Endomembrane System Overview

Endomembrane System Overview

• The endomembrane system is composed of different membranes suspended in the cytoplasm of a eukaryotic cell.
• Key organelles involved in the endomembrane system include the nuclear membrane, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles (which are present in plant cells).
• The endomembrane system is responsible for producing membranes and carrying out many biosynthetic functions.
• Approximately 50% of membranes in eukaryotic cells are part of the endomembrane system.

Endoplasmic Reticulum (ER)

• The ER is a network of membranes connected to the nuclear envelope.
• Rough ER has ribosomes attached to its cytoplasmic surface. Ribosomes on the Rough ER make proteins that are incorporated into the lumen (inner space) of the ER. The Rough ER also helps modify proteins that will be secreted from the cell, and make phospholipids for other cellular membranes.
• Smooth ER lacks ribosomes and plays a role in metabolic processes such as lipid synthesis, detoxification, and carbohydrate hydrolysis. Steroids and sex hormones are synthesized in the smooth ER. Glycogen in the liver is broken down into glucose; drugs and poisons are also detoxified in the smooth ER. In muscles there is special smooth ER called sarcoplasmic reticulum, it surrounds each myofibril and stores and releases calcium.

Golgi Apparatus

• The Golgi apparatus is found in both plant and animal cells.
• Its function is packaging proteins and chemical products.
• It has a cis face (receiving) and a trans face (shipping). Incoming transport vesicles from the endoplasmic reticulum travel to the cis face. Newly forming vesicles are then moved to the trans face. Secretory vesicles containing the final products exit from the trans face.
• The Cisternae shape (flattened membrane sacs or stacks) of the Golgi apparatus are used for modifying the proteins.

Lysosomes

• Lysosomes contain digestive enzymes and act as the cell's recycling facility.
• They break down old and unnecessary cellular structures.
• Example: White blood cells called macrophages use phagocytosis (engulfing a particle) to destroy pathogens. Lysosomes work within the macrophage to digest and break down the pathogen.

Vacuoles (Plant Cells)

• Vacuoles in plants store water and waste, isolate harmful materials, and break down macromolecules and cellular components.
• Vacuoles help maintain water balance within the plant cell.

Peroxisomes

• Peroxisomes contain enzymes involved in oxidation reactions and produce hydrogen peroxide as a by-product.
• Some functions include bile acid synthesis; purine catabolism (breakdown); lipid biosynthesis; viral innate immune defense; thermogenesis (heat production), oxidation of fatty acids and production of ROS/NOS (reactive oxygen species/nitric oxide).

Endocytosis & Exocytosis

• Endocytosis is a process where the cell takes up material by forming a vesicle from the plasma membrane to surround and engulf the material.
• Types of Endocytosis: -Phagocytosis: "cellular eating" - engulfing large particles (e.g. bacteria, viruses) -Pinocytosis: "cellular drinking"- engulfing dissolved substances.
• Exocytosis is a process where the cell discharges material by fusing a secretory vesicle with the plasma membrane.

Plasma Membrane

• The plasma membrane is selectively permeable; allowing only certain substances to pass.
• It is a phospholipid bilayer, with a hydrophobic (nonpolar) interior and hydrophilic (polar) exterior.
• The fluid mosaic model depicts the membrane as a fluid structure with proteins embedded within the phospholipid bilayer.

Membrane Transport

• Passive Transport: The movement of substances across the membrane down their concentration gradient (needs no energy).
  • Diffusion - net movement of a substance from an area of high concentration to low concentration.
  • Osmosis: A form of diffusion involving the movement of water across a semipermeable membrane. Water flows from an area of low solute concentration to high solute concentration.
  • Facilitated Diffusion: Uses membrane channel proteins to enhance diffusion of substances (no energy required).
• Active Transport: Movement of substances across the membrane against their concentration gradient (requires energy). Example: Carrier mediated active transport, endocytosis and exocytosis.

Membrane Functions

• Protection
• Communication
• Selectively allow substances
• Respond to the environment
• Recognition

Types of Passive Transport

• Diffusion
• Osmosis
• Facilitated Diffusion

Explore the endomembrane system in eukaryotic cells, including its key organelles and functions. Learn about the roles of the nuclear membrane, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles. This quiz will enhance your understanding of cellular membranes and biosynthetic processes.