Movement and Functions into and out of cells

• What is diffusion?

  • Diffusion is the movement of molecules from a region of its higherconcentration to a region of its lower concentration
  • Molecules move down a concentration gradient, as a result of their random movement
• What is diffusion in living organisms?

  • For living cells, the principle of the movement down a concentration gradient is the same, but the cell is surrounded by a cell membrane, which can restrict the free movement of the molecules
  • The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all
  • The simplest sort of selection is based on the size of the molecules (i.e. smaller molecules can diffuse across the membrane but larger molecules cannot)
• What does diffusion help living organisms do?

  • Obtain many of their requirements
  • Get rid of many of their waste products
  • Carry out gas exchange for respiration
• Examples of diffusion in living organisms:

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• Osmosis theory


  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
• Cell membrane

  • All cells are surrounded by it
  • It is partially permeable
• Water movement in and out of cells

  By osmosis
• Water moves down its concentration gradient during osmosis
• Partially permeable membrane

  Allows small molecules (like water) through but not larger molecules (like solute molecules)
• Dilute solution


  Has a high water potential
• Concentrated solution

  Has a low water potential
• Osmosis


  The movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) across a partially permeable membrane
• Animal cells


  • Lose and gain water as a result of osmosis
  • Do not have a supporting cell wall, so the results of osmosis can be severe
• What happens when an animal cell is placed in a strong sugar solution

  Cell loses water by osmosis and becomes crenated (shrivelled up)
• What happens when an animal cell is placed in distilled water

  1. Cell gains water by osmosis as it has no cell wall to create turgor pressure

  2. Cell continues to gain water until the cell membrane is stretched too far and it bursts
• Osmosis helps to regulate the water content in animal cells
• Diagram of osmosis in animal cells:


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• What is osmosis in the human body?

  • It is important that osmosis is carefully controlled in organisms to avoid damage to cells through lysis
  • The human body is adapted to maintain the optimum osmotic balance using processes such as sweating or increasing and decreasing urine concentration
  • This is all part of osmoregulation
• Osmosis in plant cells

  Plant cells lose or gain water as a result of osmosis
• Plant cells

  • Have a supporting cell wall
  • Are protected from cell lysis
• Plant cell placed in strong sugar solution

  1. Cell loses water by osmosis

  2. Vacuole gets smaller

  3. Cell membrane shrivels away from cell wall

  4. Cell becomes flaccid or plasmolysed (shrivelled up)
• Plant cell placed in distilled water

  1. Cell gains water by osmosis

  2. Vacuole gets bigger

  3. Cell membrane pushed against cell wall

  4. Cell is turgid or contains high turgor pressure (pressure of cytoplasm pushing against cell wall)
• Osmosis


  The movement of water molecules from an area of high water potential to an area of low water potential across a semi-permeable membrane
• How osmosis affects plant cells

  1. Plant cells lose water in strong sugar solution (lower water potential)

  2. Plant cells gain water in distilled water (higher water potential)
• Plant cells

  • Have a supporting cell wall
  • Protected from cell lysis
• Plant cell placed in strong sugar solution (lower water potential)

  Loses water by osmosis
• Flaccid/Plasmolysed


  Shrivelled up, vacuole gets smaller, cell membrane shrivels away from cell wall
• Plant cell placed in distilled water (higher water potential)


  Gains water by osmosis
• Turgid


  High turgor pressure, vacuole gets bigger, cell membrane pushed against cell wall
• Diagram of the affect of osmosis on plant cells:


  Here..

  
• Define active transport:

  The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
• What is the active transport theory?

  • Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion)
  • Active transport across the cell membrane involves protein carrier molecules embedded in the cell membrane

  
• Active transport in animals


  A process that allows molecules to be transported into or out of a cell against a concentration gradient, using energy
• Food molecules (such as the sugar glucose) can be absorbed across the wall of the small intestine by diffusion, but this is dependent on a concentration gradient existing between the lumen of the intestine and the bloodstream
• Active transport


  Allows molecules such as glucose to be transported into the bloodstream from the lumen of the small intestine (the gut) when the concentration of sugar molecules in the blood is higher
• Active uptake of glucose by epithelial cells in kidney tubules

  Allows for the reabsorption of glucose back into the blood so that none is lost in the urine
• Sugar molecules are used in respiration to release energy for cells to function
• What is active transport in plants?

  • Root hair cells lining the surface of plant roots need to move minerals such as magnesium ions from a region of lower concentration (the very dilute solution of minerals in the soil surrounding the roots) to a region of higher concentration (inside the cytoplasm of the cell)
  • Mineral ions are needed by plants to function
  • Magnesium ions are required to make chlorophyll
  • Nitrate ions are needed to make amino acids for protein synthesis (and subsequently growth)
• How does surface area to volume ratio affect diffusion?

  • The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface
  • Many cells which are adapted for diffusion have increased surface area in some way - e.g. root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

  
• How has the small intestine adapted for better diffusion?


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