General Biology: Cell Structure, Metabolism, and Photosynthesis Study Guide

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are the basic units of life and can be 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: Bacteria and Archaea.

• Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Plants, Animals, Fungi, and Protists.

• Key differences: Eukaryotes are generally larger, have complex internal structures, and can form multicellular organisms.

Structure and Function of Organelles

Organelles are specialized structures within eukaryotic cells that perform distinct functions necessary for cellular life.

• Nucleus: Stores genetic material (DNA) and coordinates cell 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 to break down waste.

Organelle Function and Cellular Activity

Each organelle contributes to the overall function and survival of the cell. For example, mitochondria provide energy, while the nucleus controls gene expression.

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.

Energy and Metabolism

Potential vs. Kinetic Energy in Chemical Reactions

Energy exists in two main forms:

• Potential energy: Stored energy due to position or structure (e.g., chemical bonds).

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• In chemical reactions, potential energy in bonds is converted to kinetic energy as molecules move and react.

Endergonic vs. Exergonic Reactions and Gibbs Free Energy

Reactions are classified by their energy changes:

• Endergonic reactions: Require energy input; products have more free energy than reactants ($\Delta G > 0$).

• Exergonic reactions: Release energy; products have less free energy than reactants ($\Delta G < 0$).

• Gibbs free energy ($\Delta G$): Predicts whether a reaction is spontaneous.

Enzyme Catalysis and Activation Energy

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

• They bind substrates at the active site, facilitating the conversion to products.

• Enzymes are not consumed in the reaction.

Factors Affecting Enzyme Function

• pH: Each enzyme has an optimal pH; deviations can denature the enzyme.

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

Cellular Respiration

Stages of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose. It consists of four interconnected stages:

1. Glycolysis

2. Pyruvate oxidation

3. Citric acid cycle (Krebs cycle)

4. Electron transport chain (ETC) and oxidative phosphorylation

Inputs and Outputs of Each Stage

Stage	Inputs	Outputs
Glycolysis	Glucose, 2 ATP, 2 NAD+	2 Pyruvate, 4 ATP (net 2), 2 NADH
Pyruvate Oxidation	2 Pyruvate, 2 NAD+	2 Acetyl-CoA, 2 CO2, 2 NADH
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	~26-28 ATP, H2O

Phases of Glycolysis

• Energy investment phase: Uses ATP to phosphorylate glucose.

• Energy payoff phase: Produces ATP and NADH.

Regulation of Glycolysis

• Key regulatory enzyme: Phosphofructokinase (PFK).

• Regulated by ATP (inhibitor) and AMP (activator).

Citric Acid Cycle (Krebs Cycle)

• Completes the oxidation of glucose derivatives.

• Generates NADH and FADH2 for the ETC.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• ETC uses electrons from NADH and FADH2 to pump protons, creating a gradient.

• ATP synthase uses this gradient to produce ATP (chemiosmosis).

Aerobic vs. Anaerobic Respiration

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

• Anaerobic: Uses other molecules (e.g., nitrate, sulfate) or fermentation; less efficient.

Fermentation

• Occurs when oxygen is absent.

• Regenerates NAD+ for glycolysis.

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

Photosynthesis

Inputs and Outputs of Photosynthesis

• Inputs: CO2, H2O, light energy

• Outputs: Glucose (C6H12O6), O2

• Occurs in chloroplasts: Light-dependent reactions (thylakoid membrane), Calvin cycle (stroma)

Pigments and Light Absorption

• Chlorophyll absorbs light most efficiently in the blue and red wavelengths.

• Absorption spectrum determines which wavelengths are used for photosynthesis.

Z-Scheme and ATP/NADPH Generation

• The Z-scheme describes the flow of electrons from water through photosystem II and I to NADP+.

• ATP is generated via a proton gradient (photophosphorylation), and NADPH is produced via electron transport.

Connecting Light Reactions to the Calvin Cycle

• ATP and NADPH produced in the light reactions are used in the Calvin cycle to fix carbon dioxide into sugars.

The Calvin Cycle: Phases and Key Enzyme

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

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

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

C4 and CAM Plants: Adaptations to Minimize Photorespiration

• C4 plants: Spatially separate carbon fixation and the Calvin cycle (mesophyll and bundle sheath cells).

• CAM plants: Temporally separate carbon fixation (night) and the Calvin cycle (day).

• Both adaptations reduce photorespiration and increase efficiency in hot, dry environments.