Transmembrane proteins are transported from the extracellular fluid into the cytoplasm within the membrane, and are sometimes bound to glycoproteins (proteins attached). This always happens inside the system along the concentration gradient. Through osmosis, water molecules travel across a semi-permeable membrane, from high-concentration regions to low-concentration regions, until a balance is achieved. However passive diffusion, along its concentration gradient, describes the passage of substances (small molecules and ions) across the membrane. Channel-mediated facilitated diffusionįacilitated diffusion is used to carry large molecules that are unable to cross at will the plasma membrane.
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