Key concepts:
* All organisms are made up of cells. Some may be unicellular and others multicellular.
* Anton Von Leuwenhoek first saw and described a living cell.
* Cell the fundamental structural and functional unit of all living organisms.
* Schleiden and Schwann proposed cell theory and it was modified by Virchow.
* The cell theory states that
• All living organisms are made up of cells and products of cells.
• All cells arise from pre-existing cells.
* Cells may be eukaryotic or prokaryotic.
* The eukaryotic cells have a definite nucleus, membrane bound organelles and 80S and 70S type of ribosomes.
* The prokaryotic cells do not have a definite nucleus, membrane bound organelles but contain 70S type of ribosomes.
* The eukaryotic cells differ in plants and animals in few characters.
* A plant cell has cell wall, plastids and vacuoles but lacks centrioles.
* An animal cell has no cell wall, plastids and vacuoles but contains centrioles.
Prokaryotic cells:
* They are present in Bacteria, blue-green algae and mycoplasmas.
* In bacteria these cells may be rod like (Bacillus), Spherical (Cocci), comma shaped (Vibrio) or spiral (Spirillum).
* Cell wall, cell membrane and cytoplasm are present but no definite nucleus.
* The genetic material is a naked circular DNA which is not surrounded by a nuclear membrane.
* Small circular DNA present in cytoplasm are called plasmids.
* Except ribosomes (70S) all other membrane bound organelles are absent.
* Bacteria are surrounded by a cell envelop and may be classified as gram positive or gram negative based on the chemical composition of cell envelop.
* The cell envelop shows 3 regions:
1) Glycocalyx
2) Cell wall
3) Plasma membrane
* The glycocalyx can be in the form of a loose slimy layer or a tough capsule.
* The cell wall provides shape and structure to the cell.
* The plasma membrane is semi-permeable. It forms tubular structures towards inside called “mesosomes”.
* Mesosomes play a role in cell wall formation, DNA replication, and respiration and increase the surface area of absorption.
* In Cyano bacteria the plasma membrane forms chromatophores which contain pigments.
* Some motile bacteria may contain flagella, pili and fimbriae.
* Ribosomes, polyribosomes and several types of inclusion bodies like phosphate granules, glycogen granules and gas vacuoles are found in the cytoplasm.
Eukaryotic cells:
The eukaryotes are protists, plants, animals and fungi.
The eukaryotes contain an organized nucleus with a membrane bound envelop.
An eukaryotic plant cell contains a cell wall, cell membrane, endomembrane system, mitochondria, plastids, ribosomes, cytoskeleton, nucleus and microbodies.
In addition to these the animal cell shows centrioles but lacks in plastids, cell wall and vacuoles.
Cell wall:
* Cell wall is a non-living outer covering of the plant cell and fungal cell.
* It gives shape, protects the cell and allows cell-cell interaction.
* It is usually made up of cellulose, hemicelluloses, pectin and some proteins.
* In algae it is made up of cellulose, galactans, mannans and minerals like calcium carbonate.
* It can be differentiated into three regions:
* Middle lamella made up of calcium pectates.
* Primary cell wall made up of cellulose and hemicelluloses.
* Secondary cell wall made up of lignin, suberin, pectin or cutin.
Cell membrane:
* Cell membrane is composed of lipids, proteins and less % of carbohydrates.
* The membrane proteins may be integral or peripheral.
* Singer and Nicolson proposed “Fluid Mosaic Model” to explain the structure of cell membrane.
* The cell membrane is selectively permeable and allows transport of certain molecules through it.
* The neutral solutes do not require energy for their transport through cell membrane. Such transport which occurs according to concentration gradient is called passive transport.
* Polar molecules require energy (ATP) to be transported through carrier molecules which is called active transport.
* The cell membrane also helps in cell growth, secretion, cell division, etc.
Endo membrane system:
The endo membrane system includes
1. Endoplasmic reticulum
2. Golgi complex
3. Lysosomes
4. Vacuoles.
1. Endoplasmic reticulum(ER)
* It was first coined by K.R.Porter.
* It is a network or reticulum of tiny tubular structures in the cytoplasm.
* It can be differentiated into two types, the rough ER (RER) which is studded with ribosomes on its surface and the smooth ER (SER) without ribosomes.
* The RER is concerned with protein synthesis and SER is concerned with lipid synthesis.
2. Golgi apparatus:
* Camello Golgi first observed this structure.
* The Golgi apparatus consists of many flat disc shaped sacs called cisternae.
* It is present near the nucleus and performs the functions of packaging, protein synthesis, synthesis of glycoproteins and glycolipids, and cell plate and cell wall formation during cell division.
3. Lysosomes:
* They are membrane bound vesicles formed from Golgi apparatus and contain hydrolytic enzymes.
* These enzymes are active at acidic pH and help in digesting carbohydrates, proteins etc.
* Under extreme condition the lysosomes digest the cell contents causing autolysis.
4. Vacuoles:
* It is a membrane bound space found in the cytoplasm of a plant cell.
* It stores water, metabolic bye products, waste materials etc., and this fluid is called vacuolar sap.
* The vacuole is surrounded by a single membrane called tonoplast.
* It helps in osmoregulation.
* In some organisms like Amoeba it acts as contractile vacuole and helps in excretion.
Mitochondria:
* It is a double membrane bound structure which encloses a matrix.
* It may be sausage-shaped or cylindrical.
* The outer membrane is continuous and inner membrane is folded into finger like projections called cristae.
* The two membranes have specific enzymes involved in respiration.
* The matrix contains enzymes of Kreb’s cycle, single circular DNA, few RNA molecules and 70S ribosomes.
* Cellular energy is produced in the form of ATP in the mitochondria; hence they are called “power houses of the cell”.
Plastids:
* These are the cell organelles found in plant cells and euglenoids.
* Based on the pigments present they are classified into 3 types:
1) Chromoplasts
2) Leucoplasts
3) Chloroplasts
1) Chromoplasts:
* They are the plastids that contain fat soluble pigments like carotenoids (carotene and xanthophylls)
* They give yellow, orange or red colour to plant parts like flowers and fruits.
* e.g.: Orange color of Carrot.
2) Leucoplasts:
* They are colourless plastids which contain stored nutrients. They are again classified into
* Amyloplasts that store carbohydrates, e.g.: Potato
* Elaioplasts that store oil and fats e.g.: Seeds of castor and ground nut and
* Aleuroplasts that store proteins. e.g.: Seeds of peas, beans etc.
3) Chloroplasts:
* They are usually found in the Mesophyll cells of the leaves.
* They may be lens shaped, oval, spherical, discoid or ribbon like.
* They are also double membrane bound structures enclosing a stroma.
* The stroma contains flattened membranous sacs called thylakoids.
* The thylakoids may be piled up like a stack of coins called grana.
* The membranes of thylakoids enclose a space called a lumen.
* The stroma contains enzymes required for synthesis of carbohydrates during photosynthesis.
* Circular DNA and 70S ribosomes are also present and hence the chloroplasts can perform protein synthesis.
* Chloroplasts are the organelles of photosynthesis.
Ribosomes:
* Ribosomes are granular structures made up of ribonucleic acid and proteins which are not surrounded by any membrane.
* They were first observed by George Palade.
* In Eukaryotes 80S ribosomes are present with two sub units, 60S and 40S.
* In Prokaryotes 70S ribosomes are present with two sub units, 5oS and 30S.
* The two sub units unite during protein synthesis in presence of magnesium ions.
* The function of ribosomes is protein synthesis.
Cytoskeleton:
* Cytoskeleton is the network of filamentous structures present in the cytoplasm.
* The cytoskeleton is mainly composed of micro filaments, intermediate filaments and microtubules.
* It helps in mechanical support, cell shape, cell motility, intercellular transport, signaling and karyokinesis.
Cilia and Flagella:
* Cilia and flagella are hair like outgrowths on the cell membrane.
* Cilia are small structures which help in cell movement or movement of surrounding fluid.
* Flagella are longer and help in cell movement.
* In prokaryotes the flagella have a simple monofibrillar structure.
* The eukaryotic flagellum shows a 9+2 arrangement of microtubules.
Centrosome and Centrioles:
* Centrosome is an organelle with two centrioles.
* Centrioles form the basal body of cilia and flagella and give rise to spindle fibers during cell division in animal cells.
Nucleus:
* Nucleus was first described by Robert Brown.
* It shows
* Chromatin
* Nuclear matrix
* Nucleoli and
* Nuclear envelop.
* The nuclear envelop contains two membranes which enclose perinuclear space.
* The nuclear envelop shows minute pores called nuclear pores.
* The nuclear matrix or nucleoplasm contains nucleolus and chromatin.
* The nucleolus is the site of RNA synthesis.
* The chromatin appears as a loose indistinct network of nucleoprotein fibres during interphase.
* During metaphase and anaphase it appears as distinct number of chromosomes.
* The chromatin is made up of DNA, histone proteins, nonhistone proteins and RNA.
* The chromosome shows a primary constriction called centromere.
* Depending on the position of centromere the chromosomes can be classified into 4 types:
Metacentric chromosome: Centromere is in the middle and the chromosome shows two equal arms.
Sub-metacentric chromosome: Centromere is present near to the middle with one short and one long arm of chromosome.
Acrocentric chromosome: The centromere is situated close to one end with one extremely short arm and one very long arm.
Telocentric: The centromere is present at one end.
* The nucleosome structure of chromosome is seen under electron microscope.
* The nucleosome contains 200 base pairs of DNA helix coiled around 8 histone proteins two each of H2A, H2B, H3 and H4.
* The DNA between two nucleosomes is called linker DNA.
Microbodies:
* Single membrane bound organelles peroxysomes and glyoxisomes are called microbodies.
* Peroxisomes help in synthesis of phospholipids, photorespiration and protect cell from hydrogen peroxide.
* Glyoxysomes are found in germinating seeds and contain enzymes of glyoxylate cycle.
