Rajasthan Board RBSE Class 9 Science Notes Chapter 6 Structure of Living Organisms
Basis of life – Cell:
- Cell is the smallest structural and functional unit of all the organisms. All living organisms are made up of numerous co-ordinate compartments called cells. Cell was first observed by Robert Hook, in a piece of cork in 1665. Cell word was derived from the Latin word ceila means, very small compartment. Antony Van Leeuwenhoek saw minute green particles in plant cell, chloroplasts. He discovered free red blood cells and observed their inner organisation.
- Cell is the unit of life. Cell theory was propounded by Schleiden and Schwann in 1838-39. Cells originate from the pre-existing cells.
- Purkinje used the word protoplasm for the ground substance of cell. Robert Brown discovered nucleus, in the orchid root cells.
- Cell shape may be fixed or variable. It depends upon the external and internal pressures, as well as function performed by the cell.
- The size of a cell also varies. It mostly depends on the size and number of chromosomes. The smallest cell are found in bacteria, about (0.1 m) and largest cell is the egg of Ostrich (18 cm). The longest cell is nerve cell of human-being, about a metre long. In an organism, the cell number also varies.
Unicellular and Multicellular organism:
- A unicellular organism have a single cell carrying out all the life processes. This is unable to exhibit wide range of different functions and easily susceptible to damage, that can lead to death of the organism.
- A multicellular organism have different cells carrying out different functions. They are more efficient and performs no. of activities. They have greater capacity of survival and are continuously replaced.
The cells present in the living organisms are of two types:
- Prokaryotic cells: Cells of bacteria and blue green algae, in which nuclear envelope is absent and chromosome is a single circular molecule are prokaryotic cells.
- Eukaryotic cells: Cells of higher plants and animals containing nuclear envelope, with more than one chromosome are eukaryotic cells.
Two biologists J.M. Schleiden and T. Schwann proposed that All living organism are composed of one or more cells and (b) the cell is the basic unit of life. All the cells arise from pre-existing cells. This is known as cell theory.
Structure of cell:
Cells contains following compounds:
1. Cell membrane
- Cell membrane: A cell is bound by selectively permeable membrane, the plasma membrane and contains a nucleus and cytoplasm. Cell membrane is selectively permeable and composed of double layer of lipids and proteins. In a plant cell, cell wall lies outside the plasma membrane. It is composed of cellulose. In an animal cell, cell wall is not found. Protoplasm, the living substance which is translucent, homogeneous and jelly-like colloidal material is found in all the cells. Protoplasm is differentiated into cytoplasm and nucleus.
- Cytoplasm: The cytoplasm of the cell lies outside the nucleus and bound inside the plasma-membrane. It is made up of dense, viscous and colloidal mass. It is a solution of organic compounds, like carbohydrates, proteins, lipids, vitamins and minerals. In cytoplasm, cytoplasmic vacuolar system is formed by the membrane bound spaces and consists of endoplasmic reticulum. In the cell cytoplasm, numerous living and non-living structures are present, which are collectively called cytoplasmic inclusion. Cytoplasmic matrix is the homogeneous liquid in which cell organelles and inorganic substances are found. It is granular in structure. Cytoplasm stores a number of vital chemicals. In a cell certain metabolic activities can occur. It always remains in a state of movement.
Cytoplasm contains a number of cellular organelles, such as:
Mitochondria, which are spherical or rod like structure found in all cells of animals and plants, except prokaryotic cells and matured RBCs. It is like a bag whose wall is made up of two membranes. The outer membrane is smooth and the inner is thrown into many folds, called cristae. The interior space is called matrix.
Matrix is filled with a semi-liquid substance. It contains many enzymes which release and control energy. Stalked particles, called oxysomes are found on the surface of cristae.
The main functions of mitochondria are:
- It is the site for cellular respiration.
- Energy is stored in the form of ATP, in mitochondria.
Plastids are of three types:
- Chloroplast: Plastids that contain green pigment, are called chloroplast. The color of leaves is green, due to the presence of chloroplast.
- Chromoplast: These plastids are of different colors such as red, yellow or orange. Tomato is red and flowers are of different colors, due to plastids-chromoplast.
- Leucoplast: They are colorless. They are found in underground stems and roots. They help in food storage. Each plastid is bound by double plasmic membrane like nucleus and mitochondria
The main functions of Plastids are:
- Carbohydrate is synthesized by photosynthesis, in presence of light due to chloroplasts.
- Leucoplast store sugar, oil-drops and proteins.
Lysosomes are spherical shaped bodies surrounded by a single thin membrane. It is filled with a dense liquid. The liquid contains enzymes, which help in digestion of food molecules.
The main functions of Lysosomes are:
- They digest the remains of already digested bacteria.
- They secrete the hydrolytic enzymes in the cell.
- They help in the digestion of dead cells.
- If lysosomes burst and release their enzymes, the entire cell is digested. This is called autolysis of cells and due to this, they are known as suicidal Bags.
Endoplasmic reticulum is a branching tubular system, found in cytoploasm and connected with nuclear envelope and plasma membrane.
It is of two types:
- Smooth endoplasmic reticulum
- Rough endoplasmic reticulum.
The main functions of endoplasmic reticulum are:
- They transport different soluble materials inside the cell.
- They are helpful in the synthesis of proteins and steroid hormones.
- They manufacture the nuclear membrane and Golgi bodies.
- Ribosomes were discovered by Clad and Palade reported these organelles, in animal cell. Ribosomes are small, granular particles, made up of RNAand protein, and remain attached with the membranes of rough endoplasmic reticulum.
- Ribosomes are categorised into two main groups (a) 70 S type (B) 80 S type
- 70 S types are found in Prokaryotes and 80 S types are found in Eukaryotes.
- The main function of ribosomes is synthesis of proteins.
Golgi body was discovered by an Italian biologist, Camilo Golgi. They appear like flat tubes, close to the nucleus. They contain enzymes.
The main functions of golgi body is to:
- Manufacture the membranes which surround the nucleus, the vacuoles, acrosome and cellular plate.
- It also plays a role in secretion. It is helpful in synthesis of proteins, mucous, polysaccharides and pectin.
It is also involved in the formation of:
- Cell plate, during cytokinesis in plant cells.
Centro some was discovered by Von Benden Boveri named it as centro some. It is found in all the animal cells. It is situated near the nucleus. Jelly like substance, centro sphere is found in it. Before cell division, centriole divides into two daughter centrioles. During the division of cell, spindle is formed between these two centrioles.
The main functions of centrosome are:
- It forms two daughter centrioles, which make poles on reaching the sides of the nucleus.
- centrosomes manufacture the axial fibers of the tail of sperms.
- centrosomes manufacture cilia and flagella.
Plant cell can easily be distinguished from the animal cell by the presence of cell wall and large vacuoles.
Vacuoles are storage sacs for solid liquid contents. Vacuoles are small sized, in animal cells, while plants cells have large vacuoles. In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
- Nucleus was first of all, discovered by Robert Brown in 1831. It is the largest and most important organelle of the cell.
- Nucleus is a denser and spherical body, found in cytoplasm. It is bounded by a double layered nuclear membrane. Nuclear membrane is semipermeable and helps in exchange of dissolved matter between protoplasm and the nucleus. It is the nuclear membrane which encloses thick jelly like semi-fluid, called nucleoplasm.
- The nucleoplasm shows fine thread like structures called nuclear reticulum, made up of a special substance called chromatin. Before cell division, chromatin takes the form of thread like structures called chromosomes. The number of chromosomes is fixed for all the species of animals and plants. Man has 23 pairs of chromosomes. Within the nucleus, there is a more dense spherical body called nucleolus.
- The main function of nuclear reticulum is participation in cell division. Nuclear reticulum plays an important role in the inheritance of heriditary characters. It is the controlling center of a cell.
- Formation of ribosomes takes place in nucleolus.
Plant and Animal Cell:
The basic structure of plant and animal cell is same, however, there are many vital differences between them, plant cell is comparatively larger in size, cell wall is present. Plastids are present, lysosomes are absent, centrosomes and centrioles are also absent when compared with animal cells.
Division is a characteristic feature of cell. New cells are always produced by the division of pre-existing cells. Dividing cells repeat same events that constitute the cell cycle. Cell cycle consists of preparatory phase called interphase and a dividing phase called mitotic phase.
Cell division is of three types Amitosis, Mitosis and Meiosis (reduction division).
- It is also called simple or direct division. Protozoans, bacteria and some species of algae and fungi divide by amitosis.
- In amitosis, first the nucleus elongates. Then, contraction takes place in the mid portion of the nucleus. The parent nucleus divides into two equal parts. The cytoplasm also divides, due to contraction. In this way, two daughter cells are formed. Budding is another form of simple nucleus division. Budding is found in plants like yeast and animals like hydra.
Mitosis is a kind of cell division in which mother cell forms two daughter cells by replication (duplication), and the number of chromosomes is the same as that in the mother cell. Cell division is a cyclic action. It is called cell cycle.
It is divided into three main phases:
- Division phase
- It is the resting, phase but no division starts in this phase.Nucleus and cell are at their peak of growth . It is divided into three periods on the basis of synthetic activities.
- Growth period: Necessary substances and enzymes are synthesized, for the synthesis of DNA .
- Synthetic period: In this period, DNA is synthesized.
- Growth period: In this period, central RNA, ribosomal RNA and messenger RNA are synthesized.
- In this phase, first nucleus and afterwards, cytoplasm are divided. Division of nucleus (Karyokinesis) is divided into four phases: (a) Prophase (b) Metaphase (c) Anaphase and (d) Telophase.
- Prophase: The chromosomes shorten and thicken and become distinctly visible. Nuclear membrane dissolve. Chromosomes split longitudinally into two parts, called chromatids which are attached through the centromere. Centrosome divides into two tiny bodies, called centrioles. One centriole remains on the same place and other centriole moves to opposite spot and makes the second pole. Many astral rays come out from each centriole.
- Metaphase: The spindle formation is completed. All chromosomes arrange themselves on equatorial plate, and make metaphase plate.
- Anaphase: The centromeres duplicate and split. One centromere is attached to each chromatid. Separated chromatids are now called daughter chromosomes. Daughter chromosomes move to the opposite poles, by contraction of split fibres and finally the two sets reach their respective poles.
- Telophase: All the chromosomes after reaching the poles, uncoil and elongate, Nuclear membrane is formed around them. The spindle fibres disappear. In this way, two nuclei are formed.
The division of cytoplasm is known as cytokinesis. The cytoplasm divides, after the division of the nucleus. In an animal cell, the cell constructs by cleavage and ultimately divides into two daughter cells. In plant cell, a plate is formed by arranging the particles. The plate divides the cytoplasm into two daughter cells.
Significance of Mitosis:
- Mitosis increases the number of cells in an organism.
- Mitosis brings about growth and development
- Mitosis helps in asexual reproduction.
- Mitosis is helpful in repair and regeneration
- Mitosis keeps the size and volume of a cell definite.
The main function of meiosis is to maintain a definite and constant number of chromosomes in species, generation after generation. In this type of division, the number of chromosomes in the daughter cells is reduced to half of the number of chromosomes in the parent cell. The process of meiosis takes place into two nuclear division – (A) Meiosis I and (B) Meiosis II. Meiosis I results in reduction of chromosomes to half, i.e., from 2n to n. So, this division is also called reductional division or heterotypic division.
In this division, two daughter cells are formed. Each daughter cell has half the number of chromosomes than the parent cell.
Meiosis takes place in five stages:
- Interphase: It is similar to that in mitosis. Synthetic reactions take place, as in interphase of mitosis.
- Prophase I: It is of longer duration and more complex. The chromatin substance transforms into chromosomes. Chromosomes are in the form of fine threads. Homologous chromosomes arrange in pairs, and this is known as synapsis. There is a crossover at the broken ends of chromatids of the paired homologous chromosomes.
- The paired homologous chromosomes begin to move apart, except at chaismata. Chaismata are the points where crossing over occurred.
- Spindle formation occurs, and chromosomes arrange themselves on equatorial plane. The centromeres lie towards the poles.
- Anaphase I: The two partners of homologous chromosomes completely separate from each other and move to the opposite poles. The number of chromosomes in each set is haploid.
- Telophase I: Nuclear membrane is formed at each pole, surrounding the chromosomes. Nucleolus again appears. In this way, a daughter nucleus is formed at each pole. Cytokinesis occurs by cleavage or invagination, and two haploid daughter cells are formed.
- Interphase II: The interphase that follows Meiosis-I, is of different duration in different species. This phase is not found in many living-beings. Nuclear membrane and nucleolus are not formed. This occurs in both the daughter cells formed, in Meiosis I. The Meiosis-ll is similar to that of Mitosis.
- Prophase II: Nuclear membrane and nucleolus disappear, in both the daughter cells. Both the chromatids of each chromosome begin to separate from one another and spindle formation takes place.
- Metaphase II: The chromosomes arrange themselves on metaphase plate (equatorial plate). The centromere divides with one chromatid.
- Anaphase II: Two chromatids separate and move to opposite poles.
- Telophase III: Chromosomes uncoil and form chromatin network. Nuclear membrane is formed at the pole, surrounding the centrosomes and other nuclear inclusions. As a result of two successive divisions, four daughter cells are formed, each having haploid number of chromosomes.
Significance of Meiosis.
- Due to this division, the gametes formed are haploid. Hence, meiosis is an important and essential part of the life history of living beings.
- During fertilization, the gametes on fusion form diploid zygote. Thus, original number of chromosomes in somatic cells is restored.
- New characters are introduced in the new generation, due to crossing over of the genes.
- Due to changes in parental characters, variations occur which are necessary for the evolution process.
A cellular organism-Virus:
Viruses have a structure, which reflects the structure of primordial organisms. There is a protein coat, which encloses a small nucleic acid molecule consisting of few nucleotides. They are simplest in their structure and can remain in the form of crystals. They prove the point that the earliest cells contained the gene forming nucleic acids, which were suspended freely within the cell. The freely floating nucleotide chains happened to enter a cell or alternatively, synthesised their own protein coat to assume a structure called virus.
As we know today, viruses are neither cells, nor they are organisms. They are in between the living and non¬living world. The only point that does not go in the favour of viruses as the first cells, not is the fact that they cannot replicate themselves. It is because, they do not posses a replicating machinery and that their nucleic acid cannot replicate by itself.
It is for this reason, that they have to insert their genetic material into another host cell, where they can use the replication enzymes of the host cell, to produce multiple copies of their own nucleic acid molecule and a protein coat, around each such molecule. While doing so, they also destroys the cell. Being parasitic, they lose their claim of being the first organisms, because parasitism is of later origin in evolution than the free living forms.
On the basis of nutrition, virus is differentiated into three categories:
- Animal Virus
- Plant virus
Structure of multi cellular organism:
- All living organisms are made up of cells. Some are unicellular, while other consists of large number of cells.
- In unicellular plants and animals, vital activities (like movement, intake of food, respiratory gases, respiration and excretion) are performed by a single cell.
- In multi cellular plants and animals, cells of different types, perform different functions.
A group of cells that are similar in structure or work together to achieve a particular function forms a tissue.
Major types of Animal and Plant Tissues:
- Plants are stationary or fixed, they do not move. Most of their tissues are supportive, which provide them with structural strength. Most of these tissues are dead. They require less maintenance, but can provide mechanical strength as easily as the living ones. On the other hand, animals move around in search of food, mates and shelter. Most of the tissues they contain, are living.
- The growth in plants is limited to certain regions. There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Various plant tissues can be classified as growing or meristematic tissue and permanent tissue.
- Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non-dividing regions, in animals.
Plant tissues are of two types: Meristematic and Permanent:
Meristematic tissue are present in growing regions of plants. Its cells continuously multiply so that the region grows in length and breadth.
This tissue is of three types:
- Apical meristem, found at the apex of shoots and roots. Root tip is covered by root cap.
- Lateral meristem, found on lateral sides of stem and root, e.g. cork cambium and cambium of vascular bundles.
- Intercallary meristem, a part of apical meristem, which separates off laterally from the apical meristem.
Permanent tissue arise from meristematic tissue and its cells after maturity, forms permanent tissue. It is of two types: simple tissues and complex tissues.
Simple tissue are of three types:
- Parenchyma cells: They are living and cytoplasm is vacuolated and possess intercellular spaces, with thin cell wall.
- Collenchyma cells: They are living and contain chloroplasts. Hence are capable of synthesising starch and sugar. Being elastic, they give tensile strength to the stem. No intercellular spaces are present.
- Sclerenchyma fibres or cells: They are long, narrow and pointed. On maturity, protoplasm dries off. Sclereids or stone cells or grit cells are of irregular shape. Cells lumen is narrow. They gives strong support to the stem.
- Protective tissues are epidermis of leaves, possessing stomata for exchange of gases. Cork is found on the outermost side of stem. Its cells are dead.
- Complex tissues are xylem and phloem. Xylem forms the woody part of the stem. Xylem and phloem forms the vascular bundle. Xylem is constituted by four types of cells (e.g. sclereids, tracheae and xylem fibre or sclerenchyma, dead cells and xylem parenchyma, living cells. It conducts water and nutrients from soil through roots to the leaves. Phloem is also formed of four types of cells: Sieve tubes, Companion cells, Phloem fibres and Phloem parenchyma. Sieve tubes and companion cells transport manufactured food from leaves to other parts of plants. Phloem fibres and sieve tubes are dead.
- Animal tissues include epithelial, connective, muscularine and nervous tissues.
- Epithelial tissue is found on the surface of organs and in the cavities of organs. Its cells are of various shapes and are held together with a small amount of inter cellular substance, matrix. Epithelium is of two types: Simple epithelium, formed of a single layer of cells and Stratified epithelium formed of a number of cell layers.
- Simple epithelial is squamous, cuboidal, columnar, glandular and ciliated.
- Stratified epithelium cells are arranged in layers, like epidermis of skin.
- Muscular tissue includes striped, unstriped and cardiac muscles. Their cells or fibers are long and nucleated.
- Striped muscle fibers or cells are multi nucleated and show alternate dark and light bands, due to which they are called striped. Muscles of legs and hands are of striped type. Each muscle fibre contains a number of thin and thick micro filaments within the sarcoplasm.
- Non-striped or visceral muscle fibres are also long, cylindrical and uninucleated. Each fiber also contains numerous, lengthwise arranged micro filaments in sarcoplasm. These muscles are found in the walls of internal organs of the body.
- Cardiac muscle fibers or cells are branched and uninucleated. The sarcoplasm of these cells also contains micro filaments, showing faint cross striation’s. Branch of a cell joins the branch of the adjoining cell. At the junction of adjacent cell, is present a transverse intercalated disc. In between adjacent cells, are present inter cellular spaces, having loose connective tissue. It is found in heart, due to which it contracts rhythmically.
- Connective tissue connects and anchors various organs of the body and gives support to the organs. It contains abundant jelly-like intra cellular medium (matrix), fibers (white and yellow) and various types of cells, eg. fibroblasts, macrophages, mast cells, adipose cells and immunocytes.
- Connective tissues are areolar (loose), dense and adipose tissues.
- Tendons join muscles to bones and ligaments join bone to bone. Both are dense connective tissues.
- Adipose tissue stores fat globules. Blood and lymph are fluid connective tissue. Cartilage and bones are skeletal connective tissue. Lymph is colorless and has plasma minus red blood corpuscles and certain proteins.
- Nervous tissue is formed of neurons. A neuron has a cell body, the cyton, a few small branches of cyton, the dendrons or dendrites and a single long branch called axon.
- Axon carries away impulses from cyton and transmits them to another neuron whereas, dendrons receive impulses and transmit them to cyton.
Structure of organs and system:
- Tissue system: A combination of one or more types of tissues, performing a common function, regardless of their position and continuity in the plant body, constitutes a tissue system. Three tissue system have been recognised in plants- epidermal tissue system, ground or fundamental tissue system and vascular tissue system.
- Epidermal tissue system includes epidermis and various types of epidermal appendages. Epidermis is usually single layered, but some leaves have multiseriate epidermis.
- In the aerial roots of some epiphytes, multiple epidermis forms a special tissue known as velamen.
- Stomata are abundant in the epidermis of the aerial parts, especially leaves. Each stoma has two bean shaped guard cells. These are surrounded by accessory or subsidiary cells, which are different from epidermal cells.
- Ground or fundamental tissue system, includes cortex, pericycle and pith. Cortex is further differentiated into hypodermis, general cortex and endodermis.
- Pith is the central parenchymatous region, which is quite distinct in dicot stems and monocot roots.
- Vascular tissue system includes vascular bundles which are made of xylem and phloem.
- In stems, the vascular bundles are conjoint and collateral, while in roots they are radial.
- The bundles may be open with cambium or closed (without cambium). They are endarch (in stems) or exarch (in roots).