Week 5 Lecture 1 - Metabolism

What is Life?

• A bag of chemicals is just that, but a cell is living, leading to the question of how we define life.
• Living cells and organisms exhibit various complex processes, which help define life.
• Three fundamental types of work underlie all life processes.

Three Types of Work

• Movement
• Membrane Transport
• Chemical Synthesis
• These processes enable the characteristics of life and differentiate living from artificial entities.

Movement

• Cells move organelles around (vesicles).
• Vesicles move within cells (e.g., in a nerve cell) along microtubular tracks.
• Vesicles transport proteins and membranes to specific locations, from the nucleus to other organelles or outside the cell, and vice versa.
• Vesicular trafficking is precisely regulated.
• Endocrine cells traffic hormones (e.g., estrogen, epinephrine, insulin) that are released and travel around the body.
• Large cells mix organelles, nutrients, and biomolecules through cytoplasmic streaming, common in plant, fungal, protist cells, and animal oocytes/embryos.
• Intracellular movements are powered by motor proteins like kinesin, dynein, and myosin, which move along microtubules and microfilaments (cytoskeleton).
• Kinesin, discovered after 1983, is now considered a crucial protein in eukaryotic cells.
• Entire cells move, such as neutrophils chasing bacteria.
• Cell motility is essential during development, where cells move to their correct locations in an embryo.
• Cells move by pushing the front end forward with their actin cytoskeleton and pulling the back end.
• Other cells use cilia or flagella to swim (e.g., Tetrahymena).
• Cilia also move water past cells (e.g., in airways).
• Multicellular animals move due to muscle contraction.

Membrane Transport

• Involves the movement of molecules or ions across membranes.
• Important for maintaining osmotic balance; otherwise, cells can burst or shrivel.
• Crucial for nutrient uptake, waste excretion, nerve signaling, and muscle contraction.
• Approximately 50% of the brain's energy is used to move Na^+ and K^+ across cell membranes.
• The sodium pump (Na+/K+ ATPase) is vital in nerve cells and nerve impulses.
• In muscle cells, the Ca^{2+} pump (Ca^{2+}-ATPase) maintains low intracellular Ca^{2+} levels, allowing cells to respond to sudden increases in Ca^{2+} and contract.

Chemical Synthesis

• Involves making macromolecules from monomer subunits.
• Polysaccharides from sugars, proteins from amino acids, and nucleic acids from nucleotides.

Transformation of Molecules

• Cells perform work by transforming molecules and energy.
• Molecules are modified via chemical reactions within cells.
• Reactions are controlled by enzymes, which are typically specific and catalyze reactions by decreasing the time to reach equilibrium.
• In glycolysis, glucose is converted to glucose-6-phosphate, catalyzed by hexokinase.
• In a cell, this reaction doesn't reach equilibrium because glucose-6-phosphate is used in the subsequent reaction in the metabolic pathway.

Metabolism

• Metabolism encompasses all metabolic pathways in a cell.
• Metabolic pathways can be catabolic (breaking down complex molecules into simpler ones, spontaneous, and yielding energy).
• Or anabolic (building complex molecules from simple ones, non-spontaneous, and requiring energy).
• Energy coupling involves using energy from catabolic pathways to drive anabolic pathways.
• Key energy carriers include ATP, NADH, and NADPH.
• Pathways are regulated based on the cell or organism's energy needs through enzymes, allosterically, covalently, or genetically.

Metabolic Regulation Examples

• After skipping breakfast and eating a crème egg, low energy levels cause the sugar in the egg to be broken down via catabolic pathways for energy.
• Enzymes that catalyze sugar breakdown are activated based on the energy status.
• After eating breakfast, high energy levels mean the glucose from the egg is converted to glycogen (a storage carbohydrate) through anabolic pathways.
• Enzymes that catalyze sugar breakdown are switched off.
• Cells control their metabolism by regulating enzymes, ensuring they have the materials and energy to perform work and stay alive.