Definition:

• Active transport is the movement of molecules or ions across a membrane against their concentration gradient (from low to high concentration).
• This process requires energy from ATP generated during cellular respiration.

Key Characteristics of Active Transport

1. Moves Against the Concentration Gradient: Molecules or ions are transported from regions of low concentration to regions of high concentration, allowing cells to concentrate valuable nutrients or ions.
2. Requires Energy (ATP): Unlike passive processes, active transport relies on ATP to provide the energy needed to move substances against their natural gradient.
3. Involves Specific Carrier Proteins (Pumps): Specialized proteins, often referred to as pumps, undergo conformational changes fueled by ATP to move substances across the membrane selectively.

Mechanism of Active Transport

1. Specific Carrier Proteins: Each carrier protein is selective for a particular molecule or ion.
2. Energy Usage: ATP binds to the carrier protein, and upon hydrolysis, it releases energy that causes the protein to change shape.
3. Selective Binding: The carrier protein binds to the target molecule on one side of the membrane.
4. Shape Change and Transport: ATP hydrolysis drives the protein to shift shape, moving the bound molecule across the membrane.
5. Release and Reset: The molecule is released on the opposite side, and the carrier protein returns to its original shape, ready to repeat the process.

Types of Active Transport

1. Uniport: Transports a single type of molecule in one direction.
2. Symport: Transports two different molecules in the same direction simultaneously (e.g., Na⁺ and glucose cotransport in intestinal cells).
3. Antiport: Transports two molecules in opposite directions, such as the sodium-potassium pump in animal cells.

Sodium-Potassium Pump: A Case Study

The sodium-potassium pump (Na⁺/K⁺ pump) is essential for maintaining ion balance and generating the electrical conditions required for nerve impulses. It operates as follows:

1. Loading Sodium: The pump, open to the inside of the cell, binds three sodium ions (Na⁺).
2. ATP Phosphorylation: ATP donates a phosphate group to the pump, causing it to change shape and close to the cell interior.
3. Sodium Release and Potassium Binding: The pump opens to the outside, releasing Na⁺ ions and binding two potassium ions (K⁺) from the external environment.
4. Phosphate Detachment: The phosphate group detaches, allowing the pump to revert to its original shape and release K⁺ ions inside the cell.
5. Cycle Repetition: This process repeats, helping maintain a high internal concentration of K⁺ and a low internal concentration of Na⁺, critical for nerve cell function.

Importance of Active Transport in Biological Processes

1. Kidney Function: Active transport reabsorbs essential ions and molecules from kidney tubules back into the blood, helping to maintain electrolyte balance and blood composition.
2. Digestion: In the intestines, active transport moves glucose and amino acids from the digestive tract into the bloodstream, ensuring nutrient absorption even when concentrations are lower in the intestines.
3. Plant Transport Systems:
   • Phloem Loading: Moves sugars produced in leaves to the phloem for distribution throughout the plant.
   • Root Hair Cells: Absorb minerals (e.g., potassium, nitrate) from the soil, which are often at lower concentrations in the soil compared to the root cells.

Differences Between Active Transport and Facilitated Diffusion

Bulk Transport

Endocytosis

• Definition:
  • Endocytosis is the process by which the cell membrane engulfs material from the outside, forming a vesicle (endocytic vacuole) around it to bring substances into the cell.
  • Endocytosis requires ATP.

Types of Endocytosis:

1. Phagocytosis (“Cell Eating”)
   • Description: Involves the intake of large, solid particles or whole cells.
   • Specialized Cells: Cells that perform phagocytosis are called phagocytes.
   • Example: White blood cells (phagocytes) engulf bacteria or debris, enclosing them in phagocytic vacuoles for digestion.
2. Pinocytosis (“Cell Drinking”)
   • Description: Involves the intake of extracellular fluid and dissolved substances through small vesicles.
   • Subtypes:
     • Micropinocytosis: Formation of very small vesicles to take in extracellular fluids.
   • Example: Cells absorbing extracellular fluids or nutrients by forming small vesicles around the fluid.

Exocytosis

• Definition:
  • Exocytosis is the process by which materials are expelled from the cell, essentially the reverse of endocytosis.
  • Exocytosis requires ATP.

Process:

1. Packaging in Vesicles: Materials to be expelled, such as enzymes, hormones, or structural components, are packaged into secretory vesicles by the Golgi apparatus.
2. Movement to the Membrane: These vesicles move to and fuse with the cell membrane.
3. Release: The vesicle contents are released outside the cell.

• Example: Pancreatic cells secrete digestive enzymes into the digestive tract via exocytosis.

Exam Advice:

1. Energy Requirement: Remember that endocytosis, exocytosis, and other active transport processes all require ATP from cellular respiration.
2. Specify Terms Carefully:
   • When discussing bulk transport, explicitly use terms like endocytosis or exocytosis.
   • For endocytosis, further specify the type (phagocytosis for solids, pinocytosis for liquids) to ensure full marks.

Summary Table of Bulk Transport