Lipids and Transport across membrane 3

• What are lipids?


  Lipids are a large and varied group of non-polar molecules that are insoluble in water, but dissolve easily in organic solvents, like alcohols
• What are triglycerides?


  Composed of 3 fatty acids and 1 glycerol
• Describe the formation of a triglyceride

  Condensation reaction producing 3 molecules of water. An ester bond forms between –OH groups of the glycerol and the -COOH of the fatty acids.
• Why are triglycerides not classed as polymers?

  They are not made of monomers. They are made of 2 different types of molecules
• What are 3 roles of triglycerides?

  · storage of energy

  · insulation- electrical and heat

  · protection
• How is the structure and properties of triglycerides related to their function?

  They have a high ratio of energy storing carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy.

  · They have a low mass to energy ratio, making them good storage molecules because so much energy can be stored in a small volume.

  · They are large, , non-polar molecules, insoluble in water so they have no osmotic effect and do not affect the water potential in cells.
• How does a phospholipid differ from a triglyceride?

  A phosphate group replaces one of the fatty acids
• What is the test for lipids?

  Ethanol emulsion test-

  Using a completely dry clean test tube add ethanol to the sample

  · Shake thoroughly to dissolve any lipid in the sample.

  · Then add water to the sample and shake gently.

  · A milky white emulsion indicates the presence of lipid (NB: not a precipitate).
• Define saturated fatty acid

  no double bonds between carbon atoms
• Define unsaturated fatty acid


  one or more double bonds between carbon atoms
• All cells are surrounded by a plasma membrane. In Eukaryote cells, membranes are found within the cell, as well as surrounding it.

  They may be a single phospholipid bilayer, e.g. the cell membrane and those around vesicles, vacuoles and some organelles (e.g. ER) or they may be double.
• Which organelles have a double membrane surrounding them?

  nucleus, Mitochondria and chloroplasts.
• What are the 3 main functions of the plasma membrane?


  1. To control transport of substances in and out of cell or organelles

  2. To act as a receptor site to

  recognise chemicals which need to

  enter the cell or organelle.

  3. To compartmentalise, separate off

  the cell from the environment, and the

  different reactions of the cell from each

  other by forming the organelles.
• what is the importance of the functions of the plasma membrane?

  Membranes allow certain molecules to pass through, but not others, i.e. they are partially permeable. Examples oxygen glucose water carbon dioxide.

  Allows cells to communicate and react to their internal and external environments

  Allows reactions to occur more efficiently. Eg. Aerobic respiration and photosynthesis Different concentrations can be maintained on either side of the membrane.
• What are the properties of a phospholipid?

  Hydrophilic head (glycerol and phosphate) and hydrophobic tail (fatty acids).
• What do phospholipids form in water?

  They form a two layered structure, a bilayer, where the hydrophilic head points out into the water and the hydrophobic tails all point towards each other.
• What does the bilayer act as a barrier to?

  Polar, water soluble molecules
• What does the bilayer allow through?

  non-polar, lipid soluble molecules
• Describe membrane proteins


  The membrane is a fluid structure with protein molecules floating about in the bilayer, forming a fluid-mosaic structure. Intrinsic proteins are across the whole membrane and extrinsic proteins may only be in one layer. The proteins which span the membrane often have hydrophilic channels in the middle to allow transport of some substances more easily.
• Which substances would be able to move through these hydrophilic channels?

  larger, charged, polar, water soluble molecules like glucose, sodium ions and amino acids
• What is the function of membrane proteins?

  Intrinsic proteins- transport, carrier/channel proteins.

  Extrinsic- enzymes, receptors, antigens
• Describe glycoproteins


  Short, branching carbohydrate chains are attached to proteins in membrane. These glycoproteins act as receptors for hormones and neurotransmitters or antigens.
• Describe glycolipids


  A carbohydrate bonded to a lipid. They act as recognition sites, help to maintain the stability of the membrane and help cells attach to one another, so forming tissues
• Describe cholesterol


  Restricts the movement of other molecules making up the membrane and increases strength and stability of membranes by making them less flexible.

  · It helps to prevent the loss of water and dissolved ions from the cell.
• Define simple diffusion


  Net movement of particles down a concentration gradient from a region of high concentration to lower concentration
• why is simple diffusion passive?

  It requires no ATP
• Which factors affect the rate of diffusion?

  Temperature, concentration gradient, surface area, and molecular size of diffusing molecule, water solubility and thickness of exchange surface
• What is fick‘s law?

  Rate of diffusion is proportional to surface area x difference in concentration over thickness of exchange surface
• How can we increase rate of diffusion?

  Increase surface area.

  Increase concentration gradient

  decrease thickness of exchange surface
• Which molecules can move through the phospholipid bilayer by simple diffusion?

  Small, non-polar molecules. Like CO2 or O2
• What is facilitated diffusion?

  Facilitated diffusion is the passive movement of molecules down a concentration gradient with the help of channel/carrier proteins.
• What is the rate of facilitated diffusion limited by?

  The number of carrier/channel proteins
• Which molecules move by facilitated diffusion?

  Large or polar molecules Like sodium ions or glucose
• What can increase the rate of facilitated diffusion?

  Increase in concentration gradient Or more channel proteins
• What is active transport?

  Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient, requiring energy in the form of ATP and specific protein carrier pumps
• What can limit the rate of active transport?

  Lack of O2/glucose, low temp, ATP availability and carrier proteins
• What is endocytosis and exocytosis?

  When cells need to transport large amounts of materials across their plasma membranes, they are too large to move by facilitated diffusion or simple diffusion and active transport. To do this, cells make vesicles from the plasma membrane itself to transport solids or liquids across the membrane into or out of cells. Importing materials:endocytosis, exporting materials: exocytosis. Both require ATP.
• What is osmosis?


  Osmosis is the movement of water across a partially-permeable membrane down a water potential gradient from higher to lower water potential
• What is the water potential of water?

  Zero
• What is the water potential of all other solutions?

  Negative