Membranes Properties, functions, and transport

WHAT IS THE PLASMA MEMBRANE? A selectively permeable membrane, which encloses the cell (may also be called the cell membrane).

1.Hold the cell together. 2.Control entry and exit of materials (e.g. diffusion, osmosis, active transport) 3.Protect the cell. 4.Allow the cell to recognize and be recognized (cell signaling and immunity). 5.Bind to other cells and molecules. 6.A site for biochemical reactions (enzymes, areas for reactions). Functions of Membranes

Draw and label a diagram to show the structure of membranes. Fluid Mosaic Model Fluid mosaic model - FLUID - Components (lipids and proteins) on the move. - MOSAIC – Proteins scattered about.

(channel proteins, cholesterol, glycoproteins, and integral and peripheral proteins) The diagram should show the composition as labeled above.

Membrane Protein Functions Membrane proteins dictate the different functions of different membranes. 6 General Functions - hormone binding sites - enzymatic action - cell adhesion - cell-to-cell communication - channels for passive transport - pumps for active transport

Functions Hormone Binding Sites -Proteins exposed to exterior of cell -Specific shape to fit specific hormones -Attachment results in a message to interior of the cell Enzymatic Action -Enzymes on interior/exterior that catalyze many reactions -Often grouped on the outside to facilitate a metabolic pathway

Cell Adhesion -Proteins hook together between cells to make permanent or temporary connections -Connections referred to as junctions (e.g. gap junctions or tight junctions) Cell-to-Cell Communication -Many of these proteins include carbohydrates on the exterior -Identification label for cells of different types or species

Cell to cell communication llSignaling_A.swf

Protein Channels -Span the membrane -Provide passageway for substances to pass through -Passive transport  substances move from high concentration to low Active Transport Pumps -Proteins shuttle substances by changing shape -No concentration gradient needed -Requires energy in the form of ATP

Phospholipid Bilayer Phospho  phosphates Lipid  fats Bi  two Layer  barrier

-Head of the phospholipid is attached to one ionized phosphate group which is water-loving or hydrophilic (hydrogen bonds readily form between water and the phosphate). -Phospholipids have two fatty acid (hydrocarbon) tails that are water- hating or hydrophobic. -The hydrophilic and hydrophobic nature organizes these molecules tail-to-tail. -The organization of these molecules and their orientation to the internal and external environment of the cell make a stable, strong barrier. Plasma Membrane – Lipid Bilayer

Example of the plasma membrane’s strength - Quick Quiz - online.com/objects/ViewObject.aspx?ID=AP1101

The phospholipid bilayer is selectively permeable Some molecules pass through easily (diffusion), or go through a tunnel (facilitated diffusion). Others need energy to get them through (active transport). Large molecules use their own membrane to get them through (endo- /exo-cytosis).

SOLUTIONS - Consists of solutes and solvents - Concentration is a measure of amount of solute in the solution - Hyper-/hypotonicity (use words you know like hypothermia, hyperactivity hypoxia, hypertension) CONCENTRATION GRADIENT - Difference of two solutions separated by a semi-permeable membrane is called a concentration gradient - Particles are constantly moving - Transport is movement across the membrane

Membrane Transport Passive Transport (no energy) -Diffusion -Facilitated diffusion -Osmosis Active Transport (energy) -Sodium-potassium pump -Endocytosis -Exocytosis

Diffusion Particles move from high concentration to low concentration Ex. Oxygen/CO2 exchange within human tissue hill.com/sites/ / student_view0/chapter2/a nimation__how_diffusion_ works.html hill.com/sites/ / student_view0/chapter2/a nimation__how_diffusion_ works.html

A higher concentration gradient leads to an increased rate of diffusion Increased rate of diffusion Decreased rate of diffusion

FACTORS AFFECTING THE RATE OF DIFFUSION

Facilitated Diffusion Diffusion involving a specific carrier protein that combines with a substance to aid in movement Carrier changes shape but does not require energy Very specific Levels off when total saturation occurs hill.com/sites/ / student_view0/chapter2/a nimation__how_facilitated _diffusion_works.html

Osmosis Movement of water along a concentration gradient Water moves from a hypo- osmotic solution to a hyperosmotic solution Osmosis and passive transport continue until there is equilibrium (equal concentration or a isotonic solution) Uses aquaporins or proteins with specialized channels for water hill.com/sites/ /stu dent_view0/chapter2/animati on__how_osmosis_works.htm l hill.com/sites/ /stu dent_view0/chapter2/animati on__how_osmosis_works.htm l

Net movement of water Water moves by osmosis from an area of low solute concentration (more water) to high solute concentration (less water). The solute particles cannot move to balance the concentrations as they are too large to fit through the pores of the selectively permeable membrane.

Comparison of diffusion to osmosis

Active Transport Uses energy in the form of ATP Movement against concentration gradient Use of membrane protein pumps Allows cells to maintain interior concentrations that are different from exterior

Sodium-Potassium Pump Animal cells have higher concentration of potassium ions than exterior environment and vice versa for sodium ions Potassium pumped in and sodium pumped out hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl /free/ /120068/bio 03.swf::Sodium- Potassium%20Exchange%20 Pump hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl /free/ /120068/bio 03.swf::Sodium- Potassium%20Exchange%20 Pump Protein pumps are specific to their molecules The molecule binds to the active site of the pump The release of energy from ATP results in a conformational change The molecule is pushed to the other side of the membrane

Steps in the sodium- potassium pump 1.Protein binds to 3 sodium ions (interior) 2.Sodium ions cause ATP (adenosine triphosphate) to bind with the protein 3.ATP changes to ADP (adenosine diphosphate), protein changes shape, and sodium ions are expelled to the exterior 4.Potassium (exterior) ions bind to protein and phosphate group is released 5.Loss of phosphate restores the protein’s original shape and the release of potassium ions into the interior

Exocytosis & Endocytosis - processes that allow larger molecules to move across the plasma membrane Exocytosis -exit, cell, process -Export of macromolecules from the cell -Usually begins in the ribosomes of the rough ER until secreted to the environment outside the cell Endocytosis -entry, cell, process -Import of macromolecules -Membrane changes shape to pinch off around macromolecule -Creates vesicle -Plasma membrane molecules reattach

Exocytosis 1. Protein produced by the ribosomes of the rough ER enters the membrane of the ER 2. Protein exits the ER and enters the Golgi apparatus; a vesicle carries the protein 3. Protein moves through the Golgi apparatus, it is modified, exits the Golgi apparatus, and is enveloped in a vesicle 4. Vesicle moves to and fuses with the plasma membrane – results in the secretion of the contents of the vesicle from the cell

The fluidity of the plasma membrane is essential to allow fusion and subsequent secretion of the vesicle contents. At this point the vesicle membrane is actually a part of the plasma membrane. Fluid Mosaic Model

QUICK REVIEW ions/transport1.html

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