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 they are classified as either prokaryotic or eukaryotic based on their structural features.

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

• Eukaryotic cells: Possess a true nucleus and various membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Examples include Animal, Plant, Fungi, and Protist cells.

Key differences:

• Size: Eukaryotic cells are generally larger.

• Complexity: Eukaryotic cells are more structurally complex.

• Cell division: Prokaryotes divide by binary fission; eukaryotes by mitosis/meiosis.

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; produces ATP.

• 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.

• Lysosomes: Contain digestive enzymes for breaking down waste.

Function and Cellular Activity:

• Each organelle contributes to the cell's overall function, such as energy production, synthesis of biomolecules, and waste removal.

Endosymbiosis Theory

The endosymbiosis theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

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

• Suggests that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Cellular Metabolism and Enzyme Function

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).

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

Endergonic vs. Exergonic Reactions and Gibbs Free Energy

Chemical reactions are classified based on energy changes.

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

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

• Gibbs free energy ($riangle G$): Indicates the spontaneity of a reaction.

Enzyme Catalysis and Activation Energy

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

• Activation energy: The minimum energy required to start a reaction.

• Enzymes bind substrates at their active site, stabilizing the transition state and reducing activation energy.

Factors Affecting Enzyme Function

Enzyme activity is influenced by environmental conditions.

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

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

Cellular Respiration

Stages of Cellular Respiration

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

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

• Pyruvate oxidation: Converts pyruvate to acetyl-CoA.

• Citric acid cycle (Krebs cycle): Occurs in mitochondria; produces NADH, FADH2, and ATP.

• Electron transport chain (ETC) and oxidative phosphorylation: Uses NADH and FADH2 to generate ATP.

Inputs and Outputs of Cellular Respiration

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, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP
ETC & Oxidative Phosphorylation	NADH, FADH2, O2	ATP, H2O

Glycolysis: Phases and Regulation

Glycolysis consists of two phases:

• Energy investment phase: Uses ATP to phosphorylate glucose.

• Energy payoff phase: Produces ATP and NADH.

• Regulation occurs via feedback inhibition, especially at the enzyme phosphofructokinase.

Citric Acid Cycle

The citric acid cycle is central to energy production and provides intermediates for biosynthesis.

• Acetyl-CoA combines with oxaloacetate to form citrate.

• Produces NADH, FADH2, ATP, and CO2.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

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

• Occurs in the inner mitochondrial membrane.

• Oxygen is the final electron acceptor, forming water.

Aerobic vs. Anaerobic Respiration and Fermentation

Cells can generate ATP with or without oxygen.

• Aerobic respiration: Uses oxygen; produces more ATP.

• Anaerobic respiration: Uses other electron acceptors; less ATP.

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

Photosynthesis

Inputs and Outputs of Photosynthesis

Photosynthesis converts light energy into chemical energy in plants, algae, and some bacteria.

Stage	Location	Inputs	Outputs
Light-dependent reactions	Thylakoid membrane	Light, H2O, NADP+, ADP	O2, ATP, NADPH
Calvin cycle (light-independent)	Stroma	CO2, ATP, NADPH	Glucose, NADP+, ADP

Light Absorption and Pigments

Pigments such as chlorophyll absorb light energy, which is used to drive photosynthesis.

• Chlorophyll absorbs mainly blue and red light, reflecting green.

• Other pigments (carotenoids) expand the range of absorbed light.

Z-Scheme and Electron Transport in Photosynthesis

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

• Photosystem II absorbs light, splits water, and transfers electrons to the ETC.

• Photosystem I receives electrons and reduces NADP+ to NADPH.

• ATP is generated via a proton gradient (chemiosmosis).

Calvin Cycle: Phases and Enzyme Function

The Calvin cycle fixes carbon dioxide into organic molecules using ATP and NADPH.

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

• Reduction: 3-PGA is converted to G3P using ATP and NADPH.

• Regeneration: RuBP is regenerated from G3P.

C4 and CAM Plant Adaptations

C4 and CAM plants have evolved mechanisms to minimize photorespiration and optimize carbon fixation.

• C4 plants: Spatially separate carbon fixation and the Calvin cycle; CO2 is fixed in mesophyll cells and transferred to bundle sheath cells.

• CAM plants: Temporally separate carbon fixation; CO2 is fixed at night and used during the day.

Purpose: These adaptations reduce the loss of fixed carbon due to photorespiration, especially in hot, dry environments.