Chapter 7: Cell Membrane and the Movement Across It! 2005-2006

Cell Membrane…more than just a barrier! The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins. 2005-2006

Plasma Membrane Boundary that separates living cell from its environment 8 nm thick Selectively permeable Controls traffic in and out Sandwich like Middle = phospholipids Outer = proteins

Plasma Membrane Phospholipid bilayer Proteins Carbohydrates Integral (transport) Peripheral (surface) Carbohydrates Used in cell to cell recognition Sorts cells in developing embryos Rejection of foreign cells Amphipathic

Davson-Danielli Model 1st Model Phospholipid bilayer between two protein layers Two problems Not all membranes are identical If entire outside were proteins it would be too unstable

Figure 7.2 Two generations of membrane models

Fluid Mosaic Model Not sandwich like as in first model Molecules drift laterally Must be fluid to function Cholesterol maintains membrane fluidity unsaturated phospholipids increase in low temperatures

Selectively Permeable Property of biological membranes which allows some substances to cross more easily than others Nonpolar (hydrophobic) cross easily hydrocarbons, O2, CO2 Polar (hydrophilic) pass w/ less ease Cannot dissolve - hydrophobic region Water and ethanol (small) Glucose Ions

Selectively Permeable Transport proteins Integral membrane proteins that transpose specific molecules or ions across biological membranes Key role in regulating transport Similar to a tunnel May bind

Figure 8.9 Some functions of membrane proteins

Cell (plasma) membrane Cells need an inside & an outside… separate cell from its environment cell membrane is the boundary Can it be an impenetrable boundary? NO! OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O OUT IN 2005-2006 cell needs materials in & products or waste out

Lipids of cell membrane Membrane is made of phospholipids phospholipid bilayer phosphate hydrophilic inside cell outside cell lipid hydrophobic

Phospholipid bilayer What molecules can get through directly? fats & other lipids can slip directly through the phospholipid cell membrane, but… what about other stuff? lipid inside cell outside cell salt NH3 sugar aa H2O

Membrane Proteins Proteins determine most of membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Membrane proteins: peripheral proteins = loosely bound to surface of membrane integral proteins = penetrate into lipid bilayer, often completely spanning the membrane = transmembrane proteins Integral – enzyme activity, intercellular joining, Peripheral – protein hormones 2005-2006

Membrane Protein Types Channel proteins – wide open passage Ion channels – gated Aquaporins – water only, kidney and plant root only Carrier proteins – change shape Transport proteins – require ATP Recognition proteins - glycoproteins Adhesion proteins – anchors Receptor proteins - hormones

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins. 2005-2006

Membrane Carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish neighboring cells from another important in organ & tissue development basis for rejection of foreign cells by immune system The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells. 2005-2006

Cholesterol Provides stability in animal cells Replaced with sterols in plant cells

Getting through cell membrane Passive transport No energy needed Movement down concentration gradient Active transport Movement against concentration gradient low  high requires ATP

Diffusion 2nd Law of Thermodynamics - Universe tends towards disorder Organization tends to disorder. Molecules move to disarray. Diffusion movement from high  low concentration

Simple diffusion across membrane Which way will lipid move? lipid lipid lipid inside cell lipid lipid lipid low high  lipid outside cell lipid lipid lipid lipid lipid lipid lipid

Diffusion of 2 solutes Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances Things tend to get mixed up evenly.

Facilitated diffusion Move from HIGH to LOW concentration through a protein channel passive transport no energy needed facilitated = with help 2005-2006

Gated channels Some channel proteins open only in presence of stimulus (signal) stimulus usually different from transported molecule ex: ion-gated channels when neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell ex: voltage-gated channels change in electrical charge across nerve cell membrane opens Na+ & K+ channels When the neurotransmitters are not present, the channels are closed. 2005-2006

Na+/K+ pump in nerve cell membranes Active transport Cells may need molecules to move against concentration situation need to pump against concentration protein pump requires energy ATP Plants have nitrate & phosphate pumps in their roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer. Na+/K+ pump in nerve cell membranes

Transport summary 2005-2006

How about large molecules? Moving large molecules into & out of cell requires ATP! through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” receptor-mediated endocytosis exocytosis exocytosis 2005-2006

receptor-mediated endocytosis fuse with lysosome for digestion phagocytosis non-specific process pinocytosis triggered by ligand signal receptor-mediated endocytosis 2005-2006

The Special Case of Water Movement of water across the cell membrane 2005-2006

Osmosis is diffusion of water Diffusion of water from high concentration of water to low concentration of water across a semi-permeable membrane 2005-2006

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

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

Hypotonicity animal cell in hypotonic solution will gain water, swell & possibly burst (cytolysis) Paramecium vs. pond water Paramecium is hypertonic H2O continually enters cell to solve problem, specialized organelle, contractile vacuole pumps H2O out of cell = ATP plant cell Turgid (turgor pressure) Cell wall

Hypertonicity animal cell in hypertonic solution will lose water, shrivel & probably die salt water organisms are hypotonic compared to their environment they have to take up water & pump out salt plant cells plasmolysis = wilt

Osmosis… .05 M .03 M Cell (compared to beaker)  hypertonic or hypotonic Beaker (compared to cell)  hypertonic or hypotonic Which way does the water flow?  in or out of cell 2005-2006

Water Potential Water moves from a place of greater water potential to a place of lesser water potential (net). As the concentration of a solute increases in a solution, the water potential will decrease accordingly. Which has the greater water potential: .2M or .8M? 20% or 80% water?