🧬 Nucleus
The nucleus contains the genetic material, which controls the activities of the cell. Remember, the cell's DNA is kept inside it. Because it holds the instructions, it is often called the control centre of the cell.
Cell Structure » Animal and Plant Cells
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.1.1.2
- The parts found in most animal cells and what each one does
- The extra parts found in plant and algal cells
- What plasmids do in bacterial cells
- How to use estimation to compare the size of sub-cellular structures
Parts of an animal cell
Every part inside a cell is called a sub-cellular structure. Each one has a job, and the job is linked to how the structure is built. Most animal cells have five of these parts.
Key terms:
- Sub-cellular structure: a part inside a cell that does a particular job.
- Mitochondria: the structures where aerobic respiration takes place. (One of them is a mitochondrion.)
- Ribosomes: tiny structures where proteins are made.
💧 Cytoplasm
This is where most of the chemical reactions in the cell happen. It is a watery jelly, so substances can move around in it and take part in reactions.
🔒 Cell membrane
The cell membrane controls which substances move into and out of the cell. It lets some things through and keeps others out or in, a bit like a security gate.
⚡ Mitochondria
Aerobic respiration takes place in the mitochondria. This process releases the energy that the cell needs to carry out its jobs.
🧱 Ribosomes
Ribosomes are where the cell makes proteins. Proteins are needed for growth and repair and for many jobs in the cell, so ribosomes are very important.
Extra parts in plant and algal cells
Plant cells have all the parts of an animal cell. In addition, plant cells often have chloroplasts and a permanent vacuole. Plant and algal cells also have a cell wall made of cellulose. Algae are simple organisms such as seaweed, and algal cells have a cellulose cell wall too.
Key terms:
- Chloroplasts: green structures where photosynthesis takes place.
- Chlorophyll: the green substance in chloroplasts that absorbs light.
- Permanent vacuole: a space in the cell filled with cell sap.
- Cell sap: a liquid containing dissolved substances, found in the permanent vacuole.
- Cellulose: the tough material that the cell wall is made from.
🌿 Chloroplasts
Photosynthesis takes place in the chloroplasts. They contain chlorophyll, which absorbs the light energy that photosynthesis needs. This is why the green parts of plants can make food.
📦 Permanent vacuole
The permanent vacuole is a large space in the cell that is filled with cell sap. Cell sap is a liquid with substances dissolved in it.
🧱 Cell wall
The cell wall is made of cellulose. It strengthens the cell and supports it, so the cell keeps its shape.
Common mistakes
Many students say that only plant cells have a cell membrane. This is wrong. Every cell has a cell membrane. Plant cells have a cell wall as well, and the wall is on the outside. Another mistake is to say that animal cells have no mitochondria. Most animal cells do have them, and plant cells have them too, because plants also respire.
Plasmids in bacterial cells
Bacterial cells are different from plant and animal cells. Remember, a bacterial cell may have one or more plasmids.
The job of a plasmid is to carry extra genes. These extra genes can help the bacterium to survive. For example, a plasmid can carry a gene that makes the bacterium resistant to an antibiotic.
Sub-cellular structures at a glance
Structure and function
Nucleus: contains genetic material and controls the cell.
Cytoplasm: where most chemical reactions happen.
Cell membrane: controls what enters and leaves.
Mitochondria: aerobic respiration.
Ribosomes: protein synthesis.
Chloroplasts (plant cells): photosynthesis.
Permanent vacuole (plant cells): filled with cell sap.
Cell wall (plant and algal cells): made of cellulose, strengthens the cell.
Plasmids (bacteria): carry extra genes.
Using estimations to compare sizes
Sub-cellular structures are far too small to see without a microscope, so we often work from a drawing or a micrograph. We do not always need an exact measurement. An estimation is a quick, sensible approximate answer. We use estimations to judge the relative size or area of sub-cellular structures.
You should use an estimation when:
- you only need to know roughly how much bigger one structure is than another
- an exact measurement is not possible, for example because the edges are blurred
- you want to check that an answer from a calculation is sensible
Round the numbers to make them easy, then compare them.
Worked example
On a drawing, a cell is 62 mm wide and its nucleus is 20 mm wide.
Round to easy numbers: 60 mm and 20 mm.
60 ÷ 20 = 3, so the cell is about 3 times wider than its nucleus.
Area works differently. If the two shapes are roughly the same shape, a cell that is 3 times wider has about 3 × 3 = 9 times the area. So the nucleus takes up only about 1/9 of the area of the cell, as a rough estimate.
Common mistakes
Students often compare widths and then call the answer an area. Width and area are different. If one structure is 3 times wider than another and they are roughly the same shape, it is about 9 times bigger in area, not 3 times. Also, an estimation is not a wild guess. Always round the numbers sensibly and show your working.
Exam-style question
The diagram shows a plant cell. It has a nucleus, a cell wall, chloroplasts and a permanent vacuole.
(a) Give the function of the chloroplasts. [1 mark]
(b) Name the substance that the cell wall is made from and give its function. [2 marks]
(c) State one part found in a plant cell that is also found in an animal cell, and give its function. [2 marks]
(d) A student draws a plant cell 90 mm wide. The nucleus on her drawing is 9 mm wide. Estimate how many times wider the cell is than the nucleus. [1 mark]
Model answer
(a) Photosynthesis takes place in them. (1)
(b) Cellulose (1). It strengthens the cell (1).
(c) Any one pair, for example: nucleus (1), contains the genetic material and controls the cell's activities (1). Or: mitochondria (1), where aerobic respiration takes place (1). Or: cell membrane (1), controls what moves into and out of the cell (1).
(d) 90 ÷ 9 = 10 times wider. (1)
Exam tip
In a "give the function" question, say what the part does, not just what it is. "Chloroplasts are green" earns nothing, but "photosynthesis takes place in them" earns the mark.
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