Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
Blood capillaries are very close to the outer surface of the
tubule. The blood in these capillaries has come directly
from the glomerulus, so it has much less plasma in it than
usual and has lost much of its water and many of the ions
and other small solutes.
The basal membranes of the cells lining the proximal
convoluted tubule are those nearest the blood capillaries.
Sodium–potassium pumps in these membranes move
sodium ions out of the cells (Figure 14.12). The sodium
ions are carried away in the blood. This lowers the
concentration of sodium ions inside the cell, so that
they passively diffuse into it, down their concentration
gradient, from the fluid in the lumen of the tubule.
However, sodium ions do not diffuse freely through
the membrane: they can only enter through special
co-transporter proteins in the membrane. There are several
different kinds of co-transporter protein, each of which
transports something else, such as a glucose molecule or an
amino acid, at the same time as the sodium ion.
The passive movement of sodium ions into the cell
down their concentration gradient provides the energy
to move glucose molecules, even against a concentration
gradient. This movement of glucose, and of other solutes, is
an example of indirect or secondary active transport, since
the energy (as ATP) is used in the pumping of sodium
ions, not in moving these solutes. Once inside the cell,
glucose diffuses down its concentration gradient, through
a transport protein in the basal membrane, into the blood.
All of the glucose in the glomerular filtrate is
transported out of the proximal convoluted tubule and
into the blood. Normally, no glucose is left in the filtrate,
so no glucose is present in urine. Similarly, amino acids,
vitamins, and many sodium and chloride ions (Cl − ) are
reabsorbed in the proximal convoluted tubule.
The removal of these solutes from the filtrate greatly
increases its water potential. The movement of solutes
into the cells and then into the blood decreases the
water potential there, so a water potential gradient exists
between filtrate and blood. Water moves down this
gradient through the cells and into the blood. The water
and reabsorbed solutes are carried away, back into the
circulation.
Surprisingly, quite a lot of urea is reabsorbed too.
Urea is a small molecule which passes easily through cell
membranes. Its concentration in the filtrate is considerably
higher than that in the capillaries, so it diffuses passively
through the cells of the proximal convoluted tubule and
into the blood. About half of the urea in the filtrate is
reabsorbed in this way.
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Key
active
passive
blood
plasma
endothelium
of capillary
basement
membrance
proximal convoluted
tubule cell
proximal
tubule lumen
mitochondria
nucleus
1 Na + –K + pumps in the basal
membrane of proximal convoluted tubule
cells use ATP made by the mitochondria.
These pumps decrease the concentration of
sodium ions in the cytoplasm. The basal
membrane is folded to give a large surface
area for many of these carrier proteins.
Na +
ADP +
P i
ATP
K +
glucose
and amino acids
Na +
glucose
and
amino
acids
2 Very close nearby, the blood 3 Microvilli increase surface area, helping
plasma rapidly removes absorbed
Na + , Cl – , glucose and amino
acids. This helps further uptake
from the lumen of the tubule.
uptake of solutes. Na + moves passively
into the cell down its concentration
gradient. It moves in using protein
co-transporter molecules in the membrane,
which bring in glucose and amino acids
at the same time.
Figure 14.12 Reabsorption in the proximal convoluted tubule.