Membrane Structure and Function


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Cell Membrane


The cell membrane allows nutrients to enter the cell and releases waste. Since the membrane has selective permeability it can pick and choose what comes and goes, regulating the traffic and keeping the cell from becoming too "full" or obtaining harmful material. The membrane is a fluid with numerous proteins attached to a bilayer of phospholipids. The inside of the cell is hydrophobic (water hating) and the outsides are hydrophilic (water loving). A membrane is held together mainly becuaseof the hydrophobic interactions. Integral proteins are in the hydrophobic core of the bilayer. Peripheral proteins are loosely bound to the surface of the membrane. Carbohydrates are essential in cell to cell recognition. Transport proteins transfer materials across that could not pass on their own.


Diffusion is a process that transports material across the membrane. Diffusion is the tendency of molecules to spread and even themselves out. A substance will cross over from a concentrated area to a less concentrated area meaning the substance will diffuse down its concentration gradient. This is a spontaneous process and no work is involoved. Passive transport is diffision across a biological membrane where the cell does not have to use any energy.
Diffusion
Diffusion

Osmosis
Osmosis










Osmosis is the diffusion of water across a selectively permeable membrane. Comparing two solutions the one of higher concentration is called hypertonic and the one less concentrated is called hypotonic. Solutions of equal concentration is known as isotonic. Osmoregulation is the control of water balance. An animal cell placed in a hypotonic solution will take on more water to balance out the materials, and in most cases the cell is lysed (bursts open). An animal cell in a hypertonic solution will lose its water becoming shriviled. Plants cells are equipped with larger vacoules so they can take on more water. A plant cell in a hypotonic solution is actually in its normal state which is known as turgid or very firm. In a hypertonic or isotonic solution the plant cell loses water becoming flaccid or limp which is the reason why plants wilt. In plants a phenomenon known as plasmolysis occurs when the cell is in a hypertonic solution. As the plant loses water the plasma membrane pulls away from the cell wall and is usually leathal.
Water Balance
Water Balance
Osmosis Jones
Osmosis Jones















Facilitated diffision is the spontaneous passage of molecules and ions that are attached to a carrier protein. An aquaporin is the transport protein. A gated channel allows specific ions to travel through it.


Active transport is when energy is needed to get materials across the membrane. One of these is called the sodium-potassium pump
Sodium-potassium pump
Sodium-potassium pump
which swaps sodium for potassium across the animal cell's membrane. Some pumps generate voltage across the membrane. Membrane potential is the voltage across the membrane that ranges from -50 to -200 millivolts. The diffusion gradient of an ion is called electrochemical gradient. The electrogenic pump is a transport protein that sends a voltage across the membrane which the sodium-potassium pump is the main elctrogenic pump for animals. Plants, bacteria, and fungi use a proton pump which actively transports hydrogen ions out of the cell. Cotransport is the coupling of teh downhill diffusion of one substance, uphill of the other, and all against their own gradient.



Exocytosis transport large molecules out of the cell. Endocytosis takes in the large molecules. The three types of endocytosis are phagocytosis, pinocytosis, and receptomediated endocytosis. Phagocytosis (cellular eating) is when a cell wraps its membrane around a particle creating a vacoule. This membrane enclosed particle fuses to the lysosome where it is then digested. Pinocytosis (cellular drinking) forms a vacoule around extracellular fluid and is not specific in what it may pick up. Receptor-mediated endocytosis is highly specific. Proteins attached to the membrane have specific receptor sites that have ligands bind to them.
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Receptor-mediated endocytosis
Receptor-mediated endocytosis