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 based on fatty acid chain length, degree of saturation, and head group.

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

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

  • Peripheral Proteins: Loosely attached to the membrane surface.

  • Integral Proteins: Span the membrane; some 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 involves the movement of 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 hydrolysis).

• Primary Active Transport: Direct use of ATP.

  • 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 established by primary active transport.

  • Symporters: Move two molecules in the same direction.

  • Antiporters: Move two molecules in opposite directions.

Metabolic Pathways

Overview of Metabolism

Metabolic pathways are series of chemical reactions that transform molecules within cells. They are essential for energy production, biosynthesis, and cellular maintenance.

• Catabolic Pathways: Break down molecules to release energy (e.g., cellular respiration).

• Anabolic Pathways: Build complex molecules from simpler ones (e.g., photosynthesis).

• Most metabolic pathways are conserved across life forms.

Coupling of Reactions

Cells couple exergonic (energy-releasing) and endergonic (energy-consuming) reactions to drive essential processes. ATP is the primary energy currency.

• ATP (Adenosine Triphosphate): Stores energy in high-energy phosphate bonds. Hydrolysis of ATP to ADP releases energy for cellular work.

• Free Energy Change (ΔG): Indicates whether a reaction is spontaneous. Negative ΔG means energy is released.

Redox Reactions in Metabolism

Redox (Oxidation-Reduction) Reactions

Redox reactions involve the transfer of electrons between molecules, crucial for energy extraction 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.

Cellular Respiration

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

1. Glycolysis: Glucose (6C) is split into two pyruvate (3C) molecules in the cytoplasm. Produces ATP and NADH.

2. Pyruvate Oxidation: Pyruvate enters mitochondria, converted to acetyl-CoA, producing NADH and CO2.

3. Citric Acid Cycle (Krebs Cycle): Acetyl-CoA is oxidized, generating ATP, NADH, FADH2, and CO2.

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation: NADH and FADH2 donate electrons to the ETC in the mitochondrial inner membrane, creating a proton gradient used to synthesize ATP via ATP synthase.

Equation for Cellular Respiration:

Fermentation: In the absence of oxygen, cells use fermentation to regenerate NAD+ and produce ATP (less efficient than aerobic respiration).

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which autotrophs (plants, algae, some bacteria) 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 move through the 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 convert CO2 into glucose.

• Key enzyme: Rubisco (catalyzes the fixation of CO2 to RuBP).

• Products: G3P (glyceraldehyde-3-phosphate), which can be used to form glucose and other carbohydrates.

Summary Table: Cellular Respiration vs. Photosynthesis

Autotrophs vs. Heterotrophs

• Autotrophs: "Self-feeders"; organisms that produce their own food via photosynthesis.

• Heterotrophs: "Other-feeders"; organisms that obtain food by consuming other organisms.

Key Terms and Definitions

• Phospholipid Bilayer: Double layer of phospholipids forming the basic structure of cell membranes.

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

• ATP: Adenosine triphosphate, the main energy currency of the cell.

• Redox Reaction: Chemical reaction involving the transfer of electrons.

• Glycolysis: First step of cellular respiration, breaking down glucose into pyruvate.

• Calvin Cycle: Series of reactions in photosynthesis that produce sugars from CO2.