Introduction to Biomolecules, Cell Structure, and Cellular Respiration

Exam Tip: When identifying nucleotides on an exam, always look at the 2' carbon of the pentose sugar. If there is an $-OH$ group, it is ribose (RNA); if there is only an $-H$, it is deoxyribose (DNA).

Energy and Chemical Bonds

Energy in biological systems is stored within chemical bonds. Specifically, C-H (carbon-hydrogen) bonds are the primary source of potential energy in organic molecules. When comparing the energy density of different molecules, triglycerides (fats) store significantly more energy per gram than glucose or proteins because they contain a higher proportion of these energy-rich C-H bonds.

Cell Structure and the Endomembrane System

Eukaryotic cells are highly compartmentalized, allowing different chemical reactions to occur simultaneously without interference. A critical component of this organization is the endomembrane system, a network of membranes that works together to modify, package, and transport proteins.

Proteins destined for secretion follow a specific pathway: they are synthesized in the Rough Endoplasmic Reticulum (ER) (which is "rough" due to attached ribosomes), sent to the Golgi Apparatus for modification and packaging, and finally transported via vesicles to the plasma membrane. Through the process of exocytosis, these vesicles fuse with the membrane to release their contents outside the cell.

Other essential organelles include:

• Nucleus: The repository of genetic information.
• Smooth ER: Responsible for lipid synthesis and detoxification.
• Lysosomes: Contain digestive enzymes to break down waste and macromolecules.
• Mitochondria & Chloroplasts: The energy converters of the cell.

Notably, mitochondria and chloroplasts are surrounded by a double phospholipid bilayer. This creates an inter-membrane space, which is vital for establishing the electrochemical gradients used to produce ATP.

Cellular Respiration: Harvesting Energy

Cellular respiration is the multi-step metabolic process by which cells oxidize glucose to produce ATP. This process relies heavily on electron carriers, specifically NADH and FADH2. These molecules act as shuttles: in their oxidized state ($NAD^+$ and $FAD$), they are ready to accept electrons; in their reduced state (NADH and $FADH_2$), they carry high-energy electrons to the electron transport chain.

NADH is more efficient than $FADH_2$ because it enters the electron transport chain at an earlier point, ultimately yielding more ATP per molecule.

The Stages of Respiration

The breakdown of glucose occurs in several distinct stages:

1. Glycolysis: Occurs in the cytosol. It breaks down glucose into pyruvate, producing a small amount of ATP and NADH.
2. Pyruvate Oxidation: Pyruvate enters the mitochondrial matrix and is converted into Acetyl-CoA, releasing $CO_2$ and producing NADH.
3. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix. It completes the oxidation of organic fuel, producing $CO_2$, ATP, NADH, and $FADH_2$.
4. Oxidative Phosphorylation: This occurs in the inner mitochondrial membrane and consists of two parts:
   • Electron Transport Chain (ETC): Electrons from NADH and $FADH_2$ are passed through a series of proteins. The energy released is used to pump $H^+$ ions into the inter-membrane space, creating a proton gradient.
   • Chemiosmosis: The $H^+$ ions flow back into the matrix through ATP synthase, a molecular motor that uses the flow of ions to catalyze the synthesis of ATP from ADP and inorganic phosphate.

If oxygen is absent, the cell cannot perform oxidative phosphorylation. To continue producing at least a small amount of ATP, the cell undergoes fermentation, which regenerates the $NAD^+$ needed to keep glycolysis running.

Exam Tip: Remember the relationship between oxidation and reduction. Oxidation is the loss of electrons (e.g., glucose is oxidized), while reduction is the gain of electrons (e.g., $NAD^+$ is reduced to NADH).