Plasma membrane functions

Lec. 4

Plasma membrane functions

The Plasma membrane keeps a cell intact. It allows only certain molecules and ions to inter and exit the cytoplasm freely, therefore, the Plasma membrane is said to be selectively permeable. Small molecules that are lipid-soluble, such as oxygen and carbon dioxide, can pass through the membrane easily. Certain other small molecule, such as water, is not lipid- soluble, but they still freely cross the membrane. Ion and large molecules cannot cross the membrane without assistance.

Diffusion

Is the random movement of molecules from the area of higher concentration to the area of lower concentration until they are equally distributed?. The diffusion is a passive way for molecules to enter or exit a cell meaning that no cellular energy is needed to bring it about. Like a perfume bottle may be opened in the corner of the room. The smell of perfume soon permeates the room because the molecules that make up the perfume move from where they are concentrated to all parts of the room. When equally distributed the molecule will be still be moving randomly in all directions.
The molecules that can freely cross the plasma membrane do so by diffusion. If there are molecules of a substance on both sides of membrane which way will they go? the molecules move in both directions but the net movement will be from the region of higher concentration to the region of lower concentration until equilibrium is achieved. At equilibrium as many molecules of the substance will be entering as leaving the cell. O2 diffuse across the plasma membrane and the net movement is to ward the inside the cells because a cell use O2 when it produces ATP molecule for energy purposes.

Osmosis

Is the diffusion of water across plasma membrane? Osmosis involves water and a solute (dissolved substance) that can not readily cross the plasma membrane. Tonicity is the concentration of the solute in a solution versus the concentration of water. As the amount of salt or sugar increased, the amount of water in a solution decreases. Normally body fluids are isotonic to cells. There is the same concentration of non-diffusible solutes and water on both sides of plasma membrane and cells maintain their usual size and shape. Intravenous solution medically administration is usually isotonic.
Solutions that cause cells to swell or even to burst due to an intake of water are said to be hypotonic. If RBCs are placed in hypotonic solution which a lower concentration of solute and higher concentration of water than do the cells, water enters the cell and they swell to bursting. The term lyses is used to refer to disrupted cells, hemolysis means disrupted RBCs.
A solution that causes cells to shrink or shrivel due to a loss of water are said to hypertonic if RBCs are placed in hypertonic solution which a higher concentration of solute and lower concentration of water than do the cells, water leaves the cell and they shrink. The term crenation refers to RBCs in this condition.
These changes have occurred due to osmotic pressure. Osmotic pressure is the force exerted on a selectively permeable membrane because water has moved from the area of higher to lower concentration of water (higher concentration of solute).

Facilitated transport

Many solutes do not simply diffuse across a plasma membrane; rather they are transported by means of protein carrier within the membrane. During Facilitated transport ,a molecule is transported at a rate higher than otherwise across the plasma membrane from the side of higher concentration to the side of lower concentration .This is a passive means of transport because the cell does not need to expend energy to move a substance down its concentration gradient .Each protein carrier ,sometimes called a transporter ,binds only to a particular molecule ,such as glucose .Diabetes type 2 results when cells lack a sufficient number of transporters.


Active transport
During active transport, a molecule is moving contrary to the normal direction, that is, from lower to higher concentration .For example, iodine collects in the cells of the thyroid gland, sugar is completely absorbed from the gut by cells that line the digestive tract, and sodium is some times almost completely withdrawn from urine by cells lining kidney tubules.
Active transport requires a protein carrier and the use of cellular energy obtained from the breakdown of ATP .When ATP is broken down, energy is released, and in this case, the energy is used by a carrier to carry out active transport.

*Passage of molecules into and out of cells

Name Direction Requirement Example 

Passive mechanismsDiffusion

Facilitated transport Toward lower concentration

Toward lower concentration

Concentration gradient

Carrier and concentration gradientLipid-soluble molecules ,water ,and gases

Sugar and amino acids


Active mechanisms Active transport

Endocytosis

Exocytosis Toward higher concentration

Toward inside the cell

Toward out side the cell Carrier plus energy

Vesicle formation

Vesicle fuses with plasma membrane Sugars ,amino acids ,and ions

Macromolecules

Macromolecules 

Proteins involved in active transport often are called pumps,because just as a water pump uses energy to move water against the force of gravity ,proteins use energy to move substances against their concentration gradients .One type of pump that is active in all cells ,but is especially associated with nerve and muscle cells ,move sodium ions to the out side and potassium ions to in side the cell .
The passage of salt (NaCl ) across a plasma membrane is of primary importance in cells .First ,sodium ions are pumped across membrane ;then, chloride ions simply diffuse through channels that allow their passage .Chloride ion channels malfunction in persons with cystic fibrosis ,and this leads to the symptoms of this inherited disorders .

Endocytosis and Exocytosis

During endocytosis ,a portion of the plasma membrane invaginates ,or forms a pouch ,to envelop a substance and fluid .Then ,the membrane pinches off to form an endocytic vesicle inside the cell .Some white blood cells are able to take up pathogens (disease causing agents) by endocytosis and the process is given a special name (phagocytosis).Usually ,cells take up molecules and fluid and then the process is (pinocytosis) .An inherited form of cardiovascular disease occurs when cells fail to take up a combined lipoprotein and cholesterol molecule from the blood by pinocytosis.
During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs.


The cytoskeleton
It took a high powered electron microscope to discover that the cytoplasm of the cell is criss-cross by several types of protein fibers collectively called cytoskeleton .The cytoskeleton helps maintain a cell's shape and either anchors, the organelles or assists their movement, as appropriate. The cytoskeleton includes microtubules, intermediate filaments and actin filaments .Fluorescence microscopy is another way to make these protein fibers visible.

Microtubules are much larger than actin filaments .Each is a cylinder that contains 13 longitudinal rows of a protein called tubulin .Remarkably, microtubules can assemble and disassemble .The regulation of microtubules assembly is under the control of a microtubule organizing center called the centrosome.
Microtubules begin to assemble in the centrosome and then they grow outward extending through the entire cytoplasm.
Microtubules help maintain the shape of cell and act as tracks along which organelles move .During cell division; microtubules form spindle fibers, which assist the movement of chromosomes.

Actin filaments, made of a protein called actin, are long; extremely thin fibers that usually occur in bundles or other groupings. Actin filaments have been isolated from various types of cells ,specially those in which movements occurs .Micovilli ,which project from certain cells and can shorten and extend ,contain actin filaments .Actin filaments ,like microtubules ,can assemble and disassemble .

Intermediate filaments, as their name implies, .are intermediate in size between microtubules and actin filaments .Their structure and function are different according to the type of the cell.

The nucleus and ribosomes

The nucleus and ribosomes work together to bring about protein synthesis.
The nucleus a prominent structure in cells, stores genetic information. Every cell contains the same genes but each type has certain genes or segments of DNA turned on and others tunes off. Activated DNA with RNA acting as intermediary specified the proteins in a cell. Proteins determine a cells structures and functions, particularly because the cells enzymes are proteins.

When you look at the nucleus even in an electron micrograph, you can not see DNA molecules but you can see chromatin. Chromatin undergoes coiling into rod like structures called chromosomes just before the cell divides. Each chromosome contains a specific DNA molecules and it associated proteins. Chromatin is immersed in a semi fluid medium called the nucleoplasma. A difference in PH suggests that a nucleoplasm has a different composition from cytoplasm. When you look at an electron micrograph of nucleus you will see one or more dark regions of the chromatin. These are called nucleoli (single, nucleolus) where another type of RNA called ribosomal RNA (rRNA) is produce and where rRNA joins with proteins to form the subunits of ribosome.
The nucleus is separated from the cytoplasm by a double membrane known as the nuclear envelop which is continuous with the ER. The nuclear envelop has nuclear pores of sufficient size to permit the passage of proteins into nucleus and ribosomal subunits out of the nucleus.

Ribosomes

Ribosomes are organelles composed of proteins and rRNA. Protein synthesis occurs at the ribosomes. Ribosomes are often attached to ER; they also occur free within the cytoplasm, either singly or in groups called polyribosome.
Protein synthesized by cytoplasmic ribosomes are used inside the cell for various purposes. Those produce by ribosomes attached to endoplasmic reticulum may eventually be secreted from the cell or become part of the plasma membrane.


The endomembrane system
The endomembrane system consist of the nuclear envelop, the endoplasmic reticulum, the Golgi apparatus, Lysosomes and vesicles (tiny membranous sacs).

The endoplasmic reticulum

The endoplasmic reticulum has two portions. Rough ER is studded with ribosome on the side of the membrane that faces the cytoplasm.
Here proteins are synthesized and enter the ER interior, where processing and modification begin. Some of these proteins are incorporated into membrane and some are for export. Smooth ER which is continuous with rough ER does not have attached ribosomes. Smooth ER synthesizes the phospholipids that occur in membranes and has various other functions, depending on the particular cell. In the tests it produces testosterone and in the liver it helps detoxified drugs. The ER forms vesicles in which large molecules are transport to other parts of the cell.

The Golgi apparatus

Is named for Camillo Golgi who discovered its presence in cell. The Golgi other consist of a stack of slightly curved saccules whose appearance can compared to a stack pancakes. Here proteins and lipids received from the ER are modified. For example a chain of sugars may be added to them, there by making then glycoprotein and glycolipid which are molecules found in the plasma membrane.
The vesicles that leave Golgi apparatus move to the other parts of the cell. Some vesicles proceed to plasma membrane, where they discharge there contents. Altogether is involved in processing packaging and secretion. Other vesicles that leave the Golgi apparatus are Lysosomes.

Lysosomes

Lysosomes membranous sacs produce by the Golgi apparatus contain hydrolytic enzymes. When a Lysosomes fused with such an endocytic vesicles, its contents are digested by lysosomal enzymes into simpler subunit that then enter the cytoplasm. Even parts of a cell are digested by its own Lysosomes (called auto-digestion). Normal cells rejuvenation most likely takes place in this manner, but auto-digestion is also important during development. For example, when a tadpole becomes a frog, Lysosomes digest a way the cells of the tail. The fingers of human embryo are at first webbed but they are freed from one another as a result of lysosomal action.










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