(b) Cell structure
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
2.2Cell structuresDescribe cell structures, including the nucleus, cytoplasm, cell membrane, cell wall, mitochondria, chloroplasts, ribosomes and vacuole.2.3Cell structure functionsDescribe the functions of the nucleus, cytoplasm, cell membrane, cell wall, mitochondria, chloroplasts, ribosomes and vacuole.2.4Plant and animal cell structureKnow the similarities and differences in the structure of plant and animal cells.25B Cell differentiationExplain the importance of cell differentiation in the development of specialised cells.26B Stem cells in medicineUnderstand the advantages and disadvantages of using stem cells in medicine.
Recognise the main cell structures
A cell contains distinct structures that can be described by their position, boundary and appearance before their functions are considered.
| Structure | Description |
|---|---|
| nucleus | a compartment in the cytoplasm that contains chromosomes |
| cytoplasm | the material filling the inside of the cell |
| cell membrane | a thin boundary enclosing the cytoplasm |
| cell wall | a rigid layer outside the cell membrane in plant cells |
| mitochondria | small structures distributed through the cytoplasm |
| chloroplasts | chlorophyll-containing structures in photosynthetic plant cells |
| ribosomes | very small structures in the cytoplasm |
| vacuole | a fluid-filled space; mature plant cells usually have a large central vacuole containing cell sap |
The cell wall and cell membrane are separate layers: the wall lies outside the membrane. Ribosomes are much smaller than nuclei, chloroplasts and mitochondria, so a simple light-microscope drawing may not show every structure.
Do not infer that every plant cell contains chloroplasts: only photosynthetic plant cells require them. A vacuole is a space within the cell, whereas cytoplasm surrounds the cell structures.
Match cell structures to their functions
Each cell structure has a specific job, and the abundance of a structure reflects how strongly the cell depends on that job.
| Structure | Function |
|---|---|
| nucleus | contains genetic material and controls cell activities |
| cytoplasm | site of many metabolic reactions |
| cell membrane | controls movement of substances into and out of the cell |
| cell wall | supports the cell and helps maintain its shape |
| mitochondria | site of aerobic respiration, which releases energy for cell activities |
| chloroplasts | absorb light using chlorophyll and carry out photosynthesis |
| ribosomes | site of protein synthesis |
| vacuole | contains cell sap and helps support a plant cell when full |
A cell with a high energy demand often contains many mitochondria. A palisade cell exposed to light contains many chloroplasts, while a root cell underground usually contains none because it does not photosynthesise.
Mitochondria release energy by respiration; they do not create energy. Ribosomes are the site of protein synthesis, while the nucleus supplies genetic instructions rather than assembling proteins itself.
Compare plant and animal cells
Plant and animal cells share the core structures needed for cellular control and metabolism, while plant cells have additional structures for support and photosynthesis.
| Structure | Plant cell | Animal cell |
|---|---|---|
| nucleus | present | present |
| cytoplasm | present | present |
| cell membrane | present | present |
| mitochondria | present | present |
| ribosomes | present | present |
| cellulose cell wall | present | absent |
| chloroplasts | present in photosynthetic cells | absent |
| large permanent vacuole containing cell sap | usually present | absent |
The rigid wall and large vacuole often give a plant cell a more regular outline. An animal cell has only a membrane at its outer boundary, so its outline can be less regular.
Cell shape alone is not enough for identification. Use the combination of wall, chloroplasts and large permanent vacuole, and remember that a non-photosynthetic plant cell may lack chloroplasts but still has a cellulose wall.
Explain why cells differentiate
Differentiation is the process by which an unspecialised cell develops the structures and abilities needed for a particular function.
An unspecialised cell divides, then daughter cells can differentiate into specialised types. Their structures become suited to different jobs; groups of specialised cells can therefore form tissues and organs with a division of labour.
| Without differentiation | With differentiation |
|---|---|
| cells remain similar and cannot perform the full range of body functions efficiently | different cell types perform particular jobs efficiently |
| complex tissues and organs cannot develop normally | specialised cells combine into working tissues and organs |
A stem cell is unspecialised and can continue to divide. If it differentiates into a red blood cell, it develops a specialised role in oxygen transport; if it differentiates into a muscle cell, it develops the ability to contract.
Differentiation is not ordinary growth: a cell changes what it is specialised to do, rather than merely becoming larger. Most specialised animal cells have a more limited range of possible future cell types than stem cells.
Evaluate stem cells in medicine
Stem cells can divide repeatedly and differentiate, so medicine can use them to replace damaged or missing specialised cells.
| Potential advantage | Corresponding disadvantage or condition |
|---|---|
| can produce specialised cells to repair damaged tissue | differentiation must be controlled so the required cell type forms |
| repeated division can supply many replacement cells | uncontrolled division may form a tumour |
| a patient's own cells are genetically matched, reducing immune rejection | donor cells may be rejected and may require immunosuppressant treatment |
| donor or embryonic cells may provide options when a patient's cells are unsuitable | transplanted material can carry infection; using embryonic cells raises ethical objections for some people |
For a damaged heart, stem cells may divide and differentiate into replacement muscle cells. Bone-marrow stem cells can form blood-cell types and may therefore help treat some blood conditions.
A justified medical decision weighs likely benefit against tumour, rejection, infection and ethical risks for the particular cell source. A possible treatment is not automatically safe or effective simply because stem cells can differentiate.