Every cell, from a bacterium to one of your nerve cells, has a few parts in common. Learn these three first; the other organelles come next.
Key idea
The cell membrane surrounds the cell and controls what goes in and out. The cytoplasm is the jelly-like inside where many reactions happen. In plants, animals, fungi and protists, the nucleus holds the DNA, the cell's instructions. Bacteria have no nucleus.
Why it works
A cell needs food and water to get in and wastes to get out, but it must keep its own useful molecules inside. A membrane that let nothing through would starve the cell; one that let everything through would spill it. So the membrane is selectively permeable: some things pass, others do not.
Worked examples
Worked example: Name the part
Which part of a cell holds its DNA?
- In plant, animal, fungus and protist cells, the DNA is kept inside the nucleus.
Answer: The nucleus.
Worked example: Every cell?
Does a bacterium have a nucleus? Does it have a membrane?
- Bacteria keep their DNA in a loop in the cytoplasm, with no nucleus around it.
- Every cell has a membrane, bacteria included.
Answer: No nucleus, but yes, a membrane.
Common mistake
Thinking the membrane is a solid wall that blocks everything, or that the nucleus is the cell's "brain" doing every job. The membrane lets some things through. The nucleus stores the instructions; other parts, like ribosomes and mitochondria, do the work.