Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
Biology (9700) notes A-Level - Homeostasis (CIE)
Homeostasis in mammals
● Homeostasis: It is the regulation of the internal structures of
a cell or organism to maintain optimum temperature for
function in response to internal and external changes.
● Principle of homeostasis: Receptor to detect a stimulus that is
involved with a condition. Coordination system to transfer
A2
information between different parts of the body. Effector to
carry out a response. Negative feedback-continuously
monitored. Internal and external stimuli→ Receptor detects a
stimulus that is involved with the condition → Receptor sends
s
info through the nervous system to central control in the brain
am
or the spinal cord → Central control instructs an effector to
carry out an action → the factor (stimulus) is continuously
monitored by receptors so that it fluctuates around a set
Ex
point.
● Urea is produced in the liver from the deamination of excess
amino acids. Amino group removed from each amino acid is
ck
called deamination. ( Ammonia + carbon dioxide → urea)
● Structure of human kidney: Fibrous capsule is the fairly tough
outer layer. Cortex contains the glomerulus, Bowman’s capsule,
ra
Proximal Convoluted Tubule and Distal Convoluted Tubule of
nephrons. Medulla contains the Loop of Henle and Collecting
C
duct of nephrons. Renal pelvis is where the ureter joins the
kidney. Renal artery → oxygenated blood to kidneys. Renal
vein→ deoxygenated blood away from the kidney. Ureter →
urine from kidneys to bladder. ( You should be able to identify:
Glomerulus, Bowman’s Capsule, Proximal Convoluted Tubule,
Distal Convoluted Tubule, Loop of Henle and the collecting duct
in pictures)
CRACK EXAMS A2 1
, Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
● Ultrafiltration takes place in the Bowman’s capsule; amino
acids, water, glucose, urea and inorganic materials are filtered
out of the blood capillaries of the filtrate.
● Selective reabsorption occurs in the Proximal Convoluted
Tubule; useful molecules are taken back (reabsorbed) from the
filtrate and returned to the blood as the filtrate flows along
the nephrons.
● Bowman’s Capsule is where the ultrafiltration occurs.
A2
Endothelium of capillary → squamous epithelial cells which have
thousands of membrane bound pores → small molecules pass
through. Basement membrane →collagen fibres and
glycoproteins→ mesh that has tiny spaces so that the large
s
proteins do not pass through.
am
● Afferent arteriole is wider than efferent arteriole so blood
pressure is higher in glomerular capillaries which results in
higher water potential in blood plasma in the glomerular
Ex
capillaries so the water moves down the water potential
gradient, in the glomerular capillaries. (High water potential in
Capsule tends to move the filtrate into the glomerulus)
ck
● Selective reabsorption: Single layer of epithelial cells →
microvilli (increased surface area for reabsorption),
cotransport protein (each transports a specific sol),
ra
mitochondria (energy for sodium and potassium pumps), tightly
packed (so fluid can pass through).
C
● Sodium and Potassium pumps; pump sodium ions out of the cell,
they enter the blood and move with blood flow. It results in
less concentration of sodium ions in cytoplasm. Sodium ions
enter from lumen into cells along with glucose through
cotransporters. Glucose enters against the concentration
gradient using energy from sodium ions, water potential in the
cell decreases. Water moves in the cell. Sodium ions are again
CRACK EXAMS A2 2
Biology (9700) notes A-Level - Homeostasis (CIE)
Homeostasis in mammals
● Homeostasis: It is the regulation of the internal structures of
a cell or organism to maintain optimum temperature for
function in response to internal and external changes.
● Principle of homeostasis: Receptor to detect a stimulus that is
involved with a condition. Coordination system to transfer
A2
information between different parts of the body. Effector to
carry out a response. Negative feedback-continuously
monitored. Internal and external stimuli→ Receptor detects a
stimulus that is involved with the condition → Receptor sends
s
info through the nervous system to central control in the brain
am
or the spinal cord → Central control instructs an effector to
carry out an action → the factor (stimulus) is continuously
monitored by receptors so that it fluctuates around a set
Ex
point.
● Urea is produced in the liver from the deamination of excess
amino acids. Amino group removed from each amino acid is
ck
called deamination. ( Ammonia + carbon dioxide → urea)
● Structure of human kidney: Fibrous capsule is the fairly tough
outer layer. Cortex contains the glomerulus, Bowman’s capsule,
ra
Proximal Convoluted Tubule and Distal Convoluted Tubule of
nephrons. Medulla contains the Loop of Henle and Collecting
C
duct of nephrons. Renal pelvis is where the ureter joins the
kidney. Renal artery → oxygenated blood to kidneys. Renal
vein→ deoxygenated blood away from the kidney. Ureter →
urine from kidneys to bladder. ( You should be able to identify:
Glomerulus, Bowman’s Capsule, Proximal Convoluted Tubule,
Distal Convoluted Tubule, Loop of Henle and the collecting duct
in pictures)
CRACK EXAMS A2 1
, Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
● Ultrafiltration takes place in the Bowman’s capsule; amino
acids, water, glucose, urea and inorganic materials are filtered
out of the blood capillaries of the filtrate.
● Selective reabsorption occurs in the Proximal Convoluted
Tubule; useful molecules are taken back (reabsorbed) from the
filtrate and returned to the blood as the filtrate flows along
the nephrons.
● Bowman’s Capsule is where the ultrafiltration occurs.
A2
Endothelium of capillary → squamous epithelial cells which have
thousands of membrane bound pores → small molecules pass
through. Basement membrane →collagen fibres and
glycoproteins→ mesh that has tiny spaces so that the large
s
proteins do not pass through.
am
● Afferent arteriole is wider than efferent arteriole so blood
pressure is higher in glomerular capillaries which results in
higher water potential in blood plasma in the glomerular
Ex
capillaries so the water moves down the water potential
gradient, in the glomerular capillaries. (High water potential in
Capsule tends to move the filtrate into the glomerulus)
ck
● Selective reabsorption: Single layer of epithelial cells →
microvilli (increased surface area for reabsorption),
cotransport protein (each transports a specific sol),
ra
mitochondria (energy for sodium and potassium pumps), tightly
packed (so fluid can pass through).
C
● Sodium and Potassium pumps; pump sodium ions out of the cell,
they enter the blood and move with blood flow. It results in
less concentration of sodium ions in cytoplasm. Sodium ions
enter from lumen into cells along with glucose through
cotransporters. Glucose enters against the concentration
gradient using energy from sodium ions, water potential in the
cell decreases. Water moves in the cell. Sodium ions are again
CRACK EXAMS A2 2
