VEMENT ACROSS MEMBRANES

Molecules can cross membranes in a-number of ways. They may use their own energy. Or they rely on an outside energy source. There are following types of cell movements.

SIMPLE DIFFUSION

Th movement of molecules from the area of higher concentration to the ar a of lower concentration is called simple diffusion. The molecules move ra domly. They keep on moving until they are distributed evenly. They come in a st e of dynamic equilibrium. Most of the short distance transport takes place by si pie diffusion.

Outside of cell

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FACILITATED DIFFUSION

T e movement of molecules through the protein molecules from higher to I. er concentration is called facilitated diffusion. Polar molecules are not soluble in lipids. So they diffuse through protein channels pores. The protein c annels provide a continuous pathway for movement of specific molecules. Thus passage through plasma membrane. Therefore, these molecules use facilitated diffusion. The facilitated diffusion also does not require energy. A molecule temporarily binds with a carrier protein in the plasma membrane It is transported from an area of higher concentration to the area of lower concentration.

OSMOSIS

The diffusion of water across a selectively permeable membrane from an area of higher concentration to an area of lower concentration is called osmosis. Osmosis is just a special type of diffusion.

The relative concentration of solutes in the water inside and outside the cell is called tonicity (Or. tonus. tension). There are three types of tonicity.

(a)   Isotonic solution: In this case the solute concentration is the same inside an outside the cell. For example, a red blood cell. The concentration of water molecules is also the same inside and outside the cell. Therefore, water molecules move across the plasma membrane at the same rate in both directions. Thus there is no net movement of water in either direction.

(b)   Hypertonic solution: In a solution, the solute concentration is higher outside the cell than inside. Thus concentration of water molecules inside the cell is higher than outside. So water moves out of the cell. The cell gets shrink This condition is called crenation in red blood cells.

.(c) Hypotonic solution: The solute concentration is lower outside the cell than inside. Thus, the concentration of water molecules is higher outside the cell than inside. As a result, water moves into the cell. It may cause bursting of bell.

Selectively permeable           geito.  • Water molecule

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          Filtration

Filtr tion is a process that forces small molecules across selectively per eable membranes with the .help of hydrostatic pressure. The pressure gen rated due to water in cell is called hydrostatic pressure. For example:

(i)     Blood pressure forces water and dissolved molecules through the permeable walls of capillaries by filtration. Large molecules, such as proteins, do not pass through the smaller membrane pores during filtration.

(ii)    Filtration also takes place in the kidneys. In kidney blood pressure forces water and dissolved wastes out of the blood vessels and into the kidney tubules. This is the firstestep in urine formation.


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5. ACTIVE TRANSPORT

Tin movement of molecules form the area of lower concentration to the are higher concentration by the expenditure of energy is called active­tra sport. Active transport takes place against a concentration gradient. This mo ement against the concentration gradient requires ATP energy.

Th active transport process is similar to facilitated diffusion. But the carrier pro em n in the plasma membrane must use energy to move the molecules against the r concentration gradient. There are two examples of active transport:

(a)    Sodium potassium pump: Sodium potassium pump is present in the neuron cells. It helps to maintain the high concentrations of potassium ions and low concentrations of sodium ions inside nerve cells. This transport of ions is necessary for the transmission of electrical impulses.

(b)    Calcium pump: It is another active transport mechanism. It keeps the calcium concentration hundreds of times lower inside the cell than outside.

5. ENDOCYTOSIS

T e bulk movement of material into the cell by formation of vesicle is called docytosis. The movement of molecules does not take place individually. ere are three forms of endocytosis: These are pinocytosis, phagocytosis and ,eptor mediated endocytosis:

Pinocytosis (cell drinking): The nonspecific uptake of all droplets of extracellular fluid is called pinocytosis. Any small solid dissolved in the fluid is also taken into the cell. Pinocytosis occurs when a small portion of the

plasma membrane invaginates during pinocytosis. The open end of the invagination joins to form a small vesicle. This tiny vesicle detaches from the plasma membrane and moves into the cytoplasm.

(b)   Phagocytosis (cell eating): The uptake of solid particles by invagination is called phagocytosis. It also forms vesicle called food vacuole. Lysosome combines with the food vacuole to form a phagolysosome (“digestion vacuole”). Finally the lysosomal enzymes break down the contents of vesicle.

(c)    Receptor-mediated endocytosis: Some specific receptor proteins are present on the plasma membrane. These proteins recognize an extracellular molecule and binds with it. This reaction stimulates the membrane to invaginates and create a vesicle. This vesicle contains selected molecule. Different types of molecules like cholesterol are brought into cells in this manner.

6. EXOCYTOSIS

The removal of cell secretions from the cells by out folding is called exocytosis. The Golgi apparatus packs proteins and other molecules into vesicles for secretion. These secretary vesicles fuse with membrane and release their contents into the outer environment. This process adds new membrane material. This material replaces the plasma membrane lost during endocytosis.

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(b)Phagocytosis

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