# Cell Structure, Reproduction and Development

> Edexcel International A-Level Biology · IAS Unit 2 (WBI12)
> Source: https://www.owlsprep.com/study/edexcel-ial-biology-u2-cell-structure-reproduction-and-development/

This guide covers all Edexcel IAL Biology Unit 2 (WBI12) content for Topic 3: cell ultrastructure, meiosis/mitosis, fertilisation, stem cells, gene expression, epigenetics, and the two mandatory core practicals for this topic.

**Prerequisites:** [Edexcel IAL Biology Unit 1 Topic 2: DNA and Protein Synthesis](https://www.owlsprep.com/study/edexcel-ial-biology-u1-dna-protein-synthesis/)

## Learning objectives

- Identify eukaryotic and prokaryotic ultrastructure from electron micrographs
- Explain how meiosis produces genetic variation via crossing over and independent assortment
- Describe mammalian and flowering plant fertilisation processes in correct order
- Calculate magnification and mitotic index accurately for exam calculations
- Explain differential gene expression and epigenetic effects on phenotype
- Recall procedures for Core Practical 5 (light microscopy) and Core Practical 6 (root tip squash)

## Cell Ultrastructure and Microscopy

All living organisms are made of cells, organised into a hierarchy: cells → tissues (groups of specialised cells) → organs (groups of tissues working together) → organ systems (groups of organs carrying out a shared function). Eukaryotic cells (animal, plant, fungal) have membrane-bound organelles, while prokaryotic cells (bacteria) do not.

**Eukaryotic Animal Ultrastructure** — 9 core organelles with specific functions: nucleus (stores DNA), nucleolus (produces ribosomes), ribosomes (protein synthesis), rough endoplasmic reticulum (rER: folds and processes proteins), smooth endoplasmic reticulum (sER: synthesises lipids), mitochondria (site of aerobic respiration), centrioles (form spindle fibres for cell division), lysosomes (digest cellular waste), Golgi apparatus (packages proteins for secretion).

*Example:* rER and Golgi work together to transport extracellular enzymes like amylase out of salivary gland exocrine cells.

Prokaryotic cells have a much simpler structure: peptidoglycan cell wall, protective capsule, small circular DNA plasmids, flagellum for movement, pili for attachment to surfaces, 70S ribosomes, and a single circular chromosome of DNA not contained in a nucleus.

> **tip**
>
> You must be able to identify all 9 eukaryotic organelles listed from electron micrographs for exam marks; plant organelles (chloroplasts, vacuoles, cellulose cell walls) are out of scope for this topic.

**Magnification and Resolution** — Magnification = size of image / size of real object. Resolution is the minimum distance between two points that can be distinguished as separate; electron microscopes (EM) have ~1000x higher resolution than light microscopes, allowing visualisation of organelle ultrastructure.

*Example:* Light microscopes have a maximum resolution of ~0.2µm, while transmission EM has a resolution of ~0.0002µm, enough to view ribosomes and cell membrane structure.

**Worked example:** A student views a mitochondrion under an electron microscope. The image of the mitochondrion is 8mm long, and the real length is 2µm. Calculate the magnification of the image.

1. Step 1: Convert units to be consistent. 8mm = 8,000µm.
2. Step 2: Use the magnification formula: magnification = image size / real size
3. $$magnification = 8000 / 2 = 4000$$
4. Final answer: magnification = ×4000 (always include the × sign for marks)

**Summary**

- Core Practical 5: Calibrate light microscopes using an eyepiece graticule and stage micrometer
- Draw labelled, scaled diagrams of animal cells, including a scale bar on all drawings
- Stains are used to increase contrast between transparent cell components

> **Exam tip:** Staining is required for light microscopy to increase contrast between transparent cell components; common stains include methylene blue for animal cells.

## Cell Division: Meiosis and Mitosis

Cell division includes two key processes: mitosis, which produces genetically identical diploid daughter cells for growth, repair and asexual reproduction; and meiosis, which produces genetically distinct haploid gametes for sexual reproduction, generating genetic variation in offspring.

**Meiosis Sources of Variation** — Two core mechanisms produce genetic variation in gametes, with stage labels required for marks: 1. Crossing over: prophase I, homologous chromosomes exchange sections of DNA (alleles) at chiasmata, creating new allele combinations. 2. Independent assortment: metaphase I, homologous chromosome pairs line up randomly on the spindle equator, so gametes receive a random mix of maternal and paternal chromosomes.

*Example:* Independent assortment of 23 human chromosome pairs produces ~8 million possible unique gamete genotypes before crossing over occurs.

**Worked example:** Explain why two gametes produced by the same human individual are almost never genetically identical.

1. 1. Crossing over in prophase I of meiosis: homologous chromosomes swap sections of DNA, creating new, unique allele combinations on each chromatid.
2. 2. Independent assortment in metaphase I of meiosis: each homologous chromosome pair lines up randomly, so gametes receive a random mix of maternal and paternal chromosomes.
3. 3. Random fertilisation further increases genetic variation when gametes fuse to form a zygote.

The mitotic cell cycle consists of interphase (cell growth, DNA replication, normal cell function) followed by mitosis (nuclear division) and cytokinesis (cell splitting), producing two identical diploid daughter cells with the same number of chromosomes as the parent cell.

**Mitotic Index** — The proportion of cells in a tissue sample that are undergoing mitosis, calculated as: number of cells in mitosis / total number of cells counted in the sample.

*Example:* A root tip sample has 36 cells in mitosis out of 200 total cells counted: mitotic index = 36/200 = 0.18, or 18%.

**Worked example:** A student counts 120 cells in a root tip squash. 15 cells are in prophase, 6 in metaphase, 4 in anaphase, 5 in telophase. Calculate the mitotic index of the sample.

1. Step 1: Calculate total number of cells in mitosis: 15 + 6 + 4 + 5 = 30 cells.
2. Step 2: Divide by total number of cells counted: 30 / 120 = 0.25
3. Final answer: mitotic index = 0.25 (or 25%)

> **Exam tip:** Core Practical 6 (root tip squash) uses hydrochloric acid to break down cell walls, acetic orcein to stain chromosomes dark, and gentle pressing to separate cells for viewing mitosis stages.

## Gamete Structure and Fertilisation

Gametes are specialised haploid cells adapted for fertilisation: sperm cells have an acrosome (vesicle of digestive enzymes), flagellum for movement, and large numbers of mitochondria to produce ATP for movement; egg cells have a thick protective glycoprotein layer called the zona pellucida, and cytoplasm rich in nutrients for early embryo development.

**Worked example:** Describe the sequence of events in mammalian fertilisation, in the correct order.

1. 1. Sperm reaches the egg, and the acrosome reaction occurs: digestive enzymes are released from the acrosome to digest the zona pellucida.
2. 2. Sperm cell membrane fuses with the egg cell membrane, allowing the sperm nucleus to enter the egg cytoplasm.
3. 3. Cortical reaction occurs: the egg releases cortical granules that modify the zona pellucida, preventing polyspermy (entry of additional sperm).
4. 4. Sperm and egg haploid nuclei fuse to form a diploid zygote.

Flowering plant fertilisation follows a different process: a pollen grain lands on the stigma of a flower, grows a pollen tube down the style to the ovary, and two male nuclei travel down the tube. One fuses with the egg nucleus to form a diploid zygote, the other fuses with two polar nuclei to form triploid endosperm, a food store for the developing embryo.

> **Exam tip:** Always list the acrosome reaction before the cortical reaction in fertilisation descriptions to gain full marks; incorrect order loses associated points.

## Stem Cells, Gene Expression and Epigenetics

**Stem Cell Potency** — Stem cells are unspecialised cells that can divide and differentiate into specialised cell types: 1. Totipotent: can differentiate into all cell types including extraembryonic (placental) cells, found in the morula (early embryo up to 8 cells). 2. Pluripotent: can differentiate into all embryonic cell types but not placental cells, found in the inner cell mass of the blastocyst.

*Example:* Pluripotent stem cells are used in medical research to test new drugs and develop treatments for degenerative diseases like Parkinson's and type 1 diabetes.

Cell specialisation occurs via differential gene expression: only specific genes are activated (expressed) in each cell type, producing proteins that give the cell its specialised structure and function, in both animals and plants. A single gene can produce more than one protein via post-transcriptional changes to mRNA, such as alternative splicing, where different exons are included in the final mRNA molecule, producing different polypeptide chains.

**Epigenetics** — Heritable changes to gene expression without alteration to the DNA base sequence, caused by two core mechanisms: 1. DNA methylation: addition of methyl groups to DNA, generally silences genes by preventing transcription enzymes binding. 2. Histone modification: addition of acetyl or methyl groups to histone proteins, alters how tightly DNA is wrapped around histones, changing access for transcription enzymes.

*Example:* Methylation of the gene coding for lactase in adulthood causes lactose intolerance in ~65% of the global human population.

**Worked example:** Explain how phenotype is determined by both genotype and environment.

1. Step 1: Genotype is the genetic makeup of an organism, which determines the potential range of phenotypes an individual can have.
2. Step 2: Environmental factors (e.g. diet, UV exposure, temperature) can alter gene expression via epigenetic mechanisms, modifying the final expressed phenotype.
3. Step 3: Many characteristics (e.g. height, skin colour, BMI) are polygenic (controlled by multiple genes) and influenced by environmental factors, producing continuous variation in populations.

> **Exam tip:** Epigenetic changes are not mutations, as they do not change the DNA base sequence; this is a common mark scheme distinction tested in exam questions.

## Common pitfalls

- **Wrong:** Mixing up meiosis variation stages, stating independent assortment occurs in prophase I.
  - Why it fails: Mark schemes explicitly require crossing over = prophase I and independent assortment = metaphase I, so incorrect stages lose all associated marks.
  - Correct: Memorise the stage pairings: crossing over = prophase I, independent assortment = metaphase I, always include stage labels in answers.
- **Wrong:** Defining resolution as 'how clear an image is'.
  - Why it fails: The official mark scheme definition is 'the ability to distinguish two separate points that are close together', so vague descriptions gain no marks.
  - Correct: Learn the exact definition of resolution, and clearly distinguish it from magnification (how much larger the image is than the real object).
- **Wrong:** Listing cortical reaction before acrosome reaction in mammalian fertilisation descriptions.
  - Why it fails: Fertilisation events occur in a strict sequence, and the order is a required marking point.
  - Correct: Use the mnemonic A-C-F: Acrosome reaction first, then Cortical reaction, then Fusion of nuclei.
- **Wrong:** Stating epigenetic changes are mutations because they are heritable.
  - Why it fails: Mutations alter the DNA base sequence, while epigenetic changes only modify gene expression without changing the base sequence.
  - Correct: Always explicitly state that epigenetic changes do not alter the DNA base sequence when explaining them in answers.
- **Wrong:** Including plant cell organelles (chloroplasts, cellulose cell wall, vacuole) when asked about eukaryotic animal cell ultrastructure.
  - Why it fails: Plant cell structure is out of scope for this topic, and including it wastes time and may lead to lost marks if you misidentify organelles.
  - Correct: Only list the 9 animal eukaryotic organelles specified in the syllabus for this topic.
- **Wrong:** Forgetting to include the × sign when writing magnification values in calculation answers.
  - Why it fails: Magnification is a unitless ratio, and the × sign is a required marking point for all magnification calculation questions.
  - Correct: Always write magnification answers with the × prefix, e.g. ×4000 not just 4000.

## Cheatsheet

| Concept | Key Details / Formula | Exam Marking Tip |
| --- | --- | --- |
| Eukaryotic Organelles | Nucleus, nucleolus, ribosomes, rER, sER, mitochondria, centrioles, lysosomes, Golgi | Only list animal organelles for this topic; plant organelles are out of scope |
| Prokaryotic Organelles | Cell wall, capsule, plasmid, flagellum, pili, 70S ribosomes, circular DNA (no nucleus) | No membrane-bound organelles present in prokaryotes |
| Magnification | Magnification = image size / real object size | Always convert units to match, include × sign in answer |
| Meiosis Variation | Crossing over (prophase I), Independent assortment (metaphase I) | Always include the stage name to gain full marks |
| Mitotic Index | (Number of cells in mitosis) / (Total number of cells counted) | Expressed as decimal or percentage; no units |
| Mammalian Fertilisation Order | Acrosome reaction → sperm fusion → cortical reaction → nuclear fusion | Acrosome reaction always comes before cortical reaction |
| Stem Cell Potency | Totipotent (morula, all cell types), Pluripotent (blastocyst, embryonic types only) | Do not mention multipotent stem cells as they are out of scope |
| Epigenetics | DNA methylation (silences genes), histone modification (alters transcription) | Not a mutation: no change to DNA base sequence |

## What's next

Now that you have mastered the content for Unit 2 Topic 3, you are ready to move on to the next topic in Unit 2: Classification and Biodiversity, which builds on your knowledge of cell structure to explore how organisms are grouped and how biodiversity is measured and conserved. You should also practice past paper questions focused on this topic, especially core practical questions on microscope calibration and root tip squashes, which are frequent high-mark questions in WBI12 papers. Make sure to memorise the exact definitions and stage labels specified in this guide, as mark schemes are very specific for this topic. You should also review Unit 1 content on DNA transcription and translation if you are struggling with the differential gene expression section, as this topic assumes prior knowledge of those mechanisms.

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