Pearson Edexcel International A Level Biology: Cell Ultrastructure and Microscopy (YBI11) – Revision Notes

Quick recall for the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology (YBI11) specification, Issue 2, statements 3.1 to 3.8 of Topic 3 (International AS content), including Core Practical 5. For full explanations see the study guide, then test yourself with the practice questions. The course hub, the checklist and the free diagnostics help you plan.

3.1 and 3.2 Cells and organisation

• All living organisms are made of cells.
• Common features: cell surface membrane, DNA, cytoplasm with metabolic reactions, ribosomes.
• Order of organisation: cell, tissue, organ, organ system, organism.
• Tissue: similar cells, one function. Organ: several tissues. Organ system: several organs.

3.3 Eukaryotic organelles at a glance

Organelle	Spot it by	Job
Nucleus	Double membrane with pores	Holds DNA; transcription
Nucleolus	Dark patch inside nucleus	Makes rRNA; builds ribosome subunits
Ribosome	Tiny dots	Translation
rER	Flat sacs with dots	Folds and processes proteins
sER	Tubes, no dots	Lipid synthesis and storage
Mitochondrion	Double membrane, cristae	ATP from aerobic respiration
Centriole	Pair of hollow cylinders at right angles	Spindle formation in division
Lysosome	Small, round, single membrane	Hydrolytic enzymes break down material
Golgi apparatus	Stack of curved sacs, vesicles	Modifies, packages, sorts

Must-know distinctions

• rER (ribosomes, proteins) versus sER (no ribosomes, lipids).
• Nucleus (double membrane, pores) versus mitochondrion (double membrane, cristae).
• Lysosome (single membrane, enzymes inside) versus secretory vesicle (leaves the cell).

3.4 Secretory pathway in steps

1. Nucleus: transcription; mRNA leaves via a pore.
2. rER ribosomes translate; polypeptide enters the lumen and folds.
3. Transport vesicle from rER to Golgi.
4. Golgi modifies and packages into secretory vesicles.
5. Vesicle fuses with the cell surface membrane: exocytosis.
6. Result: an extracellular enzyme (for example a digestive enzyme).

Memory line: nucleus, rER, vesicle, Golgi, vesicle, membrane.

3.5 Prokaryotic structures

Structure	Job
Cell wall	Rigid support; stops bursting
Capsule	Protection; helps adhesion
Plasmid	Small extra DNA loop; may carry resistance genes
Flagellum	Movement by rotating
Pili	Attachment; some transfer DNA
Ribosomes	Translation (70S)
Circular DNA	Main genetic material, free in cytoplasm

Prokaryote versus eukaryote

Feature	Prokaryote	Eukaryote
Nucleus	Absent	Present
DNA	Circular, free	Linear, in a nucleus
Membrane-bound organelles	None	Many
Ribosomes	70S	80S in the cytoplasm
Typical size	Around 1 to 5 micrometres	Often 10 to 100 micrometres

3.6 Reading an EM image

• Count membranes: two means nucleus or mitochondrion; one means lysosome, vesicle or Golgi sac.
• Folds inside a double-membrane oval: mitochondrion.
• Dots on flat sacs: rER.
• Remember a thin section can cut an organelle at any angle.

3.7 Microscopy

	Light	Electron
Source	Light	Electrons
Resolution	About 0.2 micrometres	Much finer
Specimen	Living or dead	Dead, in a vacuum
Colour	Possible	Black and white

• Magnification = size of image ÷ size of real object.
• Resolution = smallest separation at which two points are seen as two.
• Short wavelength gives better resolution. Electrons have a much shorter wavelength than light.
• TEM: thin sections, internal detail. SEM: surface in three dimensions.
• Staining gives contrast to near-transparent structures. Electron microscopy uses heavy metal stains.

Method for magnification

1. Measure the image in mm.
2. Put the real size in the same unit (1 mm = 1000 micrometres).
3. Divide image by real size.
4. Give the answer as ×number and no other unit.

Reminder. A cell image is 52 mm and the real cell is 8 micrometres: 52 000 ÷ 8 = ×6500. A structure is 30 mm long at ×15 000: real size = 30 ÷ 15 000 mm = 0.002 mm = 2 micrometres.

3.8 Core Practical 5

• Cheek or blood cells, stain, coverslip lowered at an angle.
• Low power first, centre, then higher power; coarse then fine focus.
• Drawing: pencil, clear lines, no shading, label lines ruled and not crossing, title and magnification.
• Graticule calibrated against a stage micrometer for each objective.
• Calibration: length covered by the stage scale ÷ number of graticule divisions.

Reminder. 100 graticule divisions match 25 stage divisions of 10 micrometres: 250 ÷ 100 = 2.5 micrometres per division. A cell 14 divisions long measures 14 × 2.5 = 35 micrometres.

Units and conversions

From	To	Rule
mm	micrometres	multiply by 1000
micrometres	nm	multiply by 1000
nm	micrometres	divide by 1000
cm	mm	multiply by 10

Rearrangements of the magnification formula:

• magnification = image size ÷ real size
• real size = image size ÷ magnification
• image size = real size × magnification

Cover the unknown in a triangle of image, real size and magnification if that helps, but always write the line of working.

Scale bar method

1. Measure the scale bar in mm with a ruler.
2. Convert the length it represents into the same unit, usually micrometres.
3. Magnification = bar length ÷ length it represents.
4. Measure the structure, then real size = structure length ÷ magnification.

A shortcut that skips the magnification is real size = (structure length ÷ bar length) × value on the bar. Both routes give the same answer.

Model answers to learn

Explain how a digestive enzyme is made and released. The gene is transcribed in the nucleus and the mRNA leaves through a nuclear pore. Ribosomes on the rER translate it, and the polypeptide folds in the rER. A vesicle carries it to the Golgi apparatus, which alters it and wraps it in a new vesicle. Finally a secretory vesicle merges with the cell surface membrane, and exocytosis frees the enzyme.

Compare light and electron microscopes. Electrons have a shorter wavelength than light, so the electron microscope has better resolution and shows ultrastructure. The light microscope can be used on living cells and gives colour, but resolves much less detail.

Describe a feature that identifies a mitochondrion. Two membranes, the inner one thrown into folds called cristae around a fluid matrix.

Command words

• State: give a short answer with no explanation.
• Describe: say what you see or what happens, in order.
• Explain: give the reason, using “because” or “so that”.
• Calculate: show the working, and give the unit unless the answer is a magnification.
• Suggest: apply what you know to an unfamiliar case.

Drawing checklist

• Pencil, sharp, single clear lines.
• No shading and no colouring.
• Large drawing that fills the space.
• Label lines drawn with a ruler, not crossing, touching the structure.
• Title, magnification and, where useful, a scale bar.
• Draw only what you see, not what a textbook shows.

Structure linked to function

• Nuclear pores: let mRNA out while the DNA stays inside.
• Cristae: a large inner membrane area for the enzymes of aerobic respiration.
• Ribosomes on rER: proteins made there enter the rER lumen directly, ready to be folded and sent on.
• Golgi vesicles: carry cargo between sacs and to the cell surface.
• Lysosome membrane: keeps hydrolytic enzymes away from the rest of the cytoplasm, so the cell does not digest itself.
• Cell wall of a prokaryote: resists the pressure that builds up when water enters.
• Pili: let a bacterium stick to a surface or to another cell.
• Capsule: a protective layer that also helps a colony stay attached.

Common exam traps

• A question that says “ultrastructure” is asking for detail seen with an electron microscope, not a light microscope.
• A question that asks for the “role” of the Golgi apparatus wants modification and packaging, not synthesis.
• Check whether a magnification question gives the image in mm or cm before you start.

Quick self-test

1. Give two features that every living cell has.
2. What is a tissue?
3. Name the organelle that makes ribosomal RNA.
4. Which organelle has cristae?
5. State the function of the Golgi apparatus.
6. Where are lysosomal enzymes stored, and what do they do?
7. Name the two kinds of DNA found in a bacterial cell.
8. Why can an electron microscope show more detail than a light microscope?
9. A cell appears 48 mm long in an image at ×1200. How long is the real cell in micrometres?
10. Why must the graticule be recalibrated when you change objective lens?
11. Why are specimens stained?
12. Give one difference between rER and sER.

Answers

1. Cell surface membrane, DNA, cytoplasm, ribosomes (any two).
2. A group of similar cells that together carry out one function.
3. The nucleolus.
4. The mitochondrion.
5. It modifies, packages and sorts proteins and lipids into vesicles.
6. Inside lysosomes; they are hydrolytic enzymes that break down worn-out organelles and taken-in material.
7. Circular DNA (the main chromosome) and plasmids.
8. Electrons have a much shorter wavelength, so the resolution is much better.
9. 48 mm = 48 000 micrometres; 48 000 ÷ 1200 = 40 micrometres.
10. Each objective gives a different magnification, so each graticule division represents a different real distance.
11. Most structures are almost transparent; a stain gives contrast so they can be seen.
12. rER has ribosomes and handles proteins; sER has none and makes lipids.

Where marks are usually lost

• Writing “makes energy” instead of “makes ATP” for mitochondria.
• Confusing magnification with resolution.
• Not converting millimetres to micrometres before dividing.
• Giving a unit with a magnification answer.
• Describing the Golgi as making proteins; ribosomes make them.
• Saying prokaryotes have no DNA.
• Drawings with shading, overlapping label lines or no magnification.
• Using one graticule calibration for every objective lens.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, Pearson Education Limited. Statements 3.1 to 3.8 of Topic 3.