General Biology Study Guide: Cell Membrane Processes, Respiration, Fermentation, and Photosynthesis

Cell Membrane Processes

Membrane Structure and Function

The cell membrane is a dynamic structure that regulates the movement of substances into and out of the cell, maintaining homeostasis and enabling communication.

• Membrane Proteins: Integral and peripheral proteins perform functions such as transport, signaling, and structural support.

• Membrane Permeability: Selective permeability allows certain molecules to pass while restricting others.

Types of Membrane Transport

Transport across membranes can be passive or active, depending on energy requirements and directionality.

• Passive Transport: Movement of substances down their concentration gradient without energy input. Includes diffusion and osmosis.

• Facilitated Diffusion: Passive movement via membrane proteins (channels or carriers).

• Active Transport: Movement against the concentration gradient, requiring energy (usually ATP).

• Water Balance: Regulation of water movement is crucial for cell survival; involves osmosis and aquaporins.

Stages of Cell Signaling

Cells communicate through signaling pathways that involve reception, transduction, and response.

• Reception: Signal molecules bind to membrane receptors.

• Transduction: Signal is relayed and amplified inside the cell.

• Response: Cellular activity is altered (e.g., gene expression, metabolism).

Types of Receptors

Receptors are specialized proteins that detect signals and initiate cellular responses.

• Membrane Receptors: Three main types: ligand-gated ion channels, G protein-coupled receptors (GPCRs), and enzyme-linked receptors.

• Intracellular Receptors: Located within the cell; bind to small or nonpolar molecules.

Epinephrine Pathway Example

Epinephrine binds to GPCRs, triggering a cascade that leads to breakdown of glycogen and elevation of blood sugar levels.

• Application: Fight-or-flight response in animals.

Energy and Enzymes

Energy Transformations in Cells

Cells convert energy from one form to another to perform work, such as movement, synthesis, and transport.

• Types of Cellular Work: Mechanical, transport, and chemical work.

• ATP: The primary energy currency of the cell; couples exergonic and endergonic reactions.

Enzymes

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

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

• Activation and Inhibition: Enzymes can be regulated by inhibitors (competitive and noncompetitive) and activators.

Respiration, Fermentation, and Photosynthesis

Overview and Equations

These processes are essential for energy conversion in living organisms. Respiration and fermentation extract energy from organic molecules, while photosynthesis captures energy from sunlight.

• Respiration Equation:

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

• Photosynthesis Equation:

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

• Fermentation: Anaerobic process producing lactic acid or ethanol and CO2.

Main Stages and Key Molecules

Each process consists of multiple stages, each with specific reactants, products, and regulatory steps.

• Respiration: Four main stages: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, and Electron Transport Chain.

• Photosynthesis: Two main stages: Light Reactions and Calvin Cycle.

• Fermentation: Occurs when oxygen is absent; produces lactic acid (in animals) or ethanol (in yeast).

Inputs and Outputs of Key Pathways

• Glycolysis: Input: Glucose; Output: Pyruvate, ATP, NADH.

• Pyruvate Oxidation: Input: Pyruvate; Output: Acetyl-CoA, CO2, NADH.

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

• Electron Transport Chain: Input: NADH, FADH2, O2; Output: ATP, H2O.

• Light Reactions (Photosynthesis): Input: H2O, Light; Output: O2, ATP, NADPH.

• Calvin Cycle: Input: CO2, ATP, NADPH; Output: Glucose.

Calvin Cycle Details

The Calvin Cycle is the set of light-independent reactions in photosynthesis that fix carbon dioxide into organic molecules.

• Inputs: CO2, ATP, NADPH.

• Outputs: Glucose, ADP, NADP+.

• Number of Carbon Atoms and Phosphate Groups: Key intermediates include 3-phosphoglycerate (3C, 1P) and ribulose bisphosphate (5C, 2P).

ATP Synthesis Mechanisms

ATP is produced by substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.

• Substrate-level Phosphorylation: Direct transfer of phosphate to ADP during glycolysis and the citric acid cycle.

• Oxidative Phosphorylation: ATP synthesis driven by electron transport and chemiosmosis in mitochondria.

• Photophosphorylation: ATP synthesis in chloroplasts during light reactions of photosynthesis.

Comparison Table: Respiration, Fermentation, and Photosynthesis

Process	Location	Oxygen Requirement	Main Products	ATP Yield
Respiration	Mitochondria	Requires O2	CO2, H2O, ATP	High (up to 38 ATP)
Fermentation	Cytoplasm	No O2 required	Lactic acid or ethanol, CO2	Low (2 ATP)
Photosynthesis	Chloroplasts	No O2 required	Glucose, O2	Uses light energy

Additional info:

• Practice figures referenced in the notes are likely diagrams of metabolic pathways and molecular complexes found in standard biology textbooks.

• Students should be familiar with the sequence of molecules and complexes in the light reactions of photosynthesis, as well as the Calvin cycle intermediates.

• Comparisons between lactic acid and alcoholic fermentation should include differences in organisms, products, and ATP yield.