Lab No (1)Lab No (1)

OsmosisOsmosis

Structural components of the cellStructural components of the cell (fig. 1, 2, 3) (fig. 1, 2, 3):: 3.5.1 Plasma Membrane:3.5.1 Plasma Membrane: the plasma membrane is a the plasma membrane is a

partially permeablepartially permeable thin molecular layer that surrounds thin molecular layer that surrounds all living cells. The plasma membrane has three basic all living cells. The plasma membrane has three basic functions: (1) separates the cell from its surroundings (= functions: (1) separates the cell from its surroundings (= a barrier keeping the constituents of the cell in and a barrier keeping the constituents of the cell in and unwanted substances out; (2) protects it from changes unwanted substances out; (2) protects it from changes in the chemical and physical environment and regulates in the chemical and physical environment and regulates the traffic of molecules into and out of the cell (= a gate the traffic of molecules into and out of the cell (= a gate allowing transport into the cell of essential nutrients and allowing transport into the cell of essential nutrients and movement from the cell of waste products; and (3) a movement from the cell of waste products; and (3) a check barrier distinguishing self- from non-self (foreign) check barrier distinguishing self- from non-self (foreign) moleculesmolecules..

Structure (fig. 4):Structure (fig. 4): The plasma membrane The plasma membrane is composed primarily of two types of is composed primarily of two types of molecules—molecules—phospholipids, and phospholipids, and proteins.proteins. The basic structural framework The basic structural framework of the plasma membrane is formed by two of the plasma membrane is formed by two sheets of lipids, each sheet a single sheets of lipids, each sheet a single molecule thick. Within this molecule thick. Within this bilayerbilayer, of , of lipids, the proteinlipids, the protein

molecules are embedded. Proteins are molecules are embedded. Proteins are responsible for a host of functions, including responsible for a host of functions, including transporting substances across the membrane, transporting substances across the membrane, aiding communication between cells, and aiding communication between cells, and carrying out chemical reactions. A phospholipid carrying out chemical reactions. A phospholipid molecule has a molecule has a headhead region at one end that is region at one end that is hydrophilichydrophilic (= water loving – i.e. it can mix with (= water loving – i.e. it can mix with water). At the other end, are two long water). At the other end, are two long tailstails that that are are hydrophobichydrophobic (= water hating – i.e. they do (= water hating – i.e. they do not mix well with water). not mix well with water).

In the plasma membrane’s bilayer construction, In the plasma membrane’s bilayer construction, phospholipid molecules are arranged so that their phospholipid molecules are arranged so that their hydrophilic heads point outward on either side of the hydrophilic heads point outward on either side of the membrane, and their hydrophobic tails point toward each membrane, and their hydrophobic tails point toward each other in the middle of the membrane. This orientation other in the middle of the membrane. This orientation keeps the hydrophobic tails away from the watery fluids keeps the hydrophobic tails away from the watery fluids that both fill and surround living cells. In fact, the plasma that both fill and surround living cells. In fact, the plasma membrane stays intact precisely because the membrane stays intact precisely because the phospholipid molecules strongly resist any change in phospholipid molecules strongly resist any change in configuration that would expose their hydrophobic tails to configuration that would expose their hydrophobic tails to the watery environment. The plasma membrane as a the watery environment. The plasma membrane as a whole is a whole is a fluidfluid structure because phospholipid structure because phospholipid molecules and some proteins can move sideways within molecules and some proteins can move sideways within the membrane. In one second, a single phospholipid the membrane. In one second, a single phospholipid molecule can travel several micrometers (µm). molecule can travel several micrometers (µm).

Proteins drift more slowly through the membrane. With Proteins drift more slowly through the membrane. With protein molecules scattered among the phospholipid protein molecules scattered among the phospholipid molecules, the plasma membrane appears to be a molecules, the plasma membrane appears to be a mosaic of phospholipids and proteins. Some of the mosaic of phospholipids and proteins. Some of the proteins (extrinsic = peripheral proteins) are found on the proteins (extrinsic = peripheral proteins) are found on the inner or outer surface of the plasma membrane, while inner or outer surface of the plasma membrane, while others (intrinsic = integral proteins) span the membrane others (intrinsic = integral proteins) span the membrane and protrude on either end. Scientists refer to this and protrude on either end. Scientists refer to this concept of the plasma membrane’s structure as the concept of the plasma membrane’s structure as the fluid fluid mosaic model.mosaic model. Cholesterol also contributes to the Cholesterol also contributes to the fluidity of the plasma membrane. fluidity of the plasma membrane.

Cholesterol is a small lipid molecule that nestles among Cholesterol is a small lipid molecule that nestles among the hydrophobic tails of the phospholipids in the interior the hydrophobic tails of the phospholipids in the interior of the membrane. It prevents phospholipid molecules of the membrane. It prevents phospholipid molecules from packing together too tightly and making the from packing together too tightly and making the membrane rigid. It also acts as “antifreeze” for the membrane rigid. It also acts as “antifreeze” for the plasma membrane, preventing the membrane from plasma membrane, preventing the membrane from freezing to a jellylike consistency at low temperatures. freezing to a jellylike consistency at low temperatures. The lipid and protein molecules that make up the plasma The lipid and protein molecules that make up the plasma membrane are manufactured inside the cell and routed membrane are manufactured inside the cell and routed to the cell surface. The membrane is a dynamic to the cell surface. The membrane is a dynamic structure, with molecules constantly being added to and structure, with molecules constantly being added to and removed from the plasma membrane as a cell moves removed from the plasma membrane as a cell moves and grows.and grows.

. Function: . Function: Transport: Transport: Only a very few molecules (such as Only a very few molecules (such as

fat soluble substances) can pass directly fat soluble substances) can pass directly through the lipid bilayer to get from one side of through the lipid bilayer to get from one side of the membrane to the other. Many substances the membrane to the other. Many substances that a cell needs in order to survive cannot cross that a cell needs in order to survive cannot cross the lipid bilayer on their own, including glucose, the lipid bilayer on their own, including glucose, amino acids, and ions, such as sodium [Na+] amino acids, and ions, such as sodium [Na+] and potassium [K+]. Molecules pass across the and potassium [K+]. Molecules pass across the plasma membrane by the following mechanisms:plasma membrane by the following mechanisms:

Passive transport: Passive transport: Passive transport is accomplished Passive transport is accomplished by by diffusiondiffusion, , the spontaneous movement of a the spontaneous movement of a substance from a region of greater concentration to substance from a region of greater concentration to a region of lesser concentration.a region of lesser concentration. The difference The difference between the concentrations of a substance in two between the concentrations of a substance in two different areas is known as a different areas is known as a concentration gradientconcentration gradient (fig. 5)(fig. 5).. Diffusion moves molecules down a concentration Diffusion moves molecules down a concentration gradient in a manner that does not require the cell to gradient in a manner that does not require the cell to expend energy. Water, oxygen, carbon dioxide, expend energy. Water, oxygen, carbon dioxide, ammonia, and a few other small molecules diffuse ammonia, and a few other small molecules diffuse directly across the plasma membrane by passing directly across the plasma membrane by passing between phospholipid molecules. between phospholipid molecules.

Passage of solvents molecules (water) across Passage of solvents molecules (water) across the plasma membrane is special case of the plasma membrane is special case of diffusion usually referred to as diffusion usually referred to as osmosisosmosis (fig. 6 & (fig. 6 & 7)7).. Substances that cannot pass directly through Substances that cannot pass directly through the plasma membrane diffuse into or out of cells the plasma membrane diffuse into or out of cells with the aid of hollow, channel-like proteins in a with the aid of hollow, channel-like proteins in a process known as process known as facilitated diffusionfacilitated diffusion (fig. 8). (fig. 8). These These channel proteinschannel proteins are shaped so that are shaped so that only one substance, or a small group of closely only one substance, or a small group of closely related substances, can pass through each type related substances, can pass through each type of protein. This specificity enables a cell to of protein. This specificity enables a cell to control precisely the molecules that travel in and control precisely the molecules that travel in and out of the cell.out of the cell.

Active transport: Active transport: = = passage of molecules across the passage of molecules across the plasma membrane in a direction not predictable from plasma membrane in a direction not predictable from their concentrations on either side of the membrane, their concentrations on either side of the membrane, and the cell must spend energyand the cell must spend energy. Active transport (fig . Active transport (fig 9) is achieved by membrane proteins called pumps, 9) is achieved by membrane proteins called pumps, which have a site that is shaped to fit a specific which have a site that is shaped to fit a specific substance. These pumps are open on either the inside substance. These pumps are open on either the inside or the outside of the cell membrane. When the proper or the outside of the cell membrane. When the proper molecule or ion attaches to this site, the pump changes molecule or ion attaches to this site, the pump changes shape so that the site moves its opening to the other shape so that the site moves its opening to the other side of the plasma membrane, releasing the molecular side of the plasma membrane, releasing the molecular load. Many pumps obtain the energy necessary to load. Many pumps obtain the energy necessary to perform this work from adenosine perform this work from adenosine triphosphate (ATP)triphosphate (ATP), a , a molecule that serves as the main energy currency of molecule that serves as the main energy currency of living cells.living cells.

Cytosis Cytosis (fig. 10)(fig. 10): = transport of large : = transport of large molecules or particulates in membrane-molecules or particulates in membrane-bound vacuoles in and out of cells.bound vacuoles in and out of cells. This This includes:includes:

Endocytosis: Endocytosis: in endocytosis, the plasma in endocytosis, the plasma membrane folds inward, forming a pouch that membrane folds inward, forming a pouch that traps molecules. The pouch continues to press traps molecules. The pouch continues to press inward until it forms a closed sac that breaks inward until it forms a closed sac that breaks loose from the plasma membrane and sinks into loose from the plasma membrane and sinks into the cell. Note that the molecule does not actually the cell. Note that the molecule does not actually cross the plasma membrane. A distinction is cross the plasma membrane. A distinction is usually made between two types:usually made between two types:

Pinocytosis = Pinocytosis = the ingestion of fluid with contained the ingestion of fluid with contained dissolved molecules into a cell by turning a portion dissolved molecules into a cell by turning a portion of the cell membrane inwards to form a of the cell membrane inwards to form a vesiclevesicle that is then pinched off to form an internal that is then pinched off to form an internal vesiclevesicle..

Phagocytosis = Phagocytosis = the ingestion into a cell in the ingestion into a cell in a membranous sac the a membranous sac the microorganismsmicroorganisms or other small insoluble particles that the or other small insoluble particles that the cell has engulfed in order to digest or cell has engulfed in order to digest or destroy them.destroy them.

Exocytosis: Exocytosis: is a reversal of endocytosis. In is a reversal of endocytosis. In reverse reverse pinocytosispinocytosis (≈ secretion) a sac (= vesicle) inside the cell (≈ secretion) a sac (= vesicle) inside the cell containing proteins and other molecules moves toward containing proteins and other molecules moves toward the outer edge of the cell until it touches the plasma the outer edge of the cell until it touches the plasma membrane. The membrane of the sac then joins with the membrane. The membrane of the sac then joins with the plasma membrane, and the contents of the sac are plasma membrane, and the contents of the sac are released from the cell. Most of the proteins released by released from the cell. Most of the proteins released by animal cells, such as animal cells, such as hormoneshormones and and antibodiesantibodies, exit , exit the cells where they are made through exocytosisthe cells where they are made through exocytosis. . A cell A cell egests solid insoluble waste by a process opposite to egests solid insoluble waste by a process opposite to phagocytosis. phagocytosis.