4: The Endomembrane System in Cell Biology

Endomembrane System

Divides the cell into compartments, channels the passage of molecules, and provides surfaces for lipid and protein synthesis.

Endoplasmic Reticulum (ER)

A network of membranes forming flattened sacs (cisternae) and tubules.

Cisternal Space (Lumen)

Interior of the ER.

Cytosol

Fluid outside the ER.

Rough ER (RER)

ER with ribosomes attached to the cytoplasmic surface.

Protein synthesis

Produces proteins destined for secretion, lysosomes, or the plasma membrane.

Glycosylation

Adds carbohydrate chains to proteins, forming glycoproteins.

Vesicle Formation

Proteins are packaged into vesicles for transport to the Golgi apparatus.

Smooth ER (SER)

ER with few or no ribosomes.

Lipid synthesis

Produces phospholipids and steroids.

Detoxification

Liver cells modify harmful substances (e.g., drugs, alcohol).

Calcium storage

Stores Ca²⁺, which is used in cell signaling (e.g., muscle contraction).

Golgi Apparatus

Stack of flattened membrane sacs called cisternae.

Cis Face

Receiving side of the Golgi apparatus (near the ER).

Trans Face

Shipping side of the Golgi apparatus (away from the ER).

Modification

Adds or modifies carbohydrate chains on proteins and lipids (glycoproteins and glycolipids).

Sorting and Packaging

Packages molecules into vesicles for transport to their final destinations.

Cell Wall Synthesis

In plants, synthesizes non-cellulose polysaccharides for the cell wall.

Vesicular Transport

Proteins and lipids move from the ER to the Golgi via transport vesicles.

Lysosomes

Membrane-bound vesicles containing hydrolytic enzymes.

Acidic pH

Maintained by proton pumps in the lysosomal membrane.

Digestion

Breaks down macromolecules (proteins, lipids, carbohydrates, nucleic acids).

Recycling

Degrades old or damaged organelles (autophagy).

Defense

Destroys pathogens engulfed by phagocytosis (e.g., in white blood cells).

Lysosomal Storage Disorders

Genetic disorders caused by defective lysosomal enzymes.

Tay-Sachs disease

Accumulation of glycolipids in nerve cells.

Vacuoles

Membrane-bound sacs.

Tonoplast

Membrane surrounding the vacuole.

Storage

Stores water, nutrients, ions, and waste products.

Water Balance

Maintains cell turgor pressure in plants.

Growth

Central vacuole expands to increase cell size.

Detoxification

Segregates harmful substances.

Central Vacuole

Large vacuole in plant cells.

Contractile Vacuole

Found in some protists, pumps water to maintain osmotic balance.

Peroxisomes

Small, spherical organelles containing oxidative enzymes.

Fatty Acid Oxidation

Breaks down fatty acids.

Detoxification (Peroxisomes)

Converts harmful substances (e.g., hydrogen peroxide) into water and oxygen using catalase.

Peroxisome Biogenesis

Forms from ER-derived vesicles or by division of existing peroxisomes.

Peroxisome Disorders

Genetic diseases caused by defects in peroxisome biogenesis (e.g., Zellweger syndrome).

Lipid Droplets

Organelles with a neutral lipid core surrounded by a phospholipid monolayer.

Energy Storage

Stores triglycerides and sterol lipids.

Membrane Synthesis

Acts as a hub for lipid metabolism.

Microbodies

A diverse group of small vesicles containing enzymes.

Endomembrane System

The ER (RER and SER) organizes biosynthetic activities, including protein and lipid synthesis.

Golgi Apparatus

Modifies, sorts, and packages proteins and lipids for transport.

Lysosomes

Contain digestive enzymes for breaking down macromolecules and old organelles.

Ribosomes on RER

Synthesize proteins destined for secretion, lysosomes, or the plasma membrane.

Cytoplasmic Ribosomes

Synthesize proteins for use within the cytoplasm, nucleus, or mitochondria.

Protein Processing Pathway

RER: Protein synthesis and initial glycosylation.

Transport Vesicles

Carry proteins to the Golgi.

Secretory Vesicles

Transport proteins to their final destinations (e.g., plasma membrane, lysosomes).