Unit 2: Cells, Development, Biodiversity and Conservation
Unit 2 — Complete A Level and AS source — IAL Edexcel Modular Revision Notes & Topical Questions
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Lesson summary
Unit 2 — Complete A Level and AS source notes — IAL EDEXCEL MODULAR notes: Summarise Unit 2: cell structure, cell division, gametes and inheritance, differentiation and variation, biodiversity and conservation.Cell theory — cells are the basic unit of life, all organisms consist …
- Eukaryotic cells
- Prokaryotic cells
- Resolution
- Magnification
- Cell cycle
- Mammalian gametes
Section 1 of 9Cell structure and organisation
Cell theory
- Cell theory cells are the basic unit of life, all organisms consist of cells and cells arise from pre-existing cells.
Eukaryotic cells
- Eukaryotic cells they have a nucleus and membrane-bound organelles.
Prokaryotic cells
- Prokaryotic cells they have no nucleus, circular DNA, smaller 70S ribosomes and a peptidoglycan (murein) wall.
Resolution
Resolution the ability to distinguish two close points as separate.
- The electron microscope has higher resolution than the light microscope.
Magnification
Magnification image size ÷ actual size.
1
DNA in the nucleus is transcribed to mRNA.
2
mRNA leaves via a nuclear pore to ribosomes on the RER.
3
The protein enters the RER lumen and is carried in a vesicle to the Golgi apparatus.
4
The Golgi modifies the protein, for example by adding carbohydrate, and packages it into vesicles.
5
Secretory vesicles fuse with the cell surface membrane and release the protein by exocytosis.
Section 2 of 9Cell division, gametes and inheritance
1Cell cycle
Cell cycle the regulated sequence of growth, DNA replication and division.
2Mitosis
- Mitosis in prophase chromosomes condense into two sister chromatids, centrioles move to the poles and the nuclear envelope breaks down.
3Meiosis
- Meiosis it produces four genetically different haploid cells.
- Crossing over is the exchange of material between non-sister chromatids of homologous chromosomes in meiosis I.
- Independent assortment of homologous pairs also creates variation.
4Mammalian gametes
- Mammalian gametes they are adapted for fertilisation.
- The sperm mid-piece contains many mitochondria.
- The cortical reaction hardens the zona pellucida and prevents polyspermy.
5Sex linkage
- Sex linkage a carrier female (XHXh) and an unaffected male (XHY) have a probability of 0.5 that a son has haemophilia.
6Linkage
- Linkage genes on the same chromosome tend to be inherited together.
Section 3 of 9Differentiation and variation
Stem cell potency
- Stem cell potency stem cells are classified as totipotent, pluripotent, multipotent or unipotent.
Differentiation
- Differentiation it depends on which genes are expressed.
Histone acetylation
- Histone acetylation acetyl groups loosen chromatin and increase transcription.
Sources of variation
- Sources of variation variation comes from genes and the environment.
- Continuous characteristics such as height show a range of values.
- Discontinuous characteristics such as blood group fall into categories.
Section 4 of 9Measures of biodiversity
| Measure | What it uses | Use |
|---|---|---|
| Species richness | Number of species | Simple comparison that ignores abundance |
| Heterozygosity index | Heterozygotes ÷ number of individuals | Genetic diversity within a population |
| Index of diversity D = N(N − 1) ÷ Σn(n − 1) | Richness and abundance | Compares habitats; higher D means more diverse |
| Endemism | Species unique to an area | Conservation priorities |
Section 5 of 9Evolution and adaptation
Hardy-Weinberg
- Hardy-Weinberg p + q = 1 and p² + 2pq + q² = 1, where 2pq is the frequency of heterozygotes.
- The assumptions are random mating, no mutation, a large population, no migration and no selection.
Adaptations
- Adaptations they may be anatomical (thick fur), physiological or behavioural (nocturnal activity).
- (concentrated urine)
Reproductive isolation
- Reproductive isolation two frog species breeding in the same pond at different times show seasonal isolation, which can lead to speciation.
Section 6 of 9Plant structures, classification and conservation
Plant structures, classification and conservation
Xylem vessels
- Xylem vessels they carry water and minerals and are lignified.
Phloem
- Phloem it has sieve tube elements and companion cells.
Starch and cellulose
- Starch and cellulose starch stores energy and cellulose gives strength through hydrogen-bonded microfibrils.
Classification
- Classification organisms are placed in domains and, for eukaryotes, in kingdoms (protoctista, fungi, plants, animals), with prokaryotes forming the kingdom Prokaryotae in the five-kingdom system.
- (Archaea, Bacteria, Eukarya)
Conservation
- Conservation it can be in situ (reserves) or ex situ, and conservation must be balanced with human needs.
- (zoos, seed banks)
- The Arabian oryx shows how captive breeding and reintroduction can rescue a species from extinction.
Section 7 of 9More from the specification
1
Calibrating Microscopy with Stage Micrometers
- An eyepiece graticule is a scale in the microscope eyepiece calibrated using a stage micrometer of known dimensions, determining the precise micrometre distance represented per graticule unit.
2
Tissue Plan Drawings
- Low-power microscopy drawings illustrate the arrangement and boundaries of different tissue layers in an organ without drawing individual cells.
- (such as a plant root or stem section)
3
High-Power Cellular Drawings
- Detailed high-power drawings show the individual cellular architecture of specific tissues, accurately representing cell walls, membranes, cytoplasm,
- and nuclear boundaries.
4
Endoplasmic Reticulum and Ultrastructure
- Electron microscopy reveals internal cytoplasm organelles such as the rough endoplasmic reticulum and smooth endoplasmic reticulum (for lipid synthesis).
- (coated with ribosomes for protein synthesis)
5
Calibrating Microscopy with Stage Micrometers
- An eyepiece graticule is a scale in the microscope eyepiece calibrated using a stage micrometer of known dimensions, determining the precise micrometre distance represented per graticule unit.
6
Tissue Plan Drawings
- Low-power microscopy drawings illustrate the arrangement and boundaries of different tissue layers in an organ without drawing individual cells.
- (such as a plant root or stem section)
7
High-Power Cellular Drawings
- Detailed high-power drawings show the individual cellular architecture of specific tissues, accurately representing cell walls, membranes, cytoplasm,
- and nuclear boundaries.
8
Endoplasmic Reticulum and Ultrastructure
- Electron microscopy reveals internal cytoplasm organelles such as the rough endoplasmic reticulum and smooth endoplasmic reticulum (for lipid synthesis).
- (coated with ribosomes for protein synthesis)
9
Graticule
- An engraved scale in the eyepiece used with a stage micrometer to measure size.
10
Tissue
- A group of similar cells working together.
11
Cytoplasm
- Jelly in which the cell's chemical reactions happen.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Resolution | The ability to distinguish two points that are close together as separate. |
| Crossing over | Exchange of genetic material between non-sister chromatids in meiosis I. |
| Cell cycle | Regulated sequence of growth, DNA replication and division. |
| Histone acetylation | Addition of acetyl groups to histones, loosening chromatin and increasing transcription. |
| Endemism | Presence of species unique to an area. |
Section 9 of 9Exam technique
1
Mixing up resolution and magnification.
2
Thinking Hardy-Weinberg holds when natural selection is strong.
3
Writing that crossing over occurs between sister chromatids.
4
Treating species richness as a full measure of biodiversity.
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