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Cell Membranes, Diffusion, and Osmosis
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Membranes Organize the Chemical Activities of Cells
Since so many reactions occur simultaneously in a cell, the cell is required to be highly organized and able to time its metabolic reactions precisely. Some of the organization comes from teams of enzymes that function like assembly lines. A product of one enzyme-catalyzed reaction becomes the substrate for a neighboring enzyme and so on until a final product is made. The right enzymes have to be present at the right times and in the right places for these reactions to occur For most cells, the plasma membrane is the border that defines it from its environment and this membrane is largely selectively permeable. The membrane allows certain substances into the cell more easily than others and blocks some substances altogether. Plasma membranes have three distinct but thin layers that function in protecting the cell. The plasma membrane also serves as a communication center with the environment around the cell.
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Cell (plasma) membrane
Cells need an inside & an outside… separate cell from its environment cell membrane is the boundary IN food - sugars - proteins - fats salts O2 H2O OUT waste - ammonia - salts - CO2 - H2O products - proteins cell needs materials in & products or waste out
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Building a membrane How do you build a barrier that keeps the watery contents of the cell separate from the watery environment? FATS LIPIDS Remember: oil & water don’t mix!! What substance do you know that doesn’t mix with water?
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Lipids of cell membrane
Membrane is made of special kind of lipid phospholipids “split personality” Membrane is a double layer phospholipid bilayer Phosphate “attracted to water” “attracted to water” phosphate inside cell outside cell lipid Fatty acid “repelled by water” “repelled by water”
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Cell membrane Function Structure separates cell from outside
phosphate “head” Function separates cell from outside controls what enters or leaves cell O2, CO2, food, H2O, nutrients, waste recognizes signals from other cells allows communication between cells Structure double layer of fat phospholipid bilayer receptor molecules proteins lipid “tail”
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Permeability to polar molecules?
Membrane becomes semi-permeable via protein channels specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell NH3
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Why are proteins the perfect molecule to build structures in the cell membrane?
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Proteins domains anchor molecule
Polar areas of protein Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane in fluid polar amino acids hydrophilic extend into extracellular fluid & into cytosol Nonpolar areas of protein
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Many Functions of Membrane Proteins
“Channel” Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor “Antigen” 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
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Membrane Proteins Proteins determine membrane’s specific functions
cell membrane & organelle membranes each have unique collections of proteins Classes of membrane proteins: peripheral proteins loosely bound to surface of membrane ex: cell surface identity marker (antigens) integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein ex: transport proteins channels, permeases (pumps)
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Semi-permeable membrane
Cell membrane controls what gets in or out Need to allow some materials — but not all — to pass through the membrane semi-permeable only some material can get in or out So what needs to get across the membrane? sugar lipids aa O2 H2O salt waste
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Crossing the cell membrane
What molecules can get through the cell membrane directly? fats and oils can pass directly through lipid inside cell outside cell salt waste but… what about other stuff? sugar aa H2O
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Cell membrane channels
Need to make “doors” through membrane protein channels allow substances in & out specific channels allow specific material in & out H2O channel, salt channel, sugar channel, etc. inside cell H2O aa sugar salt outside cell waste
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Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer 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 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
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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.
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Stem Cells So what are stem cells? Watch the following video…
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Movement across the Cell Membrane
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Protein channels Proteins act as doors in the membrane
channels to move specific molecules through cell membrane HIGH 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, aquaprorins, facilitate massive amounts of diffusion. LOW
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Movement through the channel
Why do molecules move through membrane if you give them a channel? HIGH ? LOW ?
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Molecules move from high to low
Diffusion move from HIGH to LOW concentration Movement from high concentration of that substance to low concentration of that substance.
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Brownian Motion Refers to molecules moving randomly about in a cell and collide. Molecules move about in all directions with equal frequency, bouncing off each other when they collide. When molecules collide, this causes molecules that are concentrated in one area to spread out into a less concentrated area.
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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
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Diffusion diffusion of water diffusion osmosis
Move from HIGH to LOW concentration passive transport no energy needed diffusion of water diffusion osmosis
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Passive Transport Example is putting dye in a solution and watching it slowly diffuse down its concentration gradient until an equilibrium is reached When equilibrium is reached, the molecules continue to move back and forth but there is no net change in concentration on either side Example in lungs: passive transport along concentration gradients is the sole means by which oxygen enters red blood cells and CO2 passes out of them
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Simple Diffusion LOW HIGH Move from HIGH to LOW
fat fat fat Which way will fat move? inside cell fat fat fat LOW HIGH fat outside cell fat fat fat fat fat fat fat
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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”
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Facilitated Diffusion
Move from HIGH to LOW through a channel sugar sugar sugar sugar inside cell sugar sugar LOW Which way will sugar move? HIGH outside cell sugar sugar sugar sugar sugar sugar sugar
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Simple vs. facilitated diffusion
simple diffusion facilitated diffusion lipid inside cell outside cell inside cell outside cell H2O protein channel H2O
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Active transport Cells may need molecules to move against concentration “hill” need to pump “uphill” from LOW to HIGH using energy 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. ATP
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Transport summary simple diffusion facilitated diffusion
ATP active transport
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How about large molecules?
Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” exocytosis exocytosis
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Endocytosis fuse with lysosome for digestion phagocytosis
non-specific process pinocytosis triggered by molecular signal receptor-mediated endocytosis
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The Special Case of Water Movement of water across the cell membrane
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Osmosis Water is very important, so we talk about water separately
diffusion of water from HIGH concentration of water to LOW concentration of water across a semi-permeable membrane
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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
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Osmosis Osmosis regulates cells when a membrane is permeable to water but not to other solutes. This creates two solutions of different concentrations. Water will cross the membrane until the solute concentrations are equal on both sides The direction of osmosis is determined only by the difference in total solute concentration, not by the nature of the solutes
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Keeping water balance Cell survival depends on balancing water uptake & water loss freshwater balanced saltwater
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Osmosis in Action in Your Blood Cells
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Keeping right amount of water in cell
1 Keeping right amount of water in cell Freshwater a cell in fresh water high concentration of water around cell cell gains water example: Paramecium problem: cells gain water, swell & can burst water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell plant cells turgid = full cell wall protects from bursting freshwater KABOOM! No problem, here
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Pumping water out Contractile vacuole in Paramecium ATP
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Keeping right amount of water in cell
2 Keeping right amount of water in cell saltwater I’m shrinking, I’m shrinking! Saltwater a cell in salt water low concentration of water around cell cell loses water example: shellfish problem: cell loses water in plants: plasmolysis in animals: shrinking cell solution: take up water plant cells plasmolysis = wilt can recover I will survive!
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Keeping right amount of water in cell
3 Keeping right amount of water in cell balanced That’s better! Balanced conditions no difference in concentration of water between cell & environment cell in equilibrium example: blood problem: none water flows across membrane equally, in both directions volume of cell doesn’t change example: blood cells in blood plasma slightly salty IV solution in hospital I could be better…
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Do you understand Osmosis…
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
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