Cell Membrane Structure and Function

The Fluid Mosaic Model

The cell membrane is a dynamic and complex structure that separates the interior of the cell from its external environment. The fluid mosaic model describes the membrane as a flexible layer made of lipid molecules interspersed with proteins and other components.

• Phospholipid bilayer: The fundamental structure, composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Embedded within the bilayer, cholesterol molecules help regulate membrane fluidity and stability.

• Proteins:

  • Integral proteins: Span the membrane and are involved in transport and signaling.

  • Peripheral proteins: Attached to the membrane surface, often involved in signaling or maintaining cell shape.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface, important for cell recognition and communication.

• Extracellular matrix (ECM): A network of fibers outside the cell that provides structural support and mediates cell signaling.

Example: The presence of glycoproteins on red blood cells determines blood type.

Transport Across Cell Membranes

Types of Membrane Transport

Cells regulate the movement of substances across their membranes through various transport mechanisms, which can be classified as passive or active transport.

• Passive Transport: Movement of molecules down their concentration gradient without energy input.

  • Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2) across the membrane.

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

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

  • Example: The sodium-potassium pump ( out, in) maintains electrochemical gradients in animal cells.

• Bulk Transport: Large particles or volumes are transported via vesicles in processes such as endocytosis (into the cell) and exocytosis (out of the cell).

Cell Signaling and Response

Signal Transduction Pathways

Cells detect and respond to external signals through a series of steps known as signal transduction. This process ensures that cells can adapt to changes in their environment.

• Signal Reception: A signaling molecule (ligand) binds to a receptor protein on the cell membrane.

• Signal Transduction: The signal is relayed through a cascade of intracellular molecules, often involving phosphorylation events.

• Cellular Response: The cell responds by altering gene expression, metabolism, or behavior.

Example: Insulin binding to its receptor triggers glucose uptake in muscle and fat cells.

Cellular Energy and Metabolism

Exergonic and Endergonic Reactions

Biochemical reactions in cells can either release energy (exergonic) or require energy input (endergonic).

• Exergonic reactions: Release free energy; have a negative change in Gibbs free energy ().

• Endergonic reactions: Require energy input; have a positive change in Gibbs free energy ().

• Reaction coupling: Cells often couple exergonic reactions (like ATP hydrolysis) to drive endergonic processes.

Equation:

where is the change in free energy, is the change in enthalpy, is temperature, and is the change in entropy.

Enzymes and Activation Energy

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy () required for the reaction to proceed.

• Enzyme-substrate complex: The enzyme binds to its substrate, facilitating the reaction.

• Effect on : Enzymes do not change the overall free energy change of a reaction.

Diagram: (Described) A graph showing the course of reaction with and without enzyme, where the activation energy peak is lower with enzyme, but the difference in free energy between reactants and products () is unchanged.

Overview of Cellular Respiration

Stages of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose to produce ATP. It occurs in several stages:

• Glycolysis: Occurs in the cytosol; glucose is split into two molecules of pyruvate.

• Pyruvate Oxidation: Pyruvate is converted to acetyl-CoA in the mitochondria.

• Citric Acid Cycle (Krebs Cycle): Acetyl-CoA is oxidized, producing NADH, FADH2, and CO2.

• Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis, producing the majority of ATP.

Overall equation:

ATP Production and the Proton Gradient

The electron transport chain creates a proton gradient across the inner mitochondrial membrane. ATP synthase uses this gradient to synthesize ATP from ADP and inorganic phosphate.

• Chemiosmosis: The movement of protons back into the mitochondrial matrix drives ATP production.

• Oxidative phosphorylation: The process of ATP generation using the electron transport chain and chemiosmosis.

Fermentation

When oxygen is not available, cells can generate ATP through fermentation.

• Lactic acid fermentation: Converts pyruvate to lactic acid (e.g., in muscle cells).

• Alcoholic fermentation: Converts pyruvate to ethanol and CO2 (e.g., in yeast).

Overview of Photosynthesis

Photosynthesis Process

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

• Occurs in chloroplasts of plant cells.

• Light reactions: Capture light energy to produce ATP and NADPH.

• Calvin cycle: Uses ATP and NADPH to fix CO2 into sugars.

Overall equation:

Photosystems and Electron Transport

There are two main photosystems involved in the light reactions:

• Photosystem II (PSII): Absorbs light, splits water, and transfers electrons to the electron transport chain.

• Photosystem I (PSI): Absorbs light and facilitates the production of NADPH.

Electron flow through these systems generates a proton gradient used to produce ATP (photophosphorylation).

Experimental Design and Analysis

Photosynthesis and Respiration Experiments

Experiments can be designed to measure the rates of photosynthesis and respiration by monitoring changes in CO2 and O2 levels.

• Algae in Light: In a closed flask with photosynthetic algae exposed to light, CO2 levels decrease and O2 levels increase due to photosynthesis.

• Algae in Darkness: Without light, photosynthesis stops, so CO2 increases and O2 decreases due to cellular respiration.

• Yeast with Glucose: In a closed flask with yeast and glucose, CO2 increases and O2 decreases as yeast respires aerobically. If O2 is removed, fermentation occurs, producing ethanol (or lactic acid) and CO2.

Example: Measuring gas changes in a sealed flask can help determine whether photosynthesis or respiration is occurring.

Summary Table: Comparison of Cellular Respiration and Photosynthesis