B2 Transport in Cells

• Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration.

  The bigger the concentration gradient, the faster the diffusion.

  The higher the temperature, the faster the diffusion as there is more energy.
• Cell membranes let dissolved substances move in and out by diffusion e.g. oxygen, glucose, amino acids and water. However, bigger particles such as starch and proteins can’t fit.

  The larger the surface area of the membrane, the faster the diffusion rate.
• Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration. It is a type of diffusion.
• Osmosis is used when a strong sugar solution gets more dilute as the water moves the sugar around to “even up” the concentration either side of the membrane
• Active Transport is a process that is required to move molecules against a concentration gradient. This process requires energy.
• Root Hair cells use active transport. This allows the plant to absorb minerals from a very dilute solution, against a concentration gradient. It needs energy from respiration to make it work.
• Active transport is used in the gut when there is a lower concentration of nutrients in the gut, but a higher concentration of nutrients in the blood. Active Transport allows the nutrients to be taken into the blood despite the concentration gradient being the wrong way.
• Organisms Exchange Substances with their Environment:

  • Cells can use diffusion to take in substances they need and get rid of waste products.
  • How easy this is depends on the surface area : volume ratio.
• Multicellular Organisms need Exchange Surfaces:

  • In single-celled organisms, gases and dissolved substances can diffuse directly into (or out of) the cell across the cell membrane. This is because they have a large surface area compared to their volume, so enough substances can be exchanged across the membrane to supply the volume of the cell.
  • Multicellular organisms have a smaller surface area compared to their volume. Not enough substances can diffuse from their outside surface to supply their entire volume so they need an exchange surface for efficient diffusion
• Exchange Surfaces are adapted by:

  • Thin membrane - short distance to diffuse
  • Large surface area - lots of substances can diffuse at once
  • Lots of blood vessels in animals to get stuff into and out of the blood quickly.
  • Ventilated in animals so air moves in and out.
• Gas Exchange happens in the lungs:

  • The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it.
  • To do this, the lungs contain millions of little air sacs called alveoli where gas exchange takes place.
  • The alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide. They have:
  • An enormous surface area
  • A moist lining for dissolving gases
  • Very thin walls
  • A good blood supply.
• The Villi Provide a Big Surface Area:

  • In the small intestine, there are millions of tiny projections called villi.
  • They increase the surface area so digested food can be absorbed much more quickly into the blood.
  • They have:
  • A single layer of surface cells
  • A very good blood supply to assist quick absorption.