Cell Transport
Transport Across Membranes
If a
cell
were a house, the
plasma membrane
would be walls with windows and doors. Moving things in and out of the
cell
is an important role of the
plasma membrane
. It controls everything that enters and leaves the
cell
. There are two basic ways that substances can cross the
plasma membrane
:
passive transport
, which requires no
energy
; and
active transport
, which requires
energy
.
Passive transport
is explained in this section and
Active transport
is explained in the next section,
Active Transport
and
Homeostasis
. Various types of
cell
transport are summarized in the concept map in Figure 5.7.25.7.2.
Transport Without
Energy
Passive transport
occurs when substances cross the
plasma membrane
without any input of
energy
from the
cell
. No
energy
is needed because the substances are moving from an area where they have a higher concentration to an area where they have a lower concentration. Water solutions are very important in biology. When water is mixed with other molecules this mixture is called a solution. Water is the solvent and the dissolved substance is the solute. A solution is characterized by the solute. For example, water and
sugar
would be characterized as a
sugar
solution. More the particles of a solute in a given volume, the higher the concentration. The particles of solute always move from an area where it is more concentrated to an area where it is less concentrated. It’s a little like a ball rolling down a hill. It goes by itself without any input of extra
energy
.
The different categories of
cell
transport are outlined in Figure 5.7.25.7.2.
Cell
transport can be classified as follows:
Passive Transport
which includes
Simple
Diffusion
Osmosis
Facilitated Diffusion
Active Transport
can involve either a pump or a
vesicle
Pump Transport can be
primary
secondary
Vesicle Transport
can involve
Exocytosis
Endocytosis which includes
Pinocytosis
Phagocytosis
Receptor
-Mediated Endocytosis
Figure 5.7.25.7.2: The
Cell
Transport Concept Map illustrates various types of
cell
transports that happen at the
plasma membrane
Simple
Diffusion
Diffusion
Although you may not know what
diffusion
is, you have experienced the process. Can you remember walking into the front door of your home and smelling a pleasant aroma coming from the kitchen? It was the
diffusion
of particles from the kitchen to the front door of the house that allowed you to detect the odors.
Diffusion
is defined as the net movement of particles from an area of greater concentration to an area of lesser concentration.
Figure 5.7.35.7.3. Simple
diffusion
shows as a timeline with the outside of the
cell
(extracellular space) separated from the inside of the
cell
(intracellular space) by the
cell
membrane. In the beginning of the timeline there are many molecules outside of the
cell
and none inside. Over time, they diffuse into the
cell
until there is an equal amount outside and inside.
The molecules in a gas, a liquid, or a solid are in constant motion due to their kinetic
energy
. Molecules are in constant movement and collide with each other. These collisions cause the molecules to move in random directions. Over time, however, more molecules will be propelled into the less concentrated area. Thus, the net movement of molecules is always from more tightly packed areas to less tightly packed areas. Many things can diffuse. Odors diffuse through the air, salt diffuses through water and nutrients diffuse from the
blood
to the body tissues. This spread of particles through the random motion from an area of high concentration to an area of lower concentration is known as
diffusion
. This unequal distribution of molecules is called a concentration gradient. Once the molecules become uniformly distributed, a dynamic equilibrium exists. The equilibrium is said to be dynamic because molecules continue to move, but despite this change, there is no net change in concentration over time. Both living and nonliving systems experience the process of
diffusion
. In living systems,
diffusion
is responsible for the movement of a large number of substances, such as gases and small uncharged molecules, into and out of cells.
Osmosis
Osmosis
is a specific type of
diffusion
; it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Water moves in or out of a
cell
until its concentration is the same on both sides of the
plasma membrane
.
Semi-permeable membranes are very thin layers of material that allow some things to pass through them but prevent other things from passing through.
Cell
membranes are an example of semi-permeable membranes.
Cell
membranes allow small molecules such as oxygen, water carbon dioxide, and oxygen to pass through but do not allow larger molecules like
glucose
, sucrose, proteins, and
starch
to enter the
cell
directly.
The classic example used to demonstrate
osmosis
and osmotic pressure is to immerse cells into
sugar
solutions of various concentrations. There are three possible relationships that cells can encounter when placed into a
sugar
solution. Figure 5.7.45.7.4 shows what happens in
osmosis
through the semi-permeable membrane of the cells.
The concentration of solute in the solution can be greater than the concentration of solute in the cells. This
cell
is described as being in a hypertonic solution (hyper = greater than normal). The net flow or water will be out of the
cell
.
The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation, the
cell
is in an
isotonic
solution (iso = equal or the same as normal). The amount of water entering the
cell
is the same as the amount leaving the
cell
.
The concentration of solute in the solution can be less than the concentration of solute in the cells. This
cell
is in a hypotonic solution (hypo = less than normal). The net flow of water will be into the
cell
.
Figure 5.7.4.A5.7.4.𝐴: Hypertonic solution. A solution that has a higher solute concentration than another solution. Water particles will move out of the
cell
, causing crenation.
Figure 5.7.4.B5.7.4.𝐵:
Isotonic
solution. A solution that has the same solute concentration as another solution. There is no net movement of water particles, and the overall concentration on both sides of the
cell
membrane remains constant.
Figure 5.7.4.C5.7.4.𝐶: Hypotonic solution. A solution that has a lower solute concentration than another solution. Water particles will move into the
cell
, causing the
cell
to expand and eventually lyse.
Figure 5.7.55.7.5 demonstrates the specific outcomes of
osmosis
in red
blood
cells.
Hypertonic solution. The
red blood cell
will appear to shrink as the water flows out of the
cell
and into the surrounding environment.
Isotonic solution. The
red blood cell
will retain its normal shape in this environment as the amount of water entering the
cell
is the same as the amount leaving the
cell
.
Hypotonic solution. The
red blood cell
in this environment will become visibly swollen and potentially rupture as water rushes into the
cell
.
Figure 5.7.55.7.5:
Osmosis
demonstration with Red
Blood
cells places in a hypertonic,
isotonic
, and hypotonic solution.
Facilitated Diffusion
Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules such as
glucose
all need help with
diffusion
. The help comes from special proteins in the membrane known as transport proteins.
Diffusion
with the help of transport proteins is called
facilitated diffusion
. There are several types of transport proteins, including channel proteins and carrier proteins (Figure 5.7.65.7.6)
Channel proteins form pores, or tiny holes, in the membrane. This allows water molecules and small ions to pass through the membrane without coming into contact with the hydrophobic tails of the
lipid
molecules in the interior of the membrane.
Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane.