Cell transport
For normal functioning, cells in the human body need to be in a stable environment that continually
supplies the materials they need and continually removes any materials they produce.
The immediate environment of a cell is the fluid that surrounds it; the tissue fluid or extracellular
fluid. Even cells that appear to be very close together when observed under a microscope have
a thin layer of fluid between them. This fluid allows a continual exchange of materials into and out
of cells.
Body systems work together to ensure that the cellular environment is kept constant. This is
called homeostasis. The cells are maintained at a constant temperature, surrounded by fluids with a
constant concentration.
To carry out their functions, cells need to take in certain substances from the tissue fluid. As
they process these substances, they produce materials that must then be removed from the cell.
Depending on their particular role, different cells have different requirements and produce different
materials. However, there are certain substances that all cells require and all cells produce.
During cellular respiration, glucose and oxygen are used to produce carbon dioxide, water and
energy. Therefore, cells need to be supplied with oxygen and glucose, while carbon dioxide and water
are removed.
Many cells also produce substances that will be used elsewhere in the body, such as hormones
and enzymes. Other wastes, in addition to carbon dioxide, are also produced. All these products are
released into the tissue fluid.
Structure and function of the cell membrane
Each cell is surrounded by a cell membrane that separates the internal and external environment.
Substances that enter or leave the cell must pass through this membrane; therefore, it is very
important in determining which substances will get into or out of a cell.
The cell membrane and all the membranes within the cell have a similar structure. Even with an
electron microscope the detailed structure of cell membranes is too small to be seen. For this reason,
models have been proposed to account for the behaviour and composition of the cell membrane.
In science, a model is a simple explanation of a complex idea. The currently accepted model for cell
membrane structure is called the fluid mosaic model. The membrane is said to be fluid because the
molecules of which it is made are constantly changing position, and it is said to be mosaic because it
is composed of many different kinds of molecules.
The main structure of the membrane is composed of phospholipid molecules, which are lipid
molecules containing a phosphate group. The phospholipids are arranged in two layers, known as a
bilayer. Each phospholipid molecule has a head that is hydrophilic (water-loving), and a tail that is
hydrophobic (water-hating). The phospholipids are arranged in the two layers with their heads on
the outside and tails on the inside. They drift from place to place with their heads and tails moving,
keeping the membrane fluid. Embedded in the phospholipid
bilayer of the membrane are cholesterol
and protein molecules. The cholesterol
molecules are wedged between the
phospholipids. These molecules are
important for the function integrity and
stability of the membrane. Cell membranes
have a variety of protein molecules,
including receptor proteins, channel
proteins, carrier proteins and cell-identity
markers. Some of these molecules extend
from one side of the membrane to the
other, while others are bound to the
membrane surface. Only about 2% of the
molecules in the membrane are proteins,
yet they make up about 55% of the mass of
the membrane. This is because proteins are
very large molecules.
Functions of the cell membrane
The cell membrane has the following main functions:
• It acts as a physical barrier. The membrane separates the cell cytoplasm from the extracellular
fluid around the cell. Isolation of the cytoplasm from the surrounding fluid is important because
their compositions are very different.
• It regulates the passage of materials. The membrane controls the movement of materials into
and out of the cell – for example, the entry of ions and nutrients, the removal of wastes and the
release of secretions.
It is sensitive to changes. The cell membrane is the first part of the cell affected by any changes
in the extracellular fluid. It also has receptors that are sensitive to particular molecules in its
immediate environment.
• It helps support the cell. The internal part of the cell membrane is attached to the
microfilaments of the cell’s cytoskeleton (see Figure 2.2 on page 27), thus giving support to the
whole cell. There are also connections between the membranes of adjacent cells, providing
support to the whole tissue.
Transport across the cell membrane
Cell membranes are described as being differentially permeable, semipermeable or selectively
permeable. This means that they allow certain ions and molecules to pass through but restrict the
movement of others.
Materials may pass through a cell membrane in different ways. Some transfer mechanisms are
passive processes, while others are active. Passive processes do not use energy, whereas active
processes use the cell’s energy in the form of adenosine triphosphate (ATP).
Three basic processes result in transport of materials into or out of a cell:
• Simple diffusion – a passive process resulting from the random movement of ions and molecules;
osmosis (also a passive process) is a special case of diffusion where water passes across the
membrane.
• Facilitated transport – a process that requires special proteins in the cell membrane, either
channel proteins or carrier proteins; it may be passive transport or active transport, depending
on the exact nature of the mechanism.
• Vesicular transport – an active process in which materials are moved in
membrane-bound sacs.
Simple diffusion
Diffusion is the spreading out of particles so that they are evenly distributed over the space available.
It occurs in gases and liquids because the molecules of gases and liquids are constantly moving.
They move in random directions and in straight lines until they hit another molecule or the wall of
the container. A deflected molecule then continues in a straight line until it hits another obstacle.
Molecules moving away from an area in which they are concentrated experience fewer collisions
than those moving towards the area of higher concentration. They therefore stay on their straight
paths longer and move out into areas where the concentration of those molecules is lower. In this way,
the molecules become evenly spread over the space available. The random movement of molecules
continues, but the chances of collision are the same in whatever direction the molecule is travelling.
Figure 2.13 shows how a sugar cube dissolves in water and how the molecules of sugar spread
out until they are evenly spread throughout the water. As the sugar dissolves, the sugar molecules
near the cube are more concentrated than those near the surface of the water. The difference in
concentration that brings about diffusion is called a concentration gradient, or diffusion gradient
(Figure 2.14). The greater the difference in concentrations, the ‘steeper’ the diffusion gradient and the
faster the rate of diffusion (Figure 2.15).
The movement of liquid or gas molecules from places of higher concentration to places of
lower concentration, along a concentration gradient, is more correctly called net diffusion. This is
because there will also be some molecules moving against the concentration gradient in the opposite
direction. While there is a difference in concentrations, there will be more particles moving from the
area of high concentration to the area of low concentration. Once the concentrations are the same,
the same number of particles will be moving in each direction.
Alcohol, steroids and other fat-soluble substances can easily enter cells because they can diffuse
through the lipid portions of the membrane. Oxygen and carbon dioxide can also diffuse through the
phospholipid bilayer. This type of diffusion is referred to as simple diffusion.
• Oxygen diffuses into cells because it is continually used up inside the cell for respiration.
The concentration of oxygen inside the cell is therefore lower than the oxygen concentration
outside the cell. Because of this concentration difference, there is net diffusion of oxygen into
the cell. Carbon dioxide is continually produced inside the cell by respiration. The higher concentration
of carbon dioxide inside the cell means that there will be net diffusion of carbon dioxide out
of the cell.
Water-soluble substances are unable to pass directly through the lipid portion of the membrane
and hence require other modes of transport that are discussed in the next section.
Osmosis
Osmosis is a special type of diffusion: the diffusion of a solvent through a differentially permeable
membrane in order to balance the concentration of another substance. As water is the most
important solvent in the human body, osmosis can be considered to be the diffusion of water across a
differentially permeable membrane. The water will move from an area where a solute such as sugar is
in low concentration to an area where the solute is in high concentration. As more water moves into
the high concentration, the solution will become diluted, lowering the concentration. At the same
time, as the water moves out of the area of the low concentration the concentration will increase.
This occurs because, if there are equal volumes in both areas, where there is more solute there will
be less solvent. The concentration of water is therefore lower. Conversely, the area of the lower
concentration of solute will have a higher concentration of water. In this way, water is moving from
the area of high concentration to low concentration of water.
Large polar molecules, such as glucose, and ions, such as sodium ions, are unable to cross the
cell membrane directly as they are repelled by the hydrophobic tails in the phospholipid bilayer.
However, water molecules are small enough to be able to pass through the cell membrane, since
they can fit between the lipid tails. Water also crosses the membrane by passing through protein
channels; this form of transport is discussed in the next section.
Figure 2.17 shows a beaker divided in two by a differentially permeable membrane. On one
side of the membrane is pure water; on the other side is a sugar solution. Water molecules can pass through the membrane, but the sugar molecules will stay on the same side of the membrane.
Because of the difference in concentration, more water molecules will move from the water to the
sugar solution than in the opposite direction. The sugar solution will gain water. Note that in Figure 2.17, the level of liquid on the water side of the membrane has dropped,
whereas the liquid level on the sugar side has risen. This higher level on one side of the membrane
results from a pressure, known as osmotic pressure. The higher the concentration of solute (in this
case, the sugar), the higher the osmotic pressure.
Facilitated transport
In facilitated transport, proteins in the cell membrane allow molecules to be transported across the
membrane. These proteins are channel proteins, which form protein channels, and carrier proteins,
which allow carrier-mediated transport.
Protein channels
To diffuse across a cell membrane, water-soluble molecules must pass through protein channels in
the membrane, allowing facilitated diffusion. These channels provide a pathway for the hydrophilic
particles to travel through to cross the cell membrane without coming in contact with the
hydrophobic inner portion. The protein channels are very small in diameter, but water and ions can
easily get through. Larger molecules are too big to fit through the channels.
Carrier-mediated transport
While channel proteins provide a channel through the membrane, carrier proteins are only open on
one side of the membrane at a time. When the specific substance binds to the binding site within the
protein, the protein changes shape and opens to the other side. The substance can then be released
on the side opposite to where is entered.
Some important characteristics of carrier-mediated transport are as follows:
• The carrier proteins are specific; they will only bind to a particular molecule. For example, the
carrier that transports glucose cannot transport any other molecules, even simple sugars that are
very similar to glucose.
• Carriers can become saturated. Once all the available carriers are occupied, any increase in the
concentration of molecules to be transported cannot increase the rate of movement.
• Carrier activity is regulated by substances such as hormones. Hormones are important in
coordinating the activities of carrier proteins.
There are two main types of carrier-mediated transport.
1 Facilitated diffusion occurs when substances are transported through a protein along the
concentration gradient, from a higher concentration on one side of the membrane to a lower
concentration on the other. This is a passive process, as it does not require the input of energy.
During carrier-mediated facilitated diffusion, the molecule to be transported, such as glucose,
attaches to a binding site on the specific carrier protein. The protein changes shape and the
molecule is released on the other side of the membrane.
2 Active transport requires energy in the form of ATP because substances are transported
across the membrane against the concentration gradient, from lower to higher
concentration. The process of active transport is similar to that of facilitated diffusion
via carrier proteins, but its big advantage is that it does not depend on a concentration
gradient. Using active transport, a cell can take in or pass out substances regardless of their
concentrations inside or outside the cell.
Vesicular transport
Vesicular transport is the movement of substances across the cell membrane in membranous sacs
called vesicles. This is an active process, because energy from the cell is needed to form the vesicles.
Endocytosis is taking liquid or solids into the cell by vesicular transport. The cell membrane
folds around a droplet of liquid or a solid particle until the droplet or particle is completely
enclosed. The vesicle formed then pinches off and is suspended in the cell’s cytoplasm. Taking
liquids into the cell in this way is called pinocytosis; when the vesicles contain solid particles it is
called phagocytosis.
Exocytosis is when the contents of a vesicle inside the cell are passed to the outside. A vesicle
that is formed inside the cell migrates to the cell membrane and fuses with the membrane. The
contents of the vesicle are then pushed out into the extracellular fluid.
Movement within the cell
Molecules and ions move within the cell mostly by diffusion. Remember that diffusion is the
spreading of particles so that they are evenly distributed over the space available. Thus, as molecules
of a substance are used up in one part of the cell, other molecules will spread to take their place.
For example, as oxygen is used up by the mitochondria for respiration, a lower concentration of
oxygen is created. Oxygen will then diffuse into the area of lower concentration from areas of higher
concentration within the cell.
There are also structures that transport substances. The endoplasmic reticulum is used to
transport substances within the cell – particularly proteins that the cell has made. These are
transported to the Golgi body for secretion from the cell. Microtubules are very fine tubes
that help to maintain the shape of
the cell and to hold the organelles
in place. They also act like railway
tracks, guiding organelles or molecules
to particular places within the cell.
Microtubules are not permanent
structures but are able to be broken
down or built up as needed in the
various parts of the cell.
Why are cells so small?
The cells in a human body vary greatly
in size. Most human cells are extremely
small; between 10 and 15 micrometres
(μm) in diameter (1 μm is onethousandth
of a millimetre). Nerve cells
may have extensions up to a metre long,
and muscle cells up to 30 cm long.
However, both nerve and muscle cells
are too thin to be seen with the naked eye. Human egg cells have a diameter of up to 100 μm and
may be just visible to the naked eye.
There is a limit to how big a cell can be. All the requirements and products of a cell must pass
across the membrane that surrounds the cell. Thus, the relationship between the surface area of
the cell and the volume is very important. Imagine that an apple is a cell. If the apple is cut in half,
each piece has half of the original volume, but each piece has more than half of the original surface
area. Cutting the apple in half has created extra surface area because of the two cut surfaces. If you
continue to cut the apple into smaller and smaller pieces, the surface-area-to-volume ratio of the
pieces gets bigger and bigger. In the same way, a small cell will have a larger surface-area-to-volume
ratio than a large cell.
Figure 2.24 illustrates how doubling the length of the side of a cube-shaped cell results in eight
times the volume, but only four times the surface area. As a cell grows, its ability to exchange enough
materials to support its increasing volume is diminished because the volume increases at a greater
rate than the surface area. A large cell could not support itself because it would not have enough
surface to absorb the nutrients required, and remove the wastes produced, for its large volume. To
function effectively, most cells have to be microscopic.