BIOL110: Module 2 - Lecture 2: Cellular Processes That Use Energy

Cellular Role and Structure of Phospholipid Membranes

• Cells are enclosed by phospholipid membranes, which enable selective permeability.

• Phospholipids in Water: Phospholipids spontaneously form a bilayer when placed in water. The polar heads form hydrogen bonds with water, while the non-polar (hydrophobic) tails create an interior.

• Membrane Selectivity: Biological membranes are selectively permeable, meaning only small, hydrophobic molecules (e.g., $O<em>{2}$, $CO</em>{2}$) can easily diffuse through the lipid bilayer. Larger or polar molecules (e.g., ions, carbohydrates, amino acids) require assistance from transport proteins to cross, which aids in cellular regulation.

Mechanisms of Molecular Transport Across Membranes

• Passive Movement: Occurs when molecules move along their concentration gradient without the input of energy.

  • Small non-polar molecules like $O<em>{2}$ and $CO</em>{2}$ can freely diffuse across the membrane.

  • Water, though small and polar, utilizes both simple diffusion through the bilayer and facilitated diffusion via aquaporins (water channel proteins).

• Transport Proteins: Membrane proteins facilitate the movement of ions and nutrient molecules.

  • Channel Proteins: Form pores for ions and molecules to pass through.

  • Carrier Proteins: Bind to specific substances, change shape, and shuttle them across the membrane. These are often specific to the molecules they transport.

• Bulk Transport (for large molecules):

  • Endocytosis: The plasma membrane folds inward, engulfing extracellular material (forming a vesicle).

  • Exocytosis: A vesicle fuses with the plasma membrane to release its contents outside the cell.

Moving Molecules Against a Concentration Gradient (Active Transport)

• Active Transport: This process requires energy (usually from ATP) to move substances against their concentration gradient (from an area of low concentration to high concentration).

• ATP Use in Active Transport: ATP is crucial for powering these processes.

  • $Na^{+}/K^{+}$ Pump: A significant consumer of ATP in resting mammals, it maintains the resting potential of nerve cells by moving $Na^{+}$ and $K^{+}$ ions against their gradients. It utilizes approximately $60\%$ of the ATP produced in the brain.

  • Contribution to BMR: Pumping ions (especially $Na^{+}$ and $K^{+}$) across cell membranes is the largest contributor to Basal Metabolic Rate (BMR), accounting for $19-28\%$ of ATP usage.

ATP in Protein Synthesis and Movement

• Energy Use in Protein Synthesis:

  • Transcription (mRNA synthesis): Utilizes nucleoside triphosphates (ATP, GTP, UTP, CTP). ATP is specifically used to unwind the DNA template.

  • Translation (polypeptide chain formation): ATP energy is necessary to attach each amino acid to its corresponding tRNA. GTP facilitates the movement of the ribosome along the mRNA for protein elongation.

  • Protein synthesis accounts for $25-30\%$ of ATP usage in a whole animal.

• Transport of Proteins:

  • Vesicular Transport: ATP is utilized to move protein vesicles from the endoplasmic reticulum (ER) to the Golgi apparatus and subsequently out of the cell or within larger cells.

  • Cytoskeleton and Motor Proteins: The cytoskeleton provides a network for intracellular transport.

  • Motor proteins, such as kinesin and dynein, utilize ATP to move along cytoskeletal filaments (microtubules). Kinesin moves vesicles, and both kinesin and dynein are crucial for chromosome separation during cell division.

  • Myosin motor proteins interact with actin filaments (part of the cytoskeleton), using ATP to drive muscle contraction through a series of binding and releasing mechanisms.

  • Energy used by the cytoskeleton (including muscle contraction) accounts for $2-8\%$ of total ATP usage.