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

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Cell Structure and Membrane Transport

Membrane Protein Orientation and Transport

Membrane proteins synthesized in the rough endoplasmic reticulum (ER) are transported to the plasma membrane, maintaining their orientation throughout the process.

Key Point: Vesicle fusion preserves the orientation of membrane proteins, ensuring that the cytoplasmic and extracellular domains remain correctly positioned.
Example: A protein with an extracellular domain in the ER will have the same domain facing outside the cell after vesicle fusion with the plasma membrane.

Permeability of Biological Membranes

Cell membranes selectively allow certain molecules to pass through while restricting others, based on size, charge, and polarity.

Key Point: Small nonpolar molecules (e.g., O2) diffuse easily, while ions (e.g., Na+) and polar molecules require transport proteins.
Example: Oxygen diffuses across the membrane, but glucose and sodium ions need specific channels or carriers.

Osmosis and Water Balance in Cells

Osmosis is the movement of water across a semipermeable membrane from low solute concentration to high solute concentration.

Key Point: Placing a cell in a hypotonic solution (lower solute concentration outside) causes water to enter the cell, potentially leading to swelling or lysis.
Example: Red blood cells placed in pure water will swell and may burst.

Sodium-Potassium Pump and Electrochemical Gradients

The sodium-potassium pump (Na+/K+ ATPase) maintains membrane potential by actively transporting Na+ out and K+ into the cell.

Key Point: This pump creates concentration gradients and a charge difference (membrane potential) across the plasma membrane.
Equation: per ATP hydrolyzed

Types of Membrane Transport

Cells use various mechanisms to move substances across membranes.

Passive Transport: Diffusion and facilitated diffusion move substances down their concentration gradients without energy input.
Active Transport: Requires energy (usually ATP) to move substances against their concentration gradients.
Example: Glucose uptake via facilitated diffusion; Na+/K+ pump via active transport.

Metabolism and Energy Conversion

Chemical Energy and Work

Chemical energy stored in molecular bonds can be converted into kinetic or potential energy to perform cellular work.

Key Point: Cells convert chemical energy into motion, heat, and other forms of work through metabolic reactions.
Example: ATP hydrolysis releases energy for muscle contraction.

Spontaneous Reactions and Enzyme Function

Enzymes catalyze biochemical reactions by lowering activation energy, increasing reaction rates without changing the overall free energy change ().

Key Point: Enzymes do not alter the equilibrium or of a reaction, but they allow reactions to proceed faster.
Equation: (activation energy) is reduced by enzyme action.

Enzyme Regulation and Inhibition

Enzyme activity can be regulated by molecules binding to active or allosteric sites, affecting substrate binding and catalysis.

Competitive Inhibition: Inhibitor binds to the active site, blocking substrate access.
Allosteric Regulation: Molecule binds elsewhere, changing enzyme shape and activity.

Metabolic Pathways and Energy Yield

Cells break down macromolecules to release energy, which is captured in ATP and other high-energy molecules.

Key Point: Catabolic pathways degrade molecules, while anabolic pathways build complex molecules.
Example: Cellular respiration breaks down glucose to produce ATP.

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose and other organic molecules.

Key Steps: Glycolysis, Citric Acid Cycle (Krebs Cycle), Electron Transport Chain
Equation:

Glycolysis and Fermentation

Glycolysis breaks down glucose into pyruvate, generating ATP and NADH. In the absence of oxygen, fermentation regenerates NAD+ by converting pyruvate to lactate or ethanol.

Key Point: Fermentation allows glycolysis to continue in anaerobic conditions.
Example: Yeast cells produce ethanol; muscle cells produce lactate.

Citric Acid Cycle and Electron Transport Chain

The citric acid cycle oxidizes acetyl-CoA, producing NADH and FADH2, which donate electrons to the electron transport chain for ATP synthesis.

Key Point: Oxygen is the final electron acceptor, forming water.
Equation:

Photosynthesis

Light-Dependent and Light-Independent Reactions

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

Light-Dependent Reactions: Occur in the thylakoid membranes, producing ATP and NADPH.
Light-Independent Reactions (Calvin Cycle): Use ATP and NADPH to fix CO2 into glucose.

Photosystems and Electron Flow

Photosystems I and II capture light energy, driving electron transport and ATP/NADPH production.

Key Point: Photosystem II splits water, releasing O2 and providing electrons; Photosystem I uses electrons to reduce NADP+ to NADPH.
Equation:

Calvin Cycle and Sugar Production

The Calvin cycle uses ATP and NADPH to convert CO2 into three-carbon sugars (G3P), which are used to synthesize glucose.

Key Point: The three-carbon sugar produced is glyceraldehyde-3-phosphate (G3P).

Cell Signaling and Communication

Signal Transduction Pathways

Cells communicate using signaling molecules that bind to receptors, triggering intracellular responses.

Key Point: Ligand binding activates receptor proteins, leading to changes in gene expression, metabolism, or cell behavior.
Example: Epinephrine binding to a G-protein coupled receptor activates cAMP production, leading to glycogen breakdown.

Receptor Tyrosine Kinases (RTKs)

RTKs are membrane receptors that, when activated, initiate phosphorylation cascades controlling cell division and growth.

Key Point: Mutations in RTKs can lead to uncontrolled cell division and cancer.

Tables

Comparison of Membrane Transport Mechanisms

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	O2 across membrane
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Summary of Cellular Respiration Steps

Step	Main Reactants	Main Products	Location
Glycolysis	Glucose	Pyruvate, ATP, NADH	Cytoplasm
Citric Acid Cycle	Acetyl-CoA	CO2, NADH, FADH2, ATP	Mitochondrial matrix
Electron Transport Chain	NADH, FADH2, O2	ATP, H2O	Inner mitochondrial membrane

Photosynthesis: Light-Dependent vs. Light-Independent Reactions

Reaction Type	Location	Main Inputs	Main Outputs
Light-Dependent	Thylakoid membrane	Light, H2O	ATP, NADPH, O2
Calvin Cycle	Stroma	CO2, ATP, NADPH	G3P (sugar)

Additional info:

Some context and explanations have been expanded for clarity and completeness.
Key terms and processes are defined and illustrated with examples for exam preparation.