Solutions
A solution is a liquid mixture of solute dissolved in solvent. Think of salt water, a
solution in which salt (the solute) is dissolved in water (the solvent).
The interior of a cell is also a solution. The cytoplasm is a watery jelly-like
substance (the solvent) that contains a variety of substances, like salt and minerals
(the solutes). Maintaining the concentration of solutes in the cytoplasm is critical to
cell function-too much or too little of any component causes damage to the cell. This
ideal balance of solutes within the cell is a state the cell strives to maintain through
a variety of mechanisms. The process is referred to as maintaining homeostasis.
The Cell Membrane and Cellular Transport
Hormones are chemical messengers that regulate some body functions in
multicellular organisms. One function of hormones is to help maintain homeostasis.
Other functions of hormones include the control of movement of oxygen into cells
and the removal of carbon dioxide from cells, the maintenance of the internal
temperature of an organism, and the regulation of fluids. Individual cells move
fluids and nutrients in and out through the semi-permeable membrane. They can
move these materials by either passive or active transport mechanisms to maintain
homeostasis.
Cell Membrane
The main purpose of the cell membrane is to regulate the movement of materials
into and out of the cell. The cell membrane is semi-permeable, or selectively
permeable, meaning that only certain substances can go through. Membranes can
select molecules by size and charge. For example,large or charged molecules cannot
pass.
The cell membrane is composed of a phospholipid bilayer as shown below. Each
phospholipid layer consists of phosphate groups (phosphorous bonded with
oxygen) attached to two fatty acid (lipid) tails. The layers arrange themselves so
that the phosphate heads are on the outer edges of the membrane, and the fatty acid
tails compose the interior of the membrane. Globular proteins used for various
functions, such as transporting substances through the membrane, are embedded in
the cell membrane,.. The phospholipids are free to move around, allowing the
membrane to stretch and change shape. Other molecules, like cholesterol, are
important stabilizing molecules found in the phospholipid bilayer.
Passive Transport
Passive transport is spontaneous and does not require energy. In passive transport,
molecules move spontaneously through the cell membrane from areas of higher
concentration to areas of lower concentration; they are said to move “with the
concentration gradient.” The three types of passive transport are diffusion,
facilitated diffusion and osmosis.
Diffusion is the process by which substances move directly through the cell
membrane as shown below. Osmosis is a type of simple diffusion. Facilitated
diffusion involves the help of a channel, or carrier, protein to move a substance from
one side of the cell membrane to the other. Often, large molecules or charged
molecules use channel proteins.
diffusion
Osmosis is the movement of water from an area of high water concentration to an
area of low water concentration through a semi-permeable membrane. The figure
below shows osmosis. Think of osmosis as the diffusion of water.
Osmosis can occur in either direction, depending on the concentration of dissolved
material inside and outside the cell. Defining the solution concentrations relative to
one another will predict the direction in which osmosis will occur. A hypotonic
solution has the lower concentration of solute; this may be thought of as a higher
concentration of water. A hypertonic solution has a higher concentration of
dissolved solute, which may be thought of as a lower concentration of water. If the
solute concentrates are the same inside and outside the cell membrane, the
solutions are said to be isotonic to each other. Diffusion of water (osmosis) across a
cell membrane always occurs from hypotonic to hypertonic. Three situations are
possible, as illustrated in the table below.
Placing plant cells in a hypertonic solution causes the plant cell membranes to
shrink away from the cell wall. This process is called plasmolysis. Plasmolysis can
result in plant cell death due to water loss. A wilted plant is showing signs of
plasmolysis. Placing a plant in a hypotonic solution has an opposite effect: the cell
will swell until the cell wall allows more expansion. The plant now becomes very
stiff and turgid. Placing an animal cell in a hypotonic solution causes the cell to
swell until the membrane ruptures. This will cause cell death. Kidney dialysis is an
example of a medical procedure that involves diffusion. Another example is food
preserved by salting, sugar curing or pickling. All of these examples are methods of
drawing water out of the cells through osmosis.
Active Transport
In some cases, the cell may need to move material across the cell membrane, against
the concentration gradient. To do so, the cell must expend energy. The movement
of substances from an area of low concentration to an area of high concentration is
called active transport. The movement is characterized by its directionality.
Cells can use special proteins, called carrier proteins, embedded within the
membrane during active transport. Each carrier protein is specifically shaped to
accommodate the type of molecule it transports. The molecule and the protein fit
together like a lock and key. Carrier proteins use energy from ATP to physically
change shape and move the molecule into the cell. (see the figure below)
Exocytosis is a form of active transport that removes materials from the cell. A sac
stores the material to be removed from the cell, and then moves near the cell
membrane. The cell membrane opens and the substance is expelled from the cell.
Waste materials, proteins and fats are examples of materials removed from the cell
this way. (see figure below)
Endocytosis
Endocytosis, another form of active transport, brings materials into the cell without
passing through the cell membrane. The membrane folds itself around the
substance, creates a vesicle, and brings the substance into the cell. Some unicellular
organisms, such as an amoeba, obtain food this way.
Active transport is a mechanism that allows certain organisms to survive in their
environments. For instance, sea gulls can drink salt water because their cells
remove excess salt from their bodies through active transport. However, freshwater
fish are not able to remove excess salt from their cells and, therefore, would become
dehydrated in a saltwater environment. Another example of active transport
involves blood cells which use carrier proteins to transport molecules into the cell.
Review Questions
1. Define the following:
a. Hormones
b. Semi-permeable
c. Passive transport
d. Solute
e. Solution
f. Diffusion
g. Facilitated diffusion
h. Osmosis
i. Hypotonic
j. Homeostasis
k. Hypertonic
l. Isotonic
m. Plasmolysis
n. Active transport
o. Phospholipid bilayer
p. Exocytosis
q. Endocytosis
r. Vesicle
s. Solvent
t. Phosphate group
2. What is the movement of water across a semi-permeable membrane
from an area of high water concentration to an area of low water
concentration called?
a. Active transport
b. Diffusion
c. Osmosis
d. Hypotonic
3. A cell place in a solution shrinks by the process of osmosis. What kind
of solution is outside the cell?
a. Hypotonic
b. Hypertonic
c. Active
d. Isotonic
4. If the solution surrounding a cell has a lower concentration of solutes
than inside the cell, water will move into the cell through osmosis
causing it to expand. What kind of solution is surrounding the cell?
a. Active
b. Passive
c. Hypertonic
d. hypotonic
5. How does active transport differ from diffusion?
6. Dried beans are soaked overnight in preparation for cooking. Explain
the process affecting the beans. What will happen to the dried beans?
7. Differentiate between exocytosis and endocytosis.
8. A celery stalk is placed in a solution. It begins to wilt. What is a likely
component of that solution?