Notes on Cytology

7/28/2019 Notes on Cytology

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Cytology Notes

Two basic types of cells: eukaryotic and prokaryotic.

Only bacteria have prokaryotic cells. A major difference between prokaryotic cells and

eukaryotic cells is that the DNA of prokaryotic cells is not enclosed in a nucleus. In theprokaryotic cell, the DNA is located in a limited region of the cell called a nuclear area, ornucleoid, that is not enclosed by a membrane.

The cell is highly organized and complex system composed of its own control center, internal

transportation system, power plants, factories for making needed materials, packaging plants,

and even a self-destruct system.

Cytoplasm: The part of the cell outside of the nucleus.

Nucleoplasm: The part of the cell within the nucleus.

Most prokaryotic cells have cell walls (structures that enclose the entire cell, including the cell

membrane). Many prokaryotic cells have flagella, long fibers that project from the surface of the

cells. Flagella are used in locomotion.

The dense internal material of the bacterial cell contains ribosomes, small complexes of RNA

and protein that synthesizes polypeptides as well as storage granules that hold glycogen, lipid, or

phosphate compounds. In eukaryotic cells a number of membranes are generally considered to

be part of the internal membrane system, or endomembrane system.

Some organelles have direct connections between their membranes and compartments. Others

transport materials in vesicles, small-membrane-bounded sacs. Vesicles also carry materials

from one organelle to another.

Typically, the nucleus is the most prominent organelle in the cell. Size of the nucleus is typically

5 m in diameter. The nuclear envelope consists of two concentric membranes that separate the

nuclear contents from the surrounding cytoplasm. These membranes are separated by about 20-

40 nm. At intervals the membranes come together to form nuclear pores that regulate the

passage of certain materials between nucleoplasm and cytoplasm.

Most of the cells DNA is located inside the nucleus.

DNA is associated with proteins, forming a complex known as chromatin, which appears as a

network of granules and strands in cells that are not dividing. In dividing cells, the chromatin

condenses and becomes visible as distinct threadlike structures called chromosomes.

Most nuclei have one of more compact structures called nucleoli (sing. nucleolus).


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The nucleolus is not membrane-bounded. Each nucleolus contains nucleolar organizers, made

up of chromosomal regions containing instructions for making the type of RNA in ribosomes.

This ribosomal RNA is synthesized in the nucleolus. The proteins needed to make ribosomes are

synthesized in the cytoplasm and imported into the nucleolus. Ribosomal RNA and proteins are

then assembled into ribosomal subunits that leave the nucleus through the nuclear pores.

Ribosomes manufacture proteins and consist of two main components: a large subunit and a

small subunit. Each subunit contains ribosomal RNA and several ribosomal proteins.

Protein synthesis occurs in the ribosomes.

The endoplasmic reticulum (ER) is a maze of parallel internal membrane that encircles the

nucleus and extends into many regions of the cytoplasm. The ER forms a significant part of the

total cytoplasmic volume in many cells.

The internal space enclosed by the ER membranes is the ER lumen and in most cells, the ER

lumen forms a single internal compartment that is continuous with the outer membrane of the

nuclear envelope.

The ER has distinct regions: rough ERand smooth ER.

Rough ER has ribosomes attached to it and consequently appears rough in electron micrographs.

Many proteins that are exported from the cell and those destined for other organelles are

synthesized by the rough ERs ribosomes. The ribosome contains a tunnel that connects to an

ER pore, ortranslocon. Proteins are transported through this extended tube (through the ER

membrane) into the ER lumen. In the ER lumen, proteins may be modified by enzymes that add

complex carbohydrates or lipids to them. Other enzymes (chaperones) located in the ER lumencatalyze the efficient folding of proteins into proper conformations.

The proteins are then transferred to other compartments by small transport vesicles, which bud

off of the ER membrane and are then inserted into the membrane of some target compartment.

Smooth ER is more tubular and does not have ribosomes bound to it (and thus has a smooth

appearance).

The smooth ER is the primary site of phospholipid, steroid, and fatty acid metabolism.

The Golgi complex (also known as the Golgi body or the Golgi apparatus) consists of flattenedmembranous sacs that constitute some separated compartments in addition to interconnected

compartments.

Each Golgi stack has three areas referred to as cis and trans faces, with a medial region

between. The cis face is typically located nearest to the nucleus and functions to receive

materials from the transport vesicles from the ER. The trans face, nearest to the plasma

membrane, packages molecules in vesicles and transports them out of the Golgi apparatus.


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Cells that secrete large amounts of glycoproteins have large numbers of Golgi stacks. Golgi

complexes of plant cells produce extracellular polysaccharides that are used as components of

the cell wall.

The Golgi apparatus functions primarily as a mechanism for processing, sorting, and modifying

proteins.

In animal cells, the Golgi apparatus also manufactures lysosomes.

Lysosomes are small sacs of digestive enzymes that are dispersed throughout the cytoplasm of

most eukaryotic cells. The enzymes (~ 40 different kinds) in these organelles break down

complex molecules that originate both inside and outside the cell. The enzymes are most active

under rather acidic conditions (pH ~ 5).

Primary lysosomes are formed by budding from the Golgi complex. Their hydrolytic enzymes

are synthesized in the rough ER.

One or more primary lysosomes fuse with the cell containing ingested material, forming a larger

vesicle called a secondary lysosome. In the secondary lysosome the powerful enzymes come in

contact with the ingested molecules and degrade them into their components.

Lysosomal enzymes are also released into the cell in some normal processes. Programmed cell

death (apoptosis) is a normal part of development and maintenance.

Peroxisomes are membrane-bounded organelles containing enzymes that catalyze an assortment

of metabolic reactions in which hydrogen is transferred from various compounds to oxygen.

During these reactions, H2O2 is produced as a byproduct. H2O2 is toxic to cells but peroxisomes

contain catalase, which breaks down the hydrogen peroxide, rendering it harmless.

Peroxisomes are found in large numbers in cells that synthesize, store, or degrade lipids.

Peroxisomes found in the liver and kidney cells detoxify ethanol.

In plant seeds, specialized peroxisomes (called glyoxysomes) contain enzymes that convert

stored fats to sugars. Animal cells lack glyoxysomes and cannot convert fatty acids into sugars.

Many of the functions carried out in animal cells by lysosomes are performed in plant and fungal

cells by a large, single, membrane-bounded sac called the vacuole. The vacuolar membrane,

part of the endomembrane system, is the tonoplast.

Although the terms vacuole and vesicle are sometimes used interchangeably, vacuoles are

usually larger structures.

Virtually all eukaryotic cells (plant, animal, fungal, and protest) contain complex organelles

called mitochondria which serve as the site ofaerobic respiration (a process that includes most

of the reactions that convert the chemical energy present in certain foods to ATP).


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Mitochondria are most numerous in cells that have high energy requirement.

Mitochondria vary in size, ranging from 2 to 8 m in length. Each mitochondrion is bounded by

a double membrane which forms two different compartments within the organelle: the

intermembrane space and the matrix.

The intermembrane space is the compartment formed between the outer and inner mitochondrial

membranes. The matrix is the compartment enclosed by the inner mitochondrial membrane and

it contains enzymes that break down food molecules and convert their energy to other forms of

chemical energy.

The outer mitochondrial membrane is smooth and allows many small molecules to pass

through it. The inner mitochondrial membrane has numerous folds (cristae) that extend into

the matrix, the space enclosed by the inner mitochondrial membrane. Cristae greatly increase the

surface area of the membrane, providing a surface for the chemical reactions that transform the

chemical energy in food molecules into the energy of ATP.

The shapes of cells and their ability to move are determined in large part by the cytoskeleton, a

dense network of protein fibers. In addition to providing mechanical support, the cytoskeleton

functions in transport of materials within the cell and in cell division.

The cytoskeleton is highly dynamic and constantly changing. Its framework is made of three

types of protein filaments: microtubules, microfilament (or actin filaments), and intermediate

filaments.

Both microfilaments and microtubules are fibers formed from beadlike, globular protein

subunits, which can be rapidly assembled and disassembled. Intermediate filaments are madefrom fibrous protein subunits and are more stable than microtubules and microfilaments.

Microtubules are the thickest filaments of the cytoskeleton. They play a structural role in the

formation of the cytoskeleton and are involved in the movement of chromosomes during cell

division. Microtubules consist of two very similar proteins: and tubulin. These proteinscombine to form a dimer.

Microfilaments are flexible, solid fibers. Each microfilament consists of two intertwined strings

of beadlike actin molecules. Actin molecules cross-link with one another and with other proteins

to form bundles of fibers that provide mechanical support for various cell structures.

Intermediate filaments are very stable, tough fibers made of polypeptides that vary in size among

different cell types and different species of animals. The intermediate filaments form a sheath

called the nuclear lamina (just inside the nuclear envelope).

Most eukaryotic cells are surrounded by a glycocalyx, or cell coat, formed by polysaccharide

side chains of proteins and lipids that are part of the plasma membrane.


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Many animal cells are also surrounded by an extracellular matrix (ECM), consisting of a gel of

carbohydrates and fibrous proteins. The main structural protein in the ECM is collagen. Certain

glycoproteins of the ECM, called fibronectins, bind to protein receptors that extend from the

plasma membrane. The main membrane receptors for the ECM are integrins. These proteins

have a variety of functions, including anchoring the external ECM to the microfilaments of the

internal cytoskeleton.

Notes on Cytology

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