03-08-2014, 12:52 PM
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[font]CYTOLOGY[/font]Is the study of cells structures and their functionsA cell is an organized independent mass of protoplasm (nucleus and cytoplasm) which makes the basic primary structure of an organism.CELL CONCEPT:One of the most important concept in biology is that a cell is a basic structural and functional unit of living organism.This is known as a cell theory and was proposed jointly by two scientists . A Belgian Botanist called Schleiden and the German zoologist called Schwan. They studied the plant cell and animal cell respectively and come up with the idea that plants and animals are made up by small individuals which perform different functions of the whole organism. They finally come up with what they say cell theory.The cell theory embraces four ideas, these are:
EUKARYOTIC CELLSThese are cells which possess true nucleus and nucleus material are found inside the nucleus surrounded by the two membranes, i.e nucleus envelope. Example: plant cell, animal cell. diagram: a plant cell as seen under light microscope diagram: Ultrastructure of a plant cell diagram: structure of ribosome
- Living organisms are made up of smallest sufficient unit of living matter called cell.
- The new cell is derived from pre-existing ones by cell division.
- Each cell is independent with others but function as integral part of the whole organism.
- The cell contains the hereditary material which is passed from generation to generation.
TYPES OF CELLS
There are two fundamentally different types of cells. These ae:- The prokaryotic cell
- The eukaryotic cell
CELLS AS SEEN WITH LIGHT MICROSCOPE
The cell could be described as a small unit of living protoplasm always surrounded by cell surface membrane and sometimes as in case of plants surrounded by non living cellwall of cellulose.The most conspicuous structure in the cell is the nucleus which contains a deeply staining material known as chromatin (colored materials) this is the loosely coiled form of chromosomes. Chromosomes appear as the thread like structure just before the nucleus division. They contain DNA (genetic material) that controls the cell activities and can replicate itself so that the new cells can form.The living material between the nucleus and the cell surface membrane is known as cytoplasm. Cytoplasm contains variety of cell organelles.diagram: an animal cell as seen under light microscopeULTRASTRUCTURE OF ANIMAL AND PLANT CELLS
Ultra structure of the cell is the fine structure of the cell as seen under electron microscope. In the cell, there are organelles which are suspended within an aqeuos medium and contained within plasmamembrane. The living material between the nucleus and the cell surface membrane know as cytoplasm.The fluid that remains when all organelles are removed is referred as cytosol.diagram: Ultrastructure of an animal cellDifferences between prokaryotic and eukaryotic cells
[table][tr][td]Prokaryotic cell[/td][td]Eukaryotic cell[/td][/tr][tr][td]Nucleus material are not enclosed by nuclear membrane[/td][td]Nucleus material are enclosed by nuclear membrane[/td][/tr][tr][td]Contains few organelles[/td][td]Contains many organelles[/td][/tr][tr][td]No membrane bounded organelles such as; chloroplast and mitochondria[/td][td]Has membrane bounded organelles[/td][/tr][tr][td]DNA is circular and lies free in cytoplasm[/td][td]DNA is linear and enclosed in nucleus[/td][/tr][tr][td]No mitosis or meiosis, divide by binary fission[/td][td]Mitosis and meiosis occur[/td][/tr][tr][td]It contains 70s ribosome (smaller)[/td][td]It contains 80s ribosome (larger)[/td][/tr][tr][td]Mainly unicellular[/td][td]Mainly multi-cellular[/td][/tr][/table]Note:In unicellular organisms a single cell, perform all the life activities and characteristics of living organisms, the cell organelles work as organs in an organism.COMPARTMENTS AND DIVISION OF LABOUR (CELL SPECIALIZATION)
Eukaryotic cells are far larger and more complex than prokaryotic cells and contains many organelles. The eukaryotic has been therefore compared to a factory. Efficiency is improved by division of labor (cell specialization), i.e shairing out of job in such a way that each organelle has its own rol involving its own specialized structure and chemistry. For-example; mitochondrion is the power house of the cell providing energy in the form of ATP from the specialized reactions and respiration.The cell as whole is in effect, divided up into compartments.This compartmentation is often achieved by membranes so that just as a cell surface membrane controls exchange between the cell and its environment. Each membrane bounded organelle can have its own particular unique set of chemicals and chemical reactions.Basically plant cells are very similar to animal cells but plant cells have more structures than animal cells. The organelles and structures which are common to both plant and animal cells are:-- Plasma membrane
- Nucleus
- Mitochondria
- Endoplasmic reticulum
- Golgi apparatus
CELL ORGANELLES
An organelle is a distinct part of a cell which has a particular structure and function.THE CELL MEMBRANEThe cell membrane or plasma membrane is the membrane or structure which enclose a mass of protoplasm of a cell.It is made up of many proteins and lipids. The lipids include:- Phospholipids – is the lipid which contains phosphate group
- Glycolipids – is the lipid which contains carbohydrates
- Cholesterol – is close related to lipid, made up of steroid and alcohol
STRUCTURE OF CELL MEMBRANE
Danielli-Davson structureIn 1940’s Danielli and Davson proposed that all the plasma membrane consist of lipid layer coated with protein molecules as continous layer. This suggest the triliminar or having three layers. The lipid layer is a fluid medium in which the protein coated or attracted. diag: sructure of danielli-davson cell surface membraneFluid Mosaic ModelIn 1972, Singer and Nicholas put forward the “Fluid Mosaic Model” of membrane structure in which a mosaic protein molecules floats in a fluid lipid bilayer.This model is proposed that membrane is made up of lipid and protein but the protein does not form a continuous layer covering both sides of the membrane as proposed by Danielli and Davson.In mosaic model the protein molecules are either partially (peripheral protein) or wholly embedded (integral protein). Some of these proteins that float, consist of pores that allow the passage of particular molecules or ions through the membrane. In absence of these pores, the polar molecules could be difficult to cross the membrane.According to this model, the membrane structure is not static, the lipid molecule linked to one another only by weak bond.diagram: structure of fluid mosaic model mebrane FUNCTIONS OF THE CELL (PLASMA) MEMBRANE
- It separates the contents of the cell from their external environment.
- It controls the exchange of materials between the cell and the surrounding. Eg: gases.
- It acts as the site for metabolic reactions such as energy production in mitochondria and also enzymes attached to the plasma membrane.
- Acts as a receptor site for recognizing of hormones, neurotransmitter and other chemicals.
- The membrane protein sometimes act as an enzyme, for-example; the microvilli on epithelial cells lining some parts of the gut contains digestive enzymes in their cell surface membrane.
- It contains glycoprotein which acts as cell identity markers, hence enables the cell to recognize other cells and to behave in an organized way. For example; during the formation of tissue or organ in multicellular organisms.
- It allows transportation of materials such as water, food materials and waste substances.
FUNCTIONS OF THE NUCLEUS
- It contains chromosomes which have DNA (hereditary material) for the transmission of characteristics from one generation to another.
- It controls the metabolic activities since DNA is organized into genes which control all the activities of the cell.
- Formation of the ribosomal RNA by nucleolus.
- Nuclear division gives rise to cell division hence reproduction.
- It carries the instructions for synthesis of proteins in the nuclear DNA.
FUNCTIONS OF ENDOPLASMIC RETICULUM
- Rough endoplasmic reticulum concerned with the production and storage of protein molecules before they are used inside the cell or are secreted to the exterior.
- They transport materials within the cell from one part to another.
- S.E.R involved in lipids and steroid synthesis and storing.
- The E.R provides surface or location for chemical reaction.
- Producing and storing carbohydrates (S.E.R).
FUNCTIONS OF RIBOSOMES
- It is a site for protein synthesis.
- Amyloplast – store starch.
- Lipidoplast – store lipid, eg. In sunflower seeds
- Proteiplast – store protein, eg. In beans
FUNCTIONS OF GOLGI APPARATUS
- It transports material to other parts of the cell or to the cell surface membrane for secretion.
- It makes lysosomes.
- It involves in storage, modification and packaging of excretory products.
FUNCTIONS OF LYSOSOME
- Lysosomes contain digestive enzymes which are used in digestion of reductant structure or damaged macromolecule from, within or outside the cell by autolysis.
- Lysosome destroys foreign particles such as bacteria by phagocytosis.
- It secretes the digestive enzymes.
- Lysosomes play part in autophagy, autolysis, endocytosis and exocytosis.
FUNCTIONS OF MITOCHONDRIA
- In aerobic respiration the cristae are the site of oxidative phosphorylation and electron transport.
- The matrix which contains large number of hormones and enzymes in liquid form is the site of kreb’s cycle.
- Water generally enters the concentrated cell sap by osmosis. Osmotic uptake of water is important in cell expansion during cell growth as well as in the normal water relations of plants.
- The vacuole sometimes contains pigments in solution, eg: anthocynins which are red, blue and purple and other related compounds which are yellow and ivory.
- Plant vacuole sometimes contains hydrolytic enzymes and act as lysosomes. After cell death, the tonoplast loses its partial permeability and the enzymes escape causing autolysis.
- Vacuoles contain waste products and certain secondary products of plants metabolism such as calcium oxalate, allcaloids and tannins which offer protection from consumption by herbivores.
- Vacuole acts as a food storage organelle. It stores sucrose and mineral salts which can be utilized by the cytoplasm when necessary.
- Plant cell wall contains cellulose.
- Prokaryotes cell wall contains murein.
- Fungi cell wall contains chitin.
FUNCTIONS OF CELL WALL
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It provides mechanical and skeletal support for individual cells and for the plant as a whole.
- It allows development of turgidity when water enters the cell by osmosis since it is fairly rigid and resistant to expansion.
- It prevents the cell from bursting when exposed to a dilute solution.
- It limits and helps to control cell growth and shape since the cell’s ability to stretch is determined by concentration of cellulose microfibrils.
- It acts as a major pathway for the movement of water (apoplast).
- Cell walls develop a coating of waxy cutin (cuticle) which reduces water loss and risks of infections.
- The cell walls of root endodermal cells are impregnated with suberin that forms a banner to water movements.
- The wall of xylem vessels and sieve tubes are adapted for a long distance translocation of materials through the cells.
- Some cell walls are modified as food reserves as in storage of hermicellulose in some seeds.
FUNCTIONS OF CILIA AND FLAGELLA
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They contain enzymes that produce energy to move a cell. Eg: sperm or a unicellular organism such as chlamydomonous.
- They propel fluids across cells, eg: the ciliated cells that move mucus along the bronchial lining.
- The energy is also used to acquire food, eg: feeding current generated by paramecium in its oral groove.
- They are used to sense the environment, eg: sensory hair cells.
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