Plant and Animal Cells
Senior 1 · Biology
Plant and Animal Cells
Lesson 1 of 5
25 min
In 1665 the English scientist Robert Hooke cut a very thin slice of cork, the spongy bark used to stopper bottles, and looked at it through a microscope he had built himself. He saw rows of tiny empty boxes, like the rooms of a honeycomb, and called them cells, from the Latin cella, a small room. Hooke was looking at the dead walls of plant cells. It took almost two hundred more years for scientists to agree on what his discovery meant: that every living thing, from the bacteria that turn milk into ikivuguto to a cow or a eucalyptus tree, is built of cells.
Definition
Cell: the smallest unit of a living thing that can carry out all the processes of life. Example: a single cell from the lining of your cheek.
Organism: a complete living thing. A unicellular organism is made of one cell (a bacterium, an Amoeba, a yeast cell); a multicellular organism is made of many cells (a bean plant, a goat, you).
Micrometre (symbol µm): one thousandth of a millimetre, the unit used for cells. .
Better microscopes let later scientists see cells in every tissue they examined. The botanist Matthias Schleiden (1838) found that all plants are made of cells, the zoologist Theodor Schwann (1839) found the same for animals, and the doctor Rudolf Virchow (1855) added that cells only come from other cells. Together their findings form the cell theory:
Figure 1: How big are cells? Each mark is ten times smaller than the one before it.
When biologists compare cells from many organisms, they find two basic plans. The difference is whether the DNA is enclosed in a nucleus.
Figure 2: The two basic types of cell.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | none: the DNA lies free in the cytoplasm | present, surrounded by a nuclear membrane |
| Organelles with membranes (mitochondria, chloroplasts) | none | present |
| Typical width | 0.1 to 5 µm | 10 to 100 µm |
| Cell wall | usually present (not made of cellulose) | in plants (cellulose) and fungi; absent in animals |
| Examples | bacteria, such as those that sour milk into ikivuguto | plants, animals, fungi (yeast) and protoctists (Amoeba, the malaria parasite) |
Definition
Prokaryotic cell (prokaryote): a cell with no nucleus and no membrane-bound organelles; its DNA lies free in the cytoplasm. All bacteria are prokaryotes.
Eukaryotic cell (eukaryote): a cell whose DNA is enclosed in a nucleus and which contains membrane-bound organelles such as mitochondria. Plant and animal cells are eukaryotic.
Organelle: a small structure inside a cell that does a particular job, as an organ does in a body. Example: the nucleus.
Warning
Keep your units straight. A common mistake is to write that a cheek cell is "60 mm" wide: that is 6 cm, about the length of your little finger. Cells are measured in micrometres. Before you compare two sizes, convert both to the same unit: multiply millimetres by 1000 to get micrometres.
Worked example 1: how big is the cell really?
A photograph of a cheek cell taken at a magnification of ×500 shows the cell 30 mm wide. Find its real width in micrometres.
Answer: the cell is 60 µm wide, which matches Figure 1.
Worked example 2: cells on a pinhead
A red blood cell is about 8 µm wide and the head of a pin is about 2 mm wide. How many red blood cells could lie side by side across the pinhead?
Answer: about 250 red blood cells. This is why a single cell can never be seen without a microscope.
You meet both types of cell every day. The bacteria that turn fresh milk into ikivuguto are unicellular prokaryotes: each one is a single cell only 1 or 2 µm long, and millions of them fit in a drop of milk. The yeast that ferments banana juice into urwagwa is also unicellular, but it is a eukaryote with a nucleus. At every health centre, a laboratory technician diagnoses malaria by looking at a drop of stained blood under the microscope: the parasite, Plasmodium, is a single eukaryotic cell living inside your red blood cells.
Prokaryotic vs. Eukaryotic Cells (Updated), Amoeba Sisters. Watch for the list of structures that both cell types share, then for the features that only eukaryotic cells have.
Tip
Memory hook: the Greek word karyon means "kernel", that is, the nucleus. Pro- means "before": prokaryotes are "before a nucleus". Eu- means "true": eukaryotes have a true nucleus.
Practice: the cell
Key points
Sources and credits
Figures 1 and 2, the table, the worked examples and the practice questions are written by Ganzaa.
