Cell Structure
BiologyΒ· 2.2β2.6B (2017 Issue 3)Β· 12 min read
1. Key Cell Organelles: Structure and Functionβ β ββββ± 4 min
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All living cells are made up of small, specialised structures called organelles, each with a specific role in keeping the cell alive and functioning. You are required to know the structure and function of 8 core organelles for your exam.
Organelle
A specialised sub-structure inside a cell that carries out a specific function.
Organelle | Function |
|---|---|
Nucleus | Contains genetic material (chromosomes/DNA) and controls all cell activities |
Cytoplasm | Gel-like substance where most chemical reactions in the cell take place |
Cell membrane | Partially permeable layer that controls which substances enter and leave the cell |
Cell wall | Rigid cellulose layer surrounding plant cells, provides support and strength |
Mitochondria | Site of aerobic respiration, where energy is released for the cell to use |
Chloroplasts | Found in green plant cells, contain chlorophyll and are the site of photosynthesis |
Ribosomes | Tiny structures in the cytoplasm, site of protein synthesis |
Vacuole | Large permanent structure in plant cells, contains cell sap and maintains cell turgor |
Name the organelle responsible for protein synthesis, and state one organelle found only in plant cells.
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Step 1: Recall organelle functions. Protein synthesis takes place in ribosomes.
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Step 2: Identify plant-only organelles: valid answers include cellulose cell wall, chloroplasts, or large permanent vacuole.
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Final answer: Ribosomes; chloroplast (or other valid plant-only organelle)
2. Plant vs Animal Cell Comparisonβ β ββββ± 3 min
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You will often be asked to identify differences and similarities between plant and animal cells in exam questions. Mark schemes require you to explicitly state which structures plant cells have that animal cells do not.
Shared Features (Both Cell Types) | Plant Cell Only Features |
|---|---|
Nucleus | Cellulose cell wall |
Cytoplasm | Chloroplasts |
Cell membrane | Large permanent vacuole |
Mitochondria | |
Ribosomes |
A student observes a cell under a microscope and sees a cell wall, mitochondria, and a large vacuole. State if this is a plant or animal cell, and give two reasons for your answer.
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Step 1: Identify unique features: cell wall and large permanent vacuole are only found in plant cells.
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Step 2: Confirm shared features: mitochondria are present in both cell types, so they do not contradict the conclusion.
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Final answer: Plant cell. Reasons: It has a cellulose cell wall and a large permanent vacuole, which are not found in animal cells.
3. Cell Differentiation (Biology Only, Paper 2)β β β ββHL onlyβ± 3 min
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Cell differentiation
The process by which unspecialised stem cells change structure and function to become specialised cells adapted for a specific role.
Differentiation is essential for the development of complex multicellular organisms: it allows cells to form specialised tissues (e.g. muscle, nerve, leaf palisade) that carry out specific functions, rather than all cells performing the same general role.
Explain why cell differentiation is important for the development of a human embryo.
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Step 1: Define differentiation: it is the process where unspecialised cells become adapted for specific functions.
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Step 2: Link to embryo development: an embryo starts as a single fertilised egg cell that must differentiate to form all the specialised cell types the body needs, e.g. nerve cells, muscle cells, blood cells.
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Final answer: Cell differentiation allows unspecialised embryonic cells to develop into all the specialised cell types required for a fully functional human body, including nerve, muscle and blood cells.
4. Stem Cells in Medicine (Biology Only, Paper 2)β β β ββHL onlyβ± 4 min
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Stem cells
Unspecialised cells that can divide to produce more unspecialised cells, or differentiate into other types of specialised cell.
Advantages: Can differentiate into almost any cell type, so can be used to replace damaged or diseased cells (e.g. insulin-producing cells for diabetes, nerve cells for spinal cord injuries), can be used to test new drugs without testing on human patients, and can reduce the need for animal testing.
Disadvantages: Embryonic stem cells are taken from unused embryos from IVF treatment, which raises ethical objections as some people believe embryos have a right to life, risk of immune rejection if stem cells are not a genetic match for the patient, risk of stem cells dividing uncontrollably and forming tumours in the patient's body.
Evaluate the use of stem cells to treat patients with paralysis caused by spinal cord damage.
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Step 1: State an advantage: Stem cells can differentiate into nerve cells to replace the damaged nerve cells in the spinal cord, potentially restoring movement for patients with no other treatment options.
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Step 2: State a disadvantage: If embryonic stem cells are used, there are ethical objections to using embryos for medical treatment. There is also a risk the new cells could be rejected by the patient's immune system, or form tumours.
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Final balanced conclusion: While stem cell treatment has the potential to cure paralysis, ethical concerns and clinical risks mean it is not yet a widely available treatment.
5. Common Pitfalls
Wrong move:
Calling the cell membrane "semi-permeable" or "fully permeable"
Why:
Edexcel mark schemes only accept the term "partially permeable" for the cell membrane
Correct move:
Always use the exact phrase "partially permeable" when describing the cell membrane's function
Wrong move:
Stating that animal cells have large permanent vacuoles
Why:
Animal cells only have small temporary vacuoles if any; large permanent vacuoles are unique to plant cells
Correct move:
When comparing plant and animal cells, explicitly state that plant cells have a large permanent vacuole while animal cells do not
Wrong move:
Only describing advantages of stem cells when asked to evaluate their use
Why:
Evaluation questions require a balanced response covering both pros and cons to get full marks
Correct move:
Always include at least one advantage and one disadvantage of stem cell use in evaluation answers
Wrong move:
Omitting ribosomes or vacuoles from lists of cell organelles
Why:
The specification requires you to know all 8 organelles, including ribosomes and vacuoles, which are commonly forgotten
Correct move:
Memorise the full list of 8 organelles: nucleus, cytoplasm, cell membrane, cell wall, mitochondria, chloroplasts, ribosomes, vacuole
Wrong move:
Stating that cell walls are found in animal cells
Why:
Cell walls made of cellulose are only present in plant cells, not animal cells
Correct move:
When listing features of animal cells, explicitly exclude cell walls, chloroplasts and large permanent vacuoles
6. Quick Reference Cheatsheet
Topic | Key Exam Facts |
|---|---|
Organelle Functions | Nucleus=controls cell; Mitochondria=respiration; Chloroplasts=photosynthesis; Ribosomes=protein synthesis; Cell membrane=partially permeable; Cell wall=cellulose support; Vacuole=cell sap/turgor; Cytoplasm=reaction site |
Plant vs Animal Cells | Shared: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes. Plant only: cell wall, chloroplasts, large permanent vacuole |
Cell Differentiation (B only) | Process where stem cells become specialised; essential for multicellular organism development |
Stem Cells (B only) | Advantages: treat disease, replace damaged cells. Disadvantages: ethical objections (embryos), rejection risk, tumour risk |
7. Frequently Asked
Is the cell membrane semi-permeable or partially permeable?
For Edexcel IGCSE Biology exams, you must use the term partially permeable when describing the cell membrane. 'Semi-permeable' is not accepted in official mark schemes.
What features are unique to plant cells?
Plant cells have three features never found in animal cells: 1. Cellulose cell wall, 2. Chloroplasts, 3. Large permanent vacuole (animal cells only have small temporary vacuoles if any).
Do I need to learn stem cell content for Paper 1?
No: content points 2.5B (cell differentiation) and 2.6B (stem cells) are only assessed on Paper 2 (4BI1/2B) for single award Biology students. Double award students do not need to revise these topics.
Going deeper
What's Next
Now that you have mastered cell structure, you are ready to move on to studying the organisation of cells into tissues, organs and organ systems, which builds on your knowledge of specialised cell function. You will also explore how substances move across cell membranes via diffusion, osmosis and active transport, which relies on your understanding of the partially permeable cell membrane. For Paper 2 Biology students, your next step is to learn about specialised cell adaptations, which links directly to the cell differentiation content covered in this guide. Make sure you practice past paper questions on cell structure to familiarise yourself with mark scheme expectations.