Cell Membrane Structure Plasma membrane: outer membrane: defines the cell Fluid mosaic model Phospholipid bilayer Hydrophilic (water loving) polar heads of phospholipid molecule Hydrophobic (water fearing) non-polar tails Proteins Integral proteins: go all the way through the membrane Membrane gates and channels, receptors Peripheral proteins: attached to only 1 side of membrane May be found in layer facing cytoplasm May be found in layer facing outside of cell Enzymes, Receptors Some proteins and phospholipids have carbohydrate parts attached Glycoproteins Glycolipids

Passive Transport vs. Active Transport Passive = without adding energy Watching TV is passive (but it is NOT transport) Active = energy must be added Playing soccer is active (but it is also NOT transport)

Passive Transport Does not require energy at the point of transport But some force is at work Moves substances from where they are in high concentration to where they are in low concentration Move down the concentration gradient

Types of Passive Transport Diffusion Random movement of molecules Does not REQUIRE movement through a membrane, but it CAN BE through a membrane Moves substances from where they are in high concentration to where they are in low concentration until equilibrium is reached Gasses, liquids and solids can all diffuse, but solids take a LONG time! Some materials can diffuse through some membranes Lipids can diffuse through cell membranes

Types of Passive Transport Facilitated diffusion Diffusion through a membrane, but where a “door” is needed When ions, proteins and other non-lipids move through membranes, they usually need to go through a gate or channel Gate or channel is also called a carrier protein molecule

Types of Passive Transport Osmosis Movement of WATER across a selectively permeable membrane, from where water is in high concentration to where water is in low concentration Water can move through but the dissolved substances can’t It may be easier to think of the other substances’ concentration Water moves toward the higher concentration of dissolved substances Each dissolved particle exerts about the same amount of osmotic pressure How many osmotic particles per molecule? (hint: ionic or covalent bonds?) Glucose, NaCl, CaCl2, a protein

Osmosis , cont. When one side of the membrane has a higher concentration of dissolved substances, that side is said to be hypertonic to the other side When one side of the membrane has a lower concentration of dissolved substances, that side is said to be hypotonic to the other side When both sides of the membrane have the same amount of dissolved substances, they are said to be isotonic to each other The substances on either side DO NOT have to be the same substances ( for example- salt on one side of membrane and sugar on the other side)

Osmosis , cont. Physiologic condition (PC) = the normal osmotic pressure of body fluids PC is isotonic to 5.0% glucose solution PC is isotonic to 0.9% NaCl solution But body fluids do NOT have 5% glucose and/or 0.9% NaCl! (a) A red blood cell in an isotonic solution looks normal, because the same amount of water moves into the cell as moves out (b) A red blood cell in a hypertonic solution - more water moves out of the cell than moves in, so it shrinks (crenates) (c) A red blood cell in a hypotonic solution - more water moves into the cell than moves out, so it swells & may burst (hemolysis)

Osmosis in Plants Turgor pressure results when plant cells are placed in hypotonic solution Central water vacuole gains water Cytoplasm expands Plasma membrane pushes against cell wall (cell does NOT burst) Maintains plant’s upright posture Plasmolysis results when plant cells are placed in hypertonic solution Central water vacuole loses water Plasma membrane pulls away from cell wall

Active Transport Requires energy in the form of ATP at the point of transport Moves substances across a membrane from where they are in low concentration to where they are already in high concentration Move against the concentration gradient

Types of Active Transport Active Membrane Transport Movement of materials across a cell membrane against the concentration gradient, using a carrier protein that bonds to ATP Bonding causes the protein to change shape May move one substance in one direction (uniport), or two substances in the same direction (symport) or two substances in opposite directions (antiport) The Na+/K+ pumps are an example (they move the ions in opposite directions = antiport)

Types of Active Transport The “–cytosis family” Endocytosis Moves materials into the cell by having the membrane engulf them and form a vesicle around them (selective membrane fusion) Pinocytosis = “cell drinking” Phagocytosis = “cell eating” Exocytosis Moves materials out of the cell by surrounding them in a membrane (vesicle formation) and merging the vesicle to the plasma membrane (selective membrane fusion) Secretion is a type of exocytosis

Types of Active Transport Special application of endocytosis: Receptor mediated endocytosis Essentially just endocytosis, but with special receptor proteins that trigger the formation of the vesicle