Cells

Organelle Structure and function

Nucleus  enclosed in double membrane with pores
 contains chromosomes with genes made of DNA to control protein
synthesis

Ribosomes  made of RNA and protein

 free in cytoplasm or attached to RER
 site of protein synthesis

Rough endoplasmic  interconnected sacs with ribosomes attached

reticulum  transport proteins to other parts of cell

Smooth endoplasmic  synthesis of lipids and steroids

reticulum

Mitochondria  double membrane – inner folded into cristae

 site of later stages of aerobic respiration

Centrioles  one pair found in animal cells

 made of protein microtubules
 involved in spindle formation and cellular transport

Lysosomes  digestive enzymes wrapped in membrane

 breakdown of unwanted structures or old cells

Nucleolus  dense body in nucleus

 synthesis of ribosomes

Vesicle  Small fluid-filled sac in the cytoplasm, surrounded by membrane

 Transports substances in out of the cell via the cell membrane and
between the organelles
 Some are formed by the Golgi apparatus (exocytosis) and some
by the plasma membrane (endocytosis)
Eukaryotes (such as human liver cell) Prokaryotes (such as a bacterium)
Larger cells (2-200 um diameter) Very small cells ( <2.0 um in diameter)

Linear DNA Circular DNA

Nucleus present (DNA inside) No nucleus (DNA free in cytoplasm as plasmids)

Animals have NO CELL WALL; Plants have a Cell wall made out of polysaccharide
CELLULOSE cell wall ; Fungi have a CHITIN cell
wall

Many organelles such as mitochondria/golgi Few organelles, no mitochondria

Larger (80S ribosomes) Small (70S ribosomes)

Rough endoplasmic reticulum & Golgi apparatus

 {protein / polypeptides} produced by ribosome ;

 ribosomes {held on/attached to/eq} rER ;
 proteins {stored / transported / within rER / eq} ;
 proteins {folded/assume 3-D shape/tertiary structure} within (lumen of) rER / eq;
 (rER) produce vesicles / packages proteins /eq ;
 vesicles fuse with Golgi (apparatus) / eq ;
 Golgi {modifies/processes} protein ;
 details of modification e.g. glycoprotein / carbohydrate added, trimming of
carbohydrate ;
 water removed (to concentrate) / eq ;
 Golgi produces {lysosomes / secretory vesicles}
 vesicles {move towards / fuse with} cell surface membrane / correct reference to
exocytosis / eq ;

Tissue
 {one / few / similar} cell types ;
 working together / for the { same / eq } function / often cells come from the same
origin / eq ;

Organ
 Organ has {many / several / different / eq} {cell types / types of tissue};
 Organ has {many / eq} functions ;
Group of different tissues designated / working together / eq. for a particular function = 2 marks
Cell cycle:

G1 (first gap phase) synthesis of cellular proteins and organelles

interphase

S (synthesis phase) replication of DNA

G2 (second gap phase) synthesis of spindle proteins

mitosis (nuclear division) separation of the 2 DNA helices making up the

division

chromosome

cytoplasmic division cleavage of a single cell into two daughter cells

Mitosis phases:

Prophase:
 idea that during prophase {chromosomes / chromatids} (becoming) visible ;
 idea of centrioles move to opposite poles ;
 reference to formation of {spindle /spindle-fibres / microtubules} ;
 disappearance of nucleolus / nucleoli ;
 breaking down of nuclear {envelope / membrane} (in prophase) ;

Metaphase:
 nuclear envelope is broken down by metaphase / eq ;
 (at metaphase) {chromosomes / centromeres} attached to spindle (fibres) ;
 idea of {chromosomes / chromatids} lined up at equator ;

Anaphase:
 centromeres divide / split / separate / pulled apart;
 {spindle fibres / microtubules} become {shorter / contract / condense};
 chromatids {separate / pulled apart};
 chromosomes / chromatids move towards (opposite) {poles / ends / sides /
centrioles};

Telophase:
 {spindle fibres / microtubules} have now {vanished / broken down / eq};
 chromosomes {unravel / uncoil / eq} (and become invisible) ;
 reference to reformation of {nuclear membrane / envelope / eq} / {nucleolus /
nucleoli};
Diagrammatic representation of mitosis

Order: A D C E B – note that A (interphase) is sometimes referred to as part of mitosis but

most of the time it’s listed as separate.

Difference between mitosis and meiosis

Mitosis Meiosis

(Homologous) chromosomes associate in (Homologues) independent/do not pair

pairs (IGNORE ref. separation

Crossing-over/chiasmata formation No crossing-over

Two/(nuclear stages) divisions/ → 4 offspring One/(nuclear stage) division/ → 2 offspring

cells cells;

Genetically different (product) Genetically identical (product);

Cytokinesis
 The cytoplasmic division of a cell at the end of mitosis, bringing about the separation
into two daughter cells.

Function/importance of spindle
 Attachment of centromeres;
 Separation of (daughter) chromatids;
Plants

Xylem tissue structure (Centre of the stem)

 long cells / tubes with no end walls;
 continuous water columns;
 no cytoplasm / no organelles/named organelle;
 to impede/obstruct flow / allows easier water flow;
 thickening/lignin;
 support / withstand tension / waterproof / keeps water in cells;
 pits in walls;
 allow lateral movement / get round blocked vessels;

Sclerenchyma fibres structure (Outer edge particularly)

 Bundles of dead cells running vertically;

 Hollow lumen;
 No end walls;
 Thickening/lignin;
 support / withstand tension;
 More cellulose than other plant cells;

Structure of function of starch

 Contains amylose and amylopectin;

 Amylose (not a mark) :
 Long and unbranched chains;
 1-4 glycosidic bonds;
 coiled structure;
 (so) compact;
 (so) can fit more in a small space;
 Amylopectin (not a mark) :
 (long), branched chains;
 (has) 1-4 and 1-6 glycosidic bonds;
 side branches which can be broken down quickly;
 Insoluble;
 so water doesn’t cause it swell via osmosis
 Structure of cellulose
 (cellulose) contains beta glucose / eq ;
 (glucose molecules) joined by condensation reactions ;
 appropriate reference to glycosidic bonds ;
 appropriate reference to (only) 1- 4 (glycosidic bonds) ;
 reference to inversion of alternate glucose molecules in the chain ;
 { unbranched / straight } chain ;
 idea of microfibril composed of many cellulose molecules ;
 (cellulose chains) held together by hydrogen bonds ;

Starch Cellulose
α-glycosidic bonds β-glycosidic bonds
Flexible chains Straight chains
H bonds within each chain, forming helix H bonds between chains, forming
microfibrils
Reacts with Iodine to form purple complex Doesn’t react with Iodine
Forms H-Bonds with water, so soluble Can’t form bonds with water, so insoluble
Easy to digest Difficult to digest
Storage role Structural role

Structure of a chloroplast
Organelle Diagram Description Function
 Rigid structure  Supports plant cells
Cell wall surrounding
plant cells;
 Cellulose;

Middle lamella  Outermost  Stability

layer;
 Adhesive;

Plasmodesmata  Channels in cell  Transport of substances

wall that link and communication
adjacent cells between cells
together

Pits  In very thin  Transport of substances

regions; between cells
 Arranged in
pairs;

Chloroplast  Small and  Photosynthesis

flattened; (Some in grana; some in
 Surrounded by stroma – thick fluid in the
double chloroplast)
membrane;
 Thylakoid
membranes;
 Stacked to form
grana;
 Grana linked by
thin, flat
lamellae;
Amyloplast  Small organelle  Storage of starch;
enclosed by  Converts starch back to
membrane; glucose;
 Starch
granules;

Vacuole & Tonoplast  Vacuole is a  Vacuole contains cell sap

compartment (water, enzymes,
surrounded by minerals, waste products)
a membrane
 Turgidity to prevent
called the
tonoplast wilting
 Breakdown and isolation
of unwanted chemicals
 Tonoplast controls what
enters and leaves the cell
Cell wall structure
 Ref to cellulose ;
 Cellulose arranged into microfibrils ;
 Embedded into a matrix ;
 Named component of matrix e.g. pectin / hemicellulose ;
 Ref to {primary and secondary cell walls / several layers of microfibrils} ;
 Ref to plasmodesmata ;

Plant fibres
 cellulose microfibrils (in cell wall) ;
 reference to net-like arrangement (of microfibrils) ;
 secondary cell wall ;
 reference to secondary cell wall being thicker ;
 idea that these features make them the plant fibres strong ;

Sustainability
 (NOT A MARK - Oil-based plastics and fuels are not sustainable as):
 Release carbon dioxide / ref. to global warming / pollution ;
 plastic is from oil ;
 oil is a non-renewable resource ;
 Generation of non-biodegradable waste ;
 (however) {plants / starch} are renewable ;
 plants can be re-grown / eq ;
 (fibres often biodegradable so) doesn’t accumulate / doesn’t take up landfill space /
eq ;

Starch Plant fibres

 Found in all plants  Ropes and fabrics – less fossil fuel
used up & crops can be regrown to
 Bioplastics – less fossil fuel used up maintain supply
& crops can be regrown.
 Products are biodegradable – this
 Vehicle fuel – such as bioethanol. means less accumulates and fills up
Same reasoning as above. landfills.

 Other uses include glues, plaster,  Easier to grow and process –

hair mousses and antiperspirants. cheaper and easier for developing
countries

Disadvantages
 The ropes made from plant fibres aren’t as strong;
 Bags made out of paper are less strong than plastic bags and disintegrate when wet;
 Degradation of waste requires aerobic organisms, so little happens in deep landfill
sites;
  Closer to the surface, methane (a greenhouse gas) is often produced;
Tissues of a leaf
 Lower epidermis – contains stomata to let air in and out for gas exchange
 Spongy mesophyll – full of spaces to let gases circulate
 Palisade mesophyll – most photosynthesis occurs here
 Xylem – carries water to the leaf & transports mineral ions
 Phloem – carries sugars away from the leaf
 Upper epidermis – covered in a waterproof waxy cuticle

Plants requirements of water + mineral ions

 Water;
 Photosynthesis;
 Structural rigidity / transport minerals / temperature regulation;
 Magnesium ions;
 Chlorophyll;
 Nitrate ions;
 DNA / protein / chlorophyll production;
 Required for plant growth / fruit production / seed production;
 Calcium ions;
 Plant growth;

Digitalis
 Foxglove extract;
 Treated dropsy;
 reference to digitalis as active drug;
 idea of extracting drug (from the plant / soup) / tested on patients /
 idea of try to find suitable dose / trial and error ;

Modern day drug testing

 idea that only contemporary testing will use animals / eq ;
 idea that only contemporary testing will {test on healthy people / have phase 1} ;
 idea that {phase 1 / eq} is to {detect side effects / find safe dosage} ;
 idea that only contemporary testing will pay volunteers ;
 idea that only contemporary testing may involve double-blind trials ;
 idea that after phase 1, phase 2 occurs – drugs are tested on a larger group of
people ;
 idea that this checks how effective the drug actually is ;
 idea that only contemporary testing will {use statistical analysis / reference to phase 3
/ use large number / comparison to existing treatments } ;
 idea of more regulation ;
 idea of controlling {factors /variables / eq} in tested cohort e.g. age, lifestyle ;

Placebo
 idea of two groups ;
 idea that group 1 is given {placebo / inactive substance / sugar pill / eq} ;
 reference to placebo effect ;
 idea that other group is given the drug ;
  idea that this checks the effectiveness of the drug / eq ;

Double-blind trial
 idea that neither doctors nor patients know placebo/drug group;
 idea that this reduces bias;
 reference to phase 2 / 3;

Leaf cross-section

(Phloem + xylem in vascular bundle)

Stem cross-section

P = Phloem; Q = Sclerenchyma fibres; R = Xylem; S = Cortex; T = Epidermis; U = Cambium

Adaptations in (different) Xerophytes relating to water
 shallow roots enable rapid uptake of rainfall;
 widespread/shallow roots allow collection of larger volume water/over a larger
area/rapid uptake of water;
 swollen stem for water storage;
 deep roots for accessing deep groundwater;
 small/ no leaves so little transpiration;

Structure of leaves of Xerophytes to keep water in

 Reduce number of stomata; reduce surface area
 Thick waxy cuticle; increases diffusion distance
 Leaves reduced to spines; reduces surface area
 Epidermal hairs; reduced diffusion gradient
 Sunken stomata; reduced concentration difference
 Curled leaves; reduced concentration gradient

Environmental factors the influence water loss in a leaf

 temperature ;
 increase gives more (kinetic) energy / movement to water molecules ;
 on surface of spongy mesophyll cells ;

 greater evaporation (into air spaces) ;

 increases diffusion gradient / rate (of water vapour through stomata) ;
 wind / air movement ;

 increases / speeds removal of water vapour / humid air ;

 from (leaf) surface/ stomata ;
 increases diffusion gradient (of water vapour through stomata) ;
 decreases boundary layer ;

 humidity ;
 increase means more water vapour around leaf / stomata ;
 reduces diffusion gradient (of water vapour through stomata) ;

 light intensity ;
 increase causes stomata to open / widen ;
 so more diffusion of water vapour ;
 from air spaces (in spongy mesophyll) ;

 water stress / extremely high temperature / high winds ;

 stomata close ;
 less diffusion ;
How to find surface area of a leaf
 draw round leaf on graph / squared paper / grid ;
 count squares

Explanation of water pathways as it travels from the epidermis of a root to a Xylem

vessel.

 apoplast pathway described ;

 e.g. ‘water moves through the cell walls’
 or water moves through the spaces between cells’

 symplast pathway described ;

 e.g. ‘water moves through the cytoplasm’

 reference to the vacuolar pathway;

 e.g. water moves through the vacuoles’

 reference to the endodermis and {Casparian strip / layer of suberin / waxy layer} ;
 function of the Casparian strip ;
 e.g ‘the Casparian strip is waterproof’ or ‘stops movement of water in the apoplast
pathway

Role of the epidermis

 endodermal cells have Casparian strip ;
 reference to {suberin / eq} ;
 which is waterproof / eq ;
 stops movement of water in apoplast pathway / description ;
 water directed into symplast pathway / description ;
 reference to {control of uptake / active transport of ions (into xylem)} ;

How AT of mineral ions into xylem vessels in the roots results in water entering
vessels and then being moved up the xylem tissue

 Water potential in xylem reduced (by entry of ions);

 Water potential gradient established between xylem and surrounding cells;
 Plasma membranes of surrounding cells are partially permeable;
 Water enters xylem by osmosis;
 Volume of water in xylem increases;
 Cannot move back due to gradient;
 Pressure in xylem increases (and forces water upwards);

Cohesion-tension theory
 Evaporation from leaves / transpiration;
 Water in xylem under tension*/negative pressure/pulled up;
 Water molecules cohere*/stick together/form hydrogen bonds; [Ignore: references to
adhesion]
 So water a single column;
Another mark scheme for cohesion-tension theory
 water evaporates/transpires;
 reduces water potential / creates water potential gradient / increases
osmotic gradient ;
 moves via apoplast pathway;
 water drawn out of xylem;
 creates tension/pulling effect / creates negative pressure (in context);
 cohesive forces or H bonding between water molecules / water moves as a column

Uses of water in a plant

 photosynthesis ;
 {component / eq} of {cytoplasm / sap} ;
 water as a solvent /eq ;
 water as a transport medium /eq ;
 involved in thermoregulation / eq ;
 reference to role in structural support ;
 reference to involvement in hydrolysis ;
 reference to turgor changes ;

Transpiration
 Loss of water vapour from the surface of the plant, mainly from the leaves ;

Root pressure in relation to water moving through the xylem

 Involves active transport;
 Secretion / movement of salts into xylem;
 Reference to role of endodermis ;
 Water moves along water potential gradient ;

Why less water is lost at upper surface of leaves

 more stomata on the lower surface;
 (thicker) waxy cuticle on the upper surface ;
Variation, evolution and classification

Continuous variation
 idea that individuals in a population vary within a range / no distinct categories;
 credit example e.g skin colour / mass / height ;

Discontinuous variation
 reference to two or more distinct categories;
 idea that an individual only falls into one of these categories;
 credit example e.g gender / blood group ;

Species
 Group of similar organisms ;
 able to reproduce to give fertile offspring / eq ;

Taxonomy
 Science of classification - naming & organising organisms into groups based on
similarities and differences

Classification rankings
 Kingdom
 Phylum
 Class
 Order
 Family
 Genus
 Species

Use of binomial name

 idea that confusion is avoided since species have different non-scientific names in
countries
 first = genus; second = species name

Molecular phylogeny
 Study of evolutionary history of organism groups
 Measures relation between species;
 DNA and proteins examined – the more closely related the more similar the
molecules

Domains
 Bacteria, Achaea and Eukarya ;
 Prokaryotae in Archaea and Bacteria ;
 Other kingdoms (organisms with a nucleus) in Eukarya ;
 Archaea and bacteria distantly related, so classified into two domains ;
KINGDOM EXAMPLES FEATURES
 Prokaryotes
Prokaryotae Bacteria  Single-celled
 No nucleus
 Less than 5 μm
Protocista Algae, protozoa  Eukaryotic
 Generally water
inhabitants
 Single-celled or
simple multi-cellular
 Eukaryotic
Fungi Mould, yeast, mushrooms  Chitin cell wall
 Saprotrophic (absorb
substances from
decaying organisms)
 Eukaryotic
Plantae Mosses, ferns, flowering  Multicellular
plants
 Celluse cell walls
 Contain chlorophyll
 Photosynthesis ability
 Autotrophic (produce
own food)
Animalia Insects, fish, reptiles, birds,  Eukaryotic
mammals, molluscs,  Multi-cellular
nematodes
 No cell walls
 Heterotrophic
(consume plants +
animals)

Comparison of species
 Fossil records
 Homologous features
 Evolutionary history
 DNA Base sequence
 Ecological Niches

Variation of genotype to variation of phenotype

 Individuals of a species have different genotypes;
 This means a variation in phenotype / eq;
 Idea that some characteristics are monogenic (controlled by one gene);
 Reference to discontinuous variation / example;
 Idea that most characteristics are polygenic at different loci (controlled by many
genes);
 Reference to continuous variation / example ;
Environmental influences to phenotypes
 Height – polygenic;
 (by) nutrition / example e.g. malnutrition limiting child’s maximum height;
 Monoamine Oxidase A (MAOA) – monogenic;
 Enzyme that {breaks down / eq} monoamines;
 Reference to low levels of MAOA to mental health problems;
 Reference to antidepressants / tobacco reducing MAOA levels;
 Cancer;
 (by) diet;
 Animal hair colour;
 Seasonal / temperature influences / example e.g dark hair in summer but white hair
in winter;
 Genes required to trigger the change ;

Niche
 Role of the species within its habitat;
 Interactions with other living organism / example e.g. prey and predators;
 Interactions with non-living environment / example e.g. gas exchange;
 reference to niche being unique to species;
 reference to competition if two species occupy the same niche;

Behavioural The way an organism acts to increase its chance of survival

and reproduction
Adaptations

Physiological Processes inside an organisms body that increase chance

of survival

Anatomical These are structural features of an organism’s body that

increase its chance of survival

Adaptations
 idea of selection pressure / change in environment;
 reference to competition / predation ;
 mutation (in context) ;
 idea of advantageous allele;
 idea that individuals with advantageous { alleles / characteristics / eq } survive and
breed ;
 idea of (advantageous) { allele / mutation } being passed on (to future generations) ;
 idea of more individuals with this adaptation in the population / increased frequency .
 .
 .

 
 .
 .of advantageous alleles in the population

Biodiversity and endemism

Biodiversity
 Variety of living organisms in an area

Species diversity
 Number of different species (in an area)
 Abundance of each species (in an area)

Genetic diversity
 Variation of alleles within a species

Species richness
 the number of species ;
 in {an area /one location /habitat} ;

Endemic
 idea that the ( species / eq ) is restricted in its distribution / species only found in one
specific { location / area / eq }

Habitat
 The area in which an organism lives

Measurement of species diversity

 Count the number of different species ;
 The higher the number of species, the greater the species richness ;
 Limitation – no indication of the abundance of each species ;
 (so) Count the number of different species and number of individuals ;
 Usage of biodiversity index / eq ;

Practical measurements of species diversity

 choose an area within the habitat to sample ;
 idea that the sample is random ;
 idea of reduction of bias ;
 count the number of individual species ;
 Ways: plants – quadrat; airborne insects – sweepnet; ground insects – pitfall trap;
aquatic animals – net ;
 reference to repeats ;
 Estimate total number of individuals or total number of different species ;
 idea of sampling technique being the same ;
Measurement of genetic diversity
 Genetic diversity is the variety of alleles in the gene pool of a species;
 The gene pool is the complete set of alleles in a species
 The greater the variety of alleles, the greater the genetic diversity
 Measured by phenotype;
 idea that different alleles code for different versions of the same characteristics;
 Larger the number of phenotypes, the greater the diversity;
 Measured by genotype;
 Sequence of Base pairs in DNA analysed;
 Order of bases in alleles is different;
 By sequencing DNA, similarities and differences are observed;
 Larger the number of alleles, the greater the genetic diversity

Reduction of genetic diversity

 Reduced variety/number of different alleles/DNA / reduced gene pool (in new
population);
 Founder effect;
 A few individuals from a population become isolated/form colonies
 (Genetic) bottlenecks;
 (Significant) fall in size of population
 Selective breeding / artificial selection;
 Using organisms with particular alleles/traits/phenotypes/characteristics;

Importance of storing seeds rather than plants

 take {less / smaller} space / eq ;
 can have more individuals / eq ;
 reference to {greater / more} genetic variety ;
 idea of less {maintenance / cost} ;
 likely to survive longer / eq ;
 can freeze seeds / eq ;

Importance of storing seeds from different plants

 idea of {greater / maintain} genetic variety
 e.g. wider gene pool, different alleles ;
  idea of less chance of inbreeding ;
 idea of reducing chance of storing seeds with
 {low viability / disease / eq} ;

Seeds of different sizes

 idea of maintaining or increasing {genetic diversity / size of gene pool / genetic
variation} ;
 idea of more chance of having beneficial alleles /eq ;
 increases chance of future survival {if environment changes / due to higher
adaptability } / eq ;
 less chance of all being susceptible to a disease / eq ;

How zoos maintain the genetic diversity of endangered species

 breeding programme / eq ;
 careful selection of mate / eq ;
 allowing only to mate with a different individual to previous mating / eq ;
 only allowing those with different genes to mate / eq ;
 use of genetic testing / eq ;
 record keeping (studbooks) ;
 reason for outbreeding ;
 reintroduction to the wild / eq ;

Seedbank conservation
 details of assessment of seed viability e.g. only select seeds with a living embryo,
use of X ray (to detect embryo presence) / eq ;
 idea of {cleaning seeds / surface sterilisation / eq} ;
 idea of drying (of the seed) ;
 idea of storing at low temperatures ;
 idea of regularly testing viability (during storage of seed) ;
 idea of what to do if viability decreases, e.g. if less than 75% germinate collect fresh
seed for storage

Problems with re-introduction and captive breeding

 idea that conditions are difficult to re-create in captivity / lizards won’t breed / eq ;
  ethical issues qualified e.g. cruelty / immorality ;
 reintroduced species could bring disease ;
 idea that the reintroduced species may not retain their natural behaviours in the wild ;

Sexual reproduction
Ways in which a human sperm cell is specialised for its function
 shape qualified e.g. hydrodynamic, streamlined ;
 idea of reduced resistance ;
 {acrosome / vesicle} containing {enzyme / acrosin};
 involved in {digestion / break down} of the {zona pellucida / jelly layer} ;
 {haploid / eq} nucleus ;
 allows restoration of {diploid / full complement / 46 / eq} chromosomes at fertilisation ;
 mitochondria qualified e.g. large number, correct location ;
 to supply {ATP / energy} for {movement / eq} ;
 {flagellum / eq} present ;
 for propulsion / swimming / motility / eq ;
 {markers / receptors} in cell surface membrane ;
 to bind to egg cell surface membrane / detect chemicals released by ovum / eq ;

OVUM  large cell, incapable of independent movement

 wafted along oviducts by cilia and muscular contractions of the tubes
 cytoplasm contains protein and lipid food reserves
 surrounded by a jelly-like coat – the zona pellucida – which hardens
after one sperm penetrates ovum preventing any others entering

SPERM  smaller than the ovum and motile (it can move)
 long tail for swimming, powered by energy released by mitochondria
 head contains acrosome (package of digestive enzymes) to break
down the zona pellucida

Importance of meiosis
 Halves chromosome number / diploid to haploid / 2n to n ;
 Diploid restored at fertilisation ;
 Reference to independent assortment ;
 Reference to crossing over
 Four {sperm / gametes} produced ;

Independent assortment
 Copy of each chromosome randomly assigned to each cell ;
 Leads to large number of different combinations ;

Role of digestive enzymes for the style

 idea of {breakdown / digestion / eq} of style ;
  (breaks down) protein / pectin / middle lamella ;
 reference to hydrolysis / eq ;
 easier for pollen tube to grow / reduced resistance / eq ;
 supplies {nutrients / named nutrient / energy}
 for (pollen tube) growth / eq ;

Crossing over
 (before 1st meiotic division) homologous pairs of chromosomes pair up
 chromatids {twist / tie / eq} around each other ;
 {bits / parts / eq} of the chromatid break off and join the other chromatid ;
 reference to different combination of alleles ;
 reference to different combination of alleles in the 4 daughter cells ;

Meiosis
 DNA replicated to two identical chromosomes (chromatids) ;
 DNA condenses to form (double-armed) chromosomes (from two sister chromatids) ;
 reference to homologous pairing of chromosomes ;
 reference to first division e.g. homologous pairs separated – chromosome number
halves ;
 reference to second division e.g. pairs of sister chromatids separated ;
 four new cells (gametes) which are genetically different produced ;

Fertilisation in humans
 Fertilisation in / sperm swims to {oviduct / fallopian tube / eq} ;
 reference to acrosome reaction with the zona pellucida;
 idea that digestive enzymes are released from the acrosome;
 idea that the sperm are able to move through it to the cell membrane;
 reference to fusion of cell membrane with sperm head;
 egg cell releases cortical granules into space between zona pellucida and cell
membrane ;
 idea that the zona pellucida thickens so only one sperm fertilises the egg cell;
 reference to discarding of tail / only the nucleus entering ;
 idea that nuclei of both gametes fuse ;
 zygote forms with full number of chromosomes ;

Fertilisation in flowering plants

 Firstly, a grain of pollen lands on the stigma;
 Grain absorbs water and splits open;
 Pollen tube grows out of the pollen grain down the style;
 Reference to 3 nuclei present;
 One tube nucleus on tube’s tip and two male gamete nuclei behind it ;
 idea that enzymes from tube digest surrounding cells to make a way through for
pollen tube;
 tube grows through micropyle once it reaches the ovary and into embryo sac within
the ovule;
 tube nucleus {disintegrates / breaks down / eq}and tip of pollen tube bursts to release
2 male nuclei ;
 one nucleus fuses with egg nucleus to form a zygote;
  division by mitosis to form embryo;
 other male nucleus fuses with two other polar nuclei at the centre of the embryonic
sac to form triploid nucleus;
 large nucleus forms to become a food store called endosperm ;
 reference to double fertilisation
Stem Cells
Totipotent stem cell
 totipotent (stem cells) can give rise to {all / any / 216} cell types / eq ; .
 (stem cells) are {undifferentiated / unspecialised} / eq ;
 can keep dividing / eq ;
 only present in early life of embryo ;
 no genes switched off ;

Pluripotent stem cell

 Pluripotent (stem cells) can give rise to {all / any} cell types / eq ;
 EXCEPT extraembyronic cells ;
Note – first point must be linked with second to earn marks

Differentiation
 The process by which a cell becomes specialised ;

Location of stem cells

 Embryo – differentiate into all cells needed to form a fetus ;
 Adult tissues e.g. bone marrow ;

Differential gene expression - enabling cells to have different structures and

functions.

 different genes active in different cells / different genes

 active at different times / some genes {active / inactive} / eq ;
 active genes make mRNA / eq ;
 active genes make proteins / polypeptides /eq ;
 (proteins) control cell {processes / eq} ;
 idea of permanent change (to cell) / eq

Benefits
 Replace damaged tissues (in diseases) ;
 credit example e.g. nerve tissue for spinal cord or heart tissue for heart-related
diseases ;
 idea of saving many lives e.g. growth of organs for those waiting for transplants ;
 idea of quality of live being improved e.g. replacing damaged cells in eyes in blind
people ;

Adult stem cells (multipotent)

  body tissues of an adult e.g bone marrow ;
 simple operation but discomfort to patient ;
 needle into centre of a bone ;
 small quanity of bone marrow removed ;
 idea that adult stem cells {are not flexible / are limited in use / eq} ;
Embryonic stem cells
 Obtained from early embryos
 Embyros created in IVF lab / explained
 Stem cells removed after 4-5 days
 Embryos destroyed ;
 idea that embryonic stem cells can develop into {all / any / 216 / eq} cell types ;
 more likely to cause immune rejection than {adult / somatic} stem cells ;
 totipotent until blastocyst phase (where the cells are pluripotent) ;

Ethics
 Destructed embryos were viable ;
 idea of ‘right to life’ ;
 stem cells from unfertilised embryos have fewer objections ;
 arguments that only adult stem cells should be used because there are no embryos
destroyed ;
 however adult stem cells are limited in use ;

Regulatory authorities
 Proposals of research are examined to see if they’re carried out for good reason ;
 Licensing/monitoring centres ensures that fully-trained staff carry research out ;
 Guidelines / codes of practice to ensure scientists are working in a similar manner ;
 Monitoring developments in research to ensure any changes are regulated and
guidelines are up-to-date;
  Providing advice/information to the government and professionals to promote the
science involved in the research, and aid society in understanding the benefits

Experiment-based questions
Observing mitosis

 Cut ~5mm tip from a growing root;

 Place root on a watch glass, then add hydrochloric acid;
 Add drops of stain to darken chromosomes and make them more visible under
microscope (credit example; e.g. toluidine blue);
 Warm watch glass using a Bunsen burner;
 Place root tip on a microscope slide;
 Use a mounted needled to break the tip open and spread it evenly;
 Add more drops of stain and place a cover slip over it;
 Squash the cover slip;
 Warm slide to intensify stain;
 Repeat experiment to increase reliability;

Totipotency & Tissue Culture

 Single cell taken from a growing area (credit example – e.g. root)
 idea that cell is placed in a sterile growth medium;
 idea that the growth medium contains nutrients and growth hormones;
 plant cell will {grow / divide / eq} into unspecialised cells ;
 idea that unspecialised cells will differentiate into specialised cells if the conditions
are optimum
 Cells will grow and differentiate into an entire plant

Plant tensile strength

 Attach each of the four fibres to a clamp stand at one end;

 Hang weights from other end;
 Add one weight at a time to the fibres until they break
 Record the mass taken to break each fibre;
 More mass = greater tensile strength;
 Repeat experiment to increase reliability;
  Keep variables constant – such as length of fibres, temperature, and humidity;
 Protective gear must be worn;

Investigating plant mineral deficiencies (January 2013)

 Idea of genetically similar plants e.g. clones, cuttings, explants or seedlings from
same parent plant ;
 plants all of same { age /size } (at start) ;
 reference to at least five nutrient broths with different (nitrate) concentrations ;
 sensible range of different nitrate concentrations either side of and including 200
(ppm) ;
 correct reference to any two abiotic variables that need to be kept constant (volume
of mineral solution / size of container / amount of light) ;
 idea of sensible measure of growth e.g. mass / number of leaves / length of roots
/height of plant ;
 time allowed for growth { weeks / months };
 appropriate reference to repeats, e.g. replication at each concentration or repeating
the whole experiment

Anti-microbial properties of plants

 Take plant extract by drying and grinding;
 Soak in ethanol;
 Filter liquid bit;
 Evenly spread bacteria on {agar / nutrient plate / eq};
 Dip disc of absorbent paper in plant extract;
 Soak a disc in ethanol to act as a control;
 Place discs on plate, widely apart;
 Incubate plate to allow bacterial growth;
 idea that an {inhibition zone / clear patch / eq} (on the lawn of the bacteria) will be
present if plant has antimicrobial properties;
 no inhibition zone should be present on control disc;