Cell Structure, Energy, Cellular Respiration, and Photosynthesis: Study Guide (Chapters 7–10)

Cell Structure and Function

Cellular Classification

Cells are the fundamental units of life and are classified into two main types based on their internal organization: prokaryotes and eukaryotes.

• Prokaryotes: Lack a membrane-bound nucleus and organelles. Their genetic material is located in a region called the nucleoid.

• Eukaryotes: Possess a membrane-bound nucleus and specialized organelles, allowing compartmentalization of cellular processes.

Prokaryotic Cell Structures

• Cell Wall: Provides structural support and protection.

• Plasma Membrane: Regulates the passage of molecules in and out of the cell.

• Nucleoid: Region containing the cell's DNA.

• Ribosomes: Sites of protein synthesis.

• Flagella/Pili: Structures for movement (flagella) and attachment (pili).

Eukaryotic Cell Structures and Organelles

Eukaryotic cells contain membrane-bound organelles, each with specialized functions:

Organelle	Primary Function
Nucleus	Contains genetic information; site of transcription
Endoplasmic Reticulum (ER)	Rough ER: Protein synthesis; Smooth ER: Lipid synthesis and detoxification
Golgi Apparatus	Modifies, sorts, and packages proteins and lipids for transport
Lysosomes	Contain digestive enzymes to break down macromolecules and waste
Mitochondria	Site of aerobic cellular respiration and ATP production
Chloroplasts (plants)	Site of photosynthesis; converts light energy into chemical energy
Vacuoles	Storage for water, nutrients, or waste; provides turgor pressure in plants
Peroxisomes	Involved in oxidation reactions and breaking down fatty acids

The Endomembrane System and Molecular Zip Codes

The endomembrane system coordinates the synthesis, folding, and transport of proteins and lipids. Proteins are directed to their correct destinations using molecular 'zip codes':

• NLS (Nuclear Localization Signal): Directs proteins to the nucleus.

• ER Signal Sequence: Directs proteins to the endoplasmic reticulum.

• Mannose-6-phosphate: Directs enzymes to lysosomes.

Energy, Thermodynamics, and Enzymes

Principles of Thermodynamics

Cells obey the laws of thermodynamics, which govern energy transformations:

• Enthalpy (H): Total heat content or potential energy of a system.

• Entropy (S): Measure of disorder or randomness.

• Gibbs Free Energy (G): Energy available to do work. The change in free energy ($\Delta G$) predicts reaction spontaneity.

The relationship is given by:

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

Chemical Reactions and Energy

Cells use energetic coupling to drive non-spontaneous (endergonic) reactions by pairing them with spontaneous (exergonic) reactions.

Feature	Exergonic Reaction	Endergonic Reaction
$\Delta G$ Value	$\Delta G < 0$ (Negative)	$\Delta G > 0$ (Positive)
Energy Status	Releases energy	Requires/absorbs energy
Spontaneity	Spontaneous	Non-spontaneous
Example	Cellular Respiration	Photosynthesis

ATP: The Cell’s Energy Currency

• ATP (Adenosine Triphosphate): Composed of adenine, ribose, and three phosphate groups.

• Hydrolysis of ATP releases energy for cellular work:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

Enzyme Function and Regulation

Enzymes are biological catalysts that lower the activation energy ($E_a$) of reactions, increasing reaction rates. They function by:

• Bringing substrates together in the correct orientation

• Stabilizing the transition state

Enzyme activity is regulated by:

1. Competitive Inhibition: Inhibitor competes with substrate for the active site.

2. Allosteric Regulation: Molecule binds elsewhere, causing conformational change that affects activity.

3. Feedback Inhibition: End product inhibits an earlier enzyme in the pathway.

Free Energy Diagram Analysis

• Exergonic Reaction: Products have lower free energy than reactants ($\Delta G < 0$).

• Activation Energy ($E_a$): Energy required to reach the transition state; reduced by enzymes.

• Spontaneity: If products have less energy than reactants, the reaction is spontaneous.

Cellular Respiration

Overview and Equations

Cellular respiration is the process by which cells extract energy from glucose to produce ATP.

Overall equation:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{Energy (ATP + Heat)}$

• Aerobic Respiration: Requires oxygen; yields high amounts of ATP.

• Anaerobic Respiration: Uses alternative electron acceptors; less efficient.

• Fermentation: Occurs without oxygen; regenerates NAD+ for glycolysis; produces lactic acid or ethanol.

Stages of Glucose Oxidation

1. Glycolysis

   • Location: Cytoplasm

   • Input: 1 Glucose

   • Output: 2 Pyruvate, 2 NADH, net 2 ATP (4 produced, 2 consumed)

2. Pyruvate Processing

   • Converts pyruvate to Acetyl-CoA

3. Citric Acid Cycle (Krebs Cycle)

   • Location: Mitochondrial matrix

   • Completes glucose oxidation; releases CO2

   • Produces ATP (or GTP), NADH, FADH2

4. Electron Transport Chain (ETC) and Chemiosmosis

   • Location: Inner mitochondrial membrane

   • NADH and FADH2 donate electrons to ETC; protons pumped to create a gradient

   • ATP synthase uses proton gradient to synthesize ATP (oxidative phosphorylation)

Phosphorylation Types

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP from a substrate (in glycolysis and citric acid cycle).

• Oxidative phosphorylation: ATP production via ETC and chemiosmosis.

Regulation and Energy Yields

• Regulation Example: Phosphofructokinase is an allosteric enzyme in glycolysis; high ATP inhibits it, slowing glycolysis and the citric acid cycle.

• Energy Yields: Actual ATP yield is lower than theoretical due to transport costs and inefficiencies.

Photosynthesis

Foundational Concepts

• Autotrophs: Organisms that produce their own food (e.g., plants).

• Heterotrophs: Organisms that consume others for energy (e.g., animals).

• Chloroplast Structure: Contains thylakoid membranes (site of light reactions) and stroma (site of Calvin Cycle).

Overall equation:

$6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{Light Energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2$

The Process of Photosynthesis

1. Light-Dependent Reactions

   • Location: Thylakoid membrane

   • Photosystems: Antenna complex and reaction center

   • Mechanism: Light excites electrons; water is split, releasing O2

   • Products: ATP and NADPH

   • Chemiosmosis: Proton gradient powers ATP synthase

2. Light-Independent Reactions (Calvin Cycle)

   • Location: Stroma

   • Uses ATP and NADPH to fix CO2 into sugar (G3P)

   • Product: Glucose (via G3P intermediate)

ATP Synthase Locations in Plant Cells

• Thylakoid membrane: For ATP production during photosynthesis

• Plasma membrane: (As highlighted in assessment context; typically, ATP synthase is also found in the inner mitochondrial membrane for respiration)

Additional info: In most eukaryotic cells, ATP synthase is primarily located in the inner mitochondrial membrane for cellular respiration and in the thylakoid membrane of chloroplasts for photosynthesis. The plasma membrane location is more typical in prokaryotes, but may be referenced in certain assessment contexts.