BIOL 190A Midterm Study Guide: Chapters 5-8 (Membrane Transport, Cell Signaling, Metabolism, Respiration, Fermentation, Photosynthesis)

Membrane Processes and Cell Signaling

Membrane Structure and Fluidity

The cell membrane is a dynamic structure composed primarily of a phospholipid bilayer with embedded proteins. Its fluidity is essential for proper cellular function.

• Phospholipid Bilayer: Provides a semi-permeable barrier; hydrophilic heads face outward, hydrophobic tails inward.

• Membrane Fluidity: Influenced by lipid composition (saturated vs. unsaturated fatty acids) and cholesterol content.

• Functions of Membrane Proteins: Include transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment to cytoskeleton and extracellular matrix.

Membrane Permeability and Transport Types

Cell membranes regulate the movement of substances via several transport mechanisms.

• Passive Transport: Movement of molecules down their concentration gradient without energy input. Includes diffusion and facilitated diffusion (via channel or carrier proteins).

• Active Transport: Movement against the concentration gradient, requiring energy (usually ATP). Example: sodium-potassium pump.

• Bulk Transport: Movement of large molecules via endocytosis (into cell) and exocytosis (out of cell).

• Osmosis: Diffusion of water across a selectively permeable membrane. Direction depends on tonicity (relative solute concentration).

Water Balance in Cells

• Animal Cells: Prefer isotonic environments; hypotonic solutions cause lysis, hypertonic cause crenation.

• Plant Cells: Prefer hypotonic environments; turgid state is optimal, plasmolysis occurs in hypertonic solutions.

Cell Signaling

Cell signaling enables cells to communicate and respond to their environment.

• Stages of Cell Signaling:

  1. Reception: Signal molecule binds to receptor.

  2. Transduction: Signal is relayed and amplified via intracellular pathways.

  3. Response: Cell changes its activity (e.g., gene expression, metabolism).

• Types of Receptors:

  • Membrane Receptors: Three main types:

    • G protein-coupled receptors (GPCRs)

    • Receptor tyrosine kinases (RTKs)

    • Ion channel receptors

  • Intracellular Receptors: Located in cytoplasm or nucleus; bind to hydrophobic ligands.

• Epinephrine Pathway: Epinephrine binds to GPCR, activating a cascade that leads to glycogen breakdown and increased blood sugar. Example: Fight-or-flight response.

Energy Concepts and Metabolism

Metabolic Pathways

Metabolism encompasses all chemical reactions in a cell, divided into two main types:

• Catabolic Pathways: Break down molecules, releasing energy (e.g., cellular respiration).

• Anabolic Pathways: Build complex molecules, requiring energy (e.g., protein synthesis).

Energy Profiles of Reactions

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

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

• Activation Energy (EA): Minimum energy required to start a reaction.

Role of ATP

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Energy Coupling: ATP hydrolysis drives endergonic reactions.

• Cellular Work: ATP powers mechanical, transport, and chemical work.

Enzymes

• Enzymes: Biological catalysts that lower activation energy.

• Factors Affecting Activity: Temperature, pH, substrate concentration.

• Activation and Inhibition:

  • Activators: Increase enzyme activity.

  • Inhibitors: Decrease activity; can be competitive (bind active site) or noncompetitive (bind elsewhere).

Respiration, Fermentation, and Photosynthesis

Overall Equations

• Cellular Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$

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

• Oxidation/Reduction: In respiration, glucose is oxidized; oxygen is reduced. In photosynthesis, water is oxidized; carbon dioxide is reduced.

Main Stages and Locations

• Respiration (4 Stages):

  1. Glycolysis (cytoplasm)

  2. Pyruvate Oxidation (mitochondrial matrix)

  3. Citric Acid Cycle (mitochondrial matrix)

  4. Oxidative Phosphorylation (inner mitochondrial membrane)

• Photosynthesis (2 Stages):

  1. Light Reactions (thylakoid membrane)

  2. Calvin Cycle (stroma)

Inputs and Outputs of Key Pathways

• Glycolysis: Input: glucose; Output: 2 pyruvate, 2 ATP, 2 NADH

• Pyruvate Oxidation: Input: pyruvate; Output: acetyl-CoA, CO2, NADH

• Citric Acid Cycle: Input: acetyl-CoA; Output: CO2, NADH, FADH2, ATP

• Calvin Cycle: Input: CO2, ATP, NADPH; Output: G3P (sugar), ADP, NADP+

Fermentation

• Alcohol Fermentation: Pyruvate converted to ethanol and CO2; occurs in yeast.

• Lactic Acid Fermentation: Pyruvate converted to lactate; occurs in muscle cells and some bacteria.

• Similarities: Both regenerate NAD+ for glycolysis; occur without oxygen.

• Differences: End products (ethanol vs. lactate); CO2 released only in alcohol fermentation.

Light Reactions of Photosynthesis

• Sequence of Molecules: Involves photosystems II and I, electron transport chain, ATP synthase.

• Photophosphorylation: ATP produced using light energy.

Calvin Cycle Intermediates

• Key Intermediates: RuBP (5C, 2 phosphate), 3-PGA (3C, 1 phosphate), G3P (3C, 1 phosphate).

• Number of Carbon Atoms and Phosphate Groups: See textbook Fig 8.18 for details.

ATP Synthesis Mechanisms

Process	Location	Mechanism	Example
Substrate-level phosphorylation	Cytoplasm, mitochondrial matrix	Direct transfer of phosphate to ADP	Glycolysis, Citric Acid Cycle
Oxidative phosphorylation	Inner mitochondrial membrane	ATP synthesis via electron transport chain and chemiosmosis	Respiration
Photophosphorylation	Thylakoid membrane	ATP synthesis using light energy	Photosynthesis (light reactions)

Example: During glycolysis, ATP is produced by substrate-level phosphorylation; during oxidative phosphorylation, ATP is generated by ATP synthase using the proton gradient; in photosynthesis, photophosphorylation produces ATP using light energy.

Additional info: For detailed diagrams and stepwise pathways, refer to Practice Figures in your course modules and textbook figures as indicated.