Transport between cells

• A vast majority of the membrane is phospholipid. This is the same in all membrane bound organelles. The membrane is 3D
• Cholesterol and glycolipids are the other types of phospholipid found in membranes.
• Cholesterol, whilst dangerous in veins, provide mechanical strength and provide the shape of the membrane.
• Because membranes are always moving, the more cholesterol the better. With more cholesterol contained the less mobile a membrane becomes.
• Both lipids and cholesterol are hydrophobic, making membranes impermeable in water.
• Cholesterol is vital in animal cell membranes as they lack a cell wall.
• Glycolipids are important in cell recognition. Glycolipids are antigens in cell membranes and can be used for cell signalling.
• There are 2 types of proteins found within the membrane. Carrier proteins and channel protein.
• Channel proteins allow substances to pass through the membrane. Whilst carrier proteins selectively bind to substances. They change the shape to release substances on the other side.
• The cell membrane is described as a fluid mosaic.

  Fluid: Flexible and able to move

  Mosaic: protiens within the membrane are arranged in a variety of shapes and sizes.
• Simple diffusion: The net movement of particles from a region of high concerntration to a low concerntration, down a diffusion gradient, until an equilibrium is reached.
• Facilitated diffusion requires the help of molecules such as carrier and channels proteins to take place.
• What can do simple diffusion?

  Uncharged molecules / ions

  Any hydrophobic molecules

  Small molecules
• What can do facilitated diffusion?


  Charged molecules / ions

  Any hydrophilic molecules

  Bigger molecules
• Channel protiens are mainly used by ions for facilitated diffusion. Within these protiens water is contained, as the water is slightly charged molecules will bind.
• Each ion has a specific channel. Depending on:

  Size of ion

  Charge of ion

  Channel is also regulated.
• Carrier protiens aid the transport of bigger molecules in and out of cells. For example: Glucose. Carrier proteins are also selective.
• Diffusion is proportional to:

  Surface area x difference in concerntration / length of diffusion path.
• Rate of diffusion also depends on:

  • Number of channel / carrier protiens in the membrane
  • Size of diffusing particle
• The movement of substances across cell membranes is affected by membrane structure. Describe how.

  1. Cholesterol provides rigidity in the membrane
  2. Contains carrier and channel proteins for facilitated diffusion / co-transport.
  3. Middle of the bilayer allows non-polar molecules to pass through
  4. Disallows polar molecules
  5. Carrier proteins are selective
• The curve for flask F levelled off after 20 minutes. Explain why.

  Flask F used facilitated diffusion

  An equilibrium was reached so no more facilitated diffusion would happen.
• The water potential of a plant is -400kPa. The cell is put in a solution with a water potential of -650Pa. Describe and explain what will happen to the cell.

  Water will leave via osmosis

  Water moves down water potential

  Protoplast pulls away from the membrane and the cell wall will shrink
• The carrot cylinders were left for 18 hours in the sucrose solution. Explain why they were left for a long time.

  To allow an equilibrium to be reached so no further changes happen
• Young carrots store sugars in their tissues, but in older carrots some of this is converted into starch. How would using cylinders of older carrot tissue affect the results obtained for a sucrose solution of 0.6mol dm^-3. Give a reason for your answer

  Starch is insoluble, so water potential is unaffected

  Greater difference is present so a greater decrease in volume would happen.
• Glucose is absorbed from the lumen of the small intestine into epithelial cells. Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells.

  A sodium potassium pump is contained

  This maintains a concentration gradient

  Sodium uses facilitated diffusion

  Sodium is a glucose co-transporter

  Glucose is absorbed with Na+ ions against the concentration gradient
• Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen doesn't.

  Oxygen is non-polar

  Able to dissolve into water

  Chloride is a charged molecule, making it use facilitated diffusion

  Chloride ions are soluble

  Chloride cannot pass the bilayer
• There is 2 types of active transport:

  • Primary /direct active transport
  • Secondary / Indirect active transport
• Primary Active Transport: The net movement of molecules / ions in or out of a cell, from a region of lower concentration to a higher concentration. This happens against a concentration gradient. Requiring energy in the form of ATP and carrier proteins.
• Primary active transport:

  • Uses a special carrier protein, which acts as a 'pump'
  • A molecule / ion can then bind to a carrier protein
  • The carrier protein hydrolyses ATP into ADP and Pi
  • Causing the carrier protein to change its shape, releasing the molecule on the other side of the membrane.
• Primary active transport only transports 1 molecule at a time, if more than one molecule needs to be transported, or if molecules need to go in opposite directions secondary active transport takes place.
• An example of secondary / indirect active transport is co-transport
• Secondary / indirect active transport: The movement of 2 or more molecules using an already created concentration gradient made by active transport.
• Co transport within the small intestine:

  • A sodium potassium pump is contained
  • Removing sodium and adding potassium
  • Maintaining a sodium concentration gradient for sodium to move into the lumen.
  • When this happens, sodium will couple its movement with glucose
  • Glucose then uses facilitated diffusion to enter the blood stream.
• Anthocyanins are coloured pigments found in the cell vacuole of some plant cells. A student investigated how to extract anthocyanins from blueberries.

  She investigated three different extraction solvents:

  E – Ethanol, water and acid

  F – Ethanol and water

  G – Water

  Use your knowledge of membranes to explain the results shown in the graph below.

  A higher absorbance indicates more anthocyanin

  E is greater than F because acid can denature enzymes

  Sample E extracts the most because ethanol breaks down the phospholipids found in cell membranes