1.2 Cells as Basic Units of Living Organisms
- Syllabus
- 9700–2028–2029
- Topic
- 1.2
- Level
- AS
Learning objectives
1.2.1Eukaryotic cell organelles• Recognise and describe organelles in eukaryotic cells:- Cell surface membrane- Nucleus, nuclear envelope, nucleolus- Rough and smooth endoplasmic reticulum- Golgi body- Mitochondria (with small circular DNA)- Ribosomes (80S in cytoplasm, 70S in chloroplasts/mitochondria)- Lysosomes- Centrioles and microtubules- Cilia and microvilli- Chloroplasts (with small circular DNA)- Cell wall and plasmodesmata- Large permanent vacuole and tonoplast (plant cells)1.2.2Cell images and drawings• Interpret photomicrographs, electron micrographs and drawings of plant and animal cells1.2.3Plant and animal cell comparison• Compare plant and animal cell structures1.2.4ATP in cells• State that cells use ATP from respiration for energy-requiring processes1.2.5Prokaryotic cell features (bacteria)• Prokaryotic cell features (bacteria):- Unicellular, 1–5 µm diameter- Peptidoglycan cell walls- Circular DNA- 70S ribosomes- Absence of double-membrane organelles1.2.6Prokaryotes vs eukaryotes• Compare prokaryotic and eukaryotic cell structures1.2.7Virus structure and envelope• Virus structure: non-cellular with nucleic acid core (DNA or RNA), protein capsid, some with phospholipid envelope
Eukaryotic organelles divide work between specialised compartments
Eukaryotic cells divide their cytoplasm into membrane-bound organelles. Each organelle has a structure that supports a particular job, while ribosomes carry out protein synthesis but are not membrane-bound.
- Cell surface membrane: partially controls exchange between the cell and its surroundings.
- Nucleus, nuclear envelope and nucleolus: the nucleus contains chromosomes; pores in the envelope regulate traffic; the nucleolus makes ribosomal components.
- Rough and smooth endoplasmic reticulum: rough ER has ribosomes and processes proteins; smooth ER lacks ribosomes and makes lipids.
- Golgi body and vesicles: the Golgi modifies and packages cell products; vesicles transport or store them.
- Mitochondria: folded inner membranes and matrix enzymes support aerobic respiration; mitochondria also contain small circular DNA and 70S ribosomes.
- Ribosomes: 80S ribosomes occur in the eukaryotic cytoplasm; 70S ribosomes occur in mitochondria and chloroplasts; both are sites of translation.
- Lysosomes: hydrolytic enzymes digest worn-out structures, engulfed material and cell debris.
- Centrioles and microtubules: organise microtubules and support cell shape, movement and chromosome movement during division.
- Cilia: move substances across the cell surface.
- Microvilli: increase cell-surface area for absorption.
- Cell wall: provides support outside the cell-surface membrane; plant walls contain cellulose.
- Chloroplasts: thylakoid membranes and grana absorb light for photosynthesis; chloroplasts contain small circular DNA and ribosomes.
- Plasmodesmata: cytoplasmic bridges that allow transfer between neighbouring plant cells.
- Large permanent vacuole and tonoplast: the vacuole stores cell sap and supports the plant cell; the tonoplast is its partially permeable membrane.
A structure–function link is not the same as an exclusive identity: not every eukaryotic cell contains every organelle, and centrioles are not found in flowering plants and fungi. Do not describe a ribosome as membrane-bound or treat a cell wall as the membrane controlling exchange.
Read a cell image from visible evidence, not from a memorised diagram
Interpreting a plant or animal cell image means using visible evidence to identify plausible structures. A photomicrograph or electron micrograph records a specimen, while a drawing selects and simplifies what is shown.
- Identify the source: photomicrograph, electron micrograph or drawing, and note any scale bar or magnification.
- Find the cell boundary and describe observations such as relative size, shape, position, contrast and repeated patterns.
- Match several clues to a structure: for example, a large dark region may be a nucleus, a folded internal membrane may be a mitochondrion, and a large clear compartment in a plant cell may be a vacuole.
- Use the whole pattern to distinguish a plant cell from an animal cell, including a wall, chloroplasts or a large vacuole when visible.
- State the identification as an inference from the evidence; do not add structures that are hidden, unresolved or absent from the drawing.
Several polygonal cells with a regular outer boundary and large clear internal spaces support the inference of plant cells with cell walls and vacuoles. The conclusion is stronger because multiple features agree, not because one shape was memorised.
A mitochondrion is not always cylindrical: section angle and specimen condition can change its appearance. Image resolution, staining, contrast and the drawing convention can hide structures, so distinguish what is visible from what is inferred.
Plant and animal cells share a core plan but specialise differently
Plant and animal cells are both eukaryotic, so they share a core plan of membrane, cytoplasm, nucleus, mitochondria, endoplasmic reticulum, Golgi bodies, vesicles or lysosomes, ribosomes and microtubules. Their differences reflect how each cell gains support, materials and energy.
| Feature | Plant cells | Animal cells |
|---|---|---|
| Cell surface membrane and cytoplasm | Present; membrane controls exchange | Present; membrane controls exchange |
| Nucleus, mitochondria, ER, Golgi, vesicles/lysosomes, ribosomes and microtubules | Shared eukaryotic structures | Shared eukaryotic structures |
| Cellulose cell wall | Present; provides rigid support outside the membrane | Absent |
| Large permanent vacuole | Typical; stores cell sap and supports the cell | Not typical; animal vacuoles, when present, are small and temporary |
| Chloroplasts | Present in photosynthetic cells; absorb light for photosynthesis | Absent |
| Plasmodesmata | Connect neighbouring plant-cell cytoplasm | Absent |
| Centrioles and microvilli | Not typical of flowering-plant cells | Typical comparison features; centrioles organise division machinery and microvilli increase absorptive surface area |
The wall and vacuole help a plant cell keep its shape, chloroplasts capture light energy, and plasmodesmata permit transfer between neighbouring plant cells. Animal cells rely more on a flexible surface membrane and external support, while specialised animal cells may use centrioles or microvilli for particular jobs.
Use the table as a comparison of typical structures, not as a rule for every specialised cell: a leaf cell may contain chloroplasts, while a mature red blood cell lacks a nucleus. Do not infer cell type from one feature or from shape alone.
ATP transfers usable energy immediately where a cell needs it
ATP is the immediate energy-transfer molecule used by cells. When ATP is hydrolysed to ADP and inorganic phosphate (Pi), energy is released and can be coupled to an energy-requiring process.
Respiration continually supplies energy to rebuild ATP from ADP + Pi. ATP then carries that immediately usable energy to cellular machinery; hydrolysis releases it at the site of work, and the ADP and Pi can be recycled into ATP. This links energy release in respiration to work elsewhere in the cell without treating glucose as the direct fuel for each process.
ATP from respiration is used for energy-requiring processes such as:
- building larger molecules in anabolic reactions;
- moving substances across membranes by active transport or within the cell;
- muscle contraction and nerve impulse transmission in animals.
Glucose is broken down in respiration to help make ATP, but ATP—not glucose—is the immediate energy source used directly by cellular processes. ATP is a short-term transfer molecule, not the cell’s main long-term energy store.
A prokaryote keeps its genome compact without a nucleus
A prokaryote is a unicellular cell whose genetic material is not enclosed in a nucleus and whose cytoplasm has no double-membrane organelles.
The syllabus feature hierarchy is:
- Size: typically 1–5 µm in diameter.
- Cell wall: made of peptidoglycan.
- DNA: a circular DNA molecule lies in the cytoplasm rather than inside a nucleus.
- Ribosomes: 70S ribosomes are present for protein synthesis.
- Organelles: double-membrane organelles such as a nucleus or mitochondrion are absent.
This compact organisation distinguishes a bacterial cell from a eukaryotic cell: the DNA is not separated into a nuclear compartment, and respiration or other processes are not housed in double-membrane organelles. Size supports recognition, but the structural features provide the stronger identification.
No nucleus does not mean no DNA, and a cell wall does not by itself prove that a cell is prokaryotic. Do not add a capsule, flagellum or plasmid to every bacterium: those features are not universal requirements of this objective.
The prokaryote–eukaryote contrast is about organisation, not just size
Prokaryotes and eukaryotes both have a cell-surface membrane, cytoplasm, DNA and ribosomes. The key structural contrast is that eukaryotes organise DNA and many cell processes inside membrane-bound organelles, whereas prokaryotes do not.
| Structural feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Cell organisation | Usually unicellular; no membrane-bound organelles | May be unicellular or multicellular; membrane-bound organelles present |
| Genetic material | Circular DNA in the cytoplasm, not enclosed by a nucleus | Linear chromosomes inside a nucleus |
| Ribosomes | 70S | 80S in the cytoplasm; 70S in mitochondria and chloroplasts |
| Typical size | Usually about 1–5 µm in diameter | Often larger; plant cells may be about 10–100 µm across |
| Cell wall, when present | Peptidoglycan | Cellulose in plants or chitin in fungi |
The eukaryotic nucleus and other compartments keep different reactions in controlled local conditions. A prokaryote’s simpler organisation leaves its circular DNA in the cytoplasm and has no double-membrane organelles. These are structural patterns to compare together, not a single size test.
Both cell types contain DNA, ribosomes and a membrane. A large bacterium can still be prokaryotic, and a unicellular yeast cell can still be eukaryotic; identify the cell type from DNA location, ribosome type and organelle organisation rather than apparent size alone.
All viruses have a nucleic-acid core and protein capsid
A virus is a non-cellular structure, not a prokaryotic or eukaryotic cell. Every virus has a nucleic-acid core containing either DNA or RNA, surrounded by a protein coat called a capsid.
| Viral component | Present in which viruses? | Material |
|---|---|---|
| nucleic-acid core | all viruses | DNA or RNA |
| capsid | all viruses | protein |
| outer envelope | some viruses only | phospholipids |
The core carries genetic information and the capsid surrounds it. When an outer envelope is present, it lies outside the capsid. Because a virus is non-cellular, it does not have the cellular organisation used to classify bacteria, plant cells or animal cells.
A virus contains either DNA or RNA, not both as its genetic core, and an envelope is optional rather than universal. ‘Non-cellular’ is the required structural boundary; it does not make a virus a type of prokaryotic cell.