Prokaryotic Cell Structures
A bacterium has no nucleus and no membrane-bound organelles, yet it is one of the most successful life forms on Earth. It does everything a cell must do using a small set of structures, some shared with eukaryotes, some uniquely bacterial. This lesson names each one, from the sticky capsule on the outside to the whip-like flagella, and explains the job it does.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upA bacterium has no membrane-bound organelles, yet it still respires, makes proteins, moves and reproduces.
Before reading: if a prokaryote has no nucleus and no organelles, where do you think it keeps its DNA, and where do you think protein synthesis happens?
Learning Intentions
goalsKnow
- The nine prokaryotic structures named in the syllabus
- The structure of each one
- Which structures are always present and which are optional
Understand
- The function each prokaryotic structure performs
- How a prokaryote runs every life process without organelles
- Why plasmids and pili matter for antibiotic resistance
Can Do
- Describe the structure and function of each named structure
- Label a diagram of a prokaryotic cell
- Match a described job to the correct structure
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
The Outer Layers
+5 XPCapsule, cell wall and cell membrane
A bacterium is protected and bounded by up to three concentric layers, working from the outside in.
The capsule is a slimy protective layer outside the cell wall that helps the cell resist drying out, evade the host's immune system and stick to surfaces (not all bacteria have one). The cell wall (made of peptidoglycan) is a rigid layer that gives the cell shape and prevents it bursting. The cell membrane beneath it is a partially permeable phospholipid bilayer that controls what enters and leaves, and also hosts the enzymes of respiration (since there are no mitochondria).
Pause, copy the highlighted structure-and-function statements for the three outer layers.
The slimy protective layer found outside the cell wall of some bacteria is called the _____.
Odd one out. Three of these are functions of a bacterial cell's outer layers. Which one is not?
Take it further
The next move, with the diagram to work from and two more checks.
Inside: DNA, Plasmids and Ribosomes
+5 XPCytoplasm, nucleoid, plasmid and ribosomes
We just covered the outer layers. That raises a question: with no nucleus or organelles, where does a bacterium keep its genes and make its proteins? This card answers it → everything happens in the cytoplasm.
Without a nucleus or organelles, a prokaryote runs its genetics and protein synthesis directly in the cytoplasm.
Text description
- Outside to inside: capsule (outermost layer), cell wall (peptidoglycan), cell membrane.
- In the cytoplasm: the nucleoid (circular DNA, not in a nucleus), a plasmid (small extra ring of DNA) and ribosomes.
- Projecting outward: a long flagellum for movement and short pili.
The cytoplasm is the jelly-like fluid where reactions occur and structures are suspended. The nucleoid is the region holding the single circular chromosome, the cell's essential genes, not enclosed by a membrane. A plasmid is a small separate ring of extra DNA (often antibiotic-resistance genes) that can be copied and passed between cells. Ribosomes (70S, smaller than the eukaryotic 80S) are the site of protein synthesis.
Pause, copy the highlighted inside-the-cell structures and functions into your book.
Which structure carries extra genes (such as antibiotic resistance) that bacteria can pass to one another?
Fill the gaps. A prokaryote's essential genes sit in a region called the , extra genes such as antibiotic resistance sit on a small separate ring called a , and proteins are built on .
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Label and Describe
Pattern A, Structured recall
In your book, sketch a labelled prokaryotic cell. Then write a two-column table headed Structure and Function covering all nine: capsule, cell wall, cell membrane, cytoplasm, nucleoid, plasmid, ribosomes, flagella and pili. Give each a one-line function.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
Moving and Connecting: Flagella and Pili
+5 XPThe structures that project beyond the wall
We just looked inside the cell. That raises a question: how does a bacterium move toward food or attach to a host? This card answers it → two external projections, the flagella and the pili.
Two kinds of projection reach beyond the cell wall, one for movement, one for attachment and gene transfer.
Flagella are long whip-like tails that rotate to propel the cell, allowing it to move toward favourable conditions (e.g. nutrients). Pili are short hair-like projections used to attach to surfaces and other cells, and to transfer DNA (such as plasmids) between bacteria, a route for spreading antibiotic resistance. Both are optional structures: not every prokaryote has them.
Add the functions of flagella (movement) and pili (attachment + DNA transfer) to your notes before the check.
Flagella rotate to move the bacterial cell.
Prokaryotic cells carry out protein synthesis inside membrane-bound organelles.
Pili can transfer DNA, including plasmids, between bacterial cells.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
How antibiotic resistance spreads
Antibiotic resistance is one of the biggest threats in modern medicine, and prokaryotic structures explain it. A resistance gene often sits on a plasmid. Using a pilus, one bacterium can pass a copy of that plasmid to a neighbour, even a different species, in a process called conjugation.
Describe the structure and function of the capsule, cell wall, cell membrane, nucleoid, plasmid, ribosomes, flagella and pili in a prokaryotic cell.
Describe asks for features and characteristics. Here the dot point names two things for every structure — what it is and what it does — so a named list without functions caps the answer at about half marks.
7 marks across eight named structures means roughly one sentence each, every sentence in the form "structure X is … , which allows the cell to … ". Decide the order before writing.
Work from the outside in: capsule, cell wall, cell membrane, then the cytoplasm and what sits in it — nucleoid, plasmid, ribosomes — then the external projections, flagella and pili.
The capsule is a slimy polysaccharide layer outside the cell wall, present in some bacteria but not all; it resists drying out, helps the cell evade a host’s immune defences and lets it adhere to surfaces. The cell wall is a rigid layer of peptidoglycan that gives the cell its fixed shape and prevents it bursting when water enters by osmosis. The cell membrane beneath is a partially permeable phospholipid bilayer that controls what enters and leaves, and — because there are no mitochondria — also carries the enzymes of aerobic respiration on its inner face.
The cytoplasm is the aqueous gel filling the cell, holding dissolved ions, enzymes and substrates, and it is where the cell’s metabolic reactions occur. Within it the nucleoid is the region containing the single circular chromosome, which carries the genes essential for life and is not enclosed by any membrane. A plasmid is a separate small ring of DNA carrying non-essential extra genes, commonly antibiotic resistance, and it can be replicated and passed to other cells independently of the main chromosome.
Ribosomes are small non-membranous structures of RNA and protein, smaller in prokaryotes (70S) than in eukaryotes (80S), and they are the site of protein synthesis, translating messenger RNA into polypeptides. A flagellum is a long whip-like filament anchored in the membrane that rotates like a propeller to move the whole cell, usually towards nutrients or away from harm. Pili are short hair-like protein projections used to attach to surfaces and to other bacteria, and a specialised sex pilus transfers plasmid DNA from one cell to another during conjugation.
Together these structures allow a prokaryotic cell to carry out every process needed for life — boundary control, respiration, protein synthesis, movement and inheritance — without any membrane-bound organelle, each job being done by a single structure or by the cell membrane itself.
A prokaryotic cell is bounded, from the outside in, by up to three layers. The capsule is a slimy polysaccharide layer outside the cell wall, present in some species only, which resists desiccation, helps the cell evade host immune defences and allows adhesion to surfaces. The cell wall is a rigid peptidoglycan layer that maintains the cell’s shape and prevents it bursting when water enters by osmosis. The cell membrane is a partially permeable phospholipid bilayer controlling the entry and exit of substances, and it also carries the enzymes of aerobic respiration on its inner face because the cell has no mitochondria. Inside, the cytoplasm is an aqueous gel containing dissolved ions, enzymes and substrates in which metabolic reactions occur. The nucleoid is the unbounded region holding the single circular chromosome that carries the cell’s essential genes, while a plasmid is a separate small ring of DNA carrying additional non-essential genes such as antibiotic resistance, replicated and transferred independently of the chromosome. Ribosomes are small non-membranous RNA-and-protein structures, 70S in prokaryotes, and are the site of protein synthesis. Externally, a flagellum is a long whip-like filament anchored in the membrane that rotates to propel the cell, and pili are short hair-like projections used for attachment, with a sex pilus transferring plasmid DNA between cells during conjugation. Together these structures allow the cell to perform every life process — boundary control, respiration, protein synthesis, movement and inheritance — without a single membrane-bound organelle.
Explain how a prokaryotic cell carries out respiration, protein synthesis and movement despite having no membrane-bound organelles.
Explain wants cause and effect, so each of the three processes needs a structure named and a reason that structure can do the job. "It uses its membrane instead" is an assertion until you say why a membrane will serve.
5 marks, three processes. Give one mark-worth of reasoning to each, then spend the remaining two on the principle that ties them together.
In a eukaryotic cell, respiration, protein processing and directed movement are each handed to a dedicated membrane-bound compartment. A prokaryotic cell has none of these, so every one of those jobs must be done by a structure that is either non-membranous or part of the cell membrane itself.
Glycolysis occurs in the cytoplasm, and the enzymes and electron-transport proteins for aerobic respiration are embedded in the inner face of the cell membrane rather than in a mitochondrial cristae membrane. Because the reactions require a membrane across which a proton gradient can be built rather than a mitochondrion specifically, the cell membrane can serve, and in many bacteria it is folded inwards to increase the area available.
Protein synthesis needs ribosomes, and ribosomes are not membrane-bound structures in any cell, so their absence from an organelle costs the prokaryote nothing. Free ribosomes in the cytoplasm translate messenger RNA directly; because there is no nuclear membrane separating transcription from translation, a ribosome can begin translating a transcript while it is still being made.
Movement uses the flagellum, a filament anchored in the cell membrane and driven by a rotary motor powered by the same proton gradient that respiration builds, so no cytoskeleton or vesicle transport is required. In each case the prokaryotic solution is the same: because the cell is only a few micrometres across, diffusion distributes materials fast enough without compartments, and the one membrane it has can host the processes a eukaryote would separate.
In a eukaryotic cell, respiration, protein processing and directed transport are each assigned to a membrane-bound compartment, so a prokaryote must accomplish them using structures that are either non-membranous or part of the cell membrane itself. Respiration begins with glycolysis in the cytoplasm, and the enzymes and electron-transport proteins of aerobic respiration are embedded in the inner face of the cell membrane; what those reactions require is a membrane across which a proton gradient can be established, not a mitochondrion as such, so the cell membrane serves, and it is often folded inwards to increase the available area. Protein synthesis costs the cell nothing at all, because ribosomes are non-membranous structures in every cell: free ribosomes in the cytoplasm translate messenger RNA directly, and since no nuclear membrane separates transcription from translation, translation can begin on a transcript still being made. Movement uses the flagellum, a filament anchored in the cell membrane and turned by a rotary motor driven by the same proton gradient respiration generates, so neither a cytoskeleton nor vesicle transport is needed. The common principle is size: at only a few micrometres across, diffusion supplies every part of the cell quickly enough that compartments are unnecessary, and the single membrane can host the processes a eukaryotic cell separates.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPUnderstandBand 4(4 marks) 1. Describe the structure and function of the nucleoid, plasmid, flagella and pili in a prokaryotic cell.
1 mark each: a correct structure-and-function statement for the nucleoid, plasmid, flagella and pili
ApplyBand 4(4 marks) 2. A bacterium is found living in the fast-moving water of a mountain stream, attached to a rock surface. Identify two prokaryotic structures that would help it survive there and describe the function of each.
2 marks: two appropriate structures named · 2 marks: a correct function given for each, linked to the habitat
AnalyseBand 5–6(4 marks) 3. Describe the structure and function of the plasmid and the pilus, and explain how the two together allow a useful gene to spread through a bacterial population.
1 mark: plasmid structure and function · 1 mark: pilus structure and function · 2 marks: explains transfer between cells and why this spreads a gene faster than reproduction alone
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (4 marks): The nucleoid is the region of the cytoplasm, not enclosed by a membrane, that holds the single circular chromosome carrying the cell's essential genes. A plasmid is a small, separate ring of extra DNA (often carrying antibiotic-resistance genes) that can be replicated and transferred to other cells independently of the chromosome. Flagella are long, whip-like tails that rotate to propel the cell, allowing it to move toward favourable conditions. Pili are short, hair-like projections used to attach to surfaces and other cells and to transfer DNA (including plasmids) between bacteria.
Q2 (4 marks): Pili are short hair-like projections whose function is to attach the cell to surfaces and to other cells [1], which lets the bacterium hold onto the rock instead of being washed downstream [1]. The capsule is a slimy layer outside the cell wall that helps the cell stick to surfaces and resist drying out [1], which both anchors it and protects it if the water level falls [1]. (Flagella, which rotate to propel the cell toward favourable conditions, are also an acceptable answer.)
Q3 (4 marks): A plasmid is a small circular ring of DNA that is separate from the main chromosome, and it carries extra, non-essential genes such as antibiotic resistance [1]. A pilus is a short hair-like projection from the cell surface whose functions are attachment to other cells and the transfer of DNA between them [1]. A cell with the plasmid can use a pilus to connect to a neighbouring cell and pass a copy of that plasmid across [2 marks: transfer described, and the point made that a plasmid can therefore move sideways between existing cells rather than only downward to offspring, so the gene spreads through the population far faster than reproduction alone could carry it].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now describe all nine prokaryotic structures and what each does. Next lesson moves to the bench: preparing a wet mount slide and calculating cell size under the microscope.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you predicted where a prokaryote keeps its DNA and makes proteins. The DNA sits free in the nucleoid (plus any plasmids), and protein synthesis happens on ribosomes in the cytoplasm, no nucleus or ER needed. If you assumed a plasmid was just a copy of the main chromosome, revisit the misconception box.