Cell – Structure and Function MODULE - 1
Diversity and Evolution
of Life
4
Notes
CELL – STRUCTURE AND FUNCTION
INTRODUCTION
All organisms are composed of structural and functional units of life called ‘cells’.
The body of some organisms like bacteria, protozoans and some algae is made up
of a single cell whereas the body of higher fungi, plants and animals are composed
of many cells. Human body is built of about one trillion cells.
Cells vary in size and structure as they are specialized to perform different functions.
But the basic components of the cell are common to all biological cells. This lesson
deals with the structure common to all types of the cells. You will also learn about
the kinds of cell division and the processes involved therein in this lesson.
OBJECTIVES
After completing this lesson, you will be able to :
z justify that cell is the basic structural and functional unit of all organisms;
z list the components of the cell and state cell theory;
z differentiate between prokaryotic and eukaryotic cells;
z differentiate between plant and animal cells;
z illustrate the structure of plant and animal cells by drawing labelled diagrams;
z describe the structure and functions of plasma membrane, cell wall, endoplasmic
reticulum (ER), cilia, flagella, nucleus, ribosomes, mitochondria, chloroplasts,
golgi body, peroxisome, glyoxysome and lysosome;
z describe the general importance of the cell molecules-water, mineral ions,
carbohydrates, lipids, amino acids, proteins, nucleotides, nucleic acids, enzymes,
vitamins, hormones, steroids and alkaloids;
z justify the need for cell division;
z describe various phases of cell cycle;
z explain the term karyotype and mention the karyotype analysis and its
significance.
BIOLOGY 79
, MODULE - 1 Cell – Structure and Function
Diversity and Evolution
of Life 4.1 THE CELL AND CELL THEORY
4.1.1 Landmarks in the study of a cell
Soon after Anton Van Leeuwenhoek invented the microscope, Robert Hooke in
1665 observed a piece of cork under the microscope and found it to be made of
small compartments which he called “cells” (Latin cell = small room). In 1672,
Leeuwenhoek observed bacteria, sperms and red blood corpuscles, all of which were
Notes
cells. Much later, in 1831, Robert Brown, an Englishman observed that all cells had
a centrally positioned body which he termed the nucleus.
4.1.2 The cell theory
In 1838 M.J. Schleiden and Theodore Schwann formulated the “cell theory.” Which
maintains that:
z all organisms are composed of cells.
z cell is the structural and functional unit of life, and
z cells arise from pre-existing cells.
The cells vary considerably, in shapes and sizes (Fig.4.1). Nerve cells of animals
have long extensions. They can be several centimeter in length. Muscle cells are
elongated in shape. Egg of the ostrich is the largest cell (75 mm). Some plant cells
have thick walls. There is also wide variation in the number of cells in different
organisms.
4.1.3 The Cell
A cell may be defined as a unit of protoplasm bound by a plasma or cell membrane
and possessing a nucleus. Protoplasm is the life giving substance and includes the
cytoplasm and the nucleus. The cytoplasm has in it organelles such as ribosomes,
mitochondria, golgi bodies, plastids, lysosomes and endoplasmic reticulum. Plant
cells have in their cytoplasm, large vacuoles containing non-living inclusions like
crystals, and pigments. The bacteria have neither defined cell organelles nor a well
formed nucleus. But every cell has three major components:
z plasma membrane
z cytoplasm
z DNA (naked in bacteria) and enclosed by a nuclear membrane in all other
organisms
Two basic types of cells
Cytologists recognize two basic types of cells (Fig. 4.1). Their differences have been
tabulated below in Table 4.1. Organisms which do not possess a well formed nucleus
are prokaryotes such as the bacteria. All others possess a well defined nucleus,
covered by a nuclear membrane. They are eukaryotes.
80 BIOLOGY
,Cell – Structure and Function MODULE - 1
Diversity and Evolution
Table 4.1 Differences between Eukaryotic and Prokaryotic cells of Life
Eukaryotic cell (eu = true, karyon = nucleus) Prokaryotic cell (Pro = early/primitive)
1. Nucleus distinct, with well formed 1. Nucleus not distinct, it is in the form
nuclear membrane. of a nuclear zone ‘nucleoid’. Nuclear
membrane absent.
2. Double-membraned cell organelles 2. Single-membraned cell bodies like
(Chloroplasts, mitochondria, nucleus) mesosomes present. Endoplasmic
and single membraned (Golgi apparatus, reticulum, plastids, nitochondria Notes
lysosomes, vacuole, endoplasm reticulum) microbodies like lysosomes, and
are present Golgi body absent.
3. Ribosomes - 80 S 3. Ribosomes - 70 S
4. Distinct compartments in the cell 4. No compartments.
i.e. the cytoplasm and the nucleus
5. Depending upon the species number of 5. There is only one chromosome per cell.
chromosomes per nucleus varies from
two to many.
6. Each chromosome is linear with its two 6. The chromosome is circular and remains
ends free. attached to cell membrane at one point.
7. Each chromosome has one linear double- 7. The chromosome has single double-stranded
stranded DNA complexed with histones circular DNA molecule and is not associated
with histones.
8. Each chromosome has one centromere 8. The chromosome lacks a centromere.
that divides a chromosome into two
arms. However, if the centromere is
terminal, the chromosome would have
only one arm
Fig. 4.1a Eukaryotic Cell Fig. 4.1b Prokaryotic Cell
(As seen in an electron micrograph.)
Svedberg unit
When the cell is fractionated or broken down into its components by rotating
in an ultracentrifuge at different speeds the ribosomes of eukaryotic and
prokaryotic cells sediment (settle down) at different speeds. The coefficient of
sedimentation is represented in Svedberg unit and is depicted as S.
The plant cell and the animal cell also differ in several respects as given in Table
4.2 and shown in Fig. 4.2.
BIOLOGY 81
, MODULE - 1 Cell – Structure and Function
Diversity and Evolution
of Life Table: 4.2 Differences between plant cell and animal cell
Plant cell Animal cell
1. Cellulose cell wall present external to cell 1. No cell wall, outermost structure is cell
membrane. membrane or plasma membrane
2. Vacuoles are usually large. 2. Generally vacuoles are absent and if
present, are usually small..
3. Plastids present. 3. Plastids absent.
Notes 4. Golgi body present in the form of units 4. Golgi body well developed having 2
known as dictyosomes. cisternae
5. Centriole absent. 5. Centriole present.
Fig. 4.2a Generalised plant cell Fig. 4.2b Generalised animal cell
INTEXT QUESTIONS 4.1
1. From where do new cells arise?
............................................................................................................................
2. Name the scientists who proposed the ‘cell theory’.
............................................................................................................................
3. Name an organelle which a plant cell has but an animal cell does not.
............................................................................................................................
4. Give two points of difference between a prokaryotic cell and a eukaryotic cell
............................................................................................................................
4.2 COMPONENTS OF THE CELL
The major components of the cell are (1) cell membrane, (2) cytoplasm, and (3)
nucleus.
4.2.1 Cell membrane (Plasma membrane)
Each cell has a limiting boundary, the cell membrane, plasma membrane or
plasmalemma. It is a living membrane, outermost in animal cells but internal to cell
wall in plant cells.
It is flexible and can fold in (as in food vacuoles of Amoeba) or fold out (as in the
formation of pseudopodia of Amoeba)
82 BIOLOGY
Diversity and Evolution
of Life
4
Notes
CELL – STRUCTURE AND FUNCTION
INTRODUCTION
All organisms are composed of structural and functional units of life called ‘cells’.
The body of some organisms like bacteria, protozoans and some algae is made up
of a single cell whereas the body of higher fungi, plants and animals are composed
of many cells. Human body is built of about one trillion cells.
Cells vary in size and structure as they are specialized to perform different functions.
But the basic components of the cell are common to all biological cells. This lesson
deals with the structure common to all types of the cells. You will also learn about
the kinds of cell division and the processes involved therein in this lesson.
OBJECTIVES
After completing this lesson, you will be able to :
z justify that cell is the basic structural and functional unit of all organisms;
z list the components of the cell and state cell theory;
z differentiate between prokaryotic and eukaryotic cells;
z differentiate between plant and animal cells;
z illustrate the structure of plant and animal cells by drawing labelled diagrams;
z describe the structure and functions of plasma membrane, cell wall, endoplasmic
reticulum (ER), cilia, flagella, nucleus, ribosomes, mitochondria, chloroplasts,
golgi body, peroxisome, glyoxysome and lysosome;
z describe the general importance of the cell molecules-water, mineral ions,
carbohydrates, lipids, amino acids, proteins, nucleotides, nucleic acids, enzymes,
vitamins, hormones, steroids and alkaloids;
z justify the need for cell division;
z describe various phases of cell cycle;
z explain the term karyotype and mention the karyotype analysis and its
significance.
BIOLOGY 79
, MODULE - 1 Cell – Structure and Function
Diversity and Evolution
of Life 4.1 THE CELL AND CELL THEORY
4.1.1 Landmarks in the study of a cell
Soon after Anton Van Leeuwenhoek invented the microscope, Robert Hooke in
1665 observed a piece of cork under the microscope and found it to be made of
small compartments which he called “cells” (Latin cell = small room). In 1672,
Leeuwenhoek observed bacteria, sperms and red blood corpuscles, all of which were
Notes
cells. Much later, in 1831, Robert Brown, an Englishman observed that all cells had
a centrally positioned body which he termed the nucleus.
4.1.2 The cell theory
In 1838 M.J. Schleiden and Theodore Schwann formulated the “cell theory.” Which
maintains that:
z all organisms are composed of cells.
z cell is the structural and functional unit of life, and
z cells arise from pre-existing cells.
The cells vary considerably, in shapes and sizes (Fig.4.1). Nerve cells of animals
have long extensions. They can be several centimeter in length. Muscle cells are
elongated in shape. Egg of the ostrich is the largest cell (75 mm). Some plant cells
have thick walls. There is also wide variation in the number of cells in different
organisms.
4.1.3 The Cell
A cell may be defined as a unit of protoplasm bound by a plasma or cell membrane
and possessing a nucleus. Protoplasm is the life giving substance and includes the
cytoplasm and the nucleus. The cytoplasm has in it organelles such as ribosomes,
mitochondria, golgi bodies, plastids, lysosomes and endoplasmic reticulum. Plant
cells have in their cytoplasm, large vacuoles containing non-living inclusions like
crystals, and pigments. The bacteria have neither defined cell organelles nor a well
formed nucleus. But every cell has three major components:
z plasma membrane
z cytoplasm
z DNA (naked in bacteria) and enclosed by a nuclear membrane in all other
organisms
Two basic types of cells
Cytologists recognize two basic types of cells (Fig. 4.1). Their differences have been
tabulated below in Table 4.1. Organisms which do not possess a well formed nucleus
are prokaryotes such as the bacteria. All others possess a well defined nucleus,
covered by a nuclear membrane. They are eukaryotes.
80 BIOLOGY
,Cell – Structure and Function MODULE - 1
Diversity and Evolution
Table 4.1 Differences between Eukaryotic and Prokaryotic cells of Life
Eukaryotic cell (eu = true, karyon = nucleus) Prokaryotic cell (Pro = early/primitive)
1. Nucleus distinct, with well formed 1. Nucleus not distinct, it is in the form
nuclear membrane. of a nuclear zone ‘nucleoid’. Nuclear
membrane absent.
2. Double-membraned cell organelles 2. Single-membraned cell bodies like
(Chloroplasts, mitochondria, nucleus) mesosomes present. Endoplasmic
and single membraned (Golgi apparatus, reticulum, plastids, nitochondria Notes
lysosomes, vacuole, endoplasm reticulum) microbodies like lysosomes, and
are present Golgi body absent.
3. Ribosomes - 80 S 3. Ribosomes - 70 S
4. Distinct compartments in the cell 4. No compartments.
i.e. the cytoplasm and the nucleus
5. Depending upon the species number of 5. There is only one chromosome per cell.
chromosomes per nucleus varies from
two to many.
6. Each chromosome is linear with its two 6. The chromosome is circular and remains
ends free. attached to cell membrane at one point.
7. Each chromosome has one linear double- 7. The chromosome has single double-stranded
stranded DNA complexed with histones circular DNA molecule and is not associated
with histones.
8. Each chromosome has one centromere 8. The chromosome lacks a centromere.
that divides a chromosome into two
arms. However, if the centromere is
terminal, the chromosome would have
only one arm
Fig. 4.1a Eukaryotic Cell Fig. 4.1b Prokaryotic Cell
(As seen in an electron micrograph.)
Svedberg unit
When the cell is fractionated or broken down into its components by rotating
in an ultracentrifuge at different speeds the ribosomes of eukaryotic and
prokaryotic cells sediment (settle down) at different speeds. The coefficient of
sedimentation is represented in Svedberg unit and is depicted as S.
The plant cell and the animal cell also differ in several respects as given in Table
4.2 and shown in Fig. 4.2.
BIOLOGY 81
, MODULE - 1 Cell – Structure and Function
Diversity and Evolution
of Life Table: 4.2 Differences between plant cell and animal cell
Plant cell Animal cell
1. Cellulose cell wall present external to cell 1. No cell wall, outermost structure is cell
membrane. membrane or plasma membrane
2. Vacuoles are usually large. 2. Generally vacuoles are absent and if
present, are usually small..
3. Plastids present. 3. Plastids absent.
Notes 4. Golgi body present in the form of units 4. Golgi body well developed having 2
known as dictyosomes. cisternae
5. Centriole absent. 5. Centriole present.
Fig. 4.2a Generalised plant cell Fig. 4.2b Generalised animal cell
INTEXT QUESTIONS 4.1
1. From where do new cells arise?
............................................................................................................................
2. Name the scientists who proposed the ‘cell theory’.
............................................................................................................................
3. Name an organelle which a plant cell has but an animal cell does not.
............................................................................................................................
4. Give two points of difference between a prokaryotic cell and a eukaryotic cell
............................................................................................................................
4.2 COMPONENTS OF THE CELL
The major components of the cell are (1) cell membrane, (2) cytoplasm, and (3)
nucleus.
4.2.1 Cell membrane (Plasma membrane)
Each cell has a limiting boundary, the cell membrane, plasma membrane or
plasmalemma. It is a living membrane, outermost in animal cells but internal to cell
wall in plant cells.
It is flexible and can fold in (as in food vacuoles of Amoeba) or fold out (as in the
formation of pseudopodia of Amoeba)
82 BIOLOGY
