Outline the hormonal and nervous mechanisms involved in the control of
heart rate
 Adrenaline increases heart rate
 Cardiovascular centre in medulla oblangata in the brain has a nervous
connection to the sino atrial node of the heart
 This controls the frequency of the waves of excitation across the heart
 The vagus parasympathetic nerve decreases heart rate
 The accelerator sympathetic nerve increases heart rate
 High blood pressure is detected by baroreceptors
 Low blood pH caused by high levels of CO2 detected by
chemoreceptors in aorta and carotid arteries
Describe how the medulla oblongata responds to an increase in CO2
concentration in the blood during exercise
 Carbon dioxide causes an increase in H+ concentration and therefore a
lower pH
 Chemoreceptors in medulla oblongata, carotid arteries and aorta monitor
blood pH
 Heart rate is increased – heart beats faster
 Increased frequency of impulses down the sympathetic accelerator nerve
 Increased frequency of impulses to the sino-atrial node causes increased
frequency of waves of excitation across heart
 Impulses to diaphragm and intercostal muscles causing deeper breathing
(increased tidal volume) and faster breathing
 Heart contracts more strongly – increased stroke volume
 Faster removal of CO2 so blood CO2 levels fall to normal level
 Negative feedback mechanism to maintain homeostasis
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Outline how impulses are transmitted from receptor to effector
Sensory receptors detect a change in environment
If the receptor has a generator potential greater than the threshold
potential, the depolarisation will reach +40mV and an action potential will
be transmitted
A sensory neurone transmits the impulse from the receptor to the CNS
Impulse travels through dorsal root ganglion into spinal chord
 Intermediate relay neurone connects sensory neurone to motor neurone
 At the synapse between neurones, calcium ions diffuse into presynaptic
neurone. Vesicles containing neurotransmitter acetylcholine fuse with
plasma membrane and release acetylcholine into synaptic cleft by
exocytosis. Neurotransmitter binds to specific complementary receptors
on post synaptic membrane. Sodium ions then enter the post synaptic
neurone allowing for its depolarisation
 Impulse travels through ventral root and along axon of motor neurone
 Motor neurone carries impulse to effector, connected via the motor end
plate.
Describe how an impulse is transmitted across a synapse
 Action potential causes calcium ion channels in presynaptic neurone
membrane to open
 Calcium ions diffuse into presynaptic knob through calcium ion channels
 Calcium ions causes vesicles containing neurotransmitter acetylcholine to
fuse with presynaptic membrane
 Acetylcholine is released into the cleft by exocytosis
 Acetylcholine diffuses across the cleft to the post synaptic membrane
 Binds to receptor proteins on post synaptic membrane
 Sodium ions channels in post synaptic membrane open
 Sodium ions diffuse in
 If enough sodium ions diffuse in to reach the threshold potential of 50mV, voltage gated sodium channels open and an action potential is
generated
Describe how insulin is secreted into the blood
 Beta cells of the Islets of Langerhans in the pancreas secrete insulin
 Cell membranes of Beta cells contain potassium and calcium ion channels
 K+ channels usually open, so K+ diffuses out of the cell, making the inside
of the cell negative (-70mV)
 Glucose levels around Beta cells are high, so glucose diffuses into the cell.
As it enters, the glucose is phosphorylated and metabolised to form ATP.
 K+ channels are sensitive to the amount of ATP in the cell
 As intracellular ATP concentration rises, K+ channels close
 K+ cannot diffuse out, so cell potential increases. Change in potential
difference causes calcium channels to open.
 Calcium ions diffuse in, causing vesicles containing insulin to fuse with the
plasma membrane, releasing the insulin by exocytosis.
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Describe how amino acids are excreted
Takes place in liver cells – hepatocytes
Deamination removes the amine group of the amino acid. This produces
ammonia and a keto acid. The keto acid can be used in respiration to
release its energy. The ammonia is highly toxic and highly soluble, so it
must enter the Ornithine cycle.
Ornithine cycle converts ammonia to urea:
2NH 3
+ CO2  CO(NH2) 2 + H20
Urea is less toxic than ammonia. It can be passed into the blood stream
and transported to the kidneys
In the kidneys, urea is filtered out of the blood and concentrated in the
urine
Explain how ultrafiltration takes place in the kidney
Blood arrives at glomerulus in wide afferent arteriole
Blood leaves glomerulus in narrow efferent arteriole
The difference in the diameters of the arterioles, the efferent being
narrower than the afferent, creates a high hydrostatic pressure in the
glomerulus
A 3 layered barrier exists between the capillary and the lumen of the
Bowman’s capsule:
o Endothelium of blood capillary – narrow gaps between
squamous epithelial cells through which blood plasma can pass
o Basement membrane – fine mesh of collagen and glycoproteins.
This acts as a filter to prevent passage of molecules with a
molecular mass greater than 69000. This means that most
proteins and all blood cells are held in the capillaries
o Podocytes – epithelial cells of Bowman’s capsule have finger like
projections called major processes that ensure there are gaps
between cells
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Explain how the composition of fluid in the proximal convoluted tubule
changes
Selective reabsorption occurs in the proximal convoluted tubule
Sodium ions are actively transported out of the cells of the PCT wall using
a sodium-potassium pump. This creates a low Na+ concentration in the
cell
Na+ ions then enter the cell from the PCT lumen using cotransporter
proteins. The cotransporter protein is able to bring in glucose or amino
acids along with the Na+ ions. This is facilitated diffusion.
Glucose and amino acids are able to diffuse out of the opposite side of the
cell into the tissue fluid. From there, they can diffuse into the blood.
Water is reabsorbed by osmosis: movement of ions/glucose into the cell
creates a low water potential in the cell and a high water potential in the
tubule fluid, so water will move down the water potential gradient into
the cell
Some urea is reabsorbed by diffusion. Urea becomes more concentrated
in the proximal convoluted tubule as water is reabsorbed
Describe how adrenaline brings about a change in its target organs. Give
examples of the effects of adrenaline.
Adrenaline is synthesised from amino acids and is therefore not lipid
soluble. Its target cells have receptors on their plasma membranes to
which adrenaline can bind
Adrenaline molecule attaches to specific complementary receptor on
outside of plasma membrane
Binding of adrenaline with its receptor alters the shape of the receptor
The binding of adrenaline, the 1st messenger, activates the enzyme
adenyl cyclase
Adenyl cyclase converts ATP to cAMP. cAMP is the 2nd messenger and can
go on to activate other enzymes inside the cell
Effects of adrenaline include relaxing smooth muscle in bronchioles,
increasing stroke volume and heart rate, causing vasoconstriction of blood
vessels to digestive system and vasodilation of blood vessels to brain and
skeletal muscles, stimulating glycogenolysis in hepatocytes
Describe the homeostatic mechanism that regulates blood glucose
If blood glucose is too high:
 Detected by β cells in Islets of Langerhans in pancreas
 β cells produce insulin; α cells stop secreting glucagon
 Insulin targets liver and muscle cells
 Muscle cells absorb more glucose
 Increased respiration of glucose
 Increased conversion of glucose to glycogen (glycogen synthase
enzyme)
 Blood glucose levels decrease
If blood glucose too low:
 Detected by α cells of Islets of Langerhans in pancreas
 Α cells secrete glucagon; β cells stop producing insulin
 Glucagon targets liver cells only
 Increased conversion of glycogen to glucose – glycogenolysis
 Increased rate of production of glucose from other substances –
gluconeogenesis
 Decreased respiration of glucose – increased respiration of other
substances like fatty acids
 Blood glucose levels increase
Describe the sequence of events that results in the water potential of
blood remaining constant
 Osmoregulation is the control of water potential of blood
 When water potential of blood is low, water must be conserved.
Collecting duct wall must become more permeable, more water must be
reabsorbed and less urine produced.
 Osmoreceptros in the hypothalamus detect low water potential of blood.
When water potential is low, osmoreceptros lose water by osmosis. This
causes them to shrink and stimulates Neurosecretory cells in the
hypothalamus.
 Neurosecretory cells produce and secrete ADH. ADH flows down axon to
the terminal bulb in the posterior pituitary gland. ADH is released into the
blood from the posterior pituitary gland.
 ADH binds to specific receptors in the membranes of collecting duct cells.
Binding of ADH causes a series of enzyme controlled reactions.
 Vesicles containing aquaporins fuse with the membranes of collecting
duct cells. The collecting duct cells have an increased permeability to
water.
 More water is reabsorbed by osmosis and water potential of blood
increases
 If water potential rises too high, less ADH is released and the plasma
membrane folds inwards to create new vesicles that remove aquaporins
from the membrane. Walls of collecting duct are made less permeable,
less water is reabsorbed and more water is lost from the body, decreasing
water potential of blood.
Describe how the light energy absorbed in photosystems is converted
into chemical energy in the light dependent stage of photosynthesis
 Non cyclic photophosphorylation takes place on thylakoid membranes
of chloroplasts. Both photosystem I (p700) and photosystem II (p680)
are involved
 Light energy causes a pair of excited electrons to be emitted from PSII.
Electrons are accepted by electron acceptor molecule. Electrons are
passed down a series of electron carriers.
 Energy is released as electrons passed down electron transport chain:
energy is used to pump H+ across thylakoid membrane into thylakoid
space. This creates a proton gradient. The H+ move down their
concentration gradient back into the stroma through ATPsynthase.
This is chemiosmosis and forms ATP.
 Electrons from PSII passed to PSI
 Photolysis occurs at PSII: H2O split into protons, electrons and oxygen
 Photosystem I also loses excited electrons. These are accepted, along
with the H+ produced by photolysis by NADP which becomes reduced
NADP.
 Electrons from photolysis replace those lost by PSI
 Cyclic photophosphorylation involves PSI only. Electrons are released
and returned to PSI
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Describe the main features of the Calvin cycle
Calvin Cycle occurs in stroma of chloroplast
Rubisco enzyme combines ribulose bisphosphate with carbon dioxide –
this is carboxylation
Forms an unstable 6 carbon intermediate
6 carbon intermediate splits into 2 molecules of 3 carbon glycerate-3phosphate
ATP and reduced NADP from light dependent reaction are used to form
triose phosphate
1/6 triose phosphate used to create glucose, hexose sugars, lipids
5/6 of triose phosphate used to regenerate ribulose bisphosphate
NAD, FAD and NADP are important molecules in plant cells. Describe the
role of these molecules within a palisade mesophyll cell.
NAD and FAD are involved in respiration
2 molecules of reduced NAD are formed in glycolysis when triose
phosphate is oxidised to pyruvate
1 molecule of reduced NAD is produced in Link reaction, when pyruvate
undergoes dehydrogenation to form acetate
Krebs cycle produces 3 reduced NAD and 1 reduced FAD per turn
The reduced NAD and FAD carry hydrogens to the electron transport chain
on the cristae of the mitochondria.
NADP is involved in photosynthesis
NADP is reduced in non-cyclic photophosphorylation
Hydrogens that reduce the NADP come from photolysis of water
Reduced NADP is then used in Calvin cycle when glycerate-3-phosphate is
reduced to triose phosphate.