CYTOLOGY

Cytology

A cell is the structural and functional unit of any living organism.

Development of the cell theory

The first person who made a contribution to the theory was Robert Hooke in 1665. Another person who added on information about the cells of plant and animals tissues was Dutrochet in 1824. He was able to conclude that all living organisms are made up of cells of various kinds, he also added that growth is due to increase in the number of cells and increase in the size of cells. From 1838-1839, two common biologists; MJ Schleiden and Schwann. Theodor presented convincing arguments that each cell can carry out all the functions of an organism.

Rudolf Virchow obtained the information that cells are obtained from each other by cell division.

Fundamental principles of the cell theory.

• All living organisms/things are composed of cells and are cell products.
• All cells arise from other cells by division.
• The nucleus is a fundamental and constant component of a cell and is involved in the formation of new cells. Robert Brown discovered the nucleus in 1831.
• The function of the organism as a whole is the result of the sum of activities and interactions of the component cell units.
• There are fundamental similarities in the chemical composition and metabolic activities of all cells. The genetic code is universal.

The smallest cell is a bacterial cell and the biggest is the ostrich egg cell. Cells vary in structure and size.

There are 2 classes of cells.

1. Prokaryotic cells
2. Eukaryotic cells.

Differences

GENERALIZED DIAGRAM OF A PLANT CELL

GENERALIZED STRUCTURE OF ANIMAL CELL.

Difference between plant and animal cells

Structure of plasma membrane

• Double membrane made of protein and lipid layers.
• Perforated with minute pores which are widely spaced.
• Has an approximate thickness of between 7.5nm and 8.0nm.
• Though a bilayer is not symmetrical because the inner proteins greatly differ from surface proteins.
• Semi-permeable allowing contains materials to pass through.
• May be invaginated (infolded) in some areas forming both phagocytic and pinocytic vesicles.

Structure and functions of various organelles in plant and animal cells

Plasma or cell membrane. This is a structure which surrounds the entire cells and all the organelles. It is invisible to the naked eye. Its thickness is about 0.01 micrometers. It is composed of proteins and lipids and sometimes polysaccharides.

Functions

• Controls entrance and exit of molecules and ions between the outside and inside of the cell. It does this by osmosis, diffusion and active transport. its being semi-permeable in nature enables it to carryout this function efficiently.
• In some cases, the cell membrane provides a mechanical framework on which specific enzymes can be oriented to perform specific functions.

• Provides a large surface area for absorption when it is modified into microvilli in the intestines.
• Speeds up transmission of impulses when modified into the myelin sheath.
• In come animals; it is modified into the flagella and cilia for locomotion.

A number of models have been put up to explain the structure of the plasma membrane i.e. Danielli Davson model, fluid mosaic model.

 Danielli Davson model.

According to the model, a plasma membrane is made of sandwitch of lipids and proteins with the 2 protein layers enclosing a double layer of lipids.

Diagram of the Danielli Davson models

The lipids are bi-polar in nature and one end is hydrophilic hence dissolve in water hence associates with ions. The other end of the molecule is hydrophobic and hydrophilic so dissolve lipid-soluble particles. The Danielli Davson model also has proteins and pores which are very small and allow materials of a particular size to cross.

Physical evidence that supports the Danielli Davson model.

• Strong membrane provided by the protein component.
• It is flexible and elastic because of the protein layer.
• Shows surface tension because of the lipid layer.

• Permeable to small sized particles because of the pores.

Chemical evidence

• Solubility of different molecules especially lipid-soluble particles.
• Selective absorption to different sizes of the particles and lipid insoluble particles can’t enter or enter very slowly.
• Charged particles are attracted at different rates due to the bi-polar nature of the lipid molecule.

The Danielli Davson model suggests that the cell membrane is a cell structure and it was criticized. It also suggests that the proteins form a continuous layer in the membrane and this was too criticized hence Biologists came up with another model called the fluid mosaic model. This was supported against the Danielli Davson model and the evidence is as follows.

• Protein does not form a continuous layer but occurs in form of globules arranged in a mosaic pattern. Some proteins are found on the surface while others penetrate the membrane.
• The membrane is not rigid but has a fluid consistency.

Evidence

• When the membrane is treated with chemicals that react with proteins, one observes that there are some scattered proteins and some penetrate the membrane.
• When you are the freeze etching method and observe under the electron microscope, you will see the protein scattered in form of globules and some penetrating the membrane.

The fluid mosaic model.

Functions of components of the cell membrane

Phospholipids

Affect the fluidity and permeability of the membrane.

Cholesterol

Make the membrane less fluid at higher temperature but more fluid at lower ones.

Glycolipids

Act as recognition sites e.g. the human ABO blood system is the result of different glycolipids on the cell membrane of red blood cells. Also help to make the membrane more stable.

Proteins

• Provide structural support for the membrane.
• Assist the active transport of materials across the membrane.
• Act as recognition sites.
• Act as enzymes, energy transducers and electron carries.
• Pump certain molecules across the membrane.

Act as receptors for hormones or antibody reactions

Glycoproteins

• Act as recognition sites e.g. for neurotransmissions and harmonies.

Carbohydrates

• Offer membrane specificity and enable cells to recognize each other. This is important during development when cells are re-organizing in tissues and in healing of wounds to ensure that cells of same type are involved.
• Play part in the way cells adhere to one another and interact which is important during embryonic development, agglutination of blood cells in incompatible blood transfusions is due to this.
• Play a role in the mechanism by which specific hormones and foreign substances e.g. antigens are recognized.

CYTOPLASM

This is the living part of the cell and it surrounds the nucleus. It forms the true internal environment of the cell and it has a composition of 90% H₂0 and other biochemicals of life. It carries out the synthetic functions of the cell. It contains enzymes necessary for energy production by anaerobic glycolysis. Other synthetic function include;

Protein synthesis, condensation reactions involving formation of fats and glycogen. Cytoplasm is colloidal in nature. A colloidal system is a type of dispersion system whereby the solute particles are larger than the solvent particles and the mixture forms a heterogeneous solution where particles don’t settle under the force of gravity. The colloid e.g. cytoplasm has component parts and these are the continuous and discontinuous phases. The continuous phase is made up of the solvent particles i.e. water molecules. The discontinuous phase is made up of molecules like proteins, sugar, starch and glycogen. The colloidal material doesn’t set out in the cytoplasm because they bear charges hence repel each other, so can’t form precipitates except on addition of acids and other electrolytes.

A semi-solid colloid is known as a gel. A fluid colloid is known as a sol hence cytoplasm is a sol. In order to form a gel from a sol, add an electrolyte or coagulate the sol with heat or agitate the sol. Heat cooled gel to become a sol or add water. The colloidal nature of the cytoplasm is very important in the organisms e.g. it helps in the locomotion of unicellular organisms like amoeba. The cytoplasm can also contain the spindle fibres for cell division.

In plant cells, the cytoplasm maintains communication with adjacent cells through cytoplasmic strands, through pores known as plasmodosmata. synthetic functions.

Nucleus

It is spherical in shape. It is an organelle found in all Eukaryotic cells except mature mammalian red blood cells. The nucleus is bounded by a double unit membrane. The outer unit membrane gives rise to the endoplasmic reticulum. The nuclear membrane has pores through which the nucleus communicates with the cytoplasm. The nucleus contains nuclear sap or nucleoplasm containing chromatin material and nuclear ribosomes. The chromatin material is DNA and the chromatin material controls all the activities of the cell and contains the hereditary information. The nucleus also contains the nucleolus which manufactures the ribosomes.

Functions of the nucleus

• Essential for cell division (mitosis and meiosis)
• Controls all cell activities
• Contains hereditary (genetic) material hence plays the biggest role in cell division.
• Controls metabolic activities of the cell.

• Instructions for protein synthesis in the DNA on which metabolic activities of the cell depend.

Involved in production of ribosomes and RNA

MITOCHONDRION

This organelle is found in every type of cell except the prokaryotic cells and mature mammalian red blood cells. It is oval in shape and is about 5 micrometers long and 0.2 micrometers in diameter. It has a constant diameter and this facilitates rapid diffusion of materials between the cytoplasm and the mitochondrion. The mitochondria have their own DNA, therefore can undergo division independent of the cell containing them. The mitochondrion is surrounded by a double unit membrane. The outer membrane is smooth but the inner membrane is folded, therefore has cristae and a cristae may be branched. The mitochondrion is filled with a liquid matrix and it is within this matrix that the kreb’s cycle takes place. The matrix also contains ribosomes, enzymes and ribosomal RNA.

Diagram of the mitochondrion

The number of mitochondria depends on the cell and the organism. Very active cells like the sperms, liver cells called hepatocytes, alongside contractile fibrils in the muscles contain very many mitochondria. While some muscle cells which are not very active contain very few mitochondria. Mitochondria are referred to as the power houses of cells because they produce ATP which readily supplies the cell with energy and mitochondria are the major users of oxygen. The energy produced by mitochondria is put to different uses including chemical work. The chemical work involves biosynthetic pathway e.g. building up macromolecules e.g. proteins, fats, nucleic acids and polysaccharides. The energy can also be used for active transport. The energy can also be used to do electrical work like in nervous transmissions. It can also be used for protection e.g. in the electric eel. The energy can be used for mechanical work like muscle contraction, movement of cilia and pseudopodia as well as flagella. The energy can be used for cell division.

Endoplasmic reticulum

It is found in all cells accept the mature mammalian red blood cells and the prokaryotic cells. It is a complicated network of cytoplasmic membrane which appears to be continuous with the plasma membrane and the nuclear membrane. It is called endoplasmic reticulum because under the electron microscope, it looks like a net hanging in the cytoplasm. Its flat expansions are called cisternae. These expansions increase on the S.A of flat areas within the cell. There are 2 types of endoplasmic reticulum.

• The rough endoplasmic reticulum.
• The smooth endoplasmic reticulum.

The rough E.R appears granular because it is covered with many ribosomes. The ribosomes are for the manufacturer of proteins and the rough E.R is the channel through which proteins are moved away from their site after being manufactured to areas where they are needed.

The smooth E.R isn’t covered with ribosomes and it is abundant in cells which are involved in synthesis of fatty acids and phospholipids as well as steroids. Examples of such cells include sebaceous glands (manufacture fats), endocrine glands like gonads manufacture the steroids. The smooth E.R helps to maintain a particular shape of some animal cells like the rods and cones in the eyes.

Robosomes/microsomes

Contain proteins and ribosomal RNA. They are very small and only visible under the electron microscope. They are found in all cells of living cells either lying free in the cytoplasm or attached to the outer surface of the endoplasmic reticulum. All ribosomes are composed of 2 hemi-spherical subunits of unequal sizes.

Small subunit

Larger subunit

Functionally, Ribosomes are used in cells for protein synthesis. This involves assembling of amino acids in a particular sequence. The ribosomes provide a stabilizing surface for messenger RNA and other reacting molecules during protein synthesis.

Lysomes

These are also very small organelles. They are surrounded by a single unit membrane and they contain hydrolytic enzymes and they function as suicidal bags because they have the ability to destroy other organelles within the cell. They are also associated with intracellular digestion and involved with the aging of an organism. They are found in Eukaryotic cells and more in the animal cells.

Golgi apparatus/Golgi body/ Golgi complex

This is an organelle found in the cytoplasm of the Eukaryotic cells. It consists of a stack called the dictyosome of flattened membrane bounded sacs together with vesicles that break off laterally from one end of the stack. The membrane bound stacks can be known as cisternae.

Formation of the Golgi body

• Budding off of the smooth surfaced vesicles from the R.E.R.
• Fusion of the vesicles to form the cisternae at the forming face.
• Budding off of some cisternae to form vesicles at the formation face.

When vesicles break off, new cisternae are added on as required at the opposite end. The Golgi apparatus is of diverse shapes and sizes in many secretary cells. It spreads throughout the cytoplasm though in some animals and many higher plant cells, it doesn’t spread throughout the cytoplasm.

Functions

• Concentrates and transport materials in the vesicles e.g. glycoproteins.
• In plant cells, they are involved in cell wall formation.
• Contribute to cell plate formation after mitosis in plant cells.
• Participate in secretory activities like if an organism releases slime, wax, gum and mucus. It participates in the insectivorous plants which capture insects by releasing substances which are going to capture the insects.
• Formation of lysosomes.

Chloroplasts

This is a larger plastid which contains chlorophyll. It is an organelle found in the Eukaryotic plant cells. It is a small biconvex structure measuring about 5-10 micrometers in length and 30 nanometers in thickness. It is surrounded by a double unit membrane and it has its own DNA, therefore, it can undergo its own division independent of the cell where it is found. It has the granum which is the site for the light reaction of photosynthesis it has the stroma which is the site for the dark reaction of photosynthesis. In higher plants, chloroplasts are mainly found in the palisade cells and the spongy mesophyll cells. They can also be found in the collenchyma cells found in the stems of herbaceous plants. Palisade cells contain more chloroplasts than the spongy mesophyll cells. They are positively phototactic and aren’t stationary in the presence of light.

Diagram of chloroplast

Internally, there is an elaborate membrane system making up lamellae and thylakoids distinguished as follows;   

Thylakoids are double membrane flattened sack like (disc like) structures normally piled together like coins forming the granum. Some of the membranes of thylakoids are continuous with the thylakoids of other grana. These are known as intergranal lamellae. This arrangement helps to economise space by providing a large S.A for chlorophyll which is packed in the internal membrane systems.

The main function is absorption of light by chlorophyll molecules on the lamellae or grana for photosynthesis. The thylakoid membranes hold the chlorophyll molecules in a suitable position where they can trap maximum amount of energy.

Similarities between mitochondria and chloroplasts.

Similarities

• Both are semi-autonomous and self replicating.
• Both contain their own nucleic acid DNA, similar enzymes, and the electron carriers of the ETS.
• Both are bound by double membranes which are permeable.
• Both have a fluid matrix.
• Both produce ATP
• Both are basically lipoprotein complexes.
• Both contain their own ribosomes.
• Both are found in only eukaryotic cells.
• Both contain granules, chloroplasts have starch granules, mitochondria have phosphate granules.

Differences

Microtubules

Structure

These are tubular structure with a diameter of about 25nm. They are composed of protein called tubulin and are distributed in the cytoplasm where they occur singly or in bundles. They have a remarkable ability to break up and rebuild quickly. They can therefore be assembled in one part of a cell where they are needed, then taken apart and re-assembled in another part of the cell, where a new need has arisen.

Functions

• Associated with movement (e.g. of organelles) and transport in cells.
• Provision of rigidity and support to those parts of the cell where they are located. This is because they are stiff. Together with the microfilaments, they maintain the cytoskeleton.
• Formation of the spindle fibres during cell division on which the movement of chromosomes on the spindle is certainly dependent since they are able to break up and rebuild quickly.

Formation of organelles like Centrioles and cell parts like cilia and flagella and basal bodies. Movement of cilia, Centrioles and flagella depends on them.

Microfilaments

Structure

Long, thin, solid fibres smaller than microtubules. They are of 2 types i.e. thin filaments about 4nm in diameter and the thick ones about 6nm in diameter. Like microtubules, they are made of proteins though of different types.

Functions

Protein in thin ones is actin and that in thick ones is myosin. Like microtubules, they can be readily assembled and disassembled and are useful in cell movement. They usually occur in clusters or bundles and are important in processes involving movement in the cell like;

Cytoplasmic streaming where the cytoplasm moves about. This explains movement in protozoans like amoeba, movement of white blood cells and solutes in the phloem of plants.

• Formation of vesicles by pinching off infolded portions of the cell membrane depending on the contractility provided by microfilaments.
• Formation of cytoskeleton and microtubules.
• In striated muscle e.g. skeletal muscle for contraction.

Centrioles

Structure

Found in a specialized region of cytoplasm near the nucleus known as the centrosome. They are found only in animal cells, cells of some micro-organisms and fungi. They are generally absent in the cells of higher plants.

Each centriole is a hollow cylindrical structure whose walls consists of 9 parallel groups of microtubules arranged in a ring each consisting of 3 microtubules. Most Centrioles are between 300-500nm long and 150nm wide.

They usually occur in pairs with one Centriole lying at right angles to the other. During cell division, each pair of Centrioles synthesizes and assembles a new pair of Centrioles. The pairs of Centrioles then move to opposite poles of the cell where they organize the intercellular microtubules into the spindle which helps to move groups of chromosomes apart to form nuclei.

The basal body of cilia and flagella is in fact a centriole. Though Centrioles do not occur in most plants, the division of their cells also depends on spindle formation in similar way.

 Functions

– Assemble the spindle during cell division.

– Organize microtubules in cilia and flagella.

Cilia and flagella

The structure of both cilia and flagella is basically similar.

• Consist of 9 pairs of peripheral microtubules (doublets) surrounding a central pair of microtubules normally referred to as the 9+2 pattern. These tubules run longitudinally along the whole length of a cilium or flagellum.
• Both cilia and flagella are surrounded by a membrane derived from the cell membrane.

Both grow out of the basal bodies to which they are attached. Rootlet fibres arising out of the basal bodies penetrating deeper in the cytoplasm seem to strengthen this attachment further. These rootlets are made up of the protein collagen.

T.S of cilium of flagellum

NB

The arm like processes attached to one of the peripheral microtubules and both central microtubules contain enzyme ATPase and are sites of ATP hydrolysis to yield energy for the bending of the cilium and flagellum.

Differences between cilia and flagella

• Cilia are shorter and smaller than flagella.
• Cilia usually occur in large numbers on a cell while in most cases, a cell has only single flagellum.

Both cilia and flagella are protrusions at the cell surface and are associated with movement and sometimes locomotion. Both have ability to undulate i.e. create wave like motion or lash back and forth which is linked with the structural arrangement of microtubules in them.

Both ciliary and flagellary movement are restricted to aqueous media therefore, occur on submerged or permanently wet surfaces.

Functions of cilia in organisms

• Locomotion of lower organisms particularly protozoa e.g. paramecium, ciliated larva and gametes especially of lower animals.
• Important in feeding mechanisms e.g. of paramecium i.e. help to bring minute food particles to the sites of ingestion by creating water currents.
• In mollusks, they maintain the circulation of body fluids in coelomic cavities and in the alimentary canal.
• In respiratory passages of mammals, they help to keep surfaces and passages clear of unwanted debris e.g. dust particles and secretions.
• Keep materials on the move in the animals e.g.
• Excretions in the nephridial tubules of the earthworm.
• Ova in oviducts of numerous animals.
• Nutrients in the central canals and ventricles of the central nervous system.

-Locomotion of large invertebrate animals like flatworms and sea snails where aided by muscular contractions of the body wall they enable such animals to glide on smooth surface.

In many of the above cases, the cilia are associated with the cells of the epithelium which is then known as ciliated epithelium.

Functions of flagella

These are almost invariably related to locomotion or movement of some kind e.g.

• Movement of whole organisms in case of protozoans like euglena.
• Movement of gametes e.g. Sperms in higher animals (sperm tail is flagellum) and motile gametes in lower plants e.g. the biflagellate antherozoids of mosses.

Flagella are also important in the feeding mechanisms of some organisms particularly the sponges (phylum porifera) by creating water currents within them and in the osmoregulation and excretion of flatworms where they form part of the flame cells.

Cell vacuole

A fluid filled space within the cytoplasm bounded by a single membrane similar to the plasma membrane. Membrane binding the cell vacuole is the Tonoplast and the solution contained in it is known as the cell sap which is normally isotonic (has same concentration) with the cytoplasm and consists of mineral salts, sugar, amino acids, wastes e.g. tannins, pigments e.g. anthocyanins and digestive enzymes able to digest protein in some cases. Animal cells may contain vacuoles and contractile vacuoles. In plant cells, it is a feature of common occurrence and is usually large and central in location.

Function

• Increase S.A to volume ratio by pushing the cytoplasm to the outer edge of the cell forming a thin layer in which exchange of gases and nutrients readily occurs.
• Controls cell volume, shape and support. Its membrane allows a greater concentration of sugars, salts and other substances inside it which develop a high osmotic potential inside the cell which is a prerequisite to the support of those plants or plant parts that do not have strengthening tissues in them e.g. herbaceous plants and petals of flowers respectively.

Contributes to colour of plant parts like the flower petals and fruit pericarps. This is done by concentrating pigments in the vacuoles of those parts e.g. carotenes in fruit pericarps offer it an orange colour which is important for the attraction of dispersal agents like birds.

• Acts a depository for substances which can no longer be used by the cell e.g. crystals of materials like calcium oxalate. Some of these are deposited in the vacuoles of the cells of old leaves which later fall off. This is a common method of excretion in plants.
• Storage of food. The sugars and amino acids in them may be used in future when they are not readily synthesized.

In animal cells,

• Excretion and osmoregulation especially in protozoans like amoeba (contractile vacuoles).
• Digestion e.g. in food vacuoles of protozoans.