the plasma membrane

• boundary of life- separates the cell from its surroundings

• selectively permeable membrane

  • allows some materials to cross it more easily than others

  • enables the cell to maintain a unique internal environment

fluid mosaic model

• biological membranes consist of various proteins that are attached to or embedded in a bilayer of phospholipids

  • have both a hydrophilic and hydrophobic region

• the phospholipid bilayer is fluid- always moving

• with a mosaic of proteins floating in it

phospholipid bilayer

• phospholipids self-assemble to form a double layer called a bilayer

• hydrophobic hydrocarbon tails in the center

• hydrophilic heads facing the aqueous solution on both sides of the membrane

  • internal environment of the cell (cytoplasm)- mostly H2O

  • external environment of the cell (extracellular matrix)- mostly H2O

fluidity of membranes

• membranes held together by weak hydrophobic interactions

• allows lipids and some proteins to drift laterally (side to side)

• membranes must be fluid to work properly

• if a membrane solidifies permeability changes

  • enzymatic proteins become inactive

unsaturated vs. saturated hydrocarbon tails

• phospholipids with unsaturated hydrocarbon tails maintain membrane fluidity at lower temperatures

cholesterol (steroid)

• common in plasma membrane of animals

• acts as a fluidity buffer

  • restricts movement of phospholipids

    • reducing fluidity at warmer temps

  • prevents close packing of lipids

    • enhancing fluidity at colder temps

evolution of differences in membrane lipids

• variations in membrane lipid composition is an evolutionary adaptation

  • fish that live in extreme cold have higher percentage of unsaturated hydrocarbon tails

  • bacteria/archaea in thermal hot springs have a membrane lipid composition that prevent excessive fluidity

• ability to change lipid composition in response to changing temperatures is an evolutionary adaptation

  • some plants (example: winter wheat) adjust percentage of unsaturated tails with seasonal temperature changes

    • percentage increases in autumn to prevent membrane from solidifying

membrane proteins (mosaic part of fluid mosaic model)

• each membrane has its own unique set of proteins

  • determine specific functions of the membrane

• integral proteins (transmembrane proteins)

  • extend through the membrane

  • have two hydrophilic ends and a hydrophobic midsection

• peripheral proteins

  • attached to the surface of the membrane

  • also attach to the cytoskeleton and fibers of the ECM

    • provides support for the plasma membrane

• 6 major functions of membrane proteins- transport, enzymatic activity, signal transduction, cell-cell recognition, intracellular joining, attachment

membrane carbohydrates

• ability of a cell to distinguish other cells is based on recognition of membrane carbohydrates

• covalently bond to lipid or proteins

  • glycolipid/glycoprotein

• vary from species to species, individual to individual, and cell to cell

  • example- A, B, AB, and O Blood types

    • red blood cells differ in the carbohydrate part of a glycoprotein on surface

synthesis of membranes

• membrane proteins and lipids are synthesized in the ER, modified in the Golgi, and transported by vesicles to the membrane

selective permeability

• plasma membrane permits a regular exchange of nutrients, waste products, oxygen, and inorganic ions

  • ease and rate at which small molecules pass through them differs

• hydrophobic, non-polar molecules cna dissolve and cross easily

  • hydrocarbons, CO2 and O2

transport proteins

• ions and polar molecules are stopped by the hydrophobic center

  • Na+, K+, H2O, and Glucose can not cross easily

• channel proteins

  • provides a hydrophilic passageway through the membrane

    • aquaporins

• carrier proteins

  • physically bind and change shape to shuttle molecules across

    • glucose transporter

diffusion

• movement of a substance down its concentration gradient

  • molecules move randomly, but movement of substance is directional

  • movement is from areas of high concentration to areas of low concentration

• the cell does not expend energy when substances diffuse across membranes down their concentration gradient

  • this is passive transport

osmosis

• diffusion of free water across a selectively permeable membrane

• water diffuses down its own concentration gradient

• affected by concentration of dissolved solutes

  • clustering of water molecules around solute particles lowers proportion of free water

tonicity

• tendency of a surrounding solution to cause a cell to gain or lose water- affected by concentration of dissolved solutes

• cellular environments can by hypotonic, isotonic, or hypertonic

isotonic cell environment

• solutions that have equal concentrations with no net movement of water across the membrane

  • iso means same

  • water goes in and out of the two solutions at the same rate

• an animal cell will neither gain nor lose water in an isotonic environment

hypertonic cell environment

• solution with a higher concentration of solutes

  • hyper means more

  • water will rush out of the cel, into the solution

• an animal cell placed in a hypertonic solution will lose water and shrivel

hypotonic cell environment

• solution with a lower concentration of solutes

  • hypo means less

  • water will rise into the cell, out of the solution

• if placed in a hypotonic solution, the cell will gain water, swell, and possibly lyse (burst)

plant cells (cell with cell wall)

• the cell wall of plants, fungi, and prokaryotes play a role in water balance in hypotonic environments

  • water moving into the cell causes the cel to swell against its cell wall

• creates turgor pressure- turgid cells

  • provides mechanical support for the plant

    • hypotonic environment is the healthiest environment for a plant

animal cells (cell without cell wall)

• cels without rigid walls must either live in an isotonic environment

  • salt water or isotonic body fluids

• or have adaptations for osmoregulation- regulation of water balance

• isotonic environment is the healthiest environment for an animal cell

facilitated diffusion

• passive transport aided by proteins

  • polar molecules and ions transported across membrane faster than normal

  • down concentration gradient

  • no energy (ATP) required

• channel protein- hydrophilic tunnel (aquaporin)

• carrier protein- structural match (glucose transporter)

active transport

• requires expenditure of energy (ATP) to transport a solute against its concentration gradient

  • movement of solute from low concentration to high concentration

  • essential for cell to maintain internal concentrations of small molecules

• the transport proteins for active transport are carrier proteins

membrane potential

• a voltage across the plasma membrane due to the unequal distribution of ions on either side

  • electrical potential energy results from the separation of opposite charges

• cytoplasm of a cell is negatively charged relative to the ECM fluid

  • favors diffusion of cations (+) into the cell and anions (-) out of the cell

• both the membrane potential and the concentration gradient affect the diffusion of an ion

  • an ion diffuses down its electrochemical gradient

electrogenic pumps

• membrane proteins that generate voltage across a membrane by the active transport of ions

• sodium potassium pump

  • actively transports Na+ ions out and K+ ions into the cell to maintain membrane potential in animal cells

  • 3 Na+ out for every to K+ in (maintains negative cytoplasm)

• proton pump is main electrogenic pump for plants, fungi, and protists

bulk transport (active)

• like active transport, it requires energy

• transports larger biological molecules, packaged in vesicles across the membrane

• exocytosis- cell secretes large molecules by the fusion of vesicles with the plasma membrane

  • exo- out of the cell

• endocytosis- a region of the plasma membrane sinks inward and pinches off to form a vesicle containing material that had been outside of the cell

  • endo- into the cell

3 types of endocytosis

• phagocytosis- endocytosis of food particles- cellular eating

• pinocytosis- endocytosis of fluids- cellular drinking

• receptor- a mediated endocytosis enables a cell to acquire specific substances by ligand binding (molecule to specific receptor)

  • cholesterol for membrane