Life Science · Unit 1: Cells
Plant Cells and Animal Cells
How does a plant stand up without bones, and how are its cells different from yours? You will build a model cell for a job, switch its parts on and off, and read real measurements of onion, waterweed and cheek cells. Afterwards you can name the parts plant and animal cells share, the three parts plant cells add, and the job each one does.
Where this lesson sits
- Virginia SOL LS.2 c
- Practices LS.1 b, LS.1 c, LS.1 e
Essential question: How does a plant stand up without a skeleton, and how are its cells different from mine?
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Life Science (grade 7) LS.2 c You are here
This lesson
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Biology
Preview it at high school level
In Biology you will learn that a cell works like a system, with each part passing its product to the next. You will meet three new parts. Ribosomes build proteins. The Golgi apparatus sorts and packs them and sends them on. Lysosomes are small sacs that break down worn-out cell parts. You will follow the instructions for one protein from the nucleus, then the protein itself through the endoplasmic reticulum and the Golgi, to the outside of the cell. You will also compare cells that have a nucleus with bacteria, which do not.
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Biology II Biology II: Advanced Survey of Biology Topics, BIOII.2 a
Preview where it goes next
In Biology II: Advanced Survey of Biology Topics, the cell is studied as a system of parts that depend on each other: the nucleus, mitochondria making ATP, ribosomes, the endoplasmic reticulum, the Golgi and the cytoskeleton, a set of protein fibres that gives a cell its shape and moves its parts. You also see how a cell’s shape and parts fit its job across many kinds of plant and animal tissue.
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College
Watch first A Full Vacuole Holds a Plant Up · 2:33
A cut stalk of celery left out can go limp and bendy as it loses water. Put the limp stalk in water, and it can become crisp again. You drink water every day, but a glass of water does not stiffen your body. A plant has no bones, so something inside its cells must hold it up. What is different about a plant’s cells and yours?
What do you think? Decide before you read.
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Scientists disagree: Scientists measure cells with microscopes, and the sizes vary a lot. One study found onion skin cells about 412 micrometres long. Human cheek cells are flat and about 55 micrometres across. A textbook shows one “typical” cell, but real cells come in many shapes.
What scientists think
Scientists disagree: Scientists measure cells with microscopes, and the sizes vary a lot. One study found onion skin cells about 412 micrometres long. Human cheek cells are flat and about 55 micrometres across. A textbook shows one “typical” cell, but real cells come in many shapes.
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Scientists disagree: Plant cells have both. In 2017, scientists put glowing labels on the parts of waterweed leaf cells. The labels lit up chloroplasts, and other labels lit up mitochondria in the same kind of cell.
What scientists think
Scientists disagree: Plant cells have both. In 2017, scientists put glowing labels on the parts of waterweed leaf cells. The labels lit up chloroplasts, and other labels lit up mitochondria in the same kind of cell.
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Scientists disagree: Look at an onion. The bulb grows underground, and its skin cells are not green. Scientists checked onion bulb skin with a microscope and found no developed chloroplasts. These cells are still plant cells with a cell wall.
What scientists think
Scientists disagree: Look at an onion. The bulb grows underground, and its skin cells are not green. Scientists checked onion bulb skin with a microscope and found no developed chloroplasts. These cells are still plant cells with a cell wall.
Explore: build a cell for a job
In this model you build one cell and decide which parts it gets. You can switch a cell wall, chloroplasts and a large central vacuole on or off. You can also change how much water the cell gets and turn a light on or off. One bar shows how stiff the cell is, and another shows how much food it makes.
Switch on a part to build your cell. Then change the water or the light.
In this sketch, one cell stands for a whole celery stalk. The bars are relative units, not measurements. Stiffness is 10 times the water supply when the cell has a wall and a large central vacuole, and 0 otherwise (a simple rule for this sketch, not a measurement). The food bar checks chloroplasts, light and water. Carbon dioxide is not shown.
Rule to keep: a full vacuole presses on a stiff cell wall and keeps a plant firm, and chloroplasts need light to make food.
Try this.
- Switch on all three plant parts and give plenty of water. Predict what happens to the stiffness bar when you slide the water down. Then test it.
- Predict what happens to the cell’s shape if you switch off the cell wall. Then test it, and watch the outline.
- Turn the light off but keep the chloroplasts on. Predict the food bar. Then switch the chloroplasts off with the light on, and compare.
Explain: the parts all these cells share
- organelle
- nucleus
- cell membrane
- cytoplasm
- endoplasmic reticulum
- mitochondria
- vacuole
- cell wall
- chloroplast
- central vacuole
An organelle is a small part inside a cell that does one job. You met the parts of a cell in that lesson. Plant cells and animal cells share most of them.
- The nucleus holds the cell’s DNA, its instructions, and controls what the cell does.
- The cell membrane is a thin outer layer that controls what goes in and out.
- The cytoplasm is the jelly-like part between the membrane and the nucleus.
- The endoplasmic reticulum is a folded network of membranes that helps make and move materials.
- The mitochondria release energy from sugar (glucose) in a form the cell can use.
- A vacuole is a sac that stores water and other materials. Animal cells may have small ones.
Most plant and animal cells have a nucleus. A few do not; mature red blood cells in mammals have none.
Explain: three parts plants add
Plant cells add three parts. Each one has a job.
| Part | Job | Plant cells | Animal cells |
|---|---|---|---|
| Cell wall | Supports and protects the cell | Yes, outside the membrane | No |
| Chloroplasts | Capture radiant energy to make food | In green cells | No |
| Large central vacuole | Stores water; presses out on the wall when full | Yes | No, only small vacuoles |
The cell wall is a stiff layer outside the cell membrane that supports and protects the cell. A plant cell has both a wall and a membrane. The wall contains a tough material called cellulose, along with other sugar-based materials.
A chloroplast is a green organelle that captures radiant energy, the energy in light, for making food. Inside it, the plant makes sugar (glucose) from carbon dioxide and water. You will study this in that lesson. Plant cells have mitochondria too, so they can release energy from that sugar, day and night. Cells that get no light, like most root cells and onion bulb cells, usually have no chloroplasts.
The central vacuole is one large sac that fills most of a plant cell and stores water. Now back to the celery. When water moves into a cell, the vacuole fills and presses out against the wall. The wall is stiff, so it pushes back, and the cell gets firm. Many firm cells side by side make a firm stalk. When a cell loses water, the vacuole shrinks and the stalk goes limp. Water moves into cells by osmosis, which you will study in that lesson.
A water balloon in a cardboard box is a good model. Fill the balloon, and it presses on the box until the box is tight. The model is not exactly like a real cell: a real wall can stretch a little, and water moves in and out of a cell all the time.
You are different. Your body is held up by a skeleton of bones. Your cells have no walls, so water inside them does not hold you up the way it holds up a plant. Your body also controls how much water stays inside you.
- cell wall
- large central vacuole
- chloroplast
- nucleus
- cell membrane
- cell wall
- blank
- blank
- nucleus
- blank
Word bank: cell membrane, chloroplast, large central vacuole
- blank
- blank
- blank
- blank
- blank
Check your labels
- cell wall
- large central vacuole
- chloroplast
- nucleus
- cell membrane
Explain: shape and size fit the job
Animal and plant cells can differ in shape, size and parts. Most often, these differences fit the cell’s job. Plant cells often look boxy, because the wall gives them straight sides. Animal cells are often rounder or flatter. A cheek cell is thin and flat, like a tile on a floor.
A green leaf cell is packed with chloroplasts, and it gets light. An onion bulb cell usually has none, and it grows underground where there is no light. Both still have a wall that holds the plant up.
- A plant cell can stand firm because blank presses on blank.
- A leaf cell has chloroplasts, but an onion bulb cell does not, because blank.
- My cells do not need a cell wall because blank.
Elaborate: measure real cells
Scientists have measured many of the cells you can see in a school microscope. Here are seven, from different studies. One micrometre (µm) is one thousandth of a millimetre.
| Cell | Plant or animal | Length (µm) | Width or diameter (µm) | Parts seen (sources differ by row, see below) |
|---|---|---|---|---|
| Onion bulb skin | Plant | 412 | 55 (diameter) | Nucleus with iodine; no chloroplasts |
| Waterweed leaf, normal | Plant | about 150 | about 50 | Chloroplasts (tens to hundreds); large central vacuole with a dye |
| Waterweed leaf, colorless cell | Plant | most 60 to 200 | not reported | No chloroplasts; large central vacuole with a dye |
| Potato storage cell | Plant | not reported | 212 (diameter) | Cell wall with a dye |
| Guard cell of a leaf (Arabidopsis) | Plant | 20 | 9 | Not reported |
| Human cheek cell | Animal | not reported | 55 (diameter) | Nucleus, 9 µm across, with a stain |
| Human red blood cell | Animal | not reported | 7.7 (diameter) | No nucleus |
Sources: onion, Przedpelska-Wasowicz and Wierzbicka, 2011 (6 cells), with the iodine-stained nucleus from an Ohio State University teaching page and the missing chloroplasts from Osaki and Kodama, 2017; waterweed (Egeria densa), Osaki and Kodama, 2017, and Hara and colleagues, 2015; potato, Gibson’s 2012 review (the wall dye is from Biswas and Barma, 2020); cheek, Prasad and colleagues, 2010 (five adults, 50 cells each); guard cell and red blood cell, the BioNumbers database. Each study used its own method, and most are averages.
- Which cell in the table is the largest across? Which is the smallest?
- The cheek cell and the onion cell have about the same width. How many times longer is the onion cell than it is wide?
- Which rows show that not every plant cell has chloroplasts?
Claim, evidence, reasoning
Make a claim that answers the essential question: how are plant cells different from your cells? Use this data.
Start with: The data show that plant cells blank, while animal cells blank.
Start with: The onion cell was blank µm long, but the cheek cell was blank µm across. The blank cells had no chloroplasts.
Start with: This matters because a cell wall and a full vacuole blank, while my body is held up by blank.
Compare with a model answer
Plant and animal cells differ in size, shape and parts. The onion skin cell was about 412 µm long and 55 µm wide, a long box, while the cheek cell was flat and about 55 µm across. In the table, the potato cells had walls and the waterweed cells had large central vacuoles, and walls with full vacuoles hold a plant firm. But not every plant cell had chloroplasts: the onion cell and colorless waterweed cells had none. One limit is that each value comes from a different study, with only a few cells or a few people.
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Design problem
Does the amount of water change how firm celery gets? Plan an investigation you could do in class. Name your question, variables, constants, control, trials, measurements and safety steps.
One way to do it
- Question: Does limp celery get firmer when it stands in water than when it stands in an empty cup?
- Independent variable: water or no water in the cup.
- Dependent variable: how far each stalk bends. Clamp the thick end to a table edge, hang a 20-gram mass on the thin end, and measure the bend in centimetres with a ruler.
- Constants: all stalks left out on a counter for a day first so they start limp; stalks from the same bunch, cut to the same length (15 cm), same room, same time.
- Control: stalks in empty cups, measured the same way.
- Trials: at least five stalks in each group. Measure each one at the start and after 3 hours. Find the mean bend for each group.
- Safety: your teacher cuts the stalks. Wipe up spills. Do not eat lab celery.
Evaluate: check your thinking
Limp celery gets crisp in water because water fills the large central vacuoles in its cells. Each full vacuole presses on a stiff cell wall, so the cells and the stalk stand firm. Your cells have no walls, so water inside them does not hold you up the way it does a plant; your skeleton holds you up.
Science practices you used: planning a fair test with a control, reading and comparing measurements in a table, and using a model and saying how it differs from a real cell.
How we measure it
Light microscope and wet mount
A thin piece of a plant or a few cheek cells sit in a drop of water or stain under a cover slip, and light passes through them. You see each cell’s outline and some of its parts, so you can compare plant and animal cells side by side.
Stains
Iodine stain darkens the nucleus in onion skin, and methylene blue colors cheek cells. A stain shows parts that are hard to see, so a part you cannot see may still be there.
Measuring cells with the field of view
If you know how wide the circle of light is, you count how many cells fit across it and divide the circle’s width by the number of cells. Scientists use a tiny ruler inside the eyepiece, called a micrometer, to measure each cell in micrometres.
Finish line
Test yourself
8 questions
Practice and MCAT-style, with explained answers.
Next · Lesson 4
How Cells Divide
When you scrape your knee, where does the new skin come from?
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