A2.2 Cell structure
SL & HL+ AHL sectionsSL and HL, with AHL content · Cells · Theme A
Big questions
- What do all cells have in common, and where do different kinds of cell part ways?
- How do microscopes let us see and measure the inside of cells?
Sections framed in purple are AHL (Higher Level only). SL students can hide them in Display settings.
Cells are the basic unit of life
Every organism that has been examined is made of one or more cells. Single-celled organisms such as bacteria, yeasts and Paramecium are unicellular; plants, animals and most fungi are multicellular. The cell is the smallest unit that can carry out all the functions of life on its own, so it is the basic structural unit of all living organisms. This is cell theory.
A theory, not just a fact
Cell theory began with observations. In the 17th century Robert Hooke described the “cells” of cork, and later microscopists such as Schleiden and Schwann found cells in every plant and animal tissue they examined. A theory goes further than a list of observations: it is a general explanation that applies widely.
Using cell theory deductively
Deductive reasoning moves from a general theory to a specific prediction. If all organisms are made of cells, then a newly discovered organism, from a deep-sea vent or a sample of soil, will consist of one or more cells. Each time a prediction like this is tested and found to be true, the theory is corroborated. If it were found to be false, the theory would have to be changed.
Testing a theory also means looking for awkward cases. Some cells have no nucleus and some have hundreds (see Atypical cell structure below). These challenge what a “typical” cell looks like, but none of them is a living thing that is not made of cells, so cell theory stands.
Microscopy skills and magnification
Making a slide
- Temporary mount: put a thin layer of cells or tissue (onion epidermis, a cheek-cell smear, moss leaf) on a glass slide in a drop of water or stain.
- Cover slip: lower it at an angle so that no air bubbles are trapped; blot off excess liquid.
- Staining: most cell structures are colourless, so a stain that binds to particular chemicals is added. Methylene blue binds to DNA and so shows up nuclei in animal cells; iodine solution stains starch blue-black and makes the nuclei of plant cells easier to see.
Focusing
Start with the lowest-power objective and the stage lowered. Use the coarse focusing knob to bring the specimen roughly into focus, then the fine focusing knob to sharpen it. Move to a higher-power objective and use only the fine knob, so that the lens does not hit the slide. Total magnification = eyepiece × objective, for example ×10 × ×40 = ×400.
Measuring with an eyepiece graticule
An eyepiece graticule is a scale inside the eyepiece. Its divisions have no fixed size, so they are calibrated for each objective against a stage micrometer (a slide ruled in 0.01 mm divisions). If 40 eyepiece units line up with 100 µm on the micrometer, one unit = 2.5 µm, and a cell 12 units across is 30 µm wide. Measuring in this way is quantitative observation made with an instrument.
Calculating sizes and adding a scale bar
Magnification = image size ÷ actual size. Rearranged: actual size = image size ÷ magnification, and image size = actual size × magnification. Always convert both lengths to the same unit first (1 mm = 1,000 µm; 1 µm = 1,000 nm).
A scale bar is a line on an image labelled with the length it represents. Its length = the length it represents × magnification: at ×2,000, a bar for 10 µm is 20,000 µm = 20 mm long. Photographs taken through the eyepiece, even with a phone, record specimens for later measuring.
magnification = image size ÷ actual size
I = image size · A = actual size · M = magnification. Cover the one you want: I = A × M, A = I ÷ M, M = I ÷ A.
Magnification = ×12,000
- Magnification = image size ÷ actual size
- Use the same units for both lengths: image size = 36 mm = 36,000 µm (1 mm = 1,000 µm); actual size = 3 µm.
- Magnification = 36,000 µm ÷ 3 µm = ×12,000
- The units cancel, so a magnification has no units: write it as ×12,000.
Draw a scale bar
Draw the scale bar 20 mm long
- Length of scale bar = length it represents × magnification
- = 10 µm × 2,000 = 20,000 µm
- Convert to mm to measure with a ruler: 20,000 µm ÷ 1,000 = 20 mm
Read a scale bar
Magnification = ×5,000
- Magnification = measured length of scale bar ÷ length it represents
- Same units: 25 mm = 25,000 µm
- Magnification = 25,000 µm ÷ 5 µm = ×5,000
In a micrograph at ×25,000, the length of a mitochondrion measures 75 mm. Calculate its actual length in µm.
Exam rule: convert both lengths to the same unit before dividing (1 mm = 1,000 µm; 1 µm = 1,000 nm). Magnifications have no units; write them with a × sign.
Text description of this simulation
A calculator built on one equation: magnification equals image size divided by actual size. Choose which of the three quantities to find, type in the other two with their units (millimetres, micrometres or nanometres), and the working is shown step by step: convert both lengths to the same unit, then divide or multiply. A scale bar mode works out how long to draw a scale bar for a given magnification, or the magnification of an image from a scale bar you have measured. A practice mode gives a random exam-style question to answer, then shows the worked solution.
Developments in microscopy
Magnification is how much larger an image is than the object. Resolution is the smallest distance between two points at which they can still be told apart. Light has a long wavelength, so a light microscope cannot resolve points closer than about 0.2 µm, however much the image is enlarged.
Electron microscopy
Electrons have a far shorter wavelength than light, so an electron microscope resolves around 1 nm or better for biological specimens. This reveals the ultrastructure of cells: membranes, ribosomes, cristae and thylakoids.
| Light microscope | Electron microscope | |
|---|---|---|
| Resolution | about 200 nm (0.2 µm) | about 1 nm or better |
| Useful magnification | up to about ×1,500 | ×500,000 or more |
| Specimens | living or dead | dead, in a vacuum |
| Image | colour | black and white (colour can be added) |
A transmission electron microscope (TEM) sends electrons through very thin sections to show internal structure; a scanning electron microscope (SEM) scans the surface and gives three-dimensional-looking images.
Freeze fracture
A specimen is frozen very rapidly and then cracked with a blade. The fracture follows the weakest regions, which are often the middle of membranes, between the two layers of phospholipids. The exposed surface is coated with a thin film of metal at an angle (shadowing) and carbon to make a replica, which is viewed by electron microscopy. It shows the inner faces of membranes, studded with proteins that span the bilayer, which was key evidence for the fluid mosaic model (B2.1 Membranes and membrane transport).
Cryogenic electron microscopy (cryo-EM)
A thin layer of a solution of a protein or virus is frozen so quickly, in liquid ethane, that the water forms glass-like (vitreous) ice instead of crystals. Thousands of images of individual molecules are combined by computer into a 3D structure at near-atomic resolution. Molecules are seen in their natural state, without stains, and in different shapes they adopt as they work. Cryo-EM was used to find the structure of the SARS-CoV-2 spike protein within weeks in 2020.
Fluorescent stains and immunofluorescence
Fluorescent stains absorb light of one wavelength and emit light of a longer wavelength, so stained structures glow brightly against a dark background (for example, DAPI binds DNA and glows blue). In immunofluorescence, a fluorescent dye is attached to antibodies that bind one specific protein, showing exactly where that protein is in the cell. Using several antibodies with different colours shows several proteins at once.
Structures all cells share, and prokaryote cells
- Cell wall (peptidoglycan): rigid, stops the cell bursting by osmosis
- Plasma membrane: controls what enters and leaves the cell
- Cytoplasm: water and enzymes; site of most metabolic reactions
- Nucleoid: naked DNA in a loop (one circular chromosome)
- 70S ribosomes: sites of protein synthesis
- Plasmid: small extra loop of DNA (not in all bacteria)
- Pili: attach the cell to surfaces or other cells (not in all)
- Flagellum: rotates to move the cell (not in all)
Loading 3D model…
Drag to rotate, pinch or scroll to zoom. Keyboard: click the model, then use the arrow keys to rotate, + and − to zoom, 0 to reset and L to toggle labels. Tab moves through the labels.
Your browser can't show 3D graphics (WebGL is off or unsupported), so the exam diagram and text description are shown instead.
Text description of this visual
Four cut-away models, each sliced open by a flat plane so you look into the back half of the cell. Organelles that cross the cut are cut too, so you can see their double membranes, cristae and grana; organelles behind the cut are whole. Proportions are schematic: ribosomes in particular are drawn far larger than they really are. The bacterium is a rod-shaped Gram-positive cell like Bacillus, with a thick cell wall, plasma membrane, cytoplasm with 70S ribosomes, a nucleoid containing one loop of naked DNA, plasmids, pili and a flagellum, and an optional capsule. The animal cell has a nucleus with a double membrane, nuclear pores and a nucleolus, rough and smooth endoplasmic reticulum, a Golgi apparatus, vesicles, lysosomes, mitochondria with cristae, a pair of centrioles, microtubules of the cytoskeleton and free 80S ribosomes. The plant cell has a cellulose cell wall with plasmodesmata, a plasma membrane pressed against it, a large permanent vacuole surrounded by the tonoplast, chloroplasts with grana and starch grains, mitochondria, a nucleus pushed to the side; it has no centrioles, because cells of flowering plants and conifers build their spindle of microtubules without them. The fungal hypha has a chitin wall, a septum with a central pore, several nuclei, large vacuoles in its older part, vesicles packed into its growing tip and glycogen granules, with a branching hypha of the mycelium behind it.
Bacterium
- Cell wall (peptidoglycan)
- A thick, rigid layer of peptidoglycan outside the plasma membrane. Gram-positive bacteria such as Bacillus and Staphylococcus have a thick wall and no outer membrane. The wall stops the cell bursting when water enters by osmosis.
- Plasma membrane
- A phospholipid bilayer with proteins, just inside the wall, that controls what enters and leaves the cytoplasm. Prokaryotes have no membranes inside the cell, so the cytoplasm is one undivided compartment.
- Cytoplasm
- Mostly water, with enzymes, ribosomes and other dissolved substances. Most of the cell's chemical reactions (its metabolism) take place here.
- Nucleoid (naked DNA in a loop)
- The region of cytoplasm that holds the DNA. There is usually one chromosome, a single circular DNA molecule. It is naked (not bound to histone proteins) and is not enclosed by a nuclear membrane.
- 70S ribosomes
- Sites of protein synthesis, free in the cytoplasm. Prokaryotic ribosomes are 70S (a 50S and a 30S subunit), smaller than the 80S ribosomes in eukaryotic cytoplasm. They are drawn far too large here.
- Plasmid
- A small extra loop of naked DNA, separate from the chromosome. Plasmids carry a few genes, such as for antibiotic resistance, and can be passed from one bacterium to another. Not every bacterium has them.
- Flagellum
- A long, helical protein filament that is rotated by a motor at its base in the wall and membrane, pushing the cell along. Bacillus species have several; Staphylococcus has none. It has no 9+2 microtubules.
- Pili
- Short, thin protein projections from the surface. They help bacteria stick to surfaces and to each other, and some are used to pass DNA between cells. Many, but not all, bacteria have them.
- Capsule (only some bacteria)
- A slimy layer, often of polysaccharide, outside the wall. It protects some bacteria from drying out and from being engulfed by white blood cells. Many bacteria have no capsule; it is not part of the required structure.
Animal cell
- Nucleus (double membrane)
- Holds the chromosomes, which are DNA molecules wound around histone proteins. The nuclear envelope is two membranes separated by a narrow space; the outer membrane is continuous with the rough ER.
- Plasma membrane
- A phospholipid bilayer with proteins that encloses the cell and controls what enters and leaves. Animal cells have no wall outside it, so they can change shape and take in material by endocytosis.
- Mitochondrion
- The site of aerobic respiration, which produces ATP. It has a double membrane; the inner membrane is folded into cristae, which increase its surface area. The fluid inside is the matrix.
- Rough ER (with ribosomes)
- Flattened membrane sacs (cisternae) with 80S ribosomes on their outer surface. Proteins made by these ribosomes pass into the ER, are folded and modified, and are carried in vesicles to the Golgi apparatus.
- Free ribosomes (80S)
- Ribosomes in the cytoplasm that are not attached to a membrane. They make proteins for use inside the cell. Eukaryotic cytoplasmic ribosomes are 80S (a 60S and a 40S subunit). They are drawn far too large here.
- Golgi apparatus
- A stack of curved, flattened membrane sacs. It receives proteins from the rough ER, modifies them (for example by adding carbohydrate) and sorts them into vesicles, for secretion or for lysosomes.
- Smooth ER
- A network of membrane tubules with no ribosomes. It makes lipids, including phospholipids and steroid hormones; in some cells it also stores calcium ions or breaks down toxins.
- Lysosome
- A vesicle bounded by a single membrane and filled with digestive (hydrolytic) enzymes. It digests material taken in by endocytosis and breaks down worn-out organelles.
- Vesicles
- Small sacs of single membrane that carry materials around the cell. Secretory vesicles bud from the Golgi apparatus, move to the plasma membrane and release their contents by exocytosis.
- Nucleolus
- A dense region inside the nucleus, with no membrane around it, where ribosome subunits are assembled from rRNA and proteins. The subunits leave the nucleus through nuclear pores.
- Nuclear pore
- A channel through the nuclear envelope, where its two membranes join. Pores let mRNA and ribosome subunits out of the nucleus and let proteins such as enzymes in.
- Centrioles
- Two short cylinders at right angles, each made of nine triplets of microtubules. They are part of the centrosome, which organises microtubules, including the spindle in cell division. Most plant cells have none.
- Cytoskeleton (microtubules)
- A network of protein fibres. Microtubules (shown spreading from the centrosome) and thinner actin microfilaments support the cell's shape, anchor organelles and move vesicles and chromosomes.
- Cytoplasm
- Everything inside the plasma membrane except the nucleus. Its watery fluid (cytosol) holds enzymes and the organelles, and internal membranes divide it into compartments (compartmentalization).
Plant cell
- Cell wall (cellulose)
- A tough layer of cellulose fibres outside the plasma membrane. It is freely permeable, gives the cell its shape, and resists the pressure that builds up when water enters by osmosis, so the cell does not burst.
- Plasma membrane
- Pressed against the inside of the wall by turgor pressure. Unlike the wall, it is partially permeable and controls what enters and leaves the cytoplasm.
- Large permanent vacuole
- A large sac of cell sap (water with sugars, ions, pigments and wastes) that fills most of the cell. Water drawn into it by osmosis keeps the cell turgid, which supports the plant. Animal cells have only small, temporary vacuoles.
- Tonoplast
- The single membrane around the vacuole. Its transport proteins control which substances move into and out of the cell sap.
- Chloroplast
- The site of photosynthesis. A double membrane (envelope) surrounds the stroma, which holds enzymes, starch grains, 70S ribosomes and a loop of DNA. Chloroplasts are plastids; animal and fungal cells have no plastids.
- Grana (thylakoid stacks)
- Thylakoids are flattened membrane sacs; stacks of them are called grana, and single thylakoids link the stacks. Thylakoid membranes hold chlorophyll and absorb light for photosynthesis.
- Nucleus
- Has a double membrane with pores, as in all typical eukaryotic cells. Here the large vacuole has pushed it to the side of the cell, into the thin layer of cytoplasm.
- Mitochondrion
- Plant cells have mitochondria as well as chloroplasts. They respire aerobically, day and night, to make the ATP the cell needs.
- Plasmodesmata
- Narrow channels through the cell wall, lined with plasma membrane, that join the cytoplasm of neighbouring cells, so substances can pass from cell to cell.
- Golgi apparatus
- Modifies and packages proteins, and in plant cells also makes some of the polysaccharides of the wall, which vesicles carry to the plasma membrane. (Cellulose itself is made at the plasma membrane.)
- Starch grain
- Plants store glucose made in photosynthesis as starch, in grains inside chloroplasts and in other plastids such as amyloplasts.
Fungal hypha
- Cell wall (chitin)
- Made mainly of chitin, a tough nitrogen-containing polysaccharide, with other polysaccharides, but no cellulose. Like a plant cell wall it is freely permeable and stops the hypha bursting when water enters by osmosis.
- Plasma membrane
- Lines the wall and controls what enters and leaves. Fungi digest their food outside the hypha; they release enzymes by exocytosis and absorb the small molecules that are produced.
- Septum (cross-wall)
- A cross-wall of chitin dividing the hypha into compartments, each with one or more nuclei. Fungi such as Penicillium have septate hyphae; aseptate fungi such as Rhizopus have no septa.
- Septal pore
- A hole in the middle of each septum. Cytoplasm, organelles and even nuclei can pass through it from one compartment to the next.
- Nuclei
- Each nucleus has a double membrane, as in other eukaryotes. A hyphal compartment may hold several nuclei, and aseptate hyphae have many nuclei in one continuous mass of cytoplasm.
- Vacuoles
- Large, permanent vacuoles fill the older parts of a hypha, as in plant cells. They store substances and, by taking up water, push the cytoplasm forwards towards the growing tip.
- Growing tip
- A hypha grows only at its tip. New wall and plasma membrane are added here when vesicles fuse with the membrane.
- Tip vesicles
- Vesicles from the Golgi apparatus crowd into the tip. They carry enzymes and materials for building the wall, and digestive enzymes that are released by exocytosis.
- Mitochondrion
- Fungal cells have mitochondria with cristae, like other eukaryotes, and most fungi can respire aerobically.
- Glycogen granules
- Fungi store carbohydrate as glycogen, as animals do, not as starch. They have no chloroplasts, because fungi are heterotrophs and do not photosynthesize.
- Mycelium (branching hyphae)
- Hyphae branch and grow into a network called a mycelium. A hypha is a long thread with a wall around continuous cytoplasm, not a chain of separate, typical cells.
Three things every typical cell has
| Structure | Why every cell needs it |
|---|---|
| DNA as its genetic material | Stores the genetic instructions for making the cell’s proteins (via mRNA) and is copied and passed on when the cell divides. |
| Cytoplasm, mainly water | Water is the solvent in which enzymes and their substrates dissolve, so metabolic reactions can happen. |
| Plasma membrane made of lipids | A barrier between the cell and its surroundings that controls what enters and leaves, so conditions inside (pH, concentrations) can differ from those outside. |
All cells also have ribosomes, which make proteins.
Prokaryote cell structure
Prokaryotes (bacteria and archaea) have no nucleus. The structure you need is that of Gram-positive eubacteria, such as rod-shaped Bacillus and spherical Staphylococcus, which grows in grape-like clusters:
- Cell wall of peptidoglycan, thick in Gram-positive bacteria, which have no outer membrane. It prevents the cell bursting when water enters by osmosis.
- Plasma membrane just inside the wall.
- Cytoplasm with enzymes and no membrane-bound compartments.
- Naked DNA in a loop: one circular chromosome, not bound to histones, in a paler region called the nucleoid.
- 70S ribosomes, smaller than eukaryotic 80S ribosomes.
Prokaryote structure varies. Many bacteria have plasmids, pili or flagella, and some have a capsule; Gram-negative bacteria have a thin wall and an extra outer membrane; a few, such as mycoplasmas, have no wall. You do not need details of these variations.
Eukaryote cell structure
Eukaryotes (animals, plants, fungi and protists) have a compartmentalized cytoplasm: membranes divide it into separate compartments, each with its own conditions and enzymes. Almost every eukaryotic cell has:
- a plasma membrane around the outside, holding in the cytoplasm;
- 80S ribosomes, some floating free and some bound to the rough ER;
- a nucleus wrapped in two membranes (the nuclear envelope) with nuclear pores through it; inside are the chromosomes, each a linear DNA molecule wound around histone proteins;
- organelles enclosed by membranes: mitochondria, rough and smooth endoplasmic reticulum, the Golgi apparatus, lysosomes and many vesicles and vacuoles;
- a cytoskeleton built from microtubules (hollow tubulin tubes) and microfilaments (thinner actin fibres) that gives the cell its shape and moves organelles, vesicles and chromosomes around.
Organelles by number of membranes
| Membranes | Structures |
|---|---|
| None | 80S ribosomes, cytoskeleton, centrioles |
| One | rough and smooth ER, Golgi apparatus, vesicles, lysosomes, vacuoles |
| Two | nucleus, mitochondria, chloroplasts |
Prokaryotes and eukaryotes compared
| Prokaryote | Eukaryote | |
|---|---|---|
| Nucleus | none: DNA in a nucleoid | DNA inside a double membrane with pores |
| DNA | naked, circular (plus plasmids) | linear chromosomes bound to histones |
| Ribosomes | 70S | 80S in cytoplasm (70S in mitochondria and chloroplasts) |
| Membrane-bound organelles | none | many |
| Typical size | 1–5 µm | 10–100 µm |
Compartmentalization lets incompatible processes run side by side: digestive enzymes are kept inside lysosomes, and the nuclear envelope separates transcription from translation. For how organelles work together, see B2.2 Organelles and compartmentalization.
Life processes in unicellular organisms
A unicellular organism must carry out every function of life within a single cell. Two well-studied freshwater examples are Paramecium, a heterotroph covered in cilia, and Chlamydomonas, a photosynthetic alga with two flagella.
| Function | Paramecium | Chlamydomonas |
|---|---|---|
| Homeostasis (keeping internal conditions within limits) | Contractile vacuoles pump out water that keeps entering by osmosis | Contractile vacuoles pump out excess water |
| Metabolism (all the cell’s chemical reactions) | Enzymes in the cytoplasm and organelles, such as respiration in mitochondria | Respiration and photosynthesis |
| Nutrition (obtaining food) | Cilia sweep bacteria into an oral groove; they are digested in food vacuoles | Makes its own food by photosynthesis in one large chloroplast |
| Movement | Beating cilia | Two beating flagella |
| Excretion (removing metabolic wastes) | CO₂ and ammonia diffuse out across the membrane | CO₂ (and O₂ from photosynthesis) diffuse out |
| Growth (increase in size and dry mass) | Assimilates digested food | Assimilates sugars made by photosynthesis |
| Response to stimuli | Reverses its cilia and backs away after hitting an obstacle | An eyespot detects light; it swims towards moderate light |
| Reproduction | Asexual by binary fission (mitosis); also a sexual process | Asexual by mitosis; also sexual, with gametes |
Notice that excretion means removing waste products of metabolism; getting rid of excess water is osmoregulation, part of homeostasis.
Animal, fungal and plant cells compared
All three are eukaryotic, so they share a nucleus, mitochondria, 80S ribosomes, ER, Golgi apparatus and a cytoskeleton. They differ in a few key features (compare the three eukaryotic 3D models above).
| Feature | Animal cells | Fungal cells | Plant cells |
|---|---|---|---|
| Cell wall | none | yes, mainly chitin | yes, mainly cellulose |
| Vacuoles | small and temporary (vesicles, food vacuoles, lysosomes) | often large and permanent | one large, permanent sap vacuole with a tonoplast |
| Plastids (chloroplasts, amyloplasts) | none | none | yes, though not all cells have chloroplasts |
| Centrioles | yes | absent from most | absent (except in the swimming sperm of mosses and ferns) |
| Cilia and flagella | in some cells (ciliated epithelium, sperm) | absent from most (a flagellum on the spores of chytrids) | absent from flowering plants and conifers; flagella on the sperm of mosses and ferns |
| Carbohydrate store | glycogen | glycogen | starch |
Why the differences matter
- A wall resists the pressure of water entering by osmosis, so plant and fungal cells do not burst; animal cells have no wall, which lets them change shape and take in food by endocytosis.
- In plants the large vacuole stores sap and, by taking up water, keeps the cell turgid; in fungal hyphae, vacuoles fill the older parts and push cytoplasm towards the growing tip.
- Plastids are found only in plants and algae: chloroplasts for photosynthesis, amyloplasts for storing starch.
- Centrioles help organize the spindle in animal cells, and the related basal bodies anchor cilia and flagella; plant cells build a spindle without them.
Atypical cell structure in eukaryotes
A typical eukaryotic cell has one nucleus. Counting nuclei shows that some cells are atypical:
- Anucleate cells, with no nucleus, cannot transcribe DNA, so they cannot make new proteins.
- Multinucleate cells, with many nuclei, can make more mRNA and protein for a large volume of cytoplasm.
Red blood cell (mammals) 0 nuclei
The nucleus (and the mitochondria) are lost as the cell matures, leaving more room for haemoglobin. Without DNA it cannot make new proteins, so it lives only about 120 days.
Phloem sieve tube element 0 nuclei
At maturity it has no nucleus and very few organelles, so sap flows freely from element to element through the sieve plates. A companion cell with a nucleus supplies its proteins through plasmodesmata.
Skeletal muscle fibre many nuclei
Formed when many embryonic muscle cells fuse, so one fibre can be centimetres long and contain hundreds of nuclei in a continuous cytoplasm.
Aseptate fungal hypha many nuclei
The hypha is not divided into cells, so many nuclei share one continuous cytoplasm inside a single wall, as in Rhizopus. Septate hyphae are divided by septa with pores.
| Cell | Nuclei | How it comes about, and why it helps |
|---|---|---|
| Mammalian red blood cell | 0 | The nucleus is pushed out as the cell matures; more space for haemoglobin, but no repairs, so it lives only about 120 days |
| Phloem sieve tube element | 0 at maturity | The nucleus and most organelles break down so sap flows freely; a neighbouring companion cell keeps it alive |
| Skeletal muscle fibre | many | Formed by the fusion of many cells; one fibre can be centimetres long |
| Aseptate fungal hypha | many | No cross-walls (septa), so nuclei share continuous cytoplasm, as in Rhizopus |
Micrographs: identifying and drawing cells
Identifying the type of cell
- Prokaryote or eukaryote? A nucleoid and no membrane-bound organelles means prokaryote; a nucleus (or visible chromosomes, if the cell is dividing) and organelles such as mitochondria means eukaryote. Cells that are part of a multicellular tissue are eukaryotic.
- Plant or animal? A cell wall, a large sap vacuole and chloroplasts point to a plant; no wall, only small vacuoles and perhaps microvilli or cilia point to an animal. Remember that fungi and protists are neither, and that one thin section may miss structures that are present elsewhere in the cell.
What structures look like in electron micrographs
| Structure | Appearance |
|---|---|
| Nucleus | Large, round; double membrane with gaps (pores); grainy chromatin with darker patches (heterochromatin) and a dense nucleolus |
| Chromosomes | Dense, dark threads, visible separately only during cell division |
| Mitochondrion | Oval, double membrane, inner one folded into cristae; matrix often darker than cytoplasm |
| Chloroplast | Larger, double membrane, stacks of thylakoids (grana), often pale starch grains |
| Sap vacuole | Large, very pale area bounded by a single membrane |
| Golgi apparatus | Stack of curved, flattened sacs with vesicles at the edges |
| Rough ER / smooth ER | Parallel sheets dotted with ribosomes / branching tubules without them |
| Ribosomes | Tiny dark dots, free or on rough ER |
| Cell wall / plasma membrane | Thick layer outside the membrane / a thin dark line |
| Microvilli | Many finger-like folds of the plasma membrane |
| Nucleoid, prokaryotic wall | Paler central region with no membrane / thick layer outside the membrane |
Drawing from a micrograph
- Microvilli: finger-like folds of the plasma membrane that increase the surface area for absorption
- Plasma membrane: controls what enters and leaves the cell
- Secretory vesicle: carries proteins from the Golgi to the plasma membrane for exocytosis
- Golgi apparatus: modifies, sorts and packages proteins into vesicles
- Rough ER: its ribosomes make proteins for secretion or for membranes
- Mitochondrion: aerobic respiration produces ATP; cristae increase the inner membrane area
- Nucleus: double membrane (nuclear envelope) separates the chromosomes from the cytoplasm
- Nuclear pore: lets mRNA and ribosome subunits out of the nucleus
- Chromatin: chromosomes (DNA with histones); dark patches are condensed heterochromatin
- Nucleolus: makes ribosome subunits
- Smooth ER: tubules where lipids such as phospholipids and steroids are made
- Lysosome: contains digestive enzymes
- Free ribosomes (80S): make proteins that stay in the cytoplasm
- Cell wall (cellulose): strong and freely permeable; resists turgor pressure
- Plasma membrane: pressed against the wall; controls entry and exit
- Plasmodesma: membrane-lined channel through the wall joining neighbouring cells
- Large (sap) vacuole: stores cell sap; keeps the cell turgid
- Tonoplast: single membrane around the vacuole
- Chloroplast: double membrane; site of photosynthesis
- Granum: stack of thylakoids where light is absorbed
- Starch grain: stored carbohydrate
- Mitochondrion: aerobic respiration (plant cells have both organelles)
- Nucleus: double membrane with pores, pushed to the edge
- Golgi apparatus: packages proteins and makes some wall polysaccharides
- Cytoplasm: thin layer between the vacuole and the membrane
The origin of eukaryotic cells by endosymbiosis
All eukaryotes appear to descend from one single-celled common ancestor that already had a nucleus and reproduced sexually. Mitochondria and chloroplasts were added later, by endosymbiosis: one cell living inside another.
- An aerobic bacterium was taken into the ancestral cell by endocytosis but not digested.
- It survived inside a vesicle, so it was surrounded by two membranes, and it supplied ATP from aerobic respiration while the host provided food and protection. Both benefited (mutualism).
- It divided by binary fission inside the host and was passed to daughter cells when the host divided.
- Over many generations most of its genes moved to the host nucleus and it became a mitochondrion, found in almost all eukaryotes.
- Later, in some eukaryotes, a photosynthetic cyanobacterium was taken in the same way and became the chloroplast. Its descendants include the green and red algae and the plants; some other algae gained chloroplasts later by engulfing such algae (secondary endosymbiosis).
Reduced motion is on: use the arrows to step through. Change this in Display settings.
Text description of this animation
A diagram in six steps. An ancestral single-celled eukaryote, which already has a nucleus, takes in an aerobic bacterium by endocytosis. The bacterium is not digested but survives inside a vesicle, so it is surrounded by two membranes. It respires aerobically, supplying ATP, divides by binary fission and is passed to the host's daughter cells, becoming a mitochondrion. Later, in the ancestor of green algae, red algae and plants, a photosynthetic cyanobacterium is taken in the same way and becomes a chloroplast. The last step summarizes the evidence: double membranes, naked circular DNA, 70S ribosomes, division by binary fission and a size similar to bacteria.
- A nucleated ancestor. The common ancestor of all eukaryotes was a single cell that already had a nucleus, a cytoskeleton and internal membranes, and could reproduce sexually. It probably could not respire aerobically. Nearby is a free-living aerobic bacterium, with its own plasma membrane, naked circular DNA and 70S ribosomes.
- Engulfed by endocytosis. The host cell surrounds the bacterium with its plasma membrane and takes it in by endocytosis. The bacterium ends up inside a vesicle, so it is now enclosed by two membranes, its own plasma membrane inside and the vesicle membrane from the host outside.
- Not digested, but kept. Instead of being digested, the bacterium survives and respires aerobically, supplying the host with ATP; the host supplies food molecules and protection. Both benefit (mutualism). The bacterium grows and divides by binary fission inside the host.
- It becomes a mitochondrion. When the host divides, each daughter cell receives some of the bacteria. Over many generations most of the bacterium's genes move to the host's nucleus, and it can no longer live independently. It has become a mitochondrion, found in almost all eukaryotes.
- A second event makes chloroplasts. Later, in one lineage of these eukaryotes, a photosynthetic cyanobacterium is taken in by endocytosis and kept in the same way. It becomes the chloroplast. Its descendants include green and red algae and plants, which have both mitochondria and chloroplasts; some other algae gained chloroplasts later by engulfing such algae (secondary endosymbiosis). Animals and fungi have only mitochondria.
- The evidence today. Mitochondria and chloroplasts still resemble bacteria. Each has a double membrane, its own naked circular DNA, 70S ribosomes that make some of its proteins, and a size like a bacterium (about 1–10 µm). Neither can be made from scratch; new ones form only by binary fission of existing ones.
Evidence
Mitochondria and chloroplasts:
- have double membranes, as expected if a bacterium was enclosed by a vesicle;
- contain their own naked, circular DNA, like bacteria, with genes that are transcribed;
- have 70S ribosomes and make some of their own proteins;
- replicate only by dividing in two (binary fission), so a cell cannot make them from scratch;
- are about the size of bacteria (around 1–10 µm);
- have gene sequences most similar to those of particular groups of bacteria, and some antibiotics that block bacterial 70S ribosomes also affect them.
Cell differentiation
A multicellular organism develops from a single cell, the zygote, by repeated mitosis, so almost all its cells contain the same genome. Yet they become very different: neurons, muscle fibres, red blood cells and xylem vessels. Differentiation is the process by which a cell develops a specialized structure and function, and its basis is different patterns of gene expression.
- Housekeeping genes, such as those for the enzymes of respiration, are expressed in almost every cell.
- Other genes are expressed only in particular cell types. The genes for haemoglobin are expressed only in developing red blood cells; the insulin gene only in β cells of the pancreas.
What switches genes on
Which genes a cell expresses is often triggered by changes in its environment, meaning its surroundings inside or outside the body:
- chemical signals from neighbouring cells, or concentration gradients of signal molecules that tell a cell its position in an embryo;
- hormones circulating in the body;
- external conditions: a honeybee larva fed royal jelly develops into a queen rather than a worker, with the same genes; in plants, light switches on the genes for chloroplast development.
Cells that receive the same signals develop in the same way and form a tissue: a group of cells with the same structure and function. The advantage is that each cell type can be specialized for one job and do it more efficiently. Stem cells, which have not yet differentiated, are covered in B2.3 Cell specialization.
Evolution of multicellularity
Multicellularity was not a single evolutionary event. It has evolved independently many times, by cells staying together after division (or gathering together) and becoming dependent on each other:
- all animals and all plants are multicellular (each group arose from a single multicellular origin);
- many fungi are multicellular (mushroom-forming fungi and moulds), but others, such as yeasts, are unicellular;
- many eukaryotic algae are multicellular, and multicellularity evolved separately in brown algae (such as kelps), red algae and several groups of green algae, for example Volvox.
Close relatives of multicellular groups are often unicellular, such as Chlamydomonas among the green algae, which is evidence that multicellularity arose separately in different lineages.
Advantages
- Larger body size: a multicellular organism is not limited by the size one cell can reach, which helps it to compete for light (trees, kelps), to catch prey or to avoid being eaten.
- Cell specialization: different cells can differentiate for different functions (division of labour), so each function is carried out more efficiently and more complex body forms are possible.
- Longer life: the death of a few cells does not kill the organism, because they can be replaced.
Large organisms also need cells to stick together (cell-adhesion molecules, B2.1 Membranes and membrane transport) and to communicate, so multicellularity brought new problems as well as advantages.
Must-know definitions
- Cell theory
- All living organisms are made of one or more cells, and the cell is the smallest unit that can carry out all the functions of life. From the theory we can deduce that any newly discovered organism will consist of cells.
- Magnification
- How many times larger an image is than the real object: magnification = image size ÷ actual size, with both lengths in the same unit (1 mm = 1,000 µm; 1 µm = 1,000 nm). It has no units and is written with a × sign.
- Resolution
- The smallest distance between two points at which they can still be seen as separate. Light microscopes resolve about 0.2 µm (200 nm); electron microscopes resolve around 1 nm or better, so they show the ultrastructure of cells.
- Prokaryote
- A cell with no nucleus and no membrane-bound organelles. A typical Gram-positive bacterium has a cell wall, plasma membrane, cytoplasm, naked DNA in a loop in a nucleoid region, and 70S ribosomes.
- Eukaryote
- A cell whose cytoplasm is compartmentalized by membranes, with a nucleus (double membrane with pores, chromosomes of DNA bound to histones), membrane-bound organelles, 80S ribosomes and a cytoskeleton of microtubules and microfilaments.
- 70S and 80S ribosomes
- S (svedberg) units measure how fast a particle sediments in a centrifuge, so they indicate size. Prokaryotic ribosomes are 70S (50S + 30S subunits); ribosomes in eukaryotic cytoplasm and on rough ER are larger, 80S (60S + 40S).
- Plastids
- A family of double-membraned organelles found only in plants and algae, including chloroplasts (photosynthesis) and amyloplasts (starch storage). Animal and fungal cells have none.
- Atypical cell structure
- A cell that does not fit the typical pattern of one nucleus per cell. Mammalian red blood cells and mature phloem sieve tube elements have no nucleus; skeletal muscle fibres and aseptate fungal hyphae have many nuclei.
- EndosymbiosisAHL
- The theory that mitochondria, and later chloroplasts, evolved from free-living bacteria that were taken into a nucleated host cell by endocytosis and lived on inside it. Evidence: double membranes, naked circular DNA, 70S ribosomes and division by binary fission.
- Cell differentiationAHL
- The development of cells with the same genome into specialized types with different structures and functions, because different genes are expressed in each type. The pattern of expression is often triggered by signals from the cell's surroundings.
Common exam mistakes
Loses marks Electron microscopes are better because they magnify more.
Write instead Their advantage is higher resolution. Magnifying a light-microscope image beyond about ×1,500 just makes a bigger blur; electrons resolve detail down to about 1 nm, so higher magnification becomes useful.
Loses marks A 30 mm image at ×10,000 is 30 ÷ 10,000 = 0.003 µm long.
Write instead Convert to the same unit first: 30 mm = 30,000 µm, and 30,000 µm ÷ 10,000 = 3 µm. Check the answer is sensible for the structure (3 µm suits a mitochondrion).
Loses marks Prokaryotes have no DNA (or no ribosomes) because they have no nucleus.
Write instead Prokaryotes have DNA — usually one circular, naked chromosome in the nucleoid region of the cytoplasm — and many 70S ribosomes. What they lack is a nuclear membrane and other membrane-bound organelles.
Loses marks Plant cells have chloroplasts instead of mitochondria.
Write instead Plant cells have mitochondria too, and respire all the time. Many plant cells (in roots, for example) have no chloroplasts at all, but nearly all have mitochondria.
Loses marks Fungal and bacterial cell walls are made of cellulose.
Write instead Only plant cell walls are mainly cellulose. Fungal walls are mainly chitin, and bacterial walls are peptidoglycan.
Loses marks Eukaryotic cells have 70S ribosomes in their cytoplasm.
Write instead Eukaryotic cytoplasm and rough ER have 80S ribosomes. The only 70S ribosomes in a eukaryotic cell are inside its mitochondria and chloroplasts.
Loses marks Drawing the plasma membrane as a double line, or shading organelles in a drawing from a micrograph.
Write instead Draw the plasma membrane and other single membranes as one sharp, unbroken line; use two lines for double membranes (nucleus, mitochondria, chloroplasts) and a thicker double line for a cell wall. Do not shade.
Loses marks Red blood cells and sieve tube elements disprove cell theory.
Write instead They are atypical: each develops from a cell with a nucleus and is still a membrane-bound unit of life. Atypical cells test the theory, and it has been kept because it explains so many observations.
Loses marks Chloroplasts and mitochondria came from the same engulfed bacterium.
Write instead They came from two separate events: an aerobic bacterium gave rise to mitochondria in the ancestor of all eukaryotes; later a photosynthetic cyanobacterium gave rise to chloroplasts, only in the ancestors of algae and plants.
AHLLoses marks Specialized cells are different because they contain different genes.
Write instead Almost all the cells of an organism contain the same genes. They differ because they express different genes, switched on or off in response to signals during development.
AHL
Self-test
Answer the questions, then check. Your best score and the flashcards you know are saved in this browser only.
Multiple-choice questions
Flashcards
Say the answer to yourself, then reveal it. Mark Got it or Again; cards you've got move to the back of the deck.
State cell theory and one prediction you can deduce from it.
All living organisms are made of one or more cells, and the cell is the smallest unit of life. Deduction — a newly discovered organism will consist of one or more cells.
Give the magnification equation and its two rearrangements.
Magnification = image size ÷ actual size; actual size = image size ÷ magnification; image size = actual size × magnification. Convert both lengths to the same unit first (1 mm = 1,000 µm, 1 µm = 1,000 nm).
How is an eyepiece graticule calibrated?
Line it up with a stage micrometer (a slide ruled in known divisions, e.g. 0.01 mm) and count how many eyepiece units match a known length. This must be repeated for each objective lens.
How long should a scale bar for 5 µm be on a drawing at ×4,000?
Length = length represented × magnification = 5 µm × 4,000 = 20,000 µm = 20 mm.
Magnification vs resolution
Magnification is how many times larger the image is than the object. Resolution is the smallest distance between two points that can still be distinguished — about 200 nm for light and about 1 nm or better for electron microscopes.
Freeze fracture and cryo-EM — what does each show?
Freeze fracture splits rapidly frozen cells along weak planes, often through the middle of membranes, and a metal replica of the surface is viewed by electron microscopy. Cryo-EM images molecules frozen in vitreous ice and combines thousands of images into a 3D structure at near-atomic resolution.
What is immunofluorescence?
Light microscopy using antibodies, tagged with a fluorescent dye, that bind one specific protein (antigen), so that protein glows wherever it is in the cell. Different colours can show several proteins at once.
Which three structures do all typical cells have, and why?
DNA as genetic material (instructions for proteins); cytoplasm that is mainly water (solvent for metabolic reactions); a lipid plasma membrane (barrier that controls exchange, so the inside can differ from the outside).
List the structures of a Gram-positive bacterium such as Bacillus.
Cell wall (thick peptidoglycan, no outer membrane), plasma membrane, cytoplasm, naked DNA in a loop in the nucleoid, 70S ribosomes. Plasmids, pili, flagella and a capsule are found in some but not all.
Classify eukaryotic structures by number of membranes.
None — 80S ribosomes, cytoskeleton, centrioles. One — rough and smooth ER, Golgi apparatus, vesicles, lysosomes, vacuoles. Two — nucleus, mitochondria, chloroplasts.
Name the eight functions of life a unicellular organism must carry out.
Homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction. For example, Paramecium uses contractile vacuoles (homeostasis), food vacuoles (nutrition) and cilia (movement).
Animal, fungal and plant cells — cell walls, vacuoles and plastids
Wall — none / chitin / cellulose. Vacuoles — small and temporary / often large / one large permanent sap vacuole. Plastids — none / none / chloroplasts and amyloplasts. Animals and fungi store glycogen; plants store starch.
Which cells have centrioles, cilia or flagella?
Animal cells have centrioles; some have cilia, and sperm have a flagellum. Most fungi and flowering plants have none; exceptions are the swimming sperm of mosses and ferns and the spores of chytrid fungi.
Give two anucleate and two multinucleate cells.
Anucleate — mammalian red blood cells and mature phloem sieve tube elements. Multinucleate — skeletal muscle fibres and aseptate fungal hyphae.
How should membranes and walls be drawn from an electron micrograph?
Plasma membrane and other single membranes as one sharp line; double membranes (nucleus, mitochondria, chloroplasts) as two lines, with the nuclear envelope doubled back at pores; a cell wall as a thicker double line. No shading; annotate with functions.
Give four pieces of evidence that mitochondria and chloroplasts evolved by endosymbiosis.
They have double membranes, their own naked circular DNA, 70S ribosomes like bacteria, and they replicate only by binary fission. They are also the size of bacteria.
What is the basis of cell differentiation?
Different patterns of gene expression. Cells share the same genome, but signals from their surroundings (neighbouring cells, hormones, external conditions) switch different genes on or off.
How many times did multicellularity evolve, and what are its advantages?
Many times, independently — in animals, plants, several groups of fungi and several groups of algae. It allows a larger body size and specialization of cells for different functions.
Last reviewed 19 October 2026. Spotted a mistake? Report it.