Cell structure and organellesIB MYP Sciences: Revision notes
Section 1
Cells and organelles
All living things are made of cells, the smallest units of life. Each cell contains small structures called organelles, and each organelle has its own job. Animal cells and plant cells both have a nucleus, cytoplasm, a cell membrane and mitochondria. Plant cells have extra structures that animal cells lack.
Section 2
Parts of an animal cell
- The nucleus holds the cell's DNA and controls the cell's activities.
- The cytoplasm is a jelly-like liquid where most chemical reactions happen.
- The cell membrane is a thin layer that controls what enters and leaves the cell.
- Mitochondria are where aerobic respiration happens. This releases energy from glucose using oxygen.
Cells that need lots of energy, such as muscle cells, contain many mitochondria.
Mitochondria do not 'make energy'. Energy cannot be made. They release energy from glucose during aerobic respiration.
Section 3
Extra parts of a plant cell
Plant cells have three extra structures:
- The cell wall is made of cellulose. It supports the cell and gives it a fixed shape.
- Chloroplasts contain chlorophyll and are where photosynthesis happens. They are found in the green parts of a plant, such as leaves.
- The permanent vacuole is filled with cell sap. It keeps the cell firm.
Root cells have no chloroplasts because they are underground, where there is no light.
The cell wall and the cell membrane are different. The wall is rigid and gives support. The membrane is thin and controls what passes in and out.
Section 4
Plant and animal cells compared
Both have: a nucleus, cytoplasm, a cell membrane and mitochondria.
Only plant cells have: a cell wall, chloroplasts and a permanent vacuole.
Animal cells have no cell wall, so they have no fixed shape. They are usually smaller and more irregular. Plant cells often have a regular shape because of the cell wall.
Section 5
Bacterial cells
A bacterial cell is much smaller than a plant or animal cell and has no nucleus. Its DNA is found free in the cytoplasm, often as a loop. It has a cell membrane and a cell wall.
Bacteria such as the ones in yoghurt are single-celled organisms.
If a question says 'no nucleus', think bacteria. Say the DNA is free in the cytoplasm.
Must know
- Nucleus: holds DNA, controls the cell
- Cytoplasm: where reactions happen
- Cell membrane: controls what enters and leaves
- Mitochondria: aerobic respiration
- Cell wall (cellulose): support; chloroplasts: photosynthesis; permanent vacuole: cell sap
- Bacterial cells have no nucleus
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Cell structure and organelles
- A biology class in Nairobi compares cells from three sources under a microscope: cells from a cheek swab, cells from a leaf of pondweed, and bacteria from a sample of yoghurt.Outline two ways in which the structure of a bacterial cell from the yoghurt differs from that of a cheek cell.2 marks
- Researchers compare cells from a long-distance runner's thigh muscle with skin cells from her arm. The muscle cells need a large, continuous supply of energy for running, and under the microscope they contain many more mitochondria than the skin cells.Explain why the runner's muscle cells contain many more mitochondria than her skin cells.2 marks
- A horticulturalist in the Netherlands grows lettuce. On a hot day the unwatered lettuce leaves wilt and droop, but after watering they become firm again. She examines a lettuce leaf cell and a cheek cell and notes the differences in their structures.Describe the function of each of these structures in a lettuce leaf cell: the cell wall, the chloroplasts and the permanent vacuole.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).