What you need to know
- 3.1–3.7Cell organisation, eukaryoticDescribes cells with a nucleus and membrane-bound organelles, such as animal and plant cells. and prokaryoticDescribes cells with no nucleus and no membrane-bound organelles, such as bacteria. Their DNA is free in the cytoplasm. structure, protein transport and microscopy.
Use the suggested answers to check your explanation and identify gaps.
By the end, you should be able to…
- Choose evidence that supports an organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome. identification.
- Show complete calculations and units.
- Connect structures to mechanisms and outcomes.
Build your understanding
Approach an unfamiliar image
Check the microscope type, scale and whether you see an entire cell, a thin section or a surface. Identify more than one structural clue before naming an organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome.. Lack of a visible feature may reflect the section or resolutionThe smallest distance between two points that can still be seen as separate points..
Build an explanation
Use structure → process → function. For a secretory cell, connect ribosomeA small organelle made of rRNA and protein, where translation takes place. and rER to protein synthesis, Golgi to modification and sorting, and vesicle fusion to export. Do not rely on a list of organelle names.
Calculate before rounding
Convert all lengths to matching units before applying magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size. = image size / actual size. For a resized image, use its embedded scale bar. Keep intermediate precision and round the final result sensibly.
Make your reasoning visible
Follow the command word
A comparison needs paired differences or similarities. An explanation must connect a feature to its consequence.
Distinguish direct evidence from inference
A dark spot is an observation; calling it a ribosome is an interpretation that needs size and context.
Use the model stated
Do not turn features of a typical bacterium into universal claims about every prokaryote.
Avoid this misconception
The claim: An image without a visible nucleus must show a bacterium.
The correction: A section can miss a eukaryoticDescribes cells with a nucleus and membrane-bound organelles, such as animal and plant cells. nucleus; use other structural evidence.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
An image is 32 mm long and represents a cell 8 µm long. Calculate magnification.
Show suggested answer
32,000/8 = ×4,000.
Practice question
Retrieval and explanation · suggested answer
An organelle is three times as long as a scale bar labelled 200 nm. Give its length in µm.
Show suggested answer
3 × 200 = 600 nm = 0.60 µm.
Practice question
Retrieval and explanation · suggested answer
Compare rER with smooth ER using a structural difference and a functional difference.
Show suggested answer
rER has bound ribosomes and participates in synthesis/processing of secretory-pathway proteins. Smooth ER lacks bound ribosomes and has roles such as lipid synthesis.
Practice question
Retrieval and explanation · suggested answer
A treatment blocks secretory-vesicle fusion but not earlier steps. Predict where protein may accumulate and why less is found outside.
Show suggested answer
It may accumulate in secretory vesicles. Fusion with the cell-surface membrane is needed for exocytosis, so extracellular release is reduced.
Practice question
Retrieval and explanation · suggested answer
A small cell has a circular chromosome, 70S ribosomes and no membrane-bound organelles. Give a supported classification and one shared feature with eukaryotic cells.
Show suggested answer
The combination supports a prokaryotic cell. Both have a cell-surface membrane, cytoplasm, ribosomes and DNA; any one shared feature is suitable.
Practice question
Retrieval and explanation · suggested answer
Explain why a heart is an organ rather than a tissue.
Show suggested answer
It contains several tissues, including cardiac muscle and supporting/lining tissues, cooperating in pumping blood.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A cell’s image is doubled in size but its reported magnification is not changed. What happens if you calculate actual size using the old magnification? How can an embedded scale bar help?
Show a hint
The image measurement changes while the divisor stays fixed.
Explore the reasoning
The calculated actual size is twice the correct value. If the scale bar was enlarged with the image, the object-to-bar ratio remains correct and gives the true size.
Recall this lesson
Lesson recall questions
Question 1 of 5: Give one animal example at each level: tissue, organ and organ system.
Tissues, organs, systems
Practice question
Retrieval and explanation · suggested answer
Give one animal example at each level: tissue, organ and organ system.
Lesson: 3A 5: The Organisation of Cells
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Beyond the conventional light microscope
3A · Observing CellsThe 2014 Chemistry Nobel Prize recognised super-resolved fluorescence microscopy. Specialised techniques can reveal detail beyond conventional light-microscopy limits. A research microscopist chooses a method according to the question:…Specification 3.7(i)Read & use the scienceClose article
Why this matters
The 2014 Chemistry Nobel Prize recognised super-resolved fluorescence microscopy. Specialised techniques can reveal detail beyond conventional light-microscopy limits. A research microscopist chooses a method according to the question: does the experiment need the position of a labelled molecule, the outline of an organelle, or a measured cell size? Fluorescent tags can reveal where selected proteins occur, but a bright spot is not every structure in the cell. Resolution describes whether two nearby features can be distinguished; magnification only makes the image larger. This is why a scale bar and the staining or labelling method belong beside a published micrograph.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.7(i) know how magnification and resolution can be achieved using light and electron microscopy
Use the science
Why does this not mean that digitally enlarging a school microscope image gives the same result?
Explore a possible response
The research methods collect or localise information differently. Zooming a finished image does not acquire new structural information.
Explore the source
- Nobel Prize in Chemistry 2014Nobel Prize Outreach
Interpreting a real cell section
3A · Eukaryotic Cells 1: Common Cellular StructuresLouisa Howard’s TEM image shows a Golgi region in a human leukocyte. Researchers interpret features in prepared sections rather than expecting every structure to resemble a textbook drawing.…Specification 3.6Read & use the scienceClose article
Why this matters
Louisa Howard’s TEM image shows a Golgi region in a human leukocyte. Researchers interpret features in prepared sections rather than expecting every structure to resemble a textbook drawing. A thin section shows only one slice of a three-dimensional cell. An apparent isolated circle may be part of a larger membrane system outside the plane of section. The Golgi can be identified more confidently when several flattened sacs and nearby vesicles are seen together. A clinical scientist examining a leukocyte would describe the visible evidence before naming a structure. Preparation and staining also create contrast and sometimes artefacts. The exam skill is to identify features from evidence, not from a perfect diagram.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.6 be able to recognise the organelles in 3.3 from electron microscope (EM) images
Use the science
What additional evidence would help identify an ambiguous small membrane-bound profile?
Explore a possible response
Other sections, higher-resolution detail or specific labelling could help; shape alone may fit several structures.
Explore the source
- Louisa Howard / Dartmouth: Golgi TEM, public domainWikimedia Commons
