Leeza-Marie Williams
Membrane as a dynamic structure:
The general structure of all biological membranes consists of noncovalent bonds that hold lipid and protein molecules together. The dynamic structure of cell membranes enables molecules to move within the membrane. A cluster of lipid molecules packed together forms a lipid bilayer which functions as an impermeable barrier to water soluble or hydrophilic molecules. Protein molecules are responsible for most of the functions within the membrane. Because of protein molecules, specific molecules can travel across and catalyzing membrane associated reactions like ATP synthesis are mediated. Protein molecules also serve as a means of providing structure towards connecting the cytoskeleton through the lipid bilayer to the extracellular matrix or to an adjacent cell. Some protein molecules act as receptors in detecting and transducing chemical signals in the cell’s environment.
Function of membrane proteins:
Each membrane contains a set of membrane proteins that enables the membrane to carry out specific activities. Proteins are bound to different locations such as the membrane surface or the domains of one or both sides of the membrane. Proteins that are bound to the extracellular membrane are involved in cell to cell signaling and interactions. Proteins that are bound within the membrane form channels and pores to allow for the movement of molecules across the membrane.
There are two general categories of membrane proteins based on the behavior of membrane-protein interactions: integral (intrinsic) and peripheral (extrinsic). Through embedding in the phospholipid bilayer, integral proteins contain hydrophobic residues that interact with fatty acyl groups located within membrane phospholipids which allows the protein to bind to the membrane. Since most integral proteins take up most of the space within the phospholipid bilayer, they are called transmembrane proteins. These transmembrane proteins consist of membrane spanning domains called α helices or multiple β strands. Some integral proteins have covalently bonded fatty acids that are anchored to a portion of the membrane however the polypeptide chain does not enter the phospholipid bilayer.
Peripheral membrane proteins which are also known as extrinsic proteins are indirectly bound to the membrane via interactions with lipid polar head groups or with integral membrane proteins. Therefore, extrinsic proteins do not interact with the hydrophobic core of the phospholipid bilayer. Peripheral membrane proteins located in the region where the cytosol and the plasma membrane meet contain the protein kinase C which plays a role in signal transduction by moving to and from the cytosol and the cytosolic face of the plasma membrane. Other proteins are found on the outer surface of the plasma membrane.
Phospholipid bilayers:
Phospholipids are the most abundant type of lipids which consists of a hydrophilic, polar head group and two hydrophobic, fatty acid tails that vary in length from between 14 to 24 carbon atoms. One tail contains unsaturated or cis- bonds while the other tail is saturated. Double bonds create kinks in the tail which alters the length of the tail. Since phospholipids contain a polar and a nonpolar end, there are amphipathic. Because of their amphipathic structure, when placed into water, phospholipids spontaneously arrange themselves where the hydrophilic end faces the water and the hydrophobic end is tucked into the lipid bilayer in the shape of micelles or bilayers. Micelles occur when all of the fatty acid tails face one another and form a spherical shape while bilayers occur when all of the fatty acid tails face one another and form a raft-like shape.
The amphipathic and shape of the phospholipid as well as temperature determines the fluidity of a lipid bilayer. A shorter fatty acid chain length reduces the tendency of hydrocarbon tails which means the hydrocarbon to interact whereby the formation of kinks occurs due to cis- double bonds. Therefore, at lower temperatures the membrane remains fluid. Some organisms like bacteria and yeasts maintain their fatty acid structures when the temperature fluctuates. In reference to a decrease in temperature, these organisms increase the synthesis of cis-double bonds to avoid a decrease in bilayer fluidity. Since the lipid bilayer is not 100% fat, cholesterol also impacts the fluidity of the membrane by immobilizing regions of hydrocarbon chains that are closer to the hydrophilic end of the phospholipid. Therefore, an increase in cholesterol results in a decrease in fluidity of the phospholipid.
Passive transport:
Passive transport is the most basic form of transporting molecules in and out of the cell by using a process called diffusion. Diffusion is the random movement of molecules across a membrane. Since passive transport does not require the expenditure of energy by the cell, substances diffuse from an area of high concentration to an area of low concentration across the membrane. Diffusion results in the net movement of molecules from one region to the other in presence of a concentration gradient. Small molecules like oxygen and carbon dioxide move easily across the plasma membrane through the process of diffusion. Hydrophobic substances such as triacylglycerols do not encounter problems when trying to diffuse in and out of a membrane. However, some hydrophilic molecules cannot move passively from regions of higher concentration to regions of lower concentration. Therefore, these molecules must protein transporters through the process of facilitated diffusion. The difference between facilitated diffusion and simple diffusion is that diffusion only uses the lipid bilayer to move molecules while facilitated diffusion uses a membrane transporter and bypasses the lipid bilayer.
There are two types of membrane transporters: a channel and a carrier. Channel membrane transporters allow molecules of a specific shape and charge to pass through an opening similar to a tunnel. Gated membrane channels require a chemical or an electrical signal in order to respond and allow molecules in and out of the membrane. Carrier membrane transporters bind to specific molecules and then transports them across the membrane. When molecules bind to the carrier membrane, a conformational change occurs in the membrane protein whereby it is determined if a molecule can be transported across the lipid bilayer. One membrane carrier is open to movement of molecules on one side of the cell and the other membrane carrier located on the opposite side performs a similar function.
Water molecules also are allowed to pass through by using simple diffusion the lipid bilayer because of their small size and because of aquaporins which are protein channels specifically geared towards transporting water molecules through facilitated diffusion. Solvents like water that move across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration do so through the process of osmosis. Movement of water occurs due to a force such as gravity or pressure on a cell wall.
Active transport:
In instances where molecules cannot diffuse through the membrane from regions of low concentration to regions of high concentration, the cell must expend energy in the form of ATP in order to move these molecules through the process called active transport. Naturally, glucose moves from inside of the cell to outside of the cell, therefore the cell must use active transport in order to bring in more glucose molecules from outside of the membrane. Substances are transported in and out of the cell using proteins that are embedded in the cell membrane act as pumps in moving substances against the concentration gradient. For example, as in the case of the sodium-potassium pump, the sodium concentration inside of the cell is lower than outside of the cell, however, the potassium concentration inside of the cell is higher than outside of the cell. Both sodium and potassium must use primary active transport in order to move against the concentration gradient. Primary active transport directly uses a source of chemical energy to move molecules against the concentration gradient. Sodium is pumped out of the cell while potassium is pumped into the cell. Since the protein that is used to move sodium and potassium in opposite directions, the protein transporter is referred to as an antiporter. Likewise, protein transporters that move two molecules in the same direction are called symporters or co transporters.
Through the process of secondary active transport, an electrochemical gradient is used to move molecules against the concentration gradient without the use of ATP, a chemical source of energy. The electrochemical gradient is formed when there is an increase in the concentration of small ions on one side of the membrane. The build up of potential energy can be used to move molecules across the membrane against the concentration gradient. When protons are moved across the membrane, a difference in charge occurs which causes the formation of an electrical gradient. The protons move from regions of common positive charge to regions of opposite or negative charge. An electrochemical gradient, as its name suggests, forms when a gradient has both charged and chemical aspects.
To summarize, the difference between primary active transport and secondary active transport is that primary active transport uses chemical energy of ATP to move molecules while secondary active transport uses the build up of potential energy in order to power an electrochemical gradient that enables for the movement of molecules.
Bulk transport:
When macromolecules like proteins or polysaccharides are moved across the plasma membrane in a process called bulk transport occurs. Bulk transportation has two forms, that both require the expenditure of ATP: exocytosis and endocytosis. During exocytosis, secretory vesicles formed by the Golgi apparatus are used to excrete molecules out of the cell by fusing to the unneeded molecules and transporting them to the plasma membrane. Once at the plasma membrane, the molecules are released into the extracellular space. Endocytosis is the process in which molecules outside of the cell are encapsulated by vesicle made out of a plasma membrane that folds inwards. Using specialized proteins, the pocket then pulls apart from the membrane and forms a vesicle in the cell.
The three types of endocytosis are as followed: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Nicknamed “cellular eating,” phagocytosis involves having the cell membrane pocket the macromolecule to form a phagosome, a food vacuole. Pinocytosis is nicknamed “cellular drinking” because the cell plasma membrane engulfs small amounts of extracellular fluid. Receptor-mediated endocytosis occurs when receptors proteins located on the cell’s surface cluster in coated pits which are regions of the plasma membrane capture specific target molecules that otherwise would not be captured via phagocytosis nor pinocytosis.
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