TOPIC 03 • CELL STRUCTURE
Cells, Microscopy and Organisation
All living organisms are made of cells. In multicellular organisms, cells become specialised and are organised into tissues, organs and organ systems.
Cell Organisation
- Cells are the basic structural and functional units of living organisms.
- Similar cells can form tissues.
- Tissues combine to form organs.
- Organs work together in organ systems.
Cell → Tissue → Organ → Organ system → Organism
Eukaryotic Cell Ultrastructure
Eukaryotic cells contain membrane-bound organelles. The ultrastructure of these organelles can be studied using electron microscopy.
Nucleus
Contains the genetic material. The nucleolus is involved in the production of ribosomal components.
Ribosomes
Sites of protein synthesis.
Rough Endoplasmic Reticulum
Membrane network with ribosomes attached. It is involved in the synthesis and transport of proteins.
Smooth Endoplasmic Reticulum
Involved in synthesis and processing of substances including lipids.
Mitochondria
Sites of aerobic respiration and ATP production.
Golgi Apparatus
Modifies, sorts and packages proteins and other molecules for transport.
Lysosomes
Contain hydrolytic enzymes that digest materials inside cells.
Centrioles
Associated with the organisation of spindle fibres during cell division in animal cells.
Ribosome → rough ER → transport vesicle → Golgi apparatus → secretory vesicle → cell surface
Prokaryotic Cells
Prokaryotic cells are smaller and structurally simpler than eukaryotic cells. They do not contain a membrane-bound nucleus.
- Cell wall
- Capsule
- Plasma membrane
- Circular DNA
- Plasmids
- Ribosomes
- Flagellum
- Pili
Microscopy
Microscopy allows structures that cannot be seen with the unaided eye to be observed.
Resolution
Resolution is the ability to distinguish two points as separate objects.
Electron microscopes generally have much greater resolving power than light microscopes.
Magnification makes an image appear larger. Resolution determines how much detail can actually be distinguished.
WORKED EXAMPLE
A cell image is 40 mm long. Its actual length is 20 μm. Calculate the magnification.
Convert 40 mm to micrometres:
40 mm = 40 000 μm
Magnification = 40 000 ÷ 20
Magnification = ×2000
Core Practical 5
Use a light microscope to make observations and labelled drawings of suitable animal cells, use a graticule to make measurements and understand scale.
TOPIC 03 • REPRODUCTION
Meiosis, Gametes, Fertilisation and the Cell Cycle
Cell division allows organisms to grow, reproduce and maintain their tissues. Meiosis produces genetically varied gametes, while mitosis produces genetically identical daughter cells.
Chromosomes and Loci
A locus is the position of a gene on a chromosome. Genes located on the same chromosome are linked.
Meiosis
Meiosis produces haploid cells from a diploid parent cell. It contributes to genetic variation through independent assortment and crossing over.
- Independent assortment of chromosomes during metaphase I.
- Crossing over between chromatids during prophase I.
Independent Assortment
Homologous chromosome pairs line up independently of one another. This creates different combinations of maternal and paternal chromosomes in gametes.
Crossing Over
Sections of DNA can be exchanged between chromatids of homologous chromosomes. This creates new combinations of alleles.
Specialised Mammalian Gametes
Sperm Cell
- Acrosome containing enzymes involved in penetration of the egg.
- Many mitochondria providing ATP for movement.
- Flagellum for movement.
- Haploid nucleus.
Egg Cell
- Large cell containing nutrient reserves.
- Haploid nucleus.
- Zona pellucida surrounding the cell.
Mammalian Fertilisation
The sperm reaches the egg and the acrosome reaction releases enzymes. Following successful fusion, the cortical reaction helps prevent additional sperm from fertilising the egg. The nuclei eventually fuse.
Fusion of the nuclei of two haploid gametes to produce a diploid zygote.
Flowering Plant Fertilisation
A pollen grain germinates on a compatible stigma. A pollen tube grows towards the ovule and carries the male gamete nuclei towards the embryo sac, where nuclear fusion occurs.
Mitosis and the Cell Cycle
Mitosis produces genetically identical daughter cells. It is important for growth, tissue repair and asexual reproduction.
WORKED EXAMPLE
In a root tip, 24 cells are observed. Six cells are undergoing mitosis. Calculate the mitotic index.
Mitotic index = 6 ÷ 24
Mitotic index = 0.25
As a percentage:
0.25 × 100 = 25%
Core Practical 6
Prepare and stain a root tip squash to observe the stages of mitosis and calculate mitotic indices.
Be able to connect the stages of cell division with the production of genetically identical cells and with chromosome behaviour.
TOPIC 03 • DEVELOPMENT
Stem Cells, Differentiation, Gene Expression and Phenotype
Development involves cells becoming specialised. This requires different genes to be expressed in different cells.
Stem Cells
Stem cells are unspecialised cells that can divide and differentiate into specialised cell types.
Totipotent Cells
Can give rise to all cell types required to form an entire organism.
Pluripotent Cells
Can differentiate into many different cell types but are more limited than totipotent cells.
Morula and Blastocyst
After fertilisation, repeated mitotic divisions produce an early embryo. The morula develops into a blastocyst containing cells with different developmental potentials.
Cell Differentiation
Cells with the same genome can develop different structures and functions because different genes are expressed.
Different cells activate different sets of genes, resulting in the production of different proteins and therefore different cell structures and functions.
From Gene to Protein
A gene can be transcribed to produce mRNA. The mRNA is then translated at a ribosome to produce a polypeptide.
Alternative Proteins
Post-transcriptional changes to mRNA can allow one gene to give rise to more than one protein.
Genotype and Environment
Phenotype is influenced by both genotype and environmental conditions.
Epigenetics
Epigenetic modifications can alter gene activation without changing the underlying DNA base sequence.
DNA Methylation
Methyl groups can be added to DNA and may reduce the expression of particular genes.
Histone Modification
Chemical modification of histone proteins can alter how tightly DNA is associated with histones and therefore influence gene expression.
Some epigenetic modifications can persist following cell division.
Polygenic Inheritance
Some characteristics are influenced by multiple genes. Polygenic inheritance can produce continuous variation. Environmental factors can also contribute.
EXAM THINKING
If individuals with similar genotypes show different phenotypes, consider environmental influences as well as genetic differences.
Stem Cells and Society
Stem-cell research may contribute to medical therapies, but decisions about its use involve scientific, ethical, moral and social considerations.
TOPIC 04 • PLANT STRUCTURE
Plant Cells, Tissues, Transport and Plant Products
Plant cells contain structures not found in typical animal cells and are adapted for photosynthesis, storage, support and transport.
Plant Cell Structures
- Cell wall — provides strength and support.
- Chloroplast — site of photosynthesis.
- Amyloplast — involved in starch storage.
- Vacuole — contains cell sap and contributes to turgor.
- Tonoplast — membrane surrounding the vacuole.
- Plasmodesmata — connections between adjacent plant cells.
- Pits — areas associated with transport between cells.
- Middle lamella — helps bind adjacent plant cells together.
Starch
Starch is a storage polysaccharide made from α-glucose. It includes amylose and amylopectin.
Cellulose
Cellulose is a structural polysaccharide made from β-glucose. Hydrogen bonding between chains contributes to the formation of strong cellulose microfibrils.
Hydrogen bonds between β-glucose chains help hold cellulose molecules together, forming microfibrils that provide considerable tensile strength.
Plant Tissues
Sclerenchyma
Provides mechanical support. Thickened cell walls contribute to strength.
Xylem
Transports water and mineral ions and also contributes to support.
Phloem
Translocates organic solutes through the plant.
- Sclerenchyma → support
- Xylem → water, mineral ions and support
- Phloem → organic solute translocation
Plant Mineral Requirements
Plants require inorganic ions for healthy growth. Important ions include nitrate, calcium and magnesium.
Nitrate Ions
Required to produce amino acids and therefore proteins.
Magnesium Ions
Required for chlorophyll production.
Calcium Ions
Important for cell walls and normal plant development.
Plant Fibres and Sustainability
Plant fibres can be used to produce materials and may contribute to sustainability by replacing some oil-based products.
Core Practical 7
Use a light microscope to observe, draw and label plan diagrams of roots, stems and leaves, and identify sclerenchyma fibres, phloem, sieve tubes and xylem vessels.
Core Practical 8
Determine the tensile strength of plant fibres.
EXAM COMPARISON
Xylem vessels have thickened walls that provide support while allowing water and mineral ions to be transported. Phloem is specialised for translocation of organic solutes.
TOPIC 04 • BIODIVERSITY
Biodiversity, Classification, Evolution and Conservation
Biodiversity describes the variety of life. It can be studied at different levels, including the variety of species and genetic variation within species.
Classification
Classification organises organisms according to similarities and differences in phenotype and genotype and is based around the species concept.
Three-Domain System
- Archaea
- Bacteria
- Eukarya
New molecular evidence can lead scientists to revise taxonomic groupings.
Biodiversity
Biodiversity refers to the variety of living organisms. Human activity has contributed to threats to biodiversity.
Endemism
An endemic species is naturally found only in a particular geographical area.
Species Richness
Species richness is the number of different species present within a particular habitat.
Heterozygosity Index
Index of Diversity
The index of diversity can be used to compare biodiversity between habitats.
where N is the total number of organisms and n represents the number of organisms belonging to each species.
WORKED EXAMPLE
A population contains 100 individuals, of which 20 are heterozygous for a particular gene.
Heterozygosity index = 20 ÷ 100
Heterozygosity index = 0.20
As a percentage: 20%
Niche
A niche describes the role and position of an organism within its environment.
Adaptations
Organisms may show behavioural, anatomical and physiological adaptations that increase their ability to survive and reproduce in their environment.
Hardy-Weinberg Equation
The Hardy-Weinberg equation can be used to investigate whether allele frequencies in a population are changing over time.
Here p and q represent the frequencies of two alleles. The terms p², 2pq and q² represent genotype frequencies under the assumptions of the model.
Changes in Allele Frequency
Allele frequencies can change because of mutation and natural selection.
Speciation
Reproductive isolation can prevent gene flow between populations. Over generations, different genetic changes can accumulate and potentially result in the formation of new species.
Variation → selection pressure → differential survival and reproduction → change in allele frequency → divergence of populations
Conservation
Zoos and seed banks can help conserve endangered species and their genetic diversity.
Captive Breeding
Breeding programmes can increase population numbers and help preserve genetic diversity.
Reintroduction
Individuals can sometimes be returned to suitable natural habitats.
Seed Banks
Seeds can be stored as a means of conserving plant genetic diversity.
Education and Research
Research improves understanding of endangered species, while education can encourage conservation and reduce human pressures on habitats.
Conservation methods have benefits and limitations. For example, captive breeding can increase population size but may involve limited genetic diversity and adaptation to artificial conditions.
Core Practical 9
Investigate the antimicrobial properties of plants, including aseptic techniques for the safe handling of bacteria.
Drug Testing
Modern drug testing has developed from historical approaches to controlled scientific protocols.
- Historical evidence such as William Withering's work on digitalis.
- Placebo-controlled studies.
- Double-blind trials.
- Three phases of clinical testing.
CORE PRACTICALS
Unit 2 Practical Skills
Unit 2 includes practical knowledge that can be assessed through the Unit 2 examination and contributes to preparation for Unit 3 practical skills.
Light microscopy of animal cells, labelled drawings, graticule measurements and scale.
Root tip squash and observation of mitotic stages.
Microscopy of roots, stems and leaves, including identification of xylem, phloem and sclerenchyma.
Determination of tensile strength of plant fibres.
Investigation of antimicrobial properties of plants using appropriate aseptic techniques.
Recommended Additional Practical Work
- Investigating factors affecting pollen tube growth.
- Investigating plant mineral deficiencies.
PRACTICE
Unit 2 Mixed Practice
Try each question before opening the answer.
Show answer
Eukaryotic cells contain a membrane-bound nucleus and membrane-bound organelles. Prokaryotic cells do not. Prokaryotes contain circular DNA and may contain plasmids.
Show answer
Actual size = image size ÷ magnification.
30 mm ÷ 1500 = 0.020 mm.
0.020 mm = 20 μm.
Actual size = 20 μm
Show answer
Independent assortment produces different combinations of chromosomes. Crossing over exchanges sections of DNA between chromatids, creating new combinations of alleles.
Show answer
Mitosis produces genetically identical daughter cells. Increasing cell number allows tissues and organisms to grow.
Show answer
Mitotic index = 50 ÷ 200.
Mitotic index = 0.25
Or 25%.
Show answer
Different cells express different genes. This results in different mRNA molecules being produced and therefore different proteins being synthesised. The proteins influence cell structure and function.
Show answer
Cellulose molecules form chains of β-glucose. Hydrogen bonds between chains help form strong cellulose microfibrils, giving the cell wall tensile strength.
Show answer
50 ÷ 250 = 0.20.
Heterozygosity index = 0.20
Show answer
Archaea, Bacteria and Eukarya.
Show answer
Reproductive isolation prevents gene flow between populations. Different mutations and selection pressures can then cause genetic differences to accumulate. Eventually the populations may become sufficiently different to form separate species.
Show answer
p + q = 1
p² + 2pq + q² = 1
Show answer
Examples include captive breeding, reintroduction, seed banks, scientific research and education.
QUICK REVIEW
Unit 2 Key Ideas
- Cells are organised into tissues, organs and organ systems.
- Eukaryotic cells contain membrane-bound organelles.
- Prokaryotic cells contain circular DNA and lack a membrane-bound nucleus.
- Magnification = image size ÷ actual size.
- Resolution is the ability to distinguish two points as separate.
- Meiosis produces genetically varied haploid gametes.
- Independent assortment and crossing over contribute to genetic variation.
- Mitosis produces genetically identical daughter cells.
- Mitotic index = cells undergoing mitosis ÷ total cells.
- Stem cells can differentiate into specialised cells.
- Differential gene expression causes cell specialisation.
- Phenotype is influenced by genotype and environment.
- Epigenetic modifications can affect gene expression.
- Cellulose microfibrils provide strength to plant cell walls.
- Xylem transports water and mineral ions.
- Phloem transports organic solutes.
- Biodiversity includes variation between species and genetic diversity within species.
- Species richness measures the number of different species in a habitat.
- The Hardy-Weinberg equations can be used to investigate changes in allele frequency.
- Mutation and natural selection can change allele frequencies.
- Reproductive isolation can contribute to speciation.
- Zoos and seed banks can contribute to conservation.