plasma membrane, diffusion, osmosis and active transport

Features of Lipids

• Insoluble in water
• soluble in alcohol and acetone
• contain the elements C, H and O

 Roles of lipids

• Energy source-provide twice the energy per molecule as carbohydrate
• Waterproofing as insoluble
• Insulation as fat stored below skin
• Protection as fat around the organs

Triglycerides

• Triglycerides are made up of a single glycerol molecule attached to three fatty acids
• These join in a condensation reaction which remove three water molecules

How is one triglyceride different from another?

• Glycerol is the same in all triglycerides but the fatty acids (r group) can vary which gives fats and oils different properties.
• In phosolipids (which make up cell membranes) one of the fatty acids is substituted by a phosphate group.

Saturated and unsaturated fatty acids

• Saturated fatty acids have no double bonds between carbons
• Unsaturated fatty acids have at least one double bond (C-C)

Plasma (cell curface membrane)

• All membranes around cells and organelles are known as plasma membranes
• There are two main components that make them up: Phospholipids and Proteins.

Phosopholipids

• Phosphate head allows lipids soluble substances to enter and lave the cell quickly
• Hydrophilic head prevents water soluble and non polar substances entering and leaving the cell
• Bilayer makes the membrane flexible

Proteins

• Extrinsic proteins appear on the surface or partly embedded and act as support and hormone receptors.
• Intrinsic proteins span the entire bilayer. Some are enzymes and others act as protein carriers or channels for Na, K and water soluble materials.

Fluid mosaic model of the cell surface membrane

• Fluid because the phospholipids can move giving the membrane a flexible structure
• Mosaic because the proteins embedded vary in size, shape and pattern like mosaic tiles.

Diffusion

• The net passive movement of molecules or ions from a region of high concentration to a region of lower concentration
• passive-happens naturally, requires no ATP to be done

Factors affecting diffusion are:

• the concentration gradient-faster gradient, greater conc.
• Temperature
• Thickness of the exchange surface-thinner exchange surface, faster diffusion
• surface area-larger surface area, faster rate of diffusion, e.g. microvilli, avioli
• number of channel/proteins(pores) in membranes
• size and nature of diffusing particles, e.g. lipid soluble molecules diffuse quickly through phosolipid bilayer but water solubles are diffuse slower through protein channel/carriers

• Time (speed of movement of particles)
• Only lipid soluble molecules can diffuse through the bilayer. Others must find a tunnel to diffuse through.

Fick's law

Diffuse rate is proportional to= surface areaXdifference in conc/length of diffusion pathway

Facilitated diffusion

• Large molecules (amino acids and glucose) and charged particles (chloride ions) can't diffuse directly throught the phosolipid bilayer
• Instead they move through:1. carrier proteins-large molecules 2. Protein channels-charged molecules.

Osmosis

• The passage of water molecules from a region where it has a high water potential to a region where it is has a lower water potential through a partially permeable membrane.
• Water potential-similar to concentration
• from high w.p. to low w.p.:down the water potential gradient

Water potential

• This is the pressure created by water molecules ushing against cell membranes measured in kilopascals(kPa)
• Pure water has a water potential of 0kPa.
• If any solute (salt or sugar) is added- this will lower the water potential to a more negative value. The more solute that is added the lower the water potential.
• Which direction would water molecules move?-goes to the smaller number (-80 is smaller than -60)

Isotonic solution

• This is when the water potential of the solution is the same as the cell.
• A cell will not have 0kPa (pure water) as there is always glucose present
• Amount of water transported into the cell equal to the amount of water transported out of the cell.
• These allow small molecules like waterthrough but not larger solute molecules like sugar and salt.

Osmosis and cells

Active transport and absorption

• Active transport is the movement of molecules or ions into or put of a cell from a region of lower concentration to a region of higher concentration using energy in the form of ATP and carrier molecules/proteins.

Co-transport

• Sodium ions are actively transported out of the epithelial cells, by the Na+K+ pump, into the blood. This takes place in one type of protein-carrier molecule found in the cell-surface membrane of the epithelial cells.
• There is now a much higher concentration of Na+ in the lumen of the intestine than inside the epithelial cells
• The Na+ diffuse into the epithelial cells down the concentration gradeitn through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the Na+ flood back in through this second carrier protein, they couple with the glucose molecules which are carried into the cell with them.
• The glucoe passes into the blood plasma by facilitated diffusion using another type of carrier.