Cell Membranes & Movement Across Them 2006-2007

FLUID MOSAIC MODEL

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

Lipids of cell membrane Membrane is made of special kind of lipid phospholipids “split personality” Membrane is a double layer phospholipid bilayer “attracted to water” phosphate inside cell outside cell lipid “repelled by water”

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 or 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

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

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

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

Movement through the channel Passive Transport = High Concentration to Low Concentration & No Energy Required - Diffusion - Osmosis (Diffusion of Water) - Facilitated Diffusion Concentration gradient - a difference between concentrations in a space Solvent (ex. Water) / Solute (example, salt, dissolved in solvent)

Solutions

Molecules move from high to low Diffusion move from HIGH to LOW concentration Move down the concentration gradient Movement from high concentration of that substance to low concentration of that substance.

Diffusion Move from HIGH to LOW concentration diffusion of water passive transport no energy needed diffusion of water diffusion osmosis

Moves from High  Low, until……….Equilibrium is reached Diffusion Moves from High  Low, until……….Equilibrium is reached

Simple Diffusion Move from HIGH to LOW LOW HIGH fat inside cell fat Which way will fat move? inside cell fat fat fat LOW HIGH fat outside cell fat fat fat fat fat fat fat

Facilitated Diffusion Move from HIGH to LOW through a channel sugar sugar inside cell sugar sugar LOW Which way will sugar move? HIGH outside cell sugar sugar sugar sugar sugar sugar sugar

Diffusion Move from HIGH to LOW concentration directly through membrane simple diffusion no energy needed help through a protein channel facilitated diffusion (with help) Move Down the Concentration Gradient HIGH LOW

Simple vs. facilitated diffusion simple diffusion facilitated diffusion lipid inside cell outside cell inside cell outside cell H2O protein channel H2O

Active transport Cells may need molecules to move against or Up a 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

Transport summary simple diffusion facilitated diffusion ATP active transport

Osmosis Movement of Water Across Cell Membrane 2006-2007

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

Keeping water balance Cell survival depends on balancing water uptake & water loss freshwater HypOtonic Balanced Isotonic Saltwater Hypertonic

Equilibrium 85% water 15% solute 75% water 25% solute 50% water

Plasmolysis When plant cells are placed in extremely hypertonic environment, water moves out of the cells by osmosis. This causes the plant vacuoles to shrink and the cell membrane detaches from the cell wall. This is plasmolysis. (Bendy Carrot)

Active Transport – Endocytosis (Pinocytosis; phagocytosis)

Active Transport - Exocytosis

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