Transport across the cell membrane

This article discusses different modes of transportation across cell membranes and their importance. Here, learn about the transportation process mechanisms and more.

Keywords:  Human physiology| Biology |Active |Passive | Diffusion|  primary active transport | secondary active transport| transcytosis |Phagocytosis | pinocytosis | Exocytosis | channels | ligands | 

Table of contents

1.	Introduction
2.	Classification of transportation
3.	Passive
4.	Active
5.	Links

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Bilipid layer of cell membrane -simple diffusion

                         This Photo is by Sandhya Prasad, Licensed under CCBY.

The cell membrane is the outer layer of the cell. A cell membrane is a semi-permeable and selectively permeable structure. This is a bi-lipid layer in which protein molecules are embedded. Proteins form about 60%-70% by weight of the cell membrane, while lipids contribute the rest. Substances pass the cell membrane through two distinct processes.

Sodium Potassium ATP ase pump

                        This Photo is by Sandhya Prasad, Licensed under CCBY.

 1. Active, and 2. Passive transport processes molecules or substances move across the cell membrane.

1. Passive transport: 

Characteristics of passive transport:

No energy is required

Transport occurs from high to low concentrations along the chemical gradient, a downhill movement.

Transport across along an electrical gradient that is from the positive charge. Cations move to anions ( negatively charged areas) and vice versa.

Types of passive transport

                      This Photo is by Sandhya Prasad, Licensed under CCBY.

Types: 1. Simple diffusion, and 2.Facilitated diffusion

Simple diffusion occurs for small and non-charged particles.

Charged particles cannot cross the cell membrane by this method. However, small molecules, lipids soluble drugs, and lipid-soluble molecules can move through to the cell membrane, for example, oxygen, carbon- dioxide, nitrogen, alcohol, steroid hormones, and lipid-soluble drugs across the cell membrane by simple diffusion from higher concentrations to low concentration or from positive to negative or harmful to positive charge areas.

The rate of simple diffusion is directly proportional to the cell membrane's surface area, the substance's solubility, temperature, concentration gradient, and lipids solubility.

Simple diffusion is inversely proportional to the molecular weight of the substance if the molecular weight increasing rate of diffusion decreases.

Fick's law: Rate of diffusion is = diffusion coefficient of the substance x surface area x concentration gradient/thickness of the membrane.

Water soluble molecules (ions and glucose) cross the cell membrane slowly through the aqueous channel (Aquaporin 2).

Types of channels in transportation
This Photo is by Sandhya Prasad, Licensed under CCBY.

Channels: ion channels cross the cell membrane. The ionic channels are different for different ions.

Large and charged molecules move through these channels.

Types:

Open channels or leak channels, for example, potassium leak channels.

Gates protect gated channels.

Voltage-gated channels operate by alteration of membrane potential -concentration of Ions on the cell membrane, for example, Sodium ions, Potassium ions, and calcium channels.

Ligand-gated

channels: When the gate binds a ligand, the gate opens.The ligands and maybe

External: a hormone or neurotransmitter, or

Internal: cyclic GMP, G-protein, and Calcium ions

Mechanosensitive channels open due to mechanical stretch.

Facilitated diffusion :

Characteristic features are:

More rapid than simple diffusion.

The passive transport system,

No energy is required

Carrier mediated process

Large molecules that cannot pass through ion channels.

Carrier proteins undergo repetitive configurational changes during which the binding sites for the substance are exposed to intracellular fluid and extracellular fluid alternatively.

The rate of diffusion increases with increases in the concentration of the substance. However, if the concentration of the substance rises more, all carrier proteins for that substance get saturated. At this stage, if the concentration of the substance increases, diffusion will not increase -it will form a plateau. Characteristics of carrier protein:

1. Saturation

2. Stereo specific

3. Competition, for example - transport of glucose and galactose for carrier protein, i.e., GLUT-4, across the intestinal cell membrane.

Osmosis: Water moves across Aquaporin-2 in the cell membrane from higher to low concentrations.

Active transport process

Characteristic features are:

1. Use energy

2. Low-concentration to high-concentration uphill transport.'

against concentration and electrical gradient.

Types

1. Primary active transport process

2. Secondary active transport process

3. Vesicular transport process

Primary active transport process: This process is known as a pump and uses energy obtained by hydrolysis of adenosine triphosphate( ATP).

Examples: Sodium Potassium ATPase pump, Calcium ATPase pump, Potassium Hydrogen pump, Sodium Hydrogen ATPase pump.

Detail of Sodium Potassium ATPase: The sodium Potassium pump is present in all body cells. It is the most common pump in the body and utilizes a significant part of the basal metabolic rate.

This carrier protein is formed by 3- alpha subunits and 3 beta subunits. Alpha units are responsible for Sodium and potassium transport.

ATP is converted into ADP and one high-energy phosphate by ATPase attached to the alpha-subunits of the carrier protein. This phosphorylation reaction. ADP is released.

On the intracellular side, it has binding sites for three sodium ions; on the extracellular side, it has two potassium binding sites.

Three sodium ions and ATP bind to a carrier protein inside the cell membrane. ATPase on the carrier protein separates the high-energy phosphate group from the aspartic acid residue of the alpha subunit of the carrier protein. This is the phosphorylation of carrier protein, during which configuration changes so that three sodium ions move to the external surface of the cell, and two potassium ions bind to the carrier protein on the outer side of the cell and move inside. 

The aspartic acid phosphatase bond is hydrolyzed. That is, the dephosphorylation of aspartic acid occurs. Sodium Potassium ATPase pumps hydrolyses ATP to release energy, which causes three sodium ions to move outside and two potassium ones inside. This is a 3/2 coupling.

This electrogenic pump with a coupling ratio of 3/2 causes the net movement of one positive charge out of the cell.

Dysfunction of the Sodium Potassium pump

1. When the temperature is reduced

2. Concentration of sodium and potassium ATPase is reduced when oxygen concentration is deficient due to some reasons, for example, 2,4 dinitrophenol, that prevents the formation of ATP.

3. Dopamine reduces its activity.

Factors increasing Sodium Potassium ATPase activity: Hormones: thyroid, insulin, and G-actin.

Other examples of the primary active transport processes are:

The calcium ATPase pump is present in the sarcoplasmic reticulum of muscles. Most cells' cytoplasm has a lower calcium concentration than the extracellular fluid.

Potassium hydrogen ATPase is present in gastric mucosal cells and renal tubules, where it causes the secretion of hydrogen ions.

In the secondary active transport process, primary active sodium transport ATPase is linked to the transport of other system substances.

When sodium ion binds

The carrier protein's affinity for the substance to be transported increases. The sodium ion moves downhill while the substance moves uphill, and the energy required is supplied by the Sodium Potassium ATPase pump.

Glucose sodium secondary active transport process: This carrier is in the intestinal mucosal cell membrane. Glucose and Sodium attach to their respective sites and are transported inside the cell. This is Cotransportation or symporter. Glucose is not absorbed unless Sodium is attached to the carrier protein and vice versa. Galactose competes with glucose.

When sodium and glucose molecules attach to the carrier protein, the protein configuration changes, so both molecules move inside the cell. As a result, the carrier protein is co-transported when both molecules move in the same direction.

In another instance, Sodium moves inside, that is, downhill, and hydrogen ions move uphill. This is counter transport or antiports.

The vesicular transport process is also known as transcytosis.

Types:

Pinocytosis

Phagocytosis

Receptor-mediated endocytosis and exocytosis.

Pinocytosis is a process by which the cell membrane engulfs [liquid] water with dissolved solids.

When the substance touches the cell membrane, it sends pseudopodia and invaginates, forming pinocytic vesicles. Pinocytes contain water and dissolved solids. In addition, microtubules are present with unique motor proteins called' kinesin' and Dyenisn.' These motor proteins attach with pinocytic receptors and drag the pinocytic vesicles inside the cell, releasing their contents in the cytoplasm. The motor proteins use ATP, which is a primary active transport process.

Phagocytosis is a primary active transport process, also known as cell eating. Through this process, particulate substances, bacteria, and large molecules are engulfed and digested. When these substances touch, the cell membrane sends pseudopodia, covers the substance, and forms phagosomes. 

Actin filaments drag phagosomes inside the cell. Lysosomes present in the cytoplasm fuse with phagosomes and form phagolysosomes. Different types of enzymes present in the lysosomes act on the substance. They work optimally in an acidic media and break the substance into small and digested particles. The metabolic waste material is contained in the vesicles, and metabolites are expelled from the cells.

Receptor-mediated endocytosis is also an active process. On the cell membrane, there are LDL receptors. The LDL receptors project on the cell membrane's outer surface; on the inner side, there is a unique protein,' claudin.' When LDL combines with its receptor, the cell membrane forms pseudopodia and covers it, forming a vesicle. The wall of the vesicle is also a bilipid layer and a proton pump enters the vesicles. The claudin covers the vesicle called 'claudin coded pits.' The claudin pulls it inwards. The proton pump operates in the vesicle and dissociates LDL from its receptor. The dissociated receptor moves to the cell membrane and works again. This is 'receptor-recycling.' The proton pump operates using ATP. Now, the vesicle contains LDL. Lysosomes fuse with these vesicles and cause the release of LDL and cholesterol in the cytoplasm.

Exocytosis is a process of transportation of substances outside the cell. The substances are: 1. Solid waste 2. Hormones 3. Neurotransmitters and4.Mucin.

Microtubules of the cytoplasm contain motor proteins, Kinesin, and design. These motor proteins use ATP and drag the vesicles to the cell membrane. Now, the vesicles express V-Snare protein on their surface. This combines tightly with the T-snare present on the cell membrane. In the presence of calcium ions, V-snare, and T-snare contracts, vesicles move to the cell membrane and are expelled from the cell, and the cell membrane regains its continuity. This is calcium-dependent exocytosis.

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Hashtags: Human physiology| Self study |Active |Passive | Diffusion|  primary active transport | secondary active transport| transcytosis |Phagocytosis | pinocytosis | Exocytosis | channels | ligands | 

 Internal links: https://blog.totalphysiology.com/2022/10/cell-junction-physiology.html

External Links:https://en.m.wikipedia.org>wiki/Membrane_transport