Exchange with the Environment

• Single-celled organisms have a large surface area : volume ratio, and gas exchange across their cell-surface membrane is sufficient due to the short diffusion pathway.
• Diffusion is the movement of fluid (liquid or gas) from an environment or region of higher concentration of the fluid to a region of lower concentration.
• Gas exchange occurs when there is a difference in partial pressure between two areas separated by a membrane.
• A concentration gradient exists when there is a region of high concentration leading to a region of low concentration.

  • going from high to low concentration is going down the concentration gradient
  • going from low to high concentration is going up/against the concentration gradient.
• The rate of diffusion depends on the size of the area over which it takes place, the length of time available for diffusion, the temperature, and the distance across which diffusion must occur.
• Oxygen diffuses more easily than carbon dioxide because its molecules are smaller.
• Going down a concentration gradient refers to going from regions of high concentration to regions of low concentration.
• Going up a concentration gradient means moving from regions of low concentration to regions of high concentration.
• The movement of substances up a concentration gradient requires energy in the form of ATP.
• Partial Pressure is defined as if a container filled with more than one gas, each gas exerts pressure. The pressure of any one gas within the container is called its partial pressure.
• The diffusion gradient is the gradual decreasing of a substance as you travel away from the source. For example, oxygen enters the membrane of the cell in high concentrations, but as you travel further into the cell the concentration of oxygen decreases.
• Diffusion describes the passive movement of particles in fluids (liquids and gases). Molecules move randomly but tend to move into any space available until it is evenly distributed. So, the net movement of particles will be towards the area of lower concentration or down a concentration gradient.
• Diffusion is the movement of molecules down the concentration gradient.
• Active transport involves the use of ATP to pump substances against their concentration gradient.
• Water will always flow from an area of high pressure to an area of low pressure.