Active Transport

Carrier proteins

Carrier proteins, which are transmembrane proteins, act as pumps to allow the passage of molecules. They have binding sites that are complementary to specific molecules. This makes carrier proteins highly selective for specific molecules.

The steps involved in active transport are described below.

1. The molecule binds to the carrier protein from one side of the cell membrane.

2. ATP binds to the carrier protein and is hydrolysed to produce ADP and Pi (phosphate group).

3. The Pi attaches to the carrier protein and this causes it to change its conformational shape. The carrier protein is now open to the other side of the membrane.

4. The molecules pass through the carrier protein to the other side of the membrane.

5. The Pi detaches from the carrier protein, causing the carrier protein to return to its original conformation.

6. The process begins again.

Different types of active transport

According to the mechanism of transport, there are also different types of active transport:

• "Standard" active transport: this is the type of active transport that people usually refer to when using just "active transport". It's the transport that uses carrier proteins and directly uses ATP to transfer molecules from one side of a membrane to the other. Standard is in quotation marks because this is not the name it is given, as it usually is just referred to as active transport.
• Bulk transport: this type of active transport is mediated by the formation and transport of vesicles that contain the molecules that need importing or exporting. There are two types of bulk transport: endo- and exocytosis.
• Co-transport: this type of transport is similar to the standard active transport when transporting two molecules. However, instead of directly using ATP to transfer these molecules across a cell membrane, it uses the energy generated by transporting one molecule down its gradient to transport the other molecule(s) that have to be transported against their gradient.

According to the direction of molecule transport in "standard" active transport, there are three types of active transport:

• Uniport
• Symport
• Antiport

Uniport

Uniport is the movement of one type of molecule in one direction. Note that uniport can be described in the context of both facilitated diffusion, which is the movement of a molecule down its concentration gradient, and active transport. The carrier proteins needed are called uniporters.

Fig. 1 - The direction of movement in uniport active transport

Symport

Symport is the movement of two types of molecules in the same direction. The movement of one molecule down its concentration gradient (usually an ion) is coupled to the movement of the other molecule against its concentration gradient. The carrier proteins needed are called symporters.

Fig. 2 - The direction of movement in symport active transport

Antiport

Antiport is the movement of two types of molecules in opposite directions. The carrier proteins needed are called antiporters.

Fig. 3 - The direction of movement in antiport active transport

Active transport in plants

Mineral uptake in plants is a process that relies on active transport. Minerals in the soil exist in their ion forms, such as magnesium, sodium, potassium and nitrate ions. These are all important for a plant's cellular metabolism, including growth and photosynthesis.

The concentration of mineral ions is lower in the soil relative to the inside of root hair cells. Due to this concentration gradient, active transport is needed to pump the minerals into the root hair cell. Carrier proteins that are selective for specific mineral ions mediate active transport; this is a form of uniport.

You can also link this process of mineral uptake to water uptake. The pumping of mineral ions into the root hair cell cytoplasm lowers the cell's water potential. This creates a water potential gradient between the soil and the root hair cell, which drives osmosis.

Active transport in animals

The sodium-potassium ATPase pumps (Na+/K+ ATPase) are abundant in nerve cells and ileum epithelial cells. This pump is an example of an antiporter. 3 Na + are pumped out of the cell for every 2 K + pumped into the cell.

The movement of ions generated from this antiporter creates an electrochemical gradient. This is extremely important for action potentials and the passage of glucose from the ileum into the blood, as we will discuss in the next section.

Fig. 4 - The direction of movement in the Na+/K+ ATPase pump

What is co-transport in active transport?

Co-transport, also termed secondary active transport, is a type of active transport that involves the movement of two different molecules across a membrane. The movement of one molecule down its concentration gradient, usually an ion, is coupled to the movement of another molecule against its concentration gradient.

The cotransporter uses the energy from the electrochemical gradient to drive the passage of the other molecule. This means ATP is indirectly used for the transport of the molecule against its concentration gradient.

Glucose and sodium in the ileum

The absorption of glucose involves cotransport and this happens in the ileum epithelial cells of the small intestines. This is a form of symport as the absorption of glucose into the ileum epithelial cells involves the movement of Na+ in the same direction. This process also involves facilitated diffusion, but cotransport is especially important as facilitated diffusion is limited when an equilibrium is reached - cotransport ensures all glucose is absorbed!

This process requires three main membrane proteins:

• Na+/ K + ATPase pump

• Na + / glucose cotransporter pump

• Glucose transporter

The Na+/K+ ATPase pump is located in the membrane facing the capillary. As previously discussed, 3Na+ are pumped out of the cell for every 2K+ pumped into the cell. As a result, a concentration gradient is created as the inside of the ileum epithelial cell has a lower concentration of Na+ than the ileum lumen.

The Na+/glucose cotransporter is located in the membrane of the epithelial cell facing the ileum lumen. Na+ will bind to the cotransporter alongside glucose. As a result of the Na+ gradient, Na+ will diffuse into the cell down its concentration gradient. The energy produced from this movement allows the passage of glucose into the cell against its concentration gradient.

The glucose transporter is located in the membrane facing the capillary. Facilitated diffusion allows glucose to move into the capillary down its concentration gradient.

Fig. 5 - The carrier proteins involved in glucose absorption in the ileum

Adaptations of the ileum for rapid transport

As we just discussed, the ileum epithelial cells lining the small intestine are responsible for the cotransport of sodium and glucose. For rapid transport, these epithelial cells have adaptations that help increase the rate of cotransport, including:

• A brush border made of microvilli

• Increased density of carrier proteins

• A single layer of epithelial cells

• Large numbers of mitochondria

Brush border of microvilli

The brush border is a term used to describe the microvilli lining the cell surface membranes of the epithelial cells. These microvilli are finger-like projections that drastically increase the surface area, allowing for more carrier proteins to be embedded within the cell surface membrane for cotransport.

Increased density of carrier proteins

The cell surface membrane of the epithelial cells have an increased density of carrier proteins. This increases the rate of cotransport as more molecules can be transported at any given time.

Single layer of epithelial cells

There is only one single layer of epithelial cells lining the ileum. This decreases the diffusion distance of transported molecules.

Large numbers of mitochondria

The epithelial cells contain increased numbers of mitochondria which provide the ATP needed for cotransport.

What is bulk transport?

Bulk transport is the movement of larger particles, usually macromolecules like proteins, into or out of a cell through the cell membrane. This form of transport is needed as some macromolecules are too large for membrane proteins to allow their passage.

Endocytosis

Endocytosis is the bulk transport of cargo into cells. The steps involved are discussed below.

1. The cell membrane surrounds the cargo (invagination.

2. The cell membrane traps the cargo in a vesicle.

3. The vesicle pinches off and moves into the cell, carrying the cargo inside.

There are three main types of endocytosis:

• Phagocytosis

• Pinocytosis

• Receptor-mediated endocytosis

Phagocytosis

Phagocytosis describes the engulfment of large, solid particles, such as pathogens. Once pathogens are entrapped inside a vesicle, the vesicle will fuse with a lysosome. This is an organelle containing hydrolytic enzymes that will break down the pathogen.

Pinocytosis

Pinocytosis occurs when the cell engulfs liquid droplets from the extracellular environment. This is so that the cell can extract as many nutrients as it can from its surroundings.

Receptor-mediated endocytosis

Receptor-mediated endocytosis is a more selective form of uptake. Receptors embedded in the cell membrane have a binding site that is complementary to a specific molecule. Once the molecule has attached to its receptor, endocytosis is initiated. This time, the receptor and the molecule are engulfed into a vesicle.

Exocytosis

Exocytosis is the bulk transport of cargo out of cells. The steps involved are outlined below.

1. Vesicles containing the cargo of molecules to be exocytosed fuse with the cell membrane.

2. The cargo inside of the vesicles is emptied out into the extracellular environment.

Differences between diffusion and active transport

You will come across different forms of molecular transport and you may confuse them with each other. Here, we will outline the main differences between diffusion and active transport:

• Diffusion involves the movement of molecules down their concentration gradient. Active transport involves the movement of molecules up their concentration gradient.
• Diffusion is a passive process as it requires no energy expenditure. Active transport is an active process as it requires ATP.
• Diffusion does not require the presence of carrier proteins. Active transport requires the presence of carrier proteins.