Cellular Transport
Important Properties of Water:
1. Water is Polar (Capillarity, Surface Tension, & Hydrogen Bonding)
2. Water Resists Temperature Changes
3. Water Expands When it Freezes
Diffusion = the net movement of particles from an area of higher concentration to an area of
lower concentration (i.e. across the concentration gradient)
 Diffusion results b/c of the random movement of particles
 It is a slow process b/c it relies on the random molecular motion of atoms.
 3 key factors affect the rate of diffusion:
1. concentration (= the primary controlling factor)
- The more concentrated the substances, the more rapidly diffusion occurs.
2. temperature
- An increase in temperature increases kinetic energy (or speed of molecular
movement) and, thus, increases diffusion.
3. pressure
- Increasing pressure will accelerate molecular movement and, therefore,
diffusion.
Dynamic Equilibrium = the point at which there is continuous movement but no overall
concentration change
 The results of diffusion (left unhindered)
Diffusion in Living Systems
 Ions and molecules diffuse across a concentration gradient. Once the two
concentrations are equal, diffusion stops, and dynamic equilibrium occurs.
 Diffusion is one of the methods by which cells move substances in and out of the
cell. It is also evident outside the cell and can involve substances other than
molecules in an aqueous environment (e.g. O2 into the capillaries of the lungs).
Osmosis = the diffusion of water across a differentially permeable membrane.
 There are three possible conditions in regards to a cell's water concentration relative
to its environment:
1. Isotonic = The solute concentration is the same on either side of the cell membrane.
 This is the condition of the cells in the human body.
 The fluids in our body (blood and plasma) are isotonic to the cells in our body.
2. Hypotonic = The solute concentration is lower outside of the cell than inside.
 Under these circumstances a cell is at risk of lysing. If blood cells are placed
in distilled water they will burst.
 Some protozoans have contractile vacuoles that expel excess water. Many
protists, all the fungi and all plant cells regulate the inflow of water under
these conditions by means of the cell wall. While the membrane is in contact
with the cell wall the wall will provide a static force that equals the force
generated by the tendency of water to move into the cell. This not only
prevents the cell from bursting, but, in the case of herbaceous plants, also
provides support like that provided by an inner tube in a tire.
3. Hypertonic = The solute concentration is greater outside the cell than inside.
 A cell under these conditions will tend to shrink.
 This often represents an abnormal, stressful condition (the cells of a
wilting plant, dehydration due to drinking sea water etc.).
 HOMEOSTASIS 
Isotonic Solution = The concentration of dissolved substances in the solution is the same as the
concentration of dissolved substances inside the cell. Plus, the concentration
of water in the solution is the same as the concentration of water inside the cell.
Hypotonic Solution = The concentration of dissolved substances is lower in the solution outside
the cell than the concentration inside the cell. Therefore, there tends to be
more water outside the cell than inside the cell.
Hypertonic Solution = The concentration of dissolved substances outside the cell is higher than
the concentration inside the cell.
Passive Transport = A molecule or ion that crosses the membrane by moving down a concentration
or electrochemical gradient and without expenditure of metabolic energy is
said to be transported passively.
Types of Passive Transport:
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
Types of Active Transport:
1. Endocytosis
a. Phagocytosis
b. Pinocytosis
2. Sodium-Potassium Pump (exchanges sodium for potassium
across the plasma membrane of animal cells)
3. Proton Pump
(actively transports hydrogen ions out of the cell)
Facilitated Diffusion = The passive transport of materials across the plasma membrane with the
aid of transport proteins. It is driven by a concentration gradient.
Active Transport = Movement of materials through a membrane against a concentration gradient (from
low to high). It requires energy from the cell. A type of transport protein, called a
carrier protein, first binds with the particle to be transported. In general, this binding
transforms the shape of the particle and when it is released it goes back to its
original/normal configuration.
 ATP supplies the energy for most active transport.
Endocytosis = The cellular uptake of macromolecules by localized regions of the plasma
membrane that surround the substance and pinch off to form an intracellular vesicle.
Exocytosis = The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.
Phagocytosis = A type of endocytosis involving large, particulate substances.
Pinocytosis = A type of endocytosis in which the cell ingests extracellular fuid and its dissolved solutes.
Cell Size Limitations
 Most living cells are between 2 and 200 μm in diameter. Considering this wide range of
cell sizes, why then can’t most organisms be just one giant cell?
Limitations to Cell Size:
1. Diffusion – Although it is fast and efficient over short distances, it becomes slow and
inefficient as the distances become larger.
2. DNA – There is a limit as to how quickly proteins can be copied in the nucleus and made into
proteins in the cytoplasm. The larger the cell, the more cytoplasm and, thus, the more
required supplies of enzymes/proteins.
3. Surface Area-to-Volume Ratio – As a cell’s size increases, its volume increases much faster
than its surface area. (If a cell size doubled, the cell would require eight times more nutrients
and would have eight times more waste to excrete.)
 See Problem-Solving Lab 8-1 on page 203 
Assesment Questions:
1. What factors affect the diffusion of water through a membrane by osmosis?
2. How do animal cells and plant cells react differently to osmosis in a hypotonic solution?
3. Compare and contrast active transport and facilitated diffusion.
4. How do carrier proteins facilitate passive transport of molecules across a membrane?
5. A paramecium expels water when the organism is surrounded by freshwater. What can you
deduce about the concentration gradient in the organism’s environment?
6. Osmosis is a form of diffusion. What effect do you think an increase in temperature has on
osmosis?
7. Describe how a cell’s surface area-to-volume ratio limits its size.
Diffusion:
Osmosis:
Selectively
Transport Summary:
Assessment Solutions:
1. The concentration of water on either side of the membrane and the permeability of the
membrane.
2. In a hypotonic solution, water moves into the cell. In an animal cell, the extra water
may cause the plasma membrane to burst. In a plant cell, the plasma membrane
pushes against the cell wall, providing added support.
3. Facilitated diffusion and active transport both use carrier proteins. Facilitated
diffusion does not require energy; active transport does.
4. Carrier proteins move substances that cannot diffuse through the plasma membrane
from an area of higher to lower concentration.
5. The organism is in a hypotonic environment and the concentration gradient is from
outside to inside.
6. Increasing temperature will increase the rate of osmosis, but it will not change the
final outcome because it cannot change the membrane permeability to other solutes.
7. As volume increases, surface area does not increase sufficiently to support large cells.