Cellular Exchange.

• Exchange of the substances in the body are oxygen, carbon dioxide, water, urea, mineral ions dissolved food molecules.
• Active transport requires energy
• •Unicellular (single-celled) organisms like amoeba have very large surface areas (SA) in comparison to their volumes.
• •For larger, multicellular organisms the distance between the surface of the organism to its centre is relatively long.
• •This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise
• •Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes
• •Exchange surfaces in animals include •The lungs and alveoli for gas exchange•The small intestines and villi for absorption of digested food
• •Multicellular organisms have surfaces and organ systems that maximise the exchange of materials by increasing the efficiency of exchange in a number of ways:By Having a large surface area to increase the rate of transport• A barrier that is as thin as possible to separate two regions, to provide as short a diffusion path as possible for substances to move across
• The greater difference in concentration between two regions, faster the rate of diffusion is.
• The higher the temperature is the more kinetic energy the particles of the substance will have. They will move/spread faster compared to when at at a lower temperature.
• A membrane with a greater surface area will have a greater rate of diffusion across it ( entry + exit points).
• Osmosis a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one.
• Diffusion is the movement of molecules in a fluid from areas of high concentration to areas of low concentration. This movement occurs down the concentration gradient.
• Facilitated diffusion is a type of passive transport in which molecules moves a cross a biological membrane, facilitated by these proteins, move down their concentration gradient without losing energy.
• Co-transport is the movement of molecules across a membrane in both directions.
• Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. 
• When an animal cell is placed in a solution more dilute than the fluids in the cytoplasm, water will move by osmosis across the plasma membrane and into the cell. 
• Simple diffusion, facilitated diffusion and osmosis are passive processes, meaning that they do not require energy. ​

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  Movement is always from high concentration to low concentration. 
• Simple  diffusion – small non-polar molecules (carbon dioxide, oxygen, steroid, hormones, lipids and fat-soluble vitamins). Does not require protein channels 
• Diffusion​-The movement of molecules from an area of high concentration to one of lower concentration until an equilibrium is reached.
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  • Facilitated diffusion – large Polar molecules (wate, glucose and ions) diffusion is assisted by proteins in the membrane. Requires protein channels. 
• Simple  diffusion – small non-polar molecules (carbon dioxide, oxygen, steroid, hormones, lipids and fat-soluble vitamins). ​
• Diffusion​

  • Charged particles or ions and large molecules such as glucose do not readily pass through the cell membrane because they are insoluble in lipid. ​
  • In the cell membrane intrinsic protein molecules span the membrane from one side to the other and help such particles to diffuse in or out of the cells. ​
• • Facilitated diffusion – large Polar molecules (water, glucose and ions) diffusion is assisted by proteins in the membrane. There are two types of proteins which facilitate (help) diffusion – channels and carriers.​
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• Facilitated diffusion​- Channel proteins consist of pores. This allows charged ions to pass through (such as Na+ ). Each channel protein is specific for one type of ion. They can also open and close depending on the needs of the cell (these are called gated channels).
• Carrier proteins - A particular molecule attaches to a carrier protein at its binding site and causes the carrier protein to change shape or rotate within the membrane; this action releases the molecule on the other side of the membrane.
• In biological systems osmosis is a special form of diffusion which involves the movement of water molecules only.​

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  Osmosis is the passage of water from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.​
• • Osmosis
  • At high concentrations (of water molecules) water has a greater potential energy i.e. the water molecules are completely free to move about.​
  •  When a solute, such as sugar, is dissolved in water there are proportionally fewer water molecules to move about and the water potential of the solution is lowered (becomes more negative). ​
• Co-transport mechanisms


  1. Epithelial cells lining the small intestine have carrier proteins that only transport a glucose molecule together with a sodium ion

  2. Sodium ion pump in the plasma membrane pumps sodium ions out of the epithelial cells into the blood capillaries which lowers the concentration of sodium ions inside the epithelial cells

  3. Sodium ion concentration gradient from the inside of the small intestine into epithelial cells

  4. Sodium ions diffuse into epithelial cells via a co-transport protein which brings with the glucose molecules

  5. Glucose diffuses down a concentration gradient into blood, requires a carrier protein
• Co-transport


  Helps ensure all the glucose is absorbed from the small intestine
• Co-transport is like the marriage therapist for the couple it helps the couple get back together and work together.
• Co-transport-coupled movement of one molecule down its concentration gradient with a second molecule going against its concentration.
• Simple Diffusion of oxygen in the body​

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  Oxygen moves by diffusion from an area of higher concentration inside the lungs to an area of lower concentration in the blood.​
• Facilitated Diffusion is used to transport amino acids from the bloodstream into the cell​

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  This molecule is taken in via active transport in the small intestine and then released into the bloodstream.
• Osmosis and our Kidneys​

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  The water in your blood travels to your kidneys. The kidney facilitate the reabsorption of water from the filtrate back into the body.​
• simple diffusion is known as passive transport as it requires no energy.
• Facilitated diffusion is diffusion but with the help of a transport protein in the cell membrane.