Biology Unit 2 Edexcel AS

Just trying to organise my revision, I don't have everything but I hope what I do have helps! :)

  • Created by: jadene
  • Created on: 16-04-12 16:35

The Cell Cycle - Mitosis

  • Mitosis occurs in somatic cells and is exponential
  • Mitosis occurs for growth and repair and to replace dying cells
  • In the case of growth, cells divide rather than get bigger to maintain a large surface area to volume ratio (to function adequately)
  • Mitosis is asexual so yields identical cells (if cells produced via mitosis vary it's because of a cell mutation)

The cell cycle consists of 3 stages:

1) Interphase 
2) Mitosis
3) Cytokinesis

1 of 56


1) Interphase - preparation for mitosis 

G1:- the cell is specialised and certain genes are switched on or off
      - protein synthesis occurs normally (transcription and translation)
      - the volume of the cells cytoplasm increases and organelles double

S (synthesis) :- replication of DNA stage
       - DNA doubles and chromosomes go from having 1 chromatid to 2 (held                    together at the centromere)

G2 :- transcription and translation stop
       - the mitochondria are the only organelle to continue being made
       - this is to create and provide energy for the process 

2 of 56


There are 4 stages in mitosis

1) Prophase
- chromosomes condense from being chromatin to short and fat (visible)
- the nuclear envelope dissolves into a collection of small vesicles, freeing the chromosomes to use     the whole cell 
- centrosome organelles move to opposite poles and start producing microtubles

2) Metaphase
- a short resting period where chromosomes line up along the "equator" (metaphase plate) of the cell
- spindle-like fibres/microtubles attach to the centromeres

3) Anaphase
- the spindles contract and split the centromere
- each chromatid goes from the centromere to opposite poles

4) Telophase
- once the daughter chromatids reach the centrosomes and poles they become   chromatin again
- the spindle disintegrates and the nuclear envelope reforms around 2 sets   of chromosomes
- the cytoplasm divides into 2 separate 'cells' from the equator
- everything is equal in shape and size 

3 of 56


3) Cytokinesis - cell division

  • a 'waist' forms at the equator of the cell and is constricted by a ring of contractile proteins/enzymes called actin
  • the cell splits
  • the membrane spreads out around both cells to replace lost or missing membrane
4 of 56


Meiosis is the production of gametes (sperm and egg cells)

  • it's a form of nuclear division that produces haploid gametes
  • gametes are haploid as they have 23 chromosomes (half the number in a normal human body cell)
  • during fertilisation gametes must be haploid as when they fuse it restores the diploid number which will sustain a consistent number of chromosomes for future generations and thus prevent genetic abnormalities

There are 2 parts to meiosis: Meiosis I and Meiosis II

5 of 56

Meiosis I

1) Interphase occurs (the same as in mitosis)

2) Prophase I:
- the nuclear membrane breaks down and centrioles move to opposite       poles and start producing microtubles
- chromosomes condense from chromatin to short and fat (visible)
- synapsis occurs:  

  • homologous pairs come very close to one another and form a bivalent/tetrad
  • genes in a tetrad cross over with the aim to get variation
  • non-sister chromatids shuffle alleles at the chiasmata (site of crossing over) to produce different combinations of genes (creating variation)
  • the new chromosomes made are called recombinant chromosomes

3) Metaphase I: (shortest phase)
- tetrads align on the metaphase plate in their homologous pairs
- this is random assortment as they can align themselves in different ways and   the orientation of the homologous pairs to poles is random
- spindle fibres attach to the centromere of 1 chromosome each 

  • the formula for calculating the number of ways the chromosomes can align is: 2^n (n = haploid number)
6 of 56

Meiosis I

4) Anaphase I:
- spindle like fibres contract so each homologous pair is separated
- each pair moves to opposite poles (sister chromatids remain attached to each     other)

5) Telophase I:
- chromosomes may become chromatin again and a nuclear envelope may reform     however as there's still another stage to occur this normally doesn't happen 
- Cytokinesis however does still occur in the usual way and as there's 92                 chromosomes in total, there are 46 left in each split cell 

7 of 56

Meiosis II

- a short resting period between meiosis I and II
- in meiosis II there's no interphase as you aim to create haploid cells
- meiosis II is exactly the same as mitosis

  • Prophase II:    microtubles form and centrioles move to opposite poles
  • Metaphase II: chromatids align on the metaphase plate and microtubles                                attach to their centromeres 
  • Anaphase II: microtubles contract and split the centromere, the chromatids                            move to opposite poles
  • Telophase II: the nuclear membrane reforms around each set of                                            chromosomes and then become chromatin again
  • Cytokinesis: contractile actin splits the cell identically and membrane shifts                           to replace lost membrane

Haploid gametes are produced

8 of 56

Gametes: Sperm

Sperm are made up of 3 parts:
1) Flagellum/tail - to aid locomotion
2) Mild region containing many mitochondria - to release ATP energy for           movement
3) Acrosome head - containing enzymes which can break down the egg's             membrane through hydration, allowing sperm to enter and bring its DNA 
4) it also contains a haploid nucleus

  • sperm are alkali
  • no two are the same to create variation
  • as many do not survive the long journey to the egg or may be defective, over 1000 sperm are made per *********** for a high fertility chance
  • when sperm enters the vagina it's assisted up the fallopian duct by contractions of the uterus walls (if sperm do not swim up the current being made from these contractions they may attatch to the epithelium and not make it through the cervical mucus)
  • they are streamlined and have a small cytoplasm to aid locomotion
  • sperm are attracted to the ovum as it releases chemicals (pheremones
9 of 56

Gametes: Egg

The egg is made up of (from the inside out):
1) Haploid nucleus
2) Large cytoplasm - full of energy rich materials and cortical granules             
3) Cell surface membrane
4) Zona pellucida - jelly like layer
5) Follicle membrane - made up of follicle cells 

  • eggs are acidic
  • are incapable of independent movement so it wafted along the oviducts from the ovary to the uterus by beating cillia
  • it has a large cytoplasm to store nutrients for chemical reactions which can occur to produce specific substances that the embryo needs ie respiration which will produce energy 
  • the cytoplasm contains proteins, lipids (food reserves for a developing embryo) and cortical granules
  • the cortical granules  contain enzymes that swell the zonapellucida once "meiosis is complete" to prevent polyspermy & ensure a specific diploid number of chromosomes only 
10 of 56


  • the sperm follow chemicals released by the egg (pheremones) to the oviduct where the ovum is
  • when closer, the chemicals become more intense and an acrosome reaction occurs
  • this is where the sperms acrosome head swells and releases enzymes which digest the follicle membrane via hydration
  • it then digests through the zona pellucida and the outer surface membrane
  • when the sperm reaches the cytoplasm "meiosis is complete"
  • cell surface membranes of the sperm and the egg fuse enabling the haploid nucleus of the sperm to enter the cytoplasm of the egg cell
  • cortical granules move towards and fuse with the eggs cell surface membrane
  • they release their contents via exocytosis which thickens the jelly like layer of the zona pellucida to prevent polyspermy
  • the nuclei of the sperm and the egg fuse and fertilisation will occur 
11 of 56

Fertilisation in plants

Female reproductive system - Carpel:
Stigma - recieves the pollen during fertilisation
Style - a tube on top of the ovary
Ovaries - containing ovules and 2 polar nuclei
Male reproductive system - Stamen:
Anther - which releases the pollen/sperm
Filament - holds the anther 

  • when a pollen grain is released it is partially dehydrated
  • when it gets to the stigma, chemical reactions occur to make it hydrated
  • this aids its movement down the style to the ovum/seed as it moves from an area of high water concentration to an area of low water concentration
  • the tube nucleus is stimulated to become a pollen tube
  • the tip of the growing pollen tube digests the style by hydrolysing the pectins in the lamella
  • this makes it easier for the pollen tube to grow as there's reduced resistance
  • the tube enters the passage of the ovary/embryo sac where one of the two male gametes will fuse with an ovule to create a diploid zygote (which will become an embryo)
  • the left over nucleus/male gamete will fuse with the two polar nuclei in the ovary to produce a triploid cell (which will become endosperm/seed storage tissue used to feed the zygote)
12 of 56

Prokaryotic cells (Bacteria)

  • all bacteria are prokaryotic (have no nucleus) so their genetic material can float about freely in the cell eg cyanobacteria
  • some bacteria can carry out photosynthesis
  • prokaryotic cells are the simplest type of living cell
  • they lack membrane bound organelles or internal membranes

If a bacteria name ends in:
Cocci - it's sphere shaped
Bacilli - it's rod shaped
Spirilla - it's spiral shaped 

13 of 56

Prokaryotic cells (structure and organelles)

  • Plasmids - extra chromosomal DNA. Small rings of DNA which serve as additional chromosomes and can produce proteins which help the bacteria to survive in harsh environments
  • Circular DNA - as there's no membrane bound nucleus the ring of DNA is concentrated in certain areas of the cell, so is called "nucleoid"
  • Cell surface membrane - the selective permeability of the membrane helps maintain appropriate compositions of cytoplasm
  • Pilli - thin, protein tubes which help bacteria to adhere to surfaces 
  • Flagellum - hollow, cylindrical thread-like structure is shaped like a cork-screw and aids locomotion by rotating on its axis to propel the cell
  • Slime layer- contains water and prevents desiccation, the slimy capsule also gives extra protection for the cell against ingestion by phagocytes (slime layer prevents bacteria being broken down so cannot be ingested)
  • Ribosomes - site of protein synthesis (translation)
  • Cytoplasm - site of all metabolic activities, contains food storage particles, enzymes and granules/ribosomes
  • Cell wall - gives the cell support, shape and prevents it from bursting. In plant cells the cell wall is made of cellulose, in bacteria it's murien.
  • Respiratory membrane/mesosomes - an infolding of the plasma membrane
  • Photosynthetic membrane - infolding of cell surface membrane where photosynthetic pigments (bacteriochlorophyll) are present, similar to chlorophyll.
14 of 56

Eukaryotic cells (overview)

  • these cells contain a nucleus eg animal cells, plant cells, fungi
  • all organelles in a eukaryotic cell have membranes so compartmentalise functions within the cell 
  • eukaryotic cells are always larger than prokaryotic cells
  • prokaryotes don't have intracellular organelles
15 of 56

Eukaryotic cells (structure and organelles)

  • Ribosomes - small, dense structures found either in groups (polyribosomes) or alone in the cytoplasm, involved in protein synthesis
  • Centrioles - 2 short bundles of hollow microtuble cylinders, positioned at right angles to each other. They may be involved in microtuble assembly or in the organisation of the cillia or flagella
  • Plasma membrane - allows selective receptivity of the cell
  • Mitochondria - site of aerobic respiration/produces ATP energy, is sausage shaped and has a double membrane (like the nucleus):
    1) outer membrane - semipermeable and allows what goes in/out of the cell
    2) has folds called 'cristae' - covered with stalked particles where ATP is made and respiration occurs
  • Smooth endoplasmic reticulum - complex system of flattened membrane sacs (cisternae) and tubular cavities bonded by membranes, is the site of lipids (stores, transports and makes them)
  • Rough endoplasmic reticulum - like SER but studded with ribosomes so is involved in translation, helps transport proteins across the cell 
  • Temporary vacuoles - formed by intucking of plasma membrane
16 of 56

The Nucleus

The nucleus:
is made up of 3 parts
1) Nuclear envelope - contains pores to allow passage of large molecules in/out     ie mRNA
2) Nucleoplasm - a gel matrix
3) Nucleolus - shaded regions of chromosomes 

  • The nucleus is the "control center" of a cell and is involved in the production of mRNA and in protein synthesis
  • It contains DNA so retains the genetic material of the cell in this form
  • It starts the process of cell division
17 of 56

The Golgi Apparatus

The Golgi Apparatus
is involved in the modification of lipids and proteins and the storage and packaging of materials that will be exported from the cell 

  • is made up of vesicles budding off of the RER, containing cisternae
  • these small pieces of RER join to make a golgi body
  • its job is to transport proteins from the RER to the cell membrane for export
  • some proteins however are modified in the golgi ie by combining carbohydrates and proteins (to make a glycoprotein) and packaging them in a cell/vesicle

The enclosed cisternae are stacked and each have 2 distinct faces:
1) 'cis' face - located at the top of the golgi for entry
2) 'trans' face - located at the bottom of the golgi for exit

18 of 56



  • Lysosomes are released by the golgi
  •  they contain enzymes (proteins modified by the golgi) 
  • they are small, hydrolytic vacuoles formed when pieces of the golgi body are 'pinched' off 
  • they contain hydrolytic enzymes which digest material in the cell
  • they can release these to destroy worn out organelles
  • they can digest material taken into the cell ie via phagocytosis in white blood cells
  • they can release enzymes via exocytosis to outside of the cell to digest material around it
  • they can be released in autolysis - when a cell dies a lysosome may release enzymes to digest its remains
19 of 56

from the RER to the Golgi

Proteins (made by the RER) which are destined for the golgi are packaged into vesicles at specialised sites referred to as ER exit sites on the ends of the RER:

  • They move through the cisternae of the RER towards the ER exit sites
  • These sites are studded with receptors which bind to the proteins leaving the RER
  • These proteins contain specific amino acid sequences that bond to the receptors
  • The protein is packaged into a vesicle
  • The vesicles from the RER fuse to form cisternae (containing proteins)
  • The proteins binding to the receptors induce the budding vesicles to be transported from the RER to the cis-golgi network
  • If proteins don't need to be modified they'll just leave the RER

Once modified the protein is put back into a vesicle and will exit the cell via exocytosis

20 of 56

Multiple genes and Polygenic Inheritence

1) 2 independant genes controlling 2 characteristics - dihybrid cross
2) 2 independent genes controlling 1 characteristic - polygenetic inheritence
3) 2 interacting genes controlling 1 characteristic - epistasis

Polygenetic inheritence:

  • Characteristics that show continuous variation are controlled by the combined effects of a number of genes (alleles at many loci) called polygenes 
  • Any characteristic which results from the interaction of many genes is called a polygenic characteristic

Dihybrid crosses:

  • Mating that involves parents who differ in 2 genes which effects 2 different characteristics eg flower colour and stem length
21 of 56

Genes and the environment - height

The average human height in industrialised countries is now 8cm taller than it was in 1850. This is because:

1) Some evidence shows taller men have been having more children which would result in the gradual change in the genetic make up of the population.

2) Greater movements of people have resulted in less inbreeding, resulting in taller offspring

3) Better nutrition ie increases in protein mean that there's a greater growth in children world wide

4) Improved health and more medicine means that energy used on fighting disease and illnesses can be saved and used on growth

5) Less child labour

The environment can have an effect on the expression of some alleles involved in polygenetic inheritance, meaning a given genotype may produce a range of phenotypes ie in crocodiles, the temperature eggs are incubated at determine the offspring's gender

22 of 56

Genes and the environment - melanin

Melanin is a pigment found in skin, hair roots etc.

  • it's made in melanocyte cells
  • each of these cells have receptors on their surface which has a complimentary binding site specifically for the hormone 'melanocyte stimulating hormone' (MSH)
  • once these bond with MSH, the enzyme tyrosinase is stimulated to convert the amino acid tyrosine into melanin
  • the melanocyte cell then places melanin into a vesicle, creating a melanosome which is ejected out the cell via exocytosis and into the skincells via endocytosis
  • melanosomes collect around the nucleus (in its membrane) to protect DNA from harmful uv rays

when there's more UV, more hormones and receptors are produced for skin cell protection - hence tanning.
however, your hair lightens in the sun as high levels of uv exposure can break down melanosomes and damage the chromosomes.

23 of 56

Genes and the environment - melanin (examples)

1) the Arctic Fox:

  • has a thick white coat in the winter but a thin, brown coat in summer
  • the environment affects these genes as the length of day effects the number of receptors made (this is detected in the animals brain via signals from the eyes)
  • in the Summer, the day length is greater so fewer MSH receptors are produced, therefore they produce a winter coat beneath their summer coat
  • in the Winter, the length of day is shorter so more MSH receptors are made beneath the winter coat, creating more melanin and a darker summer coat under the winter one

2) Cats:
gene 'C' controls the colour of cats coat
a normal cat is black and has the dominant gene C
siemese cats however have recessive genes (mutant) and are black and white

  • the mutant allele codes for an enzyme that synthesiese black pigment but only when below body temperature
  • the siamese cat's coat therefore is only black in colder parts of the body (ie tail, ears etc)
  • this is because the enzyme (tyrosinase) begins to denature at above 37*
  • the tyrosinase is heat sensitive 
24 of 56

Mineral Deficiencies

In plants:

  • Nitrogen:
    -nitrate ions are used to make amino acids and proteins
    -these proteins include plant enzymes which the cells require to function
    -nitrates are also needed for plants to make DNA and hormones
    -if lacking older leaves go yellow and die and the plant growth is stunted 
  • Calcium:
    -calcium ions in the middle lamella of plant cells combine with pectins to form  calcium pectate
    -this holds plant cells together
    -calcium ions also play a role in the permeability of membranes
    -if lacking the young leaves become yellow and crinkled
  • Magnesium:
    -magnesium ions are needed to produce chlorophyll as well as the activation of some plant enzymes and the synthesis of nucleic acid
    -if lacking yellow areas develop on older leaves and growth slows
  • Phosphate: 
    -phosphate ions are needed for ADP and ATP which are involved in energy transfers in cells
    -also needed in structural molecules that give support to plant cells & nucleic acid
    -if lacking leaves are dark green with purple veins and growth's stunted 
25 of 56


Stem-cells are cells which have the ability to continuously divide and differentiate/develop into various types of cell or tissue. There are 3 types of stem cell:

1) Totipotent - stem cells that can develop into an organism. These are only present in the 1-8 cell stage (early embryo) and can be extracted from the umbilical cord, 'cord blood' or from embryos left over from IVF.

2) Pluripotent - cells which can form 216 different cell types but can not develop into an organism. These can be found in blastocysts and occur during the 16-50 cell stage

3) Multipotent - cells which are differentiated but can still form a number of other tissues. These can be found in fetal tissue, cord blood and adult stem cells.

26 of 56


  • made up of 2 parts: the inner cell mass which forms the tissues of the developing embryo; the outer cell layer which forms the placenta
  • only 216 genes are left switched on so these pluripotent cells can only code for 216 different types of protein
  • blastocysts develop into tissues and then adult body cells
  • adult cells are fully differentiated so can only make 1 type of protein
27 of 56

Adult stem cells

  • most adult cells lose the capacity to develop into a wide range of cells as they're differentiated
  • some adult cells developed from blastocysts however retain the capacity to give rise to a small variety of different cell types as a few genes are left switched on. These are not fully differentiated.
  • These are multi-potent stem cells.
  • These are present in small quantities in a non-dividing state
  • These stem cells are naturally activated in and by the event of damage or disease of the organ in which they occur.

Sources of these stem cells are:
-The brain
-Bone marrow

28 of 56

Natural way of creating/obtaining embryonic stemce

  • At the cleavage stage, the zygote passes down the oviduct towards the uterus
  • cell division commences as it is swept along the oviduct
  • at early cleavage (4-8 cell stage) any of the cells can develop into any other cell/tissue/code for any protein and the totipotent cells are also able to become a living organism
  • Identical twins/triplets arise when cells at the cleavage stage become separated
  • 5 days after conception a blastocyst is formed (hollow ball of cells)
  • this embeds into the wall of the uterus
29 of 56

Artificial way of creating/obtaining embryonic ste

  • normal menstrual cycle steps are blocked temporarily (the pituitary gland is actively suppressed by a drug injection)
  • synthetic FSH is injected and ovaries are stimulated to develop many egg cells (superovulation)
  • several egg cells are removed from the ovaries using a laproscope positioned with the aid of an ultrasound
  • the laproscope cuts a 3mm hole into the ovary and sucks the eggs out so as not to damage any
  • the male (partner/doner) provides a semen sample which is processed to concentrate the healthiest sperms
  • eggs are mixed with sperms in a shallow dish and checked under a microscope to ensure fertilisation is occurring
  • zygotes are incubated at body temperature for 2-3 days
  • they undergo more microscopic examination to confirm that the embryos have reached the 4-8 cell stage 
  • up to 3 embryos are transferred into the uterus in the expectation that one will implant successfully
  • alternatively embryos can be frozen for future use
  • spare embryos from IVF may be used in stem-cell research
30 of 56


  • Stem cells from IVF/superovulation are cultured in an incubator so they can divide into identical cells
  • the incubator must be sterile so bacteria, viruses and fungi do not effect the stem cells
  • the optimum temperature (body temperature) is used when incubating
  • different hormones/transcription-factors are trialed and errored on the stem cells to see which genes each hormone can switch off until you get the gene you want switched on only
  • once differentiated, the stem cells can develop into adult cells and then eventually a tissue/organ by which time the cells have stopped dividing
  • these can then be implanted into a human 


  • the body can reject the organ tissue as it is not from the patient (however antirejection drugs can be used to help prevent this)
  • infections may be caused
  • mutations in the stem cells etc can cause cancer
31 of 56

Thereputic cloning

  • The nucleus of  the zygote is replaced with the nucleus of a cell from the patients body
  • the zygote is triggered to develop into an embryo using incubation and various hormones in a petridish
  • The adult cells/nucleus must have hormones added to it again firstly though to switch all the genes on/needed genes on 
  • these stem cells develop into different tissues and organs which can be used for medical treatment and is unlikely to be rejected from the patients body as it contains their genetic information
32 of 56

Ethical issues

For the use of embryonic stemcells:

  • potential for alleviating human suffering ie by culturing the patients own cells to provide replacement tissues/organs
  • if its banned in the UK it will still happen in other countries
  • the embryos used are spare from IVF and will alternatively be destroyed. The excess of embryos produced in superovulation can be regulated however
  • research on embryonic stemcells is needed to develop use of adult stem cells
  • embryo is not a human until it's viable

Against the use of embryonic stemcells:

  • potential babies from the point of conception, this is effectively murder
  • embryo doesnt get a say/lack of respect to embryo as a potential human
  • alot of current embryonic stem cell treatment is badly regulated and exploits suffering/encourages IVF clinics to 'create' more 'spare' embryos which puts pressure on women to produce surplus embryos
  • cloning techniques may fall into the wrong hands and lead to things like designer babies
  • it will soon be possible to just use adult stemcells so why not just wait?
33 of 56

Proving the existence of the nucleus 1

Acetabulgaria is single celled, blue green algae about 2-3 cm long.
It is made up of a hat (top), a stem (middle), and a rhizoid (a structure in plants, fungi and some other organisms that functions like roots and contains the nucleus)

Acetabulgaria was used in experiments to prove the existence of a nucleus as it's a large plant with large cells that enable microsurgery to be performed as well as dissecting sections and transferring the nucleus from one section to the other

  Ex 1)

  • scientists dissected the plant by cutting it into a rhizoid, a stem and a tip
  • the tip produced other parts of the plant and so did the rhizoid
  • the stem did not, this proved genetic info was present in the rhizoid and tip

  Ex 2)

  • the tip is cut
  • the stem is also cut from the rhizoid but this happens after a few days
  • the stem now is able to regrow its rhizoid and hat whereas it couldnt before
  • this shows there's genetic information in the rhizoid as the genetic info created a chemical which went up the cytoplasm of the stem
  • scientists however didn't know whether or not the information came from the nucleus or the cytoplasm
34 of 56

Proving the existence of the nucleus 2

  • to find out whether or not the genetic information existed in the nucleus or the cytoplasm, scientists separated the stem from the rhizoid and the hat
  • the nucleus was transferred from the rhizoid and put into the stem of the plant
  • the stem then grew into a complete plant with the hat and the rhizoid
  • the nucleus moved back down from the stem to the rhizoid again, proving that it was the nucleus that contained the genetic info
35 of 56

Plant fibre strength core p

  • extract the plant fibres via retting (soaking it til its all moist etc)
  • bacteria will accumulate as it's warm and will decay outside the stems surface (flesh) so it can be extracted:
  • suspend masses (increasing in 10g intervals) until it breaks - this tests its tensile strength
  • Keep the temperature and width of the stem the same
36 of 56


  • is made up of beta-glucose monomers and is a polysaccharide. 
  • the positions of the –H group and the –OH group on a single carbon atom are reversed in beta than in alpha.
  • In beta-glucose the –OH group is above, rather than below, which means that to form glycosidic links, alternate beta glucoses must be turned upside down
  •  cellulose has straight, unbranched chains running parallel to one another, allowing hydrogen bonds to form cross linkages between adjacent chains. 
  • Cellulose is a major component of plant cell walls and provides rigidity to the plant cell.
  • The cellulose cell wall also prevents the cell from bursting as water enters by osmosis. 
  • It does this by exerting an inward pressure that stops any further influx of water. 
  • As a result, living plant cells are turgid and push against one another, making herbaceous parts of the plant semi-ridged. This is especially important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis.
37 of 56

Testing new drugs

The stages to trial drugs are:

  • a potentially useful substance is identified and an active ingredient is used (this can be synthetically made)
  • drugs are trialed on animals to ensure efficacy and check for danger
  • they are then trialed on a small group of human, healthy volunteers to check for side effects (phase I clinical testing)
  • a small group of patients suffering the disease is then trialed on to see if it works (phase II clinical testing)
  • a much larger sample is tested on and split into two groups, one group will recieve the drug whereas the other will recieve a placebo (sugar tablet, empty capsule, water filled capsule etc)
  • Phase III clinical testing may be done as a blind trial (where patients do not know whether or not they have the real drug or the placebo), a double blind trial (where neither the patients nor the doctors know which drug is which) or an open trial where the patients know whether or not they have the drug.
  • an independant review is then made of the drug to determine and assess its risks, efficacy/effectiveness and whether or not it should be released to the public
38 of 56

Withering's methods

  • William Withering made "digitalis soup" (out of foxglove?) which he noticed alleviated people with dropsy of their symptoms (made them better)
  • he gave the soup to a small sample of patients and realised it worked
  • he then used a larger sample and recorded signs of recovery so he could get the dosage right
  • he then gave it to a larger sample of patients as treatment
  • he reviewed his own work

Differences in methods: he didn't test on animals/use a placebo/use blind, double blind trials/ he didn't test on healthy people for side effects

Similarities: he tested it on a small group of patients before using a larger sample size

39 of 56

Millennium seed bank - processes involved in stora

-as plants are threatened world wide (ie by habitat destruction, climate change, over harvesting...) ex-situ conservation like seed banks & botanic gardens help plants to survive. 
-Aim: conserve seed samples from threatened plant species (10 000 species already banked)

Most plants produce large numbers of seeds so collecting small samples will not damage a wild population
-seeds are small and easy to store
-can survive in a desiccated state for many years 

-seeds survive longer if dried and cooled. For every 1%/5degreeC reduction in seed moisture/temperature its life span doubles

  • Seeds are collected
  • they are transferred to the seed bank by a courier
  • seeds are unpacked & checked and identified
  • seeds are cleaned
  • seeds are dried and packaged
  • they are stored at -20degrees
  • about 1 month later the sample is removed and germinated on agar plates to ensure they can survive in storage/storage conditions
  • periodic germenation trials (every 10 years) occur to check on seed viability
  • if germenation falls below 75% the seeds will be grown to collect a new seed sample
  • these seeds will then be put into storage
  • testing for seed viability is carried out as school projects
40 of 56


  • Water is lost through the stomata of the leaves via evaporation 
  • Water is gained through  the root hair cells through osmosis
  • Because water has been lost, it must move up the stem through the xylem tissues (aided by increased water potential) across the leaf and down the concentration gradient towards the stoma
  • The water moving vertically up the xylem creates a continuous column of water due to cohesive forces between water molecules (as water molecules are polar)
  • Water also sticks to the surface of the xylem as it is adhesive (and both the water and xylem walls are polar so attract each other)
  • Water is drawn up the stem due to the low hydrostatic pressure at the top of the plant created by water constantly being lost 
  • This means water is continuously drawn from the soil, into the roots and up the stem to replace the lost water (from the leaves)
  • This will continue as the osmotic gradient will keep being higher in the stem than in the roots and soil so water will keep moving from the soil, to the roots and then up the stem
41 of 56

Sclerenchyma fibres

Lignin not only waterproofs the cell walls but it also makes them much stiffer/gives the plant a greater tensile strength (the taller a plant needs to grow the greater proportion of lignin present in the stem)

  • Sclerenchyma fibres are elongated fibrous cells with lignin deposited on its cell walls (once lignified the sclerenchyma fibres die, like the xylem tissues/vessels) 
  • found in any part of the cell that needs strengthening 
  • these fibres help support the plant
42 of 56

How is the xylem adapted for its job

To build tall plants (ie trees)  some cells in the stem must be stiffened to provide mechanical support, at the same time some must allow water and minerals to pass from roots to the leaves. 2 types of specialised cell fulfil these functions:

Xylem vessels-

The xylem is good at transporting water as:

  • it has hollow tubes to allow water to move up it vertically
  • it is waterproof (because of lignin) so that the water stays inside the vessels and there is less water loss 
  • it is porous to allow sideways water movement

The xylem helps support the stem as:

  • it contains lots of lignin/extra cellulose for strength
  • it has a spiralled shape for strength and flexibility
  • its made up of large cells with thick walls

They can be found in the epidermis, vascular tissue and ground tissue. Each vascular bundle contains xylem vessels and phloem sieve tubes. on the outside of the bundles are schlerenchyma fibres. The xylem vessels carry water/inorganic ions up the stem. The phloem transports sugars (from photosynthesis) in the leaves up and down the plant.

43 of 56

Plant cell structure

  • Cell wall - thick, rigid membrane structure that surrounds plant cells. It's made mainly of cellulose (carbohydrates) which supports the cell.
  • Plasmodesmata- Channels in the cell wall that links adjacent cells together. They allow transport of substances and communication between cells.
  • Vacuole- a compartment surrounded by a membrane called the tonoplast. A vacuole contains cell sap which is made up of sugar and water, enzymes, minerals and waste products. Vacuoles keep calls turgid which stops them and the plant wilting. It's also involved in the breakdown and isolation of unwanted chemicals in the cell.
  • Tonoplast - controls what enters and leaves the vacuole - is the vacuole membrane.
  • Amyloplast- small organelle enclosed by a membrane and contains starch granules. Is involved in the storage of starch grains and also converts starch back to glucose for release when the plant requires it.
  • Chloroplast - small flattened structures surrounded by double membranes and has membranes inside called thylakoid membranes. These membranes are stacked up in some parts of the chloroplast to form grana. Grana are linked together by lamella (thin, flat pieces of thylakoid membrane). Chloroplasts are the site where photosynthesis takes place. Some parts of photosynthesis happen in grana and others in stroma (a thick fluid found in chlorophyll)
  • Pits- regions of the cell wall where the wall is very thin. They're arranged in pairs: a pit in one cell is lined up with a pit in an adjacent one. This allows movement of substances between cells.
  • Middle lamella- outermost layer of cell - acts as an adhesive which sticks adjacent plant cells together giving the plant stability
44 of 56


Taxonomy (classification) is divided into a hierarchy consisting of 7 groups:

  • Kingdom (Anamalia, Plantae, Fungi, Protocista, Prokaryotae)
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species


45 of 56

Natural selection

Biodiversity decreases if a species doesn't adapt

1) mutations occur naturally in some of the population's  alleles (due to variation)

2) a change in the environment (stimulus) causes a change in selection pressure acting on a population (eg something to kill a species)

3) the survivors are adaptive

4) the resistant allele species will survive as the allele is more favourable

5) survivors breed and transfer genes so the allele frequency for resistance will increase within the population

***if there's more variation within a species geno & phenotypes they are less likely to become endangered***

46 of 56

Topic 4 definitions

  • Community - different populations of animals & plants sharing the same habitat/all the species in a given area
  • Habitat - where an animal or plant lives and is able to adapt to and survive in
  • Adaptations - features which enable the survival of an organism is one of 3 ways:
  • Population - all the members of a given species in a given area
  • Species - Individuals who share similar DNA & can produce fertile offspring are classified as a species
  • Niche - a plant or animals ecological niche is a way of life that is unique to that species. The niche describes the species role or function within their community.
  • Ecosystem - an ecosystem is made up of all the biotic (living) and the abiotic (non-living) components in a given area
  • Abiotic - anything (components of an ecosystem) that are not living ie wind
  • Biotic - anything living or influencing something living is a biotic factor
  • Competitioninter-specific competition is competition for resources between individuals of different species
  • -intra-specific competition is competition with other organisms within a species
  • Evolution - a change in allele frequency for a particular gene in a population 
  • Species richness - numbers of species in a given area
  • Species evenness - abundance of each species in a given area (the more even they are the more diverse the area is)
  • Genetic Diversity - 
  • Endemic - 
47 of 56


48 of 56

13 Explain the terms biodiversity and endemism and describe 

how biodiversity can be measured within a habitat using 

species richness and within a species using genetic 

diversity, eg variety of alleles in a gene pool.

14 Describe the concept of niche and discuss examples of 

adaptation of organisms to their environment (behavioural, 

physiological and anatomical).

15 Describe how natural selection can lead to adaptation and 


16 Discuss the process and importance of critical evaluation of 

new data by the scientific community, which leads to new 

taxonomic groupings (ie three domains based on molecular 


17 Discuss and evaluate the methods used by zoos and 

seedbanks in the conservation of endangered species 

and their genetic diversity (eg scientific research, captive 

breeding programmes, reintroduction programmes and 


49 of 56

core practicles

50 of 56

Gene expression

  • If genes are switched on all the time and are making proteins all the time they are constitutive
  • If genes are only switched on at some points they are inducible
  • Another word for an inducible gene is structural (it is only switched on when necessary) 
    ---------------------------------------------------------------------------------------------------------------Bacteria (E-Coli) makes Beta-galactosidase (enzyme) which hydrolyses lactose into becoming glucose and galactose (this glucose can be made into energy). E-coli has to code for the protein/enzyme B-Galactosidase so the gene is inducible as it isnt present all the time - only when it has to hydrolyse lactose.
  • In its 'normal state' the gene is coiled and uncoils to code for the protein
  • To uncoil it, theres a space behind genes called an "operator gene" and this activates the production of rna polymerase and uncoils the structural gene.
  • The issue however is that the space is not always there. There is a regulatory gene which codes for a repressor protein (constitutive) which competes for the space. 
  • If the repressor gets there first rna polymerase cannot get there so the gene will not be stimulated to uncoil and make beta-galactosidase 
  • When coiled, we want the repressor present/active.  The operation region is always more attracted to the repressor when it's competing with RNApolymerase (transcription factor/signalling protein)
  • to prevent the repressor winning, lactose has a complimentary binding site on the repressor which changes its shape so it cannot bind/fit into the operator gene
  • This frees it so that RNA polymerase can fit in the space to stimulate uncoiling and code for B-Galac which will hydrolyse the lactose into glucose and galactose and the glucose can be utilised for respiration.
51 of 56

Aseptic techniques

  • All work is done close to a bunsen flame because the convection current created from the heat from the bunsen flame prevents potentially contaminating particles from falling onto the agar plate
  • a petri dish containing agar is generally known as an agar plate
  • Sterilisation of apparatus occurs before use as all apparatus' & (medicine??!) is autocleaned for sterilisation. Autocleaning is a steam sterilisation process which kills any bacteria present by subjecting them to 121 degrees heat.
52 of 56

53 of 56

When gene expression goes wrong (FOP)

"master genes" send out signals to other cells (making them signalling proteins/transcription factors) ie telling them to code for a specific protein

These signals are sent to the damaged area (in the case of FOP this would be the muscles/muscle genes)

In FOP white blood cells have their bone cell switched on (all the time) the muscles get the signal from the white blood cell master genes to switch on their bone cells when repairing damaged muscle. This results in the change from muscles into bones.

Problems with this are paralysis of a person when knocked or bumped as the damaged area will continuously code for bone cells

54 of 56

Cells > Tissues > Organs > Systems

Cell - every cell has a cell membrane outside, a cytoplasm inside and organelles. A cell is singular (1 cell) and carries out 1 specific function. It is the building block of tissues. 
Every cell has an external protein on its surface. These are called adhesion or recognition proteins. This allows cells to stick together with other cells (which creates tissues).
Recognition proteins are particular and are made by switched on genes. These genes determine which cells the recognition proteins can attach to. (always the same type)

Tissues - a group of specialised cells that perform the same function. Tissues are only made up of 1 type of specialised cell.

Organs - 2 or more different types of tissue together that perform a specific function or group of functions (organs are made up of several cell types).

System - organs & tissues involved in a system work together to carry out functions to enable a process to take place eg the respiratory system allows respiration to occur.  

55 of 56


  • cellulose is a polysaccharide (a polymer of B-glucose)
  • condensation reactions between -OH groups link molecules (by a 1,4 glycosidic link)
  • in cellulose all links are 1,4 opposed to starch which can have 1,6 bonds
  • this means cellulose is long and unbranched
  • each chain typically contains 1000-10 000 glycosidic units
  • cellulose remains in straight chains, forming bundles called microfibrils (formed when neighboring cellulose chains bond together through hydrogen bonds)
  • although hydrogen bonds are weak, many of them makes cellulose a strong structure
  • microfibrils in plant walls are wound in helical arrangements around the cells and are stuck together by polysaccharide glue
  • the glue holding microfibrils together is composed of short, branched polysaccharides called hemicelluloses and pectins
  • these bind to the surface of cellulose and each other to hold the microfibrils together
56 of 56




Good set of notes, but need to cover some more topics.

cvhange the 3 slide to the volume of the (cell's) cytoplasm increases.









thank you very much they helped alot !!! <3 <3 :)



Oliver wrote:

Good set of notes, but need to cover some more topics.

cvhange the 3 slide to the volume of the (cell's) cytoplasm increases.

no :s why would i?

Daniel Williams





Helpful, but could add some diagrams to make it easier to undersatnd and more interesting :)



tbf, 55 slides, im not going to criticise because you have done a ot of work and this will be helpful, so thank you!



you do realise that many of the slides do not actually fit onto the screen or if you go to change it into a printable format.. 

as well as many important topics being left blank:P
the rest looks alright though 

Naya Patel


wow, the notes are great!!



OMG!! Thnak you for this!! This really really helped! =D <3



but you're welcome to everyone else :)



Great help, thanks alot!

Jake Da Floatilla Bird


bad boy ina london, rude boi ina england



THANK YOU SO MUCH !!!!!!! :d



Thanks these were great. And I loled at the ****'s on the sperm section :p



They're really good- but seem to give unnecessary details that aren't in the spec? :)



do you have unit 1 notes

Similar Biology resources:

See all Biology resources »