2b Cell Structure¶
Part of 2 Structure and Functions in Living Organisms.
What a cell contains decides what it can do. The comparisons that matter are between plant and animal cells and, for Biology-only students, between unspecialised cells, specialised cells and stem cells.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
The main organelles and cell structures in plant and animal cells, what each one does, and how the two kinds of cell differ. Biology-only students also learn how cells become specialised and how stem cells are used.
Organelles and Cell Structures¶
Organelles are the specialised structures inside a cell, and they are sometimes called sub-cellular structures. Five are found in both plant and animal cells:
| Structure | Function |
|---|---|
| Nucleus | Contains the genetic material (DNA), which codes for specific proteins, and is enclosed in a nuclear membrane |
| Cytoplasm | A jelly-like liquid in which many chemical reactions of metabolism occur, and which contains enzymes and the other organelles |
| Cell membrane | A partially permeable membrane surrounding the cell that controls what enters and leaves the cell |
| Mitochondria | Where aerobic respiration reactions occur, releasing energy for the cell |
| Ribosomes | Where protein synthesis takes place, found in the cytoplasm |
Plant cells also contain three structures that animal cells lack:
| Structure | Function |
|---|---|
| Chloroplasts | The site of photosynthesis. They contain the green pigment chlorophyll, which absorbs light energy |
| Permanent vacuole | Contains cell sap and helps keep the cell rigid by maintaining turgor |
| Cell wall | Made of cellulose, and gives the cell structural strength |
The number of mitochondria in a cell shows how much energy it uses. Muscle and sperm cells have many, because contraction and swimming need a large supply of energy from aerobic respiration.
Compare Plant and Animal Cells¶
Use the interactive below to compare the shared structures and the plant-only structures side by side. Open full interactive.
Plant Cells and Animal Cells¶
| Feature | Plant cell | Animal cell |
|---|---|---|
| Nucleus, cytoplasm, cell membrane, mitochondria, ribosomes | Present | Present |
| Cell wall | Cellulose wall | None |
| Chloroplasts | Present in cells exposed to light | None |
| Permanent vacuole | Large | None |
| Shape | Usually regular and box-like | Usually irregular or rounded |
| Carbohydrate store | Starch | Glycogen |
The differences follow from how the two kinds of organism live. The cellulose wall supports the cell and gives it a regular shape. Animal cells have no wall, so they are usually more irregular. Plants make their own food by photosynthesis, so they need chloroplasts, and animals do not.
Exam technique
A question about a structure usually needs its name and its function, so write both. The cell membrane controls what enters and leaves, and the cell wall supports the cell, so do not swap them. Describe ribosomes simply as structures in the cytoplasm where proteins are made.
Biology-Only Content¶
This content is required for Biology-only students and is not required for Combined Science students.
Specialised Cells¶
Cells become specialised through differentiation, in which a cell gains the organelles and structures suited to its job. In animals, most cells differentiate early in development and then keep their specialised form. In plants, many cells keep the ability to differentiate throughout the life of the plant.
| Cell | Structural adaptation | How it helps |
|---|---|---|
| Sperm cell | Streamlined head, long tail, many mitochondria, acrosome with enzymes | Swims to the egg, and the enzymes digest the egg's outer layers so the sperm can enter |
| Nerve cell (neurone) | Long axon, branching dendrites, many mitochondria at the nerve endings | Carries electrical impulses quickly over long distances and connects with many other neurones, and the mitochondria supply energy to make neurotransmitter |
| Muscle cell | Proteins (actin and myosin) that slide over each other, many mitochondria, glycogen store | Shortens to cause movement, with the energy needed for contraction |
| Root hair cell | Long extension of the cell, large permanent vacuole, mitochondria | A large surface area absorbs water and mineral ions, the vacuole draws water in by osmosis, and the mitochondria supply energy for active transport of ions |
| Xylem cell | Walls strengthened with lignin, cells hollow and dead, pits in the walls | The hollow cells join into a continuous tube that carries water and mineral ions upwards, and water can move sideways through the pits |
| Phloem cell | End walls with holes (sieve plates), companion cells alongside | Sucrose and amino acids flow from cell to cell, and the companion cells supply the energy the phloem cells need |
Worked example: explaining an adaptation
A good explanation links the feature, its effect and the function: feature → what it does → why that helps the job.
Sperm cells have many mitochondria. Mitochondria release energy by aerobic respiration. The tail needs that energy to swim to the egg.
Stem Cells¶
A stem cell is an undifferentiated cell that can divide to make many more cells, some of which then differentiate into specialised cell types. Stem cells are essential for development, growth and repair.
- Embryonic stem cells come from the early embryo, which forms when the fertilised egg first divides. They can differentiate into any type of cell in the body. Scientists can culture them and direct their differentiation, which could allow replacement of insulin-producing cells in diabetes, neural cells in Alzheimer's disease, or nerve cells in spinal cord injuries.
- Adult stem cells are found mainly in bone marrow. They can produce many cell types, particularly blood cells, but a narrower range than embryonic stem cells. Bone marrow transplants already use them to treat blood cancers such as leukaemia.
- Meristems in plants are found in root and shoot tips. They can differentiate into any plant cell type and keep this ability throughout the plant's life. They are used to make clones of plants with useful features, and to conserve rare species.
Specialisation and Stem Cells¶
Differentiation turns an unspecialised cell into a specialised one, and stem cells are the cells that keep the ability to do this. That is why they are useful in medicine, where damaged tissue needs replacing. The therapy also has limits, which is why questions on it ask for both sides.
| Benefits | Risks |
|---|---|
| Can replace damaged cells in conditions such as type 1 diabetes, multiple sclerosis and spinal cord injuries | Ethical concerns over destroying unused embryos |
| Bone marrow transplants can treat blood-cell cancers such as leukaemia | No guarantee of long-term success or absence of side-effects |
| Whole organs might eventually be grown for transplant | Mutations could arise in cultured stem cells |
| If derived from the patient's own cells, rejection is less likely | Suitable donors are difficult to find |
| Could allow drug testing without animal testing |
Exam technique
A question asking you to evaluate stem-cell use wants at least one benefit and one risk, each with a reason. A bare list of points is weaker than "stem cells can replace damaged nerve cells, but cultured cells may mutate, so the treatment may not be safe".
Common Confusions¶
- Stem cells vs all cells: Not every cell can differentiate repeatedly. Stem cells are unusual because they keep the ability to divide and specialise.
- Cell wall vs cell membrane: The membrane controls exchange with the surroundings. The wall supports the cell, and it does not replace the membrane.
- Ribosome location at this level: Ribosomes are structures found in the cytoplasm where proteins are made.
- Xylem vs phloem: Xylem transports water and mineral ions upwards. Phloem transports dissolved sugars and amino acids in both directions.
Check Yourself¶
- State the function of the nucleus and of the mitochondria.
- State three structures found in plant cells but not in animal cells.
- A student says that the cell wall controls what enters and leaves a plant cell. Explain what is wrong with this.
- Explain why plant cells are usually more regular in shape than animal cells.
- (Biology only) Explain how the structure of a root hair cell helps it absorb mineral ions.
- (Biology only) Give one benefit and one risk of treating a patient with stem cells.
Answers
- The nucleus contains the genetic material (DNA) that codes for proteins. Mitochondria are where aerobic respiration reactions occur, releasing energy.
- Any three of: cell wall (cellulose), chloroplasts, permanent vacuole.
- The cell membrane does that, because it is partially permeable. The cell wall is made of cellulose and supports the cell, and it does not control what passes in or out.
- Plant cells have a rigid cellulose wall, which supports the cell and gives it a regular, box-like shape. Animal cells have no wall, so their shape is usually more irregular.
- The long root hair extension gives a large surface area. The mitochondria release energy for the active transport of mineral ions into the cell.
- Benefit: stem cells can replace damaged cells, for example in spinal cord injuries or diabetes. Risk: cultured stem cells might mutate, or there are ethical concerns about destroying embryos, or the treatment may not work in the long term.
Key Terms¶
- Nucleus: the organelle that contains the cell's genetic material and controls cell activities.
- Cytoplasm: the jelly-like substance filling the cell in which chemical reactions occur.
- Cell membrane: a partially permeable membrane surrounding the cell that controls what enters and leaves the cell.
- Ribosome: the site of protein synthesis.
- Mitochondrion: the organelle where most aerobic respiration occurs.
- Chloroplast: the organelle that contains chlorophyll and carries out photosynthesis.
- Vacuole: a fluid-filled space in plant cells that contains cell sap and supports the cell.
- Differentiation: the process by which a cell becomes specialised for a particular function.
- Stem cell: an undifferentiated cell that can divide and differentiate into specialised cell types.
- Lignin: a strengthening material deposited in xylem cell walls.
- Sieve plate: the perforated end wall of a phloem cell through which substances can flow.
- Meristem: a region of actively dividing stem cells in plants, found at root and shoot tips.