Movement of Substances - Detailed Notes

Movement of Substances

Types of Particle Movement

Particles in nature move in four different ways:

• Passive transport
  • Diffusion
  • Facilitated diffusion
  • Osmosis
• Active transport

Diffusion

• Definition: Net movement of ions/molecules from a region of higher solute concentration to a region of lower solute concentration (down a concentration gradient) with or without a partially permeable membrane.

• Example:

  • Sugar molecules moving from a region of high sugar concentration to a region of low sugar concentration.
  • Red ink molecules spreading from a region of high ink concentration to a region of low ink concentration.
    • Initially, dye crystals are placed in water, concentrating them in one area.
    • The dye dissolves, leading to a net movement of dye molecules. Water molecules move in the opposite direction.
    • Eventually, the water and dye molecules are equally distributed.

• Particle Movement Example:

  • If there are 100 particles on the left side and 10 on the right:
    • 20 particles move from left to right, and 2 from right to left, resulting in 82 on the left and 28 on the right.
    • Then, about 16 move from left to right, and 6 move from right to left, leaving 72 on the left and 38 on the right.
    • Eventually, the gap narrows, and the two sides even out.

• Down a Concentration Gradient:

  • Particles move from a region of high solute concentration to a region of low solute concentration.

• Examples of Diffusion in Biology:

  • Movement of oxygen into red blood cells.
  • Gaseous exchange through the stomata of leaves.
  • Movement of digested food by cells in the small intestines into the bloodstream.

Factors Affecting Rate of Diffusion

• Surface area to volume ratio:
  • Larger the SA:V, faster the rate of diffusion.
• Concentration gradient:
  • The steeper the concentration gradient (higher concentration difference between two regions), the faster the rate of diffusion.
• Temperature:
  • Higher the temperature, higher the kinetic energy (KE) of molecules, thus a faster rate of diffusion.
• Thickness of membrane:
  • Thicker the membrane, the slower the rate of diffusion, as molecules take more time to move across.
• Distance between two regions:
  • Longer the distance, the slower the rate of diffusion, as molecules take more time to travel.
• Pressure differences:
  • Larger pressure difference leads to faster molecule movement, resulting in a faster rate of diffusion (from higher to lower pressure).

Concentration Gradient

• The greater the concentration difference, the faster the rate of diffusion.

• Examples:

  • Graph 1: Region A = 20 au, Region B = 2 au, Concentration Difference = 18 au
  • Graph 2: Region A = 20 au, Region B = 10 au, Concentration Difference = 10 au
  • Graph 3: Region A = 12 au, Region B = 12 au, Concentration Difference = 0 au

• Analysis:

  • Graph 1 has a larger concentration difference than Graph 2, so molecules move faster from Region A to Region B in Graph 1.
  • In Graph 3, there is no concentration difference, so there is no net movement of molecules, and no diffusion occurs.

When Diffusion Stops

• Diffusion stops when the two regions have the same concentration (= same number of solute molecules), resulting in NO NET MOVEMENT.
• The particles are still in constant and random motion, but there is the same number of particles everywhere, so the system has reached DYNAMIC EQUILIBRIUM.

Effects of Heat and Alcohol on Plant Cell Diffusion

• Heat:

  • Heat denatures the proteins on the cell membrane, making the cell membrane porous.
  • Particles/molecules move across the cell membrane faster and more easily.
  • Rate of diffusion of the solute particles increases.

• Alcohol:

  • Alcohol dissolves the membrane lipids and denatures the membrane proteins, making the cell membrane even more porous.
  • Particles move across the cell membrane even faster and more easily.
  • Rate of diffusion of the solute particles increases.

Facilitated Diffusion

• Definition: A type of passive transport that allows substances to cross membranes with the assistance of special transport proteins such as ion channel proteins and carrier proteins.

• Some molecules and ions such as glucose, sodium ions and chloride ions are unable to pass through the lipid bilayer of cell membranes and requires the help of transport proteins to help them cross the membranes.

• Mechanism:

  • Ion channel proteins allow specific ions to pass through the protein channel. These channels are regulated (open or closed) by the cell.
  • Carrier proteins bind to specific molecules, change shape, and then deposit the molecules across the membrane. The proteins then return to their original position.

Osmosis

• Definition: Net movement of water molecules from a region of lower solute concentration (= high water potential) to a region of higher solute concentration (= low water potential), against a concentration gradient across a partially permeable membrane.

• Osmosis is the passive movement of water across a membrane.

• The actual movement of water through a cell membrane is the result of two processes:

  • Simple diffusion of water (also known as osmosis).
  • Bulk flow by facilitated diffusion [protein carrier is called aquaporin].

• Water molecules can pass through the bilayer by zipping between phospholipids molecules and through protein carriers called aquaporins.

• Against Concentration Gradient:

  • Water particles move from a region of low solute concentration (high water potential) to a region of high solute concentration (low water potential).

• Examples of Osmosis in Biology:

  • Absorption of water by root hair cell.
  • Absorption of water by large intestines.

• Water Potential:

  • High water potential = more free water molecules.
  • Low water potential = fewer free water molecules.

Effects of Osmosis on Cells

• Animal Cells:

  • Isotonic solution: Normal red blood cells.
  • Hypotonic solution: Red blood cells gain water, become bloated, and may burst/lyse.
  • Hypertonic solution: Red blood cells lose water, shrink in size, and become crenated.

• Plant Cells:

  • Plasmolyzed: Cytoplasm & cell sap loses water, vacuoles decrease in size, and cell membrane shrinks away from the cell wall.
  • Bloated and Turgid: Cytoplasm & cell sap gains water, vacuole increases in size, and cell membrane pushes against the cell wall.

Osmosis in Plant Cells

• Isotonic conditions: no net movement of water.
• Hypotonic environment: vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.
• Hypertonic environment: vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell.

Turgor Pressure

• As water enters the plant cell, the vacuole and cytoplasm volume increases, the water molecules in the cytoplasm and vacuoles press against the cell membrane creating an outward pressure.

• The cell membrane exerts a pressure that pushes against the cell wall, which is called turgor pressure.

• To prevent over expansion of cell, cell wall exerts an opposite pressure.

• Plant cell in this state is said to be a turgid cell.

• Turgidity of cell with water is called turgor.

• Equilibrium: When turgor pressure becomes equal to the wall pressure, no exchanges of water takes place and equilibrium is established.

Importance of Turgor in Plant

1. Maintain the shape of soft tissues in plants; Keep stem upright (Eg of soft tissues: leaf, petals)
2. Control the opening and closing of stomata
   • When guard cells gains water → guard cell swells → curved more → stomata opens
   • When guard cells loses water → guard cell shrinks → curved less → stomata closes
3. Opens up the leaf and keep it flat.
   • Purpose: larger surface area exposed to sunlight so as to trap more sunlight for photosynthesis
4. Opens up the petals in flowers
5. Prevent plant from wilting.

Osmosis as Special Type of Diffusion

• Both osmosis and diffusion involve movement of particles from high concentration to low concentration.
• Osmosis is specific to water molecules and requires a partially permeable membrane.

Active Transport

• Definition: Net movement of ions/molecules from a region of lower solute concentration to a region of higher solute concentration (against a solute concentration gradient) through a partially permeable membrane with the expenditure of energy from cellular respiration.
• The solutes will pass through specific protein carriers on the cell membrane.
• Energy (ATP) is used to change the shape of the carrier protein in order to bring in the nutrients from lower concentration to higher concentration.

$A-P-P-P \rightarrow A-P-P + Energy$

Difference Between Facilitated Diffusion and Active Transport

• Active transport requires energy to move substances against their concentration gradient, while facilitated diffusion does not.

Examples of Active Transport in Biology

• Movement of digested food by cells in the small intestines into the bloodstream.
• Absorption of mineral salts by root hair cells.

Comparing Passive and Active Transport