CELL BIOLOGY

 

CELL BIOLOGY

PREPARED BY MR. ABHIJIT DAS

CELL: cell is the fundamental structural and functional unit of all living organisms. A cell consists of a plasma membrane enclosing a number of organelles suspended in a watery fluid known as cytosol.

PROTOPLASM: The living part of a cell surrounded by a plasma membrane. (all inside plasma membrane)

CYTOPLASM: The semifluid substance of a cell excluding the nucleus. (protoplasm – nucleus)

The human body contains about 100 trillion cells.

HISTORY

Robert Hooke discovered dead plant cell in 1665. However, it was Robert Hooke who coined the term cell.

Antonie Van Leeuwenhoek discovered living cell in 1674.

Robert brown discovered nucleus of the cell in 1831.

CELL MEMBRANE

Cell membrane or the plasma membrane is the protective semipermeable membrane, covering the cell body.

The cell membrane is composed of lipids that are arranged in a bilayer.

The lipids are arranged within the membrane with the polar head (hydrophilic head) towards the outer sides and the non-polar tails (hydrophobic tails) towards the inner side.

This ensures that the hydrophobic tail is protected from the aqueous environment.

The cell membranes also possess protein and carbohydrate.

Membrane proteins can be classified as integral or peripheral proteins.

Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane.

According to FLUID MOSAIC MODEL, the fluid nature of lipid enables movement of proteins within the bilayer.

 Figure Credit: Yostnarani Sethy

The most important function of plasma membrane is the transport of the molecules across it.

DIFFERENT ORGANELLES OF CYTOPLASM:

1.     Endoplasmic reticulum

2.     Golgi bodies

3.     Ribosome

4.     Mitochondria

5.     Lysosome

6.     Centrioles

   

 Figure Credit: Yostnarani Sethy

STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES

1.    ENDOPLASMIC RETICULUM

STRUCTURE

Ø Endoplasmic reticulum is a network of tiny tubular structures scattered in the cytoplasm.

Ø They are of two types: smooth and rough.

Ø The ER bearing ribosomes on their surface is called rough endoplasmic reticulum (RER).In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER).

    Figure Credit: Yostnarani Sethy

FUNCTION

Ø RER is involved in protein synthesis.

Ø SER is the major site for synthesis of lipid and lipid-like steroidal hormones.

2.    GOLGI APPARATUS

STRUCTURE

    Figure Credit: Yostnarani Sethy

They consists of many flat, disc-shaped sacs which are called as cisternae.

FUNCTION

Ø A number of proteins synthesized by ribosomes on the endoplasmic reticulum are modified (or processed) in the golgi apparatus.

Ø Golgi apparatus is also the important site of formation of glycoproteins and glycolipids.

3.    MITOCHONDRIA

STRUCTURE

    Figure Credit: Yostnarani Sethy

Ø Typically mitochondria is cylindrical shaped having a diameter of 0.2 - 1.0 micrometer and length 1.0 - 4.1micrometer.

Ø Mitochondria is a double membrane bound structure and is described as the power house of the cell.

Ø The inner compartment is called the matrix.

Ø The inner membrane forms a number of foldings called as cristae (singular: crista). It increases the surface area.

FUNCTION

Ø They are the sites of aerobic respiration.

Ø They produce cellular energy in the form of ATP hence they are called ‘power houses’ of the cell.

4.    RIBOSOMES

They are tiny granules composed of RNA and protein.

FUNCTION

They synthesize proteins from amino acids.

5.    LYSOSOMES

They are membrane bound vesicular structures pinched off from the golgi apparatus.

FUNCTIONS

Ø They are rich in almost all types of hydrolytic enzymes (lipases, proteases, carbohydrases). These enzymes are capable of digesting macromolecules such as carbohydrates, proteins, lipids etc.

Ø They also break down fragments of organelles inside the cell into small particles that are either recycled or removed as waste material.

Ø Lysosomes in white blood cell (WBC) contain enzymes that digest microbes.

6.    CENTROSOME

Centrosome is an organelle usually containing two cylindrical structures called centrioles. Both centrioles lie perpendicular to each other.

FUNCTION

They form spindle fibres during cell division in animal cells.

NUCLEUS

Nucleus is present in almost all eukaryotic cells.

The nucleus is the largest organelle and is covered by the nuclear membrane, a double-layered membrane with tiny pores through which some substances can pass between nucleus and the cytoplasm.

The outer layer of nuclear membrane is continuous with endoplasmic reticulum.

                                             

    Figure Credit: Yostnarani Sethy

The nucleus contains the body’s genetic material in the form of DNA.

In a non-dividing cell, DNA is present as a fine network of threads called chromatin, but when the cell prepares to divide, the chromatin forms compact structures called chromosomes.

RNA is also found in the nucleus which is involved in protein synthesis.

Within the nucleus there is another spherical structure called the nucleolus, which is responsible for synthesis of ribosomes.

CELL CYCLE

The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed as cell cycle.

Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. That’s why it becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.

PHASES OF CELL CYCLE

The cell cycle is divided into two basic phases.

·        Interphase (95% of the duration of the cell cycle)

·        M-Phase (5% of the duration of the cell cycle)

INTERPHASE

The interphase represents the phase where the cell prepares itself. That’s why it is otherwise called as preparatory phase.

The interphase is divided into three further phases.

·        G₁ Phase

·        S Phase

·        G₂ Phase

During G1 Phase, the cell continuously grows (cell growth).

During S Phase, DNA replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C, but there is no increase in the chromosome number.

During G2 Phase, proteins are synthesized in preparation for mitosis while cell growth continues.

M-PHASE

The M-Phase represents the phase when the actual cell division occurs. The M-Phase starts with the nuclear division (karyokinesis) and usually ends with division of cytoplasm (cytokinesis). It is also called as equational division.

In humans, mitotic cell division is only seen in the diploid somatic cells.

KARYOKINESIS

Karyokinesis is divided into the following four stages

·        Prophase

·        Metaphase

·        Anaphase

·        Telophase

PROPHASE

Ø Chromosomal material condenses. Chromosomes are seen to be composed of two chromatids attached together at the centromere.

At the end of prophase cells don’t show golgi complexes, endoplasmic reticulum, nuclear membrane etc.

METAPHASE

Ø Spindle fibres attach to kinetochores of chromosomes.

Ø Chromosomes are moved and get aligned along metaphase plate through spindle fibres.

ANAPHASE

Ø Centromeres split and chromatids separate.

Ø Chromatids move to opposite poles.

TELOPHASE

Ø Chromosomes gather at opposite poles.

Ø Nuclear membrane reappears around the chromosomes.

Ø Other cellular organelles reappear.

CYTOKINESIS

The cell is divided into two daughter cells by a process known as cytokinesis at the end of which cell division is complete.

This is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell into two.

SIGNIFICANCE OF MITOSIS

Ø The growth of humans is due to mitosis.

Ø Another significant contribution of mitosis is cell repair. The cells of upper layer of epidermis, cells of the lining of the GIT, blood cells etc. are being constantly replaced.

MEIOSIS

The production of offspring by sexual reproduction includes the fusion of two gametes (sperm, eggs), each with a complete haploid set of chromosomes. Gametes are formed from specialized diploid cells (spermatogonia, oogonia).

This specialised kind of cell division reduces the chromosome number by half results in production of haploid daughter cells. This kind of division is called meiosis.

Meiosis involves two sequential cycles of karyokinesis and cytokinesis called meiosis I and meiosis II but only a single cycle of DNA replication.

Four haploid cells are formed at the end of meiosis II.

MEIOSIS I

PROPHASE I

It has been further subdivided into five phases

       I.            Leptotene

    II.            Zygotene

 III.            Pachytene

IV.            Diplotene

   V.            Diakinesis

LEPTOTENE

The condensation of chromosomes continues throughout leptotene.

ZYGOTENE

During this stage the homologous chromosomes start pairing together and this process is called synapsis.

The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or tetrad.

There is also a formation of complex structure called synaptonemal complex.

PACHYTENE

At this stage crossing over occurs between non-sister chromatids of the homologous chromosomes.

Crossing over leads to recombination of genetic materials of the two chromosomes.

DIPLOTENE

During this stage, the dissolution of synaptonemal structure occurs.

The recombined homologous chromosomes separate from each other except at the sites of crossovers.

These X shaped structures are called chiasmata.

DIAKINESIS

During this state, terminalisation of chiasmata occurs.

At this stage the spindle fibres are assembled to prepare the homologous chromosomes for separation.

By the end of diakinesis, the nuclear membrane also breaks down.

METAPHASE I

The spindle fibres attach to the homologous chromosomes.

The bivalent chromosomes align at the centre.

ANAPHASE I

The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

TELOPHASE I

Chromosomes gather at opposite poles.

Nuclear membrane reappears.

Cytokinesis occurs at the end of telo phase.

MEIOSIS II

PROPHASE II

By the end of prophase II the nuclear membrane disappears.

The chromosomal material condenses.

METAPHASE II

During this stage the spindle fibres get attached to the kinetochores of sister chromatids.

The chromosomes align at the equator.

ANAPHASE II

Splitting of the centromere of each chromosome occurs allowing them to move toward opposite poles of the cell.

TELOPHASE II

In this stage the chromosomes at the opposite poles get enclosed by the nuclear membrane.

Then cytokinesis occurs resulting in the formation of four haploid daughter cells.

SIGNIFICANCE OF MEIOSIS

It increases the chances of genetic variability (mutation) in the population of organisms from one generation to the next.

Conservation of specific chromosome number of species is achieved in sexually reproducing organisms.