A Closer Look At Cell Membranes Ch.5 OCC BIO-161

Intro Video The Cell Membrane Active and Passive Transport

5.1 The Structure of Membranes Cell membrane separates living cell from nonliving surroundings thin barrier = 8nm thick Controls traffic in & out of the cell selectively permeable allows some substances to cross more easily than others hydrophobic vs hydrophilic Made of phospholipids, proteins & other macromolecules (Fig 5.1)

Phospholipids (Fig 5.1) Fatty acid tails Phosphate group head hydrophobic Phosphate group head hydrophilic Arranged as a bilayer Fatty acid

Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads

More than lipids… In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer = Fluid Mosaic Model

Filaments of cytoskeleton Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Where is the Cytoplasm? Extracellular Fluid? Filaments? Phospholipids? Cholesterol? Transmembrane Proteins? Peripheral Proteins? Glycoprotein? Glycolipid? Glycoprotein Extracellular fluid Glycolipid Transmembrane proteins The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins. Phospholipids Filaments of cytoskeleton Cholesterol Peripheral protein Cytoplasm

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

Membrane Proteins (Table 5.1) Proteins determine membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Membrane proteins: peripheral proteins loosely bound to surface of membrane cell surface identity marker (antigens) integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein transport proteins channels, permeases (pumps)

Why are proteins the perfect molecule to build structures in the cell membrane?

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

Classes of amino acids What do these amino acids have in common? polar & hydrophilic

Proteins domains anchor molecule (Fig 5.5) Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane polar amino acids hydrophilic extend into extracellular fluid & into cytosol Polar areas of protein Nonpolar areas of protein

Examples water channel in bacteria NH2 H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic) a-helices in the cell membrane Examples water channel in bacteria Porin monomer b-pleated sheets Bacterial outer membrane proton pump channel in photosynthetic bacteria function through conformational change = shape change

Many Functions of Membrane Proteins (Fig 5.5) Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made. Cell surface identity marker Cell adhesion Attachment to the cytoskeleton

Membrane carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish one cell from another antigens 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.

5.4-5.5 Movement across the Cell Membrane

Diffusion 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Movement from high concentration of that substance to low concentration of that substance. Diffusion movement from high  low concentration

Diffusion (Fig 5.11) Move from HIGH to LOW concentration “passive transport” no energy needed movement of water diffusion osmosis

Diffusion across cell membrane Cell membrane is the boundary between inside & outside… separates cell from its environment 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 cell needs materials in & products or waste out

Diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids What molecules can NOT get through directly? polar molecules H2O ions salts, ammonia large molecules starches, proteins lipid inside cell outside cell salt NH3 sugar aa H2O

Channels through cell membrane Membrane becomes semi-permeable with protein channels specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell NH3

Facilitated Diffusion (Fig 5.10) Diffusion through protein channels channels move specific molecules across cell membrane no energy needed facilitated = with help open channel = fast transport high low Donuts! Each transport protein is specific as to the substances that it will translocate (move). For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer. Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane. These channel proteins allow fast transport. For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion. “The Bouncer”

conformational change Active Transport Cells may need to move molecules against concentration gradient shape change transports solute from one side of membrane to other protein “pump” “costs” energy = ATP conformational change low high Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport. ATP “The Doorman”

Active transport Many models & mechanisms ATP ATP antiport symport Plants: nitrate & phosphate pumps in roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer Supplying these nutrients to plants Replenishing the soil since plants are depleting it antiport symport

Getting through cell membrane Passive Transport Simple diffusion diffusion of nonpolar, hydrophobic molecules lipids high  low concentration gradient Facilitated transport diffusion of polar, hydrophilic molecules through a protein channel Active transport diffusion against concentration gradient low  high uses a protein pump requires ATP ATP

Transport summary simple diffusion facilitated diffusion ATP active transport

5.6 How about large molecules? (Fig 5.13) Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” exocytosis exocytosis

Endocytosis (Fig 5.15 & 5.16) fuse with lysosome for digestion phagocytosis non-specific process pinocytosis triggered by molecular signal receptor-mediated endocytosis

Which Way Will Water Move? Movement of water across the cell membrane

Osmosis is diffusion of water (Fig 5.9) 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 across a semi-permeable membrane

Concentration of water (Fig 5.12) 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

Managing water balance Isotonic animal cell immersed in mild salt solution example: blood cells in blood plasma problem: none no net movement of water flows across membrane equally, in both directions volume of cell is stable balanced

Managing water balance Hypotonic a cell in fresh water example: Paramecium problem: gains water, swells & can burst water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP plant cells turgid ATP freshwater

Water regulation Contractile vacuole in Paramecium ATP

Managing water balance Hypertonic a cell in salt water example: shellfish problem: lose water & die solution: take up water or pump out salt plant cells plasmolysis = wilt Red Blood Cells in Hypertonic Solution Red Blood Cells in Hypotonic Solution saltwater

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