TOPIC 2

Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

Molecules will diffuse both ways but the net movement will be to the area of lower concentraion and it will continue until the particles are evenly distributed.

The particles diffuse down the concentration gradient.

Diffusion is a passive process meaning that it does not require energy.

Gas exchange surfaces are adapted for efficient diffusion.

Most gas exchange organs such as the lungs have a large surface area and are often only one layer of epithelial cells thin which provides a short diffusion pathway across the surface.

The organ also maintains a steep concentration gradient of gases.

All of these features increase the rate of diffusion.

In the lungs, oxygen diffuses out of the alveoli and into the blood and carbon dioxide diffuses into the alveoli from the blood and is breathed out.

Having a lot of alveoli means there is a large surface area for diffusion to occur across. 

All the alveoli have a good blood supply from the capillaries which constantly take away oxygen and bring more carbon dioxide.

Breathing in and out refreshes the air in the alveoli, keeping the concentration gradients high.

Fick's Law relates the rate of diffusion to the concentration gradient, the surface area and the thickness of the exchange surface.

Rate of Diffusion = Area of diffusion surface x difference in concentration 

                                        Thickness of diffusion surface

Rate of diffusion will double if the surface area or difference in concentration doubles, or the thickness of the surface halves.

The structure of all membranes are similar. They are composed of lipids and phospholipids.

In the fluid mosaic model the phospholipid molecules form a continuous double layer (bilayer), the bilayer is 'fluid' because the phospholipids are constantly moving.

Protein molecules are scattered through the bilayer and can move around within it.

Some proteins have a polysaccharide chain attached, which are called glycoproteins.

Some lipids also have a polysaccharide chain attached, which are called glycolipids.

Cholesterol is present in the membrane. It fits inbetween the phospholipids, forming bonds between them which makes the membrane more rigid.

The membrane is partially permeable meaning that small molecules can move through gaps between the phospholipids.

Phospholipid molecules have a head and a tail.

The head contains a phosphate group, it's hydrophillic meaning it attracts water.

The tail is made of two fatty acids and is hydrophobic meaning it repels water.

Because of this, the hydrophillic layer faces out towards the water and the hydrophobic tails are on the inside which makes the center of the bilayer hydrophobic.

This means that the membrane doesn't allow water-soluble substances through it.

Osmosis is the diffusion of water molecules.

It is the movement of free water molecules across a partially permeable membrane from an area of higher concentration to an area of lower concentration.

Water molecules will diffuse both ways through the membrane but the net movement will be to the side with the lower concentration.

Facilitated diffusion uses carrier proteins and channel proteins.

Some larger molecules such as amino acids and glucose and charged particles such as ions don't diffuse directly through the phospholipid bilayer of the cell membrane.

They instead diffuse through carrier proteins or channel proteins in the cell membrane.

Carrier proteins move large molecules into or out of the cell, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.

Facilitated diffusion moves particles down the concentration gradient and also doesn't use energy.

Active transport uses energy to move molecules and ions across plasma membranes against a concentration gradient.

This process involves carrier proteins.

A molecule attaches to the carrier protein which changes it's shape, this moves the molecule across the membrane, releasing it on the other side.

Energy is needed for this process to occur, which comes from ATP.

Some molecules such as proteins, lipids and some carbohydrates are too big to be taken into the cell by carrier proteins.

Instead, a cell can surround a substance with a part of its cell membrane.

The membrane then pinches off to form a vesicle inside the cell containing the ingested substance.

Some cells also take in much bigger objects by endocytosis such as white blood cells, that take in things such as microorganisms and dead cells so that they can destroy them.

This process also uses ATP.

Some substances produced by the cell such as digestive enzymes, hormones and lipids need to be released from the cell, which is done by exocytosis.

Vesicles containing these substances pinch off from the sacs of the golgi apparatus and move towards the cell membrane.

The vesicles fuse with the cell membrane and release their contents outside the cell.

Some substances aren't released outside the cell but are instead inserted straight into the cell membrane.

Exocytosis uses ATP as an energy source.

Proteins are made up from a long chain of amino acids.

The monomers of proteins are amino acids. A dipeptide is formed when two amino acids join together and a polypeptide is formed when more than two amino acids are joined together.

Different amino acids have different variable groups but they all have the same general structure.

They are made up of a carboxyl group, an amino group and a carbon-containing R group, also known as the variable side group.

All living things share a bank of 20 amino acids and the only difference is what makes up their R group.

Polypeptides are formed by condensation reactions.

Amino acids are linked together to form polypeptides.

A molecule of water is released during a condensation reaction.

The bonds formed between amino acids are called peptide bonds and the reverse of this reaction occurs during digestion.

Primary structure: this is the sequence of amino acids in the polypeptide chain. They are held together by peptide bonds.

Secondary structure: the polypeptide chain doesn't remain flat and straight and hydrogen bonds form between the amino acids in the chain. This makes them automatically coil into an alpha helix or fold into a beta pleated sheet.

Tertiary structure: the coiled or folded chain is often coiled of folded further. More bonds form between different parts of the polypeptide chain including hydrogen bonds and ionic bonds. Disulfide bonds form when two molecules of cystein come together. The tertiary structure forms their final 3D structure.

Quarternary structure: some proteins are made of different polypeptide chains held together by bonds.The quaternary structure is the way these polypeptide chains are assembled together.

A proteins primary structure determines it's 3D structure and properties.

Globular proteins are round, compact proteins made up of multiple polypeptide chains.

The chains are coiled up so that hydrophillic parts of the chains are on the outside and hydrophobic parts of chains face inwards.

This makes the proteins soluble so they're easily transported in fluids.

Haemoglobin is a globular protein made of four polypeptide chains. It carries oxygen around the body. It also has an iron-containing haem group that bind to oxygen.

Fibrous proteins are made up of long, insoluble polypeptide chains that are tightly coiled round each other to form a rope shape.

The chains are held together by lots of bonds which makes the proteins strong.

Because they are strong, they are found in supportive tissues.

Collagen is strong that forms connective tissues in animals.

Enzymes speed up reactions by acting as a biological catalyst, they catalyse metabolic reactions both at a cellular level and for the organism as a whole.

Enzymes can affect structures in an organism as well as functions.

Enzymes are highly specific due to their tertiary structure and will usually only catalyse one reaction.

Enzymes lower the activation energy of a reaction.

Each different enzyme has a different tertiary structure and so a different shaped active site. If the substrate shape doesn't match the active side, the enzyme-substrate complex won't be formed.

The tertiary structure of an enzyme may be altered by changes in pH or temperature.

Enzyme concentration affects the rate of reaction up to a certain point.

DNA and RNA carry important information.

DNA is used to store genetic information where as RNA transfers genetic information from the DNA to the ribosomes.

DNA and RNA are polymers of mononucleotides which are made from a pentose sugar, a nitrogen-containing organic base and a phosphate group.

The sugar in DNA is called deoxyribose and each DNA mononucleotide has the same sugar and phosphate group but the base can vary - there are four possible bases A, T, C and G.

The sugar in RNA is ribose, it is the same as DNA but the base T is replaced with U.

Mononucelotides join together to form polynucleotides and DNA is made of two polynucleotide chains in a double-helix structure.

Two DNA polynucleotide strands join together by hydrogen bonding between the bases.

Each base can only join with one other base which is called complementary base pairing.

A always pairs with T and C always pairs with G, which means that there are always equal amounts of each base.

Two hydrogen bonds form between A and T and three hydrogen bonds form between C and G.

DNA contains genes which are instructions for making proteins.

The order of the mononucleotide bases determine the other of amino acids in a particular protein an different sequences of bases code for different amino acids.

DNA is copied into RNA for protein synthesis.

DNA is too large to move out of the nucleus so a section is copied into mRNA, this process is called transcription.

The mRNA leaves the nucleus and joins with a ribosome in the cytoplasm, where it can be used to synthesise a protein, this process is called translation.

mRNA is made in the nucleus during transcription and its 3 adjacent bases are called a codon.

It carries the genetic code from the DNA in the nucleus to the cytoplasm, where it's used to make a protein during translation.

tRNA is found in the cytoplasm.

It has an amino acid binding site at one end and a sequence of three bases called an anti-codon.

It carries the amino acids that are used to make proteins to the ribosomes during translation.

1. Transcription starts when RNA polymerase attaches to the DNA double-helix at the beginning of the gene.

2. The hydrogen bonds between the DNA strands break which seperates the strands and they unwind.

3. One of the strands is then copied to make an mRNA copy.

4. The mRNA strand ends up being a complementary copy of the DNA template strand except the T base is replaced with U.

5. The hydrogen bonds between the unwound strands of DNA re-form once the RNA polymerase has passed by and the strands wind up again to create the double-helix.

6. When RNA polymerase reaches the stop codon it detaches from the DNA and mRNA moves out of the nucleus through a nuclear pore and attaches to the ribosome in the cytoplasm.

1. The mRNA attaches to the ribosome and tRNA molecules carry amino acids to the ribosome.

2. A tRNA molecule with an anti-codon that's complementary to the start codon on the mRNA attaches itself to the mRNA by complementary base pairing.

3. A second tRNA molecule attaches itself to the next codon in the same way.

4. The two amino acids attaches to the tRNA molecules are joined together by a peptide bond and the first tRNA molecule moves away leaving it's amino acid behind.

5. The ribosome moves along to the next codon.

6. This process continues producing a chain of linked amino acids until there is a stop codon on the mRNA molecule.

7. The polypeptide chain moves away from the ribosome and translation is complete.

1) The enzyme DNA helicase breaks the hydrogen bonds between the bases on the polynucleotide DNA strands. This makes the helix unwind to form single strands.

2) Each original single strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand.

3) Condensation reactions join the nucleotides of the new strands together which is catalysed by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original new strands.

4) Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

Mutations are changes to the base sequence of DNA and can be caused by errors during DNA replication.

The type of errors that can occur include:

• Substitution: one base is substituted with another.
• Deletion: one base is deleted.
• Insertion: an extra base is added.
• Duplication: one or more bases are repeated.
• Inversion: a sequence base is reversed.

The order of DNA bases determines the order of amino acids in a particular protein. If a mutation occurs in a gene, the primary structure could be altered which could change the final 3D shape of the protein so it doesn't work properly.

If a mutation occurs in a gene it can cause a genetic disorder, which is then passed on. Some genetic disorders can be caused by lots of different mutations.

Gene: A sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic.

Allele: A different version of a gene. Different versions have slightly different base sequences, which code for different versions of the same characteristic.

Genotype: The alleles a person has.

Phenotype: The characteristics displayed by an organism.

Dominant: An allele whose characteristics appears in the phenotype even when they're only one copy. Dominant alleles are shown by a capital letter.

Recessive: An allele whose characteristic only appears in the phenotype if two copies are present.

Incomplete Dominance: When the traits from a dominant allele isn't completely shown over the trait produced by the recessive allele so both alleles influence the phenotype.

Homozygote: An organism that carries two copies of the same allele for a certain characteristic.

Heterozygote: An organism that carries two different alleles for a certain characteristic.

Carrier: If a recessive allele can cause disease, a carrier is someone who has one dominant and one recessive alelle. They won't have the disease but they carry a copy of the allele for the disease.

Cystic fibrosis affects the respiratory system, digestive system and the reproductive system.

Within the respiratory system, the cilla are unable to move the mucus towards the throat because its so thick and sticky. This means that mucus builds up in the airways. Some airways can become completely blocked by the mucus and gas exchange cannot take place. People with CF are also more prone to lung infections.

Within the digestive system, the tube that connects the pancreas to the small intestine can become blocked with mucus preventing digestive enzymes from reaching the small intestine. The mucus can cause cysts to form in the pancreas and inhibits the absorption of nutrients in the small intestine.

In some men with CF, the tubes connecting the testicles to the penis are absent and can become blocked by thick mucus so any sperm produced can't reach the penis. In women, thickened cervical mucus can prevent the sperm from reaching the egg.

Identification of Carriers: this is offered to individuals with a family history of genetic disorders. Couples can be tested before having children to determine the chances of any future children having the disorder. It allows parents to make informed decisions about things like whether to have children and whether to carry out prenatal testing.

Preimplantation Genetic Diagnosis: PGD is carried out on embryos produced by IVF. It involves screening embryos for genetic disorders before they are implanted into the womb.

Ethical issues:

• May cause emotional stress on you and your partner.
• The tests aren't always 100% accurate and could give a false result.
• Other genetic abnormalities may be found, which could cause further stress.
• There are concerns that the results of genetic tests could be used by employers or life insurance companies.

This is carried out at 15-20 weeks of pregnancy.

A sample of amniotic fluid is obtained via the abdomen using a very fine needle.

This fluid contains fetal cells. The cells contain DNA, which can be analysed.

Amniocentesis has a 1% risk of miscarriage.

Results aren't available until 2-3 weeks after the sample is taken, although a rapid test (which only looks for a few of the most common disorders) can also be performed. The results of the rapid test are usually available in 3-4 days.

CVS is performed at 11-14 weeks of pregnancy.

A sample of cells is taken from the chorionic villi (part of the fetus that connects it to its mother). The cells contain fetal DNA, which can be analysed.

This procedure is done via either the abdomen (using a fine needle) or the vagina (using a catheter).

CVS has a 1-2% risk of a miscarriage, which is greater than amniocentisis.

Initial results are available within a few days and will tell you whether any major issues have been found, but the results of the more in-depth and detailed tests can take two weeks or more.