Transport Across Cell Membranes



Osmosis is the diffusion of water molecules across a partially permeable membrane from an area of high water potential to an area of lower water potential.

Pure water has the highest WP

Animal Cell:

Hypotonic - higher WP than cell, net movement into cell, cell bursts.

Isotonic - same WP, net movement equal, cell stays the same.

Hypertonic - lower WP than cell, net movement out of cell, cell shrinks.

Plant Cell:

Hypotonic - net movement into cell, cell becomes turgid.

Isotonic - net movement is equal, cell stays the same.

Hypertonic - net movement out of cell, cell plasmolysed.

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Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

Molecules can diffuse both ways but the net movement will be to the lower concentration.

Passive process - requires no energy

Rate of diffusion:

Concentration gradient - the higher it is, the faster the rate.

Diffusion distance - the thinner the exchange surface, the faster the rate.

Surface area - the larger te SA, the faster the rate.

Temperature - the higher the temperature, the faster the rate.

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Facilitated Diffusion

Used when larger molecules (e.g amino acids, glucose) can't diffuse directly through the phosphlipid bilayer so they diffuse through carrier or channel proteins instead.

Passive process - doesn't require energy

Moves particles down a concentration gradient, from a higher to lower concentration.

Carrier proteins move large molecules into or out of the cell. A molecule attaches to the protein, the protein changes shape and releases the molecule on the opposite side of the membrane.

Channel proteins form pores in the membrane for charged particles to diffuse through.

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Active Transport

Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient.

This process involves carrier proteins.

1 - A molecule attaches to the carrier protein

2 - The protein changes shape

3 - It is moved across the membrane

4 - Released on the other side.

ATP is used to move the solute against its concentration gradient.

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Some molecules are too large to be taken in by carrier proteins, so they use endocytosis.

It requires energy - ATP

The cell surrounds a substance with a section of its plama membrane. It then pinches off to form a vesicle inside the cell containing the ingested substance.

Used by white blood cells to engulf microorganisms and dead cells.

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Some substances produced by the cell need to be released from them.

Vesices containing these substances pinch off from sacs of the golgi apparatus and move towards the plasma membrane.

The vesicles fuse with the plasma membrane and release their contents outside the cell.

Uses ATP as an energy source.

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