Overview
Cell structure is one of the most important topics in GCSE Biology. All living organisms are made of cells, and different cells have different structures that allow them to carry out specific functions.
Understanding cell structure helps explain how organisms grow, survive and reproduce.
Cell Structure Summary
What is a Cell?
A cell is the basic unit of life.
Some organisms contain only one cell (unicellular), while others contain many cells (multicellular).
Examples:
- Bacteria are unicellular.
- Humans are multicellular.
- Plants are multicellular.
Animal Cells
Animal cells contain several important structures called sub-cellular structures.
| Structure | Function |
|---|---|
| Nucleus | Controls the activities of the cell and contains genetic material (DNA) |
| Cytoplasm | Where most chemical reactions occur |
| Cell Membrane | Controls the movement of substances in and out of the cell |
| Mitochondria | Site of aerobic respiration and energy release |
| Ribosomes | Site of protein synthesis |
Plant Cells
Plant cells contain all the structures found in animal cells plus extra structures.
| Structure | Function |
|---|---|
| Cell Wall | Supports and strengthens the cell |
| Vacuole | Contains cell sap and helps keep the cell rigid |
| Chloroplasts | Site of photosynthesis |
Bacterial Cells
Bacteria are prokaryotes.
They are much smaller and simpler than plant and animal cells.
Bacterial cells contain:
- Cell membrane
- Cytoplasm
- Cell wall
- Circular DNA
Some bacteria also contain:
- Plasmids
- Flagella
Bacteria do not have:
- A nucleus
- Mitochondria
- Chloroplasts
Eukaryotic and Prokaryotic Cells
Eukaryotic Cells
Examples:
- Animal cells
- Plant cells
Characteristics:
- Have a nucleus
- Larger and more complex
Prokaryotic Cells
Examples:
- Bacteria
Characteristics:
- No nucleus
- DNA is free in the cytoplasm
- Smaller and simpler
Microscopy
Microscopes allow scientists to observe cells.
Types:
Light Microscopes
- Used in schools and laboratories
- Produce less detailed images
Electron Microscopes
- Much higher magnification
- Higher resolution
- Show very small structures in detail
Magnification
Formula
Magnification = Image Size ÷ Actual Size
Rearranged Formula
Actual Size = Image Size ÷ Magnification
Make sure all measurements are in the same units before calculating.
Specialised Cells
Specialised cells are adapted for specific functions.
Sperm Cell
Adaptations:
- Tail for movement
- Many mitochondria for energy
- Acrosome contains enzymes to penetrate the egg
Nerve Cell
Adaptations:
- Long structure to carry impulses
- Branched ends to connect with other cells
Muscle Cell
Adaptations:
- Contains many mitochondria
- Can contract to produce movement
Root Hair Cell
Adaptations:
- Large surface area
- Absorbs water and mineral ions efficiently
Cell Differentiation
Differentiation is the process by which a cell becomes specialised.
Animals
Most animal cells differentiate early in development.
Plants
Plant cells can continue differentiating throughout life.
Stem Cells
Stem cells are unspecialised cells that can:
- Divide by mitosis
- Differentiate into specialised cells
Sources of Stem Cells
- Embryos
- Adult bone marrow
- Umbilical cord blood
Potential Uses
- Treat paralysis
- Treat diabetes
- Replace damaged tissues
Key Differences to Remember
| Animal Cell | Plant Cell |
|---|---|
| No cell wall | Cell wall present |
| No chloroplasts | Chloroplasts present |
| No permanent vacuole | Large permanent vacuole |
| Eukaryotic Cell | Prokaryotic Cell |
|---|---|
| Has nucleus | No nucleus |
| Larger | Smaller |
| More complex | Simpler |
GCSE-Style Questions
Question 1 (1 Mark)
Name the structure that controls the activities of a cell.
Question 2 (1 Mark)
Which organelle is the site of protein synthesis?
Question 3 (2 Marks)
State two structures found in plant cells but not in animal cells.
Question 4 (2 Marks)
Give two features of bacterial cells.
Question 5 (3 Marks)
Explain the function of mitochondria.
Question 6 (3 Marks)
Describe the role of the cell membrane.
Question 7 (4 Marks)
Compare plant cells and animal cells.
Question 8 (4 Marks)
Explain how a sperm cell is adapted for its function.
Question 9 (6 Marks)
Describe the differences between prokaryotic and eukaryotic cells.
Question 10 (6 Marks)
Explain why electron microscopes can provide more detailed images than light microscopes.
Question 11 (8 Marks)
A student measures the image of a cell as 40 mm. The actual size of the cell is 0.04 mm.
Calculate the magnification and explain your working.
Question 12 (8 Marks)
Describe how a root hair cell is adapted for absorbing water and mineral ions.
Question 13 (9 Marks)
Evaluate the use of stem cells in treating disease.
Question 14 (12 Marks)
Explain the importance of cell differentiation in multicellular organisms.
Question 15 (2 Marks)
What is meant by the term specialised cell?
Question 16 (3 Marks)
Explain why bacteria are classified as prokaryotes.
Question 17 (4 Marks)
Describe the function of chloroplasts.
Question 18 (6 Marks)
Explain the importance of ribosomes in cells.
Question 19 (8 Marks)
Compare the structures found in bacterial cells and animal cells.
Question 20 (12 Marks)
A scientist discovers a new cell containing cytoplasm, ribosomes, a cell membrane and circular DNA, but no nucleus.
Use this information to identify the type of cell and explain your answer.
Answers
Question 1 (1 Mark)
Recommended Length: 1 word
The nucleus.
Question 2 (1 Mark)
Recommended Length: 1 word
Ribosomes.
Question 3 (2 Marks)
Recommended Length: 2 short points
- Cell wall
- Chloroplasts
(A large permanent vacuole would also gain credit.)
Question 4 (2 Marks)
Recommended Length: 2 short points
- They do not have a nucleus.
- Their DNA is found freely in the cytoplasm.
Question 5 (3 Marks)
Recommended Length: 2 to 3 sentences
Mitochondria are the site of aerobic respiration. Respiration releases energy for the cell. Cells that need lots of energy contain many mitochondria.
Question 6 (3 Marks)
Recommended Length: 2 to 3 sentences
The cell membrane controls the movement of substances into and out of the cell. It acts as a selectively permeable barrier. This helps maintain suitable conditions inside the cell.
Question 7 (4 Marks)
Recommended Length: 4 linked points
Both plant and animal cells contain a nucleus, cytoplasm, cell membrane and ribosomes. Plant cells also contain a cell wall. Plant cells contain chloroplasts for photosynthesis. Plant cells usually have a large permanent vacuole.
Question 8 (4 Marks)
Recommended Length: 4 linked points
The sperm cell has a tail to swim towards the egg. It contains many mitochondria to provide energy. The acrosome contains enzymes that help penetrate the egg cell. It also carries genetic information in its nucleus.
Question 9 (6 Marks)
Recommended Length: 5 to 6 sentences
Eukaryotic cells contain a nucleus whereas prokaryotic cells do not. Prokaryotic DNA is free in the cytoplasm. Eukaryotic cells are generally larger and more complex. Plant and animal cells are eukaryotic. Bacteria are prokaryotic. Prokaryotes may also contain plasmids.
Question 10 (6 Marks)
Recommended Length: 5 to 6 sentences
Electron microscopes use electrons instead of light. Electrons have a much shorter wavelength. This gives a much higher resolution. Scientists can see much smaller structures. Images are more detailed than those produced by light microscopes. Therefore very small cell components can be observed.
Question 11 (8 Marks)
Recommended Length: Full calculation and explanation
Magnification = Image Size ÷ Actual Size
Magnification = 40 mm ÷ 0.04 mm
Magnification = 1000
Answer = ×1000
The image is 1000 times larger than the actual cell.
Question 12 (8 Marks)
Recommended Length: 7 to 8 sentences
Root hair cells absorb water and mineral ions from the soil. They have a long hair-like projection which increases surface area. A larger surface area increases absorption. They have a thin cell wall which reduces the distance substances travel. Many mitochondria provide energy for active transport. This allows mineral ions to be absorbed even when concentrations are low.
Question 13 (9 Marks)
Recommended Length: 8 to 9 sentences
Stem cells can develop into specialised cells. They may be used to replace damaged tissues. This could help treat diseases such as paralysis or diabetes. Stem cells offer the possibility of repairing organs. However, embryonic stem cell research raises ethical concerns. Some people believe embryos should not be used in this way. There are also risks of unwanted cell division. Adult stem cells avoid many ethical issues. Overall, stem cells have significant medical potential but also present ethical and scientific challenges.
Question 14 (12 Marks)
Recommended Length: 10 to 12 sentences
Cell differentiation allows cells to become specialised. Different cell types perform different functions. Muscle cells contract to create movement. Nerve cells transmit electrical impulses. Root hair cells absorb water and minerals. Specialisation makes organisms more efficient. It allows tissues and organs to develop. Organs can then work together in organ systems. Without differentiation, complex multicellular organisms could not function properly. Growth and development depend on differentiation. Repair processes also rely on specialised cells. Therefore differentiation is essential for survival.
Question 15 (2 Marks)
Recommended Length: 1 sentence
A specialised cell is a cell adapted to carry out a particular function.
Question 16 (3 Marks)
Recommended Length: 2 to 3 sentences
Bacteria are classified as prokaryotes because they do not have a nucleus. Their genetic material is free within the cytoplasm. They are simpler than eukaryotic cells.
Question 17 (4 Marks)
Recommended Length: 4 sentences
Chloroplasts are the site of photosynthesis. They contain chlorophyll. Chlorophyll absorbs light energy. This energy is used to produce glucose.
Question 18 (6 Marks)
Recommended Length: 5 to 6 sentences
Ribosomes are the site of protein synthesis. They use genetic instructions from DNA. Proteins are required for growth and repair. Enzymes are proteins made at ribosomes. Cells need proteins to function correctly. Therefore ribosomes are essential structures.
Question 19 (8 Marks)
Recommended Length: 7 to 8 sentences
Both bacterial and animal cells contain cytoplasm, a cell membrane and ribosomes. Bacterial cells have circular DNA. Animal cells contain DNA inside a nucleus. Animal cells contain mitochondria. Bacteria do not contain mitochondria. Bacterial cells are smaller and simpler. Some bacteria contain plasmids and flagella. These structures are not found in typical animal cells.
Question 20 (12 Marks)
Recommended Length: 10 to 12 sentences
The cell is most likely a bacterial cell. The presence of circular DNA suggests a prokaryotic cell. The absence of a nucleus is a key feature of bacteria. Bacteria contain cytoplasm, a cell membrane and ribosomes. Their DNA is not enclosed within a nucleus. Prokaryotic cells are generally smaller and simpler than eukaryotic cells. Plant and animal cells both possess a nucleus. Therefore the cell cannot be a plant or animal cell. The evidence matches the characteristics of a bacterial cell. Consequently it should be classified as a prokaryote.
