Structure and Function of Cell Membranes

Overview of Cell Membranes

Cell membranes are essential structures that regulate the passage of substances into and out of cells. They are composed of a bilayer of phospholipids with embedded proteins, carbohydrates, and cholesterol, forming a dynamic and selectively permeable barrier.

• Selective Permeability: Only certain molecules can pass through the membrane freely; others require transport proteins.

• Phospholipid Bilayer: The fundamental structure, consisting of amphipathic phospholipids (hydrophilic heads, hydrophobic tails).

• Fluid Mosaic Model: Describes the membrane as a fluid combination of lipids, proteins, and carbohydrates.

Key Components

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. The composition can differ by fatty acid chain length, degree of saturation, and head group.

• Cholesterol: A steroid that modulates membrane fluidity and stability.

• Proteins: Embedded in the bilayer, serving as channels, carriers, receptors, and enzymes.

  • Peripheral Proteins: Loosely attached to the membrane surface.

  • Integral Proteins: Span the membrane; may function as channels or transporters.

• Carbohydrates: Usually attached to proteins (glycoproteins) or lipids (glycolipids); involved in cell recognition and communication.

Transport of Molecules Across Membranes

Passive Transport (No Energy Required)

Passive transport moves substances down their concentration gradient without the use of cellular energy.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2, H2O) directly through the lipid bilayer.

• Osmosis: Diffusion of water across a selectively permeable membrane.

  • Hypertonic Solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

  • Hypotonic Solution: Lower solute concentration outside; water moves in, cell swells.

  • Isotonic Solution: Equal solute concentration; no net water movement.

• Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, ions) via specific transport proteins (channels or carriers).

Active Transport (Energy Required)

Active transport moves substances against their concentration gradient, requiring energy (usually from ATP).

• Primary Active Transport: Direct use of ATP to transport molecules.

  • ATP-driven Ion Pumps: Example: Na+/K+ ATPase pumps Na+ out and K+ into the cell.

  • ABC Transporters: Use ATP to transport various molecules, including drugs, across membranes.

• Secondary Active Transport: Uses the electrochemical gradient created by primary active transport.

  • Symporters: Move two molecules in the same direction.

  • Antiporters: Move two molecules in opposite directions.

Metabolic Pathways

Overview of Metabolic Pathways

Metabolic pathways are series of chemical reactions in cells, each catalyzed by a specific enzyme. They can be catabolic (breaking down molecules to release energy) or anabolic (building complex molecules from simpler ones).

• Each reaction is catalyzed by a specific enzyme.

• Pathways are interconnected and regulated.

• Most pathways are conserved across life forms.

Coupling of Reactions

Cells couple exergonic (energy-releasing) and endergonic (energy-consuming) reactions to drive necessary processes. ATP is the main energy currency, linking catabolic and anabolic reactions.

Energy and ATP

• ATP (Adenosine Triphosphate): The primary energy carrier in cells. Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate (Pi).

• Free Energy (ΔG): The amount of energy available to do work. Negative ΔG indicates a spontaneous, energy-releasing reaction.

Redox Reactions in Metabolism

Redox (Reduction-Oxidation) Reactions

Redox reactions involve the transfer of electrons between molecules, crucial for energy production in cells.

• Oxidation: Loss of electrons or hydrogen atoms.

• Reduction: Gain of electrons or hydrogen atoms.

• NAD+/NADH and NADP+/NADPH: Electron carriers in redox reactions.

ATP Synthesis and Chemiosmosis

• Electron Transport Chain (ETC): Series of protein complexes in mitochondria (or chloroplasts) that transfer electrons and pump protons to create a proton gradient.

• ATP Synthase: Enzyme that uses the proton gradient to synthesize ATP from ADP and Pi.

Equation for ATP Hydrolysis:

Cellular Respiration

Stages of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose and other organic molecules. It consists of several stages:

1. Glycolysis: Occurs in the cytoplasm; glucose (6C) is split into two pyruvate (3C) molecules. Produces a net gain of 2 ATP and 2 NADH.

2. Pyruvate Oxidation: Pyruvate is converted to acetyl-CoA in the mitochondrial matrix, producing NADH and releasing CO2.

3. Citric Acid Cycle (Krebs Cycle): Acetyl-CoA enters the cycle, generating NADH, FADH2, ATP, and CO2.

4. Electron Transport Chain and Oxidative Phosphorylation: NADH and FADH2 donate electrons to the ETC, driving ATP synthesis via chemiosmosis.

Overall Equation for Aerobic Respiration:

Fermentation: In the absence of oxygen, cells use fermentation to regenerate NAD+ and produce a small amount of ATP (2 ATP per glucose).

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which autotrophs (such as plants and algae) convert light energy into chemical energy stored in glucose. It occurs in chloroplasts and involves two main stages: light reactions and the Calvin cycle.

Light Reactions

• Occur in the thylakoid membranes of chloroplasts.

• Use light energy to produce ATP and NADPH.

• Chlorophyll absorbs light, exciting electrons that are transferred through an electron transport chain.

• Water is split to provide electrons, releasing O2 as a byproduct.

Calvin Cycle (Carbon Fixation Reactions)

• Occurs in the stroma of chloroplasts.

• Uses ATP and NADPH from the light reactions to fix CO2 into organic molecules (e.g., glucose).

• Key enzyme: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).

• Three phases: carbon fixation, reduction, and regeneration of RuBP.

Summary Table: Comparison of Cellular Respiration and Photosynthesis

Key Terms and Definitions

• Autotroph: Organism that produces its own food from inorganic substances (e.g., plants).

• Heterotroph: Organism that obtains energy by consuming other organisms.

• Symporter: Transport protein that moves two substances in the same direction across a membrane.

• Antiporter: Transport protein that moves two substances in opposite directions.

• ATP Synthase: Enzyme that synthesizes ATP using a proton gradient.

• RuBisCO: Enzyme that catalyzes the first step of the Calvin cycle in photosynthesis.

Additional info: Some explanations and definitions have been expanded for clarity and completeness, and a summary table was inferred to aid comparison of major processes.