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 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.
- 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
- 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.
Plasma membrane continued/Diffusion
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.
- 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.
- 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.
Diffuse rate is proportional to= surface areaXdifference in conc/length of diffusion pathway
- 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.
- 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
- 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)
- 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 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.
- 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.