Transport of Substances Across a 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 Move from HIGH to LOW concentration movement of water “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

Small nonpolar molecules such as fats, O2 and CO2 Diffuse easily across the phospholipid bilayer of a membrane (nonpolar molecules)

Osmosis is 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 across a semi-permeable membrane

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

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 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

Transport proteins may facilitate diffusion across membranes Many kinds of molecules do not diffuse freely across membranes (size, charge, polarity) For these molecules, transport proteins Provide passage across membranes through a process called facilitated diffusion Solute molecule Transport protein Figure 5.15

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 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”

Facilitated Diffusion

Ion Channels allow specific ions to pass through the protein channel. regulated by the cell and are either open or closed to control the passage of substances into the cell

Carrier Proteins bind to specific molecules, change shape and then deposit the molecules across the membrane. Once the transaction is complete the proteins return to their original position.

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 antiport symport ATP ATP 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

Sodium Potassium Pump

Cells expend energy for active transport Transport proteins can move solutes against a concentration gradient Through active transport, which requires ATP P Protein changes shape Phosphate detaches ATP ADP Solute Transport protein Solute binding 1 Phosphorylation 2 Transport 3 Protein reversion 4 Figure 5.18

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

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

Exocytosis and endocytosis transport large molecules To move large molecules or particles through a membrane A vesicle may fuse with the membrane and expel its contents (exocytosis) Fluid outside cell Cytoplasm Protein Vesicle Insulin, Crying Figure 5.19A

Membranes may fold inward Enclosing material from the outside (endocytosis) Vesicle forming Figure 5.19B

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