Symporter-transport

Active Transportation in Biology- Processes & Energy Sources

Introduction

Active transport is the movement of objects or organisms by self-exerted force. It is opposed to passive transport, which is a movement with an external source of work. In biology, active transport is the movement of molecules through a medium, usually the cell membrane.

Multicellular organisms use several active transport mechanisms to transport substances across the plasma membrane. It is the movement of molecules against a concentration gradient.

Active transport uses energy to move substances against a concentration gradient or another force (e.g., electromotility). Energy derived from ATP hydrolysis is used to transport substances from areas of low to high chemical concentration.

There are two main types of active transport mechanisms: Primary active transport and Secondary active transport.

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Primary Active Transport

Figure: Primary active transport

Primary active transport is defined as the movement of substances against a concentration gradient across a membrane. It uses ATP hydrolysis to transport substances from areas of low concentration, C, to areas of high concentration.

This method is used by the kidney and small intestine for water absorption and by the pancreas for insulin secretion. Substances transported primarily include monosaccharides (e.g., glucose), amino acids (e.g., leucine), sodium ions, potassium ions, and water itself.

The downhill movement of these substances provides the energy for ATP hydrolysis. The energy gained from this downhill gradient is used to drive the uphill movement of other substances (e.g., amino acids).

Due to the varying concentrations of substrates in the body fluids, these processes are used to transport the molecules. Molecules are also transported against a concentration gradient in this method but without energy from ATP hydrolysis.

Primary active transport is a result of energy conversion, and there is no net movement of matter. ATP is hydrolyzed into adenosine diphosphate (ADP) and inorganic phosphate.

Examples of primary active transport mechanisms are:

Proton pump, sodium-potassium pump, calcium pump, mitochondrial ATP synthase, ABC Transporter, and The Vacuolar ATPase Pump.

Proton Pump Mechanism

Figure: The proton pump in action

This mechanism is found in cells of the stomach and intestine. The H+ ions are transported from the lumen to the intracellular space across the membrane. This transport is coupled to ATP hydrolysis and requires an electrochemical gradient of protons (i.e., proton motive force or pH-gradient).

Within the lumen, gastric acid is present at a low pH (e.g., <3), and HCl is secreted into the lumen of the stomach. This proton gradient is used to drive ATP hydrolysis coupled with H+ transport across the plasma membrane. The H+ ions are transported into the cytosol and back into the lumen using an H+/K+-ATPase (i.e., V-type ATPase).

This process is targeted to the transport of H+ ions and requires no other substrates. This ATP-dependent transport process is called primary active transport, which moves molecules against a concentration gradient using energy derived from ATP hydrolysis.

The Sodium-Potassium Pump

Figure: The sodium-potassium pump

Cells also use active transport to transform energy into movement, as can be seen with membrane proteins such as ion pumps. For example, the sodium-potassium pump uses energy to move sodium out of and potassium into the cell. This movement is necessary for the propagation of action potentials in neurons.

The sodium-potassium pump works in a two-stage process: First, it takes three sodium ions out of the cell and two potassium ions into the cell. Then, it moves two potassium ions out of the cell for every three sodium ions that reenter it.

The hydrolysis of ATP illustrates this process into adenosine diphosphate (ADP) and inorganic phosphate. The ADP and inorganic phosphate are released from the inner membrane of a mitochondrion into the intermembrane space. This energy is then used to transport other substances across the membrane.

Chloroplast ATP Synthase

Figure: Chloroplast ATP synthase

This mechanism is found in the chloroplast of plants and is used for photosynthesis. The process consists of two complexes: a hydrophilic core and a hydrophobic peripheral part.

The hydrophilic core contains the membrane of the thylakoid lumen, whereas the hydrophobic peripheral part contains two lipid-soluble proteins. These proteins are embedded in the membrane at one side and leave a hole at the other, filled by water. This water allows the proton to flow through the membranes, which gives the energy required for the synthesis of ATP.

Plants use the energy of light to pump protons from the stroma through a protein complex located in the thylakoid membrane. The pumping causes an influx of sodium ions, which is accompanied by the efflux of potassium ions. The resulting electrochemical gradient is utilized to synthesize ATP from ADP and inorganic phosphate.

Calcium Pump

Figure: The calcium pump mechanism

ATP drives the active transport of calcium through the plasma membrane using an enzyme called calmodulin. This pump is found in all animal and plant tissues and transports calcium ions against its concentration gradient from the cytosol into organelles.

Calcium is the second messenger system in excitable cells. It binds to protein kinase C and phospholipase C, which results in the phosphorylation of various proteins. This mechanism is involved in signal conduction and gene expression. The two processes are essential for muscle contraction and exocytosis in the kidneys.

A calcium pump is located at different sites on the cell membrane, depending on the tissue and organelle targeted.  ATP hydrolysis energizes the process, with the resulting energy transporting calcium ions into and out of the cell. The energy released in this process is stored and used to hydrolyze ATP or transduce a signal across the plasma membrane.

The Mitochondrial ATP Synthase Pump

Figure: Mitochondrial ATP Synthase pump

This pump uses the proton gradient generated by the electron transport chain in the mitochondria to transport protons back into the mitochondrial matrix. This pump is used to make ATP inside the mitochondria by the conversion of ADP into ATP.

The mitochondrial ATP synthase contains two membranes: an outer membrane and an inner membrane connected by a channel filled with water.

The membrane contains integral proteins and peripheral proteins that are located at the top of the lipid bilayer. The peripheral part of the integral proteins is situated in the lipid bilayer and contains catalytic ATP synthesis sites. The membrane also has covalently attached phosphoryl groups that are part of the catalytic sites.

The ABC Transporter

Figure: The ABC Transporter

The ATP-binding cassette (ABC) transporter is a system found in bacteria and eukaryotic cells. It can transport a large variety of substrates by coupling ATP hydrolysis to the process. It is related to the bacterial flagella motor, which transports neurotransmitters across cell membranes.

The substrate is in the innermost membrane of the protein complex, which is linked to the peripheral membrane via a pore helix. The ABC transporter’s active site lies at the interface of the peripheral membrane and the substrate-containing compartment.

The Vacuolar ATPase Pump

Figure: Vacuolar ATPase Pump

Like the mitochondrial ATP synthase pump, the vacuolar ATPase pump uses the proton gradient to create ATP in the cell but in a different part of the cell. It is used to transport substances out of a cell, which takes place at or near the membrane.

The vacuolar ATPase pump is a powered transporter or active transport mechanism. When the energy in the form of ATP is hydrolyzed, it creates an electrochemical gradient applied to different substances in the cell.

The energy used for pumping can be generated by either non-ATP utilizing (vacuolar-type H+ -transporting ATPase) or by ATP-utilizing (solute- coupled H+-transporting ATPase) mechanisms.

The transporter structure consists of three main components: two membrane proteins and one protein located in the cytoplasm. The two membrane proteins are called V0 and V1, or hvα and hvβ in animals. The protein found in the cytoplasm is called V-ATPase.

Secondary Active Transport

Figure: Secondary active transport

Secondary active transport is defined by the movement of a substance up an electrochemical gradient created by an ion pump. An active transport protein creates this gradient (e.g., Na-K ATPase). The process occurs in both prokaryotic and eukaryotic cells.

This method is used by the cell to pump inorganic ions across the plasma membrane against a concentration gradient. It is used to maintain the concentration gradient of substances across a membrane or cell wall and create new gradients through active transport carrier proteins.

These transport processes transfer potassium from the extracellular fluid into the cell and sodium from the cell into the extracellular fluid. The pumping energy is provided either by the hydrolysis of ATP or from a proton pump. The latter uses the energy of electron transfer from a reduced redox carrier to an oxidized one.

In most bacteria, the transport proteins for secondary active transport are located in the plasma membrane. This type of active transport is divided into Symporter and Antiporter mechanisms.

Symporter

Figure: Symporter transport

Symporter is the transport of two or more different solutes across a membrane using a single carrier protein. The movement of each solute depends on the gradient of the other solute.

The direction of movement depends on the solute with the highest concentration, and it uses energy from ATP binding and hydrolysis. This transporter is associated with secondary active transport.

The symporter uses a special coupling mechanism to transport two different solutes across the membrane. However, this is only useful if there is an established gradient for the solutes on one side of the membrane.

Antiporter

Figure: Antiporter illustration

The antiporter is a special transporter where the active transport of one substance is dependent on the concentration of another substance.

It consists of two protein subunits, each with distinct binding sites that face each other across a membrane. One of its binding sites binds the substance to be transported, while the other binds the competing substance.

The transporter changes its shape to transport substance away from the area where it is most abundant and into the place where it is not present. As a result of the transport, the concentration of one substance is decreased while that of the other increases. This type of transporter is found in the membranes of mitochondria.

Secondary active transport is essential for maintaining stable sodium and potassium ions levels outside and inside cells, which helps maintain a stable membrane potential. The sodium-potassium pump is the most important of these pumps and exists in virtually all living cells.

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What is Bulk Transport?

Figure: Endocytosis versus Exocytosis

Bulk transport is the movement of substances by random molecular motion. The rate of bulk transport depends on two things: the concentration and temperature of a solution and the size of the solute.

This kind of active transport is not restricted to molecules and ions but can include larger objects like cells. Examples of bulk transport mechanisms are Exocytosis and Endocytosis.

Endocytosis

Endocytosis is a process in which a cell absorbs extracellular fluid and dissolved molecules into the cell through the plasma membrane. Cell surface receptors recognize certain specific molecules and then acquire them by sending out a signal to bring them in.

Cells ingest solid particles without digesting them first. Endocytosis uses invaginations of the plasma membrane called phagosomes to engulf solid material. Pinocytosis is used to engulf liquid material. Therefore, endocytosis is divided into phagocytosis and pinocytosis.

Phagocytosis

This process happens in three phases:

Figure: Phagocytosis

Phase 1

The process begins with the binding of a receptor to its ligand. The activated receptor then changes shape and induces a portion of the plasma membrane to invaginate or curl inwards. This new intracellular vesicle is called an endocytic vesicle and is the part of a cell that does the endocytosis.

Phase 2

The endocytic vesicle fuses with the plasma membrane and expands to form a phagosome.

Phase 3

Phagocytosis is complete when the phagosome fuses with a lysosome to form an endocytic vacuole. In eukaryotes, endocytosis involves the formation of a phagocytic cup. Phagocytosis is when a cell ingests a particle to be broken down by lysosomes.

Pinocytosis

Figure: Pinocytosis illustration

Pinocytosis is when a cell ingests extracellular fluid (and dissolved molecules) through invaginations of its plasma membrane.

The process of pinocytosis is similar to phagocytosis. In the former, a portion of the plasma membrane invaginates, but it is not phagosome-like. It forms a small intracellular vesicle called a pinocytic cup.

The pinocytic vesicle fuses with a lysosome to form a cytoplasmic vacuole. Pinocytosis is used to bring extracellular fluid into the cell, which can be used in metabolism.

Exocytosis

Figure: The Exocytosis process

Exocytosis is a process in which cells secrete substances outside of the cell. During this process, vesicles bud off from the plasma membrane and release their contents to the outside.

Exocytosis is used to transport substances too big or otherwise cannot pass through the plasma membrane, such as proteins and large polysaccharides. It’s also used to secrete extracellular enzymes and move molecules in the endocrine system.

What are Endomembrane Transporters?

Figure: Endomembrane transporters

Endomembrane transporters are membrane proteins that transport substances across biological membranes. These transporters are responsible for the active movement of molecules into, out of, or within cells.

These transporters are found in the endoplasmic reticulum (ER), Golgi apparatus, and more. The proteins involved are often ATPases and GTPases, which transport substances in an energy-dependent manner.

Endomembrane transporters are generally categorized into two groups:

Anion transporters

Figure: Anion transporters

This group moves anions like chloride, bicarbonate, and sulfate across the plasma membrane. These transporters are found in animal cells, and transport negatively charged ions from the cytoplasm into the cell or from the extracellular fluid to the cytoplasm.

Cation transporters

Figure: Cation transporters

This group moves cations like calcium, magnesium, and potassium into the cell or out of the cell. These transporters are found in fungi. They transport positively charged ions from the cytoplasm into the cell or from the extracellular fluid to the cytoplasm.

Cation transporter pumps are further divided into three groups:

  • The Na/K-ATPase is used by animal cells to pump sodium (Na) and potassium (K) ions out of the cell. It is found in animal cells and fungi, with different protein sequences reflecting a common evolutionary origin.
  • V-type H+-ATPases are used by prokaryotes to pump hydrogen ions (H+) out of the cell.
  • Voltage-gated proton pumps are used by animal cells to pump H+ or Na ions across the plasma membrane.

Concentration Gradient, Electrical Gradient, and Electrochemical Gradient

Concentration gradient

Figure: Concentration gradient illustration

A concentration (or distribution) gradient is the spatial separation of a solute in a solution. The term concentration can be replaced with the word distribution if the solution is distributed throughout a larger volume.

Concentration gradients are always from high to low concentration. The higher and the lower concentrations of solute are on opposite sides of the gradient.

The “concentration” is the number of particles in a given space or volume, including molecules, atoms, and ions.

Electrical gradient

Figure: Concentration vs Electrical gradient

An electrical gradient separates electric charges used to drive an electric current through a conducting medium. The amount of separation can vary, and the driving force for transport is always from high to low potential.

Electrochemical gradient

Figure: Electrochemical gradient

The electrochemical gradient (ion concentration gradient) is the driving force for an ion to move across a charged lipid (bilayer) membrane. However, it is not due to concentration differences, as it exists even in solutions that have uniform concentration.

The electrochemical gradient and electric gradient are not the same phenomena, and they often use different terms. However, “electric potential” and “electrochemical gradient” can be used to describe the same thing in different contexts.

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Summary

Active transport is the movement of substances across cell membranes. There are two types, facilitated and active. Facilitated transport has a carrier protein that moves other molecules or ions in one direction while remaining stationary on the membrane’s opposite side. This type occurs when there is a concentration gradient for both particles with an energy source like ATP to provide energy for this process.

The second type, active transport, uses direct coupling because there’s no need for outside help. The ions move against their concentration gradients due to adenosine triphosphate (ATP). Active transportation is a vital process in living organisms. Its importance cannot be overlooked.

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