A2.2 Cell structure

Question 1

A2.2.1 Cells as the basic structural unit of all living organisms

Answer 1

Cells as the basic unit
Every living organism is made up of one or more cells; either as unicellular (a single cell performing all life functions) or multicellular (many specialized cells working together).

All key life processes—such as metabolism, growth, reproduction, and response to stimuli—occur within cells or are coordinated by them.

NOS: deductive reasoning and cell theory
Cell theory includes the idea that all living things are composed of cells and that new cells arise from pre‑existing cells.

Using deductive reasoning, if a newly discovered organism is confirmed to be alive, cell theory allows us to predict that it must consist of one or more cells, even before it is examined in detail.

Question 2

A2.2.2 Microscopy skills

Answer 2

Actual size

Core practical skills (AOS)
Making temporary mounts:

Place a thin layer of specimen (e.g., onion peel, human cheek cells, plant tissue) on a clean slide, add a drop of water or stain, and gently lower a coverslip to avoid bubbles.

Staining:

Use stains such as iodine or methylene blue to make cell structures (nuclei, cell walls) easier to see.

Focusing:

Use the coarse adjustment to bring the specimen into approximate focus and the fine adjustment to sharpen the image at higher magnifications.

Measuring and using scale
Eyepiece graticule:

A scale inside the eyepiece is calibrated against a stage micrometer so that each division corresponds to a known length (e.g., micrometres).

Calculate actual size and magnification:

= measured size on graticule
×
× calibration factor;

image size
actual size
=
actual size
image size
​
.

Producing a scale bar:

Draw a bar on your diagram labelled with the real length (e.g., 10 μm), derived from your graticule measurement and magnification.

Taking photographs:

Capture clear images (via microscope camera or phone) and annotate them with labels, magnification, and a scale bar.

NOS: measurements as quantitative observation
Using a microscope and eyepiece graticule to measure cell size is a form of quantitative observation, because it produces numerical data (lengths in micrometres, magnification values) rather than just qualitative descriptions.

This links directly to the Nature of Science (NOS) idea that measurement with instruments turns visual observations into objective, comparable data

Question 3

A2.2.3 Developments in microscopy

Answer 3

Electron microscopy
Electron microscopes use beams of electrons instead of light, giving much higher resolution and magnification than light microscopes (down to around 0.1–0.2 nm), so viruses, organelles, and even some macromolecular complexes become visible.

This allows detailed 3D views of internal structures (e.g., mitochondria, rough ER, nuclear pores) and surfaces, greatly improving understanding of cell ultrastructure.

Freeze‑fracture electron microscopy
In freeze‑fracture electron microscopy, cells or membranes are rapidly frozen, fractured, and coated with metal to create a replica viewed under TEM.

This technique reveals the 3D arrangement and distribution of membrane proteins and lipids within the bilayer with minimal distortion, especially useful for studying membrane organisation.

Cryogenic electron microscopy (cryo‑EM)
Cryo‑EM flash‑freezes biological samples in a thin layer of vitreous (glass‑like) ice and images them with electrons at very low temperatures.

It allows high‑resolution 3D structures of proteins, viruses, and large complexes in near‑native states, without heavy chemical fixation, and has enabled near‑atomic‑level views of biological machines.

Fluorescent stains and immunofluorescence
Fluorescent stains (e.g., fluorescent dyes) bind to specific structures (DNA, cytoskeleton) and emit light when excited by certain wavelengths, making those structures visible in a fluorescence light microscope.

Immunofluorescence uses antibodies tagged with fluorophores to bind specific proteins; this lets researchers see the precise location and distribution of individual proteins within cells or tissues, greatly improving functional and spatial analysis.

Question 4

A2.2.4 Structures common to cells in all living things

Answer 4

DNA as genetic material
All typical cells use DNA as their genetic material because it can store large amounts of information in a stable, replicable form and can be accurately copied and passed on to daughter cells.

DNA also provides the instructions for building proteins, which carry out most cellular functions.

Cytoplasm as an aqueous medium
The cytoplasm is mainly water, which acts as a solvent for ions, metabolites, and enzymes, allowing metabolic reactions to occur efficiently.

Its fluid nature also lets dissolved substances diffuse and facilitates movement of vesicles and organelles within the cell.

Plasma membrane made of lipids
The plasma membrane, composed mainly of phospholipids, forms a selectively permeable barrier that separates the cell’s internal environment from the outside.

Its lipid bilayer structure allows small non‑polar molecules to cross by simple diffusion while restricting ions and polar molecules, helping to maintain concentration gradients essential for life processes.

Question 5

A2.2.5 Prokaryote cell structure

Answer 5

Core prokaryotic components
Cell wall:

Made mainly of peptidoglycan, forming a thick, rigid layer outside the plasma membrane that gives shape and protection.

In Gram‑positive bacteria, the peptidoglycan layer is especially thick and sits just outside the plasma membrane.

Plasma membrane:

A phospholipid bilayer that encloses the cytoplasm and controls what enters and leaves the cell.

It is the site of many metabolic processes, including respiration‑related electron transport in some prokaryotes.

Cytoplasm:

A gel‑like, aqueous matrix containing enzymes, metabolites, and ribosomes, where most metabolic reactions occur.

Naked DNA in a loop:

The genetic material is a single circular DNA molecule (the nucleoid) that is not enclosed by a nuclear membrane and is not bound to histones.

This “naked” DNA loop lies freely in the cytoplasm, often associated with the plasma membrane.
70S ribosomes:

Prokaryotes have 70S ribosomes (smaller than the 80S ribosomes of eukaryotes), which synthesise proteins in the cytoplasm.

Variation in prokaryote structure
Although A2.2.5 focuses on the Gram‑positive eubacterial type (Bacillus, Staphylococcus), students should recognise that prokaryote cell structure varies (e.g., Gram‑negative bacteria have a thinner peptidoglycan layer plus an outer membrane).

However, detailed differences such as the lack of a cell wall in mycoplasmas and phytoplasmas are not required for this objective.

Question 6

A2.2.6 Eukaryote cell structure

Answer 6

Basic eukaryotic layout
A plasma membrane encloses the cell, surrounding a compartmentalized cytoplasm where many organelles and 80S ribosomes are suspended.

The 80S ribosomes (larger than 70S prokaryotic ribosomes) synthesise proteins in the cytoplasm and on rough endoplasmic reticulum.

Nucleus and chromosomes
The nucleus houses the genetic material and is surrounded by a double membrane (nuclear envelope) perforated by nuclear pores that control movement of molecules.

Inside, chromosomes consist of DNA bound to histone proteins, forming chromatin that condenses during cell division.

Membrane‑bound organelles
Typical membrane‑bound organelles include:

Mitochondria: sites of aerobic respiration and ATP production.

Endoplasmic reticulum (ER): rough ER (with ribosomes) synthesises proteins; smooth ER synthesises lipids and detoxifies substances.

Golgi apparatus: modifies, sorts, and packages proteins and lipids into vesicles for transport.

Vesicles and vacuoles: transport or store materials; lysosomes contain hydrolytic enzymes for intracellular digestion.

Cytoskeleton
The cytoskeleton is a network of protein filaments including:

Microtubules (involved in cell shape, chromosome movement in mitosis, and vesicle transport), and

Microfilaments (involved in cell movement, cytokinesis, and muscle contraction)

Question 7

A2.2.7 Processes of life in unicellular organisms

Answer 7

Key life processes in unicellular organisms
Homeostasis: The cell maintains a stable internal environment (e.g., pH, ion concentrations, water balance) despite changes in the external environment.

Metabolism: All chemical reactions (such as respiration and biosynthesis) occur inside the cell to obtain energy and build molecules.

Nutrition: The cell takes in nutrients (e.g., by endocytosis, diffusion, or active transport) and uses them for energy and growth.

Movement: Many unicellular organisms move actively (e.g., using flagella, cilia, or pseudopodia) to find food, avoid danger, or respond to gradients.

Excretion: Metabolic waste products (such as CO₂ or urea analogues) are removed, often by diffusion or vesicle‑mediated exocytosis.

Growth: The cell increases in size and synthesises new organelles and macromolecules before dividing.

Response to stimuli: The cell detects changes (e.g., light, chemicals, temperature) and reacts appropriately (e.g., moving toward or away).

Reproduction: The cell reproduces, usually by binary fission (bacteria) or mitosis (many protists), producing genetically identical offspring.

Question 8

A2.2.8 Differences in eukaryotic cell structure between animals, fungi and plants

Answer 8

Cell walls
Plant cells have a rigid cell wall made mainly of cellulose, providing structural support and helping maintain turgor.

Fungal cells also have a cell wall, but it is composed mainly of chitin (not cellulose).

Animal cells lack a cell wall; they are surrounded only by the plasma membrane and extracellular matrix.

Vacuoles
Plant cells typically have a large central vacuole that stores water, ions, pigments, and waste, and helps maintain turgor pressure.

Fungal cells often have smaller, multiple vacuoles that store nutrients and waste.

Animal cells have smaller, temporary vacuoles or vesicles (for storage, transport, or digestion), but no large central vacuole.

Chloroplasts and plastids
Plant cells contain chloroplasts (green plastids for photosynthesis) and may have other plastids (e.g., amyloplasts for starch storage).

Fungal and animal cells lack chloroplasts and plastids; they are heterotrophic and do not perform photosynthesis.

Centrioles, cilia, and flagella
Animal cells usually have centrioles (involved in organising microtubules during mitosis) and may have cilia or flagella in some specialized cells (e.g., sperm cells).

Fungal and plant cells generally lack centrioles and typical cilia or flagella, though some lower fungi or algae may have flagellated reproductive cells.

Question 9

A2.2.9 Atypical cell structure in eukaryotes

Answer 9

Aseptate fungal hyphae
In aseptate (coenocytic) fungal hyphae, the cytoplasm is continuous and contains many nuclei within one long, branched tube.

This multinucleate condition allows rapid growth and distribution of materials through the hypha without cross‑walls.

Skeletal muscle fibres
A skeletal muscle fibre is a very long, cylindrical cell formed by the fusion of many smaller cells; it contains many nuclei positioned just under the plasma membrane.

This multinucleate structure supports the high protein‑synthesis demand of large, contractile muscle cells.

Red blood cells (mammalian)
Mature mammalian red blood cells are enucleate (have no nucleus).

Loss of the nucleus creates more space for haemoglobin, increasing oxygen‑carrying capacity, but limits the cell’s lifespan and ability to repair itself.

Phloem sieve tube elements
Phloem sieve tube elements are highly specialized plant cells that form sieve tubes for transport of sugars.

At functional maturity, they are enucleate or have highly reduced nuclei, and their adjacent companion cells provide metabolic support, allowing efficient long‑distance transport.

Question 10

A2.2.10 Cell types and cell structures viewed in light and electron micrographs

Answer 10

A2.2.10 trains you to recognize cell types and key structures in both light and electron micrographs.

Identifying cell type
Prokaryote cell:

Small, no visible nucleus; DNA in a nucleoid region (diffuse area without a membrane).

Often shows a prokaryotic cell wall outside the plasma membrane.

Plant cell:

Rectangular or box‑like shape, with a cell wall and usually a large sap vacuole; may show chloroplasts in photosynthetic tissues.

Animal cell:

Irregular or rounded shape, no cell wall, usually no large central vacuole, and often centrioles in the cytoplasm.

Structures to identify in electron micrographs
Students should be able to label these structures in EM images:

Prokaryote features:

Nucleoid region (no nuclear envelope).

Prokaryotic cell wall (outside the plasma membrane).

Nucleus and chromosomes:

Nucleus bounded by a double membrane with pores.

Chromosomes (as dense, condensed chromatin in dividing cells).

Organelles:

Mitochondrion (double membrane, inner cristae, matrix).

Chloroplast (double membrane, internal thylakoids/stacked grana).

Sap (central) vacuole in plant cells (large, fluid‑filled space).

Golgi apparatus (stacked, flattened sacs with vesicles at the edges).

Rough endoplasmic reticulum (RER: membrane‑bound with ribosomes attached).

Smooth endoplasmic reticulum (SER: membrane‑bound but no ribosomes).

Ribosomes:

Small, dark dots in the cytoplasm or on RER; 70S in prokaryotes, 80S in eukaryotes.

Membranes and surfaces:

Plasma membrane (thin electron‑dense line around the cell).

Cell wall (thick layer outside the plasma membrane in plants and prokaryotes).

Microvilli (finger‑like projections of the plasma membrane on some animal cells, e.g., intestinal epithelium).

Question 11

A2.2.11 Drawing and annotation based on electron micrographs

Answer 11

A2.2.11 requires you to turn electron‑micrograph views into clear, labelled diagrams with brief functional notes.

Organelles to draw and annotate
For each structure, your drawing should show typical shape and internal detail, and your annotations should include its function:

Nucleus:

Drawing: Oval or spherical with a double membrane and pores.

Annotation: Contains chromosomes made of DNA and histones; controls gene expression and cell activities.

Mitochondrion:

Drawing: Bean‑shaped with a double membrane and inner cristae.

Annotation: Site of aerobic respiration; produces ATP from glucose and oxygen.

Chloroplast:

Drawing: Lens‑shaped with a double membrane and internal thylakoids/stacks (grana).

Annotation: Site of photosynthesis; converts light energy into chemical energy (glucose).

Sap (central) vacuole:

Drawing: Large, fluid‑filled space in a plant cell.

Annotation: Stores water, ions, pigments and waste; maintains turgor pressure and cell shape.

Golgi apparatus:

Drawing: Stacked, flattened sacs (cisternae) with small vesicles nearby.

Annotation: Modifies, sorts, and packages proteins and lipids for secretion or transport.

Rough endoplasmic reticulum (RER):

Drawing: Membrane‑bound channels with ribosomes attached.

Annotation: Synthesises proteins for secretion or membrane incorporation.

Smooth endoplasmic reticulum (SER):

Drawing: Membrane‑bound channels with no ribosomes.

Annotation: Synthesises lipids and steroids; detoxifies substances.

Chromosomes:

Drawing: Dense, rod‑like or X‑shaped threads in the nucleus during mitosis.

Annotation: Carry genetic information; ensure accurate distribution of DNA to daughter cells.

Other cell structures
Cell wall:

Drawing: Thick layer around the plasma membrane (in plants/prokaryotes).

Annotation: Provides structural support and protection; maintains shape.

Plasma membrane:

Drawing: Thin line at the boundary of the cell.

Annotation: Regulates movement of substances in and out of the cell; maintains internal environment.

Secretory vesicles:

Drawing: Small circles often near the Golgi or plasma membrane.

Annotation: Transport and release proteins or other materials (e.g., hormones, enzymes) by exocytosis.

Microvilli:

Drawing: Finger‑like projections of the plasma membrane (e.g., on intestinal epithelial cells).

Annotation: Increase surface area for absorption of nutrients.