Mammalian Fertilisation

Gametes

Process

• Sperm swims through the uterus.
• If sperm enters at about the time of ovulation, sperm may meet the ovum in the oviduct.
• The sperm are attracted to the ovum by chemicals released from it.
• Once the sperm reaches the ovum, the chemicals released from surrounding cells trigger the acrosome reaction in the sperm.
• The acrosome swells, fusing with the sperm cell surface membrane.
• Digestive enzymes in the acrosome are released.
• The enzymes digest through the follicle cells and zona pellucida surrounding the ovum.
• This triggers the cortical reaction; the egg cell releases the contents of vesicles called cortical granules into the space between the cell membrane and the zona pellucida.
• These chemicals cause the zona pellucida to thicken and prevent entry from other sperm (polyspermy)
• The sperm nucleus that enters the egg fuses with egg nucleus to produce a fertilised egg, a zygote.

Protein Transport

Process

• DNA is transcribed to form mRNA
• The mRNA leaves the nucleus and moves onto one of the ribosomes on the rough endoplasmic reticulum (rER)
• The proteins are made at the ribosomes on the rER using the mRNA
• These new proteins are folded and processed as they move through the rER, assuming 3D shape en route
• Vesicles xontaining the protein are pinched off the rER
• These vesicles fuse to form the flattened sacs of the Golgi apparatus
• The proteins are modified within the Golgi apparatus
• Vesicles are pinched off the Gogli apparatus that contain the modified protein
• The vesicles fuse with the cell surface membrane releasing the protein

Helpful Video

Animal Cell (Eukaryote)

Mitochondria

• Inner membrane is folded to form finger-like projections called cristae.
• The mitochondria are the site of the later stages of aerobic respiration

Nucleus

• Enclosed by an envelope made up of 2 membranes perforated by pores
• Contains chromosomes and a nucleolus
• The DNA in chromosomes contains genes that control protein synthesis

Nucleolus

• A dense body within the nucleus where ribosomes are made

Rough Endoplasmic Reticulum (rER)

• A system interconnected, membrane-bound, flattened sacs
• Ribosomes are attached to the outer surface
• Proteins made by these ribosomes are transported through the ER to other parts of the cell

Ribosomes

• Made of RNA and protein
• Are found free in the cytoplasm or attached to endoplasmic reticulum
• The site of protein synthesis

Cell Surface Membrane

• Phospholipid bilayer containing proteins and other molecules forming a partially permeable barrier

Smooth Endoplasmic Reticulum (sER)

• Similar to rER but with no ribosomes attached
• Makes lipids and steroids, e.g. reproductive hormones

Golgi Apparatus

• Stacks of flattened, membrane-bound sacs formed by fusion of vesicles from the ER
• Modifies proteins and packages them in vesicles for transport

Lysosomes

• Spherical sacs containing digestive enzymes and bound by a single membrane
• Involved in the breakdown of unwanted structures within the cell
• Involved in destruction of whole cells when old cells are to be replaced or during development

Centrioles

• Hollow cylinders made up of a ring of 9 protein microtubules
• Involved in the formation of the spindle during nuclear division and in transport within the cell cytoplasm

Prokaryotic Cell (Bacteria)

Function
Flagellum - hollow, cylindrical, thread-like structure that rotates to move the cell.
Pili - thin, protein tubes that allow bacteria to adhere to surfaces
Capsule - slimy layer on surface for protection and to prevent dehydration
Infolding of cell membrane  - site of respiration

Unit 2: Development, Plants & The Environment

The topics that are covered in the specification are:

• Prokaryotic Cell
• Animal Cell (Eukaryote)
• Protein Transport
• Mammalian Fertilisation

Protein Synthesis

Transcription

• This process takes place in the nucleus of a eukaryotic cell.
• The DNA double helix unwinds; the enzyme helicase is used to break the hydrogen bonds between the bases, allowing the 2 strands to partly separate.
• The sequence of 1 of the strands becomes the template / antisense strand and is used in the production of mRNA.
• mRNA is built from free RNA nucleotides which line up alongside the DNA template strand.
• Due to complementary base pairing e.g. U pairs with A and C with G, the order of bases on the DNA exactly determines the order of bases on the RNA strand.
• Every triplet code on the DNA creates a complementary codon on the mRNA.
• The individual mononucleotides are joined together by phosphodiester bonds.
• The completed mRNA molecules now leave the nucleus through pores in the nuclear envelope into the cytoplasm.
• The cytoplasm is where the 2nd stage of protein synthesis, translation, takes place.

Translation

• This takes place on ribosome's on endoplasmic reticulum in the cytoplasm.
• The mRNA attaches to the ribosome.
• A transfer RNA (tRNA) molecules carrying an amino acid molecule has 3 bases called an anticodon.
• These pair with complementary bases on the mRNA codon.
• The ribosome moves along the mRNA as it pairs up with the tRNA anticodons until it reaches the stop codon.
• Then the amino acids that the tRNA carry are joined by peptide bonds formed in condensation reactions to produce a polypeptide chain.

DNA Replication

• The DNA unwinds as the enzymes, helicase, causes the hydrogen bonds between the strands to break.
• This casues the polynucleotide chains to separate, and expose the bases.
• Each chain now acts as a template.
• Free nucleotide molecules (mRNA) in the nucleus position along the exposed bases on both strands.
• Hydrogen bonds form between the complementary bases on the template strand and the nucleotide molecules.
• The enzyme DNA polymerase joins the adjacent nucleotides to form a complementary strand.
• The DNA winds up into a double helix and overall, 2 complete DNA strands have formed.
• This is known as semi-conservative replication; as each DNA molecule now contains 1 'new' strand and 1 'old'strand.

Cardiovascular Disease

Blood Clotting Process

• Clot formation is stimulated when there is damage to a blood vessel.
• Damage exposes the collagen fibres in the vessel wall.
• When platelets, a type of blood cell without a nucleus, comes into contact with the damaged vessel wall they change from flattened discs to spheres with long thin projections.
• Their cell surfaces change, causing them to stick t the exposed collagen in the wall and to each other to form a temporary platelet plug, and releases thromboplastin.
• They also release substances that activate more platelets.
• The direct contact of blood with collagen within the damaged vessel wall also triggers a complex series of chemical changes in the blood.
• A cascade of changes results in thromboplastin, when in the presence of calcium ions and vitamin K, converting soluble plasma protein called prothrombin into thrombin.
• Thrombin is an enzyme that catalyses the conversion of another soluble plasma protein fibrinogen into long insoluble strands.
• These fibrin strands form a tangled mesh that traps blood cells to form a clot.

Atherosclerosis

• This process is triggered when damage occurs to the epithelial cells lining an artery wall.
• This starts an inflammatory response and macrophages arrive at the site of damage.
• Macrophages attract chemicals and substances such as cholesterol.
• The macrophages engulf cholesterol and become 'foaming cells'.
• This leads to a fatty deposit known as an atheroma forming on the epithelial lining of the artery.
• Calcium ions, salts and fibrous tissues also build up in the area around the atheroma, turning it into hardened plaque.
• This also hardens the surrounding artery wall, causing it to lose some of its elasticity and narrows the artery lumen.
• Results in hgih blood pressure, which causes a dangerous positive feedback system, i.e. high blood pressure causes further damage to the arteries, the process is self-perpetuating.

Water

Water is the medium transport in all living things, as it is able to carry polar substances (that don't usually dissolve in organic substances) and many non-polar substances.

Structure Makes It Dipolar

• It contains 1 atom of oxygen joined to 2 atoms of hydrogen.
• The shared negative hydrogen electrons are pulled towards the oxygen atom.
• This leaves the other side of each of the hydrogen atoms with a slight positive charge.
• The unshared negative electron on the oxygen atom gives a slight negative charge.
• The negatively charged oxygen atoms of water attract the positively charged hydrogen atoms of other water molecules.
• This attraction is called hydrogen bonding, which holds the molecule together.
• The non-linear, v-shape allows the ends to attract opposite charges.

Cohesion

• Cohesion is the attraction between molecules of the same type.
• Water molecules are very cohesive (they stick together) due to their dipolar nature.
• This helps water to flow, making it great for transporting substances.

Solvent

• Waters dipolar nature makes it a solvent for other polar molecules.
• A lot of important substances in biological reactions are ionic.
• This means they're made from 1 positively charged atom/molecule and 1 negatively charged atom/molecule.
• As water is dipolar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to the positive ion.
• This means the substances will dissolve in water, as their ions will separate and become surrounded by water molecules.
• As many biological reactions within the cells happen in water, its ability to act as a solvent is very important.

Thermal Properties

• The hydrogen bonds allow water to be liquid at room temperature, unlike other small molecules like carbon dioxide which is a gas at room temperature.
• It's a liquid as the hydrogen bonds mean a lot of energy is required to warm water up and turn into gas.
• This gives water a high specific heat capacity and high latent heat of vaporisation.
• This is useful in living things as it means temperature fluctuations are small.
• This can also allow the body to cool down as sweating and transpiration take energy from the body. 

Surface Tension

• Surface tension is when liquids behave as if their surface is covered by a thin elastic skin.
• Water has a very high surface tension.
• This is due to the hydrogen bonds which hold the molecules together and pull them down.
• This attraction is not present between the different molecules where water and air meet.
• Causing the water layer to hold together to form a thin skin of surface tension.

Amphoteric

• Water molecules are amphoteric, which means they can act as both an acid and a base.
• The acid comes from its formation of H ions, a proton donor.
• The base comes from its formation of OH ions, a proton acceptor.
• As water molecules can both donate and accept protons it can act as a buffer in the biochemical reactions that take place in cells.
• This means help to prevent reactions in progress from changing the pH inside the cell.

Useful Video

Unit 1: Lifestyle, Transport, Genes & Health

The Edexcel specification covers these topics for Unit 1:

• Water
• Carbohydrates
• Lipids
• Proteins & Amino Acids
• Enzymes
• Rates of Reaction
• The Heart
• Blood Vessels
• Cardiac Cycle
• Heart Rate (Daphnia Core Practical)
• Cardiovascular Disease
• Risk Factors of CVD
• Population Studies of Risk Factors
• Prevention & Treatment of CVD
• Diet & Energy
• Cell Membrane Structure
• Structure of DNA &RNA
• DNA Replication
• Genetic Code
• Protein Synthesis
• Genes & Mutation
• Cystic Fibrosis
• Genetic Inheritance
• Genetic Screening
• Gene Therapy

Intro

I started this to help my revision but if anyone else ends up using it as well that's also really cool.
I also have an A2 Biology revision blog as well at:
Remember this revision only fits the edexcel specification.