Chapter summary
Cells and microscopy, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
Cells are the units of life
Why does a cell count as living when an isolated ribosome does not?
Key idea and reminders
A cell is the smallest unit able to carry out the coordinated processes of life; new cells arise from existing cells.
- Living organisms are cellular.
- Cells are the smallest living units.
- Present-day cells come from pre-existing cells.
Keep in mind: Chemical activity is necessary for life but does not make an isolated molecule or organelle a living cell.
02
Follow a protein through a cell
Why does a protein-secreting cell have extensive rough endoplasmic reticulum and Golgi body?
Key idea and reminders
Ribosomes make polypeptides; the endoplasmic reticulum and Golgi body process and route suitable products in membrane vesicles.
- Ribosomes: translation.
- Rough endoplasmic reticulum and Golgi body: processing and transport.
- Lysosomes: compartmentalised hydrolysis.
Keep in mind: Ribosomes synthesise the chain. Golgi enzymes modify and sort products already made.
03
Match organelles to their jobs
Which visible features justify an organelle identification?
Key idea and reminders
Identify a structure using its boundary and internal pattern, then connect those features to its function.
- Nucleolus: rRNA and ribosomal subunits.
- Cristae: inner mitochondrial membrane folds.
- Grana: stacks of thylakoids, surrounded by stroma.
Keep in mind: Mark schemes reject abbreviated organelle names, so an otherwise correct answer loses the mark. Write rough endoplasmic reticulum, smooth endoplasmic reticulum and Golgi body in full every time you name them.
04
Recognise a bacterial cell
How can a cell function without a nucleus?
Key idea and reminders
A bacterium has a membrane, cytoplasm, DNA and ribosomes, but no membrane-bound nucleus or other membrane-bound organelles.
- Typical wall: peptidoglycan.
- DNA: usually a circular chromosome, without a nuclear envelope.
- 70S ribosomes; no membrane-bound organelles.
Keep in mind: Respiratory reactions can occur in the cytoplasm and at the bacterial cell membrane.
05
Calibrate a microscope measurement
What does one eyepiece division represent at this objective?
Key idea and reminders
Calibrate the eyepiece scale at each objective before converting a specimen measurement to a real length.
- Calibration = known length / graticule divisions.
- Recalibrate after changing objective.
- Resolution means separating two close points.
Keep in mind: Eyepiece divisions have no fixed specimen length until calibrated at the selected objective.
06
Draw, measure and compare specimens
What separates a drawing that scores from a sketch that does not?
Key idea and reminders
Draw only what you see with clear, continuous single lines and no shading; label with ruled lines outside the drawing; calculate actual size = image size / magnification and give it to 2 significant figures with a unit.
- Single continuous lines, no shading, draw only what you see, keep proportions, use most of the space.
- Plan diagram: tissue outlines, no cells. Cell drawing: a few cells, double-line walls, visible organelles only.
- Labels: ruled lines, no arrowheads, ending on the structure, not crossing, labels outside.
- Actual size = image size / magnification; convert mm to micrometres; 2 significant figures with a unit.
- Differences table: feature | A | B, observable features only, both columns filled.
Keep in mind: Shading is not credited and blurs outlines. Adding structures that are not visible in the specimen loses the mark for drawing what you see.