Transport

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The cell membrane

The cell membrane is made from;

Phospholipids- hydrophillic heads point to outside of the cell and hydrophobic tails point into the center. Lipo-soluable can enter and leave thecells, prevent water soluable sources entering and leaving. Make the membrane flexible and self-sealing

Glycoproteins- recognition sites, maintain stability of membrane, help cells attach together to form tissues

Glycolipids- recognition sites, help cells recognise each other, helps cells attach together to form tissues

Proteins- protein carriers and channels, structural support, help adhere cells together, form receptors, transportation

The fluid-mosiac model shows the arrangement of materials in the cell surface membrane, the fluid is the flexible phospholipid and the mosaic is the proteins emmbedded.

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Simple Diffusion

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distrubuted

This process involves no energy as it is passive

The particles move from an area of high concentration to an area of low concentration

Through the partially permeable membrane

Small, non polar molecules such as oxygen and carbon dioxide are the only molecules which can move using simple diffusion

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Facilitated Diffusion

Some molecules can not diffuse that easily as they may be larger and polar so channel proteins and carrier proteins are needed.

This is still a passive process so without the use of ATP

Protein channels- form water-filled hydrophilic channels so water- soluable ions can pass through. These channels are selective so they open and close to allow molecules through

Carrier Proteins - the molecules are specific to each carrier protein so the molecule binds with the protein. This causes it to change shape so the molecule is released inside the cell

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Osmosis

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane

Solute- any substance dissolved in a solvent

Solution- a solute and a solvent together

Pure water has a water potential of 0

When a solute is added to water its water potantial decreases so becomes more negative

Only water can move by osmosis other molcules move using different transport systems

Water can move through proteins such as aquaporins

Animal cells such as red blood cells contain many solutes, if placed in pure water the cell will absorb the water by osmosis as water will travel to the lower water potential

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Active Transport

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

ATP is used to directly move molecules

This ATP comes from aerobic respiration in the mitochondria

Up the concentration gradient from a low to high concentration

This process is very selective as only certain substances are transported and carrier proteins are used

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Co-Transport

Co-Transport is a mixture of active transport and facilitated diffusion which moves more than one molecule at a time through the partially permeable membrane

Here a sodium Potassium pump is used to move two molecules - in this case sodium and glucose

Sodium ions are actively transported out of the epithelial cells, by the sodium- potassium pump, into the blood. This takes place in one type of protein carrier molecules

This maintains a much higher concentration of sodium ions in the lumen of the intestine than the epithelial cells

Sodium ions diffuse into the epithelial cell down the concentrayion gradient through a different (co transport) channel. As it diffuses through they carry a glucose molecule with them.

This glucose pass into the blood plasma by facilitated diffusion

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Oral rehydration therepy

This is used to treat Diarrhoea.

There are many causes of diarrhoea from damage to the epithelial cells lining the intestine to loss of microvilli due to toxins.

To treat diarrhoea the patient needs to be rehydrated however just drinking water is uneffective as the water is not absorbed in the intestine and drinking water doesnt replace the electrolytes which are being lost from the cells.

A reydration solution needs to contain- water, sodium ions (to replace lost sodium), glucose (to stimute uptake of sodium via co-transport), pottassium (to replace those lost and stimulate appetite) and other electrolytes such as chloride and citrate.

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