Cell structures and functions

Endomembrane System

• The endomembrane system is a network of connected membranes and organelles that work together to synthesize, distribute, store, and export molecules.
• The system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

Endoplasmic Reticulum (ER)

• The ER is the largest component of the endomembrane system and is an extensive network of flattened sacs and tubules.
• There are two types of ER: rough ER and smooth ER.
• Rough ER has bound ribosomes that stud the outer surface of the membrane, whereas smooth ER lacks attached ribosomes.
• Rough ER is involved in the synthesis of proteins that are to be secreted or inserted into membranes.
• Smooth ER is involved in various metabolic processes, including the production of lipids, oils, and steroids.

Diffusion

• Diffusion is the passive transport of particles from an area of higher concentration to an area of lower concentration.
• Net diffusion continues until equilibrium is reached, where there is no net change in concentration on either side of the membrane.

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.
• If a membrane separates two solutions with different solute concentrations, water will cross the membrane, moving down its concentration gradient, until the solute concentration on both sides is equal.

Facilitated Diffusion

• Facilitated diffusion is the transport of specific molecules down their concentration gradient using transport proteins.
• Hydrophobic substances can easily diffuse across a cell membrane, but polar or charged substances require specific transport proteins to cross the membrane.

Active Transport

• Active transport is the movement of solutes against their concentration gradient, requiring energy input from ATP.
• Transport proteins use ATP to pump solutes across the membrane, often against their concentration gradient.

Exocytosis and Endocytosis

• Exocytosis is the process of exporting bulky molecules out of a cell, while endocytosis is the process of importing molecules into a cell.
• Phagocytosis is a type of endocytosis where the cell engulfs large particles or cells.

Tonicity

• Tonicity refers to the ability of a surrounding solution to cause a cell to gain or lose water.
• Isotonic solutions have the same solute concentration as the cell, whereas hypotonic solutions have a lower solute concentration, and hypertonic solutions have a higher solute concentration.

Cell Junctions

• There are three types of cell junctions in animal tissues: tight junctions, anchoring junctions, and gap junctions.
• Tight junctions prevent leakage of fluid between cells, anchoring junctions fasten cells together, and gap junctions allow small molecules to pass between cells.

Cell Wall

• Plant cells have a rigid cell wall that provides support and protection.
• The cell wall is composed primarily of cellulose, with other polysaccharides and proteins embedded in the matrix.

Plasma Membrane

• The plasma membrane is a fluid mosaic model, with a phospholipid bilayer embedded with protein molecules.
• The plasma membrane exhibits selective permeability, allowing certain molecules to pass through while keeping others out.
• Phospholipids spontaneously self-assemble into simple membranes, a critical step in the origin of life.

Learn about the interconnected membranes within a eukaryotic cell that make up the endomembrane system. Discover how organelles like the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles work together in synthesis, distribution, storage, and export of molecules.