Endomembrane system: protein targeting and
sorting, endocytosis, exocytosis
By: Hari Yadav
M.Sc. (Zoology)
Department of Zoology
School of Biological Sciences
Dr. Hari Singh Gour Central University
Sagar - 470003 Madhya Pradesh, India

ENDOPLASMIC RETICULUM (ER)
Rough ER Smooth ER
Flagellum
Centrosome
CYTOSKELETON:
Microfilaments
Intermediate
filaments
Microtubules
Microvilli
Peroxisome
Mitochondrion
Lysosome
Golgi
apparatus
Ribosomes
Plasma
membrane
Nuclear
envelope
Nucleolus
Chromatin
NUCLEUS

The endomembrane system
regulates protein traffic and performs metabolic
functions in the cell
• The endomembrane system is composed of membrane bound structures that are suspended in the
cytoplasm of a eukaryotic cell.
• These membranes divide the cell into functional and structural compartments.
• Components of the endomembrane system:
– Nuclear envelope
– Endoplasmic reticulum
– Golgi apparatus
– Lysosomes
– Vacuoles
– Plasma membrane
• These components are either continuous or connected via transfer by
vesicles

The Endoplasmic Reticulum: Biosynthetic Factory
• Endoplasmic reticulum (ER) is the largest single membrane
bound intracellular compartment.
• The endoplasmic reticulum (ER) accounts for more than half of
the total membrane in many eukaryotic cells.
• The ER membrane is continuous with the nuclear
envelope.
• There are two distinct regions of ER:-
– Smooth ER, which lacks ribosomes.
– Rough ER, with ribosomes attached on its surface.

Smooth ER
Rough ER Nuclear
envelope
Transitional ER
Rough ER
Smooth ER
Ribosomes
Transport vesicle
ER lumen
Cisternae
200 nm

Functions of Smooth ER and Rough ER
• The smooth ER
– Synthesizes lipids
– Metabolizes carbohydrates
– Detoxifies poison
– Stores calcium
• The rough ER
– Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to
carbohydrates)
– Distributes transport vesicles, proteins surrounded by membranes
– Is a membrane factory for the cell

• The Golgi apparatus was first discovered in 1897 by Italian physician Camillo
Golgi.
• The Golgi complex , also termed as Golgi body or Golgi apparatus, is a single
membrane bound organelle and part of endomembrane system.
• The Golgi apparatus consists of five to eight flattened membrane bound sacs
called cisternae.
• Functions of the Golgi apparatus:-
– Modifies products of the ER
– Manufactures certain macromolecules
– Sorts and packages materials into transport vesicles
The Golgi Apparatus: Shipping and Receiving Center

cis face
(“receiving” side of
Golgi apparatus) Cisternae
trans face
(“shipping” side of
Golgi apparatus)
TEM of Golgi apparatus
0.1 µm

Lysosomes: Digestive Compartments
• Lysosomes are single membrane – bound organelles present in animal cells.
• Lysosomes have acidic internal pH (about 5) and are filled with hydrolytic enzymes.
• A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules.
• Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids.
• Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole.
• A lysosome fuses with the food vacuole and digests the molecules.
• Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a
process called autophagy.

Nucleus 1 µm
Lysosome
Lysosome
Digestive
enzymes
Plasma
membrane
Digestion
Food vacuole
(a) Phagocytosis

Vacuoles: Diverse Maintenance
Compartments
• A plant cell or fungal cell may have one or several vacuoles.
• Food vacuoles are formed by phagocytosis.
• Contractile vacuoles, found in many freshwater protists, pump
excess water out of cells.
• Central vacuoles, found in many mature plant cells, hold organic
compounds and water.

Smooth ER
Nucleus
Rough ER
cis Golgi
Plasma
membrane
trans Golgi

Cell Component Structure Function
Concept 6.4
The endomembrane system
regulates protein traffic and
performs metabolicfunctions
in the cell
Endoplasmic reticulum
(Nuclear
Golgiapparatus
Lysosome
Vacuole Large membrane-bounded
vesicle in plants
enzymes (in animalcells)
Stacks of flattened
membranous
sacs; has polarity
(cis and trans
faces)
envelope) sacs; membraneseparates
lumen fromcytosol;
continuous with
the nuclear envelope.
Extensive network of Smooth ER: synthesis of
membrane-bound tubules and lipids, metabolism of carbohy-
drates, Ca2+ storage, detoxifica-
tion of drugs and poisons
Rough ER: Aids in sythesis of
secretory and other proteins
from bound ribosomes; adds
carbohydrates to glycoproteins;
produces new membrane
Modification of proteins, carbo-
hydrates on proteins, and phos-
pholipids; synthesis of many
polysaccharides; sorting of
Golgi products, which are then
released in vesicles.
Membranous sac of hydrolytic Breakdown of ingested sub-
stances cellmacromolecules,
and damaged organelles for
recycling
Digestion, storage, waste
disposal, water balance,cell
growth, and protection

Protein Targeting / Sorting
• Protein targeting or protein sorting is the biological mechanism by which
proteins are transported to their appropriate destinations in the cell or outside it.
• Proteins can be targeted to the inner space of an organelle, different intracellular
membranes, plasma membrane, or to exterior of the cell via secretion.
• Proteins synthesized by the cytosolic ribosomes, have a specialized amino acid
sequence attached to them, which decides their fate of translocation into
different organelles.
• These sequences are targeting or sorting sequences, proteins without these signal
amino acids are permanent residents of the cytoplasm and are not transported
anywhere.
• Sorting sinals can be signal peptide.
• Protein translocation may occur co – translationally or post – translationally.

Endocytosis
 The term endocytosis was given by Christian de Duve in the year 1963.
 Endocytosis is a process whereby eukaryotic cells internalize material from their surrounding
environment.
 Internalization is achieved by the formation of membrane – bound vesicles at the cell surface that
arise by progressive invagination of the PM, followed by pinching off and release of free vesicles into
the cytoplasm.
 Classically, endocytosis has been divided into phagocytosis ( cellular eating ) and pinocytosis (
cellular drinking).
1 Phagocytosis;- Phagocytosis or cell eating describes the internalization of large particles following
particle binding to specific plasma membrane receptors and by the formation of large endocytic vesicles
called phagosomes.
2 Pinocytosis:- Pinocytosis or
cell drinking involves the
ingestion of fluid by the formation
of small endocytic vesicles.

Exocytosis
• Exocytosis is the process of moving materials from within a cell to the exterior of
the cell.
• This process requires energy and is therefore a type of active transport.
• Exocytosis is an important process of plant and animal cells as it performs the
opposite function of endocytosis.
• In exocytosis, membrane-bound vesicles containing cellular molecules are
transported to the cell membrane.