Role Of The Cell And Its Plasma Membrane Biology Essay

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A cell must take in food and other materials and must rid itself of waste products produced by metabolic reactions. Everything that enters or leaves a cell must pass through it plasma membrane. The plasma membrane regulates the passage of materials into and out of the cell. The plasma membrane must be large enough relative to the cell volume to keep up with the demands of regulating the passage of materials. The surface area of a cell must be large enough relative to its volume to permit adequate exchange of materials with the environment.

As cell become larger, its volume increases at a greater rate than its surface area (plasma membrane). If number of molecules required by the cell could not be transported into the cell fast enough to maintain its needs. The cell would not be able to regulate its concentration of various ions or efficiently export its wastes. The advantage that cells are small is that once inside molecules must be transported to the locations where they are converted into other forms. Since cells are small, the distances molecules travel within them are short, which speeds up many cellular activities. Cellular size is determined by the surface/volume ration required to maintain access to the substances and enzymes that cells need to complete their functions.

Small electrically charged molecules cannot pass through the membrane. It is difficult for charged molecules pass through the membrane by simple diffusion. Molecules' charge either positive or negative makes them to repel from charges in the membrane. Also charge electrically bind them to water molecules and making them hydrated.

Temperature, membrane surface area, size of the molecules, membrane permeability, molecular weight, and concentration gradient determines whether or not a substance will be able to actively transported through a cell membrane. A difference in the concentration of some substance on the two sides of a membrane is a form of stored energy or potential energy. As particle of the substance move across the membrane from the side of higher concentration to the side of lower concentration, the cell can convert some of this potential energy to the chemical energy of ATP molecules. The net flow from the hypotonic to the hypertonic solution. The plasma membrane must be large enough relative to the cell volume to keep up with the demands of regulating the passage of materials. The surface area of a cell must be large enough relative to its volume to permit adequate exchange of materials with the environment. As cell become larger, its volume increases at a greater rate than its surface area (plasma membrane). Since cells are small, the distances molecules travel within them are short, which speeds up many cellular activities. The plasma membrane must be large enough relative to the cell volume to keep up with the demands of regulating the passage of materials. Small molecules move faster than large molecules, since large molecules requires more energy it slows them down. Heavy molecules diffuse slowly than light molecules. In some cells more than one system may work to transport a given substance. For example, the transport of glucose form the intestine to the blood occurs through a thin sheet of epithelial cells that line the intestine. The surface that is exposed to the intestine has many microvilli that effectively increase the surface area of the membrane available for absorption. The temperature play a role on how fast molecules will diffuse. Molecules in the substance at higher temperature (more energy) move/diffuse faster than molecules at low temperature.

The outer layer of epidermis (stratum corneum) consists of dead cells rich in keratin. Keratin provides a waterproof covering preventing unnecessary water loss and evaporation. After a long time being in the water the skin start to absorb water, making keratin cells absorb and swell.

Voltage-gated ion channels open and close response depends to changes in voltage, allows the passage of particular inorganic ions across cell membranes. Voltage-gated ion channels found at the synapse and along the axon. This ion channel is an integral membrane protein and made of amino acid. It is important for sodium ion channels to open and close at the exact time in order for neurons to conduct an electrical impulse. "It allow sodium ions to run down their electrochemical gradients across the cell membrane. It functions in the generation and propagation of action potentials down axons." Voltage-gated ion channels respond to a change in membrane potential. They are found in muscle tissue and nerve. It is important in the generation and transmission of electrical signals by neurons. Depolarization is reduction in the voltage across the membrane. In order for it to occur is by a stimulus opening a sodium channel. A neuron can be depolarized by stimuli that open sodium channels. The increased inflow of Na+ makes the inside of the cell less negative. These voltage changes depends on the strength of the stimulus. A large stimulus will open more channels, producing a larger change in permeability and larger change in the membrane potential.

Voltage-gated ion channels open and close response depends to changes in voltage, allows the passage of particular inorganic ions across cell membranes. It is important for sodium ion channels to open and close at the exact time in order for neurons to conduct an electrical impulse.

Ions cross the plasma membrane by diffusion through ion channels formed in membrane proteins. Net movement of ions occur from higher concentration to lower concentration. In order for sodium ions to fit through the cell membrane they need channels.

The channels allow only specific types of ions to pass. Also to regulate the amount and measure time of ion transfer. It is important since an ions flow is used for neuron activity, osmotic pressure control and cell signaling. The neuron have passive ion channel, this channel allows the passage of specific ions such as Na+, K+, Cl-, and Ca2+. Passive ions are not controlled by gates; they are allowing ions freely flow.

In a resting neuron, one that is not transmitting an impulse, the inner surface of the plasma membrane is negatively charged compared with the outside; the membrane is polarized. The membrane is less permeable to sodium ions than to potassium ions. Sodium-potassium pumps in the plasma membrane actively pump sodium ions out of the cell and pump potassium ions in.

At resting potential Na+ activation gates are closed, and Na+ cannot pass through them into the neuron. When voltage reaches the threshold level, the protein making up the channel changes shape, opening the activation gate. Once the gate opens Na+ flow through the channel and the inside of the neuron becomes positively charged comparatively to the extracellular fluid outside the plasma membrane. When neuron is depolarized, voltage-activated sodium channels open increasing the membrane permeability to Na+. Sodium ions diffuse into the cell, moving for an area of higher concentration to a lower concentration. As Na+ flow into the cell the cytosol becomes positively charged. This charge further depolarizes the neuron so that more voltage-gated sodium channels open, further increasing membrane permeability to sodium.

If movement involved through channel proteins it is called facilitated diffusion which is part of passive transport. Some materials can pass through the cell membrane by diffusion, a process that does not required energy from the cell. It occurs because of the energy of motion (kinetic energy) of the particles. Diffusion of material through a cell membrane always occurs in both directions. The net movement of material by diffusion is from a region of higher concentration of particle to a region of lower concentration. The substance cannot pass phospholipid bilayer and passage across the membrane is facilitated by the channels. Some substances pass through membranes by facilitated diffusion, a form of carrier-mediated transport that uses the energy of a concentration gradient for the substance being transported and that cannot work against the gradient. A transport protein in the membrane binds a solute particle. the transport protein changes its shape, opening a channel thorough the membrane. A specific solute can be transported from the inside of the cell to the outside or from the outside to the inside, but net movement is always from a region of higher solute concentration to a region of lower concentration. Facilitated diffusion requires the potentials energy of a concentration gradient.

Active transport is the use of cellular energy to move particles through a cell membrane, it is occur in carrier proteins (ions pumps are form of carrier protein). The transport requires energy from the cell that supply this energy by breaking down ATP. The particles may be moved in the same direction as diffusion, or they may be moved against the normal direction of diffusion. Transport through the cell membrane is assisted by carrier proteins embedded in it. In carrier mediated active transport the cell expends metabolic energy to move ions or molecules across a membrane against a concentration gradient. The sodium-potassium pump uses ATP to pump sodium ions out of the cell and potassium ions into the cell.

In a resting neuron, one that is not transmitting an impulse, the inner surface of the plasma membrane is negatively charged compared with the outside; the membrane is polarized. A potential difference of about -70mV exists across the membrane. This is the resting potential. The segment of an axon of attractive neuron, the axon is negatively charged compared with the surrounding extracellular fluid. The membrane is less permeable to sodium ions than to potassium ions. Potassium ions diffuse out with their concentration gradient and are responsible for the voltage across the membrane. Sodium-potassium pumps in the plasma membrane pump sodium ions out of the cell and pump potassium ions in. The plasma membrane is permeable to negatively charged chloride ions, and they contribute to the negative charge. Large anions such as proteins also contribute to the negative charge.

When the axon depolarizes to about -55mV, an action potential is generated. In the resting state, voltage-activated ion channels (Na+ and K+) are closed. In a resting neuron, one that is not transmitting an impulse, the inner surface of the plasma membrane is negatively charged compared with the outside; the membrane is polarized. A potential difference of about -70mV exists across the membrane. This is the resting potential. The segment of an axon of attractive neuron, the axon is negatively charged compared with the surrounding extracellular fluid. The membrane is less permeable to sodium ions than to potassium ions. Potassium ions diffuse out with their concentration gradient and are responsible for the voltage across the membrane. Sodium-potassium pumps in the plasma membrane pump sodium ions out of the cell and pump potassium ions in. The plasma membrane is permeable to negatively charged chloride ions, and they contribute to the negative charge. Large anions such as proteins also contribute to the negative charge. As entrance, voltage-activated Na+ channels open and Na+ enters the neuron, causing further depolarization. K+ channels slowly begin to open. Inside of neuron becomes positive relative to outside. Repolarization begins when the Na+ channels close and voltage-activated K+ channels open, permitting K+ to leave the cell. Restoring negative charge to inside of cell. Resting conditions are restored. Voltage-activated Na+ and K+ channels close.

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