substance exchange

• diffusion


  movement of molecules from an area of higher concentration to an area of lower concentration
• where does diffusion occur?

  liquids and gases
• why does diffusion occur in fluids?

  because the particles in these substances are free to move around
• concentration gradient


  the difference in the concentration of a substance from one location to another
• the greater the concentration gradient, the ... the diffusion rate


  faster
• the higher the temperature, the ... the diffusion rate


  faster
• why does a higher temperature result in a faster diffusion rate?

  because the particles have more energy, so move around faster
• how are cell membranes involved with diffusion?

  because they are semi-permeable, they only allow very small molecules (such as oxygen) to diffuse through into the cell and do not allow large molecules (such as starch/proteins) in
• the larger the surface area of the cell membrane, the ... diffusion rate

  faster
• why does a larger cell membrane surface area result in a faster diffusion rate?


  because more particles are able to pass through at once
• how do particles flow in diffusion?

  randomly
• what does a random movement of particles in diffusion result in?

  particles going both ways, however this is counters by the net movement of particles from one side to another
• net movement of particles in diffusion

  the overall movement of particles from the side with higher concentration to lower concentration
• does diffusion require energy?

  no, it is passive transport
• osmosis


  the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane
• what does it mean if a solution is dilute?

  a dilute solution is one in which there is a relatively small amount of solute dissolved in the solution
• what does it mean if a solution is concentrated?

  a concentrated solution is one that has a relatively large amount of dissolved solute
• solution


  a homogeneous mixture that is made up of a solute dissolved within a solvent
• solvent


  a substance with the ability to dissolve other substances (solutes) to form a solution
• solute


  a substance that can be dissolved into a solution by a solvent
• partially permeable membrane


  a membrane that allows only certain substances to pass through because it has very small holes in it - this allows only tiny molecules (such as water) in and not larger ones (such as sucrose)
• why do water molecules pass both ways though the membrane during osmosis?


  because water molecules move about randomly at all times
• why is there a net flow of water molecules in osmosis?


  because there are more water molecules on one side than the other
• in osmosis, the concentrated solution becomes...

  more diluted
• does osmosis require energy?

  no, it is passive transport
• active transport


  energy-requiring process that moves material across a cell membrane against a concentration difference
• what process provides the energy required to transport substances against a concentration gradient?

  respiration
• how do root hair cells use active transport?

  active transport allows the root hair cells to absorb minerals from the soil (which is more diluted), against the concentration gradient
• how does the gut use active transport?

  because there is a lower concentration of glucose and amino acids in the gut and a higher concentration of nutrients in the blood, active transport is needs to get more of the nutrients into the blood, against the concentration gradient
• organisms exchange substances with their...

  environment
• How does surface area affect the rate substance exchange?

  higher surface area results in faster substance exchange as there is more space for molecules to diffuse across the membrane
• high surface area to volume ratio meaning

  a larger surface area to volume ratio means that there is more surface area available for the exchange of materials
• low surface area to volume ratio meaning

  a smaller surface area to volume ratio means that there is less surface area available for the exchange of materials
• in single-celled organisms, gases and dissolved substances can...

  diffuse directly into/out of the cell across the cell membrane because they have a high surface area to volume ratio
• multicellular organisms need...


  exchange surfaces
• multicellular organisms have a ... surface area compared to their volume

  smaller
• how are exchange surfaces adapted to maximise effectiveness?

  - thin membrane

  - moist membrane

  - large surface area

  - lots of blood vessels in animals

  - ventilated in animals
• how does having a thin membrane maximise the effectiveness of exchange surfaces?

  substances only have a short distance to diffuse across
• how does having a moist membrane maximise the effectiveness of exchange surfaces?

  gases dissolve in the moisture helping them to pass across the gas exchange surface
• how does having a large surface area maximise the effectiveness of exchange surfaces?

  allows a lot of substances to diffuse at once