Classify various modes of transport of substances across the cell membrane.

Compare and contrast modes of transport of substances across the cell-membrane with examples (osmosis, diffusion, facilitated diffusion, primary active transport, secondary active transport).  

Simple Diffusion •Diffusion through lipid bilayer •Diffusion through protein Channel •Osmosis
Carrier-Mediated Transport •Facilitated Diffusion •Primary active Transport •Secondary active transport
Vesicular Transport •Endocytosis oPinocytosis oCarrier mediated endocytosis oPhagocytosis •Exocytosis



Diffusion is the movement of a substance from an area of high concentration to an area of low concentration

It happens due to random movement of particles. 

It is a passive process means no energy required.





Movement of molecules or ions;

Through intermolecular space.


Through membrane opening(channel protein/Pores)



Composed of integral cell membrane proteins.

Form open tube through membrane and always open.

Protein pores (also known as aquaporins/watery channels).

Selectively permeable to certain substances.

Many of channels can  be opened or closed by gates that are regulated by either electrical signals (voltage gated channels) or chemicals (ligand-gated channels)

K+ channels

4 protein subunits surrounding central pore.

At the top of channel has protein loop that form selectivity filter.

This filter is lined by carbonyl oxygen.

Hydrated K ion enter the filter—–interact with carbonyl oxygen—-shed water molecule—–dehydrated K ion pass through channel.


Inner surface of this channel is lined by amino acids that are strongly negatively charged.

This strong negative charge pull sodium ions inside the channel and once inside the channel can diffuse in either direction.

Gating Of Protein Channels



Gating Of Protein Channels



Facilitated diffusion also called carrier-mediated diffusion is the transport of substance through the membrane using a specific carrier protein to help.


A  carrier protein with a pore and binding “receptor” inside the pore.

The molecule to be transported enters the pore and becomes bound.

Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein.

The  pore now opens to the opposite side of the membrane.

Binding molecule release from binding site to opposite side of membrane.

Please remember……..

The concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately, but in the case of facilitated diffusion, the rate of diffusion cannot rise greater than the V max level.


ØThe rate at which molecule can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth.

Factors That Effect The Net Rate Of Diffusion

1.Surface area of membrane (↑ diffusion)

2.Lipid solubility (↑ diffusion)

3.Molecular weight of substance (↓diffusion)

4.Distance (thickness) through which diffusion must take place(Greater the distance, the slower is the rate of diffusion).

5.Concentration gradient of substance across the membrane (movement of substances from area of higher concentration to area of lower concentration)

6.Electrical Membrane potential across the membrane (even when there is no concentration difference across the membrane→ change in voltage cause movement of ions across the membrane)

7.Pressure difference across the membrane(↑ diffusion from area of higher pressure to lower pressure).


Osmosis is the movement of water or other solvent through a plasma membrane(semi-permeable membrane) from a region of low solute concentration to a region of high solute concentration.

Osmosis is passive transport, meaning it does not require energy to be applied.

Please remember—

ØA solvent is a liquid that dissolves a solid, liquid or gaseous solute.

ØA solute is a substance dissolved in another substance.

ØA solute and a solvent make up a solution. … Water is the solvent and the salt is the solute and together they make a salt (saline) solution.


Active transport is an energy dependent process that involves the use of a carrier protein to transfer a specific substance across the membrane against its concentration gradient.

Primary Active Transport

In primary active transport, the carrier splits ATP to power the transport process.

In primary active transport, the binding site of carrier has a greater affinity for its passenger (always an ion) on the low-concentration side.


1.The carrier protein has ATPase activity, which means it splits the terminal phosphate from an ATP molecule to yield ADP and inorganic phosphate plus free energy.

2.The phosphate group then attaches to the carrier, increasing the affinity of its binding site for the ion.

3.As a result the ion to be transported binds to the carrier on the low-concentration side.

4.The carrier changes its conformation so that the ion is now exposed to the high concentration side of the membrane and released.

Example: Sodium Potassium Pump

The carrier protein of Na-k Pump is made up of  two globular proteins: a larger one (α subunit) and a smaller one (β subunit).

The larger protein has three specific features that are important for the functioning of the pump:

1. It has three receptor sites for binding sodium ions on

Inside of the cell.

2. It has two receptor sites for potassium ions on the


3. The inside portion of this protein near the sodium

binding sites has ATPase activity.

Na–K pump plays three important roles:

1. It establishes Na and K concentration gradients across the plasma membrane of all cells; these gradients are critically important in the ability of nerve and muscle cells to generate electrical signals essential to their functioning.

2. It helps regulate cell volume by controlling the concentrations of solutes inside the cell and thus minimizing osmotic effects that would induce swelling or shrinking of the cell.

3. The energy used to run the Na–K pump also indirectly serves as the energy source for secondary active transport.

Secondary Active Transport

Secondary active transport, is transport of molecules across the cell membrane against a concentration/electrochemical gradient, utilizing energy in other forms than ATP.

This energy comes from the electrochemical gradient created by pumping ions out of the cell. 


•When sodium ions are transported out of cells by primary active transport, a large concentration gradient of sodium ions across the cell membrane usually develops— high concentration outside the cell and low concentration inside.

•This gradient represents a store house of energy because the excess sodium outside the cell membrane is always attempting to diffuse to the interior.

•So, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon is called co-transport.


Carrier in this instance serves as an attachment point for both the sodium ion and the substance to be co-transported.

Once they both are attached, the energy gradient of the sodium ion causes both the sodium ion and the other substance to be transported together to the interior of the cell.


•In counter-transport, sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient.

•However, this time, the substance to be transported is on the inside of the cell and must be transported to the outside.

•Therefore, the sodium ion binds to the carrier protein where it projects to the exterior surface of the membrane, while the substance to be counter-transported binds to the interior projection of the carrier protein.

•Once both have bound, a conformational change occurs, and energy released by the sodium ion moving to the interior causes the other substance to move to the exterior.

Na-h pump

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