HUMAN PHYSIOLOGY:
11.1.4: Explain antibody production.
Actual question:Explain the production of antibodies.
Actual answer
antigens stimulate an immune response;
antibodies are produced in response to specific antigens;
antibodies are made by B-cells/lymphocytes/plasma cells;
antigen is engulfed by macrophages;
antigen is presented on macrophage membrane;
helper T-cells bind to antigen (on macrophage);
helper T-cells are activated;
helper T-cells activate B-cells;
B-cells clone;
into plasma cells and memory cells;
plasma cells produce specific antibodies to the antigen;
memory cells for long-term immunity;
a faster/stronger response later;
11.2.7: Explain how skeletal muscle contracts, including the release of calcium ions from
the sarcoplasmic reticulum, the formation of cross-bridges, the sliding of actin and myosin
filaments, and the use of ATP to break cross-bridges and re-set myosin heads.
Actual question: Explain how skeletal muscle contracts.
Actual answer
muscles/fibres/myofibrils contain (repeating) units called sarcomeres;
muscle/sarcomeres contain actin filaments and myosin filaments;
actin fibres are thin and myosin fibres are thick;
arriving action potential causes release of Ca2+ ;
from sarcoplasmic/endoplasmic reticulum;
Ca2+ binds to troponin;
causing troponin and tropomyosin to move (on actin); NOTE: troponin and tropomysin have been
removed from the current syllabus, they were required in the previous syllabus.
exposing binding sites on actin / for myosin;
ATP binds to myosin heads releasing them / breaking cross bridges;
ATP hydrolysed / split into i ADP + P ;
ATP/energy causes myosin heads to change shape / swivel / become cocked;
myosin heads bind / form cross-bridges to (exposed) actin binding sites;
myosin heads swivel / move actin (releasing i ADP + P );
myosin filaments move actin filaments towards centre of sarcomere;
sliding of filaments / actin and myosin shortens the sarcomere;
11.3.6: Explain the reabsorption of glucose, water and salts in the proximal convoluted
tubule, including the roles of microvilli, osmosis and active transport.
From Wiki
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From the Bowman’s capsule, the glomerular filtrate passes into the proximal convoluted tubule for
selective reabsorption.
In selective reabsorption, diffusion and active transport return molecules to the blood of the
peritubular capillaries from the proximal convoluted tubule.
The cells lining the proximal convoluted tubule are adapted for active reabsorption.
The cells of the proximal convoluted tubule have numerous microvilli, each about 1 um in length,
which increase the surface area for reabsorption.
In addition, the cells of the proximal convoluted tubule contain numerous mitochondria, which
produce the energy needed for active transport.
Reabsorption by active transport is selective because only certain molecules are recognized by
carrier molecules and actively reabsorbed.
Sodium and glucose are recognized by carrier molecules and returned to the blood by active
transport.
Negatively charged chloride ions and nutrients follow passively (facilitated diffusion) after the
positively charged sodium ions, and water follows these solutes passively by osmosis.
The substances that are not reabsorbed (some water, excess salts, urea) become the tubular fluid,
which enters the loop of Henle.
11.3.7; Explain the roles of the loop of Henle, medulla, collecting duct and ADH
(vasopressin) in maintaining the water balance of the blood.
From Wiki
The medulla & the loop of Henle
 The loop of Henle descends into the renal medulla. The fluid of the medulla is called the interstitial
fluid.
 Where the loop of Henle makes its turn, the interstitial fluid is salty. Thus the medulla creates a
concentration gradient for the tubular fluid, causing water to move out of the descending limb of
the loop of Henle as it approaches the turn.
 The fluid left behind (inside the loop of Henle) becomes increasingly salty until it matches the
interstitial fluid. At this point, no more water can leave the loop of Henle.
 As the tubular fluid moves up the ascending loop of Henle, sodium diffuses out through the thin
portion of the loop of Henle and is actively transported out of the thick portion. Water remains
behind because the ascending loop of Henle is not permeable to water.
The collecting duct
 Cells in the walls of the collecting duct have receptors for antidiuretic hormone (ADH).
 When water must be conserved, the hypothalamus triggers secretion of ADH from the pituitary
gland. This hormone makes the walls of the collecting duct more permeable to water. More water is,
therefore, reabsorbed, so the urine becomes more concentrated.
 When the body must lose excess water, ADH secretion is inhibited, the walls remain impermeable to
water, less water is reabsorbed, and the urine remains dilute. Alcohol inhibits ADH secretion causing
cells to dehydrate. If too much alcohol is consumed then dehydration can be severe and cause a
‘hang-over’.
11.3.8; Explain the differences in the concentration of proteins, glucose and urea between
blood plasma, glomerular filtrate and urine.
Actual question:Blood plasma, glomerular filtrate and urine have different concentrations of
solutes, suchas glucose, protein and urea. Explain the processes occurring in the kidney that
causedifferences in the concentrations of these solutes between blood plasma, glomerular filtrate
and urine.
Actual answer
(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 the surface area;
little/no urea reabsorbed
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;
PLANTS
9.1.7: Explain the role of auxin in phototropism as an example of plant growth
Actual IB mark scheme for this question from a previous year’s exam
auxin is a plant hormone;
produced by the tip of the stem/shoot tip;
causes transport of hydrogen ions from cytoplasm to cell wall;
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;
9.2.3: Explain mineral ion absorption from the soil into roots by active transport
Actual IB mark scheme from a previous year’s exam
mineral ions are absorbed by active transport;
large surface area;
branching (increases surface area);
root hairs;
root hair cells have carrier protein/ion pumps (in their plasma membrane);
(many) mitochondria in root (hair) cells;
to provide ATP for active transport;
connections with fungi in the soil/fungal hyphae;
9.2.6: Explain how water is carried by the transpiration stream
(b)
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;
replaced by osmosis from the xylem;
(diffusion of water vapour) through stomata;
water lost replaced from xylem / clear diagram showing movement of
water from xylem through cell(s) (walls) to air space;
water pulled out of xylem creates suction/low pressure/tension;
transpiration pull results;
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;
7 max
9.3.4: Explain the conditions needed for the germination of a typical seed
water needed to rehydrate the seed;
gibberellin released / active after water absorbed;
gibberellin needed to produce amylase;
water needed to allow substances inside the seedling to be transported;
oxygen needed for (aerobic) cell respiration;
warmth needed to speed up metabolism/enzyme activity;
warmth indicates that it is a favourable season for germination/spring;
some seeds need a cold period to stimulate germination;
some seeds need fire to stimulate germination;
some seeds need to pass through an animal (gut) to stimulate germination;