Membranes And Cell Transport

Concept 6.2: Membranes organize a Cell’s Activities (Plasma membrane thin barrier between cell and environment, but regulates what can enter/leave cell.) I. Membrane Structure – key to how membrane does its job A. Membrane jobs: compartmentalize teams of enzymes, regulate what enters/leaves areas

B. Parts: (of plasma membranes and all other organelle membranes) . Phospholipid molecules and proteins a. Phospholipids (see Fig. 6.7 p. 115) – phosphate head (hydrophilic) and 2 fatty acid tails (hydrophobic) (Heads attracted to water, tails repelled by water) 1.) Membranes – double layer = Phospholipid bilayer - separates 2 watery environments from each other (ex. Cytoplasm, exterior envir.)

2.) See orientation of parts on Fig. 6.8 p. 116 3.) hydrophobic tails face middle of membrane – help make membrane selective barrier 4.) Nonpolar molecules can cross phospholipids easily (O2 and CO2) 5.) Polar molecules (sugars, many ions) cannot cross as easily

Membranes Dynamic = semi-rigid, parts flow within them a. Proteins drift freely through the phospholipids, like “icebergs” float in the “sea”

II. The Many Functions of Membrane Proteins (See Fig. 6.9, p. 117) A. Membrane contains “Mosaic” of Proteins (Fluid-Mosaic Model – not in text) 1. May also contain some carbs. stuck into it, too

B. Functions of these Membrane Proteins 1. Enzyme activity – to help carry out some of the cell’s chemical rxn. 2. Cell-to-cell recognition: (Transplant issues, blood group, etc. – Ex.) 3. Cell signaling: chem. signals from 1 cell may be picked up by proteins in another cell for action 4. Transport of Materials: Serve as channels to help some materials cross the plasma membrane (Ex. Sugars)

Concept 6.3: Membranes regulate the Traffic of Molecules 1. Cell membranes are “ Gatekeepers” Control what enters and leaves cell, and what enters/leaves all internal organelles, too. (Ex. Water, nutrients, dissolved gases, CO2, & wastes)

a. Protein molecules that form chemical “pumps” and channels to move material across membrane

TYPES OF CELL TRANSPORT

PASSIVE VS. ACTIVE TRANSPORT Passive Transport: Caused by the random motion of molecules (Because all matter has kinetic energy) = “Brownian motion” Does NOT require any of the cell’s own energy Cell can be dead or alive, and will still happen http://lessons.harveyproject.org/development/general/diffusion/diffnomemb/diffnomemb.html

Active Transport: Requires the cell to use energy in the form of ATP Cell must be alive http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

(called passive transport) B. Diffusion- Movement of materials from areas of high concentration to areas of low concentration 1. All living cells have a liquid interior (cytoplasm) and are surrounded by liquid 2. Diffusion causes many substances to move across cell membrane but does not require the cell to use energy (called passive transport) Equilibrium- when conc. of molecules are equal on both sides of membrane http://www.nsf.gov/news/news_videos.jsp?org=NSF&cntn_id=105797&media_id=57229

EXAMPLES OF DIFFUSION Air Freshener (or other obnoxious odor) spreading through a room Dye spreading in a glass of water http://www.youtube.com/watch?v=ASGj_RH4Voc O2 or CO2 entering or leaving a cell http://www.indiana.edu/~phys215/lecture/lecnotes/diff.html

C. Osmosis- Diffusion of water molecules through selectively permeable membrane 1. Most membranes are selectively permeable (some materials can pass across membrane and others cannot) like “sieve”

Types of Solutions to which Cells may be exposed: Isotonic: iso = “same” , tonic refers to “strength” of solute concentration - Isotonic solutions have same conc. Of solutes as the cell’s cytoplasm Hypotonic: hypo = “less or below”, - Hypotonic solutions have a lower conc. of solutes than the cell’s cytoplasm = more water conc.!

Types of Solutions to which Cells may be exposed: Hypertonic: hyper = “more, above”, - hypertonic solutions have a higher solute conc. than the cell’s cytoplasm = less water

hypotonic solution hypertonic solution Isotonic solution 2. Effects of osmosis on cell Isotonic- “same strength” 1.) Effect on animal cells – no change 2.) Effect on plant cells – no change Ex. Contact lens solutions and IV’s for patients hypotonic solution hypertonic solution Isotonic solution

Hypotonic- “below strength” 2. Effects of osmosis on cell Hypotonic- “below strength” 1.) Effect on animal cells – in danger of bursting = “lysis” (1-celled orgs. – have contractile vacuole to prevent this!) 2.) Effect on plant cells – cell wall prevents bursting, “turgor pressure” pushes outward from water vacuole on cell wall, allows plants to stand upright! (Think fresh, crispy carrots!) hypotonic solution hypertonic solution Isotonc solution

Hypertonic- “above strength” 2. Effects of osmosis on cell Hypertonic- “above strength” 1.) Effect on animal cells = shrinking or shriveling = “crenate” 2.) Effect on plant cells = plasma membrane pulls inward away from the cell wall as the water vacuole shrinks = “plasmolysis” Hypertonic

3. Osmotic Pressure- osmosis exerts pressure on hypertonic side of selectively permeable membrane a. Cells filled with salts, sugars, proteins and other molecules- always hypertonic to fresh water b. Osmotic pressure produces movement of water into cell that is surrounded by fresh water. c. Fortunately cells don’t come into contact with fresh water. Usually bathed in isotonic solutions (e.g. blood) or protected by cell wall (e.g. bacteria) or mechanism to pump out excess water (contractile vacuole)

D. Facilitated Diffusion- membrane proteins help (faciliate) the movement of certain molecules across the cell membrane by diffusion (from high conc. To low conc.) that can’t diffuse through lipid bilayer directly.) 1. Many different protein channels for ions, sugars, salts. 2. Does not require addition of energy (passive transport)

E. Active Transport- movement of molecules and ions against a concentration difference (from low to high) Requires input of energy 2. Small molecules “pumped” across membrane (e.g. Na+ and K+ ions)

a. Endocytosis- movement into cell by means of “infolding” 1). Pinocytosis- “cell drinking” 2). Phagocytosis- “cell eating”

b. Exocytosis- removal of large amounts of material from a cell b. Exocytosis- removal of large amounts of material from a cell. Membrane of vacuole surrounding the material fuses with the cell membrane