What you'll learn
Cell structure and organelles establishes the unit of life at the level of detail CAPE requires, which is considerably beyond CSEC. At this level you are expected to know not only what each organelle does but how its structure achieves that function, how organelles cooperate in sequences such as the secretory pathway, and how the techniques of microscopy and cell fractionation allowed these structures to be discovered in the first place. By the end of this topic you should be able to distinguish prokaryotic from eukaryotic cells, describe the ultrastructure and function of every organelle, explain the secretory pathway, compare light and electron microscopy with their respective magnification and resolution, calculate magnification and actual size, describe cell fractionation, and explain the endosymbiotic theory.
Key terms and definitions
Prokaryotic cell — a cell with no nucleus and no membrane-bound organelles
Eukaryotic cell — a cell with a true nucleus and membrane-bound organelles
Ultrastructure — the detailed internal structure of a cell as revealed by the electron microscope
Organelle — a specialised structure within a cell performing a particular function
Magnification — the number of times larger an image is than the object
Resolution — the minimum distance between two points at which they can still be distinguished as separate
Cell fractionation — the separation of organelles from a cell for individual study
Homogenisation — the breaking open of cells to release their contents
Ultracentrifugation — separation of organelles by spinning at successively higher speeds
Differential centrifugation — the technique of separating components by their densities in stages
Endosymbiotic theory — the proposal that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by a larger cell
Core concepts
Prokaryotic cells
Prokaryotic cells are small, typically 0.5 to 5 micrometres across, and have no nucleus and no membrane-bound organelles.
Their features are: a cell wall of peptidoglycan, sometimes called murein, which is chemically quite different from the cellulose wall of plants; a cell surface membrane; cytoplasm; a single circular loop of DNA lying free in the cytoplasm in a region called the nucleoid; small ribosomes of 70S; and often one or more plasmids carrying additional genes, including those for antibiotic resistance.
Some also have a protective capsule of slime, flagella for locomotion, and pili for attachment and for the transfer of plasmids between cells.
Prokaryotes divide by binary fission rather than mitosis.
Eukaryotic cells and the nucleus
Eukaryotic cells are larger, typically 10 to 100 micrometres, with DNA enclosed in a nucleus and a range of membrane-bound organelles.
The nucleus is bounded by a double membrane, the nuclear envelope, perforated by nuclear pores that allow messenger RNA and ribosomal subunits to pass out and regulatory molecules to pass in. Inside, chromatin consists of DNA wound around histone proteins, condensing into visible chromosomes during division. The nucleolus is a dense region where ribosomal RNA is synthesised and ribosomal subunits are assembled.
The organelles and their structure–function relationships
The mitochondrion is the site of aerobic respiration. It has a double membrane, the inner one folded into cristae which greatly increase the surface area available for the electron transport chain, and an inner fluid-filled matrix containing the enzymes of the Krebs cycle. Mitochondria also contain their own circular DNA and 70S ribosomes. Cells with a high energy demand, such as muscle and sperm cells, contain very many mitochondria.
The chloroplast, found in plant cells, is the site of photosynthesis. It too has a double membrane. Inside, thylakoid membranes stacked into grana carry the photosynthetic pigments and are the site of the light-dependent reactions, while the surrounding stroma contains the enzymes of the light-independent reactions. The stacking of thylakoids maximises the surface area for light absorption. Chloroplasts also contain circular DNA and 70S ribosomes.
Ribosomes are the site of protein synthesis. They consist of a large and a small subunit made of ribosomal RNA and protein, and they are not membrane-bound. Eukaryotic cytoplasmic ribosomes are 80S; those in mitochondria, chloroplasts and prokaryotes are 70S.
Rough endoplasmic reticulum is a system of flattened membrane sacs studded with ribosomes. Proteins synthesised on those ribosomes enter the cisternae and are transported through the cell. Cells that secrete large quantities of protein, such as pancreatic cells and plasma cells, have extensive rough endoplasmic reticulum.
Smooth endoplasmic reticulum has no ribosomes and is the site of lipid and steroid synthesis, and of the storage and release of calcium ions in muscle.
The Golgi apparatus is a stack of flattened membrane sacs that modifies proteins arriving from the rough endoplasmic reticulum, for example by adding carbohydrate to form glycoproteins, then sorts and packages them into vesicles for secretion or for delivery elsewhere. It also produces lysosomes.
Lysosomes are vesicles containing hydrolytic enzymes. They digest material taken in by phagocytosis, break down worn-out organelles, and in some circumstances release their enzymes to destroy the cell itself, a process called autolysis. Their single membrane keeps the enzymes safely separated from the cytoplasm.
Centrioles, found in animal cells, are made of microtubules and organise the spindle during nuclear division.
The cell wall in plants is made of cellulose microfibrils, providing mechanical strength and preventing the cell bursting when water enters by osmosis. Fungal walls contain chitin instead.
The permanent vacuole in plant cells is bounded by the tonoplast and contains cell sap, maintaining turgor and storing solutes.
The secretory pathway
Examiners frequently ask candidates to trace the route of a protein, and the sequence must be given in order.
The gene is transcribed in the nucleus and the messenger RNA passes out through a nuclear pore. It is translated on ribosomes attached to the rough endoplasmic reticulum, and the polypeptide enters the cisternae. Vesicles bud off from the rough endoplasmic reticulum and carry the protein to the Golgi apparatus, where it is modified, for example by the addition of carbohydrate. Further vesicles bud from the Golgi and move to the cell surface membrane, where they fuse with it and release the protein outside the cell by exocytosis.
The pathway demonstrates that organelles function as an integrated system rather than independently, which is the point the question is usually testing.
Microscopy
The light microscope uses visible light and glass lenses. Its maximum useful magnification is about 1,500 times and its resolution about 200 nanometres, limited by the wavelength of light. Its advantages are that specimens may be living, colour images are possible, and the equipment is inexpensive and portable.
The electron microscope uses a beam of electrons, whose wavelength is far shorter, giving a resolution of about 0.1 nanometres and magnification up to about 500,000 times. Because electrons are deflected by air, the specimen must be in a vacuum, so living material cannot be examined, and preparation may introduce artefacts.
The transmission electron microscope passes electrons through an extremely thin section, producing a detailed two-dimensional image of internal structure. The scanning electron microscope reflects electrons from the surface, producing a three-dimensional image of lower resolution.
The distinction between magnification and resolution is essential and frequently tested. Magnification is how much larger the image is; resolution is the amount of detail. Magnifying beyond the resolving power of the instrument produces a larger but no clearer image, which is called empty magnification.
Magnification calculations
Magnification equals image size divided by actual size, and the relationship rearranges to give actual size equals image size divided by magnification.
Units must be converted before dividing. One millimetre is 1,000 micrometres, and one micrometre is 1,000 nanometres.
For a scale bar, measure the bar on the image, convert both measurements to the same unit, and divide the measured length by the length the bar represents to obtain the magnification.
Cell fractionation
To study organelles individually they must be separated, and the procedure has three stages, each with a reason that is examinable.
The tissue is first placed in a solution that is cold, to reduce enzyme activity and prevent organelles being digested by released hydrolytic enzymes; isotonic, so that organelles neither gain nor lose water by osmosis and therefore do not burst or shrink; and buffered, so that the pH remains constant and proteins are not denatured.
Homogenisation follows, in which the cells are broken open in a blender to release the organelles, and the homogenate is filtered to remove debris and unbroken cells.
Ultracentrifugation then separates the organelles by density. The homogenate is spun at a low speed, and the densest components — nuclei — form a pellet. The supernatant is decanted and spun at a higher speed, pelleting mitochondria and chloroplasts. Successively higher speeds pellet lysosomes, then endoplasmic reticulum, and finally ribosomes.
The order of pelleting from most to least dense — nuclei, chloroplasts, mitochondria, lysosomes, endoplasmic reticulum, ribosomes — is worth memorising.
The endosymbiotic theory
Mitochondria and chloroplasts share several features with prokaryotes: a double membrane, circular DNA, 70S ribosomes, and the ability to divide independently of the cell by binary fission.
The endosymbiotic theory proposes that these organelles originated as free-living prokaryotic cells engulfed by a larger host cell, which then formed a mutually beneficial relationship rather than digesting them. The inner membrane represents the original prokaryote's membrane and the outer one the host's engulfing vesicle.
Worked examples
Example 1: A magnification calculation (4 marks)
An electron micrograph shows a mitochondrion measuring 48 millimetres in length. The magnification is stated as 24,000 times. Calculate the actual length in micrometres.
Actual size equals image size divided by magnification, so the actual length is 48 ÷ 24,000 = 0.002 millimetres.
Converting to micrometres, multiply by 1,000: 0.002 × 1,000 = 2.0 micrometres.
This is a reasonable value, since mitochondria are typically 1 to 10 micrometres long, which confirms the calculation.
Example 2: Relating structure to function (5 marks)
A cell from the pancreas secretes large quantities of digestive enzyme. Describe three organelles you would expect to be abundant and explain why.
Rough endoplasmic reticulum would be extensive, because the enzymes are proteins and are synthesised on the ribosomes attached to it, then transported through its cisternae.
Golgi apparatus would be prominent, because the proteins must be modified, sorted and packaged into secretory vesicles before release.
Mitochondria would be numerous, because protein synthesis, vesicle transport and exocytosis all require ATP, so a cell with a high secretory output has a correspondingly high energy demand, and mitochondria are the site of aerobic respiration.
Example 3: Explaining the fractionation medium (4 marks)
Explain why the solution used in cell fractionation must be cold, isotonic and buffered.
It must be cold to reduce the activity of enzymes, particularly the hydrolytic enzymes released from damaged lysosomes, which would otherwise digest the organelles being isolated.
It must be isotonic, meaning of the same water potential as the cell contents, so that there is no net movement of water into or out of the organelles by osmosis. Otherwise they would take in water and burst, or lose water and shrink, and would no longer be representative.
It must be buffered so that the pH remains constant. A change in pH would alter the ionic and hydrogen bonding within proteins, denaturing enzymes and damaging organelle structure.
Common mistakes and how to avoid them
The most frequent error is confusing magnification with resolution. Magnification makes the image larger; resolution determines how much detail can be distinguished, and increasing magnification beyond the resolving limit gains nothing.
Students often state that electron microscopes show living specimens in colour. They require a vacuum, so specimens cannot be living, and the images are monochrome, with any colour added artificially.
In magnification calculations, failing to convert units before dividing is the standard error. Convert everything to micrometres or everything to millimetres first.
Another common slip is giving the secretory pathway out of order, or omitting the vesicles that link the organelles. The vesicles are what make it a pathway.
Finally, candidates frequently describe prokaryotic cell walls as made of cellulose. They are made of peptidoglycan; cellulose is the plant wall and chitin the fungal wall.
Exam technique for "Cell structure and organelles"
When asked to relate structure to function, name the structural feature explicitly before giving the consequence — cristae increase surface area, thylakoid stacking increases area for light absorption, the lysosome membrane isolates hydrolytic enzymes.
For calculations, write the relationship, convert the units, substitute and then state the answer with its unit. Method marks are available throughout.
In fractionation questions, give the reason alongside each condition or step. The reasons, not the steps, carry the marks.
When comparing microscopes, use the same criteria for both: magnification, resolution, whether specimens can be living, and the type of image produced.
For endosymbiotic theory questions, list the specific shared features with prokaryotes rather than asserting the relationship generally.
Quick revision summary
Prokaryotic cells lack a nucleus and membrane-bound organelles, having a peptidoglycan wall, circular DNA in a nucleoid, 70S ribosomes and often plasmids, and divide by binary fission. Eukaryotic cells have a nucleus with a double envelope pierced by pores, chromatin of DNA and histones, and a nucleolus making ribosomal subunits. Mitochondria have cristae increasing surface area for the electron transport chain and a matrix holding Krebs cycle enzymes; chloroplasts have grana of thylakoids for the light-dependent reactions and a stroma for the light-independent ones; both contain circular DNA and 70S ribosomes. Rough endoplasmic reticulum bears ribosomes and transports protein, smooth makes lipids, the Golgi modifies and packages, and lysosomes hold hydrolytic enzymes behind a single membrane. The secretory pathway runs nucleus, rough endoplasmic reticulum, vesicle, Golgi, vesicle, cell surface membrane, exocytosis. Light microscopes resolve to about 200 nanometres and electron microscopes to about 0.1 nanometres, but require a vacuum so specimens cannot be alive. Magnification is image size divided by actual size. Fractionation uses a cold, isotonic, buffered medium, then homogenisation and differential centrifugation, pelleting nuclei first and ribosomes last.