Cell structure

• Same unit on top and bottom. Resolution is set by wavelength; magnification cannot buy detail. Double membranes: nucleus, mitochondrion, chloroplast. 80S in the cytoplasm, 70S in bacteria, mitochondria and chloroplasts.
• Same unit on top and bottom, and no unit on a magnification. Recalibrate for every objective. Resolution is set by wavelength. Nucleus, mitochondrion, chloroplast: double. 80S in the cytoplasm, 70S in bacteria, mitochondria and chloroplasts. Nucleic acid core, protein capsid, sometimes an envelope.

• 1.1.1 I can make — make temporary preparations of cellular material suitable for viewing with a light microscope
• 1.1.2 I can draw — draw cells from microscope slides and photomicrographs
• 1.1.3 I can calculate — calculate magnifications of images and actual sizes of specimens from drawings, photomicrographs and electron micrographs (scanning and transmission)
• 1.1.4 I can use — use an eyepiece graticule and stage micrometer scale to make measurements and use the appropriate units, millimetre (mm), micrometre (µm) and nanometre (nm)
• 1.1.5 I can define — define resolution and magnification and explain the differences between these terms, with reference to light microscopy and electron microscopy
• 1.2.1 I can recognise — recognise organelles and other cell structures found in eukaryotic cells and outline their structures and functions, limited to: • cell surface membrane • nucleus, nuclear envelope and nucleolus • rough endoplasmic reticulum • smooth endoplasmic reticulum • Golgi body (Golgi apparatus or Golgi complex) • mitochondria (including the presence of small circular DNA) • ribosomes (80S in the cytoplasm and 70S in chloroplasts and mitochondria) • lysosomes • centrioles and microtubules • cilia • microvilli • chloroplasts (including the presence of small circular DNA) • cell wall • plasmodesmata • large permanent vacuole and tonoplast of plant cells
• 1.2.2 I can describe — describe and interpret photomicrographs, electron micrographs and drawings of typical plant and animal cells
• 1.2.3 I can compare — compare the structure of typical plant and animal cells
• 1.2.4 I can state — state that cells use ATP from respiration for energy-requiring processes
• 1.2.5 I can outline — outline key structural features of a prokaryotic cell as found in a typical bacterium, including: • unicellular • generally 1–5 µm diameter • peptidoglycan cell walls • circular DNA • 70S ribosomes • absence of organelles surrounded by double membranes
• 1.2.6 I can compare — compare the structure of a prokaryotic cell as found in a typical bacterium with the structures of typical eukaryotic cells in plants and animals
• 1.2.7 I can state — state that all viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some viruses have an outer envelope made of phospholipids

Why the two shorthands are replaced. “Makes things look bigger” hides the one idea that explains every limit of the light microscope: two structures closer together than about 200 nm merge into one however large the image. “Non-living particle” is an opinion about life; “non-cellular, nucleic acid core, protein capsid” is a structural description that can be checked against an electron micrograph, and it is the one the syllabus states.

• “An electron microscope shows more detail because it magnifies more.” Correct It shows more detail because it has a higher resolution: electrons have a far shorter wavelength than light, so points about 0.5 nm apart can be distinguished instead of about 200 nm. Magnifying a light image beyond its resolution gives a bigger blur, not more detail. Resolution is not magnification.
• “Magnification = 36 mm ÷ 60 µm = 0.6.” Correct Put image size and actual size in the same unit before dividing: 36 000 µm ÷ 60 µm = ×600. A magnification has no unit.
• “I calibrated the graticule on low power, so I can use that value on high power.” Correct The graticule sits in the eyepiece and never changes size, but the image of the specimen does. Recalibrate for every objective: an eyepiece unit worth 10 µm at ×10 is worth 2.5 µm at ×40.
• “Ribosomes and lysosomes both have a single membrane.” Correct Count membranes exactly. Double: nucleus, mitochondrion, chloroplast. Single: RER, SER, Golgi body, lysosome, vesicles, tonoplast, cell surface membrane. None: ribosomes, centrioles, microtubules, nucleolus.
• “Mitochondria have 80S ribosomes, like the rest of the cell.” Correct 80S ribosomes are in eukaryotic cytoplasm, free or on the RER. 70S ribosomes are in mitochondria, chloroplasts and prokaryotes, and mitochondria and chloroplasts also contain small circular DNA.
• “Mitochondria make energy for the cell.” Correct Energy is never produced. Respiration transfers energy into ATP; hydrolysis of ATP to ADP and phosphate releases energy for energy-requiring processes.
• “A virus is a very small bacterium.” Correct A virus is non-cellular: a nucleic acid core (DNA or RNA) in a protein capsid, with a phospholipid envelope in some. It has no cytoplasm, no ribosomes and no cell surface membrane.
• “Plant cells have a cell wall instead of a cell membrane.” Correct Plant cells have both: the fully permeable cellulose wall lies outside the partially permeable cell surface membrane. A comparison answer states both cells for every feature.
• “Resolution is how much bigger the image is.” Repair That is magnification. Resolution is the ability to distinguish two points that are close together as separate.
• “An electron microscope shows more detail because it magnifies more.” Repair It shows more detail because it has a higher resolution, since electrons have a much shorter wavelength than light.
• “Magnification = 36 mm ÷ 60 µm = 0.6.” Repair Convert to the same unit first: 36 000 µm ÷ 60 µm = ×600.
• “1 µm = 100 nm.” Repair 1 µm = 1000 nm, and 1 mm = 1000 µm. Every step on the ladder is a factor of 1000.
• “The magnification is ×20 000 µm.” Repair A magnification is a ratio of two lengths in the same unit, so it has no unit: ×20 000.
• “I calibrated the graticule at low power, so I can use the same value at high power.” Repair Recalibrate for every objective; at ×40 each eyepiece unit covers a quarter of the length it covered at ×10.
• “The eyepiece graticule is marked in micrometres.” Repair Its divisions are arbitrary eyepiece units. The stage micrometer carries the real scale, typically 0.01 mm = 10 µm per division.
• Drawing ribosomes and ER on a drawing made with a light microscope. Repair Draw only what you can see; these are below the light microscope’s resolution.
• Shading the nucleus and drawing the cell wall as one thick line. Repair No shading; the wall is two lines, three where two cells touch.
• “Ribosomes have a single membrane.” Repair Ribosomes have no membrane; neither do centrioles, microtubules or the nucleolus.
• “Microvilli are small cilia.” Repair Microvilli are folds of the cell surface membrane with no microtubule ring, and they increase surface area; cilia contain 9 + 2 microtubules and beat.
• “Mitochondria make energy for the cell.” Repair Respiration in mitochondria transfers energy into ATP; energy is never produced.
• “Bacteria have no DNA because they have no nucleus.” Repair Bacteria have circular DNA lying free in the cytoplasm.
• “Prokaryotic cells have 80S ribosomes.” Repair They have 70S, like mitochondria and chloroplasts; 80S ribosomes are in eukaryotic cytoplasm.
• “Bacteria have a cellulose cell wall, like plants.” Repair The bacterial wall is made of peptidoglycan; cellulose is the plant cell wall.
• “A virus is a very small bacterium.” Repair A virus is non-cellular: a nucleic acid core in a protein capsid, with no cytoplasm, ribosomes or metabolism.
• “All viruses have a phospholipid envelope.” Repair Some viruses do (HIV); all viruses have a nucleic acid core and a protein capsid.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• From a photomicrograph as well as a slide. The outcome says both. With a photomicrograph you cannot refocus, so draw only the structures that are in focus in the image, and take proportions from ruler measurements on it.
• One sentence, both cells. Write “plant cells have a cellulose cell wall; animal cells do not”, never “plant cells have a cell wall” on its own. A comparison states the feature for each cell side by side, and compare asks for similarities as well as differences.
• Interleave with the chapters that use this one. Topic 4 (membranes, endocytosis and exocytosis): re-answer “outline the secretory pathway”. Topic 5 (mitosis): re-answer “outline the structure and function of centrioles and microtubules”. Topic 6 (protein synthesis): re-answer “where are 80S and 70S ribosomes found?”. Topic 7 (plant transport): re-answer “what are plasmodesmata?” and re-read the drawing rules. Topic 10 (infectious diseases): re-answer the bacterium and virus statements. Topics 12 and 13 (respiration and photosynthesis): re-draw the mitochondrion and chloroplast close-ups. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 1 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A multiple-choice item on this topic can turn on one exact fact or one exact step: which organelles have a double membrane, where 80S and 70S ribosomes are found, which unit conversion a calculation needs, or whether a graticule was recalibrated. A structured question asks you to identify structures on an image, calculate a magnification or an actual size, define resolution and magnification, explain why an electron microscope shows more detail, compare two kinds of cell point by point, or state the syllabus’s own sentences about ATP and viruses.
• The evidence is an image: a photomicrograph, a transmission or scanning electron micrograph, or a drawing, usually with a scale bar or a stated magnification. You decide what kind of image it is, name what it shows, measure with a ruler in millimetres and convert through the ladder mm → µm → nm. The numerical skills are the magnification relationship (recalled, never provided), unit conversion, standard form and choosing a unit that gives a sensible number.
• Paper 3 always includes work with a light microscope, and this topic supplies its core: making a stained temporary preparation, drawing cells to the syllabus’s rules, calibrating the eyepiece graticule for the objective in use, measuring cells in eyepiece units and converting to micrometres, and calculating an actual size or the magnification of a drawing. The rest of the microscope work the syllabus lists — plan diagrams of tissues (Topics 7 to 9) and estimating numbers of cells in an area by sampling fields of view — builds on these skills. The main source of error is judging where a cell edge falls between graticule divisions; the most damaging mistake is a calibration made on the wrong objective.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.