Cell Structure Explained: H2 Biology Guide (9477)

Quick answer: Eukaryotic cells (plant and animal) contain membrane-bound organelles that separate incompatible reactions and create specialised internal conditions; prokaryotic (bacterial) cells don’t. Plant cells have a cellulose cell wall, often a large central vacuole, and chloroplasts in photosynthetic tissue — but they still have a cell-surface membrane inside the wall, controlling exchange the same way an animal cell’s membrane does. This guide covers every required organelle, how to identify structures in real micrographs (not just idealised diagrams), and the comparisons examiners test most often.

Cell structure is easier to learn when every organelle is treated as part of a working system rather than a label to memorise. A diagram may help you remember where a structure sits, but exam questions usually ask how its features support a particular function — which is the reasoning this guide is built around. It follows the Singapore-Cambridge H2 Biology syllabus 9477 for the first examination in 2026 and pairs with the broader H2 Biology Syllabus guide if you need the full Core Idea map that this topic sits within.

The most useful revision pattern is simple: identify the structure, describe the feature that matters, and connect that feature to the biological process it supports.

What Is Cell Structure?

Cell structure describes the organisation of a cell and the specialised parts that keep it alive. Some structures form boundaries, some store genetic information, and others create the conditions needed for chemical reactions.

Eukaryotic cells contain membrane-bound organelles — internal compartments that separate incompatible reactions, concentrate enzymes, and create different conditions within the same cell. Animal and plant cells are both eukaryotic, sharing a nucleus, mitochondria, endoplasmic reticulum, Golgi body, and ribosomes.

Exam focus: don’t stop at naming an organelle. A complete answer usually needs a structure-function link — folded inner membranes providing a large surface area for proteins involved in oxidative phosphorylation, for example, not just “mitochondria make energy.”

The Cell Theory

Cells are the smallest units of life; all cells arise from pre-existing cells; living organisms are composed of cells. Viruses challenge simple definitions because they contain genetic material and can evolve, but they aren’t cellular and can’t reproduce independently — they depend on a host cell for replication, protein synthesis, and many metabolic processes.

Animal and Plant Cell Structure

Animal cells have flexible outer boundaries, since the cell-surface membrane isn’t surrounded by a rigid wall — their shape varies with function, so a neurone, muscle cell, and secretory cell can look quite different despite sharing the same basic organelles.

Plant cells share the eukaryotic organelles found in animal cells, but add a cellulose cell wall. Many mature plant cells have a large central vacuole, while photosynthetic tissues contain chloroplasts. The cell wall sits outside the cell-surface membrane, so plant cells still use the membrane to control movement into and out of the cytoplasm — the wall doesn’t replace that function.

Feature	Plant cell	Animal cell
Cell-surface membrane	Present inside the cell wall	Present as the outer cell boundary
Cellulose cell wall	Present	Absent
Nucleus	Present in most living cells	Present in most living cells
Chloroplasts	Present in photosynthetic tissues	Absent
Large central vacuole	Common in mature cells	Absent as a large permanent structure
Mitochondria	Present	Present
Centrioles	Usually not shown in a typical plant-cell diagram	Present in the syllabus: animal-cell model
Shape	Often more regular because of the wall	Often flexible and variable

Common mistake: plant cells don’t replace the cell-surface membrane with a cell wall — they have both structures, and each has a different function.

Cell Organelles and Their Functions

The organelle list is easier to remember grouped by task: the nucleus stores information, the endomembrane system produces and transports materials, energy-transforming organelles provide usable energy, and the cell boundary manages exchange.

Nucleus, nuclear envelope, and nucleolus: the nucleus holds most of the cell’s DNA as chromatin. The nuclear envelope is a double membrane with pores regulating exchange between the nucleus and cytoplasm, allowing transcription and RNA processing to occur in a controlled compartment. The nucleolus produces ribosomal RNA and assembles ribosomal subunits — often visible as a darker region in a micrograph.

Ribosomes: the sites of translation. Free ribosomes produce proteins that usually function in the cytosol; ribosomes attached to the rough endoplasmic reticulum produce proteins entering the endomembrane system. Eukaryotic cytoplasmic ribosomes are 80S; bacterial ribosomes are 70S — and ribosomes aren’t surrounded by a membrane, so calling them “membrane-bound organelles” is incorrect.

Rough and smooth endoplasmic reticulum: rough ER is flattened membrane sacs with attached ribosomes, important for synthesising, folding, and initially modifying proteins destined for secretion, membrane insertion, or specific organelles. Smooth ER lacks attached ribosomes, and its functions include lipid synthesis and, in some cells, detoxification or calcium storage.

Golgi body and vesicles: stacks of flattened membrane sacs that receive material from the ER, modify it, sort it, and package it into vesicles. A secreted protein follows a linked pathway — rough ER ribosome → transport vesicle → Golgi body → secretory vesicle → fusion with the cell-surface membrane. Learn the order and each stage’s role, not just the organelle names.

Mitochondria: the main sites of aerobic respiration. The inner membrane folds into cristae, providing a large surface area for electron-transfer proteins and ATP synthase; the matrix holds enzymes for the link reaction and Krebs cycle. In an electron micrograph, look for a double boundary and internal folds, not just an oval outline.

Chloroplasts carry out photosynthesis. Thylakoid membranes hold photosynthetic pigments and light-dependent reaction components; stacked thylakoids form grana, while the stroma holds Calvin-cycle enzymes. A chloroplast isn’t simply “a green mitochondrion” — both transform energy and have extensive internal membranes, but with different structures, reactants, and products.

Lysosomes: membrane-bound vesicles containing hydrolytic enzymes, digesting materials taken into the cell, breaking down damaged components, and keeping potentially harmful enzymes separated from the cytoplasm.

Centrioles: cylindrical structures built from microtubules, helping organise microtubules and associated with spindle formation during animal-cell division — often shown as a pair positioned at an angle in diagrams.

Cell-surface membrane and cytoplasm: the membrane forms a selectively permeable boundary, controlling exchange, holding receptors, and supporting cell recognition. The cytoplasm — cytosol plus suspended structures — provides the environment for many enzyme-controlled reactions.

Cellulose cell wall and vacuole: the wall’s cellulose microfibrils provide tensile strength, supporting the cell, maintaining shape, and resisting excessive expansion when water enters. A large central vacuole contains cell sap and contributes to turgor, pushing the cytoplasm toward the cell boundary — don’t confuse it with an empty-looking space caused by poor staining.

Quick Organelle Comparison Table

Structure	Recognition feature	Main function
Nucleus	Large compartment with a double envelope; may show a darker nucleolus	Stores DNA, controls gene expression
Rough ER	Flattened membranes with small ribosome dots	Produces and processes proteins entering the endomembrane system
Smooth ER	Tubular membranes without ribosome dots	Synthesises lipids, supports specialised metabolism
Golgi body	Stacked curved sacs with nearby vesicles	Modifies, sorts, and packages materials
Mitochondrion	Double boundary with folded inner membrane	Aerobic respiration and ATP production
Chloroplast	Double boundary with thylakoid stacks	Photosynthesis
Lysosome	Small membrane-bound vesicle, dense contents	Digests material using hydrolytic enzymes
Ribosome	Very small particle; free or attached to rough ER	Translates mRNA into a polypeptide
Centriole	Paired cylinders made from microtubules	Helps organise spindle microtubules

Bacterial Cell Structure

Bacterial cells are prokaryotic: small, unicellular, with a peptidoglycan cell wall, circular DNA, and 70S ribosomes. The DNA sits in the cytoplasm rather than inside a nucleus. The absence of membrane-bound organelles doesn’t mean the cell lacks organisation — its cell-surface membrane manages exchange while the cytoplasm holds the enzymes and ribosomes needed for metabolism and protein synthesis.

Feature	Plant cell	Animal cell	Bacterial cell
Cell type	Eukaryotic	Eukaryotic	Prokaryotic
Nucleus	Present	Present	Absent
DNA	Linear chromosomes in nucleus	Linear chromosomes in nucleus	Circular DNA in cytoplasm
Ribosomes	80S in cytoplasm	80S in cytoplasm	70S
Membrane-bound organelles	Present	Present	Absent
Cell wall	Cellulose	Absent	Peptidoglycan
Mitochondria	Present	Present	Absent
Chloroplasts	Present in photosynthetic tissue	Absent	Absent

How to Identify Organelles in Micrographs

Real micrographs don’t look as neat as textbook diagrams — structures may be cut at an angle, partly hidden, or shown at an unfamiliar scale. Base identification on visible evidence:

• Start with the boundary. A thick outer wall suggests a plant or bacterial cell; a flexible membrane alone is consistent with an animal cell.
• Look for internal compartments. A nucleus and other membrane-bound organelles indicate a eukaryotic cell.
• Use internal detail. Cristae support a mitochondrion identification; grana support a chloroplast identification.
• Check for attached ribosome dots. Rough ER has them; Golgi stacks don’t.
• Use scale before deciding. A small dot at low magnification may not contain enough detail for a confident organelle label.

Exam wording: write “identified as a mitochondrion because folded inner membranes or cristae are visible,” not just “it is oval.” Shape alone is weak evidence.

Magnification and Cell Size

Cell-structure questions may combine recognition with measurement. Image size and actual size must use the same unit before calculating.

Magnification = image size ÷ actual size. Actual size = image size ÷ magnification

Convert millimetres to micrometres (× 1,000), micrometres to nanometres (× 1,000). Show working, include units, and use an appropriate number of significant figures. An eyepiece graticule has no fixed unit until calibrated with a stage micrometer at the selected objective lens — changing the objective changes the calibration.

Common Cell-Structure Exam Mistakes

Mistake	Why it loses marks	Better approach
Listing an organelle without a function	The question may need explanation, not just identification	Link one visible feature to the process it supports
Calling ribosomes membrane-bound	Ribosomes have no surrounding membrane	Describe them as particles made from rRNA and proteins
Saying bacterial cells have no DNA	They lack a nucleus, not genetic material	State that circular DNA lies in the cytoplasm
Saying only plant cells have membranes	Plant cells have a membrane inside the wall	Separate the selectively permeable membrane from the supporting wall
Identifying mitochondria by shape alone	Many components can look oval in section	Use cristae or a double boundary as evidence
Assuming every plant cell has chloroplasts	Non-photosynthetic tissues may not contain them	Refer to photosynthetic plant cells when chloroplasts are relevant
Confusing Golgi body with rough ER	Both contain flattened membranes	Check for attached ribosomes and nearby budding vesicles
Ignoring units in magnification	Different units create an incorrect value	Convert first, calculate second, state the final unit

A Step-by-Step Exam Method

1. Read the command word and decide whether the task is identify, describe, compare, or explain.
2. Use the diagram or micrograph as evidence instead of recalling an idealised picture.
3. Name the structure precisely — inner membrane, nuclear envelope, peptidoglycan — where relevant.
4. Connect the feature to a biological process with a clear cause-and-effect sentence.
5. Check units, labels, and comparison language before moving on.

For a comparison, keep the same feature in both halves of the sentence: “A plant cell has a cellulose cell wall, while an animal cell does not” is clearer than two unrelated lists.

Cell Structure Revision Checklist

□ I can outline the three statements in the cell theory. I can label a typical animal cell and plant cell without notes. I can explain the function of every required organelle. I can link the nucleus, ribosomes, rough ER, Golgi body, and vesicles in a protein-secretion pathway. I can distinguish mitochondria from chloroplasts using internal membranes. I can compare plant, animal, and bacterial cells feature by feature. I can explain why a bacterial cell has DNA but no nucleus. I can identify organelles from micrograph evidence rather than shape alone. I can calculate magnification or actual size after converting units. I can explain why viruses challenge the cell theory.

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Cell structure supports later topics — membranes, respiration, photosynthesis, gene expression, cell division. If the organelle links are weak, later explanations feel like separate facts instead of one connected system. At Pamela’s Place, Biology revision is organised around labelled-diagram recall, structure-function explanations, micrograph practice, and targeted correction of common mistakes, in small Omakase groups capped at 7 students — the aim is more precision, not more memorisation.