Each and every cell derives nutrition from its immediate surrounding which is the extracellular fluid. Apart from nutrients a cell also requires oxygen and water for its survival. Not all substance can enter the cell membrane because of its semi-permeability property. Therefore different transport mechanisms are necessary for the substance of importance to enter into the cell.
The various transport mechanisms are of three types
It is the mechanism of transport of substances along the gradient without the expenditure of any. The gradient can be concentration gradient, electrical gradient or pressure gradient. Since the transport of substances occurs along the gradient it is also called as down-hill movement. There are two types of passive transport mechanisms.
It refers to passive transport of molecules from area of higher concentration to areas of lower concentration. Diffusion through cell membrane is divided into two subtypes
Diffusion of substance occurs either through the lipid layer or proteins in the cell membrane.
Substances soluble in lipids such as oxygen, carbon dioxide and alcohol can easily pass through the cell membrane along the concentration gradient. The rate of diffusion is directly proportional to the concentration gradient and solubility of the substance in lipids.
Simple diffusion through protein layer
Substances which are not soluble in lipids can also pass through the cell membrane via proteins channels.
There are two types of protein channels according to their property
Eg: Na channel selectively allows Na to pass through, K channel selectively allow K to pass through.
Gated channels: They open when they are required to be opened. There are three types of gated channels
There channels open when there is a change in the potential
Eg: Opening on Ca channel in the presynaptic membrane in the NMJ or synapse when the action potential reaches the terminal button.
Ligand gated channel
These channels open in the presence of a ligand which could be a hormone.
Eg: Opening of Na channel in the postsynaptic membrane of NMJ of Synapse after the binding of neurotransmitter to the receptor.
Mechanically gated channel
These channels are opened due to mechanical factors such as pressure
Eg: Opening of K channels in cochlea of ear by movement of cilia of hair cells.
Facilitated or Carrier mediated diffusion
Water soluble large molecules such as glucose and amino acids are transported in this mechanism. Along with gradient there is a transport protein or carrier protein in involved in the process
Eg: Carrier protein glucose transporter (GLUT) is required for transport of glucose
There are three special types of passive transport
Bulk flow: Diffusion of large quantity of substances from a region of high pressure to the region of low pressure – due to pressure gradient. Eg: Exchange of gases across the respiratory membrane in lungs
Filtration: Movement of water and solutes from an area of high hydrostatic pressure to an area of low hydrostatic Pressure. : Filtration at arterial end of the capillaries
Osmosis: Movement of water or any other solvent from an area of lower concentration to an area of higher concentration of a solute, through a semipermeable membrane. Semipermeable membrane permits passage of only water or other solvents but not the solutes.
Active transport mechanism
It is movement of substances against the gradient; chemical, electrical or electrochemical. It is an uphill process and requires energy. Energy is derived from breakdown of high energy compounds such as ATP. Substances transported by active transport mechanism are:
Sodium, Potassium, Calcium, Iodine, Glucose, Amino acids and Urea
Carrier proteins of active transport are of three types
In this process energy is used directly due to breakdown of ATP.
Eg: Sodium-Potassium ATPase Pump (Na-K pump)
Sodium-Potassium ATPase Pump
It is known as sodium-potassium ATPase pump. It is present in almost all the cells in the body. It pumps out 3 Sodium ions from inside to outside of the cell and 2 Potassium ions from outside to inside. The carrier protein is Na-K ATPase which also acts as an enzyme for breakdown of ATP. It is responsible for distribution of Sodium and Potassium ion concentration outside and inside the cell respectively and helps in maintenance of Resting Membrane Potential.
Mechanism of action of Na-K ATPase pump
Calcium ATPase Pump
It is similar to Na-K ATPase pump. It is present in sarcoplasmic reticulum (SERCA, or sarco/endoplasmic reticulum Ca2+-ATPase, or SR Ca2+-ATPase) and mitochondria.
Hydrogen ion ATPase Pump (Proton Pump)
It is similar to Na-K ATPase pump. It is present in Parietal cells of Stomach, Epithelial cells of DCT in Kidney
Secondary Active transport mechanism
In this mechanism, the energy is derived secondarily from the energy which has been stored in the form of ionic concentration differences between the two sides of a membrane, created in the first place by primary active transport. Transport of some other substance is coupled with the active transport of Na+, i.e. the same carrier protein which is involved in the active transport of Na+ also secondarily transports some other substance.
The secondary active transport of substance may occur in the form of sodium co-transport or sodium counter-transport.
Sodium co-transport mechanism
The carrier protein here acts as a symport, i.e. transports some other substance along with the sodium
Eg: Glucose, Amino acids, chloride and idodine
Sodium countertransport mechanism
The carrier protein here acts as a antiport, i.e. transports some other substance along with the sodium in opposite direction. Eg:
Sodium–Calcium countertransport
Sodium–Hydrogen countertransport: PCT of Kidney
Sodium–Potassium counter transport
Sodium–Magnesium countertransport
Calcium–Magnesium countertransport
Chloride–Bicarbonate countertransport system
Vesicular transport
This mechanism is involved in transport of macromolecules such as large protein molecules which cannot be transported by passive and active transport mechanisms
It includes
It is the process in which the substance is transported into the cell by infolding of the cell membrane around the substance and internalising it. It is of three types
It is the process of engulfing liquid substance by invagination of cell membrane. It is also called as cell drinking. Eg:
Reabsorption by tubular epithelial cells
Phagocytosis
It is the process of engulfing solid particles such as bacteria, dead cells, foreign body etc. It is also called as cell eating.
The process of phagocytosis involves three steps:
In this process the substance to be transported binds with the special receptor protein present on the cell surface. The receptor protein–substance complex is then engulfed by the cell membrane by the process of endocytosis.
Eg: Transport of iron and cholesterol into the cells
Exocytosis
It is reverse of endocytosis. In this process, the substances which are to be extruded are collected in the form of granules or vesicles which move towards the cell membrane. Their membrane then fuses to the cell membrane. The area of fusion breaks down releasing the contents to the exterior and leaving the cell membrane intact.
Eg: Release of hormones and enzymes by secretory cells
Transcytosis
Vesicular transport within the cell is called transcytosis.
It involves three steps
The various transport mechanisms are of three types
- Passive transport
- Active transport
- Vesicular transport
It is the mechanism of transport of substances along the gradient without the expenditure of any. The gradient can be concentration gradient, electrical gradient or pressure gradient. Since the transport of substances occurs along the gradient it is also called as down-hill movement. There are two types of passive transport mechanisms.
- Diffusion
- Osmosis
It refers to passive transport of molecules from area of higher concentration to areas of lower concentration. Diffusion through cell membrane is divided into two subtypes
- Simple diffusion
- Facilitated diffusion
Diffusion of substance occurs either through the lipid layer or proteins in the cell membrane.
- Simple diffusion through lipid layer
- Simple diffusion through protein layer
Substances soluble in lipids such as oxygen, carbon dioxide and alcohol can easily pass through the cell membrane along the concentration gradient. The rate of diffusion is directly proportional to the concentration gradient and solubility of the substance in lipids.
Simple diffusion through protein layer
Substances which are not soluble in lipids can also pass through the cell membrane via proteins channels.
There are two types of protein channels according to their property
- Selective Permeability
- Gated Channels
Eg: Na channel selectively allows Na to pass through, K channel selectively allow K to pass through.
Gated channels: They open when they are required to be opened. There are three types of gated channels
- Voltage-gated channels
- Ligand-gated channels
- Mechanically gated channels
There channels open when there is a change in the potential
Eg: Opening on Ca channel in the presynaptic membrane in the NMJ or synapse when the action potential reaches the terminal button.
Ligand gated channel
These channels open in the presence of a ligand which could be a hormone.
Eg: Opening of Na channel in the postsynaptic membrane of NMJ of Synapse after the binding of neurotransmitter to the receptor.
Mechanically gated channel
These channels are opened due to mechanical factors such as pressure
Eg: Opening of K channels in cochlea of ear by movement of cilia of hair cells.
Facilitated or Carrier mediated diffusion
Water soluble large molecules such as glucose and amino acids are transported in this mechanism. Along with gradient there is a transport protein or carrier protein in involved in the process
Eg: Carrier protein glucose transporter (GLUT) is required for transport of glucose
There are three special types of passive transport
Bulk flow: Diffusion of large quantity of substances from a region of high pressure to the region of low pressure – due to pressure gradient. Eg: Exchange of gases across the respiratory membrane in lungs
Filtration: Movement of water and solutes from an area of high hydrostatic pressure to an area of low hydrostatic Pressure. : Filtration at arterial end of the capillaries
Osmosis: Movement of water or any other solvent from an area of lower concentration to an area of higher concentration of a solute, through a semipermeable membrane. Semipermeable membrane permits passage of only water or other solvents but not the solutes.
Active transport mechanism
It is movement of substances against the gradient; chemical, electrical or electrochemical. It is an uphill process and requires energy. Energy is derived from breakdown of high energy compounds such as ATP. Substances transported by active transport mechanism are:
Sodium, Potassium, Calcium, Iodine, Glucose, Amino acids and Urea
Carrier proteins of active transport are of three types
- Uniport: In this process the carrier protein allows substance to pass through in only one direction.
- Symport: In this process the carrier protein allows two substances to pass through in one direction.
- Antiport: In this process the carrier protein allows two substances to pass through in opposite direction.
- Primary active transport mechanism
- Secondary active transport mechanism
In this process energy is used directly due to breakdown of ATP.
Eg: Sodium-Potassium ATPase Pump (Na-K pump)
Sodium-Potassium ATPase Pump
It is known as sodium-potassium ATPase pump. It is present in almost all the cells in the body. It pumps out 3 Sodium ions from inside to outside of the cell and 2 Potassium ions from outside to inside. The carrier protein is Na-K ATPase which also acts as an enzyme for breakdown of ATP. It is responsible for distribution of Sodium and Potassium ion concentration outside and inside the cell respectively and helps in maintenance of Resting Membrane Potential.
Mechanism of action of Na-K ATPase pump
Calcium ATPase Pump
It is similar to Na-K ATPase pump. It is present in sarcoplasmic reticulum (SERCA, or sarco/endoplasmic reticulum Ca2+-ATPase, or SR Ca2+-ATPase) and mitochondria.
Hydrogen ion ATPase Pump (Proton Pump)
It is similar to Na-K ATPase pump. It is present in Parietal cells of Stomach, Epithelial cells of DCT in Kidney
Secondary Active transport mechanism
In this mechanism, the energy is derived secondarily from the energy which has been stored in the form of ionic concentration differences between the two sides of a membrane, created in the first place by primary active transport. Transport of some other substance is coupled with the active transport of Na+, i.e. the same carrier protein which is involved in the active transport of Na+ also secondarily transports some other substance.
The secondary active transport of substance may occur in the form of sodium co-transport or sodium counter-transport.
Sodium co-transport mechanism
The carrier protein here acts as a symport, i.e. transports some other substance along with the sodium
Eg: Glucose, Amino acids, chloride and idodine
Sodium countertransport mechanism
The carrier protein here acts as a antiport, i.e. transports some other substance along with the sodium in opposite direction. Eg:
Sodium–Calcium countertransport
Sodium–Hydrogen countertransport: PCT of Kidney
Sodium–Potassium counter transport
Sodium–Magnesium countertransport
Calcium–Magnesium countertransport
Chloride–Bicarbonate countertransport system
Vesicular transport
This mechanism is involved in transport of macromolecules such as large protein molecules which cannot be transported by passive and active transport mechanisms
It includes
- Endocytosis
- Exocytosis
- Transcytosis
It is the process in which the substance is transported into the cell by infolding of the cell membrane around the substance and internalising it. It is of three types
- Pinocytosis
- Phagocytosis
- Receptor mediated endocytosis
It is the process of engulfing liquid substance by invagination of cell membrane. It is also called as cell drinking. Eg:
Reabsorption by tubular epithelial cells
Phagocytosis
It is the process of engulfing solid particles such as bacteria, dead cells, foreign body etc. It is also called as cell eating.
The process of phagocytosis involves three steps:
- Attachment stage
- Engulfment stage and
- Killing or degradation stage
In this process the substance to be transported binds with the special receptor protein present on the cell surface. The receptor protein–substance complex is then engulfed by the cell membrane by the process of endocytosis.
Eg: Transport of iron and cholesterol into the cells
Exocytosis
It is reverse of endocytosis. In this process, the substances which are to be extruded are collected in the form of granules or vesicles which move towards the cell membrane. Their membrane then fuses to the cell membrane. The area of fusion breaks down releasing the contents to the exterior and leaving the cell membrane intact.
Eg: Release of hormones and enzymes by secretory cells
Transcytosis
Vesicular transport within the cell is called transcytosis.
It involves three steps
- Vesicle formation
- Vesicle transportation
- Docking in the cell
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