Transport through cell membranes

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What is transport?

Transport can be subcategorised as passive or active

There are 5 types of transport:

Diffusion

Osmosis

Active transport

Exocytosis

Endocytosis

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Passive transport

Passive transport is movement of molecules through the membrane in which no energy is required, molecules move in response to a concentration gradient.

Diffusion  is movement of molecules from high concentration to low concentration.  For example: movement of oxygen across the wall of an alveolus.

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Diffusion

Selective permeability: integral membrane proteins allow the cell to be selective about what passes through the membrane

Channel proteins have a polar (hydrophillic) interior allowing polar molecules to pass through

Facilitated diffusion is movement of a molecule from high to low concentration with the help of a carrier protien. It is specific, passive, and saturates when all carriers are occupied. The carrier protein changes shape, facilitating diffusion. For example: release of glucose from liver cells into the bloodstream

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Channel proteins

Ion channels allow the movement of ions (charged or molecules) which are associated with water.

Gated channels are opened or closed in response to a stimulous

The stimulous may be chemical or electrical  

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Osmosis

In an aqeuas solution: water is the solvent

                                    dissolved substances are the solvents

Osmosis is the movement of water from an area of high concentration to low concentration of water.

Movement of water toward an area of high solute solution for emample reabsorption of water molecules from the kidney tubule.

Involves a semi-permeable membrane.

When 2 solutions have different osmotic concentrations:

the hypertonic solution has a higher solute concentration

the hypotonic solution has a lower solute concentration 

Water will move towards hypertonic solutions through aquaporins

Isotonic solution has a normal level

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Active transport

Requires energy- ATP is used directly or indirectly to fuel active transport. It moves substances from low to high concentration and it requires the use of carrier proteins.For example: pumping of calcium ions into storage vesicles inside muscle cells.

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Carrier Protiens

Carrier Protiens used in active transport include:

Channels

Uniporter- move one molecule at a time

Symporter- move two molecules in the same direction

Antiporter- move two molecules in opposite directions

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Bulk Transport

Bulk transport of substances is accomplished by:

  • Endocytosis- movement of substances into the cell
  • Exocytosis- movement of materials out of the cell
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Endocytosis

phygocytosis- the cell takes in particular cell

pinocytosis- the cell takes in only fluid

receptor- mediated endocytosis: specific molecules are taken in after they bind to a receptor

Endocytosis is active

Example: Cholesterol is taken up into cells

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Exocytosis

Exocytosis occurs when material is discharged from the cell. Vesicles in the cytoplasm fuse with the cell membrane and release their contents to the exterior of the cell

  • Used plants to export cell wall material
  • Used in animals to secrete hormones, neurotransmitters, digestive enzymes

For example: Insulin is released into the bloodstream

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