Cellular Membranes and Transport

Structure and Function of Cellular Membranes

Cellular membranes are essential for maintaining the internal environment of the cell and mediating interactions with the external environment.

• Fluid Mosaic Model: Cellular membranes are composed of a fluid mosaic of lipids and proteins, allowing for flexibility and dynamic function.

• Selective Permeability: The structure of the membrane results in selective permeability, meaning only certain substances can pass through freely.

Types of Membrane Transport

• Passive Transport: The diffusion of a substance across a membrane with no energy investment. Examples include simple diffusion, facilitated diffusion, and osmosis.

• Active Transport: Uses energy (usually from ATP) to move solutes against their concentration gradients.

• Bulk Transport: Large molecules and particles are transported across the plasma membrane by exocytosis (export) and endocytosis (import).

Role in Cell Signaling

• The plasma membrane plays a key role in most cell signaling processes, allowing cells to communicate and respond to their environment.

Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions that occur within an organism, transforming matter and energy to sustain life.

• Metabolic Pathways: A series of chemical reactions that either build up (anabolic) or break down (catabolic) molecules.

• Free Energy Change: Determines whether a reaction occurs spontaneously. Reactions with a negative free energy change are spontaneous.

• ATP: Adenosine triphosphate is the cell's main energy currency, coupling exergonic (energy-releasing) reactions to endergonic (energy-consuming) ones.

• Enzymes: Biological catalysts that speed up metabolic reactions by lowering activation energy barriers.

• Regulation: Enzyme activity is regulated to control metabolism and respond to cellular needs.

Aerobic Respiration, Glycolysis, and Fermentation

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP.

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

• Glycolysis: The first step in cellular respiration, occurring in the cytoplasm, where glucose is broken down into two molecules of pyruvate, producing a small amount of ATP and NADH.

• Pyruvate Oxidation and Citric Acid Cycle: After glycolysis, pyruvate is oxidized, and the citric acid cycle completes the energy-yielding oxidation of organic molecules, generating more NADH and FADH2.

• Oxidative Phosphorylation: During this stage, the electron transport chain uses electrons from NADH and FADH2 to produce ATP via chemiosmosis.

• Fermentation: In the absence of oxygen, fermentation allows glycolysis to continue by regenerating NAD+, producing lactic acid or ethanol as byproducts.

Key Equations

• Overall Cellular Respiration:

• Fermentation (Lactic Acid):

Photosynthesis: Light Reactions and Calvin Cycle

Overview of Photosynthesis

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

• Light Reactions: Capture light energy to produce ATP and NADPH, which are used in the Calvin cycle.

• Calvin Cycle: Uses ATP and NADPH to convert carbon dioxide into glucose and other organic molecules.

• Importance: Photosynthesis feeds the biosphere by providing the chemical energy and organic molecules required by most living organisms.

Key Equation

Summary Table: Cellular Respiration vs. Photosynthesis