Active and passive cell transport is the transfer of solutes from one side of the cell membrane to the other. Transport is passive when it does not require a source of metabolic energy such as ATP, while transport is active when it uses ATP as an energy source .
Cell membranes are mainly composed of a lipid bilayer that makes it difficult for certain types of substances to pass through. This barrier function allows the cell to maintain solute concentrations in the cytosol different from the extracellular environment or intracellular compartments.
| Passive transport | Active transport | |
|---|---|---|
| Definition | Solute transfer across the lipid membrane without energy. | Transfer of solutes through the lipid membrane associated with an energy source. |
| Concentration gradient | In favor. | Against. |
| Membrane Proteins | Channels and conveyors. | Conveyors or bombs. |
| Driving force | Electrochemical gradient. | ATP. |
| Examples | Water transport through aquaporinas. | Transport of sodium Na + ions by sodium-potassium ATP-ase. |
Index
What is passive cell transport?
Passive transport is the process that allows molecules and ions to pass through the cell membrane without an energy source.
The concentration gradient or concentration difference of a species between the two sides of the membrane is the impulse that determines the movement and direction of passive transport.
When the solute is charged (positive or negative), the potential difference between the two sides of the membrane (membrane potential) can also drive transport. In this case, the concentration gradient and the electrical gradient combined form the electrochemical gradient driving force .
By generating a difference in ionic concentrations across the lipid layer, the cell membrane can store potential energy in the form of electrochemical gradients. Electrochemical gradients are used to:
- drive various transport processes,
- transmit electrical signals in electrically excitable cells and
- produce the majority of ATP in the mitochondria, chloroplast, and bacteria.
Passive transport characteristics
- The movement of solutes follows the concentration gradient, from higher concentration to lower concentration.
- It depends on the concentration gradient, the size of the particles and the temperature.
- Ions and small molecules are mobilized.
- It does not require hydrolysis of ATP.
- It is mediated by transmembrane proteins, channels and transporters, in facilitated diffusion.
Passive transport types
Molecules and ions can passively pass through the membrane through different mechanisms: simple diffusion, facilitated diffusion or osmosis.
Simple diffusion
Small nonpolar molecules such as oxygen O 2 and carbon dioxide CO 2 dissolve easily in lipid membranes. Small uncharged polar molecules such as water H 2 O and urea also diffuse through the membrane in a slow or restricted manner. In general, lipophilic or fat-like molecules can cross the membrane by simple diffusion.
Facilitated dissemination
Cells developed mechanisms for transfer of water-soluble molecules and ions across the membrane. Through specialized transmembrane proteins (they cross the membrane) ions and molecules are transported. As the diffusion from higher concentration to lower concentration occurs with the help of “passageways”, we speak of facilitated diffusion. Thus:
- essential nutrients enter the cell;
- remove metabolic waste products, and
- regulate intracellular ion concentrations.
The two main classes of membrane proteins that facilitate the movement of molecules in and out through the lipid membrane are:
- the transporters : they are proteins that have moving parts, like doors of the membrane that open and close allowing the solute to pass through. They are like revolving doors in the membrane.
- the channels : they form narrow hydrophilic pores that allow passive movement, mainly of small inorganic ions. Although water can diffuse through lipid membranes, all cells contain protein channels called aquaporins that increase the permeability of these membranes to water.
Osmosis
Osmosis is the movement of water through a semi-permeable membrane, when on one side there is a solute that cannot cross the membrane. Only water movement occurs in osmosis.
What is active cell transport?
Active transport is the process by which the cell transports material against its concentration gradient, using ATP as an energy source.
Active transport characteristics
- It is made through integral membrane proteins.
- It is specific to the solute.
- It experiences saturation, that is, when all the binding sites of the solute are occupied, no matter how much more substrate is added, the flow remains constant.
Types of active transport proteins
In cells, at least three types of proteins are described with the capacity to carry out active transport. Below its description.
ATP Pumps
The ATP pumps carry out the transport of the solute coupled to the hydrolysis of ATP, that is, the ATP releases a phosphate group (PO 4 -3 ) and is transformed into ADP. The energy released in hydrolysis is what “pumps” the solute from one side of the membrane to the other.
Active transport driven by ATP hydrolysis is also known as primary active transport .
There are three types of ATP pumps:
- P-type pumps : the protein is phosphorylated (a phosphate group is attached to the protein) in the process of transport. Examples: sodium-potassium pumps, calcium pumps.
- Type-F pumps : also called ATP synthetases as they use the proton gradient to synthesize ATP from ADP and phosphate. Examples: chloroplast ATP synthetase associated with the light-dependent phase of photosynthesis.
- ABC transporters : they are membrane proteins that transport small molecules. Examples: the cholesterol transporter ABCG1, the transporter MDR (multidrug resistance).
Coupled conveyors
The transport of an ion or molecule is concomitant with another solute. In this case, the solute in higher concentration on one side of the membrane passes to the other side and promotes the movement of the solute from lower to higher concentration. Ion gradient driven transporters are also called secondary active transport .
It is carried out by carrier proteins known as symporters and anti-carriers. A symporter or cotransporter transports a solute following its concentration gradient in the same direction as another solute against the concentration gradient.
For example, the small intestine sodium-dependent glucose cotransporter. In this case, glucose and sodium from inside the intestine are absorbed into the intestinal cell.
The epithelial cells of the intestine or kidney have a large number of symporters that are driven by the gradient of the Na + sodium ion , being more concentrated outside the cell.
In bacteria, lactose transport is coupled to H + hydrogen ion transport .
An anti-carrier or exchanger carries out the transfer of solutes in opposite directions. For example, the sodium / proton Na + / H + anticarrier sodium enters the cell and proton exits the cell.
Light activated pumps
Predominant in bacteria and archaea, this transport of solutes takes place from lower to higher concentration thanks to the capture of light energy. For example, the bacteriorhodopsins and halorhodopsins are proton pumps activated by light.
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