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CYTOLOGY

March 7, 2011

Is the study of cells structures and their functions

A 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:

  1. Living organisms are made up of smallest sufficient unit of living matter called cell.
  2. The new cell is derived from pre-existing ones by cell division.
  3. Each cell is independent with others but function as integral part of the whole organism.
  4. 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

PROKARYOTIC CELLS

These are the type of cells which do not have true nucleus. Their nucleus material (DNA) lies free in a region known as nucleoid. Example: Bacteria.

diagram:a typical structure of a prokaryotic cell.


EUKARYOTIC CELLS

These 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.

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 microscope


diagram: a plant cell as seen under light microscope


ULTRASTRUCTURE 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 cell


diagram: Ultrastructure of a plant cell


Differences between prokaryotic and eukaryotic cells

Prokaryotic cell Eukaryotic cell
Nucleus material are not enclosed by nuclear membrane Nucleus material are enclosed by nuclear membrane
Contains few organelles Contains many organelles
No membrane bounded organelles such as; chloroplast and mitochondria Has membrane bounded organelles
DNA is circular and lies free in cytoplasm DNA is linear and enclosed in nucleus
No mitosis or meiosis, divide by binary fission Mitosis and meiosis occur
It contains 70s ribosome (smaller) It contains 80s ribosome (larger)
Mainly unicellular Mainly multi-cellular

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

The chief differences between animal and plant cells are the presence of cellwall, chloroplast and a large vacuole in plant cells.

CELL ORGANELLES

An organelle is a distinct part of a cell which has a particular structure and function.

THE CELL MEMBRANE

The 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

Both phospholipids and glycolipids have polar head and non-polar tails. Cholesterol is slightly polar at one end.

The plasma membrane and all other membranes bounded organelles contains phospholipids and proteins. The lipids have hydrophilic head and hydrophobic tail which always occur in pair.

The ‘hydrophilic’ head is a polar molecule and have an affinity to water (hydrophilic i.e water loving) and the ‘hydrophobic’ tail is non-polar and do not mix with water (hydrophobic i.e water hating).

The plasma membrane has an extra proteins which are special carrier molecules that act as receptors for hormones and immunological identity such as blood group antigen.

STRUCTURE OF CELL MEMBRANE

Danielli-Davson structure

In 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 membrane

Fluid Mosaic Model

In 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

  1. It separates the contents of the cell from their external environment.
  2. It controls the exchange of materials between the cell and the surrounding. Eg: gases.
  3. It acts as the site for metabolic reactions such as energy production in mitochondria and also enzymes attached to the plasma membrane.
  4. Acts as a receptor site for recognizing of hormones, neurotransmitter and other chemicals.
  5. 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.
  6. 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.
  7. It allows transportation of materials such as water, food materials and waste substances.

NUCLEUS

Is the most important organelle in the cell and the largest one. It is enclosed by an envelope of two membranes that is perforated by nuclear pores. In some cells , it has a relatively fixed position, usually near to centre of the cell but in other it may move freely and be found almost anywhere in the cell.

Within the nucleus, there is a matrix called nucleoplasm which contain the chromatin and nucleolus.

The chromatin materials are coiled DNA bounded by protein called histones. The term chromatin means colored materials. There are two types of chromatin in the nucleus, these are:

i.            Heterochromatin – Tightly coiled and continue to stain intense.

ii.            Euchromatin – The looser coiled and more scattered chromatin during the interphase.

The outer membrane is continuous with the endoplasmic reticulum and may be covered by ribosomes for protein synthesis.

diagram: structure of the nucleus


FUNCTIONS OF THE NUCLEUS

  1. It contains chromosomes which have DNA (hereditary material) for the transmission of characteristics from one generation to another.
  2. It controls the metabolic activities since DNA is organized into genes which control all the activities of the cell.
  3. Formation of the ribosomal RNA by nucleolus.
  4. Nuclear division gives rise to cell division hence reproduction.
  5. It carries the instructions for synthesis of proteins in the nuclear DNA.

ENDOPLASMIC RETICULUM

A system of flattened membrane bounded sacs called cisternae forming sheet-like rather than tubular. It originates from outer membrane of the nucleus to which it’s often remains attached.

The outer surface of some endoplasmic reticulum carries numerous ribosome which gives granular appearance and forming rough endoplasmic reticulum (R.E.R), other endoplasmic reticulum does not contain ribosomes and form smooth endoplasmic reticulum (S.E.R).

Diagram: rough and smooth endoplasmic reticulum

FUNCTIONS OF ENDOPLASMIC RETICULUM

  1. 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.
  2. They transport materials within the cell from one part to another.
  3. S.E.R involved in lipids and steroid synthesis and storing.
  4. The E.R provides surface or location for chemical reaction.
  5. Producing and storing carbohydrates (S.E.R).

NOTE:

The muscle cells have specific type of smooth endoplasmic reticulum called sarcoplasmic reticulum.

RIBOSOMES

Ribosomes are very small organelles made up of protein and ribonucleic acid (ribosomal R.N.A) from nucleoli.

Ribosomes occur in both prokaryotic and eukaryotic cells. The ribosmes of prokaryotic cells are distinctly smaller (70’s ribosomes) than those of eukaryotic cells (80’s ribosomes).

Each ribosome consists of two units, small sub-unit and large sub-unit. When several ribsomes occur along a common strand of mRNA, the whole structure is known as Polyribosomes or Polysomes.

diagram: structure of ribosome

FUNCTIONS OF RIBOSOMES

  1. It is a site for protein synthesis.

PEROXISOMES OR MICROBODIES

Peroxisomes or microbodies are spherical organelles bounded by a single membrane commonly found in eukaryotic cells. They are slightly smaller than mitochondria. They are believed to derived from endoplasmic reticulum.

The peroxosimes are like the lysosomes containing the powerful enzymes but the enzymes in peroxisome are oxidative rather than digestive enzymes. Eg: catalase which catalyses the decomposition of hydrogen peroxide to water and oxygen. Hydrogen peroxide as a by product of certain cell oxidation reaction, is very toxic and therefore must be eliminated immediately.

In the liver cells contain large number of peroxisomes which are involved in oxidative metabolic activities. In plants peroxisomes are site of the glycolate cycle (photorespiration).

daigram: structure of peroxisome

PLASTIDS

Plastids are ovoid or spherical shaped organelles found in plant cells and in certain unicellular organism like algae. They are surrounded by two membranes which form an envelope. There are two main types of plastids, these are:

i.            Chloroplast

ii.            Chromoplast

iii.            Leucoplast

The choloplast

The chloroplast bounded by two concentric membranes. It consists of many flattened fluid filled sacs called thylakoids which form stocks called grana. Between one grana and the other there is a membrane called intergranal lamella.

The internal system membrane is suspended in an aqeous matrix called stroma which contain protein, nucleic acid, starch grains and lipid globules.

The thylakoids contain chlorophyll (green pigments) which is responsible for absorbing energy from the sunlight and convert it to chemical energy.

diagram: structure of a chloroplast

Function:

Chloroplast is the site for photosynthesis in plants.

The chromoplast

Chromoplasts are plastids containing mainly red, orange or yellow pigments which are commonly known as carotenoids. They are non-photosynthetic pigments which are found mainly in fruits and flowers, their bright color attracts insects and birds for pollination and seed dispersal.

The leucoplast

The leucoplasts are colorless plastids which have no pigments. They are numerous in storage organs such as roots, seeds and young leaves where there they store food.

Examples of leucoplasts include:

  • Amyloplast – store starch.
  • Lipidoplast – store lipid, eg. In sunflower seeds
  • Proteiplast – store protein, eg. In beans

GOLGI APPARATUS

A stack of flattened membrane bounded sacks  called cisternae, consists of a tubular parallel smooth membrane with membrane vesicles at their tips called golgi vesicles.

The golgi apparatus are found in all eukaryotic cells and they are free within cytoplasm.

At one end of stack, new cisternae are constantly being formed by vesicles from the smooth E.R. this outer outer or ‘forming’ face is convex while the other end is concave inner or ‘maturing’ face where the cisternae break up into vesicles again.

diagram: structure of golgi apparatus

FUNCTIONS OF GOLGI APPARATUS

  1. It transports material to other parts of the cell or to the cell surface membrane for secretion.
  2. It makes lysosomes.
  3. It involves in storage, modification and packaging of excretory products.

LYSOSOMES

A simple spherical sac bounded by a single membrane and contains a mixture of digestive enzymes such as protease, nuclease and lipase which break down proteins, nucleic acids and lipids respectively.

The enzymes contained within lysosomes are synthesized on rough E.R and transported to the golgi apparatus. Golgi vesicles containing the processed enzymes later bud off to form the lysosomes.

In plant cells the large central vacuoles may act as lysosome although bodies similar to the lysosome of animal cells sometimes seen in the cytoplasm of plant cell.

diagram: lysosome

FUNCTIONS OF LYSOSOME

  1. Lysosomes contain  digestive enzymes which are used in digestion of reductant structure or damaged macromolecule from, within or outside the cell by autolysis.
  2. Lysosome destroys foreign particles such as bacteria by phagocytosis.
  3. It secretes the digestive enzymes.
  4. Lysosomes play part in autophagy, autolysis, endocytosis and exocytosis.

Autolysis is the self digestion of a cell by releasing the contents of lysosome within the cell. For this reason, lysosomes sometimes called ‘suicide bags’ or ‘self breaking down’.

Autophagy is the process by which unwanted structures within the cell are engulfed and digested within lysosome.

Endocytosis occurs by an infolding or extension of the cell surface membrane to form vesicles or vacuoles. It is of two types, these are:

Phagocytosis – ‘cell eating’. Material taken up is in solid form.

Pinocytosis – ‘cell drinking’. Material taken up is in liquid form.

Exocytosis is the process in which waste materials may be removed from cells. It is the reverse of endocytosis.

diagram: the three possible uses of lysosom

MITOCHONDRIA

Are rod shaped or cylindrical organelles surrounded by an envelope of two membranes. The outer membrane is a smooth membrane and the inner membrane folded to form cristae. The cristae provide large surface area for biochemical activities. It enclose a matrix (mixture of protein, lipid and nucleic acid) with few ribosomes, a circular DNA molecule and phosphate granules.

diagram: structure of mitochondria

FUNCTIONS OF MITOCHONDRIA

  1. In aerobic respiration the cristae are the site of oxidative phosphorylation and electron transport.
  2. The matrix which contains large number of hormones and enzymes in liquid form is the site of kreb’s cycle.

CYTOPLASM

An aqeous substance containing a variety of cell organelles and other structures such as insoluble wastes and storage products.

The soluble part of the cytoplasm forms the ‘back ground material’ or ‘ground substances’ between the cell organelles.

It contains about 90% water and forms a solution which contains all the fundamental biochemicals of life. Some of these are ions and small molecules in true solution, others are large molecules such as proteins which form colloidal solutions.

VACUOLE

A vacuole is fluid filled sac bounded by a single membrane. Animal cells contain relatively small vacuoles, such as phagocytic vacuoles, food vacuoles, autophagic vacuoles and contractile vacuoles. Typically plant cells have one or two large vacuoles filled with fluid known as cell sap and surrounded by a membrane called tonoplast. The cell sap is a watery fluid containing water, sugar, organic acids, mineral salts, pigments and toxic substances.

FUNCTIONS OF VACUOLE

  1. 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.
  2. The vacuole sometimes contains pigments in solution, eg: anthocynins which are red, blue and purple and other related compounds which are yellow and ivory.

They are responsible for colors in flowers, fruits, buds and leaves. They are important in attracting insects, birds and other animals for pollination and seed dispersal.

  1. 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.
  2. 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.
  3. Vacuole acts as a food storage organelle. It stores sucrose and mineral salts which can be utilized by the cytoplasm when necessary.

CELL WALLS

Both plant cells, prokaryotic and fungi are surrounded by a relatively rigid wall which is secreted by living cell (the protoplast) within. The plant cell wall differ in chemical composition from those of the prokaryotes and fungi:-

  • Plant cell wall contains cellulose.
  • Prokaryotes cell wall contains murein.
  • Fungi cell wall contains chitin.

The wall formed during cell division of plants is called the primary wall which is later thickened to become a secondary wall.

The primary wall consists of cellulose fibrils running through a matrix of other polysaccharides.

Cellulose is a polysaccharide which has a high tensile strength which approaches that of steel.

The matrix consists of pectins and hermicellulose. Pectins are acidic and have a relatively a solubility. The middle lamella that hold neighboring cell walls together is composed of sticky gel-like magnesium and calcium salts of pectins.

Hermicellulose are mixed groups of alkali soluble pollysacharides which form less organized, shorter and more branched chain like molecules.

About 60%-70% of mass of cell wall is water which can move freely through free space in the cell wall.

FUNCTIONS OF CELL WALL

  1. It provides mechanical and skeletal support for individual cells and for the plant as a whole.
  2. It allows development of turgidity when water enters the cell by osmosis since it is fairly rigid and resistant to expansion.
  3. It prevents the cell from bursting when exposed to a dilute solution.
  4. It limits and helps to control cell growth and shape since the cell’s ability to stretch is determined by concentration of cellulose microfibrils.
  5. It acts as a major pathway for the movement of water (apoplast).
  6. Cell walls develop a coating of waxy cutin (cuticle) which reduces water loss and risks of infections.
  7. The cell walls of root endodermal cells are impregnated with suberin that forms a banner to water movements.
  8. The wall of xylem vessels and sieve tubes are adapted for a long distance translocation of materials through the cells.
  9. Some cell walls are modified as food reserves as in storage of hermicellulose in some seeds.

CENTRIOLES

Cenrioles are small hollow cylinders that occur in pair in most animal cells.

In centrosome (poorly defined structure which initiates the development of microtubules), the two centrioles lie right angle to each other. Each contains a 9+0 pattern of microtubule triplets, i.e: a ring having nine sets of triplets with none in the middle.

Before  an animal cell divides, the centrioles replicate, then each pair becomes part of a separate centrisome. During cell division the centrosomes move apart so that each new cell has its own centrosome. Plant cells have the equivalent of a centrosome but it does not contain centrioles.

 

diagram: structure of a centriole

The functions of the centriole as microtubule organizing centre is to control separation of chromatids or chromosomes by a sliding motion.

CYTOSKELETON

The cytoskeleton is a network of interconnected filaments and tubules that extends from the nucleus to the plasma membrane in eukaryotic cells.

The cytoskeleton contains three types of elements, these are:-

Actin filaments, intermediate filaments and microtubules.

ACTIN FILAMENTS

Actin filaments or microfilaments are long extremely tin fibres that occur in bundles meshlike network. It contains two chains of globular actin monomers twisted about one another in a helical manner. It plays a structural role and involved in the movement of the cell and its organelles.

INTERMEDIATE FILAMENTS

They are rope like assembly of fibrous polypeptides but specific types varies according to the tissue. They are intermediate in size between the actin filaments and microtubules.

Intermediate filament supports the nuclear envelope and plasma membrane and take part in the formation of cell to cell junction.

In the skin, the intermediate filament is made up of protein keratin which gives mechanical strength to the skin cells.

MICROTUBULES

Microtubules are straight un-branched hollow cylinders which are usually short in length. They occur I most plant and animal cells.

Microtubules are involved in the movement of cytoplasmic components within the cell. They also occur in centrioles, in the spindle, in cilia and flagella and in the basal bodies.

Microtubules are made up of proteins. They help to maintain the shape of the cell and act as routes along which organelles can move.

CILIA AND FLAGELLA

Flagella and cilia are organelles that project from the surface of cells but are connected to a basal body just below the plasma membrane. Flagella occur singly or in small number where as cilia occur in large number on large cells and are typically shorter that flagella. Simultaneously, flagella and cilia are almost identical and both are able to move.

Flagella and cilia are enclosed to plasma membrane and internally they consist of microtubules arranged in an outer ring of nine pairs surrounding one central pair.

FUNCTIONS OF CILIA AND FLAGELLA

  1. They contain enzymes that produce energy to move a cell. Eg: sperm or a unicellular organism such as chlamydomonous.
  2. They propel fluids across cells, eg: the ciliated cells that move mucus along the bronchial lining.
  3. The energy is also used to acquire food, eg: feeding current generated by paramecium in its oral groove.
  4. They are used to sense the environment, eg: sensory hair cells.

END

For better understanding, visit some other websites and don’t forget our beloved BIOLOGICAL SCIENCE (BS)

Also read some text books from your school library

 


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