Cells, Organelles, Metabolism, Enzymes, and Energy in Biology

Unit 2: Cells and Organelles, Metabolism, Enzymes, Photosynthesis, and Cellular Respiration

Cells and Organelles

Cells are the fundamental units of life. They can be classified as prokaryotic or eukaryotic based on their structural features and complexity.

• Prokaryotic cells are generally smaller and simpler. They lack membrane-bound organelles, including a true nucleus. Their DNA is found in a single circular chromosome located in the nucleoid region.

• Eukaryotic cells are larger, contain a nucleus that houses linear chromosomes, and possess membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Prokaryotes reproduce primarily through binary fission, while eukaryotes use mitosis and meiosis for cell division.

Organelle	Structure	Organisms	Function
Nucleus	Membrane-bound, contains DNA	Eukaryotes	Stores genetic information, controls cell activities
Ribosome	Non-membrane-bound, made of RNA and protein	All cells	Protein synthesis
Endoplasmic Reticulum (ER)	Network of membranes (rough and smooth)	Eukaryotes	Protein and lipid synthesis, transport
Golgi Apparatus	Stack of flattened membranes	Eukaryotes	Modifies, sorts, and packages proteins and lipids
Lysosome	Membrane-bound vesicle with enzymes	Eukaryotes (mainly animals)	Digestion and waste removal
Mitochondrion	Double-membraned, contains own DNA	Eukaryotes	ATP production (cellular respiration)
Chloroplast	Double-membraned, contains chlorophyll	Plants, algae	Photosynthesis
Cell membrane	Phospholipid bilayer	All cells	Controls entry/exit of substances

Energy and Thermodynamics in Biology

Energy is essential for all biological processes. Organisms exchange matter and energy with their surroundings, and the study of these exchanges is governed by the laws of thermodynamics.

• Energy is the capacity to perform work or bring about some sort of change. It is not material and does not have mass or occupy space.

• Potential energy is stored energy (e.g., energy in chemical bonds, such as glucose).

• Kinetic energy is the energy of motion (e.g., a falling object, moving molecules).

• Entropy (S) is a measure of disorder or randomness. Nature favors an increase in entropy, meaning systems tend toward greater disorder.

Example: The breakdown of glucose (C6H12O6) into CO2 and H2O increases entropy and releases energy.

Key Thermodynamic Equations

• The First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• The Second Law of Thermodynamics: Every energy transfer increases the entropy of the universe.

The change in Gibbs free energy ($\Delta G$) determines whether a reaction is spontaneous:

$\Delta G = G_{\text{products}} - G_{\text{reactants}}$

• If $\Delta G < 0$, the reaction is exergonic (releases energy, spontaneous).

• If $\Delta G > 0$, the reaction is endergonic (requires energy input, non-spontaneous).

Types of Metabolic Reactions

Type	Description	Example
Anabolic	Builds larger, complex molecules from smaller ones; requires energy input	Protein synthesis
Catabolic	Breaks down complex molecules into smaller ones; releases energy	Cellular respiration
Oxidation	Loss of electrons or hydrogen atoms	Glucose oxidation
Reduction	Gain of electrons or hydrogen atoms	NAD+ to NADH

ATP and Energy Coupling

Adenosine triphosphate (ATP) is the primary energy currency of the cell. It stores energy in the high-energy phosphate bonds, which can be broken to release energy for cellular work.

• ATP consists of adenine, ribose, and three phosphate groups.

• The negative charges on the phosphate groups repel each other, creating high potential energy.

• Phosphorylation is the transfer of a phosphate group from ATP to another molecule, coupling exergonic and endergonic reactions.

Example: The hydrolysis of ATP ($\text{ATP} \rightarrow \text{ADP} + \text{P}_i$) releases energy that can drive endergonic reactions.

Enzymes: Structure, Function, and Regulation

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering the activation energy required for the reaction to proceed.

• Substrate: The reactant molecule(s) upon which an enzyme acts.

• Active site: The region of the enzyme where the substrate binds and the reaction occurs.

• Induced fit: The enzyme changes shape slightly to fit the substrate more closely upon binding.

• Enzymes are not consumed in the reaction and can be reused.

Factors Affecting Enzyme Activity

• Substrate concentration: Increasing substrate increases reaction rate until the enzyme is saturated.

• Temperature and pH: Each enzyme has an optimal temperature and pH; deviations can reduce activity or denature the enzyme.

• Inhibitors:

  • Competitive inhibitors: Bind to the active site, blocking substrate binding.

  • Allosteric (noncompetitive) inhibitors: Bind elsewhere, changing the enzyme's shape and reducing activity.

• Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, regulating the pathway's activity.

Cofactors and Coenzymes

• Cofactors: Inorganic molecules (e.g., metal ions) required for enzyme activity; can be detached.

• Coenzymes: Organic molecules (e.g., vitamins) that assist enzymes; can be detached.

• Prosthetic groups: Inorganic or organic molecules permanently attached to proteins.

Example: The enzyme hexokinase requires Mg2+ (a cofactor) to catalyze the phosphorylation of glucose in glycolysis.

Additional info: These notes provide foundational knowledge for understanding metabolism, photosynthesis, and cellular respiration, which are covered in subsequent sections of a General Biology course.