plasma membrane, diffusion, osmosis and active transport

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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

























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Lipids continued

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)
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Plasma membrane

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.
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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.
  • 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
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Diffusion continued

  • 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.
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  • 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)
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Osmosis continued

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

All cells(plant and animal) contain many different molecules (sugars and salts) which LOWER their water potential.
Osmosis and plant cells (see sheet)
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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.
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