Download AQA A-level Biology Paper 1: Large Molecules, Structure and Function and more Exams Biology in PDF only on Docsity!
large molecules often contain carbon. why?
they can readily form bonds with other carbon atoms. this forms a 'backbone'. other atoms can then attach.
describe benedicts test for reducing sugars
add equal volumes of the sugar sample and benedicts reagent.
heat the mixture in a water bath for 5 minutes.
brick red precipitate will form.
what is a reducing sugar?
a sugar that can reduce (give electrons to) another chemical.
describe the benedicts test for non reducing sugars
with food sample, add an equal volume of dilute hydrochloric acid in a test tube.
place test tube in water bath for 5 mins.
add sodium hydrogencarbonate solution.
heat resulting solution with an equal volume of benedicts. solution for 5 minutes.
turns orange/brown.
what does dilute hydrochloric acid do in the benedicts test for non reducing sugars?
it hydrolyses any disaccharide into monosaccharides, thus also forming reducing sugars.
describe the structure of starch
chains of alpha glucose
some chains are branched= amylopetcin
some chains are coiled= amylose
what purpose does the structure of amylose serve?
makes the molecule compact so it is stored more easily.
what purpose does the structure of amylopectin serve?
more surface area for enzymes to work on, thus glucose molecules can be released at a much quicker rate.
how is the structure of starch suited to its function?
- starch is insolube so it doesnt affect water potential
- it is large enough that it doesnt diffuse out of cells
- compact enough so that a lot of it can be stored in a small space
- branched= each end can simultaneously be acted on by enzymes, so glucose is quickly released
how is the structure of glycogen suited to its function?
- insoluble, so no effect on water potential of cell
- insoluble, so doesnt diffuse out of cells
- compact, a lot of it can be stored in a small space
- very highly branched= each end can simultaneously be acted on by enzymes, so glucose is quickly released
why do animals need highly branched glycogen?
how does the structure of phospholipids relate to their properties?
- polar, therefore will position itself to form a bilayer
- the structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. these are important in cell recognition.
draw the structure of an amino acid
draw how a dipeptide forms
what is meant by 'primary sequence' of a protein
The unique sequence of amino acids that make up a protein or polypeptide chain
what is meant by 'secondary sequence' of a protein
The way in which the primary structure of a polypeptide chain folds e.g. alpha helix or beta pleated sheets. shape is held by H bonds.
what is meant by 'tertiary sequence' of a protein
This is the overall 3-D structure of the protein.
The shape of the protein is held together by H bonds and ionic bonds and disulfide bridges.
what is meant by 'quaternary sequence' of a protein
If a protein is made up of several polypeptide chains that are linked in various ways, the way they are arranged is called the quaternary structure.
describe the test for proteins
add equal volumes of sample and sodium hydroxide to a test tube.
add a few drops of dilute copper (II) sulfate and mix.
if solution turns purple, protein is present.
what are fiberous proteins made up of?
made of long molecules arranged to form fibres (e.g. in keratin). Several helices may be wound around each other to form very strong fibres.
what are globular proteins made up of?
made of chains folded into a compact structure. One of the most important classes are the enzymes. Although these folds are less regular than in a helix, they are highly specific and a particular protein will always be folded in the same way.
what kind of protein is collagen?
fiberous
what kind of protein is haemoglobin?
globular
what is haemoglobin made up of?
two α polypeptide chains
two β polypeptide chains
an inorganic prosthetic haem group (Fe2+)
how is DNA adapted to carrying out its function?
- it is very stable, thus rarely mutates
- the H bonds can easily be separated during DNA replication and protein synthesis
- extremely large, thus carries a lot of genetic information
- base pairing allows DNA to replicate and transfer info as mRNA.
describe semi conservative replication
- DNA helicase unwinds separates the two strands of DNA into single strands by breaking the H bonds between the bases
- this therefore forms 2 base templates (the bases on each strand are exposed)
- free complimentary nucleotides align with the bases according to complimentary base pairing
- they are joined via condensation reaction with DNA polymerase
- the new DNA molecule is formed
what is meant by resolution
the minimum distance apart that two objects can be in order for them to appear as separate items.
before cell fractionation, why is the tissue placed in a cold, isotonic buffer?
cold: reduce enzyme activity that may break down the organelles
isotonic: same water potential as tissue, this prevents organelles shrinking/bursting due to osmotic loss/gain of water
buffered: so the pH doesn't fluctuate, so that nothing alters the organelle structures or affects enzyme functioning
what are the advantages of an electron microscope?
electron beam has a short wavelength and the microscope can therefore has high resolution.
the negatively charged beam can be focused using electromagnets.
what are the limitations of using a transmission electron microscope?
the whole system must be in vacuum, so living specimens cant be studied.
complex staining process
the image may contain artefacts
what is the function of SER?
synthesises, stores and transports lipids and carbohydrates
what is the function of RER?
provides a large surface area to synthesise proteins and glycoproteins
provides a transport pathway for materials throughout the cell
what is the function of golgi apparatus?
add carbs to proteins to form glycoproteins
produce secretory enzymes
secrete carbohydrates
transport, modify and store lipids
form lysosomes
what is meant by 'organ'?
a combination of tissues that are coordinated to perform a variety of functions.
describe humoral immunity
- the surface antigen of a pathogen is taken up by B cell
- the B cell processes it and presents the antigen on its surface
- T-helper cells attach to the processed antigens, thus activating the B cell
- B cells divide by mitosis to produce a clone of plasma cells
- these secrete the specific antibody that can bind to the pathogens antigen
- the antigens destroy the pathogen
- some B cells develop into memory cells
what is passive immunity and how is it produced?
produced by the introduction of antibodies into individuals from an outside source.
immunity is acquired immediately.
no contact with pathogen/s needed.
short term immunity: antibodies aren't replaced when broken down, memory cells not formed.
e.g. anti-venom
what is active immunity and how is it produced?
produced by stimulating the production of antibodies by the individuals own immune system.
contact with pathogen/s needed.
immunity takes time to develop
natural active immunity
results from an individual becoming infected with a disease under normal circumstances.
long term
body produces its own antibodies
artificial active immunity
involves inducing an immune response in an individual
WITHOUT them suffering the symptoms of the disease.
e.g. vaccinations
memory cells produced
what are the features of a successful vaccination programme?
- vaccine must be economically available in sufficient quantities
- must have few side effects
- available means of producing, storing and transporting the vaccine
- must be possible to produce herd immunity
why can vaccinations not be effective?
- immunity may not be induced in people with defective immune systems
- individuals may develop disease after the vaccination but before their immunity levels are high enough to prevent it.
- pathogen may mutate frequently, so vaccines cant prevent them from harming people
- certain pathogens conceal themselves from the body's immune system e.g. by hiding within cells
- individuals may object to vaccinations due to religious, ethical or medical reasons
what are antibodies made up of?
4 polypeptide chains:
- 2 are 'heavy chains'
- 2 are 'light chains'
describe and explain the countercurrent exchange principle
-the blood and the water flow in opposite directions
-blood that is loaded with O2 meets water, which already has maximum O
-blood with little O2 meets with water with O2 removed
-so, diffusion concentration gradient established and maintained across the entire width of the gill lamellae
-so, 80% of O2 in water absorbed into fish blood
how does the structure of a plant leaf link to gas exchange?
-all cells are close to stomata, and therefore surface, therefore a short diffusion pathway
-inter-connecting air spaces in the mesophyll so that gases can readily contact mesophyll cells
describe and explain why plant stomata links to gas exchange and its benefits
stomata are like minute pores, on the underside of leaves.
each has guard cells, which control the rate of gas exchange
guard cells close when water loss is excessive
how do insects limit water loss?
-small surface area to volume ratio
-waterproof coverings
-spiracles which can close
how do xerophytes limit water loss?
-thick waxy cuticle
-rolled up leaves: traps water vapour
-hairy leaves: traps moist air
-stomata in pits/grooves: traps moist air
-reduced surface area to volume ratio
by trapping air, water potential gradient is reduced
describe the process of inspiration
external intercostals contract
internal intercostals relax
ribs move up and outwards, increasing thorax volume
diaphragm contracts, further increasing thorax volume
air pressure in lungs becomes lower than atmospheric pressure, so air is forced in
describe the process of expiration
internal intercostals contract
external intercostals relax
ribs movie in and downwards
diaphragm relaxes
this decreases thorax volume
air pressure in lungs becomes greater than atmospheric pressure, so air is forced out
structure of haemoglobin
4 polypeptide chains
each chain has a haem group which contains a ferrous Fe2+ ion.
longer chain fatty acids: combine with bile salts to form micelles, which break down into fatty acids and monoglycerides which diffuse into epithelial cells. once there, they recombine into triglycerides which can form chylomicrons.
artery structure
thick muscle layer so that they can dilate and contract to smooth blood flow.
thick elastic layer so artery can stretch and recoil with systole and diastole, smoothing blood flow.
overall thickness of wall is large so that it doesn't burst under pressure.
why do arterioles have thinner elastic layer than arteries?
blood pressure is lower.
vein structure
muscle layer is thinner, because they carry blood away from tissues, and so have less need to dilate.
elastic layer is thinner because of lower blood pressure, which doesn't need recoil.
overall thickness of wall is thinner due to low blood pressure.
valves at regular intervals to ensure there's no backflow of blood.
why do veins have thinner walls than arteries?
thinner walls because blood is at lower pressure, and so less stretch and recoil with each contraction of ventricles.
capillary structure
one cell thick endothelium. allows rapid diffusion.
numerous, highly branched, so provides a larger surface area.
narrow lumen and diameter.
spaces in lining to allow tissue fluid to form.
how does water travel into/through plants?
the water particles have higher water potential than the root hair cell, and so they enter the cell by osmosis down a water potential gradient.
the water potential of the root hair cell is now higher than the cortex cell, and so water enters the cortex cell by osmosis.
symplast pathway: water enters root hair cell by osmosis, and travels from the cytoplasm of one cell to another by plasmodesmata
apoplast pathway: water enters the root by entering the spaces in cell walls. water moves up xylem due to H bonding in water. water travels until it reaches the casparian strip, where water needs to travel via symplast pathway.
what is plasmodesmata?
strands of cytoplasm linking one cell to another that passes through pores in cellulose cell walls
explain cohesion-tension theory
water is drawn from cell to cell by osmosis. it moves across the root cortex to keep a constant column of water. this creates 'tension' on the column of water in the xylem.
water shows cohesion due to its H bonding. this makes the column very strong. so, water is pulled up the xylem to replace the water lost by evaporation.
water moves between the cells of a leaf along the water potential gradient.
how can you investigate mass flow using aphids?
- cut across their mouthparts.
the mouthparts act as a sampling tube that they collect the phloem contents.
- If the phloem is punctured with a hollow tube then the sap oozes out, showing that there is high pressure (compression) inside the phloem (this is how maple syrup is tapped).
- If the xylem is punctured then air is sucked in, showing that there is low pressure (tension) inside the xylem.
how can you investigate transport using radioactive tracers?
a plant is supplied with 14C, which becomes incorporated into organic substances in the plant.
The plant is then frozen in liquid nitrogen to kill and fix it quickly, and placed onto photographic film in the dark. The resulting autoradiograph shows the location of compounds containing 14C.
this shows that organic compounds (presumably sugars) are transported downwards from the leaf to the roots.
define genome
the complete set of genes in a cell
define proteome
the full range of proteins that a cell is able to produce
describe the process of transcription
DNA unwinds and separates (H bond between bases broken)
template strands form due to exposed bases
free RNA nucleotides align with template strand through complimentary base pairing
RNA polymerase bonds the nucleotides together, forming a molecule of mRNA complimentary to the DNA
mRNA leaves nucleus via nuclear pore
splicing occurs- introns removed
describe the process of translation
- mRNA associates with a ribosome in the cytoplasm
- tRNA molecules with complimentary anticodons align with the first 2 mRNA codons
- peptide bond forms between the amino acid molecules
- the tRNA that bound to the first codon is free to leave the ribosome
- the ribosome moves along the mRNA by one codon and now a new tRNA with a complimentary anticodon enters the ribosome
- this continues until the ribosome reaches a stop codon
the 2 mechanisms that mean that meiosis will result in genetic variation
independent segregation
crossing over
what happens during independent segregation?
during meiosis 1
each chromosome aligns with its homologous partner at the equator of the cell
the combination of maternal/paternal chromosomes aligns randomly
one chromosome of each pair goes to daughter cell- by chance
