Cell Biology Study Guide: Membrane Transport, Metabolism, Photosynthesis, and the Endomembrane System

Transport Across Membranes

Diffusion and Membrane Permeability

Transport across biological membranes is essential for maintaining cellular homeostasis. The movement of substances depends on their concentration gradients and the properties of the membrane.

• Diffusion: The passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Selectively Permeable Membrane: Allows certain molecules to pass while restricting others, based on size, charge, and solubility.

• Prediction of Diffusion Direction: Substances diffuse down their concentration gradient until equilibrium is reached.

Tonicity and Osmolarity

• Tonicity: The ability of a solution to cause a cell to gain or lose water, determined by the concentration of non-penetrating solutes.

• Osmolarity: The total concentration of solute particles in a solution.

• Water Movement: Water moves from areas of low solute concentration (hypotonic) to high solute concentration (hypertonic) across a selectively permeable membrane.

Mechanisms of Membrane Transport

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer without energy or membrane proteins.

• Facilitated Diffusion: Passive transport of molecules via membrane proteins (channels or carriers); no energy required; moves substances down their concentration gradient.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP) and membrane proteins.

Comparison of Transport Mechanisms

• Simple vs. Facilitated Diffusion:

  • Simple diffusion is non-specific, unregulated, and not saturable.

  • Facilitated diffusion is specific, can be regulated, and is saturable (rate-limited by number of transporters).

• Facilitated Diffusion: Channels vs. Carriers

  • Channels: Form hydrophilic pores; transport ions or water rapidly; usually gated.

  • Carriers: Bind specific molecules, undergo conformational change; transport is slower and can be saturated.

Examples of Membrane Transport Proteins

• GLUT1 Transporter: A carrier protein that facilitates glucose transport across the plasma membrane by facilitated diffusion.

• Sodium-Potassium Pump (Na+/K+ ATPase): An example of primary active transport; pumps 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

• Sodium-Glucose Symporter: An example of secondary active transport; uses the Na+ gradient (established by Na+/K+ ATPase) to drive glucose uptake against its gradient.

Types of Active Transport

• Primary (Direct) Active Transport: Uses ATP hydrolysis directly to transport molecules (e.g., Na+/K+ ATPase).

• Secondary (Indirect) Active Transport: Uses the energy stored in ion gradients established by primary active transport (e.g., Na+-glucose symporter).

Transport ATPases

• Definition: Enzymes that hydrolyze ATP to drive active transport.

• Major Types: P-type, V-type, F-type, and ABC transporters.

Glycolysis and Fermentation

Overview of Metabolism

Metabolism encompasses all chemical reactions in a cell, divided into anabolic (building up) and catabolic (breaking down) pathways. These pathways are interconnected and often coupled to enable cellular functions.

• Anabolic Pathways: Synthesize complex molecules from simpler ones; require energy.

• Catabolic Pathways: Break down complex molecules to release energy.

ATP: Structure and Function

• Structure: Adenosine triphosphate (ATP) consists of adenine, ribose, and three phosphate groups.

• Hydrolysis: ATP hydrolysis releases energy: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

• Function: ATP acts as the primary energy currency in cells, coupling exergonic and endergonic reactions.

REDOX Reactions in Metabolism

• Definition: Oxidation-reduction (REDOX) reactions involve the transfer of electrons between molecules.

• Importance: Central to energy metabolism, as electrons from glucose are transferred to electron carriers (e.g., NAD+).

Glycolysis

• Definition: The metabolic pathway that converts glucose into pyruvate, generating ATP and NADH.

• Location: Cytosol of the cell.

• Pathway: Consists of 10 enzyme-catalyzed steps; key irreversible steps catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase.

• Net Reaction: $\mathrm{Glucose} + 2\ \mathrm{NAD}^+ + 2\ \mathrm{ADP} + 2\ \mathrm{P}_i \rightarrow 2\ \mathrm{Pyruvate} + 2\ \mathrm{NADH} + 2\ \mathrm{ATP} + 2\ \mathrm{H}_2\mathrm{O}$

• ATP Production: Occurs via substrate-level phosphorylation.

• Regulation: Key enzymes are regulated by allosteric effectors and energy charge of the cell.

Fate of Pyruvate

• Aerobic Conditions: Pyruvate enters mitochondria and is oxidized to acetyl-CoA for the citric acid cycle.

• Anaerobic Conditions: Pyruvate is reduced to lactate (lactate fermentation) or ethanol and CO2 (alcoholic fermentation) to regenerate NAD+.

Fermentation

• Lactate Fermentation: Occurs in animal cells; pyruvate is reduced to lactate.

• Alcoholic Fermentation: Occurs in yeast; pyruvate is converted to ethanol and CO2.

Gluconeogenesis

• Definition: The synthesis of glucose from non-carbohydrate precursors.

• Relation to Glycolysis: Not a simple reversal; bypasses irreversible steps of glycolysis with alternate enzymes.

Regulation and the Cori Cycle

• Key Regulatory Enzymes: Hexokinase, phosphofructokinase, pyruvate kinase (glycolysis); fructose-1,6-bisphosphatase (gluconeogenesis).

• Fructose 2,6-bisphosphate: A potent regulator of glycolysis and gluconeogenesis.

• Cori Cycle: The cycle of lactate produced by muscles transported to the liver, converted to glucose, and returned to muscles.

Aerobic Cellular Respiration

Overview and Major Phases

Aerobic respiration is the process by which cells completely oxidize glucose to CO2 and H2O, using oxygen as the final electron acceptor, and generate large amounts of ATP.

• Overall Reaction: $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2 \rightarrow 6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{energy}$

• Major Phases:

  1. Glycolysis (cytosol)

  2. Pyruvate processing (mitochondrial matrix)

  3. Citric Acid Cycle (mitochondrial matrix)

  4. Electron Transport System (inner mitochondrial membrane)

  5. ATP synthesis by oxidative phosphorylation

Mitochondrial Structure and Function

• Mitochondria: Double-membraned organelles; site of aerobic respiration in eukaryotes.

• Localization: Enzymes for the citric acid cycle are in the matrix; electron transport chain is in the inner membrane.

• Bacteria: Carry out aerobic respiration using enzymes in the plasma membrane.

Pyruvate Processing and Citric Acid Cycle

• Pyruvate Transport: Pyruvate enters mitochondria via a transport protein.

• Pyruvate Dehydrogenase Complex: Converts pyruvate to acetyl-CoA, producing NADH and CO2; regulated allosterically and by covalent modification.

• Citric Acid Cycle: Series of 8 reactions; acetyl-CoA is oxidized to CO2, generating NADH, FADH2, and GTP/ATP.

• Regulation: Controlled by substrate availability, product inhibition, and allosteric regulation.

Metabolic Integration

• Fats: Broken down by β-oxidation to acetyl-CoA.

• Proteins: Amino acids can be converted to citric acid cycle intermediates.

• Anabolic Pathways: Citric acid cycle intermediates serve as precursors for biosynthesis.

Electron Transport System (ETS) and ATP Synthesis

• Malate-Aspartate Shuttle: Transfers electrons from cytosolic NADH into mitochondria.

• ETS: Series of electron carriers (complexes I-IV, coenzyme Q, cytochrome c) organized by standard reduction potential.

• Proton Gradient: Electron flow drives proton pumping, creating an electrochemical gradient across the inner mitochondrial membrane.

• ATP Synthase: Enzyme complex that synthesizes ATP as protons flow back into the matrix; operates via the binding change mechanism.

• Phosphorylation Types: Substrate-level (direct transfer of phosphate) vs. oxidative (driven by proton gradient).

• ATP Yield: Aerobic respiration produces more ATP than anaerobic respiration or fermentation.

Photosynthesis

Overview and Importance

Photosynthesis is the process by which photoautotrophs convert solar energy into chemical energy, producing organic molecules from CO2 and H2O.

• Organisms: Plants, algae, and some bacteria.

• Relationship: Photoautotrophs provide organic carbon and energy for chemoheterotrophs.

• Overall Reaction: $6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

Chloroplast Structure and Function

• Chloroplasts: Organelles with double membranes and internal thylakoid membranes; site of photosynthesis in plants and algae.

• Localization: Light reactions occur in thylakoid membranes; Calvin cycle in the stroma.

Light Absorption and Photosystems

• Wavelength and Energy: Shorter wavelengths have higher energy.

• Pigments: Chlorophylls and carotenoids absorb light at specific wavelengths.

• Photosystems: Complexes of pigments and proteins that capture light energy and initiate electron transport.

Electron Flow and ATP/NADPH Production

• Noncyclic Electron Flow (Z-scheme): Involves both photosystem II and I; produces ATP, NADPH, and O2.

• Cyclic Electron Flow: Involves only photosystem I; produces ATP but not NADPH or O2; occurs when ATP demand is high.

• Proton Gradient: Generated across thylakoid membrane; drives ATP synthesis.

Calvin Cycle and Carbon Fixation

• Stages: Carbon fixation, reduction, and regeneration of RuBP.

• Requirement: Three turns of the cycle produce one G3P molecule.

• Rubisco: Enzyme that catalyzes CO2 fixation; can also catalyze photorespiration.

Photorespiration and Adaptations

• Photorespiration: Occurs when Rubisco fixes O2 instead of CO2, leading to energy loss.

• Glycolate Pathway: Salvages some carbon lost during photorespiration.

• C4 and CAM Plants: Adaptations to minimize photorespiration by spatial or temporal separation of CO2 fixation.

The Endomembrane System

Structure and Function of Major Components

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; site of protein synthesis and initial glycosylation.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Endosomes and Lysosomes: Involved in sorting, recycling, and degradation of cellular materials.

Membrane Biosynthesis and Glycosylation

• ER Role: Synthesizes membrane lipids and proteins; initial steps of N-glycosylation.

• Golgi Role: Further modifies glycoproteins; sorts and targets proteins to their destinations.

• N-glycosylation: Addition of oligosaccharides to asparagine residues of proteins.

Vesicular Transport and Protein Targeting

• Anterograde Transport: Movement from ER to Golgi to plasma membrane or lysosomes.

• Retrograde Transport: Movement from Golgi back to ER.

• Protein Targeting: Involves retention/retrieval tags, mannose-6-phosphate pathway for lysosomal targeting.

Secretory Pathways and Endocytosis

• Exocytosis: Fusion of vesicles with plasma membrane to release contents outside the cell.

• Endocytosis: Uptake of materials via vesicle formation; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

• Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor-ligand interactions and clathrin-coated vesicles.

Coated Vesicles and Vesicle Targeting

Coat Protein	Main Function
Clathrin	Endocytosis and transport from Golgi to endosomes/lysosomes
COPI	Retrograde transport from Golgi to ER
COPII	Anterograde transport from ER to Golgi

• SNAREs and Rab GTPases: Mediate vesicle targeting and fusion with target membranes.

Lysosomes and Peroxisomes

• Lysosomes: Contain hydrolytic enzymes for degradation of macromolecules; formed by fusion of vesicles from Golgi with endosomes.

• Peroxisomes: Organelles involved in fatty acid oxidation and detoxification of hydrogen peroxide.