1.3Unit 1 · Living Organisms & Cells
Cell Structure — Unit 1 — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 4 parts Lesson 4 of 6 Unit quiz
Lesson summary
Describe the functions of the main cell structures.All cells with a nucleus contain the same genes; differentiation depends on which genes are switched on.Plant cells store starch; animal cells store glycogen.Examples of specialised animal cellsred blood cells, sperm, nerve cells…
- Examples of specialised plant cells
- Similarities and differences
- Importance of cell differentiation
- Stem cells in medicine — advantages
- Stem cells — disadvantages
- Light vs electron microscope
Section 1 of 4Cell Structure — Unit 1
Cell structures:
the nucleus holds DNA and controls the cell; cytoplasm is the site of most reactions; the cell membrane controls entry and exit; mitochondria release energy by aerobic respiration; ribosomes make proteins.- Plant cells also have a cellulose cell wall for support, chloroplasts for photosynthesis and a vacuole containing cell sap.
Similarities and differences:
animal and plant cells share nucleus, cytoplasm, membrane, mitochondria and ribosomes.- Only plant cells have a cell wall, chloroplasts and a large permanent vacuole.
- Animal cells usually have irregular shapes; plant cells are more regular due to the wall.
Importance of cell differentiation:
multicellular organisms need different cells for different jobs.- During development, cells differentiate by expressing only certain genes, producing specialised cells such as nerve cells (long axons) or palisade cells (many chloroplasts).
- This division of labour makes the organism more efficient.
- Most animal cells lose the ability to differentiate early, while many plant cells keep this ability throughout life in meristems at shoot and root tips.
Stem cells in medicine — advantages:
embryonic stem cells can form almost any cell type and could replace damaged cells, e.g. insulin-producing cells for diabetes or nerve cells after spinal injury.- Adult stem cells from bone marrow are already used to treat leukaemia.
Stem cells — disadvantages:
using embryos raises ethical concerns because the embryo is destroyed;- stem cells may divide uncontrollably and form tumours;
- transplanted cells may be rejected by the immune system;
- cultures could transfer viral infections;
- adult stem cells can make only a limited range of cells.
Light vs electron microscope:
Nucleus, cell wall, vacuole and chloroplasts can be seen with a light microscope, but mitochondria and ribosomes are too small and are only visible using an electron microscope.Red blood cell adaptations:
Red blood cells have a biconcave disc shape giving a large surface area to volume ratio for oxygen diffusion. They have no nucleus, leaving more room for haemoglobin, the protein that carries oxygen.Ciliated and root hair cells:
Ciliated cells lining the trachea and bronchi have tiny hairs (cilia) that beat to sweep mucus and trapped particles up towards the throat.- Root hair cells have a long extension that increases surface area for absorbing water and mineral ions.
Stem cells divide by mitosis:
A stem cell can divide by mitosis many times.- Each new cell either stays unspecialised as another stem cell or differentiates into a specialised cell.
- Adult stem cells (e.g. in bone marrow) mainly replace and repair cells such as blood and skin cells.
Avoiding rejection:
Cells grown from donor stem cells carry different antigens, so the patient must take immunosuppressant drugs to prevent rejection.- Using the patient's own stem cells avoids this, because the new cells have the same genes and antigens.
- Bone marrow stem cells can produce red cells, white cells and platelet-forming cells.
Section 2 of 4Process & sequence
- A zygote divides by mitosis to form a ball of identical unspecialised cells.
- Cells receive signals that switch particular genes on or off.
- Only certain genes are expressed, so specific proteins are made.
- The cell changes shape and gains structures suited to its function.
- Specialised cells group into tissues, then organs.
From stem cell to organ — write these steps in this order; each step is a marking point.
- Unspecialised stem cell
- Genes switched on or off
- Specialised cell forms
- Cells form tissue
- Tissues form organ
Section 3 of 4Compare
| Structure | Animal cell | Plant cell |
|---|---|---|
| Nucleus, cytoplasm, membrane | Yes | Yes |
| Mitochondria and ribosomes | Yes | Yes |
| Cellulose cell wall | No | Yes |
| Chloroplasts | No | Yes (in green parts) |
| Large permanent vacuole | No (small temporary vacuoles only) | Yes |
| Source | Can form | Issues |
|---|---|---|
| Embryonic stem cells | Almost any cell type | Ethical objections; risk of rejection |
| Adult stem cells (e.g. bone marrow) | Limited range of cell types | Fewer ethical issues; limited use |
| Plant meristems | Any plant cell type | Used for cloning plants |
Section 4 of 4Worked examples
Explain why a root hair cell does not contain chloroplasts.
- Root hair cells are underground and receive no light, so they cannot photosynthesise.
- The genes for chloroplast development are not expressed, as the cell has differentiated to absorb water and minerals instead.
- Root hair cells are underground and receive no light, so they cannot photosynthesise.
Give one argument for and one against using embryonic stem cells to treat paralysis.
- For: embryonic stem cells can differentiate into nerve cells, which could replace damaged cells and restore movement.
- Against: the embryo is destroyed, which many people think is unethical, and there is a risk of tumour formation or rejection.
- For: embryonic stem cells can differentiate into nerve cells, which could replace damaged cells and restore movement.
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