Transport Mechanisms in Biology

Transport Mechanisms

• Transport mechanisms are processes that enable substances to move across cell membranes.
• Different types of transport mechanisms exist, including passive and active transport.
• Passive transport does not require energy input. It occurs along the concentration gradient.
• Active transport requires energy input, often in the form of ATP. It moves substances against their concentration gradient.

Learning Objectives

• Enumerate various types of transport mechanisms across cell membranes.
• Explain each transport mechanism, including passive and active transport.
• Highlight the differences in function between passive and active transport methods.
• Discuss their physiological importance.

Solutes and Membranes

• Solutes, such as ions, glucose, and gases, can cross cell membranes via various routes.
• Diffusion (simple or facilitated) is one way solutes cross the cell membrane.
• Active transport is characterized by primary and secondary processes, as well as endocytosis and exocytosis.

Diffusion

• Diffusion is a passive process where substances move from areas of higher concentration to regions of lower concentration.
• Factors influencing diffusion include molecular size, lipid solubility, and charge.
• Diffusion across cell membranes can occur via simple diffusion or facilitated mechanisms.

Simple Diffusion

• Simple diffusion involves substances crossing the lipid bilayer directly.
• Lipid-soluble substances (e.g., O₂, N₂, alcohols), water, and small uncharged water-soluble molecules (e.g., CO₂) can undergo simple diffusion.
• Factors affecting simple diffusion include lipid solubility, concentration gradient, surface area, temperature, and membrane thickness.

Facilitated Diffusion

• Facilitated diffusion is a passive process but requires transport proteins.
• Specific carrier proteins (e.g., glucose transporters GLUT) are crucial for the movement of certain molecules.
• Various types of facilitated diffusion exist, including uniport, symport, and antiport.
• These mechanisms are regulated by factors like hormones.

Characteristics of Facilitated Diffusion

• Requires a carrier protein.
• Specific to certain substances.
• May demonstrate competitive inhibition by similar molecules.
• Exhibit saturation, where the rate peaks due to maximum carrier use.

Channel Proteins

• Channel proteins provide pathways for small, dissolved particles (especially ions) to passively cross cell membranes.

Primary Active Transport

• Primary active transport moves substances against their concentration or electrical gradients, demanding energy.
• Carrier proteins, like the Na⁺-K⁺ pump and H⁺ pump, are vital in this process.
• ATP hydrolysis fuels primary active transport.

Na⁺-K⁺ Pump

• The Na⁺-K⁺ pump is a crucial example of primary active transport, maintaining ion gradients across cell membranes, creating electrical gradients across the membrane, and controlling cell volume.
• It uses energy from ATP to move three sodium ions out and two potassium ions into the cell.

Secondary Active Transport

• Secondary active transport harnesses the electrochemical gradient established by primary active transport (like the Na⁺ gradient created by the Na⁺-K⁺ pump) to move other substances across the membrane.
• It's a co-transport mechanism using a carrier protein to move two or more substances simultaneously.
• This process is frequently involved in nutrient uptake and ion regulation.

Endocytosis and Exocytosis

• Endocytosis and exocytosis are mechanisms for transporting large substances into or out of cells.
• Pinocytosis (cell drinking) and phagocytosis (cell eating) are types of endocytosis, while exocytosis is a reverse process.
• Endocytosis involves internalizing substances by forming vesicles, while exocytosis involves releasing substances by fusing vesicles with the membrane.

Filtration

• Filtration is a passive transport mechanism driven by hydrostatic pressure.
• It forces water and small solutes through a selectively permeable membrane, like in the kidneys.
• The rate of filtration depends on pressure gradients and membrane properties.

Osmosis

• Osmosis describes the passive movement of water across a semipermeable membrane from a region of lower solute concentration to one of higher concentration.
• The osmotic pressure counteracts such movement.
• Osmosis is governed by the number of solute particles in a solution.

Osmolarity and Solutions

• Osmolarity refers to the total concentration of osmotically active solutes in a solution.
• Isotonic solutions have the same osmolarity as the fluids inside cells, while hypertonic solutions have a higher osmolarity and hypotonic solutions have a lower osmolarity.