FYJC Jacob’s Notes of Biology 1

Unit 2. Structure of the .

You have probably seen that bricks placed one above the other help in the construction of a building. Therefore, we can say that the basic unit of a building is a brick. Living beings are called organisms. Organisms display an immense diversity in form and size. Yet they have an underlying unity in their basic structure and functions. They are made up of one or many units called cells.

The body of organisms may be made up of a single cell (unicellular) or many cells (multicellular). In unicellular organisms, the characteristic features of life are expressed by the single cell. Eg. Amoeba, Chlamydomonas, Paramaecium etc. In multicellular organisms, all the functions of life are carried out by one or other group of cells. In fact, the cells are the binding blocks of life. A cell is a structural and functional unit of life.

There is division of labour in the cells constituting the body of multicellular organisms. Therefore, in their body tissues, organs, and organ systems have developed. Group of cells with similar structure and functions are known as tissues. Many types of tissues together constitute organs which perform one or many specific functions. In higher organisms several organs which are closely related with each other constitute organ system which is responsible for a typical life process. Digestive system, as important life process, is constituted of many organs as salivary glands, stomach, intestine, pancreas, liver, gall bladder etc. These organs are further made up of tissues like epithelial, connective, and muscular.

CELLS TISSUES ORGANS ORGAN SYSTEMS BODY

Characteristics of cell 1. Cell is the basic unit of all organisms. 2. The cell is surrounded by plasma membrane in all organisms. 3. The basic structure and functional properties cell membranes are similar in all types of cells. 4. The genetic material in all the cells consists of nucleic acids which is responsible for transmission of hereditary characters. 5. All cells arise from preexisting cells. 6. The basic biochemical reactions like nucleic acids synthesis, synthesis, enzyme actions, etc. are similar in all the cells.

Types of Cells Cells can be grouped into two main types: a) Prokaryotic cells & b) Eukaryotic cells. a. Prokaryotic Cell & its organization

The prokaryotic cells are small, simple and most primitive. The prokaryotes consist of the bacteria and blue green algae. They multiply rapidly and their cells vary in shape and size. A prokaryotic cell is a one envelope system. Every prokaryotic cell is bound by a complex . A plasma membrane is present below the cell wall. The plasma membrane invaginates to form internal membrane structures. Membrane bound are absent. The genetic material is called . It is not separated from the surrounding by any limiting membrane. are scattered in the cytoplasmic matrix. Each is composed of two subunits – 50S (larger) and 30S (smaller), that make up 70S for the complete ribosome. The ribosome is the site of protein synthesis. Many bacteria are motile and possess one or many flagella for locomotion.

b. Eukaryotic Cell & its organization Eukaryotic cells have a well-defined nucleus in which nuclear materials are enclosed in a double layered membrane. In addition they possess all cell organelles such as , Golgi complex,

Jacob Thomas / 09322380093 / FYJC / B O T A N Y / 1 FYJC Jacob’s Notes of Biology 2 mitochondria, , etc. Cytoplasm has a high complex organization. Motile cells have additional appendages for locomotion. Plant cells have a well-defined rigid cell wall.

The size of the cells varies in plants and animals. The smallest cells are of pleuropneumonia-like organisms (PPLO) as Mycolasma gallisseptecum. The Ostrich egg is the largest animal cell. In plant algae have large sized cells. Ex. Acetabularia.

Units of Measurement: The cells and its organelles are measured in millimeter. A millimeter is one thousandth of a meter. One thousandth of a millimeter is known as micron or micrometer. One thousandth of micron is called millimicron or nanometer. Angstrom is one tenth of millimicron.

1. CELL WALL The outer, rigid, porous, non-living covering of plant cells is known as ‘cell wall’. Cell wall is present only in plant cells.

The cell walls are composed of carbohydrate known as cellulose. Besides this, various chemical substances like hemi cellulose, pectin, lignin, cutin and chitin also occur with cellulose. The cell wall is complex in nature and is differentiated into: i. Primary cell wall – This thin outer most layer is composed of intricate network of micro fibrils of cellulose, pectic polysaccharides etc. ii. Secondary Cell wall - This layer is placed just inside the primary wall. It is composed of compactly arranged micro fibrils of the cellulose. It is thick and near the plasma membrane. iii. Tertiary Cell wall - The cell wall layer present beneath the secondary cell wall of some plants is known as tertiary cell wall. It is relatively thin and lack cellulose micro fibrils. It is made up of cellulose and a chemical substance known as xylan.

The cell wall is perforated by narrow pores or pits at many places. Through these narrow pores, fine strands of cytoplasm are extending. These strands are called plasmodesamta. Plasmodesmata connect the cytoplasm of adjacent cells and facilitate the movement of substances between cells.

Functions of cell wall: 1. It maintains the shape and gives mechanical support to the cells. 2. it allows the materials to pass in and out of cells. 3. It helps to maintain the balance of intracellular osmotic pressure with that of its surroundings. 4. It helps to check loss of water from the cells surface.

2. (PLASMA MEMBRANE) The thin, flexible, semi permeable living membrane that surrounds the protoplasm of a cell is called ‘cell membrane’ or ‘plasma membrane’ or ‘plasma lemma’. The term cell membrane was coined by Nageli and Cramer in 1885. Cell membrane is present in all plant and animal cells, it forms the boundary of the cell, while in plant cells it is located between the cell wall and protoplasm. Electron microscopic study reveals that the membrane consists of a double lipid layer sandwiched between two protein layers and hence it is termed as unit membrane. The membrane is selectively permeable and hence it controls the entry into and exit of selected molecules from the cell.

3. CYTOPLASM An amorphous, translucent, homogenous colloidal ground substance lying between the plasma membrane and the nucleus is known as cytoplasm. The cytoplasm is a complex material composed of membranes, particles and various organelles excluding nucleus. The fluid part of the cytoplasm is known as hyaloplsam or cytoplasmic matrix. This part contains various inorganic molecules like water, salts etc. and organic compounds like carbohydrates, lipids, nuclic acids etc.

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The cytoplasm shows two distinct regions – the outer non-granular, transparent thin layer being known as and the inner granular viscous portion called , where cell organelles are located. Functions : 1. The nucleus, various cell organelles and non-living cytoplasmic remain enclosed within the cytoplasm. 2. It supplies proper nutrition to the living parts of a cell. 3. It is the seat of many vital chemical reactions which are essential for life, eg. Protein synthesis, respiration, etc. 4. It is the seat of many catabolic pathways e.g. Glycolysis.

4. CELL ORGANELLES

The cell organelles are sub-cellular protoplasmic structures. They are usually covered by membrane. These are of two types: extracytoplasmic cell organelles like nucleus and cytoplasmic cell organelles like Endoplasmic reticulum, Golgi complex, Mitochondria, , Ribosomes, etc. i. Endoplasmic reticulum (ER)

It is an interconnected system of membrane bound channels in the cytoplasm. It was first reported by Keith Porter in 1953. The whole structure of ER freely communicates with one another and their lumen is filled with a fluid. The membrane of the ER has a lipoprotein composition. The ER is of two types: smooth or agranular and rough or granular. Rough ER possess rough walls because the Ribosomes attached to its membranes. They are found in those cells which are active in protein synthesis. Smooth ER possess smooth walls because of the absence of the absence of Ribosomes. Functions: i. ER acts as an intracellular structural framework and gives mechanical support to the cytoplasm. ii. It acts as a transporting system carrying various substances to various organelles. iii. ER performs various syntheitic and metabolic activities. iv. They provide surface area for various enzyme activities. v. They regulate the movement and exchange of materials within the cell. vi. It involves in the formation of new nuclear membrane after each cell division.

II. Golgi complex or Golgi bodies or Golgi apparatus are a group of membrane bound bodies occur abundantly in glandular cells. The Golgi Complex was discovered by Camillo Golgi in 1898. The Golgi Complex is a closely packed disc shaped structures formed of three components namely: Cisternae – Parallel and flattened plate like components; Tubules – a network of interconnected tubules arise from periphery of cisternae and Vesicles – small droplet like sacs found attached to the tubules.

Functions: i. It is involved in cellular secretion. ii. It plays an important role in the formation of cell walls in plant cells. iii. It has a role in formation of plasma membrane. During cellular secretion, the membrane of the secretary vesicles is added to the plasma membrane. iv. It helps in the formation of primary lysosomes. v. The Golgi complex gives rise to the in sperms.

III. Lysosomes The lysosomes are small membrane bound vesicular bodies. They were first reported by de Duve in 1955. The lysosomes are membranous vesicles bounded by a unit membrane of lipoproteins. The lumen of the lysosomes is filled with a granular fluid rich in digestive enzymes, the acid hydrolases. This acid can

Jacob Thomas / 09322380093 / FYJC / B O T A N Y / 3 FYJC Jacob’s Notes of Biology 4 breakdown all major macromolecules present in the cell. These are formed by budding off vesicles from Golgi apparatus. Under certain pathological and special physiological conditions, it may digest their own cytoplasm and organelles. This process is called . Lysosomes are often called suicidal bags of the cell because of their autophagic activity.

Functions:

i. Digestion of extracellular or intracellular substances like stored food materials of the cytoplasm during starvation. ii. Digestion of harmful and unwanted materials. They also help to remove the dead cells and debris that accumulate at the site of injury. iii. Autophagy. iv. During fertilization the lysosomes enzymes of the sperm dissolve the membrane for the easy entry of the sperm.

IV. Cytoplasmic

The cytoplasmic vacuoles are the non cytoplasmic areas present inside the cytoplasm. They are small in animal cells, whereas plant cells have large vacuoles. It is supposed that they are the highly expanded endoplasmic reticulum. In plant cells vacuoles are bound by a single semi permeable membrane called tonoplast. They contain cell sap in which they store and concentrate mineral salts as well as nutrients.

V. Ribosomes Ribosomes are tiny granular particles of ribonucleoprotein. They were first reported by Palade (1955). They remain attached to the outer surface of ER and nucleus or occur freely in the cytoplasm. 80S ribosome is found in eukaryotes which is formed of a large 60S subunit and a small 40S subunit.

Functions: i. Ribosomes are called protein factories because these are sites of protein synthesis. ii. They also store rRNAs. iii. They protect the mRNA. iv. They regulate the translation of genetic code.

VI. Mitochondria (Sing. ) Mitochondria are small granular or filamentous membrane bound organelles. They are called Power houses of the cell. They are the main source of energy for all of the metabolic activities of the cell. They are absent in prokaryotic cells. They were first observed by Kolliker in 1880. They were named by Benda in 1898.

The mitochondrion is bound by a double membrane envelope. The membranes are unit membranes as it is made up of lipids and . The two membranes are separated by a narrow fluid filled space called the outer compartment or perimitochondrial space. The outer membrane is smooth. The inner membrane surrounds the inner compartment which is filled with matrix. Into the matrix the inner membrane is thrown up into a series of folds called mitochondrial cristae. Which bear oxysomes.

Functions: i. Mitochondria are the seat of cellular respiration. During cellular respiration, energy present in the food is released by step wise oxidation. ATP molecules are synthesized from ADP molecules using this energy. This process is called oxidative phosphorylation. On this account the mitochondria are called power houses of the cell.

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ii. Mitochondria produce its own proteins from its DNA and Ribosomes. iii. They help in the synthesis of important biomolecules like cytochromes, amino aciods, steroids and chlorophyll etc.

VII. Plastids are membrane bound cytoplasmic organelles found in the plant cells and in some unicellular organisms such as Euglena. The term plastid was introduced by Haeckel in 1866. These organelles contain their own DNA, RNA, and Ribosomes.

Based on the pigment present they are classified into three main types: i) ii) iii) . Plastids are bound by two unit membranes. i. Chloroplast. These are green colored plastids containing the green pigment chlorophyll. They are the photosynthetic organelles of green plants. The chloroplast is covered by a double membrane covering a space filled with a proteinaceous matrix called stroma. Both outer and inner membranes are smooth. DNA, RNA, Ribosomes and many enzymes are present in the matrix. Many flat membranous structures called thylakoids are present in the stroma. About 20 to 50 thylakoids which are closely packed one above the other to form the grana. The thylakoids of adjacent grana are interconnected by membranous tubules called stroma lamellae or frets. ii. Chromoplast The are coloured plastids other than chloroplast. They contain fat soluble yellow orange and red pigment called carotenoid. Carotenoids include pigments such as carotene and xanthophylls. They are responsible for yellow, orange and red colouration of plants. Chromoplasts are mainly found in flowers and fruits. They are formed either from chloroplast or from leucoplast.

iii. Leucoplast These are colourless plastids found in the storage organs. They are found in large numbers in the cells of fruits, seeds, tubers etc. They lack grana and photosynthetic pigments. They store reserve food materials. These are of three types: a. – store starch grains & found in potato tubers, rice, wheat etc. b. Proteinoplast - store proteins & found in maize grains, c. - store lipids & found in the endosperm of castor seeds.

Functions: i. Chloroplast trap radiant energy of sunlight during photosynthesis, and store as chemical energy in food stuffs. Oxygen is also produced as a by-product. ii. Chromoplasts give colours to flowers to attract pollinators and to fruits to attract certain animals for dispersal. iii. Leucoplast store reserve food materials.

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VIII. Nucleus

The nucleus is the most important of the cell. It is a dense round shaped structure. It contains the genetic material, DNA and controls various metabolic activities of the cell. The nucleus was discovered by Robert Brown in 1831. The prokaryotic cells have no true nucleus. However some cells lack nucleus (anucleate) at maturity eg. Mammalian RBCs, and sieve tube cells in plants. Normally the cells contain a single nucleus. Some cells are binucleate with 2 nuclei (Paramecium) some others are multinucleated and the number of nuclei may vary from cell to cell. In some organisms, several nuclei are present in their body which is not divided into cells. They are called coenocytic, eg. Rhizopus, Vaucheria etc.

The nucleus is conspicuous only when the cell is at the interphase. The nucleus at the interphase is called interphase nucleus. The interphase nucleus is surrounded by a thin nuclear envelope or karyotheca. The nuclear envelope is filled with a semi-fluid matrix called nuclear sap or nucleoplasm. Twisted filaments called the chromatin are suspended in the nuclear sap. The nuclear sap also contains one or more spherical bodies called nucleoli.

The nuclear envelope separates the nucleoplasm from the cytoplasm. It is composed of outer and inner unit membranes. The two membranes are separated by a perinuclear space. The nuclear membrane is perforated by numerous minute pores called nuclear pores. .

Functions: i. The nuclear envelope regulate the flow of materials between the nucleus and cytoplasm through the pore complex. ii. Nuclear sap is the seat of synthesis of DNA, RNA and ribosomal subunits. iii. is the active site of RNA synthesis.

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