The Cell Membrane AP Biology 2007 2008 Phospholipids

The Cell Membrane AP Biology 2007 -2008

Phospholipids § Phosphate head u “attracted to water” hydrophilic § Fatty acid tails u Phosphate hydrophobic § Arranged as a bilayer Fatty acid “repelled by water” Aaaah, one of those structure–function examples AP Biology

Arranged as a Phospholipid bilayer § Serves as a cellular barrier / border sugar polar hydrophilic heads nonpolar hydrophobic tails H 2 O salt impermeable to polar molecules polar hydrophilic heads waste AP Biology lipids

Cell membrane defines cell § Cell membrane separates living cell from aqueous environment u thin barrier = 8 nm thick § Controls traffic in & out of the cell u allows some substances to cross more easily than others § hydrophobic (nonpolar) vs. hydrophilic (polar) AP Biology

Permeability to polar molecules? § Membrane becomes semi-permeable via protein channels u specific channels allow specific material across cell membrane inside cell NH AP Biology 3 salt H 2 O aa sugar outside cell

Cell membrane is more than lipids… § Transmembrane proteins embedded in phospholipid bilayer u create semi-permeable channels “sandwich model” AP Biology protein channels in lipid bilayer membrane

Cell membrane must be more than lipids… § In 1972, S. J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model! AP Biology

Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic AP Biology

Classes of amino acids What do these amino acids have in common? I like the polar ones the best! AP Biology polar & hydrophilic

Proteins domains anchor molecule § Within membrane u Polar areas of protein nonpolar amino acids § hydrophobic § anchors protein into membrane § On outer surfaces of membrane in fluid u polar amino acids § hydrophilic § extend into extracellular fluid & into cytosol AP Biology Nonpolar areas of protein

H+ H+ Examples Retinal chromophore NH 2 aquaporin = water channel in bacteria Porin monomer H 2 O b-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH H+ H + Cytoplasm proton pump channel in photosynthetic bacteria H O AP Biology 2 function through conformational change = protein changes shape

Many Functions of Membrane Proteins “Channel” Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell adhesion Attachment to the cytoskeleton “Antigen” AP Biology Cell surface identity marker

Membrane Proteins § Proteins determine membrane’s specific functions u cell membrane & organelle membranes each have unique collections of proteins § Classes of membrane proteins: u peripheral proteins § loosely bound to surface of membrane § ex: cell surface identity marker (antigens) u integral proteins § penetrate lipid bilayer, usually across whole membrane § transmembrane protein § ex: transport proteins w channels, permeases (pumps) AP Biology

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein AP Biology Cytoplasm Transmembrane proteins Filaments of cytoskeleton 1972, S. J. Singer & G. Nicolson proposed Fluid Mosaic Model

Membrane carbohydrates § Play a key role in cell-cell recognition u ability of a cell to distinguish one cell from another § antigens important in organ & tissue development u basis for rejection of foreign cells by immune system u AP Biology

Any Questions? ? AP Biology

Movement across the Cell Membrane AP Biology 2007 -2008

Diffusion § 2 nd Law of Thermodynamics governs biological systems u universe tends towards disorder (entropy) § Diffusion u AP Biology movement from HIGH LOW concentration

Simple Diffusion § Move from HIGH to LOW concentration “passive transport” u no energy needed u AP Biology diffusion movement of water osmosis

Facilitated Diffusion § Diffusion through protein channels u u channels move specific molecules across cell membrane facilitated = with help no energy needed open channel = fast transport HIGH LOW AP Biology “The Bouncer”

Active Transport § Cells may need to move molecules against concentration gradient u u u conformational shape change transports solute from one side of membrane to other protein “pump” “costs” energy = ATP LOW conformational change ATP HIGH AP Biology “The Doorman”

Getting through cell membrane § Passive Transport u Simple diffusion § generally nonpolar, hydrophobic molecules w across the phospholipid bilayer w HIGH LOW concentration gradient u Facilitated transport § generally polar, hydrophilic molecules § through a protein channel w HIGH LOW concentration gradient § Active transport u diffusion against concentration gradient § LOW HIGH u u AP Biology uses a protein pump requires ATP

Transport summary simple diffusion facilitated diffusion active transport AP Biology ATP

How about large molecules? § Moving large molecules into & out of cell through vesicles & vacuoles u endocytosis u § phagocytosis = “cellular eating” § pinocytosis = “cellular drinking” u AP Biology exocytosis

Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by molecular signal AP Biology

The Special Case of Water Movement of water across the cell membrane AP Biology 2007 -2008

Osmosis is just diffusion of water § Water is very important to life, § so we talk about water separately Diffusion of water from HIGH concentration of water to LOW concentration of water u AP Biology across a semi-permeable membrane

Concentration of water § Direction of osmosis is determined by comparing total solute concentrations u Hypertonic - more solute, less water u Hypotonic - less solute, more water u Isotonic - equal solute, equal water AP Biology hypotonic hypertonic net movement of water

Managing water balance § Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater

1 Managing water balance § Hypotonic u a cell in fresh water u high concentration of water around cell § problem: cell gains water, swells & can burst § example: Paramecium w ex: water continually enters KABOOM! Paramecium cell § solution: contractile vacuole w pumps water out of cell ATP w ATP u plant cells No problem, here § turgid = full § cell wall protects from bursting AP Biology freshwater

Pumping water out § Contractile vacuole in Paramecium ATP AP Biology

2 Managing water balance § Hypertonic I’m shrinking, a cell in salt water I’m shrinking! u low concentration of water around cell u § problem: cell loses water & can die § example: shellfish § solution: take up water or pump out salt I will u AP Biology plant cells § plasmolysis = wilt § can recover survive! saltwater

3 Managing water balance § Isotonic u u animal cell immersed in mild salt solution That’s perfect! no difference in concentration of water between cell & environment § problem: none w no net movement of water flows across membrane equally, in both directions I could w cell in equilibrium be better… n w volume of cell is stable § example: blood cells in blood plasma w slightly salty IV solution in hospital AP Biology balanced

Aquaporins § Water moves rapidly into & out of cells through water channels § protein channels allowing flow of water across cell membrane AP Biology

Do you understand Osmosis… . 05 M . 03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic AP Biology Which way does the water flow? in or out of cell

Any Questions? ? AP Biology

Ghosts of Lectures Past (storage) AP Biology 2007 -2008

Diffusion through phospholipid bilayer § What molecules can get through directly? u fats & other lipids inside cell NH 3 § What molecules can lipid salt NOT get through directly? u polar molecules § H 2 O u outside cell sugar aa H 2 O ions (charged) § salts, ammonia u large molecules § starches, proteins AP Biology

Membrane fat composition varies § Fat composition affects flexibility u membrane must be fluid & flexible § about as fluid as thick salad oil u % unsaturated fatty acids in phospholipids § keep membrane less viscous § cold-adapted organisms, like winter wheat w increase % in autumn u AP Biology cholesterol in membrane

Diffusion across cell membrane § Cell membrane is the boundary between inside & outside… u separates cell from its environment Can it be an impenetrable boundary? NO! OUT IN food carbohydrates sugars, proteins amino acids lipids salts, O 2, H 2 O AP Biology OUT IN waste ammonia salts CO 2 H 2 O products cell needs materials in & products or waste out