Diffusion: Cell biology

• Diffusion


  The movement of substances such as gas particles or substances in solution, from a higher concentration to a lower concentration

  
• Cell membrane


  Controls the movement of substances in and out of a cell, should be permeable to allow molecules to move through the membrane

  
• Gas exchange


  When oxygen and carbon dioxide move in and out of cells by diffusion

  
• Purpose of gas exchange

  Organisms need oxygen for aerobic respiration, they also need to remove carbon dioxide which is a waste product in some organisms

  
• Sites of gas exchange

  The alveoli of the lungs and respiring cells around the body

  
• Concentration gradient

  The difference in concentration of a substance

  
• Surface area


  The total area of the surface of an object

  
• Factors affecting diffusion

  Concentration gradient, temperature and surface area

  
• Temperature


  Particles move more and diffuse quicker in warmer environments

  
• Surface area to volume ratio

  The amount of surface area in relation to how large something is

  
• Large surface area to volume ratio


  Leads to faster diffusion rates, as there is more room for particles to diffuse through a membrane

  
• An effective exchange surface


  Has a large surface area, a good blood supply, is well ventilated for gas exchange and has a thin membrane for diffusion

  
• Structures adapted for exchanging materials

  Small intestine, kidneys, lungs, gills in fish, roots and leaves in plants all have adaptations such as thin membranes and increased surface area

  
• Specific cells, tissue and sacs adapted for exchange

  Alveoli in mammal lungs, ciliated epithelial cells in the intestines, gill filaments in fish gills and some amphibian gills, root hair cells in plants, guard cells in plants

  
• Need for transport systems


  Larger organisms have smaller surface area to volume ratios and are unable to directly obtain useful substances from their environment like single-celled organisms can

  
• Transport system


  A system that is used for transporting substances around a multicellular living organism, such as the circulatory system in animals or the xylem and phloem in plants

  
• Soluble food molecules


  Are absorbed by diffusion or active transport in the small intestine, which has highly folded structures called villi to increase surface area for absorption

  
• Urea


  A waste product that is filtered and removed from the blood plasma via diffusion in the kidneys

  
• Gas exchange in plants

  Carbon dioxide diffuses into the plant in exchange for oxygen that diffuses out of the plant, this process is regulated by guard cells in the leaves that can open and close the stomata efficiently

  
• Water and mineral ions in plants


  Needed for cell functioning and proper plant development, they diffuse passively or actively into root hair cells with large surface areas and are transported to the leaves via the xylem

  
• Gills in fish

  Have a large surface area for oxygen absorption due to the presence of gill arches and filaments, oxygenated water passes over the gills as fish swim in a specific direction

  
• Lungs

  Are specialised in gas exchange due to the presence of many tiny sacs called alveoli that are adapted for diffusion due to being moist, one cell thick and surrounded by capillaries