Transport in Cells

• Transport processes used for exchange

  • Diffusion
  • Osmosis
  • Active transport
• Diffusion

  The spreading out of the particles of any substance in solution or gas, resulting in a net movement from higher to lower concentration
• Particles in a solution or a gas are always moving about randomly
• Diffusion is an entirely passive process
• Concentration gradient

  The difference in concentration of a substance between two areas
• Oxygen and carbon dioxide can diffuse across the cell membrane during gas exchange
• Starch cannot diffuse across the cell membrane because it is too big
• Substances that enter cells by diffusion

  • Oxygen
  • Carbon dioxide
• Substances that leave cells by diffusion

  • Urea
  • Carbon dioxide
• A photosynthesising leaf cell will likely have a lower carbon dioxide concentration on a bright day compared to its environment
• At night, when the plant is not photosynthesising, the carbon dioxide concentration inside the leaf cell will be higher than the environment
• Cell membrane


  A barrier that separates the inside of the cell from its outside environment
• Cell membrane

  • Partially permeable
  • Allows some substances to enter or leave the cell
• Active transport

  The movement of substances against a concentration gradient, requiring energy
• Osmosis


  The diffusion of water across a selectively permeable membrane
• Cell Biology


  The study of cells, their structure, function, and processes
• Diffusion in Multicellular Organisms

  Large, multicellular organisms like humans have relatively small surface areas in comparison to their volumes
• Diffusion in Multicellular Organisms

  Process by which substances move from an area of higher concentration to an area of lower concentration
• Large, multicellular organisms like humans have relatively small surface areas in comparison to their volumes
• This means that the distance between the surface of the organism to its centre is relatively large
• Larger organisms usually have exchange surfaces and transport systems
• Diffusion, osmosis, and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise
• Many cells which are adapted for diffusion have increased surface area in some way
• Cells adapted for diffusion

  • Root hair cells in plants
  • Cells lining the ileum in animals
• Small intestine

  • Highly folded surface increases surface area
  • Most absorption of digested food molecules occurs across the wall
• Alveolus


  • Gas exchange surface in humans
  • Wall is one cell thick with a moist lining and excellent blood supply
• Gills

  • Gas exchange surface in fish
  • Each gill is made from lots of smaller plates called filaments
• Root hair cell

  • Highly branched root network increases surface area
  • Surface covered in root hair cells with projections increases surface area
• Leaves

  • Contain most of a plant's photosynthetic cells
  • Stomata allow air to circulate inside the leaf
• Multicellular organisms have surfaces and organ systems that maximise the exchange of materials
• Having a large surface area increases the rate of transport
• A barrier that is as thin as possible provides a short diffusion path for substances to move across
• Animals have a large network of blood vessels throughout the body
• Gas exchange surfaces that are well ventilated maintain concentration gradients
• We talk about the ‘wall of the alveoli’ or the ‘wall of the intestines’ when describing structures that maximise exchange
• Both of the walls above are made from animal cells and animal cells do not have cell walls
• Osmosis


  The movement of water into and out of cells through a partially permeable membrane
• The Process of Osmosis

  1. Occurs when two solutions are separated by a partially permeable membrane

  2. Water moves from a dilute solution to a concentrated solution

  3. Water molecules move randomly like any other particle that can diffuse
• Osmosis

  • Special type of diffusion
  • Water moves from more dilute to more concentrated solution
• Water will always move to even up the concentration of water molecules on each side of the membrane