CHAPTER 3 The Plasma Membrane & Membrane Potential
Plasma Membrane: is an extremely thin layer of lipids and proteins that forms the outer boundary of every cell and
encloses the intracellular contents. ( it maintains the fluid composition inside and outside of the cell)
-controls the entry of nutrients of molecules and the exit of secretory and waste products
-it also maintains differences in ion concentrations between the cell’s interior and exterior
-also participates in joining of cells to form tissues and organs
-also plays a key role in the ability of a cell to respond to changes or signals in the cells environment.
For every cell these membrane functions are important:
(1). The cell’s survival
(2). Its ability to maintain homeostasis
(3). The cells’ ability to function cooperatively and in coordination with surrounding cells
MEMBRANE STRUCTURE AND COMPOSITION
- appears to be trilaminar structure in electron microscope with two dark layers separated by a light middle layer.
-consists of mostly lipids (fats), proteins and some carbs.
-most abundant lipid is phospholipid
-Phospholipids have polar ( charged, hydrophilic) charge and non-polar(neutral, hydrophobi c) fatty acid tails.
-The two sided molecules form a lipid bilayer ( a two-sided double layer )
-The outer surface of the layer is exposed to ECF (Extracellular fluid) and inner surface is in contact with Intracellular
Fluid ( ICF)
-Lipid bilayer is NOT SOLID but fluid
-Phospholipids not held by strong chemical bonds can twirl around as well as move about within their own half of the
-also contributes to the fluidity and stability of the membrane Chapter
-Cholesterol helps stabilize the phospholipid position
-attached to or insterted within the lipid bilayer
-Plasma membrane contains approx. 50 times as many lipid molecules as protein molecules
-Fluidiity of the lipid bilayer enables many memraben proteins to float around freely
-The structure of the membrane is known as fluid mosaic model (because of membrane fluidity and ever
changing mosaic pattern)
-Small amount of carbohydrates present on the outer surface which makes it “sugar coated”
-Glycoproteins and Glycolipids are the sugar combinations
The different components of plasma membrane carry out a variety of functions including
(1) lipid bilayer acts as a barrier to diffusion
(2) proteins perform specialized functions
(3) carbohydrates act as self recognition molecules for cell-cell interactions.
1. Some proteins pass through the membrane through water -filled pathways or Channels
-Channels are specific for ONE protein; water soluble substances; they are small in diameter (<0.8nm)
-Smalls ions (Na+ (23nm) and K+ (39nm) )
-Water, O2, CO2, diffuse through the lipid bilayer ( since they are small or non -polar)
2.Carrier molecules, transfer specific substances across the membrane that are unable to cross on their own. They
are needed because the material might be very big and highly charged therefore they help. Chapter
3.Other proteins located on the inner membrane surface serve as Docking-Marker Acceptors that bind lock and
key fashion with the docking markers . (their size is 58nm-59nm)
4.Surface located proteins function as membrane- bound enzyme that control chemical rxns specialized in the
types of enzymes embedded within their plasma membrane.
5.Many proteins on the outer surface serve as receptor sites (recognize and bind with specific series of membrane
and intracellular events)
6.Cell Adhesion Molecules (CAM)
-Many CAMS project out from the outer membrane surface and forms loops or hooks that the cells use to grip each
other and grasp to the connective tissue fibers. (ex cadherins)
7. Proteins on the outer membrane surface with carbohydrates (glycoproteins) are important in cells’ ability to
recognize “self” (cell of the same type ) and in cell to cell interactions.
-Short sugar chains on the outer membrane su rface serve as self-identity markers that enable cells to identify and
interact with one another.
(i)Diff cells have diff markers which allow them to make a distinction (ie between muscle cell and nerve cell)
(ii)Carbohydrate-containing surface markers are involved in tissue growth,
(typically cells don’t trespass the boundaries of neighboruing cels, the exception is the dieases of cancer, where
cancer cells with their abnormal cell surface carbohydrate markers, spreads uncontrobally invading space of
2.CELL TO CELL ADHESION
-Plasma membranes also serve as the outer boundaries of cell and also PARTICIPATE in CELL ADHESION
-Cells are held together by (3) different means
(i) The extracellular matrix
(ii) cell adhesion molecules in the cells’ plasma membrane
(iii) specialized cell junctions
-Tissues are not made of solely cells being in contact with each other but also by EXTRACELLULAR MATRIX
(ECM) --( an intricate meshwork of fibrous proteins embedded in a watery , gel -like substance composed of complex
-THE ECM serves as the biological glue
-The diffusion of nutrients, wastes and other water soluble traffic occurs goes in the interstitial fluid which is
between blood and tissue cells.
-Within the gel there are 3 types of protein fibers called : (i) Collagen (ii) Elastin (iii) Fibronectin
-forms cable like fibers or sheets that provide tensile strength (resistance to longitudinal stress)
-In scurvy condition caused by Vitamin C defiancy these fibers(collagin) are not completely formed.
(Scurvy is spots on the skin, spongy gums, and bleeding from the mucous membranes. Spots are most abundant on
the thighs and legs, and a person with the ailment looks pale, feels depressed, and is partially immobilized. As scurvy
advances, there can be open, suppurating wounds, loss of teeth, jaundice, fever, neuropathy and death.)
-rubber like protein fibre most abundant in tissues that must be cpable of easily stretching and then recolilng after the
stretching force is removed.
(found in lungs, recoils as air moves in and out)
-promotes cell adhesion and holds the cells in position
-Reduced amounts of this protein have been within certain types of cancerous tissues
-Rough endoplasmic reticulum is needed for fibronec tin
-Most of the abundant matrix in connective tissue is secreted by fibroblasts (“fiber formers”)
-ECM also helps regulate behavior and functions of the cells with which it interacts.
-When cells are close to each other they stick
together by CAMs or by tissue cohesion
-Some cells are directly linked to each other by 3
types of specialized cell junctions
(i) Desmosomes (adhering junctions)
-act like spot rivets that anchor together two closely
adjacent but non-touching cells.
(i) a pair of dense , button like cytoplasmic
known as plaque, located on the inner surface of
each two adjacent cells
(ii) strong glycoproteins filaments containing Chapter
cadherins(a type of CAM) that extends across the space between two cells and att ach to the plaque on both sides.
-Most abundant in tissues (that are subject to stretching), such as skin, the heart, and the uterus.
-intermediate, is kertain (foundin skin) plaque to plaque)
(ii)Tight Junctions (impermeable junctions)
-Adjacent cells firmly bind with each other
-Epithelial tissues covers the surface of the body and lines all its internal
-All epithelial tissues serve as highly selective barriers between two
compartment with diff chemical composition. (digestive tract sep food and
potent digestive juices within the inner cavity (lumen)
-IF THE CELLS ARE NOT JOINED BY TIGHT JUNCTIONS,
UNCONTROLLED EXCHANGE OF MOLECULES COULD TAKE PLACE
BETWEEN THE COMPARTMENTS by unpoliced traffic through th e spaces between ADJACENT CELLS.
* Passage across must take place THROUGH THE CELL not BETWEEN THEM.
Impermeable- How do Substances get in?
-Go through Channels
-Receptor mediated channels
(iii) Gap Junctions (communicating junctions)
- Do not let ions or molecules great er than 0.8.
-Gap exists between adjacent cells which are linked by small, connecting tunnels formed by connexons.
-Connexon (made up of six protein subunits, arranged in a hollow tube like structure)
-Gap junctions are especially abundant in cardiac muscle and smooth muscle.
-Also found in non-muscle tissues where they permit unrestricted passage of small nutrient molecules between cells.
-Also serve as avenues for the direct transfer of small signaling molecules from one cell to the next.
OVERVIEW OF MEMBRANE TRANSPORT
-Permeable: if the substance can cross it
-Impermeable: if the substance cannot pass through it
The Plasma mem is Selectively Permeable meaning it allows only certain things to pass
-Two prop influence whether they can pass through (permeate ) the membrane are
i. the relative solubility of the particle in lipid
ii. the size of the particle
* Charge does not matter for this membrane* Chapter
-Uncharged polar molecules (O2, CO2, and fatty acids) are highly lipid soluble and easily go through the
-Charged or non-polar molecules (ions such as Na+ and K+) and polar molecules such as (glucose and proteins)
have low lipid solubility but VERY soluble in water.
-The lipid bilayer serves as an impermeable barrier to particles poorly soluble in lipid.
-For water soluble and thus lipid insoluble ions less than 0.8nm in diameter are transported by protein channels
-Particles that have low lipid solubility and are too large for channels cannot permeate the membrane on their own.
(example is Glucose)
-Two general types of forces involved in accomplishing transport across the membrane:
(i) forces that do not require the cell to expend energy to produce movement (PASSIVE FORCES)
(ii) forces that do require the cell to expend energy (ATP) to transport a substance across the membrane (ACTIVE
UNASSISTED MEMBRANE TRANSPORT
-Diffusion & Osmosis
-Molecules or ions that penetrate plasma mem on their own are driven on their own by TWO forces
(i)diffusion down a concentration gradient
(ii) movement along an electrical gradient
(i) diffusion down a concentration gradient
-greater molecular concentration of a substance in a solution , the greater the likelihood of collisions
-the difference between two concentrations of the two adjacent areas is called concentration gradient.
-after the molecules are evenly spread out, there will be NO MORE CONCENTRATION GRADIENT and no
diffusion will be taking place this will be called steady state ( though the molecules will move but they
are evenly distributed)
Passive diffusion of particles
- molecules at continuous motion at temp above zero due to heat energy (mostly in liquids and gasses as there is
-molecules collide with each other and diffuse without energy
-process of diffusion is crucial to the survival of every cell and plays an important role in many homeostatic
activities. (Blood carried to lungs is in low concentration of O 2,nce all the blood has been give to body tissues for
cell metabolism. Therefore the air in the lungs is in High concentration of O ,2a net diffusion of O 2 occurs from the
lungs into the blood as blood flows through the lungs and when it leaves the lungs)
-Diffusion plays an imp role in
-Exchange of O2 and CO2 between air and lung s
-Movement of substances across the kidney tubules Chapter
-In diffusion ions are somewhat different situation (e.g Na+, K+, Ca++, Cl -) ; They are charged but the positive move
towards the negative and like charges repel.
Several factors in addition to the concentration gradient influence rate of net diffusion across the membrane and can
be described by FICK’s LAW OF DIFFUSION ( NO EQUATION to memorize)
FACTOR Effect on RATE net of diffuscion
Magnitude (steepness) of the concentration gradient
Permeability of the membrane to the substance
Surface area of the membra