Plus One Botany Notes Chapter 5 Cell The Unit of Life is part of Plus One Botany Notes. Here we have given Kerala Plus One Botany Notes Chapter 5 Cell The Unit of Life.
|Text Book||NCERT Based|
|Chapter Name||Cell The Unit of Life|
|Category||Plus One Kerala|
Kerala Plus One Botany Notes Chapter 5 Cell The Unit of Life
Cell is the fundamental structural and functional unit of all living organisms. All organisms are composed of cells. Anton Von Leeuwenhoek first saw and described a living cell. Some organisms are composed of a single cell and are called unicellular organisms while others, like us, composed of many cells, are called multi-cellular organisms.
The cell theory was formulated by two German scientists, Matthias Schleiden and Theodore Schwann independently. Thus the cell theory states that,
- All living organisms are composed of cells and products of cells.
- All cells arise from pre-existing cells.
- Cells show similarity in chemical composition and metabolic activities.
- Cells are the structural and functional unit of living organism.
An overview of cell
Each cell consists of,
- Cell contents in place, gives from to the cell,and controls the entry and exit of material into and out of the cell.
- Inside each cell is a dense membrane bound structure called nucleus.
- Nucleus contains the chromosomes which in turn contain the genetic material, DNA.
- Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic.
- A semi-fluid matrix called cytoplasm occupies the. volume of the cell. The cytoplasm is the main area of cellular activities in both the plant and animal cells.
- The eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), the golgi complex, lysosomes, mitochondria, microbodies and vacuoles.
- The prokaryotic cells lack membrane bound organelles.
- Ribosomes are non-membrane bound organelles found in all cells of both eukaryotic and prokaryotic.
- Cells differ greatly in size, shape and activities.
- The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
- The four basic shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).
- The prokaryotes have a cell wall surrounding the cell membrane. The fluid matrix filling the cell is the cytoplasm. There is no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane.
- In addition to the genomic DNA, many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.
- A specialised differentiated form of cell membrane called mesosome is the character-istic of prokaryotes.
Cell envelope and its modification
Cell envelop is the outermost covering of protoplasm of the bacterial cell. It is known to protect the cell from mechanical shocks and injuries. It is composed of following three layers, which perform specialised function.
It is the outermost layer, made up of macro-molecules that gives strictly character to the cell. Glycocalyx differ in composition and thickness among different bacteria. It could be in the form of loose mucilaginous sheath called slime layer or thick and rough covering called capsule. It help in resisting phagocytosis.Bacteria can be classified into two groups on the basis of the differences in the cell en-velopes and the manner in which they respond to the staining procedure developed by Gram viz., those that take up the gram stain are Gram positive and the others that do not are called Gram negative bacteria.
ii. Cell wall
It is present just below the glycocalyx made up of peptidoglycan or murein in all eubacteria and cyanobacteria. It is a rigid and solid covering that gives shape and strong structural support to the cell. The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing.
iii. Plasma membrane
It is the innermost layer of the cell envelope. It is semi-permeable in nature and is responsible for interaction of the celi with the outside world. It help in the regulation of the exchange of specific materials between the cytoplasm and extracellular medium. Prevent loss of components from the cells through leakage.
Prokaryotic cells lack the complex membrane bound organelles. However, some other special membranous structures are found in them. There are,
Which is formed by the exten-sions of plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae. They help in cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.
- Bacterial cells may be motile or non-motile.
- If motile, they have thin filamentous extensions from their cell wall called flagella.
- Bacterial flagellum is composed of three parts filament, hook and basal body.
- The filament is the longest portion and extends from the cell surface to the outside.
- Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility.
- The pili are elongated tubular structures made of a special protein. The fimbriae are small bristle like fibres sprouting out of the cell.
Ribosomes and inclusion bodies
Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the mRNA into proteins. Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bounded by any membrane system and lie free in the cytoplasm, e.g., phosphate granules, cyanophycean granules and glycogen granules.
A cell which has a well organised nucleus with a nuclear envelope and several membrane bound organelles is called eukaryotic cell. The eukaryotes include all the protists, plants, animals and fungi. Eukaryotic cells possess an organised nucleus with a nuclear envelope.
In addition, eukaryotic cells have a variety of complex locomotory and cytoskeletal structures. Their genetic material is organised into chromosomes. All eukaryotic cells are not identical. Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles.
Differences is given below.
Compounts of a eukaryotic cell
i. Cell membrane
- Every living cell is covered by a thin, elastic, transparent, semi-permeable and regenerative membrane called cell membrane also called plasma membrane or plasmalemma.
- The plasma membrane seperates the internal environment of the cell from external environment. As this membrane helps in regulating the entrance and exit of molecules into and out of the cell.
- Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane.
Fluid mosaic model
An improved model of the structure of cell membrane was proposed by Singer and Nicolson widely accepted as fluid mosaic model.
- According to this, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.
- Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.
- Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to the lower. Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis.
- A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration. Such a transport is an energy dependent process, in which ATP is utilised and is called active transport.
ii. Cell wall
- A non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants.
- Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection.
- Cell wall of algae made of cellulose, galactans, mannans and minerals like calcium carbonate, in other plants it consists of cellulose, hemicellulose, pectins and proteins.
On the basis of the structure, cell wall is differentiated into the following three parts,
- Middle lamella.
It is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together.
- Primary cell wall.
It is produced inner to the middle lamella in a young and growing cell. It is capable of growth and extension. It tends to diminish gradually as the cell attain maturity.
- Secondary cell wall.
The thick secondary wall is formed inner towards membrane to the primary wall. As the cell gets fully matured. Its composition is similar to the primary wall.
iii. Endomembrane system
The membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated. The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles.
1. Endoplasmic reticulum
- A network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER).
- ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
- The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). RER is frequently observed in the cells actively involved in protein synthesis and secretion.
- In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER). The smooth endoplasmic reticulum is the major site for synthesis of lipid.In animal cells lipid-like steroidal hormones are synthesised in SER.
2. Golgi apparatus
- They consist of many flat, discshaped sacs or cisternae of 0.5μn to 1.0μn diameter.
- These are stacked parallel to each other. Varied number of cisternae are present in a Golgi complex
- The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming Cisternae m face and concave trans or the maturing face.
- The golgi apparatus is involved in the formation of lysosomes, vesicle that contain proteins and remains within the cell.
- It perform the function of packaging material.
- It acts as an important site for the formation of glycoprotein and glycolipids.
- It help in the formation of cell wall
These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases- lipases, proteases, carbohydrases) optimally active at the acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acid.
- The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell.
- The vacuole is bound by a single membrane called tonoplast.
- In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell.
- In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists,food vacuoles are formed by engulfing the food particles.
- Mitochondria are membrane bound cell organelles, essential for aerobic respiration of eukaryotic cells. These are also known as power house of the cell. Thus, they produces cellular energy in the form of ATP. The number of mitochondria per cell is variable depending on the physiological activity of the cells.
- Each mitochondrion is a double membrane- bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment.
- The inner compartment is called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae towards the matrix.
- These are semi-autonomous organelles that have double membrane envelop. Plastids have their own genetic material. Due to their large size, they are easily seen under the microscope.
- Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
- Chloroplasts contain chlorophyll and carotenoid pigments.
- A number of organised flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana or the inter- granal thylakoids.
- There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
- The membrane of the thylakoids enclose a space called a lumen. The stroma of the chlo- roplast contains enzymes required for the synthesis of carbohydrates and proteins.
- Chromoplasts contain fat soluble carotenoid pigments like carotene, xanthophylls etc.
- Leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients.
Ribosomes are small granular structure. They occur either freely in the matrix of mitochondria, chloroplast and cytoplasm or remain attached with the membranes of the endoplasmic reticulum and nucleus. They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane.
An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton. The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.
viii. Cilia and Flagella
Cilia and flagella are hair-like outgrowths of the cell membrane.
|Cilia are short, hair-like organelles||Long, whip-like organelles|
|They occur relatively in large numbers per cell||They are usually fewer per cell|
|They often cover the entire cell or the entire exposed surface of a cell||They are often at one end of a cell|
|They show sweeping or rowing” motion||They show undulatory motion|
- The electron microscopic study of a cilium or
- the flagellum show that they are covered with plasma membrane. Their core called the axoneme.
- The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke.
- The peripheral doublets are also interconnected by linkers. Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.
ix. Centrosome and centrioles
- Centrosome is an organelle usually containing two cylindrical structures called centrioles.
- They are surrounded by amorphous pericentriolar materials.
- Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.
- The adjacent triplets are also linked. The central part of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.
- Nucleus as a cell organelle was first described by Robert Brown.
- Nucleus is the controlling centre of all vital activities. It is spherical or oval in shape.
Normally, there is only one nucleus per cell, but variations in the number of nuclei can also be seen in various organisms. Nucleus is differentiated into following four parts,
1. Nuclear envelope.
It is a double membrane bound envelope that surround the nucleus and seperates the latter from the cytoplasm.
It is a clear, non-staining, fluid material present in the nucleus, which contains raw materials, enzymes and metal ion for the synthesis of RNAs and DNA. The nuclear matrix or the nucleoplasm is composed of nucleolus and chromatin.
It is a naked, round and slightly irregular structure, which is attached to the chromatin at a specific region. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure.
It is named so, because it has the ability to get strained with certain basic dyes. It is known to be the hereditary DNA protein fibrillar complex. The chromatin fibres are distributed throughout the nucleoplasm.
The nucleus in the interphase has a loose and indistinct network of nucleoprotein fibres called chromatin. But during different stages of cell division cells show structured chromosomes in place of nucleus. The chromosomes are meant for the equal distribution of genetic material. Their number is fixed and is same in all individuals of a species. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA.
Each and every chromosome is composed of a primary constriction or the centromere. On the sides of which the discshaped structures are present known as kinetochores.
On the basis of the position of the centromere, the chromosomes can be classified into four following types.
- Metacentric chromosome.
It has chromosome with equal arms and centromere lies in the centre.
- Sub-metacentric chromosome.
It has one shorter arm and one longer arm with centromere slightly away from the middle of the chromosome.
It forms one extremely short and one very long arm and centromere is located near the end of the chromosome.
It has the terminal centromere, centromere is placed at an extreme end.
a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.
Many membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells.These are minute vesicles found in both plant cells and animal cells,
e.g., in liver, kidney, protozoa, yeast and many other types of cells. Their shape can be ovoid, spherical, granular, etc.
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