never stops.
What costs marks
The idea: Three about plants and one about energy.
- Saying plants do not respire.
- Saying animal cells have chloroplasts, or that plant cells have no mitochondria.
- Calling diffusion active. It needs no energy.
- Saying energy is recycled. Matter cycles; energy passes through.