Transport across the cell membrane: 4 ways | biology (2023)

Transport across the cell membrane is divided into four types: 1. Diffusion (passive transport) 2. Osmosis 3. Active transport 4. Vesicular transport.

The cell membrane acts as a barrier to most but not all molecules. Cell membranes are semi-permeable barriers that separate the internal cellular environment from the external cellular environment. Because the cell membrane consists of a lipid bilayer with proteins attached to the surface and also passing through the cell membrane, the potential for transport across this membrane exists.

All fat-soluble substances can enter and exit easily and without hindrance, e.g. EITHER₂and company₂. While water-soluble substances like ions, glucose, and macromolecules find a dedicated transport pathway to facilitate movement with the help of integral and transmembrane proteins that serve as binding sites, channels, and gates.

Pathway #1. Diffusion (passive transport):

The net movement of a substance (liquid or gas) from an area of ​​higher concentration to one of lower concentration without expenditure of energy is called diffusion.

Diffusion can be subdivided as follows:

A. Simple diffusion

B. Facilitated Diffusion.

A. Simple diffusion:

It is further divided into two categories:

I. Diffusion of lipid-soluble substances through the lipid bilayer.

ii. Diffusion of lipid-insoluble substances through protein channels.

I. Diffusion of fat-soluble substances through the lipid bilayer:

Substances such as oxygen and carbon dioxide and alcohols are highly lipid soluble and readily dissolve in the layer and diffuse across the membrane. The rate of diffusion is determined by the solubility of the substance. For example, gas exchange in the lungs.

ii. Diffusion of fat-soluble substances through protein channels:

This is possible either through selective channel protein permeability or through gated channels.

Selective protein channel permeability:

Only one type of ion can pass through this channel. Selectivity is based on the diameter, shape, and electrical charges along the inner surface of the channel.

For example:

A. Sodium channels:

The sodium channel is a tetramer with a pore diameter of 0.3-0.5 nm that is selective for sodium. It has a strong negative charge on the inner surface that allows dehydrated sodium ions to diffuse in any direction from high to low concentration.

B. Potassium channels:

It is selective for potassium. The pore of this channel is smaller than the sodium channel and has no negative charge. But the hydrated form of potassium ions is smaller than that of sodium, allowing selective diffusion of potassium ions.

Diffusion through activated protein channels:

A portion or bump of a protein channel behaves like a gate and can open or close in response to a change in voltage, ligand (chemical), mechanical stimuli such as touching and pulling, termed voltage-gated, ligand-gated, and mechanical. . closed channels, respectively, become .

A. Voltage controlled channels:

These channels open and close in response to a change in electrical potential across the cell membrane.

Example:

excitable cells such as neurons and muscle cells. When the voltage across the membrane changes, voltage-gated sodium channels open, allowing sodium ions to flow into the cell, causing a depolarization phase of the action potential, and potassium efflux via the voltage-gated potassium channel results in repolarization. . This is the basis of the action potential in an excitable cell.

B. Ligand gate channel (chemisch):

Some channels open in response to a chemical. They may be internal ligands with the binding site on the cytosolic side of the channel. For example Second Messenger. There may also be an external ligand that binds to a site on the extracellular side of the channel.

Example:

Neurotransmitters such as acetylcholine, gamma-aminobutyric acid that transmit impulses at a synapse.

C. Mechanically controlled channels:

They respond to mechanical stimuli and deformation due to mechanical stimuli opens or closes the channel.

Example:

Pressure receptors, when put under pressure, open the sodium channel and cause a potential receptor to develop. This helps us feel the pressure.

B. Facilitated dissemination:

Also called carrier-mediated diffusion. Highly charged or large molecules that cannot pass through protein channels require carrier proteins to facilitate diffusion. The carrier protein is selective for that particular substance. When a substance to be transported binds to a carrier protein on one side, a conformational change occurs in the shape of the protein, which transports the substance into the cell by opening on the other side of the membrane. It also obeys the law of diffusion (higher to lower concentration).

Example:

Glucose transporter (GLUT) and amino acid transporter.

Factors Affecting the Net Diffusion Rate:

Factors directly proportional to diffusion:

I. Concentration gradient across the membrane

ii. Electrical and pressure gradient across the membrane

iii. substance solubility

IV. body temperature

v. cell membrane permeability.

Factors inversely proportional to diffusion:

I. cell membrane thickness

ii. Ion/molecule size.

Applied Physiology:

Local anesthetics act directly on the gates of the sodium channel, making it difficult to open and thus reducing cell excitability. The impulse does not continue and causes anesthesia.

Ion channel mutations are called channelopathies.

They mainly affect muscle and brain tissue:

I. Sodium channel disease:

Muscle spasms and Liddle's syndrome.

ii. Potassium channel disease:

Arrhythmia, neonatal seizures and congenital deafness.

iii. Chloride channel disease:

kidney stones and cystic fibrosis.

Far #2. Osmosis:

Osmosis is the net movement or diffusion of water molecules across a semipermeable membrane from an area of ​​higher concentration to a lower concentration of water (solvent), or in other words, the movement of water from an area of ​​low concentration of solutes (ie, salts and electrolytes) at a higher solute concentration.

osmotic pressure:

When pressure is applied to the sodium chloride solution, the osmosis of water in the solution stops, reverses, or slows down. The pressure required to stop osmosis is called osmotic pressure. The osmotic pressure is determined by the number of particles per unit of liquid and not by the mass of the particle.

Osmolality and Osmolarity:

A mole is the molecular weight of a substance in grams. An osmole is equal to the molecular weight of a substance in grams divided by the number of particles in a solution. Osmolarity is the number of osmoles per liter of solution. Osmolality is the number of osmoles per kg of solvent. The osmotically active substance dissolves in body water, so osmolarity is expressed in milliosmoles (mOsm) per liter of water.

colloidal osmotic pressure:

It is the pressure exerted by the colloids present in the solution.

oncotic pressure:

The colloid osmotic pressure exerted by plasma proteins is called the oncotic pressure.

tonicity:

It is used to describe the osmolarity of a solution relative to plasma. If a solution has the same osmolality or a higher or lower osmolality than plasma, it is called an isotonic, hypertonic, or hypotonic solution.

Applied Physiology:

With any solution used for fluid replacement, the shade of the solution should be considered depending on the clinical situation.

Far #3. Enable Transport:

When a substance moves across the cell membrane under energy input against the concentration or electrical gradient (uphill), it is called active transport. Energy is obtained from the breakdown of high energy compounds such as ATP.

They are divided into primary and secondary active transport depending on the energy source used. The transporter involved here is also a transporter protein. But it is different from facilitated diffusion. Here, the carrier protein can energize the transported substance to move against the gradient.

I. Primary active transport:

In primary active transport, energy is released directly from the breakdown of ATP, and the transporter protein involved here is called a pump. The enzymes that catalyze the hydrolysis of ATP are called ATPases. Therefore, these pumps are called ATPases.

Sodium-potassium pumps or sodium-potassium ATPases:

Location:

Almost all cells have Na⁺k⁺pumped mainly in all excitable cells.

Structure:

It has two subunits, namely the α and β subunits.

Separation of the subunit abolishes the activity, but the function of the β subunit is unknown, the α subunit has:

A. Three receptor sites for binding sodium ions on the protein that protrudes into the cell.

B. Two receptor sites for potassium ions on the outside of the cell.

C. A site on the ATPase enzyme that is close to the sodium binding site.

mechanism of action:

The function is to pump excess Na⁺from intracellular fluid and extract K⁺in cell. Since there are 3 digits for Na⁺and 2 digits for K⁺, the pump is activated only when three Na⁺ion and two K⁺Ion adheres to the internal or external surface of the cell. For every three sodium ions expelled from the cell, two potassium ions are attracted to it. Therefore, there is a net loss of positive charge (ion) from the cell, which initiates water osmosis from the cell and prevents each cell from swelling.

The above mechanism also creates positivity outside the cell, but leaves a deficit of positive ions inside the cell. hence the no⁺k⁺The pump is called electrogenic because when it pumps, it creates an electrical potential across the cell membrane. This is necessary for the development of the resting membrane potential (RMP), which is the membrane potential across the resting cell membrane.

Characteristics:

I. Controls the volume of cells.

ii. It maintains the resting membrane potential.

Applied Physiology:

Digitalis is a medicine used to treat congestive heart failure. Inhibits the sodium-potassium pump. This leads to an increase in ICF sodium. This reduces calcium outflow through the sodium-calcium antiport by reducing sodium influx. This ultimately increases the concentration of calcium in the myocardial cells, which increases myocardial contractility.

Hydrogen and potassium ATPases:

Location:

Gastric glands of the stomach and distal convoluted tubules of the nephron.

Characteristics:

I. Transports hydrogen ions in the parietal cells of the gastric glands. At the secretory end of these cells, hydrogen is pumped into the stomach along with chloride ions to form hydrochloric acid, which is the main composition of gastric juice.

ii. Cells embedded in the distal tubules of the nephron pump hydrogen ions for urine formation and control the body's pH.

ii. secondary active transport:

In some places due to active transport of Na⁺of cells per Na⁺k⁺A large sodium concentration gradient usually develops in the bomb, with a higher concentration on the outside than on the inside. This gradient stores free energy that is used to transport other substances such as glucose and amino acids and other ions against their concentration gradient. The energy expended is not directly due to ATP hydrolysis, but is energy stored due to primary active transport.

There are two types of secondary active transport:

(a) Co-Transport,

(b) Return Transportation.

(a) Co-Transport:

It is also called symport. Here sodium and other substances to be transported move in the same direction.

Example:

Sodium-glucose cotransport in the proximal convoluted tubule of the nephron: Here the transporter protein undergoes a conformational change and is only ready for transport if sodium and glucose are bound to it and both move in the same direction. The energy is derived from the energy stored due to the transport of sodium through Na⁺k⁺It pumps over the basolateral membrane of the tubule. This creates a high concentration gradient of sodium ions within the tubular cell. The energy stored due to the gradient is used for both sodium and glucose transport along the luminal side of the tubule.

(b) Return Transportation:

It is also called an antiport. Here sodium and other substances to be transported move in the opposite direction.

Example:

Sodium-calcium antiport in myocardial cells.

Far #4. Vesicular transport:

They are classified as:

I. transport of vesicles within the cell

ii. endocytosis

iii. Exocytosis

bow. transcytose

I. transport of vesicles within the cell:

Vesicles, which help transport proteins from one organelle to another within the cell, have protein envelopes, namely caveolin, clathrin 1, clathrin 2, etc. These protein coats are specific for transport to specific organelles. A specific protein in the vesicle binds to its corresponding paired protein in the target, allowing the vesicle to make sure it latches on to the correct target. In general, the vesicles move along microtubule motors such as dynamin.

ii. endocytosis:

Endocytosis and exocytosis can also be considered in vesicle transport, since this type of transport occurs through vesicle formation. Endocytosis is a process in which cells engulf substances.

For example:

Bacteria and dead tissue engulfed by WBC.

Receptor-mediated endocytosis:

Endocytosis can also be specific when mediated by receptors, termed receptor-mediated endocytosis. Here, the molecule or ligand binds to specific receptors on the cell membrane, which are present in wells called clathrin wells in the cell membrane. Clathrin molecules have three legs emanating from a central point that surrounds the endocytic vesicle and pinches toward the cytoplasm. Once the vesicles are formed, the clathrin falls off and is reused. The vesicle then reaches the target.

Example:

Vitamins, transferrin and cholesterol enter the cell.

Endocytosis mechanism:

Some mechanisms of endocytosis are:

1. The materials to be swallowed come into contact with the cell membrane.

2. The cell membrane invaginates along with the material.

3. The invagination is pinched into the cells.

4. The pinched material inside the cell forms a vesicle, leaving the cell membrane intact.

A. When it is a solid material it is called phagocytosis (eating cells).

B. If it is a solution, it is called pinocytosis (cell drinking).

iii. Exocytosis:

It is reverse pinocytosis in which substances synthesized in secretory cells are secreted out of the cell. The secretory vesicle moves inside the cell membrane and fuses with it. The contents are extruded and the vesicle membrane becomes part of the cell membrane. For example, the release of neurotransmitters. Both endocytosis and exocytosis preserve the surface of cell membranes.

bow. Transcytose:

It is also known as cytopepsia. The mechanism involves endocytosis of the vesicle on one side of the membrane and exocytosis on the opposite side. The vesicle attachment site has cavities lined with caveolin. For example, the transport of nutrients by endothelial cells from the blood vessels to the interstitial fluid.