1.2 Cells as Basic Units of Living Organisms

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.

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.

1. Identify the source: photomicrograph, electron micrograph or drawing, and note any scale bar or magnification.
2. Find the cell boundary and describe observations such as relative size, shape, position, contrast and repeated patterns.
3. 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.
4. Use the whole pattern to distinguish a plant cell from an animal cell, including a wall, chloroplasts or a large vacuole when visible.
5. 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 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.

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 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 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.

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.

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.

A virus is a non-cellular infectious particle, not a prokaryotic or eukaryotic cell. Its basic structure is genetic material—a DNA or RNA core—inside a protein capsid; some viruses also have a phospholipid envelope.

The capsid protects the viral genome. If an envelope is present, it is derived from host-cell membrane phospholipids and carries attachment proteins. Those proteins bind a suitable host-cell receptor, allowing the virus to deliver its genome and use the host cell’s machinery to make new virus particles. Without a host cell, the particle cannot reproduce independently.

• Nucleic-acid core: contains either DNA or RNA.
• Capsid: protein coat surrounding and protecting the core.
• Envelope: present in some viruses only; a phospholipid outer layer acquired from a host cell.
• Scale and boundary: virus particles are much smaller than typical cells and have no cytoplasm, ribosomes or independent metabolism.

Do not call a virus a cell or a living organism, and do not assume every virus has an envelope. Its ability to reproduce by using host-cell machinery is the key boundary from cellular organisms.