TRANSPORT ACROSS CELL MEMBRANES
Membranes are selectively permeable --- small, uncharged molecules can diffuse freely through the phospholipid bilayer. O2, CO2
Also some small polar molecules H2O, ethanol and some small relatively hydrophobic molecules such as benzene.
Passive diffusion
Molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane. No membrane proteins involved in process. The net movement of molecules is simply according to the concentration gradient, with molecules moving from an area of higher concentration to an area of lower concentration.
Water is actually a rather special case - readily diffusable according to concentration gradient. Process is known as osmosis.
Larger polar molecules such as glucose cannot cross by passive diffusion.
Charged ions also cannot [Na+, Ca++, K+, Cl-], even H+ cannot.
Facilitated Diffusion
Like passive diffusion, involves the movement of molecules in a net direction determined by concentration gradient---but with the assistance of specialized proteins.
Channel Proteins
Proteins which form open pores allowing for free passage of any molecule of appropriate size and charge by free diffusion. Form a passage through the lipid bilayer, allowing polar or charged molecules to cross without interacting with the hydrophobic fatty acid chains of the phospholipids.
ION CHANNELS-specific example of a channel protein.
not permanently open [GATED]
very wide variety of ion channels
All are integral membrane proteins that surround an aqueous pore.
Bidirectional flow of ions based upon the electrochemical concentration gradient
Most channel proteins are said to be gated meaning that they can exist in open or closed conformation.
Carrier Proteins [Transporter Proteins]
Selectively bind and transport specific small molecules such as glucose. The molecules bind and the protein undergoes a conformational change that allows specific molecules to pass through.
facilitated diffusion of sugars, amino acids, and nucleosides.
Active transport
Carrier proteins also provide a mechanism through which the energy changes associated with transporting molecules can be coupled to the use or production of other forms of metabolic energy.
Molecules can be transported against a concentration gradient if the transport is coupled to ATP hydrolysis, the absorption of light, the transport of electrons, or the flow of other substances down a gradient - as a source of energy.
Typically the K+ concentration inside a mammalian cell is about 100 mM, while that outside the cell is only 5mM. Diffusion of potassium out of the cell is favored. Sodium ions and Calcium ions have the opposite concentration gradient. Such gradients are maintained by active transport. ION PUMPS
Depends on integral membrane proteins that are capable of selectively binding a particular solute and moving that substance across the membrane- driven by changes in the protein's conformation.
Uptake of macromolecules and particles ENDOCYTOSIS
The Endocytic Pathway
Pinocytosis effectively internalizes portions of the cell membrane, any proteins or receptors on the membrane, and any ligands attached to those receptors. Fate of these receptors and their ligands varies after endocytosis. When soluble Ig binds to antigen, both receptor and ligand are directed to the lysosmes. The other possible route is that vesicles may be transported to another region of the membrane where the ligand is released.
One form of pinocytosis involves the use of coated pits [receptor-mediated endocytosis]. Ligand binds to receptor. Complex travels laterally through the membrane to a coated pit region. Complexes are retained and concentrated in these pits. Clathrin is a protein whose subunits form the surface of the pit. Pit then invaginates and eventually forms a vesicle known as a clathrin coated vesicle. The clathrin coated vesicle first fused with vesicles known as early endosomes. Fusion with the early endosome brings the pH of the clathrin coated vesicle down to between pH 6 and 6.2. This shift to an acidic pH allows the clathrin and the receptor to be transported back to the surface. Late endosomes then fuse with the vesicle, further lowering the pH to between 5.5-6.0. Finally, lysosomes fuse bringing the pH down to about 5.0. Lysosomes also release a battery of hydrolytic enzymes that digest the material remaining in the vesicle. The other major form of endocytosis is termed: Phagocytosis
Phagocytosis can only be carried out by phagocytic cells such as macrophages and neutrophils. Phagocytosis involves the internalization of particles such as bacteria, protozoa, etc. In the process of phagocytosis, the particle first binds to the phagocytic cell, then the cell sends out extensions of the cyotplasm known as pseudopodia which surround the particle. THe formation of psuedopodia is dependent upon the polymerization of the cytoskeletal protein, actin. The internalized particle is now enclosed in a membrane-bound structure known as a phagosome. Lysosmes subsequently fuse with the phagosome forming a phagoslysosome. The hydrolytic enzymes released by the lysosome function in the digestion of the internalized particle.
Exocytosis
Material enclosed in a cell vacuole is passed to the extracellular fluid by fusion of the vacuole with the plasma membrane. Secretory process and a mechanisim of replenishing lipids and proteins of plasma membrane.
We will be focusing our discussion then on Eukaryotic Cell Structure.
All eukaryotic cells possess membrane-bound organelles that divide them into compartments and each represents a structure with a distinct function.
In addition, eukaryotic cells possess a distinct nucleus surrounded by two membranes that constitute the nuclear membrane.
Cytoplasm - region between the nucleus of the cell and the plasma membrane. Term includes the cytosol and all of the organelles except for the nucleus.
Cytosol - refers to the fluid component of the cytoplasm.
Nucleoplasm- corresponding material within the nucleus.
Cells have to grow, replace worn-out parts, and repair themselves. They must synthesize the proteins and other complex molecules of which they are made.
ENDOPLASMIC RETICULUM Extensive maze of branching membranes that extends throughout the cytoplasm. Evidence suggests that the ER is continuous with the plasma membrane and with the outer membrane surrounding the nucleus. [Intramembrane network]
Therefore, the membranes of the ER divide the cytoplasm up into interconnected compartments in which different types of reactions take place.
The ER plays an important role in the synthesis of proteins and some lipids. Expanded regions of the ER may serve as storage areas. The ER also functions as a system for transporting materials from one part of the cell to another.
Smooth ER
The smooth ER is more tubular and its outer membrane surfaces have a smooth appearance. It is the primary site of phospholipid, steroid, and fatty acid metabolism.
The smooth ER also contains enzymes that detoxify harmful chemicals, breaking them down into water-soluble substances that can be excreted.
Rough ER
The Rough ER has a more granular appearance under EM due to the presence of ribosomes.
Ribosomes of course serve as the sites of protein synthesis and can be found assoc. with the ER or free in the cytoplasm. RER is especially abundant in cells that synthesize proteins destined for exocytosis.
Golgi Complex
factory for packaging proteins
Consists of stacks of flattened membranous sacs that may be distended in certain regions because they are filled with cell products.
The golgi complex processes, sorts, modifies, and packages proteins.
Proteins that are 1) secreted from the cell 2) plasma membrane proteins 3) and proteins routed to other intracellular organelles pass through the golgi.
Such proteins are synthesized on ribosomes associated with the RER. From the ER they are transported to the Golgi complex in small membrane-bound vesicles formed from the ER membrane.
These vesicles then fuse with the membranes of the Golgi complex. The proteins then pass through the separate layers of the Golgi complex moving by way of membrane transport vesicles.
During their passage- proteins are modified in different ways Carbohydrates are often added here or previously added carbohydrates are modified.
In some cases, the carbohydrates and other modifications act as "sorting signals" allowing the GOLGI to route the protein to different parts of the cell. The GOLGI of plant cells also produces some of the extracellular polysaccharides used in components of the cell wall.
The Golgi Complex has a distinct polarity. The cis face of the golgi {closest to the nucleus} is the site where membrane-bound vesicles from the ER first fuse. The golgi stack refers to the middle portion of the golgi. The trans face of the golgi refers to the portion of the golgi closest to the plasma membrane and serves as the site where membrane-bound vesicles bud and then exit the golgi complex.
The Golgi also performs important functions in non-secreting cells by packaging intracellular digestive enzymes in the little organelles known as lysosomes.