CYTOLOGY & HISTOLOGY Lecture Four DR. ASHRAF SAID

Review Of the third lecture

Concept 3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes

Start Of this lecture

Objectives of lecture three The Endoplasmic Reticulum: Biosynthetic Factory Lysosomes: Digestive Compartments

The endomembrane system Concept 4 The endomembrane system regulates protein traffic and performs metabolic functions in the cell The endomembrane system Includes many different structures

The Endoplasmic Reticulum: Biosynthetic Factory The endoplasmic reticulum (ER) Accounts for more than half the total membrane in many eukaryotic cells

The ER membrane Is continuous with the nuclear envelope Figure 1.12 Smooth ER Rough ER ER lumen Cisternae Ribosomes Transport vesicle Transitional ER 200 µm Nuclear envelope Figure 1.12

There are two distinct regions of ER The ER membrane There are two distinct regions of ER Smooth ER, which lacks ribosomes Rough ER, which contains ribosomes

Functions of ER The rough ER The smooth ER Rough Smooth Has bound ribosomes Produces proteins and membranes, which are distributed by transport vesicles The smooth ER Synthesizes lipids Metabolizes carbohydrates Stores calcium Detoxifies poison

The Golgi Apparatus: Shipping and Receiving Center Receives many of the transport vesicles produced in the rough ER Consists of flattened membranous sacs called cisternae

Functions of the Golgi apparatus include Modification of the products of the rough ER Manufacture of certain macromolecules

Functions of the Golgi apparatus cis face (“receiving” side of Golgi apparatus) Vesicles move from ER to Golgi Vesicles also transport certain proteins back to ER Vesicles coalesce to form new cis Golgi cisternae Cisternal maturation: Golgi cisternae move in a cis- to-trans direction Vesicles form and leave Golgi, carrying specific proteins to other locations or to the plasma mem- brane for secretion Vesicles transport specific proteins backward to newer Cisternae trans face (“shipping” side of 0.1 0 µm 1 6 5 2 3 4 Golgi apparatus Figure 1.13 TEM of Golgi apparatus

Membrane proteins and lipids Are synthesized in the ER and Golgi apparatus Figure 7.10 Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus Vesicle glycoprotein Membrane glycolipid Plasma membrane: Cytoplasmic face Extracellular face Secreted 4 1 2 3 ER

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large proteins Cross the membrane by different mechanisms

Exocytosis In exocytosis Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents

Endocytosis In endocytosis The cell takes in macromolecules by forming new vesicles from the plasma membrane

Three types of endocytosis In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane- enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. EXTRACELLULAR FLUID Pseudopodium CYTOPLASM “Food” or other particle Food vacuole 1 µm of amoeba Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). PINOCYTOSIS Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). 0.5 µm In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. Plasma membrane Vesicle PHAGOCYTOSIS Figure 7.20

RECEPTOR-MEDIATED ENDOCYTOSIS Ligand Coat protein Coated pit vesicle A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Plasma membrane Coat protein Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

Lysosomes: Digestive Compartments A lysosome Is a membranous sac of hydrolytic enzymes Can digest all kinds of macromolecules

Lysosomes carry out intracellular digestion by phagocytosis (a) Phagocytosis: lysosome digesting food 1 µm Lysosome contains active hydrolytic enzymes Food vacuole fuses with lysosome Hydrolytic enzymes digest food particles Digestion Food vacuole Plasma membrane Lysosome Digestive Nucleus Figure 1.14 A

(b) Autophagy: lysosome breaking down damaged organelle Lysosome containing two damaged organelles 1 µ m Mitochondrion fragment Peroxisome Lysosome fuses with vesicle containing damaged organelle Hydrolytic enzymes digest organelle components Vesicle containing damaged mitochondrion Digestion Lysosome Figure 1.14 B Autophagy

Intercellular Junctions

Animals: Tight Junctions, Desmosomes, and Gap Junctions In animals, there are three types of intercellular junctions Tight junctions Desmosomes Gap junctions

Types of intercellular junctions in animals Tight junctions prevent fluid from moving across a layer of cells Tight junction 0.5 µm 1 µm Space between cells Plasma membranes of adjacent cells Extracellular matrix Gap junction Tight junctions 0.1 µm Intermediate filaments Desmosome Gap junctions At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continu- ous seals around the cells, tight junctions prevent leakage of extracellular fluid across A layer of epithelial cells. Desmosomes (also called anchoring junctions) function like rivets, fastening cells Together into strong sheets. Intermediate Filaments made of sturdy keratin proteins Anchor desmosomes in the cytoplasm. Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for commu- nication between cells in many types of tissues, including heart muscle and animal embryos. TIGHT JUNCTIONS DESMOSOMES GAP JUNCTIONS Figure 1.31

Tank you End of first Lecture