1 · Cell Structure

2. Foundations in Biology — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

20 min read 35 parts Lesson 1 of 13 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

Foundations in Biology — notes: Describe cell structure and microscopy, biological molecules, nucleic acids and protein synthesis, enzymes, membranes and transport, the cell cycle and cell differentiation, with the related practical skills.FeatureLightTEMSEMResolutionAbout 0.2 µ…

Magnification
Liquid specimen
Solid specimen
Stains for light microscopy
Stains for TEM
Limitations

Part 1 of 32Light and electron microscopes

Feature	Light	TEM	SEM
Resolution	About 0.2 µm (200 nm)	About 0.2 to 0.5 nm	3 to 10 nm
Useful magnification	Up to ×1500 to 2000	Over ×500 000 to millions	Lower than TEM
Image	Colour, living or dead	Black and white, 2D, dead only	Black and white, 3D surface, dead only
Shows	Whole cells, nuclei, mitochondria, chloroplasts	Internal structure of organelles (membranes, ribosomes, ER, lysosomes)	Surface of objects (for example E. coli)
Practical	Small, cheap, simple preparation	Large, expensive, complex preparation, vacuum	Large, expensive, vacuum

TEM

TEM electromagnets transmit electrons through the specimen; denser parts absorb more and appear darker.

SEM

SEM electrons scanned over the surface and the electrons that bounce back are detected.

Resolution

Resolution light is limited by wavelength (diffraction);
- electrons have a smaller wavelength, so points much closer together can be separated.

Magnification

Magnification how many times larger the image is; for a light microscope, eyepiece × objective.

Part 2 of 32Preparing slides, stains and graticules

Preparing slides, stains and graticules

Liquid specimen

Liquid specimen drops with a pipette, coverslip lowered at an angle, gloves worn.

Solid specimen

Solid specimen cut a thin piece and stain;
- or fix in formaldehyde, dehydrate with ethanol, impregnate with paraffin or resin, cut thin slices, remove paraffin, stain and mount;
- or freeze in CO₂ or liquid nitrogen, slice, stain.

Technique

Technique always start on low power;
- a drop of water stops drying;
- blurred image: go to low power, use coarse focus, check thickness and contamination.

Graticule

Graticule an eyepiece ruler with no units; calibrate each objective with a stage micrometer.

Stains for light microscopy

Stains for light microscopy dyes absorb some colours so structures become visible; differential staining uses several dyes; chloroplasts need no stain.
- (toluidine blue gives blue, phloroglucinol red/pink)

Stains for TEM

Stains for TEM heavy-metal compounds absorb electrons; colour in electron micrographs is added by software.
- (osmium tetroxide, ruthenium tetroxide)

Limitations

Limitations cell sizes differ with section plane, some structures cannot be seen, and preparation may alter structure.

Part 3 of 32Drawing cells and magnification calculations

Conventions

Conventions title and magnification, sharp HB pencil on plain paper, single clear lines, no shading, large and proportional, label lines ruled, not crossing, no arrowheads, on one side.
Cell drawings are at high power; plan drawings (low power) show tissue only, never individual cells.

Magnification

Magnification image size ÷ actual size.

Units
- 1000 nm = 1 µm;
  - 1000 µm = 1 mm;
  - 1000 mm = 1 m;
  - 10 mm = 1 cm.
Convert to the same unit first50 mm image of a 2000 µm leaf section is 50 000 µm ÷ 2000 µm = ×25.
A 30 mm image at ×3000 is 0.01 mm = 10 µm.

Part 4 of 32Eukaryotic organelles

Organelle	Structure and function
Cell surface membrane	Partially permeable phospholipid bilayer (about 10 nm) controlling exchange
Cell wall	Outside the membrane, freely permeable; cellulose in plants, peptidoglycan in bacteria; plasmodesmata link plant cells
Nucleus	Double membrane with pores; chromatin (DNA and histones); nucleolus makes ribosomes
Mitochondria	Double membrane, inner membrane folded into cristae; matrix has enzymes, circular DNA and ribosomes; aerobic respiration and ATP
Chloroplasts	Double membrane; thylakoids stack into grana joined by lamellae; light-dependent stage in thylakoids, Calvin cycle in stroma; own DNA and ribosomes
Ribosomes	rRNA and protein, no membrane; 80S in eukaryotes, 70S in prokaryotes, mitochondria and chloroplasts; translation
Rough ER	Ribosomes on a membrane continuous with the nuclear envelope; processes proteins
Smooth ER	No ribosomes; makes, processes and stores lipids, carbohydrates, steroids
Golgi apparatus	Flattened sacs; modify proteins and lipids and package them into vesicles
Vacuole	Plant: large permanent, tonoplast; animal: small and temporary
Vesicle and lysosome	Vesicle: transport and storage; lysosome: hydrolytic enzymes digesting worn-out organelles and in the immune system and apoptosis
Centrioles	Microtubule hollow fibres in pairs forming a centrosome that organises the spindle; absent from flowering plants and fungi
Microtubules, microvilli, cilia and flagella	Microtubules (25 nm, from α and β tubulin, 13 protofilaments) form the cytoskeleton; microvilli increase surface area; cilia move substances; flagella move the cell

Plant versus animal

Plant versus animal plants have cellulose walls, large vacuoles and chloroplasts;
- animals have centrioles and microvilli.

Electron micrographs

Electron micrographs show more detail than photomicrographs because of higher resolution.

Part 5 of 32Protein production pathway and the cytoskeleton

DNA stored in the nucleus; the nucleolus makes ribosomes; transcription produces mRNA.

mRNA leaves and binds to a ribosome, where translation builds a polypeptide.

Rough ER folds and processes the polypeptide into protein.

Vesicles carry it to the Golgi apparatus, which modifies and packages it.

Vesicles fuse with the cell surface membrane and secrete the protein by exocytosis.

Cytoskeleton
- microfilaments (actin, solid) and microtubules, plus intermediate filaments.
  - (tubulin, hollow)
- Support and mechanical strength, holding organelles in place.
- Tracks for moving vesicles and chromosomes using ATP.
- Cell movement through cilia and flagella.

Part 6 of 32Prokaryotes versus eukaryotes

Prokaryote

100 to 1000 times smaller;
- no membrane-bound organelles;
- 70S ribosomes;
- circular DNA free in cytoplasm, not bound to protein;
- cell wall of murein (a glycoprotein).

Extras

Extras plasmids, a slime capsule (protects from drying and immune attack) and flagella (rotate like propellers).
- (small DNA loops, for example antibiotic resistance)

Exam note

Exam note size is not a structural difference; not all features are present in all prokaryotes.

Part 7 of 32Water and its properties

Water and its properties

Polar molecule

Polar molecule oxygen attracts electrons more strongly, giving a dipole; hydrogen bonds form between molecules.
- (oxygen of one to hydrogen of another)

Solvent

Solvent polar substances and ions dissolve, so reactions happen and substances are transported.

High specific heat capacity

(4200 J kg^-1 °C^-1) — stable habitats and enzyme temperatures; blood plasma transfers heat.

High latent heat of vaporisation

High latent heat of vaporisation evaporation cools.
- (sweat, transpiration)

Ice is less dense

Ice is less dense it floats and acts as a habitat.

Cohesion, adhesion and surface tension

Cohesion, adhesion and surface tension columns in xylem, pond skaters walk on water.

Reagent

Reagent water takes part in hydrolysis and is released in condensation.

Part 8 of 32Monomers, polymers, bonds and elements

Monomers, polymers, bonds and elements

Monomer, polymer, macromolecule

Monomer, polymer, macromolecule not all macromolecules are polymers; lipids are not polymers.

Condensation

Condensation joins monomers and removes water; hydrolysis adds water to break bonds.

Carbon

Carbon forms four stable covalent bonds in chains, branches and rings.

Elements

CarbohydratesC, H, O in the ratio C_x(H₂O)_y.
LipidsC, H, O with little oxygen.
ProteinsC, H, O, N and sometimes S.
Nucleic acidsC, H, O, N, P.

Functions

Carbohydratesenergy source and store, structure (cellulose).
Lipidsenergy source and store, insulation, membranes.
Proteinsgrowth and repair, structure, enzymes, hormones, antibodies, carriers.
Nucleic acidscarry the genetic code.

Part 9 of 32Carbohydrates

Monosaccharides

Monosaccharides trioses (3C), pentoses (5C, ribose) and hexoses (6C, glucose C₆H₁₂O₆).
Glucose is soluble, the main respiratory substrate, with α and β isomers.
Reducing sugars (glucose, fructose, galactose) donate electrons (OILRIG); sucrose is non-reducing.
Deoxyribose lacks the oxygen on carbon 2 of ribose.

Glycosidic bond

Glycosidic bond condensation between two OH groups; broken by hydrolysis with different enzymes.
Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose.

Starch

Starch amylose and amylopectin (70 to 90%, 1,4 and 1,6 bonds, branched, many free ends); stored in plastids.
- (10 to 30%, unbranched helix, compact and digestion-resistant)

Glycogen

Glycogen more branched than amylopectin;
- animals and fungi;
- granules in liver and muscle;
- free ends allow quick release or storage.

Cellulose

β-glucose chains joined by hydrogen bonds into microfibrils; strong, freely permeable wall resisting turgor pressure; gives fibre.
- (alternate units rotated 180°)

Storage polysaccharides

Storage polysaccharides are compact and insoluble, so there is no osmotic effect.

Part 10 of 32Lipids

Lipids

Triglyceride

Triglyceride glycerol + three fatty acids (RCOOH) joined by ester bonds in condensation; saturated or unsaturated (cis and trans).
- (three waters released)

Functions

Energy store
- 37 kJ per gram compared with 17 kJ for carbohydrate or protein;
  - hydrophobic so no osmotic effect;
  - releases metabolic water (desert animals, bird and reptile embryos).
Insulationmyelin sheath, adipose tissue, blubber.
Buoyancy and protection of organs.

Phospholipid

Phospholipid glycerol, two fatty acids and a phosphate; amphipathic,
- forming bilayers; the hydrophobic core is a barrier and saturated tails make it less fluid.

Cholesterol

Cholesterol made in the liver, hydrophobic and hydrophilic regions;
- disrupts close packing and stabilises at high temperature,
- reducing permeability to ions; precursor of steroid hormones.
- (keeps membranes fluid in the cold)

Part 11 of 32Proteins

Amino acids

20 types with central carbon, amine group, carboxyl group, hydrogen and R group; peptide bond by condensation, broken by hydrolysis.
- (C=O carbon to N)

Structure levels

Primarysequence of amino acids from DNA.
Secondaryα-helix or β-pleated sheet from hydrogen bonds in the backbone.
Tertiaryhydrogen, ionic, disulfide (strongest, cysteine, broken by oxidation or reduction) and hydrophobic interactions between R groups.
Quaternaryseveral polypeptides, for example haemoglobin.

Globular proteins

Globular proteins compact and soluble with hydrophobic R groups inward, hydrophilic outward;
- enzymes, antibodies, insulin and haemoglobin.
Haemoglobin
- two α and two β chains each with a haem group containing Fe²⁺;
  - four O₂ per molecule;
  - sickle cell anaemia replaces glutamic acid with valine,
- making it less soluble.
Insulinchains of 21 and 30 amino acids joined by three disulfide bridges.

Fibrous proteins

Fibrous proteins long, repetitive, insoluble and strong: keratin (hair, nails), elastin, collagen (triple helix, tendons, skin, bone).
- (stretches and recoils)
Collagen strength comes from hydrogen bonds, staggered ends and stable proline and hydroxyproline.

SAFES

SAFES compare proteins by Shape, Amino acid sequence, Function, Examples and Solubility.

Part 12 of 32Inorganic ions

Ion	Role
H⁺	Determines pH (normal body pH about 7.4); changes denature enzymes
Ca²⁺	Regulates synapses, triggers muscle contraction (binds troponin), regulates channels, activates enzymes, clotting factor
Fe²⁺/Fe³⁺	Binds oxygen in haemoglobin and myoglobin; electron transfer in cytochromes
Na⁺	Co-transport of glucose and amino acids; nerve impulses
K⁺	Repolarisation of axons, kidney water reabsorption, guard cells
NH₄⁺	Intermediate in deamination
NO₃^-	Nitrogen for plant protein synthesis
HCO₃^-, Cl^-	Carbon dioxide transport and pH balance in blood
PO₄^3-	DNA, RNA, ATP and phospholipids
OH^-	Hydrogen bonding in DNA, proteins and water

Cofactors
- some ions act as non-protein helpers for enzymes.

Part 13 of 32Biochemical tests and quantitative methods

Biochemical tests and quantitative methods

Benedict's test

Benedict's test excess reagent, heat;
- blue → green → yellow → orange → brick-red;
- for non-reducing sugars boil with HCl, neutralise with sodium hydrogencarbonate then test.

Iodine in potassium iodide

Iodine in potassium iodide blue-black for starch.

Emulsion test

Emulsion test ethanol then water; milky for lipid.

Biuret

Biuret sodium hydroxide then copper(II) sulfate;
- blue to lilac/mauve;
- needs at least two peptide bonds;
- use a control.

Serial dilution

Serial dilution standards for comparison by eye, calibration curve or colorimeter;
- also for counting microbes.

Colorimeter

Colorimeter filter of contrasting colour, zero with a blank, plot absorbance against known concentration and read unknowns.
- (blue light for orange Benedict's)

Alternative

Alternative time to first colour change; shorter means higher concentration.

Part 14 of 32Chromatography

Principle

Principle solutes separate by solubility between the mobile phase (solvent) and stationary phase (paper);
- larger or less soluble molecules move less.

Method

Method spot sample and standards on a pencil line, dry, suspend in solvent, mark the solvent front;
- stain colourless sugars;
- spray amino acids with ninhydrin (blue-violet).

Rf distance moved by solute ÷ distance moved by solvent; less than 1, no units.

Carotenoids near 1, chlorophyll a in the middle, chlorophyll b lower.

Use

Use after a positive biuret test, chromatography shows which amino acids are present, for example in forensics or food spoilage.

Part 15 of 32Nucleotides and DNA

Nucleotide

Nucleotide pentose sugar, phosphate and nitrogenous base.
DNA
- deoxyribose, A, C, G, T;
  - RNA: ribose, A, C, G, U.
Purines (A, G) have two rings; pyrimidines (C, T, U) have one.

Phosphodiester bond

Phosphodiester bond condensation between phosphate and sugar, forming the sugar-phosphate backbone;
- hydrolysed when polynucleotides break down.

ATP

ATP adenosine + three phosphates; ADP two, AMP one; universal energy currency.
- (adenine is a base, adenosine a nucleoside)

DNA

DNA two antiparallel strands with bases inwards held by hydrogen bonds: A–T (two) and G–C (three).
- (5' to 3' and 3' to 5')
If given the number of one base, work out the others (A = T, C = G).

Part 16 of 32DNA extraction (Marmur preparation)

Chop onion and add to detergent solution; heat at 60 °C for 15 minutes to disrupt membranes and denature DNA-digesting enzymes.

Cool in ice water for 5 minutes, stirring, so the DNA does not break down.

Blend for 5 seconds only, to release more DNA without breaking it.

Filter through coffee filter paper to remove debris.

Add protease to remove histone and other proteins.

Add ice-cold ethanol; DNA is insoluble and precipitates as a white layer.

Part 17 of 32DNA replication and the genetic code

1Semi-conservative replication

Semi-conservative replication in the S phase

DNA helicase breaks hydrogen bonds and unwinds the helix.
Activated nucleotides pair with exposed bases on each template strand.
DNA polymerase joins them by condensation, forming phosphodiester bonds.
Hydrogen bonds form between complementary bases.

2Importance

Importance one conserved strand ensures accuracy and genetic continuity;
- copying errors give random spontaneous mutations.

3Genetic code

Genetic code a gene codes for a polypeptide; triplets code for amino acids.
- (for example TAC = start/methionine)
4³ = 64 codons for 20 amino acids, so degenerate (limits mutation effects).
Non-overlapping and universal, so genetic engineering is possible.

4Codon and anticodon

Codon and anticodon mRNA triplets are codons; tRNA has complementary anticodons.

Part 18 of 32Transcription and translation

Transcription (nucleus)

Part of the DNA unwinds.
Free RNA nucleotides pair with the template strand.
RNA polymerase forms the mRNA backbone (phosphodiester bonds).
The mRNA leaves through a nuclear pore.
mRNA matches the coding strand (U for T);
- RNA polymerase moves 3' to 5' along the template and mRNA grows 5' to 3'.

Translation (cytoplasm)

Translation (cytoplasm) mRNA attaches to the ribosome (rRNA aligns it).
tRNAs bring specific amino acids; two fit at a time.
The anticodon pairs with the codon.
A peptide bond forms between adjacent amino acids, catalysed by rRNA.
At a stop codon translation ends and the polypeptide is released.

Do not confuse

DNA polymerase (replication) with RNA polymerase (transcription).

Part 19 of 32Enzymes: structure and action

Enzymes: structure and action

Enzymes

Enzymes are globular protein catalysts that lower activation energy by destabilising bonds,

giving an alternative pathway; virtually every metabolic reaction needs one.

Location

Location intracellular (inside cells) or extracellular.
- (such as digestive enzymes)

Specificity

Specificity the active site shape comes from the tertiary structure and is complementary to one substrate;
- the enzyme-substrate complex is temporary.

Models

Lock and key (Fischer, 1890s): rigid shapes.
Induced fit (1959): the active site, and sometimes the substrate, change shape (conformational change) for the best fit.

Denaturation

Denaturation extremes of heat or pH change the active site; do not say enzymes are "killed".

Part 20 of 32Cofactors, coenzymes and practicals on enzymes

Cofactors

Cofactors non-protein helpers: inorganic ions, prosthetic groups (permanently bound; zinc in carbonic anhydrase) and coenzymes (organic, often from vitamins).
- (chloride for amylase)

Coenzymes

Coenzymes carry electrons or groups between enzymes.

NAD, FAD (B vitamins) and NADP transfer hydrogen in respiration and photosynthesis.
ATP transfers phosphate groups; coenzyme A transfers acetyl groups.

Amylase and pH

Amylase and pH samples every 10 seconds into iodine until it stays orange-brown;
- equal volumes and concentrations;
- temperature controlled by a water bath;
- a colorimeter is more objective.

Catalase

Catalase hydrogen peroxide → water + oxygen; measure oxygen volume in a set time.

Colorimetry for starch

Colorimetry for starch calibrate with weak iodine, use a red filter, make starch standards by serial dilution and plot a calibration graph.
- (100% transmission)

Part 21 of 32Membrane structure (fluid mosaic model)

Membrane structure (fluid mosaic model)

Roles

Roles separate the cell from its environment, compartmentalise organelles, control exchange (diffusion, facilitated diffusion, osmosis, active transport) and act in cell signalling.
- (lysosomal enzymes must be contained)

Fluid mosaic model (1972)

Fluid mosaic model (1972) fluid because phospholipids and proteins move;
- mosaic because proteins are scattered.

Components

Phospholipid bilayer
- barrier to water-soluble substances;
  - can act as signalling molecules.
Cholesterol
- stops tight packing at low temperature and stabilises at high temperature;
  - adds strength.
Glycoproteins and glycolipidsreceptors (hormones, neurotransmitters), endocytosis, adhesion, and antigens such as ABO.
Channel and carrier proteinsspecific transport.

Cholesterol

Cholesterol can raise or lower fluidity depending on temperature, so read the stem.

Part 22 of 32Membrane permeability and the beetroot practical

Temperature

Temperature more fluidity makes the membrane less of a barrier;
- around 40 °C many proteins denature, irreversibly disrupting the membrane. Solvents dissolve lipids.

Beetroot method

Beetroot method five equal cubes, rinsed, in 5 cm³ water at 10 to 50 °C for 30 minutes;
- measure absorbance with a colorimeter.
Higher absorbance means more pigment, so higher permeability.
Freezing can damage channels and pierce the membrane.
Limitations
- cuvette differences, uneven pieces and pigment distribution;
  - repeat and average.

Part 23 of 32Diffusion, facilitated diffusion and active transport

Diffusion, facilitated diffusion and active transport

Diffusion

Diffusion net movement down a concentration gradient from random kinetic energy;
- rate depends on gradient, temperature, surface area, distance and molecule properties.

Facilitated diffusion

Facilitated diffusion channel proteins or carrier proteins (switch shape) for large polar molecules and ions.
- (water-filled gated pores)

Active transport

Active transport against the gradient via specific carrier proteins changing shape using ATP;
- examples are kidney reabsorption, digestion products, phloem loading and root hair ion uptake.

Endocytosis and exocytosis

Endocytosis and exocytosis bulk transport using energy.
Phagocytosis
- bulk intake of solids (white blood cells engulfing bacteria);
  - pinocytosis: liquids.
Exocytosissecretory vesicles from the Golgi fuse with the membrane (pancreatic enzymes).

Naming

Naming state the specific term, not just "bulk transport".

Part 24 of 32Diffusion practicals

Visking tubing

Visking tubing a non-living partially permeable cellulose membrane;
- starch and glucose inside;
- test the surrounding water at intervals: glucose but not starch diffuses out.
Quantify with Benedict's and standards or a colorimeter.

Agar cubes

Agar cubes cubes (0.5, 1, 2 cm) with indicator in HCl;
- rate = 1 ÷ time;
- higher SA:V gives faster diffusion because volume (cubed) grows faster than surface area (squared).

Part 25 of 32Osmosis and water potential

Osmosis

Osmosis net movement of water from higher to lower water potential through a partially permeable membrane; pure water is 0 kPa and solutions are negative.
- (more negative is lower)
Water can pass the bilayer because molecules are small.

Animal cells

Animal cells hypertonic: crenation; hypotonic: cytolysis; isotonic: no net change.

Plant cells

Plant cells water loss causes plasmolysis; water gain makes cells turgid, supporting the plant, while wilting is loss of turgor.
- (protoplast pulls from the wall)
Animal cells do not plasmolyse.
In plasmolysis the external solution fills the space between wall and protoplast, since the wall is freely permeable.

Potato cylinders

Potato cylinders equal cylinders, blotted and weighed, in at least five sucrose solutions for about 30 minutes at 30 °C;
- percentage change in mass.
Positive change
- solution had higher water potential and cells are turgid;
  - negative: flaccid or plasmolysed;
  - zero: isotonic.
The x-intercept gives the potato's sucrose concentration.

Onion epidermis

Onion epidermis red onion strips in a series of solutions, count the percentage plasmolysed under a microscope.

Part 26 of 32The cell cycle and checkpoints

1Cell cycle

Cell cycle interphase (G1, S, G2), mitosis and cytokinesis;
- cyclins trigger the transitions;
- onion root tip about 20 hours, human intestine about 10 hours.
G1: growth, RNA, enzymes and proteins; signal to divide (peak mRNA synthesis).
S: DNA replication to two sister chromatids.
G2: growth, DNA checked, tubulin made for the spindle.

2Cytokinesis

Cytokinesis animal cells: constriction of cytoplasm; plant cells: a new wall forms.

3Checkpoints

G1: damage check before S phase.
S: all chromosomes replicated.
G2: second damage check.
Metaphasechromosomes correctly attached to spindle fibres.

4DNA molecules

46 chromosomes become 92 chromatids after S phase.
- (92 DNA molecules)

Part 27 of 32Stages of mitosis and its significance

Stage	Events
Prophase	Chromosomes condense (two sister chromatids at a centromere); centrosomes move to poles; spindle forms; nuclear envelope breaks down; nucleolus disappears
Metaphase	Chromosomes line up on the equator, spindle fibres attached to centromeres via kinetochores
Anaphase	Centromeres divide; spindle fibres shorten; chromatids pulled to opposite poles
Telophase	Chromosomes decondense; nuclear envelopes and nucleoli reform; spindle breaks down

Root tip practical

Root tip practical onion or garlic root tips (about 1 cm), stain with warm acidified acetic orcein, squash gently, draw and annotate.
Prophase and telophase are hard to tell apart; several nuclei in a cell indicate telophase.

Significance

Significance growth, replacement and repair, and asexual reproduction (Amoeba, Hydra and yeast budding, strawberry runners).
- (skin, gut; regeneration in zebrafish fins and axolotl limbs)

Part 28 of 32Meiosis and its significance

Meiosis I

Prophase I
- homologous pairs form bivalents;
  - non-sister chromatids cross over at chiasmata.
Metaphase Ibivalents line up with independent assortment.
Anaphase I
- whole chromosomes separate;
  - centromeres intact.
Telophase I and cytokinesistwo haploid cells (cell plate and middle lamella in plants).

Meiosis II

Meiosis II no DNA replication; prophase II, metaphase II (single file), anaphase II (centromeres divide) and telophase II give four haploid cells.
- (new spindle at right angles)

Variation

Crossing overswapping of alleles between non-sister chromatids, more likely away from the centromere.
Independent assortment2ⁿ combinations (2²³ = 8 388 608 in humans).
Random fertilisation, plus mutation.

Counting

Counting chromosomes are counted by centromeres, not chromatids.

Part 29 of 32Specialised animal cells

Cell	Adaptations
Erythrocyte	Biconcave disc, haemoglobin, no nucleus, flexible membrane for capillaries
Neutrophil	Flexible shape and lobed nucleus; pseudopodia; many lysosomes; phagocytosis
Sperm	Haploid nucleus, acrosome enzymes, mitochondria in mid-piece, rotating tail
Ciliated epithelium	Cilia beat to move mucus; goblet cells secrete mucus trapping dust and microbes
Squamous epithelium	Single flattened cells on a basement membrane; short diffusion path; permeable to gases
Muscle (skeletal, smooth, cardiac)	Protein filaments slide; many mitochondria; skeletal fibres fuse and are multinucleate
Cartilage	Strong flexible tissue, for example C-shaped tracheal rings keeping the airway open

Part 30 of 32Specialised plant cells and organisation

Cell	Adaptations
Root hair cell	Extension for surface area, thin wall, concentrated vacuole sap, mitochondria for active transport, no chloroplasts
Palisade cell	Many chloroplasts; tall thin shape lets light penetrate deeper and packs densely
Guard cell	Inner wall thicker than outer; bends when turgid; many chloroplasts and mitochondria
Xylem vessel	No end walls, dead, lignin thickening
Phloem	Living sieve tube elements with sieve plates and companion cells; few organelles

Levels of organisation
- specialised cells form tissues, tissues form organs and organs form organ systems.
- Examples
  - red blood cell to blood;
    - heart (cardiac muscle, vessels, connective tissue) to circulatory system.

Part 31 of 32Stem cells and their uses

Stem cell

Stem cell divides indefinitely;
- daughter cells can remain stem cells or differentiate;
- potency is the ability to differentiate.
Totipotentany embryonic and extra-embryonic cell (zygote and cells up to 16-cell stage).
Pluripotentany cell in the embryo but not the placenta.
Multipotentlimited range in one tissue (adult stem cells).

Bone marrow stem cells

Bone marrow stem cells form erythrocytes (lose nucleus, biconcave, more haemoglobin, flexible membrane) and neutrophils (lobed nucleus, more hydrolytic enzymes and lysosomes).

Plant meristems

Plant meristems cambium forms xylem and phloem sieve tubes (less cytoplasm, fewer organelles, sieve plates).
- (lignin, loss of cytoplasm and end walls)

Medical uses

Medical uses repair tissue, treat Alzheimer's and Parkinson's, and study development and drug effects on embryos.
- (skin for burns, neurones for spinal cord, pancreas cells for type 1 diabetes, retina cells)

Evaluation

Evaluation ethical concerns over embryos that could become humans, immune rejection unless tissue-matched,
- and uncontrolled division leading to cancer.

Part 32 of 32Worked examples

Calculate the magnification of a drawing 60 mm long of a cell measured as 40 µm. [2]2 marks
Explain why a non-competitive inhibitor lowers Vmax but a competitive inhibitor does not. [3]3 marks
Describe the structure of a DNA molecule. [4]4 marks
A tissue cylinder gains 12% in mass in a solution. Explain what this shows. [3]3 marks
Explain how crossing over and independent assortment cause variation. [4]4 marks

Key terms

Term	Meaning
Resolution	The ability to distinguish two points as separate.
Glycosidic bond	A covalent bond between monosaccharides formed by condensation.
Peptide bond	A covalent bond between amino acids formed by condensation.
Phosphodiester bond	The bond linking nucleotides in the backbone.
Induced fit	Model in which the active site changes shape to bind the substrate.
Cofactor	A non-protein substance an enzyme needs to function.
Water potential	The tendency of water to move out of a solution; pure water is 0 kPa.
Potency	The ability of a stem cell to differentiate into other cell types.
Chiasma	The point where non-sister chromatids cross over in meiosis I.

Avoid these mistakes

Saying DNA polymerase forms hydrogen bonds — it forms phosphodiester bonds.

Saying enzymes are killed — they are denatured.

Saying animal cells plasmolyse — only plant cells do.

Confusing chromosomes with chromatids when counting.

Using size as a structural difference between prokaryotes and eukaryotes.

Last updated: 14 October 2026