THE CELL

CELL ORGANELLS

Definition:

The cell is the structural and functional unit of all living organisms

It is a small mass of protoplasm enveloped by cell membrane and containing a nucleus

CELL

cell membrane protoplasm

nucleusCytoplasm

nucleoplasmNuclear membrane

Cytosol Organelles cytoskeleton Inclusions

NUCLEUS

The nucleus is membrane-limited compartment that contains the genetic material of the cell.

It is essential for the life of the cell The nucleus is normally seen in all cells except mature red

blood cells. NUMBER: Most cells contain a single nucleus but some

cells may be binucleate (eg: some liver cells) or multinucleate (eg: skeletal muscle cells)

POSITION IN THE CELL: may be central or peripheral (eccentric)

SIZE: its diameter usually ranges from 3 to10 µm SHAPE: The shape of the nucleus varies and largely

depends on the size and shape of the cell.

NUCLEUS

All nuclei are surrounded by a nuclear envelope formed by two unit membranes with a perinuclear cistern (space) between them.

The nuclear envelope serves as a boundary between the nucleoplasm and the cytoplasm of the cell.

The nucleoplasm is the material exclusive of the chromatin and the nucleolus.

Each nucleus may contain one or more nonmembranous nucleolus

The nucleolus is the site of ribosomal RNA synthesis The nuclear envelope possesses small circular openings of

about 70-80 nm in diameter called nuclear pores. Which allow bidirectional nucleocytoplasmic transport.

Chromatin

When examined under L/M they appears in the form of fine to coarse granules distributed irregularly within the nucleus

Chromatin is a complex of DNA and proteins which on condensation form discrete bodies/ flexible rod like structures called chromosomes. Chromosomes may be straight or uncoiled called

euchromatin or it may be folded/ coiled called heterochromatin.

The euchromatin is metabolically active with regard to RNA synthesis , while the heterochromatin is considered to be relatively inactive

CYTOPLASM

Basically the cytoplasm constituents can be divided into four main groups: A. Cytosol

B. Organelles or little organs are either membrane-bound or nonmembranous. (Organelles are mostly permanent )C. Inclusions are nonliving components of a cell.(while inclusions are temporary constituents of a cell.) D. cytoskeleton

CYTOSOL

The terms like cytosol, cytoplsmic matrix and cytomatrix are all used for that component which occupies the space between organells, inclusions and elements of the cytoskeleton

It contains many soluble proteins, soluble enzymes and small molecules and ions

ORGANELLES The organelles are specialized living structural components of the

cell they include

1. Rough endoplasmic reticulum

2. Ribosomes

3. Smooth endoplasmic reticulum

4. Golgi complex

5. Lysosomes

6. Peroxisomes

7. Mitochondria

8. Microtubules

9. Filaments

10. Centrioles

Ribosomes

Are 15-20 nm granules and contain RNA (60%) and proteins (40%).

Are chromophilic because they take up deep basophilic stain. Are found in all cells except mature red blood cells and are plenty

in developing cells (myoblast, osteoblast, myeloblast, etc.). They are found to occur either free in the cytoplasm or attached

with rough endoplasmic reticulum Groups of ribosomes called polyribosomes or polysomes Free polyribosomes synthesize proteins that remain in the cell as

cytoplasmic, structural or functional components.

Rough Endoplasmic Reticulum (RER)

Appears as a series of interconnected, membrane-limited flattened sacs called cisternae.

Surfaces of cisternae are covered with ribosomes.

Acts as the site of protein synthesis, therefore, it is most developed in secretory cells.

Nissl substance in the cell body of neurons is RER and the basophilic staining of Nissl substance is due to the presence of ribosomes.

Smooth Endoplasmic Reticulum (SER)

Appears as short anastomosing/ closed meshed network of branching tubules.

Surfaces are not associated with ribosomes It is the site for lipid and glycogen

metabolism and helps in detoxification of drugs and conjugation of noxious substances.

In skeletal and cardiac muscle, it also functions as a storehouse for Ca2+ needed for muscle contraction.

Golgi Complex/ Apparatus

In the light microscope it appears typically as a clear area near the nucleus surrounded by ER.

In electron micrographs the Golgi complex appears as stacks of flattened membrane-limited sacs or cisternae that are closely associated with vesicles.

SHAPE: Dinner plate FUNCTION: is concentration and packing of the

secretory products of the cell These products may be protein or lipid in nature

Lysosomes These are small roughly spherical single

membrane-bound vesicles about 250-500 nm (0.25 to 0.8 µm) in diameter.

They contain about hundred different hydrolytic enzymes that function in the low-pH environment created within the lysosome.

They play an important role in digestion of extra cellular material brought into the cell by endocytosis and in autophagy (degradation of cellular components isolated from the cytoplasmic matrix by newly formed membrane).

The indigestible residues of lysosomal action, the residual bodies, accumulate in the cells as lipofuscin pigment. Residual bodies are a normal feature of an aging

cell.

Peroxisomes (MICROBODIES) Peroxisomes or microbodies are membrane-

bound vacuoles of about 500 nm (0.3 to 0.6 µm) in diameter.

These structures are present in all nucleated cells particularly in liver cells and in cells of the proximal convoluted tubules in kidneys.

They contain oxidative enzymes, particularly catalase responsible for breakdown of hydrogen peroxide.

They contain enzymes responsible for ß oxidation of fatty acids.

FUNCTION: Detoxification

Mitochondria

In light microscope mitochondria appear as granules, rods or filaments.

These occur in large numbers in cells that generate and expend large amounts of energy.

In electron microscope each mitochondrion appears to be bounded by two membranes SHAPE: Cigar like which delimits two compartments, a large intracristal space

within the inner membrane and a smaller intercristal space between the inner and outer membranes.

The inner membrane at intervals produces shelf-like folds called cristae. These cristae vary in number and arrangement according to the energy requirements of the cell.

The interior which is not occupied by cristae is filled with a fine granular material called mitochondrial matrix.

Mitochondria are present in all cells except red blood cells and differentiated keratinocytes.

They are capable of self-replication. They constitute the major energy source of the cell They are the chief organelles of cell respiration and

contain enzymes of Kreb’s Citric Acid cycle and energy liberated in this process is captured through oxidative phosphorylation.

The end product of these reactions is the high energy compound adenosine triphosphate (ATP)

Centrioles

These are paired, short, cylindrical bodies seen in all cells that lie at right angles to each other but are not connected. The wall of each centriole is composed of nine triplets of microtubules that are oriented parallel to the long axis of the organelle.

In the centrioles are seen close to the nucleus in a specialized area of the cytoplasm called the centrosome or cell center.

CYTOSKELETON

A. Microtubules

These are long slender hollow cylindrical elements of the cytoskeleton that maintain cell shape.

The outer diameter measures 25 nm while the luminal diameter is 15 nm

They are formed by proteins called tubulin These are involved in cell elongation, migration,

intracellular transport of secretory granules, movement of chromosomes during mitosis and meiosis, and beating of cilia and flagella.

B. Filaments

Two basic types of filaments occur within cells, microfilaments and intermediate filaments.

Microfilaments are of two types: actin or thin filaments (6-8 nm in diameter) and myosin or thick filaments (12-15 nm in diameter).

Contraction in all cells involves interactions of actin and myosin.

Actin filaments associated with plasma membrane help in anchorage and movement of membrane proteins, movement of plasma membrane, extension of cell processes, locomotion of cells, and structural core of microvilli and stereocilia.

Intermediate filaments are heterogenous group of 8-10 nm Cytoskeleton elements found in various cell types.

The Cytoplasmic inclusions are :

1. Secretory granules2. Pigment granules3. Lipid droplets4. Glycogen granules

Secretory granules and Pigment granules are surrounded by membrane

Lipid droplets and glycogen granules are not surrounded by membrane.

Secretory granules and neutral fat may fill most of the cytoplasmic volume, compressing the other constituents into a thin rim at the margin of the cell.

Pigments such as melanin, lipofuscin, and hemosiderin are present in some cells.

Schematic showing the cytoplasm, with major components of a typical animal cell

(organelles):(1) nucleolus(2) nucleus(3) ribosomes (indicated by purple dots)(4) vesicle(5) rough endoplasmic reticulum (ER)(6) Golgi apparatus(7) cytoskeleton(8) smooth ER(9) mitochondria(10) vacuole(11) cytosol(12) lysosome(13) centrioles within centrosome