2.2Structures and Functions in Living Organisms
Cell Structure — Edexcel Linear - OL Revision Notes & Topical Questions
10 min read 12 parts Lesson 17 of 25 1 quiz
Cell Structure, Plant and Animal Cells and Specialised Cells — EDEXCEL LINEAR - OL notes: Describe the functions of cell structures, compare plant and animal cells, and explain how cells differentiate and are adapted to their functions.Cells are surrounded by a cell membrane, whi…
- organelles
- Cell differentiation
- Worked example 1
- Worked example 2
- Worked example 3
- Worked example 4
Section 1 of 12Structures in cells and their functions
Cells are surrounded by a cell membrane, which separates them from the external environment.
| Structure | Function |
|---|---|
| Nucleus | Contains genetic material (chromosomes) |
| Cytoplasm | Site of many chemical reactions; contains water and solutes |
| Cell membrane | Controls substances entering and leaving the cell |
| Cell wall (plants) | Cellulose; gives extra support and shape, prevents bursting |
| Mitochondria | Site of aerobic respiration |
| Chloroplasts | Site of photosynthesis; contain chlorophyll |
| Ribosomes | Site of protein synthesis |
| Permanent vacuole | Contains cell sap; supports plant cells keeping them firm (turgid) |
Section 2 of 12Animal and plant cells compared
| Structure | Animal cell | Plant cell |
|---|---|---|
| Nucleus, cytoplasm, cell membrane | Yes | Yes |
| Mitochondria, ribosomes | Yes | Yes |
| Cell wall | No | Yes (cellulose) |
| Chloroplasts | No | Yes (in leaves and stems) |
| Vacuole | Small and temporary | Large and permanent |
A plant cell showing chloroplasts would be found in the leaves of a plant.
Section 3 of 12Cell differentiation and specialised cells
Cell differentiation is the process by which a cell changes to become specialised, developing the structure and subcellular structures it needs for its function.
| Specialised cell | Adaptation | Function |
|---|---|---|
| Ciliated cell | Hair-like cilia that beat | Move mucus and trapped particles in the trachea and bronchi towards the throat |
| Nerve cell | Long, with extensions | Conduct impulses over long distances |
| Red blood cell | Biconcave shape, no nucleus, packed with haemoglobin | Transport oxygen efficiently |
| Root hair cell | Long extension, thin wall | Absorb water and minerals with a large surface area |
| Palisade mesophyll cell | Column-shaped, many chloroplasts, packed beneath the upper epidermis | Photosynthesis |
Section 4 of 12Using the microscope and reading cell diagrams
A light microscope shows the nucleus, cytoplasm, cell membrane, cell wall, chloroplasts and the vacuole, but not ribosomes or the detailed structure of mitochondria, which need an electron microscope with much higher magnification and resolution.
- Cell size in plants is usually larger than in animal cells, and the regular shape of plant cells is due to the cell wall.
When comparing cells, always give a similarity before a difference, for example: both animal and plant cells have a nucleus, but only plant cells have a cell wall. Always link an adaptation to its function, such as many mitochondria in a sperm tail for energy.
Section 5 of 12Worked examples
State the function of the mitochondria. [1]1 mark
The site of aerobic respiration, releasing energy.
Give three structures found in plant cells but not in animal cells. [3]3 marks
Cell wall, chloroplasts and a permanent vacuole.
Explain how a red blood cell is adapted to carry oxygen. [3]3 marks
It is biconcave giving a large surface area to volume ratio, has no nucleus giving room for haemoglobin, and is packed with haemoglobin which binds oxygen.
Define differentiation. [1]1 mark
The process by which a cell becomes specialised for a particular function.
Explain why palisade cells contain many chloroplasts. [2]2 marks
Chloroplasts contain chlorophyll that absorbs light for photosynthesis; many of them give a high rate of photosynthesis.
Section 6 of 12What are stem cells?
A stem cell is an undifferentiated cell of an organism that can divide by mitosis an unlimited number of times.
| Embryonic stem cells | Adult stem cells | |
|---|---|---|
| Source | Embryos | Some tissues and organs in animals, e.g. bone marrow |
| Potential | Can differentiate into all the different cell types | Can differentiate into a limited number of cell types |
Section 7 of 12Stem cell therapy
Stem cells can be used to treat illnesses where cells in the body are not functioning properly and need replacing or repairing: stem cell therapy.
Example human blood cells are produced from adult stem cells in the bone marrow, which can differentiate only into blood cells: red blood cells, white blood cells and platelet-forming cells.
Stem cells from a healthy donor’s bone marrow can be transplanted into a patient’s bone marrow.
- so the patient must take immunosuppressant drugs to prevent rejection.
- so no rejection occurs and no immunosuppressants are needed.
Section 8 of 12Evaluating the use of stem cells
| Benefits | Risks and concerns |
|---|---|
| Can replace damaged cells and treat diseases that are otherwise hard to treat | Embryonic cells need embryos, which raises ethical objections |
| Embryonic stem cells can form any cell type | Donor cells may be rejected unless immunosuppressants are used |
| Patient’s own cells avoid rejection | Risk of infection or of uncontrolled division (tumours) |
| Adult stem cells avoid destroying embryos | Adult stem cells form only limited types of cell; treatments can be expensive |
You do not need to recall specific diseases that can be treated with stem cells, but you do need to describe what a stem cell is, the differences between embryonic and adult stem cells, and the advantages and disadvantages of their use. The main issue with embryonic stem cells is that they can only be sourced from embryos, which many people consider unethical.
Section 9 of 12Stem cells in context
Stem cells in a developing embryo form all the specialised cells of the body.
Section 10 of 12Worked examples
Define a stem cell. [2]2 marks
An undifferentiated cell that can divide by mitosis repeatedly and can differentiate into specialised cells.
Give one difference between embryonic and adult stem cells. [1]1 mark
Embryonic stem cells can become any cell type; adult stem cells only a limited number of cell types.
Explain why a patient receiving donor bone marrow stem cells needs immunosuppressant drugs. [2]2 marks
Donor cells carry different antigens, so the patient’s immune system would reject them.
Give one ethical objection to using embryonic stem cells. [1]1 mark
Obtaining them destroys embryos, which some people regard as potential human lives.
Describe one use of stem cells from bone marrow. [2]2 marks
They can form red blood cells to treat anaemia, white blood cells to improve immunity, or platelets for clotting problems.
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Organelle | A structure inside a cell with a specific function. |
| Mitochondrion | Organelle where aerobic respiration takes place. |
| Chloroplast | Organelle where photosynthesis takes place. |
| Differentiation | The process of a cell becoming specialised. |
| Specialised cell | A cell with adaptations for a particular function. |
| Turgid | Firm because the vacuole is full of water. |
| Term | Meaning |
|---|---|
| Stem cell | An undifferentiated cell that can divide by mitosis and differentiate. |
| Embryonic stem cell | A stem cell from an embryo that can become any cell type. |
| Adult stem cell | A stem cell in some adult tissues that can form a limited number of cell types. |
| Stem cell therapy | Using stem cells to replace or repair faulty cells. |
| Antigen | A protein on the cell surface that the immune system can recognise. |
| Immunosuppressant | A drug that reduces the immune response to prevent rejection. |
Section 12 of 12Exam technique
Marked practiceQuizzes for this lesson
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Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.