Cell Structure, Metabolism, and Photosynthesis: Key Concepts in General Biology

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

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

• Prokaryotic cells lack a nucleus and membrane-bound organelles. Their genetic material is located in the nucleoid region.

• Eukaryotic cells possess a true nucleus and various membrane-bound organelles, such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Example: Bacteria are prokaryotes; plants and animals are eukaryotes.

Structure and Function of Organelles

Organelles perform specialized functions that contribute to overall cellular activity.

• Nucleus: Stores genetic information and coordinates cell activities.

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis in plant cells.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

Endosymbiosis Theory

The endosymbiosis theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence includes double membranes, their own DNA, and similarities to prokaryotic ribosomes.

Energy and Metabolism

Potential vs. Kinetic Energy in Chemical Reactions

Potential energy is stored energy, while kinetic energy is energy of motion. In chemical reactions, potential energy is stored in chemical bonds and released as kinetic energy during reactions.

Endergonic vs. Exergonic Reactions & Gibbs Free Energy

Reactions are classified based on energy changes:

• Endergonic reactions: Require energy input; $\Delta G > 0$.

• Exergonic reactions: Release energy; $\Delta G < 0$.

• Gibbs free energy equation: $\Delta G = \Delta H - T\Delta S$

Enzyme Function and Catalysis

Enzymes are biological catalysts that speed up reactions by lowering the activation energy.

• Enzyme activity is affected by pH and temperature.

• Extreme conditions can denature enzymes, reducing their function.

Cellular Respiration

Stages of Cellular Respiration

Cellular respiration consists of four interconnected stages:

• Glycolysis

• Pyruvate oxidation

• Citric acid cycle (Krebs cycle)

• Electron transport chain (ETC) and oxidative phosphorylation

Glycolysis

Glycolysis occurs in the cytoplasm and consists of two phases:

• Energy investment phase: Consumes ATP to phosphorylate glucose.

• Energy payoff phase: Produces ATP and NADH.

Citric Acid Cycle

The citric acid cycle occurs in the mitochondrial matrix and completes the oxidation of glucose derivatives, generating NADH, FADH2, and ATP.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC uses electrons from NADH and FADH2 to create a proton gradient across the inner mitochondrial membrane, driving ATP synthesis via oxidative phosphorylation.

Aerobic vs. Anaerobic Respiration & Fermentation

• Aerobic respiration: Uses oxygen as the final electron acceptor.

• Anaerobic respiration: Uses other molecules as electron acceptors.

• Fermentation: Occurs when oxygen is absent; regenerates NAD+ by converting pyruvate into lactate or ethanol.

Photosynthesis

Inputs and Outputs of Photosynthesis

Photosynthesis occurs in chloroplasts and consists of two major stages:

• Light-dependent reactions: Occur in the thylakoid membranes; produce ATP and NADPH.

• Calvin cycle (light-independent reactions): Occur in the stroma; use ATP and NADPH to fix CO2 into sugars.

Pigments and Light Absorption

Chlorophyll and other pigments absorb light energy, which excites electrons and initiates photosynthesis. The absorption spectrum determines which wavelengths are most effective.

Z-Scheme and Electron Transport

The Z-scheme describes the flow of electrons through photosystems II and I, generating ATP (via a proton gradient) and NADPH (via electron transport).

Connecting Light Reactions to Calvin Cycle

ATP and NADPH produced in the light-dependent reactions are used as energy and reducing power in the Calvin cycle.

Calvin Cycle Phases and RuBisCO

The Calvin cycle has three phases:

• Fixation: CO2 is attached to RuBP by the enzyme RuBisCO.

• Reduction: ATP and NADPH are used to convert 3-PGA into G3P.

• Regeneration: RuBP is regenerated for the cycle to continue.

C4 and CAM Plants

C4 and CAM plants have adaptations to minimize photorespiration by separating carbon fixation from the Calvin cycle.

• C4 plants: Spatial separation; CO2 fixation occurs in mesophyll cells, Calvin cycle in bundle sheath cells.

• CAM plants: Temporal separation; CO2 fixation at night, Calvin cycle during the day.

Summary Table: Key Differences in Cell Types and Metabolic Pathways

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Absent	Present
Examples	Bacteria, Archaea	Plants, Animals, Fungi

Pathway	Inputs	Outputs	Location
Glycolysis	Glucose, 2 ATP	2 Pyruvate, 4 ATP, 2 NADH	Cytoplasm
Citric Acid Cycle	Acetyl-CoA	CO2, NADH, FADH2, ATP	Mitochondrial Matrix
Photosynthesis (Light Reactions)	Light, H2O	ATP, NADPH, O2	Thylakoid Membrane
Calvin Cycle	CO2, ATP, NADPH	Glucose, ADP, NADP+	Stroma

Additional info: Academic context and explanations have been expanded for clarity and completeness, including definitions, examples, and summary tables for comparison.