Biology HL IB Section B, 8 marker questions (part a)

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Blood plasma, glomerular filtrate and urine have different concentrations of solutes, such as glucose, protein and urea. Explain the processes occurring in the kidney that cause differences in the concentration of these solutes between blood plasma, glomerular filtrate and urine. (8)

- (filtrate formed by) ultrafiltration;
- glucose / amino acids / soluble components enter Bowman's capsule;
- proteins in blood plasma but not in filtrate / proteins not filtered out (of blood);
- glucose not in urine (normally);
- (selective) reabsorption (of glucose); in the proximal convoluted tubule; by active transport / microvilli increase surface area;
- little/no urea reabsorbed; means concentration increases / urea more concentrated in urine than in blood plasma;
- water reabsorbed from filtrate; by osmosis; in descending limb of nephron / in proximal convoluted tubule;
- salts actively transported into the medulla (from filtrate); creating concentration gradient/hypertonic medulla;
- collecting duct permeability altered depending on blood solute concentration.

Photosynthesis and transpiration occur in leaves. Explain how temperature affects these processes.

- photosynthesis rate increases as temperature rises (up to an optimum temperature); due to increased rate of enzyme catalysed reactions/light independent reactions/the Calvin cycle;
- (steep) drop in rate of photosynthesis above the optimum; as high temperatures makes enzymes/Rubisco/RuBP carboxylase denature;
- graph with correctly labelled axes showing relationship between temperature and rate of photosynthesis;
- transpiration rate increases as temperature rises;
- energy/heat leads to more evaporation of water (in the leaf);
- faster diffusion of water vapour at higher temperatures;
- relative humidity falls as temperature rises / warmer air can hold more water vapour;
- stomata may close at very high temperatures reducing the transpiration rate;
- some plants open their stomata at very high temperatures to cool by transpiration.

Translation occurs in living cells. Explain how translation is carried out, from the initiation stage onwards. (9)

- translation involves initiation, elongation/translocation and termination;
- mRNA binds to the small sub-unit of the ribosome;
- ribosome slides along mRNA to the start codon;
- anticodon of tRNA pairs with codon on mRNA;
- complementary base pairing (between codon + anticodon);
- (anticodon of) tRNA with methionine pairs with start codon / AUG is the start codon;
- second tRNA pairs with next codon;
- peptide bond forms between amino acids;
- ribosome moves along the mRNA by one codon;
- movement in 5 to 3 direction;
- tRNA that has lost its amino acid detaches;
- another tRNA pairs with the next codon/moves into A site;
- tRNA activating enzymes; link amino acids to specific tRNA;
- stop codon (eventually) reached.

Explain the methods and aims of DNA profiling. (8)

METHOD:
- DNA sample obtained; from hair/blood/semen/human tissue;
- DNA amplified / quantities of DNA increased by PCR/polymerase chain reaction;
- satellite DNA/highly repetitive sequences are used/amplified;
- DNA cut into fragments; using restriction enzymes;
- gel electrophoresis is used to separate by size;
- number of repeats varies between individuals / pattern of bands is unique to the individual;
AIMS:
- forensic use / crime scene investigation;
- e.g. DNA obtained from the crime scene/victim compared to DNA of suspect;
- paternity testing use e.g. DNA obtained from parents in paternity cases;
- biological father if one half of all bands in the child are found in the father;
- genetic screening;
- presence of particular bands correlates with probability of certain phenotype / allele.

Explain the role of auxin in phototropism. (8)

- auxin is a plant hormone; produced by the tip of the stem/shoot tip;
- it causes transport of hydrogen ions from the cytoplasm to the cell wall;
- leads to a decrease in pH / H+ pumping breaks bonds between cell wall fibres;
- makes cell walls flexible/extensible/plastic/softens cell walls;
- auxin makes cells enlarge/grow;
- gene expression also altered by auxin to promote cell growth;
- (positive) phototropism is growth towards light;
- shoot tip senses direction of (brightest) light;
- auxin moved to side of stem with least light/darker side causes cells on dark side to elongate/cells on dark side grow faster.

Explain why diabetes could be detected through the analysis of urine. (8)

- urine of diabetics contains glucose; whereas urine of non-diabetics contains no glucose;
- glomerular filtrate contains glucose / glucose filtered out;
- glucose (normally) reabsorbed from filtrate/into blood; through wall of / in the proximal convoluted tubules;
- blood glucose concentration higher than normal in diabetics;
- reabsorption not completed / pumps cannot reabsorb all glucose in diabetics;
- glucose in urine can be detected using test strips;
- type I diabetes is lack of insulin secretion / lack of β cells;
- type II diabetes is body cells not responding to insulin / not absorbing glucose.

Explain how an error in meiosis can lead to Down syndrome. (8)

- non-disjunction;
- chromosomes/chromatids do not separate / go to same pole;
- non-separation of (homologous) chromosomes during anaphase I; due to incorrect spindle attachment;
- non-separation of chromatids during anaphase II; due to centromeres not dividing;
- occurs during gamete/sperm/egg formation;
- less common in sperm than egg formation / function of parents' age;
- Down syndrome due to extra chromosome 21;
- sperm/egg/gamete receives two chromosomes of same type;
- zygote/offspring with three chromosomes of same type / trisomy / total of 47 chromosomes.

Antibiotic resistance in bacteria is an example of evolution in response to environmental change. Using another example, explain how an environmental change can lead to evolution. (8)

- natural selection (in correct context);
- better-adapted individuals survive/more likely to survive;
- more reproduction/genes passed on by better adapted individuals;
- e.g. great tit;
- bird that lays its eggs in spring;
- global warming/climate change;
- more caterpillars (on trees) in early spring;
- laying eggs earlier in spring;
- time of egg laying is (partly) genetically controlled;
- eggs laid early hatch at start of period of greatest food abundance;
- more young can be fed/young grow faster/fewer deaths.

Describe how water is carried by the transpiration stream. (7)

- transpiration is water loss (from plant) by evaporation;
- flow of water through xylem from roots to leaves is the transpiration stream;
- evaporation from spongy mesophyll cells;
- water is replaced by osmosis from the xylem;
- (diffusion of water vapour) through stomata;
- water lost replaced from xylem; through cell walls to air space;
- water pulled out of xylem creates suction/low pressure/tension results in transpiration pull;
- water molecules stick together/are cohesive; due to hydrogen bonding/polarity of water molecules;
- xylem vessels are thin (hollow) tubes;
- adhesion between water and xylem due to polarity of water molecules; creates continuous column/transpiration stream.

Explain how flowering is controlled in long-day and short-day plants. (7)

- flowering affected by light; phytochrome;
- exists in two (interconvertible) forms/Pfr and Pr;
- Pr (red absorbing/660nm) converted to Pfr (far-red/730nm absorbing) in red or day light;
- sunlight contains more red than far red light so Pfr predominates during the day;
- gradual reversion of Pfr to Pr occurs in darkness;
- Pfr is active form / Pr is inactive form;
- in long-day plants, flowering induced by dark periods shorter than a critical length / occurs when day is longer than a critical length;
- enough Pfr remains in long-day plants at end of short nights to stimulate flowering;
- Pfr acts as promoter of flowering in long-day plants;
- short-day plants induced to flower by dark periods longer than a critical length/days shorter than a critical value;
- at end of long nights enough Pfr has been converted to Pr to allow flowering to occur;
- Pfr acts as inhibitor of flowering in short-day plants.

Explain the effect of light intensity and temperature on the rate of photosynthesis. (8)

- both light and temperature can be limiting factors;
- other factors can be limiting;
- graph showing increase and plateau with increasing light / description of this;
- graph showing increase and decrease with increasing temperature / description of this;
LIGHT:
- affects the light-dependant stage;
- at low intensities, insufficient ATP (produced from chemiosmosis);
- and insufficient NADPH produced; stops the Calvin cycle operating (at maximum rate)
TEMPERATURE:
- affects light-independent stage / Calvin cycle;
- temperature affects enzyme activity;
- less active at low temperatures / maximum rate at high temperatures;
- but will then be denatured (as temperature rises further).

Explain the function and structure of the placenta. (8)

- transfer of foods/nutrients/glucose from mother to fetus;
- fetal gas exchange/transfer of oxygen from mother to fetus;
- transfer of excretory products/CO2 from fetus to mother;
- secretion of estrogen/progesterone; from approximately 12 weeks / when ovary/corpus luteum stops secretion;
- it's a disc shaped structure; connected to the fetus by an umbilical cord;
- embryonic tissue invades/grows into the uterine wall;
- placental villi increase the surface area (for exchange);
- fetal capillaries in placenta/placental villi;
- inter-villus spaces/sinuses through which mother's blood flows;
- small distance between fetal and mother's blood/narrow placental barrier.

Explain prokaryotic DNA replication. (8)

- DNA replication is semi-conservative / each strand of DNA acts as template;
- DNA helicase separates two strands/forms a replication fork;
- new strand built / nucleotides added in a 5' to 3' direction;
- (deoxy)nucleoside triphosphates hydrolysed to provide energy for nucleotide formation/base pairing;
- on one strand, DNA polymerase III builds continuous strand;
- on other strand, short chains of DNA/Okazaki fragments are formed;
- each short chain starts with RNA primer; added by RNA primase;
- remainder of DNA chain is built by DNA polymerase III;
- DNA polymerase I removes RNA primer and replaces it by DNA;
- DNA ligase joins DNA fragments together forming a complete strand;
- replication only occurs at a single replication fork.

Discuss the benefits and risks associated with vaccination programs. (8)

BENEFITS:
- immunity results can limit pandemics/epidemics/spread of diseases;
- diseases can be eradicated/smallpox eliminated;
- reduces mortality/deaths due to disease;
- can protect vulnerable groups/young/old/week;
- decreases crippling effects of diseases (such as polio);
- decreased health care costs;
RISKS:
- may produce (mild) symptoms of the disease;
- human error in preparation/storage/administration of vaccine;
- individual may react badly to vaccine / defective immune system / hypersensitive/allergic reaction;
- immunity may not be life-long / booster required;
- possible toxic effects of mercury-based preservatives.

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