Transport through the cell membrane
DiffusionDiffusion through the cell membrane is divided into two subtypes called simple diffusion and facilitated diffusion.
Simple diffusion means that movement of molecules or ions through the cell membrane opening without any interaction with carrier proteins in the membrane according to concentration gradient (from area of high concentration of ion to the area of low concentration of that ion).
Simple diffusion can occur through the cell membrane by two pathways:
(1) through the lipid bilayer if the diffusing substance is lipid soluble like oxygen, nitrogen,carbon dioxide, and alcohols.
(2) through protein channels that penetrate all the way through the cell membrane
The protein channels are distinguished by two important characteristics:
a-they are often selectively permeable to certain molecules or ions .This depend on the characteristics of the channel itself, such as its diameter, its shape, and the nature of the electrical charges
b- Many of the channels can be opened or closed by gates.The opening and closing of gates are controlled by :
I-Voltage gating. In this case opening of the gate responds to the electrical potential across the cell membrane. example is sodium channel when the inside of the cell membrane is negative, this could cause the sodium gates to remain closed; conversely, when the inside of the membrane loses its negative charge, these gates would open suddenly and allows sodium to pass inward through the sodium pores. This is the basic mechanism for eliciting action potentials in nerves.
II-Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance with them; this causes aconformational change in the protein molecule that opens or closes the gate. Example for this is acetylcholine channel.
Facilitated Diffusion
also called carrier-mediated diffusion because a substance transported in this manner diffuses through the membrane using a specific carrier protein to help the substance to move to the other side like glucose and the amino acids.
Factors That Affect Net Rate of Diffusion
1-Concentration Difference
2-The electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement.
3-Effect of a Pressure Difference across the Membrane. The net movement of molecules from the high-pressure side toward the low-pressure side. This occurs at the blood capillary membrane
Osmosis
Diffusion of Water Across Selectively Permeable Membranes from a region of low solute concentration (high water concentration) to one with a high solute concentration (low water concentration)
Water is the most abundant substance that diffuses through the cell membrane is water
ACTIVE TRANSPORT
Active transport means movement of a substance from area of low concentration of this substance to the area of high concentration for that substance usually need energy
Active transport is divided into two types according to the source of the energy used to cause the transport into primary active transport and secondary active transport
In primary active transport, the energy is derived directly from ATP .In secondary active transport, the energy is derived secondarily from energy that release in simple diffusion
In both instances, transport depends on carrier proteins that penetrate through the cell membrane,
Primary Active Transport
Among the substances that are transported by primary active transport are sodium, potassium, calcium ,hydrogen, chloride, and a few other ions
Sodium-Potassium Pump (Na+-K+) pump
Transport process that pumps sodium ions outward through the cell membrane of all cells and at the same time pumps potassium ions to the inside the cell. This pump is responsible for maintaining the sodium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells.
Carrier protein in this pump has three receptor sites for binding sodium ions inside of the cell and two receptor sites for potassium ions on the outside.Near the sodium binding sites has ATPase activity cleaves one molecule of ATP, splitting it to (ADP) and liberating energy
This liberated energy is then cause a conformational change in the carrier protein ,extruding the three sodium ions to the outside and the two potassium ions to the inside.net of appositive charge is moved from the interior of the cell to the exterior. This creates positivity outside the cell and negativity inside the cell.Therefore, the Na+-K+ pump is called electrogenic
Calcium pump. Calcium ions are normally maintained at low concentration inside the cell. This is achieved mainly by two primary active transport calcium pumps. One is in the cell membrane and pumps calcium to the outside of the cell. The other pumps calcium ions into the sarcoplasmic reticulum of muscle cells and the mitochondria in all cells. The carrier protein penetrates the membrane has a binding site for Ca and enzyme ATPase,
Hydrogen Ions pump
At two places in the body, primary active transport of hydrogen ions is very important:
(1) in the gastric glands of the stomach
(2) in the late distal tubules and cortical collecting ducts of the kidneys.
Secondary Active Transport
1-Co-Transport and Counter-Transport
When sodium ions are transported out of cells by pump cause excess sodium outside the cell membrane which attempt to diffuse to the inside the cell. this diffusion of sodium can pull other substances with the sodium through the cell membrane.This phenomenon is called co-transport; it is one form of secondary active transport.
The carrier in this instance serves as an attachment point for both the sodium ion and the substance to be co-transported. Once they both are attached, the energy gradient of the sodium ion causes both the sodium ion and the other substance to be transported together to the interior of the cell Co-Transport of Glucose and Amino Acids with Sodium Ions.
2- counter-transport, sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, this time, the substance to be transported is on the inside of the cell and must be transported to the outside. Therefore, the sodium ion binds to the carrier protein and the substance to be transported a conformational change occurs, and energy released by the sodium ion moving to the interior causes the other substance to move to the exterior.
sodium-calcium counter-transport
sodium-hydrogen counter-transport.
Sodium-calcium counter-transport .
Active Transport Through Cellular Sheets
At many places in the body, substances must be transported all the way through a cellular sheet instead of simply through the cell membrane. Transport of this type occurs through the(1) intestinal epithelium,
(2)epithelium of the renal tubules
(3) epithelium of all exocrine glands
(4) epithelium of the gallbladder, and
The basic mechanism for transport of a substance through a cellular sheet is
(1) active transport through the cell membrane on one side of the transporting cells in the sheet, and then (2) either simple diffusion or facilitated diffusion through the membrane on the opposite side of the cell.
sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels, water diffuses from a region of low solute concentration( high water concentration) to one with a high solute concentration( and low water concentration)