Cell Biology Study Guide: Membrane Transport, Glycolysis, Aerobic Respiration, and Photosynthesis

Transport Across Membranes: Overcoming the Permeability Barrier

Membrane Structure and Selective Permeability

Cell membranes possess a hydrophobic interior that acts as a barrier to most molecules and ions, maintaining distinct internal and external environments. In eukaryotic cells, membranes also define organelles by retaining specific molecules and ions necessary for their functions. Membranes are selectively permeable, allowing controlled passage of specific molecules and ions, which is crucial for cellular and organelle function.

• Hydrophobic Barrier: Prevents free passage of most solutes.

• Selective Permeability: Enables regulated movement of substances.

• Homeostasis: Maintains constant internal environment.

• Transport Mechanisms: Simple diffusion, facilitated diffusion, and active transport.

Mechanisms of Solute Transport

Solutes cross membranes via three main mechanisms: simple diffusion, facilitated diffusion, and active transport. Each mechanism differs in energy requirements, directionality, and specificity.

• Simple Diffusion: Direct, unaided movement down the concentration gradient.

• Facilitated Diffusion: Protein-mediated movement down the gradient; exhibits saturation kinetics.

• Active Transport: Protein-mediated movement against the gradient; requires energy input.

Comparison Table: Simple Diffusion, Facilitated Diffusion, and Active Transport

Feature	Simple Diffusion	Facilitated Diffusion	Active Transport
Energy Requirement	No	No	Yes
Directionality	Non-directional	Non-directional	Directional
Membrane Protein Required	No	Yes	Yes
Saturation Kinetics	No	Yes	Yes
Competitive Inhibition	No	Yes	Yes
Solute Types	Small nonpolar/polar	Large polar/ions	Large polar/nonpolar/ions

Energetics of Transport

The movement of solutes is determined by concentration gradients (for uncharged solutes) or electrochemical potential (for charged solutes). Transport down a gradient is exergonic and spontaneous; transport up a gradient is endergonic and requires energy.

• Concentration Gradient: Drives diffusion for uncharged solutes.

• Electrochemical Potential: Combines concentration and charge gradients for ions.

• Membrane Potential: Most cells have a negative membrane potential, favoring inward movement of cations.

Simple Diffusion

Simple diffusion is the unassisted net movement of solutes from high to low concentration. Only small, nonpolar molecules and very small polar molecules (e.g., water, glycerol, ethanol) can cross membranes by simple diffusion.

• Examples: Oxygen, carbon dioxide, ethanol.

• Equilibrium: Diffusion continues until concentrations equalize.

• Driving Force: Entropy and minimization of free energy.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, driven by differences in solute concentration.

• Hypertonic Solution: Water leaves the cell, causing shrinkage.

• Hypotonic Solution: Water enters the cell, causing swelling or lysis.

• Isotonic Solution: No net water movement.

Factors Affecting Diffusion

• Size: Smaller molecules diffuse more readily.

• Polarity: Nonpolar molecules cross more easily.

• Charge: Charged solutes are generally impermeable due to hydration shells.

Diffusion Rate Equation

The rate of inward diffusion ($J_{in}$) is given by:

$J_{in} = P \Delta C$

where $P$ is the permeability coefficient and $\Delta C$ is the concentration gradient.

Facilitated Diffusion

Facilitated diffusion is protein-mediated movement of solutes down their concentration or electrochemical gradient. It is exergonic and saturable.

• Carrier Proteins: Bind solute, undergo conformational change, and transport solute across membrane.

• Channel Proteins: Form hydrophilic channels for rapid solute passage.

• Specificity: Transporters are specific for their substrates.

• Kinetics: Saturation kinetics similar to enzymes; described by Michaelis–Menten equation.

Michaelis–Menten equation for transport rate ($v$):

$v = \frac{V_{max} [S]}{K_m + [S]}$

Types of Carrier Protein Transport

Type	Solute(s) Transported	Direction
Uniport	One solute	One direction
Symport	Two solutes	Same direction
Antiport	Two solutes	Opposite directions

Channel Proteins

• Ion Channels: Highly selective, gated, rapid transport.

• Porins: Larger, less specific, allow passage of hydrophilic solutes.

• Aquaporins: Water channels, facilitate rapid water movement.

Active Transport

Active transport moves solutes against their concentration or electrochemical gradient, requiring energy input (usually ATP hydrolysis).

• Direct Active Transport: Coupled directly to ATP hydrolysis (primary active transport).

• Indirect Active Transport: Uses ion gradients established by primary transport to drive cotransport (secondary active transport).

Types of Transport ATPases

Type	Main Function	Example
P-type	Ion transport, phosphorylated intermediate	Na/K pump
V-type	Proton transport into organelles	Lysosome acidification
F-type	ATP synthesis using proton gradient	Mitochondrial ATP synthase
ABC-type	Multidrug resistance, nutrient import/export	CFTR, MDR proteins

Energetics of Transport

For uncharged solutes, the free energy change ($\Delta G$) depends only on the concentration gradient:

$\Delta G = RT \ln \frac{[S]_{in}}{[S]_{out}}$

For charged solutes, $\Delta G$ depends on both concentration and membrane potential:

$\Delta G = RT \ln \frac{[S]_{in}}{[S]_{out}} + zF \Delta \Psi$

where $z$ is the charge, $F$ is the Faraday constant, and $\Delta \Psi$ is the membrane potential.

Chemotrophic Energy Metabolism: Glycolysis and Fermentation

Metabolic Pathways

Metabolism encompasses all chemical reactions in a cell, divided into anabolic (synthetic, energy-requiring) and catabolic (degradative, energy-releasing) pathways.

• Anabolic Pathways: Synthesize cellular components; endergonic.

• Catabolic Pathways: Break down molecules; exergonic.

• Distinct Enzymes: Anabolic and catabolic pathways use different enzymes and intermediates.

ATP: The Primary Energy Molecule

ATP is the main energy currency in cells, powering cellular activities. It contains two energy-rich phosphoanhydride bonds.

• Structure: Adenine, ribose, three phosphates.

• Hydrolysis: Highly exergonic; $\Delta G^\circ = -7.3$ kcal/mol.

• Charge Repulsion: Negative charges on phosphates contribute to bond energy.

• Resonance Stabilization: Products (ADP, $P_i$) are stabilized by resonance.

• Entropy: Hydrolysis increases entropy and solubility.

ATP Hydrolysis Equation

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

Glycolysis: ATP Generation Without Oxygen

Glycolysis is a ten-step pathway converting glucose to pyruvate, generating ATP and NADH. It occurs in the cytosol and is found in nearly all organisms.

• Phase 1: Preparation and cleavage; investment of 2 ATP.

• Phase 2: Oxidation and ATP generation; substrate-level phosphorylation.

• Phase 3: Pyruvate formation and additional ATP generation.

Net Glycolysis Reaction

$\text{Glucose} + 2 \text{ADP} + 2 P_i + 2 \text{NAD}^+ \rightarrow 2 \text{Pyruvate} + 2 \text{ATP} + 2 \text{NADH} + 2 \text{H}_2\text{O}$

Fermentation

In the absence of oxygen, pyruvate undergoes fermentation to regenerate NAD+, allowing glycolysis to continue. Two main types are lactate and alcoholic fermentation.

• Lactate Fermentation: Pyruvate reduced to lactate.

• Alcoholic Fermentation: Pyruvate converted to ethanol and CO2.

• ATP Yield: Only 2 ATP per glucose.

Lactate Fermentation Equation

$\text{Glucose} + 2 \text{ADP} + 2 P_i \rightarrow 2 \text{Lactate} + 2 \text{ATP}$

Alcoholic Fermentation Equation

$\text{Glucose} + 2 \text{ADP} + 2 P_i \rightarrow 2 \text{Ethanol} + 2 \text{CO}_2 + 2 \text{ATP}$

Regulation of Glycolysis and Gluconeogenesis

Key enzymes are regulated allosterically by metabolites such as ATP, AMP, citrate, and fructose-2,6-bisphosphate. Reciprocal regulation prevents futile cycles.

• AMP: Activates glycolysis, inhibits gluconeogenesis.

• ATP: Inhibits glycolysis, activates gluconeogenesis.

• Fructose-2,6-bisphosphate: Activates PFK-1, inhibits F1,6BPase.

Chemotrophic Energy Metabolism: Aerobic Respiration

Cellular Respiration Overview

Aerobic respiration uses oxygen as the terminal electron acceptor, allowing complete oxidation of glucose to CO2 and H2O, and maximizing ATP yield.

• Stages: Glycolysis, pyruvate oxidation, citric acid cycle, electron transport, ATP synthesis.

• Mitochondria: Central organelle for aerobic respiration in eukaryotes.

Mitochondrial Structure and Function

• Outer Membrane: Contains porins; permeable to small molecules.

• Inner Membrane: Impermeable; contains electron transport chain and ATP synthase.

• Cristae: Infoldings increase surface area for ATP generation.

• Matrix: Contains enzymes for citric acid cycle, pyruvate oxidation, and fatty acid oxidation.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle oxidizes acetyl CoA to CO2, generating NADH, FADH2, and ATP (or GTP).

• Acetyl CoA: Entry point for carbohydrates, fats, and proteins.

• Oxidative Decarboxylation: Pyruvate converted to acetyl CoA by pyruvate dehydrogenase.

• Cycle Steps: Citrate formation, decarboxylations, oxidations, regeneration of oxaloacetate.

Citric Acid Cycle Net Reaction

$\text{Acetyl CoA} + 3 \text{NAD}^+ + \text{FAD} + \text{ADP} + P_i \rightarrow 2 \text{CO}_2 + 3 \text{NADH} + \text{FADH}_2 + \text{ATP} + \text{CoA}$

Electron Transport Chain (ETC)

The ETC transfers electrons from NADH and FADH2 to oxygen, pumping protons across the inner membrane to generate a proton gradient.

• Complexes: I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome bc1), IV (cytochrome c oxidase).

• Mobile Carriers: Coenzyme Q (ubiquinone), cytochrome c.

• Proton Pumping: Complexes I, III, IV pump protons; creates electrochemical gradient.

Reduction Potential and Free Energy

Electron flow is driven by differences in reduction potential ($E^\circ$). The relationship between free energy and reduction potential:

$\Delta G^\circ = -nF \Delta E^\circ$

where $n$ is the number of electrons transferred, $F$ is the Faraday constant, and $\Delta E^\circ$ is the difference in reduction potential.

ATP Synthesis: Chemiosmotic Model

ATP synthesis is driven by the proton gradient established by the ETC. The ATP synthase (F0F1 complex) couples proton flow to ATP production.

• Proton Motive Force (pmf): Combination of membrane potential and pH gradient.

• ATP Synthase: Rotational mechanism; three ATP synthesized per full rotation.

Proton Motive Force Equation

$\text{pmf} = \Delta \Psi - \frac{2.3RT}{F} \Delta pH$

Maximum Theoretical ATP Yield

Stage	ATP Yield
Glycolysis	2
Citric Acid Cycle	2
Oxidative Phosphorylation (NADH)	3 per NADH
Oxidative Phosphorylation (FADH2)	2 per FADH2
Total (per glucose)	~30–32 ATP (actual yield)

Phototrophic Energy Metabolism: Photosynthesis

Overview of Photosynthesis

Photosynthesis converts solar energy into chemical energy (ATP, NADPH) and fixes carbon dioxide into carbohydrates. It consists of energy transduction (light reactions) and carbon assimilation (Calvin cycle).

• Photoautotrophs: Use solar energy to synthesize organic molecules from CO2 and H2O.

• Photoheterotrophs: Use sunlight for energy but require organic carbon sources.

Chloroplast Structure

• Outer Membrane: Permeable to small molecules.

• Inner Membrane: Permeability barrier; contains transport proteins.

• Stroma: Contains enzymes for carbon assimilation.

• Thylakoid Membrane: Houses photosystems, electron transport chain, ATP synthase.

• Grana: Stacks of thylakoids; increase surface area for light reactions.

Light Harvesting

Chlorophylls and accessory pigments absorb photons, transferring energy to reaction centers in photosystems. Two photosystems (PSII and PSI) operate in series in oxygenic phototrophs.

• Chlorophyll a: Primary pigment; absorbs blue and red light.

• Accessory Pigments: Carotenoids, phycobilins; broaden absorption spectrum.

• Photosystems: Functional units containing pigments and proteins; associated with light-harvesting complexes (LHCs).

Photosynthetic Electron Transport

Electrons flow from water to NADP+ via PSII, cytochrome b6f complex, PSI, and ferredoxin-NADP reductase. This flow is coupled to proton pumping, generating a proton gradient across the thylakoid membrane.

• PSII: Oxidizes water, releases O2, reduces plastoquinone.

• Cytochrome b6f: Transfers electrons, pumps protons.

• PSI: Reduces ferredoxin, which reduces NADP+ to NADPH.

• ATP Synthase: Uses proton gradient to synthesize ATP (photophosphorylation).

Noncyclic Electron Flow Equation

$2 \text{H}_2\text{O} + 2 \text{NADP}^+ + 3 \text{ADP} + 3 P_i \rightarrow 2 \text{NADPH} + 2 \text{H}^+ + 3 \text{ATP} + \text{O}_2$

Cyclic Photophosphorylation

When extra ATP is needed, electrons from PSI are cycled back to the cytochrome b6f complex, increasing ATP production without NADPH or O2 formation.

Calvin Cycle: Carbon Assimilation

The Calvin cycle fixes CO2 into organic molecules using ATP and NADPH. It consists of three stages: carboxylation, reduction, and regeneration.

• Rubisco: Catalyzes carboxylation of ribulose-1,5-bisphosphate.

• Reduction: 3-phosphoglycerate reduced to glyceraldehyde-3-phosphate.

• Regeneration: Ribulose-1,5-bisphosphate regenerated for continued CO2 fixation.

Calvin Cycle Net Reaction

$3 \text{CO}_2 + 9 \text{ATP} + 6 \text{NADPH} \rightarrow 1 \text{G3P} + 9 \text{ADP} + 6 \text{NADP}^+ + 8 P_i$

Regulation of the Calvin Cycle

Calvin cycle enzymes are regulated by light, pH, magnesium ions, and thioredoxin-mediated redox changes. Rubisco activase removes inhibitors from rubisco's active site, responding to light and ATP/ADP ratio.

Carbohydrate Synthesis

• Sucrose: Synthesized in the cytosol from triose phosphates exported from the chloroplast.

• Starch: Synthesized in the chloroplast stroma from glucose-1-phosphate.

• Regulation: Enzymes are regulated by metabolites and energy status.

Rubisco's Oxygenase Activity and Photorespiration

Rubisco can catalyze the addition of O2 to ribulose-1,5-bisphosphate, producing phosphoglycolate, which is salvaged via the glycolate pathway (photorespiration). C4 and CAM plants minimize photorespiration by concentrating CO2 or fixing it at night.

• C4 Plants: Use Hatch–Slack cycle to concentrate CO2 in bundle sheath cells.

• CAM Plants: Fix CO2 at night, store malate, and refix CO2 during the day.

Summary Tables

Membrane Transport Mechanisms

Mechanism	Energy Required	Direction	Protein Required
Simple Diffusion	No	Down gradient	No
Facilitated Diffusion	No	Down gradient	Yes
Active Transport	Yes	Up gradient	Yes

ATP Yield from Glucose Catabolism

Pathway	ATP Yield
Glycolysis	2
Citric Acid Cycle	2
Oxidative Phosphorylation	~26–28
Total	~30–32

Photosynthetic Pathways

Pathway	Key Features
Noncyclic Electron Flow	Produces ATP, NADPH, O2
Cyclic Electron Flow	Produces ATP only
Calvin Cycle	Fixes CO2 into carbohydrates
Photorespiration	Salvages carbon from phosphoglycolate
C4 Pathway	Concentrates CO2 in bundle sheath cells
CAM Pathway	Fixes CO2 at night, minimizes water loss

Key Equations

• Diffusion Rate: $J_{in} = P \Delta C$

• Free Energy Change (Uncharged): $\Delta G = RT \ln \frac{[S]_{in}}{[S]_{out}}$

• Free Energy Change (Charged): $\Delta G = RT \ln \frac{[S]_{in}}{[S]_{out}} + zF \Delta \Psi$

• Michaelis–Menten Kinetics: $v = \frac{V_{max} [S]}{K_m + [S]}$

• Proton Motive Force: $\text{pmf} = \Delta \Psi - \frac{2.3RT}{F} \Delta pH$

• Reduction Potential and Free Energy: $\Delta G^\circ = -nF \Delta E^\circ$

Additional info:

• Tables and equations have been reconstructed and expanded for clarity.

• Some details about protein structure, regulatory mechanisms, and metabolic integration have been inferred for completeness.

• Key examples and applications (e.g., clinical relevance of GLUT1, importance of photorespiration, efficiency of aerobic respiration) have been added for academic context.