Diffusion & Osmosis

Diffusion, faciliated diffusion

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Diffusion is a passive transport. Passive means the energy comes from the natural, inbuilt motion or particles rather than from some external source.

All particles are constantly in motion due to the kinetic energy they possess

This motion is random with no set patterns to the way the particles move around

Particles are constantly bouncing off one another as well as off other objects e.g. sides of vessel in which that are contained.

Diffusion is defined as the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower.

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Rate of diffusion

Number of factors that affect the rate at which molecules/ions diffuse:

Concentration gradient - the greater the difference in the concentration gradient of molecules/ions on either side of an exchange surface, the faster rate of diffusion.

Area over which diffusion takes place - the larger the area of an exchange surface, the faster rate of diffusion.

Thickness of exchange surface - The thinner the exchange surface, the faster the rate of diffusion.

Diffusion is proportional to:

Surface area x difference in concentration

Length of diffusion path

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Diffusion is also affected by:

The nature of the plasma membrane - Its composition and number of pores

The size and nature of diffusing molecule -  Small molecules diffuse faster that larger ones.

Fat soluble molecules (glycerol) difuse faster than water soluble ones

Polar molecules diffuse faster than non polar molecules

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Faciliated Diffusion

Passive process. Relies only on inbuilt motion (kinetic energy) of diffusing molecules. Theres no external output of energy. It occurs down a concentration gradient but differs in that it occurs at specific points on the plasma membrane where there are special protein molecules.

Proteins form water filled channels (protein channels) across the membrane. Allow water soluble ions and molecules such as glucose and amino acids to pass through.

Channels are selective, each opening only in presence of a specific molecule. If particular molecule isnt present, then the channel remains closed. This way theres some control over entry and exit of substances.

Alternative form of faciliated diffusion involves carrier proteins that span the plasma membrane. When particular molecule is specific to the protein present, it binds with the protein. Causes is to change shape in such a way that the molecule is released to inside of the membrane. No external energy is needed for this.

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Passage of water from a region where it has a higer water potential to a region where it has a lowe water potential through a partially permeable membrane.

Water potential is the pressure created by water molecules. Under standard conditions (25degrees and 100kPa) pure water is said to have a water potential of zero.

Addition of a solute to pure water will lower its water potential. Water potential of a solution (water+solute) must always be less than zero i.e. negative value. More solute that is added (more concentrated the solution) the lower (negative) its water potential. Water will move by osmosis from a region of higer (less negative) water potential to one of more lover (more negative) water potential.


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Osmosis and animal cells

If a red blood cell is placed in pure water, it will absorb water by osmosis because it has a lower water potential. Cell surface membrane will therefore break bursting the cell and releasing its contents. If a red blood cell is placed in a solution with water potential lower than its own, water leaves by osmosis and the cell shrinks and becomes shrivelled.

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Active transport

Allows cells to exchange molecules against a concentration gradient. Metabolic energy is required for the process.

The movement of molecules or ions into/out of a cell from a region of lower concentration to a region of higher concentration using energy and carrier molecules.

Differes from passive forms of transport:

Metabolic energy in form of ATP is needed

Materials are moved against a concentration gradient, from lower to higher

Carrier protein molecules which act as pumps are involved

Process is very selective with specific substances being transported.

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Active transport uses ATP in 1 of 2 ways: Using ATP directy to move molecules, using a concentration gradient that has already been set up by direct active transport. Also known as co-transport.

Direct active transport of a single molecule or ion:

Carrier proteins span the cell surface membrane and accept the molecules or ions to be transported on one side of it.

Molecules or ions bind to receptors on the channels of the carrier protein

Inside of the cell, ATP binds to the protein causing it to split into ADP and phosphate molecule. As a result, protein molecule changes shape and opens to opposite side of the membrane.

Molecules or ions are then released from the protein and recombines with the ADP to form ATP.

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This causes the protein to revert to its original shape ready for the process to be repeated.

Both use carried proteins but faciliated diffusion occurs DOWN a concentration gradient while active transport occurs AGAINST a concentration gradient. Metabolic energy needed in active transport is provided in the form on ATP.

Sometimes more than 1 molecule or ion may be moved in the same direction at the same time by active transport.

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