Transport across membranes

Endocytosis

In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis: Phagocytosis ("cellular eating") Pinocytosis ("cellular drinking") Receptor-mediated endocytosis, occurs when specific receptor helps substances across.

Phagocytosis

In phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membranous sac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes.

Pinocytosis

In pinocytosis, the cell "gulps" droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplets. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.

Active transport 2.

Active transport has directionality. An active transport system that moves a soluteacross a membrane in one direction will not usually move that solute in the other direction. (unidirectional process) Direct / Primary active transport: the accumulation of solute molecules or ions on one side of the membrane is coupled directly to an exergonic chemical reaction, most commonly the hydrolysis of ATP. Transport proteins driven directly by ATP hydrolysis are called transport ATPases or ATPase pumps. Indirect / Secondary active transport: also requires energy, but depends on the simultaneous transport of two solutes, with the favourable movement of one solute down its gradient, driving the unfavourable movement of the solute up its grad. This results in an overall decrease in free energy. This dual transport process can be described as either a symport or an antiport, depending on whether the 2 solutes move in the same or opp. directions.

Active transport

Active transport makes it possible to move solutes away from thermodynamic equilibrium (up a concentration gradient, or against an electrochemical potential). Always requires an input of energy, coupling thermodynamically unfavourable processes (up conc. grad), to an exergonic process (usually ATP hydrolysis). 1.) Act. Trans. makes it possible to take up essential nutrients from the environment, even when their concentrations in the cell are already higher 2.) Allows secretory products and waste materials to be removed from the cell or organelle, even when the concentration is greater on the outside than it is on the inside. 3.) It enables the cell to maintain constant, nonequilibrium I.C. concentration of specific organic ions, namely K+,Na+,Ca2+,H+.

SoKP. Active Transport

Active transport—the uphill transport of large, polar molecules and ions—requires a protein transporter and an input of energy. It may be powered by ATP hydrolysis, the electrochemical potential of an ion gradient, or light energy. Active transport powered by ATP hydrolysis utilizes four major classes of transport proteins: P-type, V-type, F-type, and ABC-type ATPases. One widely encountered example is the ATP-powered Na+/K+ pump (a P-type ATPase), which maintains electrochemical potentials for sodium and potassium ions across the plasma membrane of animal cells. Active transport driven by an electrochemical potential usually depends on a gradient of either sodium ions (animal cells) or protons (plant, fungal, and many bacterial cells). For example, the inward transport of nutrients across the plasma membrane is often driven by the symport of sodium ions that were pumped outward by the Na+/K+ pump. As they flow back into the cell, they drive inward transport of sugars, amino acids, and other organic molecules.

Facilitated Diffusion

Assisted by transport proteins that mediate the movement of solute molecules across the membrane. If such a process is exergonic, it is called facilitated diffusion. Exergonic because, the solute still diffuses in the direction dictated by the concentration gradient, (for uncharged molecules), or by the electrochemical gradient (for ions). Therefore, no input of energy needed.

Indirect Active

Driven by the movement of an ion down its electrochemical gradient. (Either Na+ in animal or H+ for most bacterial, fungal and plant cells) In animals - relatively high extracellular concentration of Na+ maintained by the NaK pump, serves as the driving force for the uptake of a variety of sugars and amino acids. Even though this is indirect active trans., ultimately, the uptake still depends on ATP because the NaK pump that maintains the sodium ion gradient is itself driven by ATP hydrolysis. ! The continuous outward pumping of Na+ by the ATP-driven NaK pump, and the inward movement of Na+ by symport (coupled to the uptake of another solute) establishes a circulation of Na+ ions across the plasma membrane of every animal cell. In addition to the symport uptake of organic molecules such as sugars and amino acids, Na+ or H+ gradients can be used to drive the export of other ions, including Ca2+ and K+. This type of indirect active transport is usually antiport and may involve the exchange of potassium ions for protons or the exchange of Ca2+ ions for Na+. PAGE 213 FOR NaK pump!

Direct Active

Most common mechanism involved in direct active transport involves ATPases, that link active transport to the hydrolysis of ATP. (P, V, F, ABC type ATPases)

Porins

Proteins that allow rapid passage of various solutes. They are found in the outer membranes of mitochondria, chloroplasts. These pores are formed by multipass transmembrane proteins. Closed beta-sheet conformation called beta-barrel. The pore allows passage of various hydrophilic solutes.

SoKP. Diffusion

Simple diffusion through biological membranes is limited to small or nonpolar molecules such as O2, CO2, and lipids. Water molecules, although polar, are small enough to diffuse across membranes in a manner that is not entirely understood. Membranes are permeable to lipids, which can pass through the nonpolar interior of the lipid bilayer. Membrane permeability of most compounds is directly proportional to their partition coefficient—their relative solubility in oil versus water. The direction of diffusion of a solute across a membrane is determined by its concentration gradient and always moves toward equilibrium. The solute will diffuse down the gradient from a region of high concentration to a region of low concentration. If the membrane is impermeable to the solute, water will move by osmosis from the area of low solute concentration (higher [H2O]) to the area of high solute concentration (lower [H2O]).

SoKP. Cells andTransport Processes

The selective transport of molecules and ions across membrane barriers ensures that necessary substances are moved into and out of cells and cell compartments at the appropriate time and at useful rates. Nonpolar molecules and small, polar molecules cross the membrane by simple diffusion. Transport of all other solutes, including ions and many molecules of biological relevance, is mediated by specific transport proteins that provide passage through an otherwise impermeable membrane. Each such transport protein has at least one, and frequently several, hydrophobic membrane-spanning sequences that embed the protein within the membrane and often act as the channel itself. Typically, a separate regulatory domain controls channel opening and closing.

Channel proteins:

Tiny pore lined with hydrophilic atoms, creating an "ion channel" through the membrane that allow the passage of solutes without a major change in the conformation of the protein. Some of these are pores, formed by transmembrane proteins called porins, and allow selected hydrophilic solutes to diffuse across the membrane. Also highly selective, as they only allow the passage of only one kind of ion, so separate channels are needed for ions such as Na+, K+, Ca2+, Cl-. Most ion channels are gated, which means that the pore opens and closes in response to stimulus. 1.) Voltage-gated - open and close in response to changes in memb. pot. 2.) Ligand-gated - are triggered by the binding of specific substances to channel protein. 3.) Mechanosensitive channels respond to mechanical forces that act on the membrane. Muscle contraction and many cellular responses require regulation of Ca2+ levels via Ca-specific ion channels. Also, the transmission of electrical signals by nerve cells depends critically on rapid, controlled changes in the movement of Na+ and K+ ions through their respective channels.

Aquaporins

Transmembrane channels that allow rapid passage of water. Whereas water can diffuse slowly across cell membranes in the absence of a protein transporter during diffusion, some cells in specific tissues require the capability to do this in a much more rapid fashion. (Proximal tubules in the kidneys reabsorb water as part of urine formation, and cells in this tissue have a high density of AQP in their plamsma membrane, allowing the kidney to filter more than 100L of H2O per day. (also erythrocytes)

Transport Proteins

Transport Proteins involved in fac. diff., are integral membrane proteins that contain several transmembrane segments, and therefore traverse the membrane multiple times. 2 classes:

SoKP. Facilitated Diffusion

Transport can either be downhill or uphill in relation to an uncharged solute's concentration gradient. For an ion, we must consider its electrochemical potential—the combined effect of the ion's concentration gradient and the charge gradient across the membrane. Downhill transport of large, polar molecules and ions, called facilitated diffusion, must be mediated by carrier proteins and channel proteins because these molecules and ions cannot diffuse through the membrane directly. Carrier proteins function by alternating between two conformational states. Examples include the glucose transporter and the anion exchange protein found in the plasma membrane of the erythrocyte. Transport of a single kind of molecule or ion is called uniport. The coupled transport of two or more molecules or ions at a time may involve movement of both solutes in the same direction (symport) or in opposite directions (antiport). Channel proteins facilitate diffusion by forming transmembrane channels lined with hydrophilic amino acids. Three important categories of channel proteins are ion channels (used mainly for transport of H+,Na+,K+,Ca2+,Cl-,and HCO3-), porins (for various high-molecular-weight solutes), and aquaporins (for water).

Carrier proteins / permeases 2:

When a single solute is transported across membrane: Uniport. If 2 substances, and 1 can't go through without the other: coupled transport. Furthermore, symport if same direction, or antiport if opposite direction. (Carrier protein in this case is called symporter/antiporter).

Carrier proteins / permeases:

bind one or more solute molecules on one side of the membrane, and then undergo a conformational change that transfers the solute to the other side of the membrane. For carrier proteins, the most likely explanation is the alternating conformation model, in which the carrier protein is an allosteric protein that alternates between two conformational states. They are similar to enzymes, as is an enzyme catalysed reaction, fac. diff. involves an initial binding of the substrate to a specific site on a protein surface), and the ES complex being the solute bound to the carrier. Like enzymes, carrier proteins can be regulated by external factors that bind and modulate their activity. Furthermore, carrer proteins are highly specific, even stereospecific. (meaning only D-/L- isomer of glucose for erythrocytes), also they have the ability to be saturated at high levels of substrate, and are sensitive to specific inhibitors of transport.

Osmosis

is the diffusion of water across across a differentially permeable membrane.