Biology 12

MINI LAB

The Effect of Salt Concentration

Common table salt (NaCl) is an important component of
cells and the fluids that surround them in the body. Life
evolved in the salt water environment of the ocean and
salt plays an important role in cellular function. The
concentration of salt affects osmosis or the movement
of water in the body’s cells. This movement can be
demonstrated in other cells, for example, in those of an
onion. In this MiniLab, you will use a piece of coloured
onion to observe the effect of increased salt concentration.
Make a wet mount slide of a thin layer of red onion skin.
Draw a diagram of two of the cells that you can see at 100x
or 400x. Label the cell wall, cell membrane, cytoplasm, and
nucleus (if visible). This usually works better if you use the

microscope’s diaphragm to decrease the amount of light.

Lift the cover slip and put two drops of saturated salt (NaCl)

solution on the onion cells. After two minutes, examine the

onion cells again.

CAUTION: Handle the microscope slides and cover

slips carefully. Wash your hands after completing

the MiniLab.

Analyze

1.Draw two of the cells that have changed. Label the cell

wall, cell membrane, cytoplasm, and nucleus.

2.Explain why the cytoplasm changed in the presence of

the salt solution.

Chapter 4 Homeostatic Mechanisms • MHR 115

process by which materials required by the body
are removed from the filtrate and returned to the
bloodstream. Osmosis, diffusion, and active
transport draw water, glucose, amino acids, and ions
from the filtrate into the surrounding cells. From
here the materials return to the bloodstream. This
process is aided by active transport of glucose and
amino acids out of the filtrate. The lining of the
proximal tubule is covered with tiny projections
(like the villi of the small intestine) to increase
the surface area and speed up the process of re-
absorption. When the filtrate reaches the end of
the proximal tubule, the fluid is isotonic with the
surrounding cells, and the glucose and amino acids
have been removed from the filtrate. We say a fluid
is isotonicwhen it has the same concentration of
water and solutes as that in the cells surrounding it.
From the proximal tubule, the filtrate moves
to the loop of Henle. The primary function of the
loop of Henle, which first descends into the inner
renal medulla and then turns to ascend back
towards the cortex, is to remove water from the
filtrate by the process of osmosis (see Figure 4.11).
The cells of the medulla have an increased
concentration of sodium ions (Na+). These ions
increase in a gradient starting from the area closest
to the cortex and moving toward the inner pelvis of
the kidney. This increasing gradient acts to draw
water from the filtrate in the loop of Henle. This
process continues down the length of the
descending loop due to the increasing level of Na+
in the surrounding tissue. You will observe a
similar process in the MiniLab below.

Figure 4.11As the filtrate travels down the descending

loop of Henle, water moves out by osmosis. What prevents

the water from being re-absorbed into the ascending loop?

The high levels of Na+in the surrounding medulla

tissue are the result of active transport of Na+out of

the ascending loop of Henle. The amount of water

removed from the filtrate by the time it reaches the

bottom of the loop of Henle results in an increased

concentration of all of the materials dissolved in

the remaining filtrate, including Na+. Thus, as the

filtrate moves up the ascending loop of Henle, Na+

loop of the Henle urea

collecting

duct

outer

medulla

inner

medulla

ascending loop

increasing concentration in renal medulla

cortex

descending loop

HO 2

HO 2

HO 2

HO (^2) HO 2
Na HO^2

• Na+Cl−
  Na+Cl−
  Na+Cl−