CYTOLOGY & HISTOLOGYLecture 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 threeThe Endoplasmic Reticulum: Biosynthetic FactoryLysosomes: Digestive Compartments

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

Smooth ER

Rough ER

ER lumenCisternae

RibosomesTransport vesicle

Smooth ER

Transitional ER

Rough ER 200 µm

Nuclearenvelope

Figure 1.12

The ER membrane

There are two distinct regions of ER–Smooth ER, which lacks

ribosomes

–Rough ER, which contains ribosomes

Functions of ER

The smooth ER– Synthesizes lipids

– Metabolizes carbohydrates

– Stores calcium

– Detoxifies poison

The rough ER– Has bound ribosomes

– Produces proteins and membranes, which are distributed by transport vesicles

Smooth Rough

The Golgi apparatus– Receives many of the transport vesicles produced in

the rough ER– Consists of flattened membranous sacs called

cisternae

The Golgi Apparatus: Shipping and Receiving Center

Functions of the Golgi apparatus include

Modification of the products of the rough ER

Manufacture of certain macromolecules

Golgiapparatus

TEM of Golgi apparatus

cis face(“receiving” side ofGolgi apparatus)

Vesicles movefrom ER to Golgi Vesicles also

transport certainproteins back to ER

Vesicles coalesce toform new cis Golgi cisternae

Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection

Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion

Vesicles transport specificproteins backward to newerGolgi cisternae

Cisternae

trans face(“shipping” side ofGolgi apparatus)

0.1 0 µm16

5

2

3

4

Functions of the Golgi apparatus

Figure 1.13

Membrane proteins and lipids– Are synthesized in the ER and Golgi apparatus

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

EXTRACELLULARFLUID

PseudopodiumCYTOPLASM

“Food” or other particle

Foodvacuole

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium viaphagocytosis (TEM).

PINOCYTOSIS

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM).

0.5 µm

In pinocytosis, the cell “gulps” droplets of extracellular fluid into tinyvesicles. It is not the fluiditself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports.

Plasmamembrane

Vesicle

In phagocytosis, a cellengulfs 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.

Three types of endocytosis

Figure 7.20

PHAGOCYTOSIS

0.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coatedpit

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).

Plasmamembrane

Coatprotein

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

(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome containsactive hydrolyticenzymes

Food vacuole fuses with lysosome

Hydrolyticenzymes digestfood particles

Digestion

Food vacuole

Plasma membrane

Lysosome

Digestiveenzymes

Lysosome

Nucleus

Lysosomes carry out intracellular digestion by phagocytosis

Figure 1.14 A

(b) Autophagy: lysosome breaking down damaged organelle

Lysosome containingtwo damaged organelles 1 µ m

Mitochondrionfragment

Peroxisomefragment

Lysosome fuses withvesicle containingdamaged organelle

Hydrolytic enzymesdigest organellecomponents

Vesicle containingdamaged mitochondrion

Digestion

Lysosome

Autophagy

Figure 1.14 B

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