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

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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.
Eukaryotic cells have a true nucleus enclosed by a nuclear membrane and possess various membrane-bound organelles.
Examples: Bacteria and Archaea are prokaryotes; plants, animals, fungi, and protists are eukaryotes.

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: Conduct photosynthesis in plant cells.
Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.
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 by performing specialized tasks.

Energy production: Mitochondria and chloroplasts generate ATP through cellular respiration and photosynthesis, respectively.
Protein synthesis and transport: Ribosomes, ER, and Golgi apparatus work together to produce and distribute proteins.

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 Enzymes in Biological Systems

Potential vs. Kinetic Energy in Chemical Reactions

Energy exists in two main forms relevant to biology: potential and kinetic.

Potential energy: Stored energy due to position or structure (e.g., chemical bonds).
Kinetic energy: Energy of motion (e.g., movement of molecules).
Example: Glucose has high potential energy; during cellular respiration, this is converted to kinetic energy and then to ATP.

Endergonic vs. Exergonic Reactions and Gibbs Free Energy

Chemical reactions can be classified based on energy changes, using Gibbs free energy ().

Exergonic reactions: Release energy; (spontaneous).
Endergonic reactions: Require energy input; (non-spontaneous).
Equation:

Enzyme Catalysis and Activation Energy

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

Activation energy: The minimum energy needed to start a reaction.
Enzyme function: Enzymes bind substrates at their active site, stabilizing the transition state.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental conditions.

pH: Each enzyme has an optimal pH range.
Temperature: Higher temperatures increase activity up to a point, after which denaturation occurs.

Cellular Respiration

Stages of Cellular Respiration

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

Glycolysis
Pyruvate oxidation
Citric acid cycle (Krebs cycle)
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 NADH, 2 CO2
Citric Acid Cycle	2 Acetyl-CoA, 6 NAD+, 2 FAD	4 CO2, 6 NADH, 2 FADH2, 2 ATP
ETC & Oxidative Phosphorylation	NADH, FADH2, O2	ATP, H2O, NAD+, FAD

Phases and Regulation of Glycolysis

Glycolysis consists of two phases: the energy investment phase and the energy payoff phase.

Regulation: Key enzymes such as phosphofructokinase regulate glycolysis in response to cellular energy levels.

Citric Acid Cycle

The citric acid cycle completes the oxidation of glucose derivatives, producing NADH and FADH2 for the ETC.

Key role: Generates high-energy electron carriers and CO2.

Electron Transport Chain and Oxidative Phosphorylation

The ETC uses electrons from NADH and FADH2 to create a proton gradient, driving ATP synthesis via ATP synthase.

Oxidative phosphorylation: The process of ATP generation using the energy from the proton gradient.

Aerobic vs. Anaerobic Respiration and Fermentation

Cells can generate ATP with or without oxygen.

Aerobic respiration: Uses oxygen as the final electron acceptor.
Anaerobic respiration: Uses other molecules as electron acceptors.
Fermentation: Regenerates NAD+ in the absence of oxygen, producing lactic acid or ethanol.

Photosynthesis

Inputs, Outputs, and Stages of Photosynthesis

Photosynthesis converts light energy into chemical energy in two main stages: the light-dependent reactions and the Calvin cycle.

Inputs: Light, H2O, CO2
Outputs: O2, ATP, NADPH, glucose
Location: Light reactions occur in the thylakoid membranes; Calvin cycle occurs in the stroma of chloroplasts.

Pigments and Light Absorption

Pigments such as chlorophyll absorb specific wavelengths of light, initiating the light-dependent reactions.

Chlorophyll: Absorbs red and blue light, reflects green.
Absorption spectrum: Range of wavelengths absorbed by a pigment.

The Z-Scheme and ATP/NADPH Generation

The Z-scheme describes the flow of electrons through photosystems II and I, leading to the production of ATP and NADPH.

ATP: Generated via a proton gradient (chemiosmosis).
NADPH: Produced by the reduction of NADP+ at the end of the electron transport chain.

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, Reduction, and Regeneration.

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 for the cycle to continue.
RuBisCO: The key enzyme that catalyzes the first step of carbon fixation.

C4 and CAM Plant Adaptations

C4 and CAM plants have evolved mechanisms to minimize photorespiration by separating carbon fixation from the Calvin cycle in space (C4) or time (CAM).

C4 plants: Fix CO2 in mesophyll cells and transfer it to bundle-sheath cells where the Calvin cycle occurs.
CAM plants: Fix CO2 at night and store it as malate, releasing it during the day for the Calvin cycle.