Lecture 5:Cytoplasmic Organelles pdf - د . قيصر ناصر

Cytoplasmic organelles

Plasma membrane

 phospholipids

cholesterol

proteins

, and

oligosaccharide

chains

covalently linked to

phospholipid

and

protein

molecules.

 7.5 nm thick
 consists of two layers, known as the

lipid bilayer

that contain

associated

integral

and

peripheral

proteins.

 The inner layer of the plasma membrane faces the cytoplasm, and

the outer layer faces the extracellular environment.

Structure

• Phospholipid molecules spontaneously orient to form a bilayer

in which the

hydrophobic tails

are pointed

inwards

. The

hydrophilic, ionic head

groups are in the

exterior

and are thus in

contact with the surrounding aqueous environment.

Structure

Cytoplasm represents

everything enclosed by the plasma membrane

with the exclusion of the nucleus.

• It consists of a viscous fluid medium that includes

salts, sugars,

lipids, vitamins, nucleotides, amino acids, RNA, and proteins

which

contain the

protein filaments, actin microfilaments, microtubules,

and intermediate filaments.

• These filaments function in animal and plant cells to provide

structural stability and contribute to cell movement.

Cytoplasm

Function:

1. Energy production through metabolic reactions,

2. Biosynthetic processes, and photosynthesis in plants.

3. Storage place of energy within the cell.

Cytosol

: is the fluid of the cytoplasm, refers only to the

protein-rich fluid environment, excluding the organelles.

Structure:

Complex array of protein fibers found in three forms:

(1)

Microtubules

: hollow structure, with an outer

diameter of 25 nm and a wall 5 nm thick,

give the

rigidity

to help maintain cell shape.

Cytoskeleton

(2)

Microfilaments

: composed of actin, allow cellular

motility

and most contractile

activity in cells

(3)

Intermediate filaments

: intermediate in size between the other two and with a

diameter averaging 10 nm. The intermediate filaments are much more

stable

than

microtubules and actin filaments, composed of different protein subunits in different cell

types.

Functions of

Cytoskeleton

1. Structural

• Provides structural support to cell
• Stabilizes junctions between cells

Movement

• Assists with cytosol streaming and cell motility;
• helps moving organelles and materials throughout cell;
• helps moving chromosomes during cell division.

Structure: The nucleus, the largest organelle of

the

cell,

includes

the

nuclear

envelope,

nucleolus, nucleoplasm, and chromatin and

contains the genetic material encoded in the

(DNA) of chromosomes.

Nucleus

1. The nuclear envelope: surrounds the nuclear material and consists of

two parallel membranes separated by a narrow

perinuclear space

. These

membranes fuse at intervals, forming openings called

nuclear pores

the nuclear envelope.

2. Nucleolus:

spherical

, highly

basophilic

, actively making

proteins

. The intense basophilia of nucleoli is due the

presence of

heterochromatin

and the presence of

densely

concentrated ribosomal RNA (rRNA)

that is transcribed,

processed, and complexed into ribosomal subunits in

nucleoli.

3. Nucleoplasm:

the

protoplasm

within

the

nuclear

envelope. It consists of a matrix and various types of

particles.

4. Chromatin: consists of

double-stranded DNA

complexed

with

histones

and

acidic proteins

. It resides within the

nucleus

heterochromatin

and

euchromatin

The

euchromatin/heterochromatin ratio

is higher in malignant

cells than in normal cells

• Chromatin is responsible for RNA synthesis.

Mitochondria

Structure:
• rod-shaped organelles that are 0.2 um wide and

up to 7 um long.

• They occupy about

20%

of the cytoplasmic

volume.

• They possess an outer membrane, which

surrounds the organelle, and an inner
membrane, which folded to form

cristae

which

provide a large surface area for attachment of
enzymes involved in respiration.

• The

matrix space enclosed

by the inner

membrane is

rich in enzymes

and contains the

mitochondrial DNA

Function: mitochondria generate ATP.

Structure:

• flattened sheets of membranes that extend throughout the

cytoplasm of eukaryotic cells and enclose a large

intracellular space called

lumen

• There is a continuum of the lumen between membranes of

the nuclear envelope.

• The rough endoplasmic reticulum (rough ER):

is close to

the nucleus, and is the site of

attachment of the ribosomes

• Ribosomes

are small and dense structures, 20 nm in

diameter, that are present in great numbers in the cell,

mostly attached to the surface of rough ER, but can float

free in the cytoplasm.

Endoplasmic Reticulum:

Function:

• Rough ER

responsible for

protein synthesis

• Smooth ER

is the primary site of

synthesis of lipids and sugars

• contains

degradative enzymes

, which

detoxify

many organic

molecules

• The rough ER

transitions into

a smooth endoplasmic reticulum (smooth

ER)

, which is generally more

tubular

and

lacks attached ribosomes.

• They are manufactured in the nucleolus of the nucleus on a DNA

template and are then transported to the cytoplasm.

Ribosomes are

the sites of protein synthesis.

Golgi apparatus:

Structure.
• consists of several membrane-

bounded

cisternae

(

saccules

)

arranged in a

stack

• positioned and held in place by

microtubules.

• Cisternae are disk-shaped and

slightly curved, with flat centers
and dilated rims, but their size
and shape vary.

Function:

• modifying, sorting,

and packaging of
proteins for secretion

• delivery to other

organelles

• for

secretion outside

of the cell

Lysosomes:

These are vesicles of

hydrolytic enzymes

and are

single-

membrane bound

. They have an

acidic interior

and contain about

hydrolytic enzymes

involved in intracellular digestions.

Peroxisomes:

These are membrane-bound vesicles containing

oxidative enzymes

that

generate and destroy hydrogen peroxide

•

Peroxisomes participate in many different metabolic activities,

including the

oxidation of fatty acids

, the

breakdown of purines

, and the

biosynthesis of cholesterol

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