paper 1

• reducing sugars can be tested for using a Benedict's test:

  • add 2cm^3 of the sample to a test tube
  • add an equal volume of Benedict's solution
  • heat in a water bath for 5 minutes
  • positive = colour change from blue to brick-red
• the test for non-reducing sugars is:

  • carry out normal Benedict's test, produces a negative result
  • add 2cm^3 of sample to 2cm^3 of hydrochloric acid in a test tube
  • heat in water bath for 5 minutes
  • add sodium hydrogencarbonate to neutralise the solution
  • carry out normal Benedict's test
  • positive = colour change from blue to brick-red
• the test for starch is:

  • place 2cm^3 of sample in a test tube
  • add 2 drops of iodine solution
  • positive = colour change from orange-brown to black
• the test for proteins is:

  • add the sample to a test tube
  • add an equal volume of Biuret's reagent and mix
  • positive = colour change from blue to purple
• the test for lipids is:

  • place 2cm^3 of sample in a dry and grease-free test tube
  • add 5cm^3 of ethanol and shake gently
  • add 5cm^3 of water and shake gently
  • positive = cloudy-white emulsion
• the induced fit model of enzyme action is:

  • substrate binds to active site
  • enzyme undergoes a conformational change as the substrate binds, making it complementary
  • enzyme-substrate complex is formed
  • enzyme breaks down substrate into product molecules
• the process of homogenisation in cell fractionation:

  • cells are broken up by a homogeniser, releasing their organelles
  • this produces a fluid called homogenate
  • homogenate is filtered to remove complete cells and large pieces of debris
• the process of ultracentrifugation in cell fractionation:

  • tube of filtered homogenate is spun at low speed in a centrifuge
  • heaviest organelles are forced to the bottom and form a pellet
  • the fluid at the top is called supernatant
  • supernatant is separated and respun at a higher speed
  • the next heaviest organelles are forced to the bottom and form a pellet
  • the process continues until the desired organelles are separated
• mitosis:

  • prophase = the nuclear membrane breaks down, spindle fibres appear, chromosomes condense
  • metaphase = chromosomes line up along the equator, spindle fibres attach to chromosomes
  • anaphase = spindle fibres contract pulling chromosomes towards poles, centromeres divide, sister chromatids move to opposite poles
  • telophase = nuclear membrane reforms, chromosomes decondense, spindle fibres disappear
• binary fission process:

  • the circular DNA molecule replicates, both copies attach to the cell membrane
  • plasmids replicate
  • cell membrane grows between the two DNA molecules, dividing the cytoplasm in two
  • new cell wall forms between the two DNA molecules, creating two identical daughter cells
  • each daughter cell has a single copy of the circular DNA and a variable number of copies of plasmids
• the cell cycle:

  • interphase = the cell grows and prepares to divide, chromosomes and some organelles are replicated, chromosomes begin to condense
  • nuclear division = mitosis or meiosis
  • cytokinesis = the organelles move to opposite sides of the cell, the cytoplasm divides, producing 2 or 4 daughter cells
• active transport process:

  • the specific ion or molecule binds to a receptor site on an intrinsic carrier proteins
  • ATP binds to the protein, it is hydrolysed into ADP and Pi
  • the protein undergoes a conformational change, changing shape and opening to the opposite side of the membrane
  • the ion or molecule is released to the opposite side of the membrane
  • the phosphate molecule is released from the protein so it reverts to its original shape and the process can be repeated
• the process of co-transport of glucose/amino acids in the ileum is:

  • Na+ ions are actively transported out of epithelial cells by the sodium-potassium pump and into the blood
  • the concentration of Na+ ions in the lumen is greater than in epithelial cells, establishing a concentration gradient
  • Na+ ions diffuse into epithelial cells down the concentration gradient through a co-transport carrier protein, carrying glucose/amino acids with them
  • the glucose/amino acids pass into the blood by facilitated diffusion using a carrier protein
• the process of phagocytosis:

  • chemoattractance, chemicals released by the pathogen or by dead cells attract the phagocyte to the pathogen
  • phagocyte receptors on the cell-surface membrane recognise and attach to the pathogen
  • the phagocyte engulfs the pathogen, forming a vesicle called a phagosome
  • lysosomes move towards the phagosome and fuse with it
  • lysosomes release hydrolytic enzymes called lysozymes into the phagosome, they hydrolyse the pathogen, destroying it
  • soluble hydrolysis products are absorbed into the cytoplasm of the phagocyte
• cell-mediated immunity process:

  • pathogens invade body cells or are engulfed by phagocytes
  • body cells/phagocytes present the antigens on their surface
  • receptors on a specific T helper cell are complementary to the antigens
  • the T helper cell receptors bind to the antigen
  • this activates the T helper cell to divide rapidly by mitosis, forming genetically identical clones
  • the cloned T helper cells become T cytotoxic cells, T memory cells, or more T helper cells
• humoral immunity process:

  • B cells have antibodies on their cell-surface membrane
  • B cells take up complementary antigens, process them and present them
  • T helper cells attach to the antigens on the B cells, activating the B cells
  • B cells divide by mitosis into B memory cells and plasma cells
  • plasma cells secrete complementary antibodies
  • memory cells remain in the body until reexposure
• HIV replication process:

  • a HIV particle reaches a T helper cell with a CD4 protein, it binds using its attachment proteins
  • protein capsid fuses with cell-surface membrane
  • RNA and enzymes are inserted into the T helper cell
  • reverse transcriptase converts the virus’ RNA into DNA
  • viral DNA is inserted into the cell’s DNA
  • viral mRNA is created using the cell’s enzymes, containing instructions for new HIV particles
  • viral mRNA passes out through a nuclear pore and the cell synthesises new HIV particles
  • new HIV particles break away, taking part of its membrane as their lipid envelope
• ELISA test process:

  • apply the sample to a surface so that the antigens are fixed
  • wash the sample to remove unfixed antigens
  • add the antibody specific to the particular antigen
  • wash again to remove excess antibody
  • add a second antibody complementary to the first antibody, with an enzyme attached
  • add the colourless substrate of the enzyme
  • if the antigen is present, the enzyme acts on the substrate and produces a colour change
  • amount of antigen present is relative to colour intensity
• inspiration process:

  • external intercostal muscles contract, internal intercostal muscles relax
  • the ribs are pulled upwards and outwards, increasing the thorax volume
  • the diaphragm muscles contract, it flattens, increasing the thorax volume
  • the increased thorax volume decreases air pressure in the lungs
  • atmospheric pressure > pulmonary pressure so air is forced into the lungs
• expiration process:

  • internal intercostal muscles contract, external intercostal muscles relax
  • the ribs are pulled downwards and inwards, decreasing the thorax volume
  • the diaphragm muscles relax, it unflattens, decreasing the thorax volume
  • the decreased thorax volume increases air pressure in the lungs
  • atmospheric pressure < pulmonary pressure so air is forced out of the lungs
• carbohydration digestion process:

  • saliva containing amylase is mixed with food during chewing
  • amylase hydrolyses starch to maltose
  • maltose reaches small intestine, membrane-bound disaccharidases such as maltase hydrolyse disaccharides such as maltose into alpha-glucose
• lipid digestion process:

  • bile salts emulsify lipids into micelles to increase their surface area
  • lipases hydrolyse triglycerides into fatty acids and monoglycerides
• protein digestion process:

  • endopeptidases hydrolyse central peptide bonds in a polypeptide forming smaller polypeptides
  • exopeptidases hydrolyse terminal peptide bonds in a polypeptide forming dipeptides
  • membrane-bound dipeptidases hydrolyse peptide bonds in dipeptides, into amino acids
• absorption of triglycerides process:

  • monogl. and fatty acids are in micelles
  • micelles reach epithelial cells in ileum, break down into monogl. and fatty acids
  • monogl. and fatty acids are non-polar so diffuse across the membrane into the epithelial cells
  • endoplasmic reticulum reforms trigl. from monogl. and fatty acids
  • trigl. associate with cholesterol and lipoproteins to form chylomicrons
  • chylomicrons move out of epithelial cells by exocytosis, enter lymphatic caplillaries, pass into the bloodstream
  • enzymes in capillary endothelium hydrolyse triglycerides so they can diffuse into cells
• cooperative binding process:

  • the initial shape of haemoglobin means it is difficult for the first oxygen to bind, so at low partial pressure of oxygen, little oxygen can bind to haemoglobin
  • the binding of the first oxygen molecule changes the quaternary structure of the haemoglobin, the new shape is easier for the next oxygens to bind to
  • a smaller increase in partial pressure is required for the second and third oxygens to bind due to the change in shape of haemoglobin
  • the fourth oxygen is more difficult to bind as there are fewer binding sites available and fewer oxygens available
• cardiac cycle:

  • diastole = atria and ventricles relax, elastic recoil of the heart lowers the pressure inside the chambers, atria fill with blood from the vessels, atrial pressure increases until atrioventricular valves open, ventricles fill with blood, semi-lunar valves stay closed
  • atrial systole = atria contract, remaining blood is forced into ventricles
  • ventricular systole = ventricles contract, atrioventricular valves close, semi-lunar valves open, blood moves out of the ventricles into the vessels
• tissue fluid formation process:

  • the heart pumping blood creates hydrostatic pressure at the arterial end of capillaries
  • hydrostatic pressure causes tissue fluid to move out of the blood plasma
  • this movement is opposed by hydrostatic pressure of tissue fluid already outside the capillaries and the water potential outside the capillaries being higher than inside
  • overall, the force pushes tissue fluid containing oxygen and nutrients out of capillaries at the arterial end, called ultrafiltration
  • proteins and large molecules do not leave as they do not fit, only small molecules leave
• return of tissue fluid process:

  • the loss of tissue fluid from the capillaries reduces the hydrostatic pressure inside them
  • at the venous end of capillaries, hydrostatic pressure inside is lower than outside
  • tissue fluid containing waste materials and carbon dioxide is forced back into capillaries by the hydrostatic pressure gradient
  • water is small so left but proteins are large so stayed, this means the water potential inside is lower than outside
  • water moves back into capillaries along the water potential gradient
• cohesion-tension theory process:

  • water molecules form hydrogen bonds between each other so tend to stick together, this is cohesion
  • water forms a continuous unbroken column across the mesophyll cells and down the xylem
  • water evaporates from the stomata of mesophyll cells
  • more water is pulled up behind because of cohesion, the column of water moves up the xylem
  • this creates negative pressure within the xylem, this is tension
• mass flow theory:

  • H+ ions are actively transported from comp cells to source, H+ ions and sucrose move into comp cells by co-transport
  • sucrose is actively transported into phloem by comp cells
  • water pt of sieve cells decreases, water enters by osmosis
  • hydro pressure in sieve cell increases, so mass movement towards sink
  • sucrose moves into sink by active transport/diffusion to be used/stored
  • water pt of sink increases, water enters xylem by osmosis
  • hydro pressure of sieve tubes decreases near sink, creating a hydro pressure gradient down phloem
  • there is mass flow of sucrose down the gradient
• transcription:

  • DNA helicase breaks hydrogen bonds between complementary bases
  • one DNA strand is used as a template
  • free nucleotides line up by complementary base pairing
  • RNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides
  • the process stops when a STOP codon is reached
  • after RNA polymerase moves away, DNA is joined together again, exposing only 12 bases at a time
  • mRNA leaves the nucleus through a pore and attaches to a ribosome, pre-mRNA is spliced
• splicing:

  • introns are removed
  • exons are kept
  • a strand of mRNA is produced
  • mRNA leaves the nucleus through a pore and attaches to a ribosome
• translation:

  • mRNA attaches to the START codon of a ribosome
  • tRNA brings amino acids from the cytoplasm to the ribosome by binding its end chain to them
  • the tRNA molecule with the complementary anticodon binds to the mRNA codon
  • the next tRNA molecule with the next complementary anticodon binds to the next mRNA codon, and so on
  • the ribosome moves along the mRNA pairing up more tRNA molecules
  • adjacent amino acids on tRNA molecules are joined by peptide bonds
  • tRNA molecules are released and can collect more amino acids
  • the process stops at a STOP codon
• meiosis process:

  • (meiosis 1)
  • homologous chromosomes pair up
  • chromatids twist round each other
  • potential for crossing over
  • cell divides into two daughter cells, each with one homologous pair of chromosome
  • (meiosis 2)
  • chromatids move apart
  • daughter cells divide into two, producing four daughter cells each with one chromatid
• natural selection:

  • there must be a variety of different phenotypes within the population
  • an environmental change occurs so the selection pressure changes
  • some individuals have advantageous alleles, they are more likely to survive and reproduce
  • advantageous alleles are passed onto their offspring
  • over time, the frequency of alleles in the populations changes, this is evolution
• semi-conservative replication:

  • DNA helicase breaks hydrogen bonds between complementary bases, separating the two DNA strands
  • each strand can be used as a template
  • free nucleotides line up by complementary base pairing
  • DNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides
  • 2 identical double-stranded DNA molecules can be produced