Cells and Cell Structure
Grade 6 · Science · Study Guide
Explore the basic building blocks of all living things.
Every living thing on Earth — from a tiny bacterium to a blue whale — is made of cells. Cells are the smallest units of life. Some organisms, like bacteria, are made of just one cell. Others, like you, are made of trillions of cells all working together.
Cells come in two major types: prokaryotic and eukaryotic. Prokaryotic cells are simpler and have no nucleus. Bacteria are prokaryotes. Eukaryotic cells are more complex and have a membrane-bound nucleus that holds DNA. All plant and animal cells are eukaryotes.
Inside a cell, tiny structures called organelles each have a specific job. The nucleus is the control center — it holds the genetic instructions (DNA) that tell the cell what to do. The mitochondria generate energy for the cell by breaking down food molecules — they are often called the powerhouse of the cell. The cell membrane is a thin, flexible barrier that controls what enters and leaves the cell.
Plant cells have a few special features that animal cells lack. They have a rigid cell wall outside the membrane for extra support, chloroplasts that capture sunlight for photosynthesis, and a large central vacuole that stores water and keeps the plant firm. Animal cells don't have any of these, but they do have centrioles that help during cell division.
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Yes | No |
| Chloroplasts | Yes | No |
| Large central vacuole | Yes | No (small vacuoles) |
| Centrioles | No | Yes |
| Cell membrane | Yes | Yes |
| Mitochondria | Yes | Yes |
Key terms
- Cell
- The smallest unit of life; the basic building block of all organisms.
- Nucleus
- The organelle that contains DNA and controls cell activity.
- Mitochondria
- Organelles that produce energy for the cell through cellular respiration.
- Cell membrane
- A thin, flexible layer that surrounds all cells and controls what passes in and out.
- Chloroplast
- An organelle found in plant cells that captures sunlight to make food.
- Organelle
- A specialized structure inside a cell that performs a specific function.
- Prokaryote
- An organism whose cells lack a membrane-bound nucleus (e.g., bacteria).
- Eukaryote
- An organism whose cells have a membrane-bound nucleus (e.g., plants, animals).
Flashcards
- Cell
- The smallest unit of life; all living things are made of cells.
- Nucleus
- Cell's control center; holds DNA with instructions for the cell.
- Mitochondria
- Produces energy for the cell; called the powerhouse of the cell.
- Cell membrane
- Flexible barrier surrounding every cell; controls what enters and exits.
- Cell wall
- Rigid outer layer found in plant cells (not animal cells) for structure.
- Chloroplast
- Plant cell organelle that captures sunlight to make food via photosynthesis.
Practice questions
Which organelle is known as the 'powerhouse of the cell' because it produces energy?
- Nucleus
- Mitochondria
- Chloroplast
- Vacuole
Show answer
Answer: Mitochondria
Mitochondria break down glucose to release ATP energy the cell can use.
Plant cells have a cell wall but animal cells do not.
Show answer
Answer: True
The cell wall is a rigid structure unique to plant cells; it provides extra support and protection.
Which type of cell has NO membrane-bound nucleus?
- Plant cell
- Animal cell
- Prokaryotic cell
- Eukaryotic cell
Show answer
Answer: Prokaryotic cell
Prokaryotes like bacteria have no membrane-bound nucleus — their DNA floats freely in the cytoplasm.
The ___ is the control center of the cell and contains DNA.
Show answer
Answer: nucleus
The nucleus houses the cell's DNA and directs all cell activities.
Chloroplasts are found in both plant and animal cells.
Show answer
Answer: False
Chloroplasts are found only in plant cells (and algae). Animal cells cannot photosynthesize.
Fun facts
- The human body contains about 37 trillion cells — that's more cells than there are stars in the Milky Way.
- Red blood cells are the only cells in the human body that have no nucleus.
- Some nerve cells in your body can be over a meter long!
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