The plasma membrane is the boundary of life; this selectively permeable membrane allows the cell to maintain a unique internal environment and to control the movement of materials into and out of the cell. The phospholipid bilayer (fluid mosiac model) with specific membrane proteins accounts for he selective permeability of the membrane and passive and active transport mechanisms. Membrane phospholipids are amphipathic. Cell membranes are phospholipid bilayers with the hydrophobic hydrocarbon tails in the center and the hydrophilic heads facing the aqueous solution on both sides of the membrane. The phospholipid bilayer also called the fluid mosiac model has proteins embedded in the phospholipid bilayer with their hydrophilic regions extending out into the aqueous environment. Membranes are held together by weak hydrophobic interactions that allow the lipids and some of the proteins to drift laterally. Phospholipids with unsaturated hydrocarbon tails maintain membrane fluidity at lower temperatures. The steroid cholesterol restricts movement of phospholipids, therefore, reducing fluidity at warmer temperatures. Cholesterol also prevents the close packing of lipids and thus enhances fluidity at lower temperatures.
The Essay on A selectively membrane means that the cell membrane can only control over certain things
Osmosis is the diffusion of water through a selectively permeable membrane. A selectively membrane means that the cell membrane can only control over certain things that come in and out through the membrane. A membrane is just the structure surrounding the cell. Throughout osmosis, the cell could either get larger, or smaller. If the cell gets larger it means that it was put into a hypotonic ...
The plasma membrane permits regular exchange of nutrients, waste products, oxygen, and inorganic ions. Membranes are selectively permeable which means that they allow some substances to cross more easily than others. Hydrophobic molecules, such as hydrocarbons, can dissolve in and cross through a membrane. Very small polar molecules, including water, can cross a plasma membrane easily. Ions and polar molecules may move across the plasma membrane with the aid of transport proteins, which may provide a hydrophilic channel or may physically bind and transport a specific molecule. The selectively permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane.
The structure of membranes is directly related to the transport of materials across a membrane. Diffusion is the spontaneous movement of a substance down its concentration gradient. The cell does not expend energy when substances diffuse across membranes therefore, the process is called passive transport. Osmosis is the diffusion of water across a selectively permeable membrane. Water diffuses down its own concentration gradient and water will move from a hypotonic solution across a membrane into a hypertonic solution. Specific proteins facilitate the passive transport of selected solutes. Facilitated diffusion involves the diffusion of polar molecules and ions across a membrane with the aid of transport proteins, which are similar to enzymes. The binding of the solute to the transport protein may cause a conformational change that serves to translocate the binding site and the attached solute. A transport protein can provide a selective channel across the membrane, functioning as gated channels that open in response to electrical or chemical stimulation. Active transport is the pumping of solutes against their gradients. The expenditure of energy is required by the cell in this process and is essential for the cell to maintain internal concentrations of small molecules that differ from environmental concentrations.
The Essay on Why are cells microscopic?
... membrane. Cell membranes act as barriers to most, but not all molecules (Transport in and out of cells). The development of a cell membrane ... for moving ions or molecules against the concentration gradient. Active transport is used to move molecules or ions from ... move in (Membranes). _Ions can only cross the membrane using integral membrane proteins called ion channels, or proteins. This figure shows ...
The work of active transport is performed by specific proteins embedded in membranes and ATP supplies the energy for most active transport. Exocytosis and endocytosis transport large molecules. In exocytosis, the cell secretes macromolecules by the fusion of vesicles with the plasma membrane. In endocytosis, a region of the plasma membrane sinks inward and pinches off the form a vesicle containing material that had been outside the cell. Phagocytosis is a form of endocytosis in which pseudopodia wrap around a food particle, creating a vaculoe that then fuses with a lysosome. In pinocytosis, droplets of extracellular fluid are taken into the cell in small vesicles.
Chemiosmosis is the production of ATP using energy of hydrogen-ion gradients across membranes to phosphorylate ADP and powers most ATP synthesis in cells. An electron transport chain assembled in a membrane pumps protons across the membrane as electrons are passed through a series of carriers. Electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a membrane. ATP synthase complexes are built into the membrane which couple the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP. Chloroplasts are the site of photosynthesis and in chloroplasts, the thylakoid membrane pumps protons from the stroma into the thylakoid space, which functions as the H+ reservoir. As the hydrogen ions leak back across the membrane through the ATP synthase, phosphorylation of ADP occurs on the stroma side of the membrane. The proton gradient, or pH gradient, across the thylakoid membrane is substantial. When chloroplasts are illuminated, the pH in the thylakoid space drops to 5, and the pH in the stroma increases to 8. This gradient corresponds to a thousand-fold difference in H+ concentration.
