Cell Transport Mechanisms: Diffusion, Osmosis, and Active Transport

OSMOSIS, DIFFUSION AND ACTIVE TRANSPORT

These are the primary processes by which substances enter and leave cells.

Three Processes

• Diffusion
• Osmosis
• Active Transport

Requirements of Diffusion and Osmosis

• A concentration gradient
• Molecules/particles

Concentration Gradient

The concentration gradient refers to the difference in the number of particles between two or more environments, typically inside and outside the cell.

Diffusion

Diffusion is the movement of molecules or particles (solute/gas) from a region of high concentration to a region of low concentration. This occurs down the concentration gradient and is passive, meaning it doesn't require energy expenditure.

Diffusion Illustration

• High concentration area separated by a membrane from a low concentration area.
• Molecules move from high to low concentration.
• The concentration gradient exists over the membrane.

Rate of Diffusion

The rate of diffusion depends on several factors:

• Concentration gradient: A higher concentration difference leads to faster diffusion.
• Distance: Shorter distances result in faster diffusion.
• Surface area to volume ratio: A higher surface area to volume ratio increases the rate of diffusion.
• Temperature: Higher temperatures (more kinetic energy) lead to faster diffusion.

Osmosis

Osmosis is the diffusion of water through a semi-permeable membrane from a region of high water potential to a region of lower water potential. It is a passive process, meaning the cell does not expend energy.

Concentration of Solutions

• Balanced: The concentration of solute is the same inside and outside the cell.
• Concentrated (Hypertonic): The outer environment has a higher solute concentration.
• Dilute (Hypotonic): The outer environment has a lower solute concentration.

Effect of Osmosis on Animal Cells

• Hypertonic Solution:
  • Red blood cells have higher water potential than the solution.
  • Net movement of water out of the cell.
  • Cells shrivel.
• Isotonic Solution:
  • Water potential is equal between red blood cell and solution.
  • No net movement of water.
  • Normal cells.
• Hypotonic Solution:
  • Red blood cells have lower water potential than the solution.
  • Net movement of water into the cell.
  • Cells swell and may lyse (burst).

Effect of Osmosis on Plant Cells

Plant cells do not burst due to the presence of a cell wall. When a plant cell gains water through osmosis, the cytoplasm and vacuole swell. The cell wall prevents the cell from bursting by withstanding the water pressure. Water moves out of the cytoplasm and vacuole through osmosis, causing them to shrink away from the cell wall. In a balanced state, the number of water molecules moving in equals those moving out, resulting in no net change.

Active Transport

Active transport is the movement of solutes against their concentration gradient, from a region of low concentration to a region of high concentration. This process is active, requiring energy or ATP.

Characteristics of Active Transport

• The process is not reversible and occurs only in living things.
• Cells carrying out active transport are typically metabolically active and contain many mitochondria.
• Root cells and villi absorb nutrients through active transport.

Active Transport Mechanism

1. ATP binds to a protein gate/protein carrier.
2. This changes the shape and position of the protein carrier.
3. The protein carrier moves the particles from a region of low concentration to a region of high concentration.