Chapter 3: 3.3 Transport Across the Plasma Membrane

Bulk-phase endocytosis (pinocytosis)

"Cell drinking." A form of endocytosis in which tiny droplets of extracellular fluid are taken up. No receptor proteins are involved; all solutes dissolved in the ECF are brought into the cell. Within the cell, the vesicle fuses with a lysosome, where enzymes degrade the engulfed solutes. The resulting smaller molecules, such as amino acids and fatty acids, leave the lysosome to be used elsewhere in the cell. Pinocytosis occurs in most cells, especially absorptive cells in the intestines and kidneys.

Carrier-mediated facilitated diffusion

*CARRIER* A *carrier* (AKA transporter) moves a solute down its concentration gradient across the plasma membrane. Since this is a passive process, no cellular energy is required. The solute binds to a specific carrier on one side of the membrane and is released on the other side after the carrier undergoes a change in shape.

Channel-mediated facilitated diffusion

*ION CHANNEL* A solute moves down its concentration gradient across the lipid bilayer through a membrane channel *ION CHANNEL* Most membrane channels are ion channels, integral transmembrane proteins that allow passage of small, inorganic ions that are too hydrophilic to penetrate the nonpolar interior of the lipid bilayer.

Two sources of energy used to drive active transport

1) Energy obtained from hydrolysis of ATP is the source in primary active transport 2) Energy stored in an ionic concentration gradient is the source in secondary active transport

Factors that influence diffusion across the plasma membrane

1) Steepness of the concentration gradient 2) Temperature 3) Mass of the diffusing substance 4) Surface area 5) Diffusion distance

Pseudopods

A "false foot" or temporary bulge of cytoplasm used for feeding and movement in some protozoans. (Engulfs the microbe)

Isotonic solution

A RBC (red blood cell) - for example - will maintain it's normal shape and volume. A solution in which the concentration of solutes is essentially equal to that of the cell which resides in the solution. *WATER MOLECULES MOVE FROM HIGHER WATER CONCENTRATION TO LOWER WATER CONCENTRATION - FROM AN AREA OF LOWER SOLUTE CONCENTRATION TO HIGHER SOLUTE CONCENTRATION*

Hypertonic solution

A RBC (red blood cell) - for example - will shrink A solution that has a higher concentration of solutes than does the cytosol inside the RBC's *WATER MOLECULES MOVE FROM HIGHER WATER CONCENTRATION TO LOWER WATER CONCENTRATION - FROM AN AREA OF LOWER SOLUTE CONCENTRATION TO HIGHER SOLUTE CONCENTRATION*

Hypotonic solution

A RBC (red blood cell) - for example - will swell and burst. A solution that has a lower concentration of solutes than the cytosol inside the RBC's (RBC has higher concentration). Water molecules enter the cells faster than they leave, causing RBCs to swell and burst. *WATER MOLECULES MOVE FROM HIGHER WATER CONCENTRATION TO LOWER WATER CONCENTRATION - FROM AN AREA OF LOWER SOLUTE CONCENTRATION TO HIGHER SOLUTE CONCENTRATION*

Transcytosis

A combination of endocytosis and exocytosis. Transport into, across, and then out of cell.

Concentration gradient

A difference in the concentration of a substance across a distance

Tonicity

A measure of the solution's ability to change the volume of cells by altering their water content

Vesicle

A membrane bound spherical sac. A variety of substances are transported within these from one structure to another within cells. They also import materials from and release materials into ECF.

Equilibrium

A state of balance. Occurs after particles have become evenly distributed throughout the solution. The particles continue to move about randomly due to their kinetic energy, but their concentrations do not change. (1 particle in, 1 particle out)

Phagocytes

A type of white blood cell that ingests invading microbes. Two main types: 1) Macrophages: Located in body tissues 2) Neutrophils: A type of white blood cell

Primary active transport

Active transport that relies directly on energy derived from the hydrolysis of ATP This changes the shape of the carrier protein, which "pumps" a substance across a plasma membrane against its concentration gradient. A typical body cell expends ~40% of the ATP it generates on primary active transport. Extra info: Chemicals that turn off ATP production - for example, the poison cyanide - are lethal because they shut down active transport in cells throughout the body

Pumps

Carrier proteins that mediate primary active transport

Phagocytosis

Cell eating. A form of endocytosis in which a cell engulfs large solid particles, such as worn-out cells, whole bacteria, or viruses.

Osmosis

Diffusion of *water* through a selectively permeable membrane. Like other types of diffusion, it is a passive process. *WATER MOLECULES MOVE FROM HIGHER WATER CONCENTRATION TO LOWER WATER CONCENTRATION - FROM AN AREA OF LOWER SOLUTE CONCENTRATION TO HIGHER SOLUTE CONCENTRATION*

Active transport

Energy-requiring process that moves material across a cell membrane *against a concentration gradient* Some polar or charged solutes that must enter or leave body cells cannot cross the plasma membrane through any form of passive transport because they would need to move "uphill." This is considered an active process because energy is required for carrier proteins to move solutes across the membrane against a concentration gradient.

Substances that move across the plasma membrane (and an example of how) via carrier-mediated facilitated diffusion

Glucose, fructose, galactose, and some vitamins. 1) Glucose binds to a specific type of carrier protein called the glucose transporter (GluT) on the outside of the membrane. 2) As the transporter undergoes a change in shape, glucose passes through the membrane 3) The transporter releases glucose on the other side of the membrane

Receptor-mediated endocytosis

Highly selective type of endocytosis by which cells take up specific ligands (molecules that bind to specific receptors). A vesicle forms after a receptor protein in the plasma membrane recognizes and binds to a particular particle in the ECF. Ex: Cells take up cholesterol-containing low-density lipoproteins (LDLs), transferrin (an iron-transporting protein in the blood), some vitamins, etc.

Secondary active transport

Movement of material that is due to the electrochemical gradient established by primary active transport. There is indirect use of energy obtained from the hydrolysis of ATP.

Simple diffusion

Passive process in which substances move freely through the lipid bilayer of the plasma membranes of cells without help of the membrane transport proteins. Nonpolar, hydrophobic molecules move across the lipid bilayer through the process of simple diffusion.

Hydrostatic pressure

Pressure exerted by a volume of fluid against a wall, membrane, or some other structure that encloses the fluid.

Endocytosis

Process in which materials move into a cell in a vesicle formed from the plasma membrane. Active process - require energy supplied by ATP.

Exocytosis

Process in which materials move out of a cell by the fusion with the plasma membrane of vesicles formed inside the cell. Active process - require energy supplied by ATP.

Facilitated diffusion

Requires transport protein. Still uses concentration gradient. Movement of specific molecules across cell membranes through protein channels (integral membrane protein). Solutes that are too polar or highly charged to move through the lipid bilayer by simple diffusion can cross the plasma membrane through this process. An integral membrane protein assists a specific substance across the membrane. The integral membrane protein can be *either a membrane channel or a carrier* Two types: 1) *Channel-mediated* facilitated diffusion 2) *Carrier-mediated* facilitated diffusion

Hemolysis

Rupture of RBC's as a result of placement in a hypotonic solution Pure water is very hypotonic and causes rapid hemolysis

Moving "down a concentration gradient"

Something moves from high to low concentration. If a particular solute is present in high concentration in one area of a solution and in low concentration in another area, solute molecules will diffuse toward the area of lower concentration. They move "down their concentration gradient."

Passive process

Substances cross the membrane. NO ENERGY OR CELL WORK 3 types: 1) Simple diffusion 2) Facilitated diffusion 3) Osmosis

Osmotic pressure

The amount of pressure needed to restore the starting condition equals the osmotic pressure. Pressure that must be applied to prevent osmotic movement across a selectively permeable membrane (force exerted by the solution w/ impermeable solute) The osmotic pressure of a solution is proportional to the concentration of the solute particles that cannot pass through the membrane. The higher the solute concentration, the higher the solution's osmotic pressure.

Electrochemical gradient

The diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ion across a membrane and its tendency to move relative to the membrane potential.

Steepness of the concentration gradient (influence of diffusion across the plasma membrane)

The greater the difference in a concentration between the two sides of a membrane, the higher the rate of diffusion. When charged particles are diffusing, the steepness of the electrochemical gradient determines the diffusion rate across the membrane.

Diffusion distance (influence of diffusion across the plasma membrane)

The greater the distance over which diffusion must occur, the longer it takes. Diffusion across a plasma membrane takes only a fraction of a second because the membrane is so thin. In pneumonia, fluid collects in the lungs; the additional fluid increases the diffusion distance because oxygen must move through both the fluid and the membrane to reach the bloodstream.

Temperature (influence of diffusion across the plasma membrane)

The higher the temperature, the faster the rate of diffusion. All of the body's diffusion processes occur more rapidly in a person with a fever.

Mass of the diffusing substance (influence of diffusion across the plasma membrane)

The larger the mass of the diffusing particle, the slower its diffusion rate. Smaller molecules diffuse more rapidly than larger ones.

Surface area (influence of diffusion across the plasma membrane)

The larger the membrane surface area available for diffusion, the faster the diffusion rate. Ex: The air sacs of the lungs have a large surface area available for diffusion of oxygen from the air into the blood. Some lung diseases, such as emphysema, reduce the surface area. This slows the rate of oxygen diffusion and makes breathing more difficult.

Sodium-potassium pump

The most prevalent primary active transport mechanism. It expels sodium ions (Na+) from cells and brings potassium ions (K+) in. Because a part of the sodium-potassium pump acts as an ATPase, an enzyme that hydrolyzes ATP, another name for this pump is Na+ - K+

Diffusion

The passive movement of molecules or particles along a concentration gradient, or from regions of higher to regions of lower concentration.

Lysis

The rupture of other types of cells due to placement in a hypotonic solution

Transport maximum

The upper limit on the rate at which facilitated diffusion can occur, placed by the number of carriers available in a plasma membrane. Once all of the carriers are occupied, the transport maximum is reached, and a further increase in the concentration gradient does not increase the rate of facilitated diffusion. (Like a completely saturated sponge not being able to absorb more water, it will exhibit saturation)

Antiporters

Transport two different substances crossing the plasma membrane in the opposite direction (within a carrier protein)

Symporters

Transport two different substances crossing the plasma membrane in the same direction (within a carrier protein)

Process of osmosis

Water molecules pass through a plasma membrane in two ways: 1) Moving between neighboring phospholipid molecules in the lipid biplayer via simple diffusion 2) Moving through AQP's (aquaporins), integral membrane proteins that function as water channels.