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Cell Membranes and Movement Across Membranes (Transport)
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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.
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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 cell needs materials in & products or waste out
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Lipids of cell membrane
Membrane is made of phospholipids phospholipid bilayer phosphate hydrophilic inside cell outside cell lipid hydrophobic
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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
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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
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Membrane Protein Types
Channel proteins – wide open passage Ion channels – gated, only active under specific conditions Aquaporins – move water only, found primarily in animal kidneys and plant roots Carrier proteins – conformational change (change in shape occurs) Transport proteins – require ATP
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Membrane Protein Types
Recognition proteins – glycoproteins, carbohydrates are attached (too hydrophilic to be directly on membrane) Adhesion proteins – anchors to surrounding cells/tissues Receptor proteins – hormones; receptors that start signal transduction pathways
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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.
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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.
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Cholesterol Provides stability in animal cells
Replaced with sterols in plant cells
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Getting through cell membrane
Passive transport No energy needed Movement down concentration gradient high low Active transport Movement against concentration gradient low high requires ATP Think: does it take more energy to move water down a waterfall (from high to low) or up a waterfall (from low to high)?
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Diffusion Diffusion movement from high low concentration Organization tends to disorder. Molecules move to disarray. 2nd Law of Thermodynamics - Universe tends towards disorder
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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
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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.
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Facilitated diffusion
Movement from HIGH to LOW concentration through a protein channel passive transport; no energy needed facilitated = with help
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Gated channels Some channel proteins open only in the 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.
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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
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Transport summary
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How about large molecules?
Moving large molecules into & out of cell requires ATP! through vesicles & vacuoles Endocytosis (taking molecules in) phagocytosis = “cellular eating” pinocytosis = “cellular drinking” receptor-mediated endocytosis Exocytosis Molecules exiting Exocytosis
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receptor-mediated endocytosis
fuse with lysosome for digestion phagocytosis non-specific process pinocytosis triggered by ligand 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 is diffusion of water
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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Managing water balance
Cell survival depends on balancing water uptake & loss freshwater balanced saltwater
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Hypotonicity animal cell in a hypotonic solution will gain water, swell & possibly burst (cytolysis) Paramecium vs. pond water Paramecium is hypertonic H2O continually enters cell specialized organelle, contractile vacuole, pumps excess H2O out of cell; requires ATP plant cell in a hypotonic solution is turgid Turgor pressure Cell wall exerts force
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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 cell in a hypertonic solution experiences plasmolysis Vacuole shrinks; cell membrane pulls away from cell wall Wilting
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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
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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?
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