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Endocytosis and

Exocytosis

Mr. Gunjankumar Mehta,

Dept. of Biotechnology, Shree M. & N. Virani Science College,

Rajkot- 360005, Gujarat, IndiaEmail: gjmehta@vsc.edu.in

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Contents• Endocytosis & Exocytosis• Process & functions of phagocytosis• Receptor mediated endocytosis• Caveolae• Exocytosis• Membrane fusion during endocytosis and

exocytosis• Biological importance of exocytosis• References

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Endocytosis & Exocytosis• Active transport mechanisms• Endocytosis:

The process of taking material into the cell such as liquids or fairly large molecules by engulfing (surrounding) them in a membrane.

• Exocytosis:

It is the release or expelling of substances out of a cell by the fusion of a vesicle WITH the membrane.

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Endocytosis• The term "endocytosis" was coined by

Christian deDuve in 1963 which includes internalization of solid as well as liquid materials both.

• Phagocytosis(Cell eating): Large solid particles engulfed by specialized cells

• Pinocytosis(Cell drinking)/Bulk phase endocytosis: Non- specific Internalization of liquid material

• Receptor mediated endocytosis: Specific internalization of liquid material

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Steps of Endocytosis

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Process of Phagocytosis• During phagocytosis cells engulf large

particles. • Binding of the particle to receptors on the

surface of the phagocytic cell triggers the extension of pseudopodia—an actin-based movement of the cell surface.

• The pseudopodia eventually surround the particle and their membranes fuse to form a large intracellular vesicle (>0.25 µm in diameter) called a phagosome.

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Process of Phagocytosis[Cont.]

• The phagosomes then fuse with lysosomes, producing phagolysosomes in which the ingested material is digested by the action of lysosomal acid hydrolases.

• During maturation of the phagolysosome, some of the internalized membrane proteins are recycled to the plasma membrane.

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1- 2 µm

Ephagy/ egestion

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Examples of phagocytic cells (A) An amoeba engulfing another protist. (B) Macrophages ingesting red blood cells

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Kinds of Phagocytosis• Types on the basis of physical and chemical

nature of foreign substance.

1. Ultraphagocytosis/ colloidopexy: Process in which plasma membrane ingests small colloidal particles. Example: Leukocytes and Macrophages

2. Chromopexy: Cell ingests colloidal chromogen particles phagocytotically. Example: Some mesoblastic cells

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Functions of phagocytosis

1. Many amoebas use phagocytosis to capture food particles.

2. Phagocytes are specialized cells which defend against invading pathogens and eliminated aged/ damaged cells from body.

3. Mammalian professional phagocytes- Macrophages and Neutrophils. The macrophages of the human spleen and liver are responsible for the disposal of more than 1011 aged blood cells on a daily basis.

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Pinocytosis(Cell drinking)/Bulk phase endocytosis

• Non- specific uptake of small droplets of extracellular fluid by endocytic “Pinosomes” formed due to plasma membrane invaginations.

• First observed by Edward in amoeba and Lewis(1931) in cultured cells.

• Diameter: 0.1- 0.2 µm• In amoeba, from the end of tiny pinocytic

channels pinosomes are pinched off and combined with lysosome to make “Food vacuoles” Digestion of ingested material occurs.

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Micropinocytosis(Cell drinking)/Transcytosis

• Micropinocytosis: Pinocytosis occurring at sub- cellular or sub- microscopic level.

• Plasma membrane invaginates to form small vesicles of 650 A° dm. They are non clathrin- coated vesicles and move across the plasma membrane of endothelial cells(blood capillary lining) discharging their content. This is called transcytosis.

• [Note: Such transcytosis also known to occur in other cells also such as Schwan cells, setellite cells of nerve ganglion, macrophage, muscle and reticular cells but vesicles are clathrin coated. ]

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Transcytosis of IgA into Lungs from Plasma cells to mucous secreting cells

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Receptor mediated endocytosis• A specific receptor on plasma membrane

recognizes and binds with extracellular macromolecules(Ligands).

• Best-characterized form of pinocytosis. • Examples of ligands:

– Viruses– Small proteins(Insuline, vittelogenin, Ig, transferrin

etc)

– Vitamin B12, Hormones, Enzymes, Blood- born proteins

– Cholesterol containing LDL– Oligosachharides

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Receptor mediated endocytosisMechanism

1. Interaction of ligands and cell surface receptors

2. Formation of coated pits and coated vesicles

3. Fusion of endocytic vesicles and endosome

4. Endolysosome formation and digestion

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1. Interaction of ligands and cell surface receptors• More than 25 different types of receptors

are involved in receptor mediated endocytosis.

• Receptor mediated endocytosis begins at specialized regions of plasma membrane known as Coated- pits.

• Coat is made up of Clathrin and certain other protein.

• Clathrin protein:– 3 large polypeptide chain– 3 small polypeptide chain– Three legged structure– Assemble on cytoplasmic surface like a

basket like network of hexagons- pentagons.

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1. Interaction of ligands and cell surface receptors• The macromolecules binds to

complementary cell surface receptor.

• These receptors are transmembrane protein having two specific binding sites:

1. Ligand binding site- external surface of plasma membrane

2. Coated pit binding site- Cytosolic face of membrane.

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2. Formation of coated pits and coated vesicles• Coated pits- depressions on plasma

membrane.• Ligand loaded receptors diffuse into

coated pits.• In fact, coated pit may accommodate

1000 receptors selectively and act as filtration/ concentrating device increasing the efficiency of internalization of particular receptor by 1000X.

• Life time of each pit is short- within a min. it invaginates the membrane and piches off to form coated vesicle.

• Process is assisted by membrane associated GTP binding protein- Dynamin.

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3. Fusion of endocytic vesicles and endosome• Clathrin dissociates from coated vesicles and return

back to the plasma membrane and form new coated pits.

• Resultant endocytic vesicle gets fused with pre- existing endosome and ultimately its contents are utilized.

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Endosome/ Receptosome• Heterogenous membrane bound vesicles• Extends from periphery of cells to the

perinuclear region near to golgi body.• Two types: • 1. Peripheral endosomes• 2. Perinuclear endosomes• The interior of endosomes is acidic(pH= 5- 6)

due to the presence of ATP driven proton pump, which pumps in the H+ into lumen. It causes dessociation of Receptor- Ligand complex.

• They lack degradative enzyme.

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4. Endolysosome formation and digestion• Coated vesicles devivers receptor- ligand

complexes to peripheral endosomes.• Which later on become perinuclear endosome. • These perinuclear endosomes get converted

into endolysosome and lysosome eventually due to following procedures:

• 1. Fusion of golgi originated transport vesicle• 2. Membrane retrieval• 3. Increased acidification

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Example of receptor mediated endocytosis- LDL

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Caveolae• Caveolae are the most common reported non-

clathrin-coated plasma membrane buds, which exist on the surface of many, but not all cell types.

• They consist of the cholesterol-binding protein caveolin (Vip21) with a bilayer enriched in cholesterol and glycolipids.

• Approx. 50 nm in diameter) flask-shape pits- shape of a cave (hence the name caveolae).

• Abundant in smooth muscle, type I pneumocytes, fibroblasts, adipocytes, and endothelial cells.

• Uptake is receptor mediated.

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Energy utilization by phagocytosis and endocytosis

• Pinocytosis- Constitutive process• Phagocytosis- induced process• Both are active mechanisms• During phagocytosis- Leukocyte oxygen

consumption, Glucose uptake and glycogen breakdown significantly increases

• Cytoskeletal microfilaments- actin- myosin causes plasma membrane invaginations which is active process.

• Cytochalasin B- endocytosis

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Exocytosis• Opposite of endocytosis, it is the release or

expelling of substances out of a cell by the fusion of a vesicle WITH the membrane.

• Also known as Emeiocytosis/ Cell vomiting.• Secretary vesicles carrying new plasma

membrane and cellular secretions arise from golgi and fuse with plasma membrane.

• RERProteinsERlumenGlycosylatedTransported to GolgiPackaged into vesicles and pinched off from trans golgiPlasma membrane fusion.

• Waste material discharge.

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Process of Exocytosis

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Steps in ExocytosisExocytosis

Vesicle trafficking

Vesicle tethering

Vesicle docking

Vesicle primingVesicle fusion mediated

by fusogenic proteins

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Membrane fusion during phagocytosis and Endocytosis

Membrane fusion:•Region of Lipid bilayers come into

close proximity•Bilayer adherence•Bilayer joining•Mediated by specific types of

proteins: Fusogenic protein.

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Biological importance of exocytosis• Secretion of proteins like enzymes, peptide

hormones and antibodies from cells.• Turnover of plasma membrane• Release of neurotransmitter from presynaptic

neurons• Placement of integral membrane proteins• Acrosome reaction during fertilization• Antigen presentation during the immune

response.• Recycling of plasma membrane bound

receptors.

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References

• The Cell- A Molecular Approach- 4th edition, GEOFFREY M.COOPER, ROBERT E. HAUSMAN.

• Cell and Molecular biology- 6th edition, GERALD KARP

• Cell Biology, Genetics, Molecular Biology, Evolution and Ecology-1st Edition, Verma and Agrawal.

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