CYTOLOGY

Cell Components

NucleusCytoplasmCytoskeletonCell surface

Plasma Membrane

All eukaryotic cells are enveloped by a limiting membrane composed of phospholipids, cholesterol, proteins, and chains of oligosaccharides covalently linked to phospholipids and protein molecules.The cell, or plasma, membrane functions as a selective barrier that regulates the passage of certain materials into and out of the cell and facilitates the transport of specific molecules.

Membrane phospholipids, such as lecithin and cephalin, consist of two long, nonpolar chains linked to a charged (hydrophilic) head group. Cholesterol is also a constituent of cell membranes. Within the membrane, phospholipids are most stable when organized into a double layer with their hydrophobic (nonpolar) chains directed toward the center of a membrane and their hydrophilic (charged) heads directed outward.

Cholesterol breaks up the close packing of the phospholipid long chains, and disruption makes membrane more fluid. The cell controls the fluidity of the membranes through the amount of cholesterol present.

Plasma membraneThe fluid mosaic model of membrane

structure

Main components;1.Phospholipids2.Cholesterol3.Proteins4.Chains of oligosaccharides

Plasma membrane

Plasma membrane Phospholipids-phosphatidylcholine(lecithin)-phosphatidylethanolamine(cephalin) Structure: 2 nonpolar (hydrophobic)

hydrocarbon chains linked to one charged(hydrophilic) head group

Cholesterol-breaks up the close packing of the

phospholipid long chains,and as a consequence plasma membranes become more fluid

Proteins1. Integral proteins-directly incorporated within the lipid bilayer2. Peripheral proteins-exhibit a looser association with membrane

surfaces.They can be easily removed from the cell membranes with salt solutions

In order to regulate the transport of molecules, there are two types of proteins in the cell: - carrier proteins - transport proteins.

The two types of transport proteins are:- channel - carrier protein. Transport is either active or passive. Active transport is moving molecules against the concentration gradient and energy is required in the form of ATP.

Glycocalyx-a fuzzy carbohydrate-rich region on the

external surface of the cell-composed of carbohydrate chains linked to

a membrane proteins and lipids and of cell-secreted glycoproteins and proteoglycans

Plasma membrane

Plasma membrane-transportThe plasma membrane is the site at which

materials are exchanged between the cell and its environment

Endocytosis-bulk uptake of material through the plasma membrane

Passive transport is moving molecules down the concentration gradient and no energy is required. Examples of passive transport are diffusion, which moves from high concentration to low concentration and osmosis, which is the diffusion of water molecules.

The Nucleus

It’s enclosed by the nuclear envelope and contains: - nuclear lamina- nucleolus- chromatin

Nucleus

•The nucleus is the site of deoxyribonucleic acid (DNA) replication and trascription of DNA into precursor ribonucleic acid (RNA) molecules. It contains all of the enzymes required for replication and repair of newly senthesized DNA, as well as for trascription and processing of precursor RNA molecules.

Nuclear envelope

-Is double membrane containing pores-The outer nuclear membrane is continuous with the endoplasmic reticulum

Nucleolus contains three morphologically distinc zones:- granular zone – found at periphery; contains ribosomal precursor particles in various stages of assembly- fibrillar zone – centrally located; contains ribonuclear protein fibrils- fibrillar center- contains DNA that is not being transcribed

Chromatin Is a complex of:DNA histone proteins and nonhistone proteins

DNA

A double-stranded helical molecule that carries gentic information of the cell

Histone proteins

Positively charged proteins enriched with lysine and arginine residues. They are important in forming two types of structures in chromatin:

- nucleosomes- solenoid fibers

The nucleosomes are the basic repeating units of the chromatin fiber.

Schematic representation of a nucleosome. This structure consists of a core of 4 types of histones (2 copies of each)—H2A, H2B, H3, and H4—and one molecule of H1 or H5 located outside the DNA filament.

Nonhistone proteinsInclude enzymes involved in nuclear

functions such as:- replication- transcription- DNA repair- regulation of chromatin function

They are acidic of neutral proteins.

Forms of chromatin

- Heterochromatin - highly condensend, transcriptionally inactive

- Euchromatin – a more extended form of DNA, which is potentially transcriptionally active.

Cytoplasm components

-Endoplasmic reticulum

-Golgi Apparatus

-Lysosomes

-Peroxisomes

-Mitochondria

Endoplasmatic Reticulum

Endoplasmatic Reticulum

Exists in two forms:

-Rough endoplasmatic reticulum (RER)

-Smooth endoplasmatic reticulum (SER)

Rough endoplasmatic reticulum

- RER is a single, lipid bilayer continuous with outer nuclear membrane. It’s organized into stacks of large flattened sacs called cisternae that are studded with ribosomes on the cytoplasmic side.- RER synthesizes proteins that are destined for the Golgi apparatus, secretion, the plasma membrane and lysosomes.- RER is very prominent in cells that are specialized of proteins destined for secretion (e.g., pancreatic cells)

Smooth endoplasmic reticulumSER is a network of membranous sacs,

vesicles, and tubules continuous with the RER, but lacking ribosomes.

SER contains enzymes involved in the biosynthesis of phospholipids, triglycerides, and sterols.

Functions of SER 1. Detoxification Reactions /

hepatocytes /These are reactions that make compounds soluble so that they can be excreted.Two types of reactions that increase solubility are:- Hydroxylation reactions- conjugation reactions

Functions of SER

2. Glycogen Degradation and Gluconeogenesis

3. Steroid synthesis / Leydig cells in testis and adrenal gland cells /

Functions of SER4. Reactions in Lipid Metabolism

Lipolysis begins in the SER with the release fatty acid from triglyceride.The SER is also the site where lipoprotein particles are assembled

Functions of SER5. Sequesration and Release of

Calcium IonsIn striated muscle the SER is known as sarcoplasmic reticulum (SR).The sequestration and release of calcium ions takes place in the SR.

Golgi Apparatus

Consists of disc-shaped smooth cisternae that are assembled in stacks (dictyosomes) Associated with numerous small membrane-bound vesicles.

The Golgi apparatus has two distinct faces:The cis (forming) face associated with RERThe trans (maturing) face is often oriented toward the plasma membrane. The trans-most region is a network of tubular structures known as the trans-Golgi network.

Functions of Golgi apparatusProteins and Lipids

AG is the site of posttranslational modification and sorting of newly synthesized proteins and lipids.

GlycoproteinsFurther modification of the carbohydrate moiety of glycoproteins produces complex and hybrid oligosaccharide chains. This determines which proteins remain in the Golgi apparatus or leave the Golgi apparatus to become secretory proteins, lysosomal proteins, or part of the plasma membrane.

Mitochondria

Mitochondria

- two membranes- synthesizes adenosine triphosphate (ATP)- contain their own double-stranded circular DNA, and make some of their own proteins- have several compartments

Outer membrane

- is smooth, continuous, and highly permeable- contains an abundance of porin, an integral membrane protein that forms channels in the outer membrane

Inner membrane

Is impermeable to most small ions and small molecules.-The inner membrane has numerous infoldings, called cristae. The cristae greatly increase the total surface area.They contain the enzymes for electron transport and oxidative phosphorylation.The number of mitochondria and the number of cristae per mitochondrion are proportional to the metabolic activity of the cells in which they reside.

Intermembrane compartment

It is the space between the inner and outer membranes.Contains enzymes that use ATP to phosphorylate other nucleotides.

Matrix

Is enclosed by the inner membrane and contains:

-dehydrogenases- oxidize many of the substrates in the cell , generating reduced NADH (nicotinamide adenine dinucleotid) and reduced FADH2 (adenine dinucleotide) for use by the electron transport chain and energy generation.

-a double-stranded circular DNA genome- encodes a few of the mitochondrial DNA is always inherited from the mother, resulting in the maternal transmission of diseases of energy metabolism.

-RNA, proteins, and ribosomes- although there is some protein synthesis, most mitochondrial proteins are synthesized in the cytoplasm and are transferred into the mitochondria.

Ribosomes

Ribosomes

Composed of RNA and proteinConsist of large (60s) and small (40s) subunitsAssembled in the nucleus and transported to the cytoplasm through the nuclear poresThe large ribosomal subunits are synthesized in the nucleolus, whereas the small subunits are synthesized in the nucleus.

Polysomes

Ribosomes often form polysomes, which consist of a single messenger RNA (mRNA) that is being translated by several ribosomes at the same time. The ribosomes move on the mRNA from the 5’ end toward 3’ end. The two ribosomal subunits associate on the mRNA with the small subunit binding first.

Forms of ribosomes

-Free polysomes are the site of synthesis for proteins destined for the nucleus, peroxisomes, or mitochondria.-Membrane-associated polysomes are the site of secretory proteins, membrane proteins, and lyzosomal enzymes.

Peroxisomes

Peroxisomes are a heterogeneous group of small, spherical organelles with a single membrane.- Contain a number of enzymes that transfer hydrogen atoms from organic substrates to molecular oxygen with the formation of hydrogen peroxide.

Catalase, the major peroxisomal protein, degrades the hydrogen peroxide to water and oxygen.- Peroxisomal enzymes are synthesized on free polysomes. After translation, the enzymes are incorporated directly into peroxisomes.

Peroxisomes functions1. synthesis and degradation of hydrogen

peroxide2. β-oxidation of very long chain fatty acids

(>C24) starts in the peroxisomes and proceeds until the carbon chain has been reduced to length of approximately 10 carbons.Oxidation of the residual 10 carbons is completed in the mitochondria.

3. Phospholipid exchange- peroxisomes contain enzymes that convert phosphatidylserine and phosphatidylethanolamine

4. Bile acid synthesis

Lysosomes

Spherical membrane-enclosed organelles Contain enzymes required for intracellular digestion

Lysosomes formsPrimary lysosomes have not yet

acquired the materials to be digested. They are formed by budding from the trans side of the Golgi apparatus.

Secondary lysosomes are formed by the fusion of the primary lysosome with the substrate to be degraded and have contents that are in various stages of degradation.

Lysosomes contain approximately 60 hydrolytic enzymes. These include nulceases for degrading DNA and RNA, lipases for degrading lipids, glycosidases for degrading glycoconjugates , proteases and peptidases for degrading proteins, and a variety of phosphatases

all lysosomal enzymes are acid hydrolases, with optimal activity at pH of approximately 5.0

the synthesis of the lysosomal hydrolases occurs in the RER; the hydrolases are transffered to the Golgi apparatus, where they are modified and packaged into lysosomes.

Proteasomes

Proteasomes

are multiple-protease complexes that digest proteins targeted for destruction by attachment to ubiquitin. Protein degradation is essential to remove excess enzyme and other proteins that become unnecessary to the cell after they perform their normal functions, and also to remove proteins that were incorrectly folded. Protein encoded by virus should also be destroyed.

Proteasomes deal primarily with proteins as individual molecules, whereas lysosomes digest bulk material introduced into the cell or whole organelles and vesicles.

The proteasome has a core particle with the shape of a barrel made of four rings stacked on each other. At each end of the core particle is a regulatory particle that contains ATPase and recognizes proteins with ubiquitin molecules attached.

Cytoskeleton

Provides a supportive network of tubules and filaments in the cytoplasm of eukaryotic cells. It is composed of :- microtubules - intermediate filaments- microfilaments.

The cytoskeletonMicrotubules- 25 nmMicrofilaments (actin filaments)-6 nmIntermediate filaments- 10 nm

MicrotubulesTubular structuresVariable in lengthComposed of subunits-heterodimersEach heterodimer is composed of alfa and

beta tubulin molecules

Alfa-tubulin protofilaments microtubules

Beta-tubulin

Microtubules

MicrotubulesThe main functions of microtubules-play role in the development and

maintenance of cell shape-participate in the the intracellular transport

of organelles and vesicles-provide the basis for cytoplasmic

components ;centrioles,basal bodies, cilia,flagella

Microtubules-functionchromosomal movement during meiosis

and mitosis. Microtubule assembly is an important event in spindle formation

intracellular vesicle and organelle transport

ciliary and flagellar movement

Microtubules

CentriolesCylindical structures composed of highly

organised microtubulesEach centriole consists of nine sets of

microtubules arranged in tripletsCentrosome- an important role during cell

division

Cilia and flagella-motile processes, covered by cell membraneCilia-usually a large number of cilia on one

ciliated cell,play role in sweeping fluid from the surface of

cell sheets.Flagella-in humans only spermatozoa possess a

flagellum (only one)typical core organisation, contains nine pairs of

microtubules, possesing a 9+2 pattern=AXONEME

Adjacent peripheral pairs are linked to each other by protein bridges-nexins

Cilia

Are apical cell surface of cell membrane that contain microtubules. They are inserted on centriole-like basal bodies present below the membrane surface at the apical pole.Cilia contain two central microtubules surrounded by a circle of nine peripheral microtubule doublets.

Cilia and flagella

Movement of Cilia

Cilia move back and forth to propel fluid and particles in one direction. They are important in clearing mucus from respiratory tract.

Immotile cilia syndromeCause; mutations in the proteins of cilia and

flagella

Immotile spermatozoa- male infertilityLack of the cleansing action of cilia in the

respiratory tract-respiratory infections

Basal body-at the base of each cillium and flagellum-Structure; similar to a centriole-Function; controls the assembly of the

axoneme

MicrofilamentsThe thinnest filaments of the cytoskeletonFound in cytoplasm of all eukaryotic cellsBuilt up of a protein-ACTIN (actin filaments)Are formed by the head-to-tail

polimerization of actin monomers (globular G-actin) forming a long filamentous F actin polymer.

- The polymers are twisted around each other in a double helical formation

Microfilaments

MicrofilamentsIn skeletal muscle they interract with

myosin filaments which is an essential process in the mechanism of contraction.

In most cells actin filaments form a thin sheath just beneath the plasmalemma, called the cell cortex-associated with membrane activities like endocytosis,exocytosis and cell migratory activity

A role in moving and shifting cytoplasmic components

Microfilaments

-are composed of actin. Each actin filament (F-actin) consists of two strands of actin twisted into helical pattern

Microvilli

Are apical cell surface evaginations of cell membranes that function to increase the cell surface area available for absorption. A thick glycocalyx coat covers them. The core of each microvillus contains actin microfilaments. It is anchored in the apical cell cytoplasm to the terminal web, which itself is anchored to the zoula adherens of the cell membrane

Intermediate filamentsThey are intermediate in thickness between

microtubules and microfilaments. They function primarily in structural roles and contain several types of tissue-specific proteins:

cytokeratins- found in epithelial tissuedesmin- found in smooth muscle; Z disks of

skeletal and cardiac musclevimentin- found in cells of mesenchymal origin

(endothelial cells, fibroblasts, chondroblasts, vascular smooth muscle)

neurofilaments- found in neuronsglial fibrillaty acid protein (GFA)- found in

astrocytes

Intermediate filaments

FILAMENT TYPE CELL TYPE EXAMPLES

keratins epithelium Both keratinizing and nonkeratinizing epithelia

vimentin Mesenchymal cells Fibroblasts, macrophages

desmin muscle Striated and smooth muscle(except vascular smooth muscle)

Gilial

Neurofilaments

Glial cells

neurons

Astrocytes

Nerve cell body and processes