Cytology

Question 1

Contribution of microscope to the expansion of knowledge on cells and cellular organization

Answer 1

Advancement of the cytology is mostly based on the microscopy.
The discovery and early study of cells progressed with the invention of microscope.

Question 2

Light microscope

Answer 2

Visible light is passed through the
specimen and then through glass lenses.

The lenses refract the light in such a way that the image of the specimen is
magnified as it is projected into the eye.

The simplest microscope is a single lens.

Question 3

The compound light microscope

Answer 3

Compound light microscopes are commonly used in school laboratories and it is used in medical laboratories as a diagnostic tool.

Question 4

Resolution power and magnification?

Answer 4

Resolution power and magnification are important parameters which can be seen in a microscope.

Magnification is ratio of an object’s image size to its actual size.
Usually the maximum magnification of light microscope is 1000 times the actual size of the specimen)

Resolution power is minimum distance between two points that can be distinguished as separate points (resolution power of light microscope is 0.2μm). It is a measure of the clarity of the image.

Magnification is limited due to the resolution.

Question 5

Light from an object (specimen on the slide) passes first through objective lens. Then produce a magnified image.
Above image then acts as an object for the second lens (the eye piece lens) which further magnifies it.
The total magnification is hence the product of the magnification of each lens.
Total magnification =
Magnification of Magnification of objective lens X objective lens
e.g- .If magnification of Objective lens = ×40, eyepiece =×15 Total is =15 × 40= ×600 time magnified

Answer 5

Y Not

Question 6

The Electron Microscope

Answer 6

The limitation imposed upon the resolution power of the light microscope by the
wavelength of light.

The resolution power is inversely proportional to the wavelength.
Due to this, scientists considered the use of
other forms of radiations with comparatively shorter wavelengths.

As a result, electron microscopes were developed.
In electron microscopy, a beam of electrons is focused through the specimen or on to its surface.

This means, that in theory, the electron microscope should be able to magnify objects up to
1×108 times. In practice, it magnifies just over
5×105 times.

Electron microscopes have revealed many organelles and other sub cellular structures those were impossible to resolve with the light microscopes.

Question 7

There are two types of electron microscopes.

Answer 7

1. Transmission electron microscopes (TEM)

2. Scanning electron microscopes(SEM)

Question 8

Transmission electron microscopes

Answer 8

It is used to study the
internal structures of cells.In this microscope, a
beam of electrons is passed through a
thin, especially prepared slice of material. A
very thin specimen is used. Specimens
stained with heavy metals which
attach more to certain cellular structures than other areas.

Image reflects the pattern of electrons passed through the specimen, displays on a screen.
While electrons pass through the specimen,
more electrons may get displayed in regions where structures were densely stained.

Question 9

Scanning electron microscopes
In this instrument, a fine beam of electrons is reflected from the surface of specimen. Specimen is mostly coated with gold prior to observation. Here the specimen scatters many electrons whereas others are absorbed. This instrument is ideal to observe the surface view in three dimensional appearances.

Answer 9

In this instrument, a
fine beam of electrons is reflected from the surface of specimen. Specimen is mostly
coated with gold prior to observation. Here the specimen
scatters many electrons whereas others are absorbed. This instrument is ideal to observe the
surface view in 3D

Question 10

Light Microscope vs Electron microscope

Answer 10

FELC ROS

Glass lenses are used to focus the light rays
Powerful magnets are used to focus beam of electrons

Image is directly detected by naked eye
Not directrly detected by naked eye, micrographs are used

Living and non living objects can be observed
Only non-living objects are observed

Actual color of the object can be observed
Actual color cannot be observed.
Images are developed

Dyes used to stain the object
Heavy metals are used to stain the object

Question 11

Historical background of the cell and analyses the structure and functions of the sub cellular units

Cell theory

Answer 11

All organisms are composed of cells.

Recall the hierarchy of life, the levels of organization mentioned earlier.

The basic unit which can be called “living” is the cell, which may form a single celled organism

(e.g.Chlamydomonas, Yeast) or a multi-cellular plant or animal.

The cell is the basic structural and functional unit of life.

The level of organization of matter represented by a cell shows all the characteristics of life.

Any stage below level of a cell cannot be considered living, whether it is a single celled organism or multi-cellular plant or an animal.

Robert Hooke (1665) examined a cork using simple microscope and gave the term “CELL” to describe the basic units.

Anton Van Leeuwenhook (1650), a contemporary of Robert Hooke, was the first to describe and record living single celled organisms, Euglena & bacteria

Matthias Schleiden (1831), a botanist, studying plant tissues concluded that all plants are made up of cells.

Theodore Schwann a zoologist (1839) concluded that animal tissues are also made up of cells.

Rudolf Virchow (1855) showed that all cells arise from pre-existing cells by cell division,

Question 12

Schleiden, Schwann and Virchow presented the ‘Cell Theory’ which included the following.

Answer 12

1. All organisms are composed of one or more cells.
2. The basic structural and functional unit of organisms is the cell.
3. All cells arise from pre-existing cells.

Question 13

Organization of cells two kinds of cellular organization?

Answer 13

• Prokaryotic and Eukaryotic

Question 14

All cells share certain basic features. They are;

Answer 14

Plasma membrane, Cytosol, DNA, Ribosome

• All cells are bounded by a plasma membrane which is a selective barrier
• Within the cell have, a semifluid, jelly like substance which is called cytosol. Subcellular components are suspended within the cytosol.
• They carry DNA as genetic materials.
• Ribosomes are found in all cells

Question 15

The differences between Prokaryotic cells and Eukaryotic cells

Feature
Prokaryote
Eukaryote?

Answer 15

OSFED GROW PiN

organism

Bacteria, Archaebacteria
Protists,Fungi, plants, animals

Cell size
Average diameter 1-5μm
10μm-100μm diameter

Form
Mainly unicellular
Mainly multicellular (except most of protista and some fungi are unicellular)

Evolutionary origin

3. 5 billion years ago
4. 8 billion years ago ,evolved from prokaryotes

Cell division
Binary fission, no mitosis and meiosis
Mitosis, meiosis, or both;

Genetic material
DNA is circular and lies free in the cytoplasm. This region is called nucleoid, DNA is naked and not associated with proteins
DNA is linear and contained in a nucleus. DNA is associated with proteins

Type of ribosomes
70s ribosome (smaller)
Both 70s (Mitochondria and Chloroplast) and 80s ribosomes (larger) present (may attach to endoplasmic reticulum)

  Organelles Few organelles, none are surrounded by membrane Internal membranes scarces; if present usually associated with respiration, photosynthesis and N2 fixation. Many organelles, membrane bounded organelles present. Great diversity of organelles. e.g. nucleus, mitochondria, chloroplasts bounded by two membranes. e.g. Lysosomes, Vacuole, bounded by single membrane.

 Cell walls Peptidoglycan present in Bacteria and cyanobacteria, polysaccharide and protein present in Archae bacteria Cell walls of green plants and fungi are rigid and contain polysaccharides; cellulose in plant cell walls and chitin in fungal walls (none in animal cells)

 Flagella Simple, lacking microtubules; extracellular (not enclosed by cell surface membrane) 20nm diameter Complex, with ‘9+2’ arrangement of microtubules; intracellular (surrounded by cell surface membrane) 200nm diameter

 Respiration Mostly by mesosomes Mitochondria for aerobic respiration

   Photosynthesis No chloroplasts; takes place on membranes which show no stacking Chloroplasts containing membranes which are usually stacked into lamellae or grana

 Nitrogen fixation Some have the ability None have the ability

Bacteria, Cyanobacteria and Achaea are prokaryotic cells. All the other organisms have eukaryotic cells.

Question 16

Structures and functions of organelles and other subcellular components

Plasma membrane?

Answer 16

Plasmamembrane is the outer limit of cytoplasm.

All cellular membranes resemble the ultra structure of plasma membrane.

In 1972, Singer and Nicolson put forward the fluid mosaic model of cell membrane. It is mainly composed of;
• Phospholipids (most abundant type of lipid in plasma membrane)
• Protein

The Plasma membrane has the following features.

It is about
7nm thick.
It is mainly made up of a
phospholipid bilayer. Phospholipids are
amphipathic molecules.
The hydrophilic heads of the phospholipids face outwards into the aqueous environment of both inside and outside of the cell.
The hydrophobic hydrocarbon tails face inwards and create a hydrophobic interior.

Plasmamembrane is compared to the
fluid mosaic model.
Since phospholipid molecules are moveable, they provide the fluid nature to the membrane.
Protein molecules embedded randomly contribute to its mosaic nature.

Some of the protein molecules penetrate all the way through the membrane, called
transmembrane proteins and some others penetrate only part of the way into the membrane. These are called

integral proteins.

Most of the integral proteins are transmembrane proteins which have hydrophilic channels.
These act as pores through which ions and certain polar molecules can pass.

Some proteins are not embedded in the lipid bilayer at all, and are loosely bound to the inner surface of the membrane, called
peripheral proteins.

Some proteins and lipids have short branching carbohydrate chains like antennae, forming glycoprotein and glycolipids, respectively.

Animal’s cell membrane may contain few
cholesterol molecules randomly integrated into the lipid bilayer.

These cholesterol molecules provide
flexibility and stability to the membrane by
reducing membrane fluidity at moderate temperatures and
prevent membrane solidification at low temperatures.

The two sides of the membrane may differ in composition and function.

Question 17

Functions - Plasma membrane

Answer 17

•The plasma membrane surrounds the cytoplasm of living cell
physically separating the intracellular components from the extracellular environment.

• Plasma membrane is
selectively permeable and able to
regulate the exchange of material needed for survival.

• Proteins embedded in the plasma membrane
identify the cell, enabling nearby cells to
communicate with each other (involved in
cell recognition).

• Some protein molecules act as
receptor molecules for interacting with specific biochemical, such as hormones, neurotransmitters and immune proteins.

• Some proteins in the cell membrane
attach to some cytoskeletal fibers and help to maintain the shape of the cell.

• Some proteins in the
membrane act as enzymes.
(e.g.Microvillus on epithelial cell lining of some parts of the gut contains digestive enzymes in their cell surface membrane.)

Question 18

Subcellular components

Answer 18

There are many sub-cellular components in the cell. Some of them are organelles,
which are bound by membranes
and suspended in the cytosol of eukaryotic cell
to perform specialized functions.