3.2.3 Transport across cell membranes

Question 1

What are the functions of cell membrane ?

Answer 1

1. Separation between the extracellular fluid and the internal components of the cell
2. Communication with other cells
3. Recognition of external substances
4. Structural support
5. Transport of materials

Question 2

What are all the transport mechanism called ?

Answer 2

● Simple diffusion
● Facilitated diffusion
● Osmosis
● Active Transport
● Co-transport

Question 3

Define simple diffusion

Answer 3

‒ Diffusion, also called passive transport, is the movement of particles from an
area with higher concentration to an area of lower concentration.
‒ This is the primary mode of transport for small molecules and gases like oxygen
and carbon dioxide. It is important that these two gases readily diffuse through
the membrane because oxygen is needed by the cells for metabolism, and carbon
dioxide need to be expelled by the cell for eventual release to the environment.

Question 4

Define facilitated diffusion

Answer 4

‒ Facilitated diffusion is the spontaneous transport of material across the cell
membrane via membrane proteins embedded in it.
‒ The transport is dependent on the interaction of material and the channel or carrier
protein.
‒ This type of transport is particularly important in moving large polar molecules and
ions that cannot readily diffuse via simple diffusion.
‒ The main difference between carrier and channel proteins is carrier proteins are
not open readily to both intracellular and extracellular environments, while a
channel protein is open to both.
‒ In carrier proteins, binding sites are present where molecules to be transported
can bind. The protein then undergoes conformational change eventually opening
the protein molecule to the other side of the cell membrane. Eventually, the solute
molecule is released to the other side of the membrane.
‒ Channel proteins interact weakly with the material to be transported. If the channel
is open, specific solutes can freely pass through them.

Question 5

Define active transport

Answer 5

‒ This is the type of transport where solute is transported from an area of lower
concentration to an area of higher concentration.
‒ This type of transport is needed to accumulate solute in a specific area where
cellular metabolic process may occur.
‒ Active transport may be classified as either primary or secondary active transport.
Primary active transport requires energy in the form of ATP for the transport of
material. Secondary active transport, on the other hand, uses an electrochemical
gradient for the transport of material to occur.
‒ An example of active transport is the transport of potassium and sodium ions
through the sodium potassium pump. An electrochemical gradient is present
because of the imbalance in the concentration of positive charges across the
membrane. ATP provides the energy needed to produce conformational change
for the eventual release of the ions to the other side of the membrane.

Question 6

Define osmosis

Answer 6

‒ This is the type of transport where water molecules (solvent) diffuse through the
membrane down its concentration gradient.
‒ In the process of transport, the concentration in both sides of the membrane is
equalised.
‒ This type of transport occurs when there the solute concentrations are not equal,
and water molecules have higher tendency to diffuse than the solute molecules.
‒ Depending on the condition of the extracellular environment, different things can
happen to the cell. If the cell is exposed to an isotonic environment (same
concentration inside and outside the cell), the movement of water into and out of
the cell occur at the same rate. If the cell is exposed to a hypertonic environment
(outside of the cell has higher solute concentration than the inside), the cell will
shrivel because of loss of water. If the cell is exposed to a hypotonic environment
(inside of the cell has higher concentration than outside), the cell take up more
water and becomes bloated and will eventually burst.

Question 7

Define co-transport

Answer 7

‒ This is the type of transport where two substances are simultaneously transported
across a membrane.
‒ This is a specific type of secondary active transport.
‒ This is facilitated by symporters, which can transfer two substances in the same
direction. An example of a symporter is the sodium-glucose symporter. It uses the
sodium ions to move glucose into the cell. The flow of sodium ions through the
symporter provides the needed energy for the glucose to move also through the
symporter.