Ch 2 · 1.2 Cells as the basic units of living organisms · AS & A Level Biology 9700
Topic 2 of 2 in Cell structure — AS & A Level Biology 9700 — 74 lessons.
Introduction
Cells are the basic structural and functional units of all living organisms, and Cambridge examiners expect the ultrastructure revealed by electron microscopy to be recalled precisely, organelle by organelle, for both animal and plant cells. This lesson also compares eukaryotic cells with the much simpler prokaryotic cells of bacteria, and contrasts both with viruses, which are not cells at all. (Syllabus section 1.2.)
Key ideas — Ultrastructure of an animal cell
An electron micrograph of a typical animal cell shows a nucleus bounded by a double nuclear envelope with pores, containing one or more dense nucleoli; rough endoplasmic reticulum (RER), a folded membrane system studded with ribosomes; smooth endoplasmic reticulum (SER), a similar but ribosome-free tubular system; a stack of flattened sacs forming the Golgi body, often with vesicles budding from its edges; numerous mitochondria with a folded inner membrane; free ribosomes scattered in the cytoplasm; membrane-bound lysosomes; a pair of centrioles, small cylindrical structures near the nucleus that organise the spindle fibres during cell division; and a cell surface membrane enclosing the whole cell. Animal cells have no cell wall, chloroplasts or large permanent vacuole, and centrioles are usually seen only in animal cells.
🧑🏫 Teacher's explanation
Think of a cell as a highly organized factory. The nucleus is the administrative office holding the blueprints (DNA), while the mitochondria are the power plant providing the electricity (ATP) needed for every machine to run. The endoplasmic reticulum and Golgi body act as the assembly lines and packaging department, ensuring proteins are folded and shipped to the correct destination.
📖 Definitions
🔍 Worked example — Easy
A student observes an electron micrograph of a cell. They identify a structure with a double membrane, inner folds called cristae, and contains its own 70S ribosomes and circular DNA. Identify this organelle.
🔍 Worked example — Standard
Calculate the magnification of an image of a mitochondrion if the actual diameter of the mitochondrion is 2 mum and the diameter in the micrograph is 40 mm.
🔍 Worked example — Exam-style
Compare the structure of a typical bacterium with a typical animal cell regarding their genetic material and ribosomes.
⚠️ Common errors
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🎯 Exam question
Key ideas — Ultrastructure of a plant cell
A plant cell shows all the structures above, plus features unique to plants.
🧑🏫 Teacher's explanation
Think of a plant cell like a high-security warehouse. The cell wall is the thick, rigid outer brick wall that keeps the building's shape. Inside, the large vacuole is like a massive central water tank that pushes everything else against the walls to keep the structure firm. The chloroplasts are the solar panels on the roof, converting light into energy. While animal cells are more like flexible, mobile tents, plant cells are built like sturdy, stationary buildings to support their height and weight.
📖 Definitions
🔍 Worked example — Easy
Identify three structures present in a typical plant cell but absent in a typical animal cell.
🔍 Worked example — Standard
A student observes an electron micrograph of a cell. They identify a double-membrane organelle containing stacks of membranes (grana) and circular DNA. Identify this organelle and state its function.
🔍 Worked example — Exam-style
Compare the structure of a typical bacterium with a typical plant cell regarding their genetic material and ribosomes.
⚠️ Common errors
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🎯 Exam question
(a) Describe two features that would allow the student to distinguish this plant cell from an animal cell. [2] (b) The student observes a small, circular piece of DNA inside the chloroplast. State the name of this DNA. [1] [3 marks]
Key ideas — Functions: the nucleus and the membrane systems
🧑🏫 Teacher's explanation
Think of a cell as a highly organized factory. The nucleus is the CEO's office, holding the master blueprints (DNA) and issuing instructions. The cell surface membrane is the security gate, deciding which raw materials enter and which finished products exit. The endoplasmic reticulum (ER) and Golgi body act as the assembly lines and packaging department, ensuring proteins are folded correctly and wrapped in protective bubbles (vesicles) before being shipped to their destination.
📖 Definitions
🔍 Worked example — Easy
A student observes an electron micrograph of a cell. They identify a large, central organelle surrounded by a single membrane called a tonoplast. Identify this organelle and state its function in plant cells.
🔍 Worked example — Standard
Compare the ribosomes found in the cytoplasm of a eukaryotic cell with those found in a typical bacterium.
🔍 Worked example — Exam-style
An electron micrograph shows a series of flattened, membrane-bound sacs that are not studded with ribosomes. Describe the likely function of this organelle and the type of molecules it processes.
⚠️ Common errors
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🎯 Exam question
Key ideas — Functions: energy, protein synthesis and movement
🧑🏫 Teacher's explanation
Think of a cell as a highly organized factory. To keep the factory running, you need power (ATP), specialized machines to build products (ribosomes), and a logistics system to move goods around (microtubules and cilia).
📖 Definitions
🔍 Worked example — Easy
A student observes an electron micrograph of a cell. They identify a structure that is a site of protein synthesis and contains 70S ribosomes. Identify the two possible organelles this could be.
🔍 Worked example — Standard
Compare the structure of a typical bacterium with a typical eukaryotic animal cell regarding their genetic material and ribosomes.
🔍 Worked example — Exam-style
An electron micrograph shows a cell with numerous folds in the cell surface membrane, significantly increasing the surface area for absorption. Name this structure and provide one biological example of where it is found.
⚠️ Common errors
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🎯 Exam question
(a) State two structural features that distinguish this bacterium from a typical plant cell. [2] (b) Describe the structure of a virus. [3] [5 marks]
Key ideas — Prokaryotic cells compared with eukaryotic cells
Eukaryotic cells have a nucleus enclosed by a nuclear envelope, membrane-bound organelles, linear DNA associated with histones, and 80S ribosomes; they are typically 10–100 μm across. Prokaryotic cells have no nucleus or membrane-bound organelles, a single circular loop of DNA lying free in the cytoplasm (not associated with histones), smaller 70S ribosomes, and are typically only 1–5 μm across.
🧑🏫 Teacher's explanation
Think of a eukaryotic cell like a large, highly organized factory. It has specialized rooms (organelles) like a manager's office (nucleus) and specialized assembly lines (ER/Golgi) to keep everything efficient. Because it is so large and complex, it needs internal walls to separate different chemical reactions.
📖 Definitions
🔍 Worked example — Easy
A student observes an electron micrograph of a cell. The scale bar represents 5 μ m. The cell measures 40 μ m in length. Calculate the magnification of the micrograph.
🔍 Worked example — Standard
Compare the ribosomes found in a typical eukaryotic animal cell with those found in a typical bacterium.
🔍 Worked example — Exam-style
A virus consists of a DNA core, a protein capsid, and a phospholipid envelope. Describe the structure of this virus and state whether it is cellular.
⚠️ Common errors
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🎯 Exam question
Key ideas — Structure of a bacterium
A bacterium is unicellular, generally 1–5 µm in diameter, and has no organelles surrounded by a double membrane (no nucleus, mitochondria or chloroplasts). It has a cell wall made of peptidoglycan (murein), which lies outside a cell surface membrane; a cytoplasm containing many 70S ribosomes; a single circular chromosome of DNA forming a nucleoid region, not enclosed by any envelope; and often one or more small circular plasmids carrying additional genes. Many bacteria also have a protective outer capsule and thread-like flagella used for motility.
🧑🏫 Teacher's explanation
Think of a bacterium as a tiny, self-contained studio apartment. Unlike a large mansion (a eukaryotic cell) which has separate rooms for cooking, sleeping, and laundry (organelles like the nucleus or mitochondria), a bacterium is one open-plan space. Everything happens in one room: the DNA sits in the corner, and the machinery for making proteins is scattered throughout. Because it is so small and simple, it doesn't need complex internal transport systems; it just relies on simple diffusion and efficient, small-scale organization to survive.
📖 Definitions
🔍 Worked example — Easy
A bacterium is observed under an electron microscope. If the actual diameter of the bacterium is 2 mum and the diameter shown in the micrograph is 40 mum, calculate the magnification.
🔍 Worked example — Standard
Compare the structure of the genetic material in a typical bacterium with that of a typical eukaryotic animal cell.
🔍 Worked example — Exam-style
An electron micrograph shows a cell with a cell wall made of peptidoglycan, 70S ribosomes, and no visible nucleus. Determine the type of cell and justify your answer using three specific structural features.
⚠️ Common errors
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🎯 Exam question
Key ideas — Comparing viruses with cells
Viruses are not cells: they have no cytoplasm, no ribosomes, no cell surface membrane and no metabolism of their own. All viruses are non-cellular. A virus consists only of a core of nucleic acid, either DNA or RNA, surrounded by a protein coat called a capsid; some viruses also have an outer envelope made of phospholipids, taken from the host cell's membrane. Viruses are far smaller than bacteria (typically 20–300 nm) and can only replicate by infecting a living host cell and hijacking its ribosomes and enzymes.
🧑🏫 Teacher's explanation
Think of a cell like a fully functioning, self-sustaining factory. It has walls (cell membrane), a management office (nucleus), machines (ribosomes), and its own power supply (mitochondria). It can take in raw materials and produce goods independently.
📖 Definitions
🔍 Worked example — Easy
A bacterium has a diameter of 2 μ m. A typical virus has a diameter of 50 nm. Calculate how many times larger the bacterium is than the virus.
🔍 Worked example — Standard
Compare the genetic material and protein content of a typical eukaryotic cell with a typical virus.
🔍 Worked example — Exam-style
Explain why a virus is described as 'obligate intracellular parasites' based on its structural limitations.
⚠️ Common errors
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🎯 Exam question
(a) State two structural features that distinguish a virus from a bacterium. [2]
Worked example
Question. An electron micrograph shows a mitochondrion 30 mm long at a magnification of ×20 000. A nearby ribosome on the same micrograph measures 0.5 mm across. Calculate the actual length of the mitochondrion and the actual diameter of the ribosome, in nanometres. [4]
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