Core Concepts in Cell Structure, Bioenergetics, and Photosynthesis

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life, and they are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a membrane-bound nucleus and organelles. Their DNA is located in a region called the nucleoid. Examples include Bacteria and Archaea.

• Eukaryotic cells have a true nucleus enclosed by a nuclear membrane and possess various membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Examples include plant, animal, fungal, and protist cells.

• Key differences: Eukaryotes are generally larger, have complex internal structures, and can form multicellular organisms, while prokaryotes are usually unicellular and structurally simpler.

Structure and Function of Organelles

Organelles are specialized structures within eukaryotic cells that perform distinct functions necessary for cellular survival and activity.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Found in plant cells; site of photosynthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

Endosymbiosis Theory

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

• Evidence: Both organelles have their own DNA, double membranes, and reproduce independently within the cell.

• Significance: This theory highlights the evolutionary relationship between prokaryotes and eukaryotes.

Bioenergetics and Enzyme Function

Potential vs. Kinetic Energy in Chemical Reactions

Energy in biological systems exists in two main forms:

• Potential energy: Stored energy, such as chemical energy in bonds.

• Kinetic energy: Energy of motion, such as the movement of molecules.

• During chemical reactions, potential energy stored in molecules can be converted to kinetic energy and vice versa.

Endergonic vs. Exergonic Reactions and Gibbs Free Energy

Chemical reactions are classified based on their energy requirements and changes in Gibbs free energy ($\Delta G$):

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

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

• Gibbs free energy equation:

$\Delta G = \Delta H - T\Delta S$

• Where $\Delta H$ is the change in enthalpy, $T$ is temperature in Kelvin, and $\Delta S$ is the change in entropy.

Enzyme Catalysis and Activation Energy

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

• Activation energy ($E_a$): The minimum energy required to initiate a chemical reaction.

• Enzymes bind substrates at their active sites, stabilizing the transition state and reducing $E_a$.

Factors Affecting Enzyme Function

Enzyme activity is influenced by environmental conditions:

• pH: Each enzyme has an optimal pH; deviations can denature the enzyme or alter its activity.

• Temperature: Higher temperatures increase reaction rates up to a point, but excessive heat can denature enzymes.

Cellular Respiration

Stages of Cellular Respiration

Cellular respiration is a multi-stage process that converts glucose into ATP:

1. Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate.

2. Pyruvate oxidation: Converts pyruvate to acetyl-CoA in the mitochondria.

3. Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, producing NADH and FADH2.

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation: Uses electrons from NADH and FADH2 to generate ATP.

Inputs and Outputs of Each Stage

Stage	Inputs	Outputs
Glycolysis	Glucose, 2 NAD+, 2 ADP	2 Pyruvate, 2 NADH, 2 ATP
Pyruvate Oxidation	2 Pyruvate, 2 NAD+	2 Acetyl-CoA, 2 NADH, 2 CO2
Citric Acid Cycle	2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP
ETC & Oxidative Phosphorylation	NADH, FADH2, O2	ATP, H2O, NAD+, FAD

Glycolysis: Two Phases

• Energy investment phase: Consumes ATP to phosphorylate glucose and its intermediates.

• Energy payoff phase: Produces ATP and NADH by substrate-level phosphorylation and oxidation.

Regulation of Glycolysis

• Key regulatory enzymes include hexokinase, phosphofructokinase, and pyruvate kinase.

• Regulation occurs via allosteric effectors and feedback inhibition.

Citric Acid Cycle

• Central metabolic pathway that oxidizes acetyl-CoA to CO2 and generates high-energy electron carriers (NADH, FADH2).

• Key role: Provides electrons for the ETC and intermediates for biosynthetic pathways.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• ETC is a series of protein complexes in the inner mitochondrial membrane.

• Electrons from NADH and FADH2 are transferred through the chain, pumping protons to create a gradient.

• ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi (chemiosmosis).

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Uses oxygen as the final electron acceptor; produces more ATP.

• Anaerobic respiration: Uses other molecules as electron acceptors; less efficient.

Fermentation

• Occurs when oxygen is absent; regenerates NAD+ for glycolysis.

• Types: Lactic acid fermentation (in animals), alcoholic fermentation (in yeast).

Photosynthesis

Inputs and Outputs of Photosynthesis

• Inputs: CO2, H2O, light energy

• Outputs: Glucose (C6H12O6), O2

• Occurs in chloroplasts; two major stages: light-dependent reactions and Calvin cycle (light-independent reactions).

Pigments and Light Absorption

• Chlorophyll: Main pigment absorbing light energy for photosynthesis; absorbs mainly blue and red wavelengths.

• Absorption spectrum determines which wavelengths are most effective for photosynthesis.

Z-Scheme and ATP/NADPH Generation

• The Z-scheme describes the flow of electrons through photosystem II and photosystem I, resulting in the production of ATP (via a proton gradient) and NADPH (via electron transport).

Connecting Light Reactions to the Calvin Cycle

• ATP and NADPH produced in the light-dependent reactions are used as energy and reducing power in the Calvin cycle to fix carbon dioxide into sugars.

The Calvin Cycle: Phases and Key Enzyme

• The Calvin cycle consists of three phases:

  • Fixation: CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme RuBisCO.

  • Reduction: ATP and NADPH are used to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P).

  • Regeneration: RuBP is regenerated from G3P, allowing the cycle to continue.

• RuBisCO: The key enzyme that catalyzes the first step of carbon fixation.

C4 and CAM Plant Adaptations

• C4 plants: Spatially separate carbon fixation and the Calvin cycle to minimize photorespiration (e.g., maize).

• CAM plants: Temporally separate these processes by fixing CO2 at night and running the Calvin cycle during the day (e.g., cacti).

• Both adaptations help plants survive in hot, dry environments by reducing water loss and photorespiration.