CELL STRUCTURE,

ORGANELLES & INTERCELLULAR CONNECTIONS

BY

DR. K. AMBAREESHA PhD.,Assistant Professor, Department of Physiology

MNR Medical College, Sangareddy

CONTENTS1. Introduction of cell

2. Cytoplasm and its Organelles- endoplasmic Reticulum (ER), Mitochondria, Golgi Complex, Lysosomes, Peroxisomes, Centrioles or Centrosomes & Microtubules and Microfilaments .

3. The nucleus

4. Junctional complexes/ Cell junctions – Tight Junction, Desmosomes or Adherens Junction & Gap junctions

INTRO- THE CELL

INTRO- THE CELL Cell: The cell is the ultimate

structural and functional unit of the body.

The three principal constituents of the cell are:

1. Cell membrane2. Cytoplasm and its organelles3. Nucleus

Cell membrane

Cell membrane

Cell membrane

Channels

Functions of cell membrane

• It forms outermost boundary of cell organelles.

• It takes food and excretes waste products.

• The selective permeability helps in maintaining difference of composition between ICF & ECF.

• It provides framework for the arrangement of all protein molecules (Enzymes, pumps, receptors, ions, channels).

• It links adjacent cells together by juctional complexes to form tissues.

*2m(Jan-2011)Cytoplasm and its

organelles: Mitochondria

Cytoplasm and its organelles: Mitochondria

• Are sausage shaped structures covered by two protein lipid membranes an outer and an inner which has many folding forming shelves.

• The mitochondria contain many enzymes and is the power house of the cell responsible for the release of energy.

• The outer layer has enzymes concerned with biological oxidation and provide raw material for the reactions that occur inside the mitochondria. The inside contains enzymes that oxidize the nutrients glucose , amino acids, fatty acids via the citric acid cycle to form H2O and CO2. The energy is utilized to synthesize the high energy phosphate compound ATP.

• Mitochondria also contain a special form of DNA and hence self replicative and when required can form a second and third mitochondria. They are derived from the mother through ovum, as spem makes no contribution in the zygote.

Cytoplasm and its organelles- Endoplasmic reticulum

Cytoplasm and its organelles- Endoplasmic reticulumRER/ Granular

• Contains granules called ribosomes which are attached to the cytoplasmic side of the membrane, 3-5 ribosomes clump together to from polyribosomes or polysomes.

• It is the site of protein synthesis e.g., hormones that are secreted by the cell; and proteins that are found in enzymes.

• Free ribosomes are also found in the cytoplasm, they synthesize cytoplasmic protein e.g., Haemoglobin.

SER/ A Granular

• It is site of steroid (Adrenocortical hormone) synthesis in steroid secreting cells and the site of detoxification processes in other cells.

• As the sarcoplasmic reticulum, it plays important role in skeletal and cardiac muscle.

Cytoplasm and its organelles- RIBOSOMES

*4m (Sep,Oct – 2005)Cytoplasm and its organelles- Functions of

RibosomesThey are tiny organelles are made up of RNA and proteins. It consists of 2 sub units one about half the size of the other, both are made separately in the nucleolus combine to form a single unit in cytoplasm.

The are present in free form in the cytoplasm or attached to ER.

They synthesize different proteins, for hormones and enzymes of lysosomes

Cytoplasm and its organelles- Golgi Apparatus

Cytoplasm and its organelles- Golgi Apparatus/ Golgi Bodies)

• These are membranous tubules and vesicles prominent in actively secreting gland cells.

• Wrapping and packaging department of the cell.

• Produces secretion granules, which store hormones and enzymes in secreting cells.

• Site of formation of lysosomes.

Cytoplasm and its organelles- Lysosomes

Cytoplasm and its organelles- Lysosomes

• Large irregular structures, filled with large number of small granules, contain variety of enzymes, called lysozymes.

• The interior is kept acidic (near pH 5.0), they function best at acidic pH.

• Acts as a form of digestive (lytic) system for the cell mainly macromolecules, Engulf worn out components of the cells in which they are located.

• Engulf exogenous substances e.g bacteria etc. and degrade them.

• When a cell dies, lysosmal enzymes cause autolysis of the remnant i.e why lysosomes are called as suicidal bags.

Cytoplasm and its organelles- Peroxisomes

• Is similar to that of lysosomes, contain oxidases (enzymes that produce H2O2) rather than hydrolases.

• They destroy certain products formed form oxygen, especially hydrogen peroxide, that can be toxic to the cells.

• The alcohol, a person drinks is mainly detoxified by the peroxisomes of the liver cells.

Cytoplasm and its organelles- Centrioles or Centrosomes

Cytoplasm and its organelles- Centrioles or Centrosomes

• Two short cylinders called ‘centrioles’ visible only during cell division.

• They are located at each pole near the nucleus and are so arranged at right angles to each other.

• Tubules in group of three (triplets) run longitudinally in the walls of the centrioles. There are nine of these triplets spaced at regular intervals.

• They are concerned with the movement of the chromosomes during cell division.

Cytoplasm and its organelles- Microtubules and Microfilaments

Cytoplasm and its organelles- Microtubules and Microfilaments

Microtubules are long hollow structures, make up tracts of chromosomes, mitochondria and secretion granules move from one part of the cell to another.

Microfilaments are long solid fibers. They comprise the contractile protein actin and are responsible for the cell motion.

They are involved in movements of the chromosomes; Cell movement; move secretion granules and movement of proteins within the cell membrane.

Cytoplasm and its organelles- CYTOSKELETON

Cytoplasm and its organelles- CYTOSKELETON

Cells have filaments in cytoplasm, which form the flexible network is cytoskeleton

They are: 1. Microfilaments made up of actin2. Intermediate filaments,3. Microtubules – tubulin protein4. Molecular motors:cause movement of organelles and movement of cell itself. The are of 3 types: kinesin, Dynein and Myosin.kinesin, Dynein: help in movement of vesicle along with cilia and flagella.

Myosin I along with actin filaments help in movement of villi, Myosin II help in contraction of skeletal and cardiac muscles by causing sliding action.

They all give mechanical support to the cells, form imp. transport system in nerve cells, some organelles move to different parts of the cell by them and chromosomal movements occur during cell division by them.

THE NUCLEUS

THE NUCLEUS• It is surrounded by membrane called nuclear membrane (or envelope) with the

pores.

• It is made up of chromosomes, which is made up of -DNA, on which genes are present. Gene is a portion of DNA molecule which carries a complete blue print for all the heritable characteristics of an animal.

• Between cell divisions the irregular clumps of dark material called chromatin are the only evidence of their presence.

• It contains a nucleolus which is patch work of granules rich in Ribonucleic Acid (RNA). They synthesize the RNA for the ribosomes. DNA in nucleus serves as a ‘template’ (block) for synthesis of RNA, which then moves to the cytoplasm where it regulates the synthesis of proteins by the cells.

• The information coded into the DNA molecules is conveyed from the nucleus to the cytoplasm by messenger RNA where actually the synthetic work of the cell takes place.

JUNCTIONAL COMPLEXES BETWEEN CELLS

Tight junction

Adherens junction

Desmosome

TJ

Gap junctions

Hemidesmosome

INTERCELLULAR CONNECTIONS-Tight Junction:

INTERCELLULAR CONNECTIONS-Tight Junction:

Here membranes of two cells become opposed and layers of the membranes fuse strongly, thus obliterating the space between the cells.

This type of junction is characteristically seen in the intestinal mucosa, the walls of the renal tubules, and the choroid plexus.

Tight junction forms a barrier to the movement of ions and other solutes from one side of the epithelium to the other.

INTERCELLULAR CONNECTIONS- Desmosomes or Adherens Junction

Here two membranes are separated by a 15-20 nm space.

There is dense accumulation of proteins on both the surfaces of the membrane with fibers extending from the cytoplasmic surface of each membrane into the cell.

The holds, adjacent cells firmly together in areas that are subjected to stretching. Such as the skin

INTERCELLULAR CONNECTIONS- Gap Junction/ Nexus

INTERCELLULAR CONNECTIONS- Gap Junction/ Nexus

There is 2-20 nm space between the opposing membrane.

The gap is filled with densely packed particles through each of which there appears to be a channel that connects the two cells.

The diameter of each channel is regulated by intracellular Ca, pH and voltage.

It permits rapid propagation of electrical potential changes from one cell to the another, e.g. cardiac and smooth muscle cells.

The permits the direct transfer of ions and other small molecules e.g. sugar, amino acids between the cell without traversing the extra cellular space.