Cell structure and organellesIB MYP Biology: Revision notes
Section 1
Animal cell organelles
All living things are made of cells. Inside a cell are small parts called organelles, each with its own job. A typical animal cell has five main ones:
- Nucleus: contains the genetic material (DNA) and controls the activities of the cell.
- Cell membrane: a thin layer around the cell that controls which substances enter and leave.
- Cytoplasm: a jelly-like liquid where most chemical reactions of the cell take place.
- Mitochondria: where aerobic respiration takes place, releasing energy for the cell.
- Ribosomes: tiny structures where proteins are made.
Cells that use a lot of energy, such as muscle cells, contain many mitochondria.
Mitochondria do not make energy. Energy cannot be made. They release energy from glucose during respiration.
Section 2
Extra structures in plant cells
Plant cells have all the organelles of an animal cell plus three more:
- Cell wall: a rigid layer outside the cell membrane that supports the cell and keeps its shape.
- Chloroplasts: contain a green pigment called chlorophyll, which absorbs light for photosynthesis.
- Permanent vacuole: a large space filled with cell sap (a solution of sugars and salts) that helps to keep the cell firm.
Chloroplasts are found in the green parts of a plant, such as leaves, but not in roots, which are underground and get no light.
The cell membrane and cell wall are different. The wall is rigid and supports the cell. The membrane is thin and controls what goes in and out.
Section 3
Bacterial cells
Bacteria are single-celled organisms. Their cells are much smaller and simpler than animal and plant cells. A bacterial cell has:
- a cell wall, a cell membrane, cytoplasm and ribosomes
- a loop of DNA lying free in the cytoplasm, because there is no nucleus
- one or more plasmids: small extra rings of DNA
Bacterial cells do not have mitochondria or chloroplasts.
Section 4
Comparing animal, plant and bacterial cells
- All three have a cell membrane, cytoplasm and ribosomes.
- Animal cells: nucleus and mitochondria. No cell wall, chloroplasts or permanent vacuole.
- Plant cells: nucleus, mitochondria, cell wall, chloroplasts and a permanent vacuole.
- Bacterial cells: cell wall, but no nucleus. DNA is a loop in the cytoplasm, plus plasmids.
When you compare, always write one point for each cell type in the same sentence, for example: "Plant cells have a nucleus but bacterial cells do not."
Do not say bacteria have no DNA. They do have DNA, but it is not enclosed in a nucleus.
Section 5
Linking structure to function
Examiners often ask you to apply your knowledge to an unfamiliar cell. Ask two questions: what does this cell do, and which organelle helps it do that?
- A very active cell needs lots of energy, so it has many mitochondria.
- A cell that makes lots of protein has many ribosomes.
- A cell that photosynthesises has many chloroplasts.
This is also how antibiotics work. Some stop bacteria building their cell wall. Human cells have no cell wall, so the drug harms the bacteria but not the patient.
Must Know
- Nucleus: genetic material. Membrane: controls entry and exit. Cytoplasm: reactions. Mitochondria: aerobic respiration. Ribosomes: protein synthesis
- Plant only: cell wall, chloroplasts, permanent vacuole
- Bacteria: cell wall, loop of DNA, plasmids, no nucleus
- Link the number of organelles to what the cell does
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Cell structure and organelles
- A sports scientist is studying cells taken from the leg muscle of a marathon runner and from the lining of the runner's cheek. Under the microscope she sees that the muscle cells contain far more mitochondria than the cheek cells.Explain why the muscle cells contain many more mitochondria than the cheek cells.2 marks
- A student compares a cell from the leaf of a pondweed plant with a cell from the lining of a human cheek. The pondweed cell has a rigid, regular outline, large green structures and a large fluid-filled space. The cheek cell is irregular in shape and has none of these features.Describe the function of the chloroplasts and of the cell wall in the pondweed cell.2 marks
- A student wants to find out whether cells in different parts of a plant contain different structures. She makes microscope slides of a green leaf and of a white root from the same potted plant and views both with a light microscope. She notices that the leaf cells contain many green structures but the root cells seem to contain none.State a testable hypothesis about the number of chloroplasts in leaf cells and root cells, with a scientific reason, and identify the independent variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).