Lesson 9 - Movements of Materials across Membranes
Student Performance Objectives
1. Define each of the following terms: filtration, diffusion, facilitated diffusion, osmosis, active
transport, pinocytosis, and phagocytosis.
2. Describe each of the processes in objective #1 utilizing an example from the human body.
3. Describe an experiment that illustrates Brownian motion and provide a scientific explanation
for this phenomenon.
4. Explain why diffusion occurs utilizing knowledge of Brownian motion.
5. Define hemolysis and crenation.
A. Define and discuss the following concepts: permeability of membranes, impermeability,
selective permeability (sometimes called semipermeability). Discuss within the framework
of membrane structure including hydrophilic and hydrophobic issues, and membrane and
membrane channel electrical charges.
B. Passive Mechanisms - movement based on relative concentrations of ions/molecules.
1. Filtration - the passage of molecules through a membrane based on unequal pressures
on the opposite sides of the membrane. The force of gravity helps to explain filtration
in a drip-type coffee maker; pressure developed by the beating heart helps to explain
filtration at the capillary wall and also in the filtration units of the kidneys.
2. Diffusion - movement of small particles (e.g., ions, molecules) from regions where
there are more of them (higher concentration) to regions where there are less
of them (lower concentration). An example is the movement of the respiratory
gases, O2 and CO2, between the lung's air sacs (alveoli) and the blood in capillaries
surrounding the alveoli.
3. Facilitated diffusion - the movement of a molecule from high to low concentration
across a membrane with the help of a transport protein. The entry of glucose into
the intracellular fluid of cells from the surrounding interstitial fluid of the tissue space
is an example of facilitated diffusion.
4. Osmosis - the diffusion of water across a membrane. Osmosis occurs across the
membranes of all cells since cell membranes are generally permeable to water.
C. Active Mechanisms - movement of molecules or ions based on work and energy being
expended at the cell membrane, usually in the form of ATP. - Active Transport
D. Engulfing Mechanisms
1. Pinocytosis - cells absorb proteins and other molecular material from the surrounding
fluids by way of membrane vesicle formation. A good example of this is the
absorption of antibodies (from its mother's milk) by an infant's intestinal epithelium
from the intestinal fluid.
2. Phagocytosis - cells engulf and digest large particulates from the surrounding fluids.
The engulfment and digestion of bacteria by white blood cells is an example of
1. The basis for the movements of all molecular-sized particles -
Brownian motion. Utilize a microscope with a drop of whole milk on a microscopic slide. The random giggling of the particles of casein and lipoproteins in the milk is due to the bombardment of these particles with water molecules that are, themselves, too small to be observed. The "invisible" water molecules' effects can, however, be seen in the larger particles being bumped-into by the randomly moving water molecules.
2. Osmosis: students utilize sheep's blood and hypotonic distilled water,
isotonic 0.9% NaCl, and hypertonic 5% NaCl solutions. One drop of
blood is placed into separate test tubes containing each of the solutions
indicated. The opacity of the solutions is noted. Then a drop of liquid
from each tube is observed under the microscope and the presence or
absence of cells, and the shape of cells, are noted. The ideas of
hemolysis and crenation are discussed.