Words to know
organelle
A small structure inside a cell that has a special job. The word means "little organ".
nucleus
The control centre of the cell. It contains DNA, the instructions for the cell's activities, and directs growth and cell division.
cell membrane
The thin, flexible layer around the cell. It controls what enters and leaves the cell.
cytoplasm
The jelly-like fluid that fills the cell and holds the organelles. Many chemical reactions happen here.
mitochondria
Organelles that release energy from food by cellular respiration. One is called a mitochondrion.
vacuole
A sac surrounded by a membrane that stores water, food or wastes. Animal cells have small vacuoles, if any.
DNA
The hereditary material: a long molecule in the nucleus that carries instructions for building and running the cell and the organism.
A city in a cell
One of your cells is far too small to see, yet it is as busy as a city. Materials come in and go out, energy is released, wastes are stored and removed, and a control centre gives instructions. Each job is done by a different part. The parts with special jobs are called organelles, which means "little organs". Just as your body has organs, a cell has organelles.
The parts of an animal cell
Cell membrane. A thin, flexible layer that surrounds the cell. It holds the cell together and controls what enters and leaves. Useful things like oxygen, water and sugar get in; wastes like carbon dioxide get out. Because it lets some substances through but not others, we say it is selectively permeable.
Cytoplasm. A jelly-like fluid, mostly water, that fills the cell. The organelles sit in it, and many chemical reactions of the cell happen there. Materials move through the cytoplasm from one part of the cell to another.
Nucleus (plural: nuclei). The control centre. It contains DNA, the instructions for everything the cell does. The nucleus directs the cell's activities, its growth and its cell division. It has its own membrane around it.
Mitochondria (singular: mitochondrion). The energy releasers. Remember from Grade 7: cellular respiration uses sugar (glucose) and oxygen to release energy, making carbon dioxide and water as wastes. This happens in the mitochondria. Cells that need lots of energy, like muscle cells, have many mitochondria.
Vacuoles. Small sacs, each surrounded by a membrane, that store water, food or wastes. Animal cells have small vacuoles, or none at all.
Coming up in high school
Cells have even more organelles that you will study in Grade 11 Biology, such as ribosomes (where proteins are made), the endoplasmic reticulum (a network of membranes that moves materials) and the Golgi apparatus (which packages materials to be sent out). For Grade 8, focus on the nucleus, cell membrane, cytoplasm, mitochondria, vacuoles, and, in the next lesson, the cell wall and chloroplasts.
Hands-on activityA cell is like…: an analogy poster
You need: paper or poster board, markers, a ruler.
- Choose a place you know well: your school, a hockey arena, a farm, a grocery store, or your home.
- Make a three-column table: Cell part, Its job in the cell, What it's like in my place. Include all five parts: nucleus, cell membrane, cytoplasm, mitochondria, vacuoles. Example (school): nucleus = principal's office, where decisions are made and records are kept.
- Draw and label an animal cell next to your table, using straight label lines that don't cross.
- Write one way your analogy does not fit. (Example: a school's doors open to anyone at recess, but a cell membrane is choosy all the time.)
- Present your poster to someone at home in under 2 minutes, using the science words.
What to look for: each part matched to a place with the same job (for example, mitochondria ↔ furnace or power source; membrane ↔ doors with a hall monitor), a correct, labelled drawing, and a clear limit of the analogy.