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IB Biology - Passive Transport (Diffusion, Facilitated Diffusion, Osmosis)

Chapter 1.4 - Membrane Transport

May 3, 2020

  • Particles move across membranes by simple diffusion, facilitated diffusion, osmosis

  • Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.

  • Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

In the last IB biology chapter 1.3 we mention the cell membrane is semipermeable, guarding the material exchange in the cell membrane. Passive transport is one of the methods to cross the membrane.

Simple Diffusion

  • Small and non-polar molecules move across the membrane directly 
  • The molecules in the body are not evenly distributed, some molecules are in higher concentration in the external environment but are lower inside the cell --> Concentration Gradient 

Concentration gradient occurs when the concentration of molecules in two regions are imbalance

  • The molecules from the high concentration region move towards the low concentration region until the molecules are evenly distribute --> Simple Diffusion 

Simple diffusion = molecules move down by the concentration gradient until the molecules are evenly distributed

  • **it is always misunderstood that when the molecule is evenly distributed, the molecule stops moving. THIS IS WRONG! Actually, the molecule still moves from one side to the other, but there is no NET gain or loss of molecules in both side.  
  • energy free, the driving force of the movement is the kinetic energy in the concentration gradient
  • Imagine you drop a dye in a cup of water, the dye diffuses to the colourless water until the whole cup of water is coloured 
  • e.g., the oxygen concentration is high in the extracellular fluid but low inside the cell. The oxygen diffuses directly across the cell membrane by the concentration gradient, supplying oxygen to the cell

Carrier Mediated Simple Diffusion

  • the cell membrane is semipermeable, small polar or charged molecules required channel protein to cross the membrane because the hydrophobic phosphate tails expel them 
  • the interior channel protein is hydrophilic, so the polar and charged molecules pass through it 
  • some the channel protein is non-gated while some are gated which means only open when it is stimulated or needed 
  • the channel protein is specific, only the target molecules can pass through it
  • energy free also because the movement is driven by the concentration gradient 
  • e.g., potassium ion (K+)channel. The is K+ charged. Inside our nerve cell, the K+ is abundant but it cannot diffuse out the cell because the K+ channel is gated so the K+ is piled up. Once the stimulant, the nerve impulse reaches the nerve cell, the channel is open. The K+ diffuses along the concentration gradient, releasing to the cell. 
  • the gated channel can help to pile up the molecules, creating the concentration gradient. Such creation is essential in some muscle cell and nerve cell signalling which you will learn in IB biology syllabus
  • the non-gated channel allows the molecule move freely, the molecules cannot be accumulated, so they are usually found in the membrane where the concentration gradient does not need to be maintained

Facilitated Diffusion

  • large polar molecules required carrier protein to bypass the semipermeable membrane 
  • the carrier protein is different from the channel protein, the carrier protein requires binding of the molecules 
  • the large polar molecules bind with the carrier protein, the protein then release it on the other side 
  • the carrier protein is specific 
  • boots up the diffusion pace 
  • energy free as the movement is driven by the concentration gradient 
  • e.g., the glucose is large and polar, it diffuses into the cell by the carrier protein 

Osmosis

  • osmosis is the diffusion of water 
  • the water moves from the region with low solute concentration to the region with high solute concentration

osmosis = water moves across the partially permeable membrane from the low solute concentration area to the high solute concentration

Osmolarity

  • osmolarity is the measurement of solute concentration in a 1L solution 
  • high osmolarity = high solute concentration
  • low osmolarity = low solute concentration 

osmolarity = the description of the concentration of solute in 1L solution

  • Hypertonic, Isotonic, and Hypotonic are the description of the osmolarity of one solution to another 
  • Hypertonic: the solution has greater osmolarity than the other
  • Isotonic: the solution has the same osmolarity as the other 
  • Hypotonic: the solution has lower osmolarity than the other 

Water potential

  • water potential describes the tendency of water to move from one place to another 
  • low water potential, the water is likely to stay 
  • high water potential, the water is likely to move and leave 

Water potential = the tendency of water to move and leave
increase the osmolarity, decrease the water potential

Summary 

  • cell in hypotonic solution (the salt conc. is lower in solution) --> water moves from solution to cell
    the solution is high in water potential (water moves and leaves), the cell is low in water potential
     
  • cell in isotonic solution (two salt conc. are the same) --> no net water movement 
    the water potential of the cell and solution are the same 
     
  • cell in hypertonic solution (the salt conc. is higher in solution) --> water moves from cell to solution 
    the solution is low in water potential, the cell is high in water potential (water moves and leaves)

Examples:

  • the extracellular fluid is isotonic to our cell cytoplasm, so there is neither water gain or water loss
  • in the hospital, 0.9% normal saline (9g sodium chloride/1L solution) is used in intravenous drips (IVs). The normal saline is isotonic to our cell cytoplasm, so the cells do not shrink or burst. 
  • Plant cell has a vacuole (IB Biology chapter 1.2), it functions to support the rigidity of the cell 
  • Plant cell in hypotonic solution, the water move in the vacuole. Expanded vacuole pushes the cell membrane against the cell wall and the cell wall pushes back due to its non-flexible property. No net water movement until the cell is expanded and turgid. 
  • Plant cell in isotonic solution, no net gain of water in the cell. The vacuole does not push the cell wall but the cell membrane is still able attaching the cell wall. The cell is flaccid. 
  • Plant cell in hypertonic solution, the water leaves the vacuole. The shrink vacuole cannot push out the cell wall. The cell membrane detaches from the cell wall. Such phenomenon called plasmolyzed. The cell shrinks.
  • The force of cell wall pushing on the cell is called pressure potential. The greater the pressure potential, the more turgid the cell. 

Experiment to determine the Osmosis

How to determine if a substance lets say potato is hypertonic or hypotonic to a solution?

  • measure the weight of a cube a the substance e.g. a potato cube
  • place it in the solution with different osmolarity 

0% salt, 0.1% salt, 0.2% salt, ........., 0.9% salt, 1% salt

  • reweight the potato cube, calculate the percentage change of the weight 

    % change = (change in the mass / original mass )x100

  • plot a graph where % change (y-axis) against the concentration of the salt (x-axis) 
  • from the graph, we can determine the 0.35% salt solution is isotonic to potato cube, hence the solute conc. in cytoplasm in the potato cell is around 0.35%
  • salt concentration below 0.35% is hypotonic, water enters the cell, so potato gains weight 
  • salt concentration above 0.35% is hypertonic, water leaves the cell, so potato losses weight 

In this IB biology chapter, you have to:

1. how things moved by the diffusion

2. distinguish the differences in different formats of diffusion

3. be awarded that things are equally distributed doesn't mean they don't move, actually, they do but just with no net movement

4. In diffusion, the molecules move from high to low conc. area, but in osmosis, water moves from low salt to high salt area

5. the relationship between osmolarity and water potential

That is the end of the topic.

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