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Cell Membranes & Movement Across Them
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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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Fluid Mosaic Model The membrane contains many types of molecules
Lipids Proteins Carbohydrates A “mosaic” of molecules Molecules are always shifting and moving They are “fluid” Thus… the “fluid mosaic model”!
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Remember: oil & water don’t mix!!
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 Hydrophilic (“water loving”) phosphate inside cell outside cell Fatty acids Hydrophobic “water fearing”
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Proteins in the membrane
Transport proteins Carrier proteins Transport specific substances Channel proteins “Holes” that molecules can pass through Other proteins Marker/recognition proteins Cell “fingerprints” for immune system Receptor proteins Receive signals from outside (example: hormones)
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Selectively permeable membrane
Cell membrane controls what gets in or out Need to allow some materials — but not all — to pass through the membrane Selectively 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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How do you build a semi-permeable cell membrane?
Channels are made of proteins proteins both “like” water & “like” lipids bi-lipid membrane protein channels in bi-lipid membrane
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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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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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Diffusion Move from HIGH to LOW concentration diffusion of water
passive transport no energy needed diffusion of water diffusion osmosis
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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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Simple Diffusion Move from HIGH to LOW LOW HIGH 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
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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More facilitated diffusion
Carrier proteins Molecule binds on outside Protein changes shape Molecule passes through Example: glucose Ion channels Let charged particles through Examples: Sodium (Na+), Potassium (K+), Calcium (Ca+2), Chlorine (Cl-) Important in muscle contraction and brain signaling
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Glucose channel
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Diffusion summary Move from HIGH to LOW concentration
directly through membrane simple diffusion no energy needed help through a protein channel facilitated diffusion (with help) Factors affecting diffusion Size of molecule Type of molecule Type of membrane HIGH LOW
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Osmosis Movement of Water Across 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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Hypotonic (freshwater)
Keeping water balance Cell survival depends on balancing water uptake & water loss Hypotonic (freshwater) Isotonic (balanced) Hypertonic (salt water)
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Keeping right amount of water in cell
Hypotonic a cell in fresh water high concentration of water around cell cell gains water problem: cells gain water, swell & can burst Animal cells: cytolysis Plant cells: turgor pressure example: Paramecium solution: contractile vacuole pumps water out of cell KABOOM! freshwater No problem, here
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Controlling water Contractile vacuole in Paramecium
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Keeping right amount of water in cell
Hypertonic 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 I’m shrinking, I’m shrinking! saltwater I will survive!
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Keeping right amount of water in cell
Isotonic 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 That’s better! balanced I could be better…
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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 Examples: amino acids, sodium- potassium pump in neurons 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.
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Transport summary simple diffusion facilitated diffusion
ATP active transport
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Endocytosis and exocytosis
Some molecules too big for proteins Food Waste Cell “eats” the molecules Endocytosis Cell expels the molecules Exocytosis
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Endocytosis Cell membrane surrounds substance and forms a vesicle
Can ingest: Large particles or other cells Phagocytosis Examples: white blood cells, amoebas Liquids Pinocytosis
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Exocytosis Vesicles join with the cell membrane Can expel:
Release contents outside the cell Can expel: Waste Proteins Other molecules Examples: hormones, neurotransmitters
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