Edexcel AS Biology Unit 2 : Topic 3

The whole of Edexcel Bilogy's unit 2 (topic 3). Everything you need to know!

Key scientific terms are in red

Important points are highlighted in yellow

HideShow resource information
  • Created by: Lu
  • Created on: 27-05-13 13:42

Prokaryotic Cells

  • Prokaryotic cells DO NOT have membrane-bound organelles
    • This includes the nucleus
  • Bacteria and Cyanobacteria make up the Prokaryotae kingdom
  • Extremely small (0.5 to 5μm)
  • Their DNA is NOT associated with any proteins and lies free in the cytoplasm
  • Prokaryotes ALWAYS have:
    • Ribosomes
    • Circular DNA
    • Cell wall (containing peptidoglycan, a combined polypeptide and polysaccharide)
    • Cytoplasm
    • Infolding cell-surface membrane (site of respiration)
  • Prokaryotes SOMETIMES have:
    • Plasmid (small circle of DNA)
    • Capsule (slimy layer on surface for protection and to prevent dehydration)
    • Pili (thin, protein tubes allowing bacteria to adhere to surfaces)
    • Flagellum (hollow cylindrical "tail" which rotates to move the cell)
1 of 19

Eukaryotic Cells

  • Contain discrete membrane-bound organelles
    • including nuclei, mitochondria and chloroplasts
  • Larger than prokaryotic cells ( 20μm or more)
  • Organisms with Eukaryotic cells are called Eukaryotes
  • Structures in a generalised animal cell:
    • Centrioles (Hollow cylinders involved in formation of spindle during nuclear division)
    • Mitrochondrion (Has folded inner membrane, site of the later stages of aerobic respiration)
    • Nucleus (Enclosed by pored double membrane [envelope] contains chromosomes and a nucleolus. Chromosomes contain genes controlling protein synthesis)
    • Nucleolus (Dense body within the nucleus where ribosomes are made)
    • Ribosomes (Made of RNA and protein, site of protein synthesis)
    • Rough endoplasmic reticulum (interconnected membrane-bound flattened sacs with ribosomes on surface. Transports proteins made by ribosomes across cell)
    • Cell surface membrane (Phospholipid bilayer forming partially permeable barrier)
    • Smooth endoplasmic reticulum (Like RER w/o ribosomes. Makes lipids and steroids)
    • Golgi apparatus (Modifies proteins and puts in vesicles for transport)
    • Lysosome (Sacs containing digestive enzymes. Break down unwanted structures)
2 of 19

Dynamic Cells

  • Cells are not static; there is continual movement of molecules within them
  • Once proteins have been synthesised they are processed and move to the part of the cell where they are needed
  • Many proteins (including extracellular enzymes, hormones, and signal proteins) are released from cells
  • The steps of protein synthesis and movement through the cell are:
    • 1) Transcription of DNA to mRNA
    • 2) mRNA leaves nucleus through nuclear pore
    • 3) Protein made on ribosomes enter RER
    • 4) Protein moves through RER assuming 3D shape en route
    • 5) Vesicles pinched off the RER contain the protein
    • 6) Vesicles from RER fuse to form the flattened sacs of the Golgi apparatus
    • 7) Proteins are modified within the Golgi apparatus
    • 8) Secretory vesicles pinched off the Golgi apparatus contain the modified protein
    • 9) Vesicle fuses with cell surface membrane, releasing protein (exocytosis)
3 of 19


  • Egg cell (Ovum):
    • Largest human cell (around 100μm)
    • Incapable of independent movement
    • Simple structure of an ovum:
      • Cytoplasm
      • Haploid nucleus
      • Lysosomes
      • Follicle cells (From ovary)
      • Zona Pellucida (Jelly-like coating)
      • Lipid droplets
      • Cell surface membrane
  • Sperm cell:
    • Much smaller (around 5μm) One of the smallest human cells
    • Motile (Able to move)
    • Made up of 3 parts: 
      • The head (the nucleus and the acrosome)
      • The middle (containing a large mitrochondrion for respiration)
      • The tail (flagellum)
4 of 19

Fertilisation in Mammals

1) Sperm reach the Ovum

2) Chemicals released from cells surrounding ovum, triggering acrosome reaction

3) Acrosome swells, fusing with the sperm cell surface membrane

4) Digestive enzymes in acrosome released

5) Enzymes digest through follicle cells

6) Enzymes digest through zona pellucida surrounding ovum

7) Sperm fuses with ovum membrane

8) Sperm nucleus enters ovum

9) Enzymes released from lysosomes in the ovum thicken the zona pellucida, preventing other sperm from entering

10) Nuclei of sperm and ovum fuse

5 of 19


  • Human body cells contain 46 chromosomes (The diploid number)
    • 22 homologous pairs and one pair of sex chromosomes
  • Human gametes contain 23 chromosomes (The haploid number)
  • Gametes are produced through meiosis, which occurs in the ovaries and testes of animals
  • Steps of meiosis:
    • Chromosomes replicate before division.
      • After replication, each chromosome is made up of two strands of genetic material: the two chromatids
    • Meiosis I (First division)
      • Homologous chromosomes pair up and then separate into two daughter cells
    • Meiosis II (second division)
      • Chromatids separate and gametes are formed
      • There are four daughter cells
      • Each gamete has half the number of chromosomes as original cell
  • Meiosis is important for two reasons:
    • Creating haploid cells for sexual reproduction
    • Creating genetic variation through independent assortment and crossing over
6 of 19

Independent Assortment and Crossing Over

  • Independent assortment
    • During meiosis only one chromosome from each pair ends up in each gamete
    • This process is random
    • The daughter cells of the first meiotic division contain different assortments of chromosomes depending on the alignment of the chromosomes during meiosis I
    • This produces genetic variable gametes
  • Crossing over
    • During the first meiotic division, homologous chromosomes come together as pairs and all four chromatids come into contact
    • At points where they make contact (chiasmata) the chromatids break and rejoin
    • The non-sister chromatids exchange corresponding sections of DNA
    • Several of these often occur along the length of each pair
    • Homologous chromosomes separate leaving chromosomes that contain new combinations of alleles from both parents
7 of 19

Fertilisation in Flowering Plants I

  • In the ovary:
    • There is one megaspore (mother cell) which is diploid
    • This undergoes meiosis leaving four haploid megaspores (three of which disintegrate)
    • This remaining megaspore undergoes mitosis three times leaving eight haploid daughter cells  (these share cytoplasm and are known as the embryo sac)
    • Cell walls form
      • Three of these nuclei create an antipodal cell, facing the micropyle
      • Three move above the micropyle - two of which are synergids and the central one is the egg
      • The two remaining nuclei got to the centre of the embryo sac and are known as the polar nuclei.
      •  It is the two polar nuclei and the egg cell that are involved in the double fertilisation event
      • The other five are present for structural purposes
8 of 19

Fertilisation in Flowering Plants II

  • In the anther:
    • The four haploid cells produced in meiosis undergo mitosis leaving each pollen grain with two haploid nuclei
    • Most of the grain is made up of the tube nucleus, and the other is the generative nucleus
  • The pollen grain lands on the stigma
  • Germination takes place:
    • Tube nucleus controls the development of a long structure (the pollen tube) which goes down the style and enters the ovary
    • Generative nucleus follows and undergoes mitosis into two haploid sperm cells
    • These sperm cells are released through the micropyle into the embryo sac
    • One sperm fertlises the egg cell, creating a diploid zygote which becomes the embryo
    • The other sperm fuses with the polar nuclei forming a triploid endosperm
    • The endosperm is used for food during early development of the embryo
9 of 19

The Cell Cycle

1) INTERPHASE - New organelles synthesised, DNA replication occurs, cell contains enough contents to produce two new cells.

2) PROPHASE* - Chromosomes condense, become shorter and thicker, visible as two strands (chromatids). Two strands identical (apart from mutations) joined by centromere. Microtubules from cytoplasm form spindle. Centrioles move around nuclear envelope, position themselves at opposite sides of cell, form two poles of spindle. Spindle fibres form.

3) METAPHASE* - Centromeres attach to spindle fibres

4) ANAPHASE* - Centromeres split, spindle fibres shorten pulling two halves in opposite directions to the two poles. Spindle breaks down.

5) TELOPHASE* - Chromosomes unravel, nuclear envelope reforms.

6) CYTOPLASMATIC DIVISION - Animals: Cell surface membrane constricts around centre, ring of protein filaments contract until cell divided into two new cells. Possibly uses actin and myosin. Plants: A new cell plate is synthesised between two new cells.

*Stages of nuclear division

10 of 19

Why Mitosis is Important

  • Achieves genetic consistency through:
    • DNA replication
    • Identical arrangement of chromosomes and separation of chromatids
  • Growth and repair
    • Ensures all body cell's genetic information is identical
    • Regenerate lost or damaged parts of body (In some organisms)
    • Replace old/damaged cells with identical new copies
    • Ensures body does not reject cells
  • Asexual reproduction
    • Many organisms reproduce without producing gametes
    • Create exact copies of themselves through mitosis
    • Occurs in:
      • Bacteria
      • Hydra
      • Fungus
      • Plants (vegetative reproduction)
    • Some organisms reproduce both asexually and sexually at different parts of their life cycle
11 of 19

Stem Cells

  • Totipotent - Can give rise to any type of cell
    • In a human zygote after 3 complete cell cycles
    • Eight cell stage
    • All cells identical
    • Causes twins
    • Most plant cells remain totipotent
      • Allows plant tissue culture
  • Pluripotent - Can give rise to most types of cell
    • Embryonic stem cells
    • Five days after conception
    • Blastocyst formed
      • Outer layer forms placenta
      • Inner mass of around fifty cells are pluripotent
  • Multipotent - Can give rise to a few different types of cell
    • Adult stem cells
      • Neural stem cells
      • White blood stem cells (found in bone marrow)
12 of 19

Uses, Risks, and Ethics concerning Stem Cells

  • Potential uses:
    • Produce universal donor cells - providing new cells, tissues or organs for transplantation
      • Tissues may be rejected by immmune system of patient, ways to get around this problem:
        • Tissue typing
        • Use of drugs
        • Theraputic cloning - removing a somatic cell from patient and fusing it with an empty ovum to create a genetically identical zygote
  • Ethical concerns:
    • No ethical concerns about use of adult stem cells
      • These do not provide enough information to understand how stem cells can be used
      • These cannot become many types of cell so are not very useful to medicine
    • Use of an embryo is unethical because:
      • It can be considered a human in its own right
      • It cannot give concent
      • It cannot go on to live after stem cells have been taken
13 of 19

Regulatory Authorities

Regulatory authorities:

  • Parliament - House of commons and house of lords
    • Advised by specialists - Advisory comitees
  • HFEA regulates research on human embryos
  • HFEA only allows reasearch to be done on embryos if it is necissary to promote advances in: 
    • The treatment of infertility
    • The causes of congenital disease
    • The causes of miscarriage
    • Methods of contraception
    • Methods of detecting and treating gene abnormalities in embryos
    • The treatment of serious disease
  • Since 2007, the use of animal-human embryos has been alllowed
14 of 19

Cell Differentiation

  • The nucleus of every cell contains the same genetic information
  • As an embryo develops into a multicellular organism, its cells become specialised
  • Specialisations means a cell takes on a specific form and has a specific function
  • This is because different cells express different genes
  • This causes the production of different proteins
  • Other genes are "switched on" meaning a special type of protein is made
    • Only when RNA polymerase has attached to the section of adjacent DNA to be transcribed (the promoter region) will the transcription proceed
    • The attachment of a regulator protein is usually also required to start transcription
  • Some genes are silenced so proteins are not made
    • Protein repressor molecules attach to promoter region, blocking attachment site
    • Can also attach to regulator proteins preventing them from attaching
    • Gene not transcribed within cell, so proteins not synthesised
  • A group of the same type of cells forms a tissue
  • A group of the same (or similar) tissues form an organ
  • A group of organs working together for the same purpose form an organ system
  • Organ systems form a multicellular organism
15 of 19

Controlling Development: Acetabularia

  • Acetabularia is a green alga consisting of a single cell.
  • It has three parts:
    • The rhizoid (containing the nucleus)
    • The stem
    • The "hat"
  • Experiment 1 - Cell was cut into three sections which were allowed to develop separately
    • The tip developed a new hat
    • The stem did not develop
    • The rhizoid developed a new stem and hat
  • Experiment 2 - Tip and rhizoid removed, and the nucleus was moved into the stem
    • The stem developed a new hat and rhizoid
    • The new rhizoid grew so that the nucleus was now contained by it
  • Experiment 3 - There were two different species of acetabularia, with different looking hats.
  • The hats and tips were discarded and the stems were switched
    • Both cells grew 'intermediate' hats that were a cross between the two original hats
    • The intermediate hats were cut off. Hats grew that matched the species of the nucleus.
  • Therefore the nucleus controls development, but sends the signal chemicals up the stem. If there is still some of the signal in the stem that has been switched, the cell will grow a hat influenced by both the signal in the stem and the nucleus.
16 of 19

Genes and Polygenic inheritance

  • Phenotypes (Characteristics of an organism) are determined by:
    • The genotype (the genetic make up)
    • The environment
  • Discontinuous variation - characteristics controlled by genes at a single locus and are NOT affected by the environment
  • Continuous variation - characteristics affected by the environment
  • Polygenic inheritance - A number of genes are involved in the inheritance of a characteristic
    • Many human characteristics showing polygenic inheritance have alleles with additive effects
  • Multifactorial - Conditions where several genetic and one or more environmental factors are involved
  • The degree of similarity between identical twins is a measure of the influence that genes have on that characteristic
17 of 19

Gene and Environment Interaction I

  • Height - The average height has increaesed in industrialised countries over the last 150 years
    • People may have the genes to reach a certain height, but may not reach that height due to malnutricion, disease etc.
  • Hair & skin colour - These are determined by genes but are changed by exposure to UV light
    • The dark pigment in skin and hair is called melanin
      • This is made in melanocytes which are activated by the melanocyte-stimulating hormone
      • There are receptors for this on the surface of melanocyte cells
      • Melanocytes places melanin into melanosomes which collect around the nucleus of hair and skin cells to protect the DNA from damage from UV light
      • Exposure to UV light increases the amount of MSH which causes the melanosomes to collect thus darkening the skin.
      • Hair doesn't appear darker as too much UV light damages melanin, lightening the hair
    • To make melanin, animals use an enzyme called tyrosinase
      • Some animals (e.g. siamese cats and himalayan rabbits) have mutant alleles for the production of tyrosinase - the enzyme is made but is inactive at normal body temperature
      • This results in the extermities (paws, ears, tips of tails) being much darker than the rest of the body
18 of 19

Gene and Environment Interaction II

  • MAOA
    • Monoamine oxidase A is an enzyme that catalyses the breakdown of a neurotransmitter in the brain involved in the regulation of behavior, including response to stress
    • People that have a mutation in the MAOA gene where the enzyme is not produced show violent and aggressive behavior
    • The gene for MAOA occurs on the X chromosome so boys only have one allele and are more affected by the mutation
    • There is NO clear link between MAOA production and violent behavior, however for people that have been abused or mistreated as children AND have low levels of MAOA, levels of antisocial behavior and violent crimes are much higher
  • Cancer - genes can increase susceptibility to cancer, and environment can trigger it
    • Cancer occurs when the rate of cell multiplication is higher than the rate of cell death, resulting in a tumour
    • Cancer is caused by damage to DNA caused by exposure to UV light, asbestos etc.
    • Cancer cells do not respond to control mechanisms:
      • Oncogenes - these code for proteins that stimulate the transitions between stages of the cell cycle. Mutations cause the cell cycle to be continually active.
      • Tumour suppressor genes - these code for proteins that stop the cell cycle
    • Environmental factors that affect cancer are: diet, smoking, virus infection, UV light etc.
19 of 19


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all resources »