Mode Of Transport Across Plasma Membrane

Passive transport

It is a sort of diffusion in which an ion or molecule that has traversed a membrane goes down its electrochemical or concentration gradient. Passive transportation requires no metabolic energy. The following are the three types of passive transport:

1. Osmosis

The plasma membrane is water molecule permeable. The back and forth migration of water molecules through the plasma membrane is caused by changes in the solute concentration on each side of the membrane.
Osmosis (Greek: osmos=pushing) is the process by which water molecules move across a membrane from an area of higher water concentration to a region of lower water concentration.
Endosmosis is the process by which water molecules enter a cell, whereas exosmosis is the process by which water molecules leave a cell.
In plant cells, severe exosmosis causes the cytoplasm and plasma membrane to recede from the cell wall. This process is called plasmolysis (Greek: plasma = shaped, lysis = releasing).
The mammalian erythrocytes contain the ions of potassium (K+), calcium (Ca+), phosphate (PO4 —), dissolved haemoglobin, and numerous other compounds.
When put in a 0.9% solution of sodium chloride (NaCl), the erythrocyte neither contracts nor swells. Because intracellular and extracellular fluids contain the same concentration, osmosis does not occur in this scenario.
This sort of extracellular fluid or solution is called an isotonic fluid or solution. Due to severe exosmosis, erythrocytes shrink if the concentration of NaCl solution is increased over 0.9%.
Hypertonic solutions are those that contain a higher concentration of solutes than intracellular fluids.
Additionally, if the concentration of NaCl solution falls below 0.9%, endosmosis will cause the erythrocytes to expand. Hypotonic solutions are extracellular solutions with a lower concentration of solutes than the cytoplasm.
Through endosmosis or exosmosis, water molecules enter or leave the cell. Hydrostatic pressure is caused by the amount of water contained within a cell.
The hydrostatic pressure induced by osmosis is referred to as osmotic pressure. The plasma membrane maintains an equilibrium between the intracellular and intercellular osmotic pressures.

2. Simple diffusion

In simple diffusion, transport across the membrane occurs unassisted, i.e., molecules of gases such as oxygen and carbon dioxide and tiny molecules (e.g. ethanol) enter the cell by traversing the plasma membrane without the assistance of a permease.
During simple diffusion, a tiny molecule in aqueous solution dissolves into the phospholipid bilayer, traverses it, and then dissolves into the aqueous solution on the other side. The procedure has minimal specificity.
The concentration gradient across the membrane will determine the relative diffusion rate of the chemical across the phospholipid bilayer.

3. Facilitated diffusion

This is a specialised form of passive transport in which particular permeases in the membrane enable the rapid passage of ions or molecules across the membrane.
Similar to simple diffusion, assisted diffusion happens only in the direction of a concentration gradient and does not require metabolic energy.
Facilitated diffusion is distinguished by the following unique characteristics: (1) The transport rate of the molecule through the membrane is far higher than would be predicted by simple diffusion. (2) Each facilitated diffusion protein (also known as a protein channel) transports only a single ion or molecule species. (3) There is a maximum transport rate, i.e., when the concentration gradient of molecules across the membrane is minimal, a rise in concentration gradient causes a proportional increase in transport rate.
Currently, it is assumed that transport proteins build the membrane channels that allow particular ions or molecules to traverse the membrane.

Examples of Facilitated Diffusion

(i) Ionic transport through charged pores

Nerve conduction is carried over the axonal membrane by action potential, which controls the opening and closing of two major types of ion channels (proteins with water-filled pores): Na+ channels (or voltage-gated Na+ channels) and K+ channels (or k+ leak channels).
At the moment of stimulation, Na+ permeability increases abruptly by several hundred-fold, reaching its peak in 0.1 millisecond (i.e., the membrane potential may depolarize from –90 mV and overshoot to +50 mV).
At the conclusion of the interval, the membrane once again becomes essentially impermeable to Na+, but the K+ permeability increases and this ion escapes the cell, repolarizing the nerve fibre.
In other words, during the rising phase of the spike, Na+ enters through the Na+ channels, whereas K+ is expelled through the K+ channels during the falling phase. These ion channels can also be found in muscle, sperm, and unfertilized ovum cells.
They are not connected to an energy source (ATP), hence the transport they mediate is always passive (“downhill”), allowing certain ions, primarily Na+, K+, Ca2+, and Cl——- to flow across the lipid bilayer in the direction of their electrochemical gradient.
In addition, ion channels are composed of integral proteins of the brain membrane. This protein possesses two functional domains: (1) a selective filter that specifies the type of ion that will be transported; (2) a gate that regulates the ion flow by opening and shutting the channel.
In both Na+- and K+- channels, the gating mechanism is electrically driven and controlled only by the membrane potential. In the steady-state situation, both Na+ and K+ channels are closed.
The Na+ channel opens during depolarization and closes during repolarization, allowing the K+ channel to open.
In axonal membranes and other membranes, calcium ion channels (Ca2+- channels) allow Ca2+ ions to enter the cell.
Ca2+ ions have a crucial part in a variety of cellular processes, including exocytosis, endocytosis, secretion, cell motility, cell growth, fertilisation, and cell division.
A variety of Ca2+ channels in the neuronal membrane are driven by the membrane potential and are required for the release of neurotransmitters (acetylcholine).

2. D-hexose permease of erythrocyte

There are unique channel proteins in the plasma membrane of mammalian erythrocytes and other body cells that assist the diffusion of glucose into the cells.
These proteins are known as glucose transporter, glucose permease, and D-hexose permease. After glucose enters the erythrocyte, it is promptly phosphorylated (by the hexokinase enzyme and ATP) to glucose-6-phosphate. Once glucose has been phosphorylated, it no longer leaves the cell, and the concentration of simple glucose within the cell decreases.
As a result, the glucose concentration gradient across the membrane increases, allowing facilitated diffusion to continue importing glucose.
Since no cellular membrane (with the exception of mitochondrial membranes) contains permease for facilitated diffusion of phosphorylated compounds, a cell can retain any type of molecule by phosphorylating it, e.g., ATP and phosphorylated nucleosides are never released from cells with a normal, intact plasma membrane.
However, mitochondrial membranes contain permeases for ATP and ADP that allow these molecules to pass.
Erythrocyte D-hexose permease is an integral, transmembrane protein with a molecular weight of 45,000 daltons. It comprises 2% of the erythrocyte membrane protein.
D-hexose permease acts, most likely, as follows: the binding of glucose to a location on the outer surface of the permease causes a conformational change in the polypeptide.
This modification creates a pore in the protein that permits the glucose to pass past the membrane.

3. Anion exchange permease of erythrocyte

Band 3 polypeptide of plasma membrane of erythrocytes and other cells is an ion exchange permease protein that catalyses anions such as chloride (Cl-) and bicarbonate (HCO3 -) exchange across the membrane (called chloride shift; erythrocyte has 100,000 times more permeability of Cl——- than other cells).
The rapid movement of anions within the erythrocyte increases the transport of CO2 from the tissues to the lungs via the blood.
The waste CO2 that is discharged from the cell into the capillary blood diffuses over the erythrocyte membrane.
In its gaseous form, CO2 dissolves poorly in aqueous solutions such as blood plasma, but the powerful enzyme carbonic anhydrase transforms it into a bicarbonate anion within the erythrocyte:

Mode of Transport Across Plasma Membrane

While the haemoglobin in the erythrocyte is releasing oxygen into the blood plasma, this process occurs. The loss of oxygen from haemoglobin causes a conformational shift that allows a globin histidine (amino acid) side chain to interact with the proton produced by the carbonic anhydrase enzyme. The bicarbonate anion produced by carbonic anhydrase is expelled from the erythrocyte in exchange for chloride (Cl—-) anions:

As the total volume of blood plasma is approximately double that of erythrocyte cytoplasm, this exchange triples the amount of bicarbonate that blood may carry.
Without the presence of an anion exchange protein (i.e., band 3 protein), bicarbonate anions formed by carbonic anhydrase would linger within the erythrocyte, preventing blood from transporting all of the CO2 produced by tissue.
In 50 milliseconds (ms), 5 109 HCO3 – ions are exported from the cell, completing the exchange process. In the lungs, the process is inverted: HCO3 – diffuses into the erythrocyte in exchange for Cl-.
The release of the haemoglobin proton is caused by the binding of oxygen to haemoglobin. CO2 diffuses out of the erythrocyte and is eventually exhaled during respiration. The precise process by which the Band 3 protein transports anions remains unknown.

Active transport

Active transport employs particular transport proteins, known as pumps, that utilise metabolic energy (ATP) to move ions or molecules against their concentration gradient.
In both vertebrates and invertebrates, the concentration of sodium ion in the blood is around 10 to 20 times higher than within the cell.
Inside the cell, the concentration of potassium ion is typically 20 to 40 times greater. The sodium-potassium pump keeps the intracellular sodium content at such a low level.
There are various sorts of pumps for various ions and molecules, such as the calcium pump, proton pump, etc.

Examples of Active Transport

1. Na+- K+- ATPase.

It is an ion pump or cation exchange pump that uses the energy of one ATP molecule to export three Na+ ions from the cell in return for the import of two K+ ions.
This enzyme or pump is extremely abundant in the electrical organs of eels. N+- K+- ATPase is a transmembrane protein that is a dimer composed of two subunits: one smaller unit that is a glycoprotein of 50,000 daltons M.W. with an unknown function; and one larger unit with 1,200,000 daltons M. W.
The bigger subunit of Na+- K+- ATPase performs the actual function of cation transport. On its extracytoplasmic surface, it possesses three sites: two for K+ ions and one for the inhibitor ouabain.
On its cytosolic side, the bigger subunit includes three sites for three Na+ ions and also has one catalytic site for an ATP molecule.
It is hypothesised that the hydrolysis of a single ATP molecule induces conformational changes in the Na+- K+- ATPase, allowing the pump to transport three Na+ ions outside the cell and two K+ ions inside.

2. Calcium ATPase

Calcium pump or Ca2+-ATPase pumps Ca2+-ions out of the cytosol, hence maintaining a low Ca2+ concentration in the cytosol.
Calcium pumps are found in the plasma membrane of some cell types, such as erythrocytes, and act to transfer Ca2+ ions out of the cell.
In contrast, Ca2+-ion pumps in muscle cells are situated in the ER or sarcoplasmic reticulum membrane. Ca2+-ATPase transfers Ca2+ from the cytosol to the interior of the sarcoplasmic reticulum to induce muscle cell relaxation.
The contraction of muscle cells is caused by the release of Ca2+ ions from the sarcoplasmic reticulum into the cytoplasm of the muscle cells.
Ca2+ ions are concentrated and stored in the sarcoplasmic reticulum with the aid of two types of reservoir proteins: (1) Calsequestrin, a protein with a molecular weight of 44,000 daltons that binds up to 43 Ca2+ ions. (2) Ca2+-binding protein with high affinity that binds Ca2+ ions, reduces the concentration of free Ca2+ ions in sarcoplasmic reticulum vesicles, and reduces the amount of energy required to pump Ca2+ ions into it from the cytosol.
A calcium pump is a polypeptide of 100,000 M.W. that makes up 80% of the integral membrane protein of the sarcoplasmic reticulum. In it, the hydrolysis of a single ATP molecule transfers two Ca2+ ions and one Mg2+ ion.

3. Proton pump or H+- pump

The lysosomal membrane contains the ATP-dependent proton pump that transfers protons from the cytosol into the lumen of the organelle, maintaining the highly acidic environment within lysosomes (pH 4.5 to 5.0). The approximate pH of the cytosol is 7.
In mitochondria and chloroplasts, proton pumps contribute to the synthesis of ATP from ADP.
They also cause gastric acidity in mammals. In the apical membrane of parietal or oxyntic cells (which produce HCl or H+ Cl—-), ATP-dependent proton pumps are situated.
Hydrolysis of ATP is associated with the efflux of H+ ions from the cell (into stomach lumen). Thus, HCl synthesis requires three types of transport proteins: anion-exchange protein, chloride (Cl—-) permeases, and an ATP-dependent proton (H+) pump.

Uniport, symport and antiport

Uniports refer to carrier proteins that carry a single substance from one side of the membrane to the other.
Others operate as coupled transporters, in which the transfer of one solute is dependent on the transfer of a second solute in the same direction (symport) or in the opposite way (antiport) (antiport).
Cotransport is composed of both symport and antiport. Most animal cells, for instance, must take up glucose from the extracellular fluid, where the sugar concentration is quite high, via passive transport via glucose carriers (such as D-hexose permease) that function as uniports.
In contrast, intestinal and kidney cells must absorb glucose from the lumen of the gut and renal tubules, where the sugar concentration is low.
These cells actively transport glucose via symport with the very concentrated Na+ ions found extracellularly. Human erythrocyte anion exchange permease functions as an antiport to the exchange of Cl- for HCO3 -.

Bulk transport by the plasma membrane

Large molecules are routinely imported and exported through the plasma membrane by cells. Exocytosis is the process by which macromolecules are secreted from the cell whereas phagocytosis and endocytosis are the processes by which macromolecules are taken into the cell from the outside.

1. Exocytosis

You may know it by its alternative names, emeiocytosis and cell vomiting. Secretory vesicles constantly transport proteins, lipids, and carbohydrates (e.g., cellulose) from the Golgi apparatus to the plasma membrane or to the cell exterior via the process of exocytosis in all eukaryotic cells.
RER is responsible for synthesis of secreted proteins (RER). They enter the ER lumen, where they are glycosylated before being shuttled to the Golgi apparatus by vesicles that originate in the ER.
Proteins are refined, concentrated, further glycosylated, sorted, and packaged in the Golgi apparatus before being secreted via vesicles that pinch off from trans Golgi tubules and go to the plasma membrane, where they fuse and are released.
Histamine, for example, is actively transported from the cytosol (where it is synthesised on the free ribosomes) into preformed vesicles, where it is complexed to specific macromolecules (such as a network of proteoglycans, in the case of histamine; Lawson et al., 1975) so that it can be stored at high concentration without generating an excessive osmotic gradient.
The plasma membrane acquires some of the characteristics of the vesicle membrane during exocytosis. When a cell is stimulated to secrete, it may temporarily add a large amount of secretory vesicle membrane to the plasma membrane. For example, when a pancreatic acinar cell releases digestive enzymes, about 900 m2 of vesicle membrane is inserted into the apical plasma membrane (whose area is only 30 m3).

2. Phagocytosis

Occasionally, the cell absorbs huge food particles or foreign particles via its plasma membrane.
Phagocytosis (Greek: phagein = to eat, kytos = cell or hollow vessel) is the process by which a cell consumes large-sized solid objects (such as bacteria and fragments of shattered cells).

Occurrence of phagocytosis

The process of phagocytosis occurs in the majority of protozoa and specific multicellular cells.
In multicellular creatures like mammals, phagocytosis occurs particularly vigorously in granular leucocytes and mesoblast-derived cells.
Cells of the macrophagic or reticuloendothelial system are collectively characterised as cells of mesoblastic origin.
Histiocytes of connective tissue, reticular cells of the hemopoietic organs (bone marrow, lymph nodes, and spleen), and endothelial cells that line the capillary sinusoids of the liver, adrenal gland, and hypophysis comprise the macrophagic system.
Through the process of phagocytosis, macrophages can consume bacteria, Protozoa, cell debris, and even colloidal particles.

Process of phagocytosis

During phagocytosis, the target particle first binds to certain receptors on the cell’s surface (a process known as adsorption), after which the plasma membrane stretches along the particle’s surface and eventually engulfs it.
The vesicle created by phagocytosis is known as a phagosome, and it normally has a diameter of 1 to 2 m or greater, far larger than those formed by pinocytosis and receptor-mediated endocytosis.
The phagosomes move to the inside of the cell and combine with the lysosomes that are already present (to form phagolysosome).
The hydrolytic enzymes (acid hydrolase) of the lysosomes digest the food, and the digested food is eventually disseminated into the surrounding cytoplasm.
In addition to the standard collection of lysosomal hydrolases, the lysosomes of macrophages contain enzymes that produce hydrogen peroxide (H2O2) and other harmful compounds that aid in the destruction of bacteria.
Through the processes of ephagy or egestion, undigested food is ejected from the plasma membrane. In macrophages, residual bodies consist of undigested portions of ingested material, such as the cell walls of microorganisms.
The short lifespan of macrophages may be attributable in part to the accumulation of residual bodies (i.e., less than a few days).

Types of phagocytosis

According on the physical and chemical characteristics of the foreign substance, the following phagocytosis types have been identified:

Ultraphagocytosis or colloidopexy: The process in which plasma membrane ingests tiny colloidal particles is known as colloidopexy or ultraphagocytosis, such as in leucocytes and mammalian macrophages.
Chromopexy: Some mesoblastic cells, for instance, engage in chromopexy when they swallow colloidal chromogen particles via phagocytosis.

3. Endocytosis

Small sections of the plasma membrane invaginate or fold inwards during endocytosis, forming new internal membrane-limited vesicles.
There are two types of endocytosis that can occur in eukaryotes: pinocytosis and receptor-mediated endocytosis.

(i) Pinocytosis

Pinocytosis (Greek: ; ‘cell drinking’) is the non-specific ingestion of minute droplets of extracellular fluid by endocytic vesicles or pinosomes with a diameter between 0.1 m and 0.2 m.
Any dissolved item in the extracellular fluid is absorbed in proportion to its concentration in the fluid.
Edward initially detected pinocytosis in amoebae, and Lewis (1931) recognised the process in cultivated cells.
Light microscopy has revealed that invagination of the plasma membrane continuously forms minute pinocytic channels at the cell surface of amoeba.
From the inner end of each channel, little vacuoles or pinosomes are snatched and transported to the cell’s centre, where they combine with primary lysosomes to form food vacuoles.
Small breakdown products, such as sugars and amino acids, permeate into the cytosol during digestion of the ingested material.

Micropinocytosis

(ii) Receptor-mediated endocytosis

A particular receptor on the surface of the plasma membrane “recognises” and binds with an external macromolecule during this sort of endocytosis.
The chemical that binds to the receptor is referred to as the ligand. Viruses, tiny proteins (e.g., insulin, vitellogenin, immunoglobin, transferrin, etc.), vitamin B12, cholesterol including LDL or low density lipoprotein, oligosaccharides, etc. are examples of ligands.
Endocytosis occurs at the area of the plasma membrane containing the receptor-ligand complex.

Events of Receptor-mediated endocytosis

The entire process of receptor-mediated endocytosis comprises the subsequent steps:

1. Interaction of ligands and cell surface receptors

The macromolecules (ligands) attach to receptors on the cell surface. There are more than 25 different types of receptors involved in the receptor-mediated endocytosis of various molecule types.
Such a receptor is a transmembrane protein with two specialised binding sites: (1) ligand-binding site at the plasma membrane’s outer surface; and (2) coated-pit binding site at the plasma membrane’s inner or cytosolic face.

2. Formation of coated-pits and coated-vesicles

Coated-pits are specific areas of the plasma membrane where the endocytic cycle begins.
Coated-pits are plasma membrane depressions with a coat of bristle-like structure on their cytosolic side.
Receptors loaded with ligand diffuse into these coated-pits. A coated-pit may accommodate around one thousand receptors of various types.
In actuality, coated-pits function as molecular filters and selective concentrating devices, as they tend to gather specific receptors while leaving others behind.
In addition to transporting small extracellular fluid components, they boost the internalisation efficiency of a specific ligand by more than a thousandfold.
Within a minute or so of formation, each coated-pit invaginates into the cell and pinches off, forming coated-vesicles.
The coating of coated pits and coated vesicles is composed of the protein clathrin and other proteins.
Clathrin molecules are made of three big polypeptide chains and three smaller polypeptide chains, which create a triskelion, or three-legged configuration.
A number of triskelions form a network of hexagons and pentagons resembling a basket on the cytoplasmic surface of the membranes.

3. Fusion of endocytic vesicle and endosome

Upon formation of a coated vesicle, clathrin and related proteins detach from the vesicle membrane and return to the plasma membrane to form a new coated-pit.
The resulting endocytic vesicle fuses with preexisting endosomes, and its contents are ultimately consumed by the cell.

Endosome or receptosome

Endosomes are a recently discovered collection of heterogeneous membrane-bound tubes and vesicles that extend from the cell’s periphery to the perinuclear region, where they are located near to the Golgi apparatus.
Consequently, there are two types of endosomes: I peripheral endosomes just below the plasma membrane, and (ii) perinuclear or internal endosomes.
Due to the presence of ATP-driven proton (H+) pumps in its membrane that pump H+ ions into the lumen from the cytosol, the interior of the endosome is acidic (pH 5-6). Endosomes low in degradative enzymes.
Thus, ligands are supplied to peripheral endosomes via receptor-mediated coated vesicles, which slowly migrate inside to become perinuclear endosomes.
These perinuclear endosomes are transformed into endolysosomes and ultimately lysosomes as a result of the following three processes:
- the fusing of Golgi apparatus transport vesicles (Note: Transport vesicles capture a cargo of molecules, such as proteins, from the lumen of one compartment as they pinch off from its membrane, and then release that cargo into another compartment as they fuse with it. Consequently, in this type of vesicular transport, the proteins are moved from one lumen to another without passing through any membranes.
- a constant process of retrieving membranes.
- higher levels of acidity.
Furthermore, the endosomal compartment is the primary sorting site along the endocytic route. Dissociation of ligands from their receptors occurs in the acidic endosome environment.
These ligands, along with the rest of the endosome’s non-membrane-bound contents, are headed for degradation in the lysosomes.
The receptor-proteins either return to the plasma membrane domain they originated from or are transported to the lysosomes where they are destroyed.

Example of receptor-mediated endocytosis

The majority of animal cells have a regulatory mechanism for cholesterol absorption. The majority of cholesterol is conveyed in the blood as low-density protein particles, or LDL.
Each of these huge spherical particles (22 nm in diameter) is composed of a core of approximately 1500 cholesteryl ester molecules surrounded by a lipid monolayer and a single big protein molecule (apoprotein).
When the cell requires cholesterol for membrane production, it synthesises and inserts LDL particle receptor proteins into its plasma membrane.
Only 50 amino acid residues protrude from the cytoplasmic side of the plasma membrane to form the coated-protein-binding site of the human LDL receptor, which is a single-pass transmembrane glycoprotein.
The receptor’s LDL-binding site is exposed on the cell surface. The LDL receptors migrate laterally within the lipid bilayer until they are connected with newly generated coated-pits.
Due to the continual formation of coatedvesicles by coated-pits, LDL particles are coupled to receptors in coated-pits and swiftly absorbed.
The endocytic vesicles deliver their contents to endosomes after shedding their clathrin coverings.
LDL particles and their receptors are separated in endosomes, with the receptors returning to the plasma membrane and LDL entering the lysosomes.
In the lysosomes, the cholesteryl esters in LDL particles are digested into free cholesterol molecules, which are then available for membrane formation.
If an excessive amount of free cholesterol accumulates in a cell, this inhibits the cell’s cholesterol synthesis and the synthesis of LDL-receptor proteins, resulting in less cholesterol being produced and taken up by the cell.

Energy utilisation by phagocytosis and endocytosis

In contrast to pinocytosis, which is a constitutive process that happens continually, phagocytosis is a triggered process in which active receptors transfer signals to the interior of the cell to initiate the reaction.
Both phagocytosis and pinocytosis are active systems in that they require cellular energy to function.
During phagocytosis, leukocytes greatly increase their oxygen consumption, glucose uptake, and glycogen breakdown.
Endocytosis involves the contraction of actin and myosin microfilaments in the peripheral cytoplasm (ectoplasm), which causes the plasma membrane to invaginate and form the endocytic vacuole (endosome/phagosome).
By inhibiting endocytosis and disorganizing actin microfilaments, the medication cytochalasin B demonstrates the involvement of actin microfilaments. Fusion of the membrane during exocytosis and endocytosis.
Exocytosis and endocytosis both involve the fusion of initially separate sections of lipid bilayer in at least two steps: first, the two bilayers come into close proximity, known as bilayer adhesion, and then they merge (This is called bilayer joining).
Both of these processes are believed to be mediated by particular proteins known as fusogenic proteins.