Words to know
cell wall
A stiff layer outside the cell membrane of plant cells. It is made mostly of cellulose and gives support and protection.
cellulose
A tough material made by plants from sugar. It forms plant cell walls; wood, cotton and paper are mostly cellulose.
chloroplast
An organelle in some plant cells that contains the green substance chlorophyll and carries out photosynthesis.
chlorophyll
The green substance in chloroplasts that absorbs light energy for photosynthesis.
central vacuole
The large vacuole in a plant cell. It stores water and pushes outward to keep the cell firm.
turgor pressure
The pressure of water in the central vacuole pushing outward against the cell wall. It keeps plants firm and upright.
Why doesn't a tree flop over?
You have a skeleton to hold you up. A 30-metre white pine has no bones at all, yet it stands straight through storms and heavy snow. A tree also doesn't eat. It makes its own food from sunlight. Both of these facts are explained by structures found in plant cells but not in animal cells.
Plant cells have all the parts you learned last lesson: a nucleus, cell membrane, cytoplasm and mitochondria. (Yes, plants need mitochondria too! Plant cells do cellular respiration to release energy from the sugar they make.) But they also have three special features.
Three plant-cell features
1. Cell wall. A stiff layer outside the cell membrane, made mostly of cellulose. It gives the cell a fixed, often box-like shape, supports the plant and protects the cell. Water and dissolved substances pass through it easily; the cell membrane, just inside it, is still the part that controls what enters the cell. Wood, paper and cotton are mostly cellulose from plant cell walls.
2. Chloroplasts. Green organelles containing chlorophyll. Remember from Grade 7: photosynthesis uses light energy to turn carbon dioxide and water into sugar (glucose) and oxygen. This happens in the chloroplasts. Only plant cells that get light, mostly in leaves and green stems, have chloroplasts. Root cells and the cells of an onion bulb have none.
3. Large central vacuole. One big vacuole that can fill much of the cell. It stores water (plus some sugars, salts and wastes). When it is full, it pushes outward against the cell wall. This turgor pressure keeps the cell firm, like air in a bike tire. When a plant doesn't get enough water, its vacuoles shrink, the cells go soft and the plant wilts.
Structure fits function
The differences make sense when you think about how plants and animals live:
- Plants make their own food using light, so they need chloroplasts. Animals eat food, so they don't.
- Plants don't have skeletons. Stiff cell walls plus water-filled vacuoles pushing outward hold the plant up. Animals have skeletons (or shells) for support, and their flexible cells let them move and change shape.
- Plants can't walk to a lake for a drink, so the large vacuole stores water between rains.
Note: fungi cells (like mushrooms and yeast) also have cell walls, but they are made of a different material, and fungi have no chloroplasts.
Contributions to science: where did mitochondria and chloroplasts come from?
In 1967, the American biologist Lynn Margulis published the idea that mitochondria and chloroplasts were once free-living bacteria that were taken inside other cells long ago and stayed, helping each other survive. Many scientists rejected her idea at first. But evidence built up: mitochondria and chloroplasts have their own DNA and divide on their own inside the cell, much like bacteria. Today her idea (called endosymbiosis) is widely accepted. It shows how a scientist with a new perspective, who kept gathering evidence, changed what everyone thought.
Hands-on activityEngineering design: build a 3-D cell model
Challenge: design and build a 3-D model of a plant cell or an animal cell that clearly shows all its parts and how it differs from the other type.
Criteria: all the parts for your cell type are shown and labelled; sizes and positions make sense (for example, a plant's vacuole is large); it can be handled without falling apart; someone at home can tell from the model whether it is a plant or an animal cell.
Possible materials: a clear zip-top bag (cell membrane); a small clear plastic container or box (cell wall); clear hair gel, or water with a little cornstarch (cytoplasm); a grape, plum or ball of modelling clay (nucleus); dried beans or jelly beans (mitochondria); green peas or green beads (chloroplasts); a smaller water-filled zip-top bag (central vacuole); paper flags on toothpicks (labels).
- Plan: sketch your design and list which material is each part, and why.
- Build it.
- Test: ask someone at home to (a) name each part and (b) decide whether it's a plant or animal cell. Write down what they got wrong or found confusing.
- Improve one thing based on the test, and explain what you changed.
- Communicate: write 3 sentences comparing your cell with the other type.
Safety: small beads, beans and peas are choking hazards for young children; keep them away from little brothers and sisters. Don't eat materials that have been in the model. Toothpick points are sharp.
What to look for: a plant cell model shows the wall outside the membrane, a large central vacuole and chloroplasts; an animal cell model has none of these. The test-and-improve steps are recorded.