Membranes and Transport Chapters 7.3

Plasma Membrane Fluid Mosaic Model Phospholipids Bilayers Membrane Proteins Cholesterol

Transport across Membranes Gradient Passive Transport diffusion osmosis facilitated diffusion Active Transport Cell Surfaces

Fluid Mosaic Model describes both the structure and function of biological membranes states that the membrane is a “fluid” (not static) structure, membrane molecules “float” or “slide” past one another also is a “mosaic” - art term describing a large picture made of many small parts

1st Major Part: Phospholipids are amphipathic Contain two regions: head phosphate polar, hydrophilic, interacts with water tails (2) fatty acids (saturated or unsaturated) nonpolar, hydrophobic, repelled by water

Major Part 2-Membrane Proteins are either inserted through the membrane = integral proteins transport proteins - channels and carriers enzymes receptor proteins or are associated w/ the membrane surface = peripheral proteins act as sites for attachment to the cytoskeleton or extracellular matrix

Major Part 3 - Cholesterol: is the basis of all steroid molecules acts to stabilize the structure of membranes affects the “fluidity” of the membrane

Bilayers: are arranged from two layers of phospholipids; arranged tail-to-tail are the basic structure for all membranes have a hydrophobic region (lipid tails) that acts as a selectively permeable barrier give membranes a condition known as selective permeability

SELECTIVE PERMEABILITY Some things can cross the membrane; other things cannot... This means the membrane is important for HOMEOSTASIS-keeps the internal part of the cell stable. The Membrane is the Cell’s BOUNCER

Cross Easily small molecules nonpolar molecules water oxygen carbon dioxide nonpolar molecules lipids steroids

Doesn’t Cross Easily Large molecules Polar and ionic compounds DNA, Proteins, etc. Polar and ionic compounds sugar salts

Gradient definition: difference in concentrations, ions, etc. over a distance helps describe how substances move from one place to another Due to the random movement of molecules leads to movement from areas of high concentration to areas of low concentration

Passive Transport any movement of molecules “down” gradient does not require energy diffusion: movement of a substance from high concentrations to low concentrations osmosis: diffusion of water across a selectively permeable membrane facilitated diffusion: diffusion of a molecule across a membrane through a membrane protein

Osmosis: diffusion of water across a selectively permeable membrane hypertonic: solution with a higher concentration of solutes than its surroundings hypotonic: solution with a lower concentration of solutes than its surroundings isotonic: solution with concentrations equal to the surroundings

Active Transport movement of molecules against or “up” their concentration gradient REQUIRES ENERGY = ATP compare:both are types of transport contrast: active requires energy; moves substances up the gradient and from low to high concentration passive doesn’t require energy; moves down the gradient and from high to low concentration

Explain the movement… of water across a selectively permeable membrane in terms of hyper-, iso-, and hypotonic solutions water moves “down” its concentration gradient water’s concentration gradient is opposite that of its solutes, SO water moves from regions of low solute concentration to regions of high solute concentration water moves from hypotonic to hypertonic

Water Balance w/out walls hypertonic-lose water and shrivel (crenation) hypotonic-gain water, swell, and possibly burst (lyse) must live in isotonic environments or have adaptations for osmoregulation

Water Balance - cont’d. with walls hypotonic-water moves into the cell and presses against the wall, causing a turgid cell isotonic solutions-flaccid cells hypertonic-plasmolysis - membrane pulls away from the cell wall as it shrivels

Exocytosis movement of large molecules out of the cell requires vesicles for transport

Endocyotsis movement of large molecules into the cell phagocytosis: cell eating-one cell engulfs another pinocytosis: cell drinking-cells bring in fluid from the surroundings

Transport Terms to Know Diffusion Osmosis Facilitated Diffusion Active transport Exocytosis Endocytosis