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Phospholipid bilayer. Phospholipid molecules form a bilayer with the hydrophilic heads exposed to the aqueous environments on both side of the membrane, and hydrophobic tails on the inside, away from water.

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The contractile vacuole of the freshwater protist Paramecium is an evolutionary adaptation for osmoregulation that offsets osmosis in a hypotonic environment by bailing water out of the cell. _Vid_Campbell7e/ParameciumVacuole-V.swf

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Cotransport: active transport driven by a concentration gradient.

A special carrier protein such as this sucrose–H⁺ cotransporter is able to use the diffusion of H⁺ down its electrochemical gradient into the cell to drive the uptake of sucrose.

The H⁺ gradient is maintained by an ATP–driven proton pump that concentrates H⁺ outside the cell.

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The diffusion of solutes across a membrane. The dye diffuses down a concentration gradient from where it is more concentrated to where it is less concentrated, leading to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.

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Receptor-mediated endocytosis Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are clustered in coated pits lined by a fuzzy layer of coat proteins. When extracellular substances (ligands) bind to these receptors, the coated pit forms a vesicle containing the ligand molecules.

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Exocytosis. Many secretory cells use exocytosis to export their products such as hormones.

facilitated_diffusion-carrier.html: 07_15FacilitatedDiffusionB.jpg
Facilitated diffusion: carrier proteins.
A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

facilitated_diffusion-channel.html: 07_15FacilitatedDiffusionA.jpg
Facilitated diffusion: channel proteins.
A channel protein has a channel through which water molecules or a specific solute can pass.

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The fluid mosaic model for membranes. Proteins also have hydrophilic and hydrophobic regions and are embedded in the bilayer to provided various functions.

laysan_albatross.html: 07_laysan_albatross.jpg

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The fluid mosaic model. The plasma membrane is a fluid structure with a “mosaic” of proteins embedded in or attached to a bilayer of phospholipids In animal cells, glycoproteins such as collagen comprise the Extracellular Matrix ( ECM ).

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Osmosis. Two sugar solutions of different concentrations are separated by a semipermeable membrane, which the solvent (water) can pass through but the solute (sugar) cannot. Water molecules move randomly and may cross through the pores in either direction, but overall, water diffuses from the solution with less concentrated solute to that with more concentrated solute.

paramecium.html: 07_14ContractileVacuole.jpg

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Phagocytosis A cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

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Pinocytosis The cell “gulps” droplets of extracellular fluid, together with molecules dissolved in the droplet, into tiny vesicles. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.

plant_water_balance.html: 07_13WaterBalanceP.jpg
Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. Plants become flaccid in a isopotonic environment and plasmolyzed in a hypertonic environment.

sodium-potassium_pump.html: 07_16SodiumPotassiumPump.jpg
The sodium-potassium pump moves 3 sodium ions out of the cell for every 2 potassium ions pumped in. The active transport moves the ions against their concentration gradient and is powered by ATP.

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A transmembrane protein has helices within the hydrophobic (lipid soluble) core of the membrane. The hydrophilic segments of the protein are in contact with the aqueous solutions on the extracellular and cytoplasmic sides of the membrane.

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For a plant in a hypotonic environment, the inflow of water results in turgor pressure against the cell wall, and the cells become turgid, contributing to rigidity and support. In isotonic environments plants become flaccid ( limp ), and in hypertonic environments the cell membrane pulls away from the wall in plasmolysis.

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Polar water molecules pass through the plasma membrane via channel proteins called aquaporins.

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Animal cell.
An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

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Plant cell.
Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

water_balance.html: 07_13WaterBalanceA.jpg
An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. Plant cells fare best in a hypotonic environment.