transport across cell membranes

• What affects the rate of diffusion?

  - temperature

  - concentration gradient

  - surface area

  - diffusion distance

  - size of molecule

  - stirring/moving of molecules
• What are the 2 types of transport across membranes?

  - passive transport - does not require external energy

  - active transport - requires energy in the form of ATP
• What is the definition of diffusion

  The net movement of particles from a region of high concentration to a region of lower concentration until evenly distributed (down a concentration gradient)
• What are the q types of diffusion

  - simple diffusion

  - facilitated diffusion
• What is simple diffusion?

  Small, lipid-soluble, uncharged molecules

  Across the phospholipid bilayer
• What is facilitated diffusion?

  Small, water-soluble molecules (mainly ions or polar)

  Using channel proteins
• What is facilitated diffusion using carrier proteins?

  Larger, water soluble molecules
• What is a phospholipid

  One of the fatty acids of a triglyceride is substituted by a phosphate containing group
• What are cell membranes made of?

  Phospholipid bilayer
• Describe the phospholipid bilayer

  A phosphate molecule consists of a hydrophilic polar head due to the phosphate group ans hydrophobic non-polar tails due to the fatty acids.

  
  
• describe the fluid mosaic model of the phospholipid bilayer

  Fluid-molecules can move horizontally in the membrane

  Mosaic-proteins are embedded in the phospholipid bilayer
• What are the differences between carrier and channel proteins?

  Channel proteins are only involved in facilitated diffusion and osmosis whereas carrier proteins are involved in both facilitated diffusion and active transport
• What is the definition of osmosis

  The net movement of water from a region of high water potential to a region of lower water potential through a semi-permeable membrane
• Pure water has the water potential of…

  0kPa
• Adding a solute to water makes the water potential…

  Lower (negative)
• Describe active transport

  - it allows sun]stances to be transported against their concentration gradient

  - it uses carrier proteins

  - energy is provided by the hydrolysis of ATP

  - most ATP comes from aerobic respiration
• What enzyme breaks down ATP

  ATP hydrolase
• What is co-transport?

  The transport of 2 different substances simultaneously using the same carrier protein
• Describe the co-transport of glucose from the ileum into the blood

  - ileum is part of the small intestine

  - sodium ions pass from the lumen into the epithelial cells by facilitated diffusion using a carrier protein

  - as the sodium ions pass through thus carrier protein, the protein changes shape so that glucose is simultaneously brought in against its concentration gradient, a process called co-transport

  - glucose leaves the blood by facilitated diffusion through the base of the cell via carrier proteins

  - sodium ions actively transported to blood out of the base of the epithelial cell (using ATP)

  - this removal of sodium ions maintains the sodium concentration gradient needed to maintain uptake of glucose
• Describe how glucose is absorbed across an epithelial cell from the ileum into the blood (4)

  - sodium ions and glucose absorbed by co-transport

  - via a carrier protein

  - sodium ions removed by active transport into blood

  - maintains low concentration of sodium ions
• What are the common adaptations of cells to maximise absorption

  - folded cell surface membrane so large surface area

  - large number of co-transport/carrier/channel proteins so fast rate of absorption

  - large number of co-transport/carrier/channel proteins for facilitated diffusion

  - large number of mitochondria so make more ATP by respiration

  - large number of mitochondria for aerobic respiration

  - large number of mitochondria to release energy for active transport