πŸš‡ Cell transport mechanisms


The plasma membrane

Passive transport

Diffusion

Facilitated diffusion

Osmosis

Endocytosis and exocytosis

Active transport

ATP and ADP


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The plasma membrane

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Armed with this knowledge of lipids, as well as carbohydrates and proteins, we can now explore the structure of plasma membranes, specifically in the context of the fluid-mosaic model. Phospholipids have a hydrophilic (water loving) head, and hydrophobic (water repelling) tails. This results in the formation of a phospholipid bilayer (double layer), which forms the basis for the plasma membrane.

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The name of fluid-mosaic model comes from:

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  • Fluid = the arrangement of proteins contained in the membrane is always changing
  • Mosaic = the proteins present are spread around in a mosaic-like fashion.

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It’s pretty isn’t it? The proteins are crucial to cell communication as well as the selective permeability of the membrane. The glycoproteins’ (sugars/carbohydrates attached to a protein) side chains act as receptors. Lipid soluble stuff such as vitamins A, D and K, as well as oxygen and carbon dioxide, can pass freely though the membrane. Cholesterol can be part of the membrane to restrict the movement of other components.

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The main properties of molecules that determine how they may be transported across a membrane are solubility, size and charge.

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Large molecules can’t cross the membrane, charged molecules also can’t, and naturally, lipid-repelling (or water-attracting) molecules can’t. Conversely, small molecules can cross the membrane barrier, alongside molecules with no charge (nonionised) as well as lipophilic (hydrophobic) molecules.

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It’s important to understand the role of microvilli. These are elongations of plasma membrane which increase the surface area available for reaction or absorption…