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CYTOLOGY

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:

y The prokaryotic celly 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.

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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

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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.

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diagram: Ultrastructure of an animal cell

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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.

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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 providingenergy 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:-

y Plasma membraney Nucleusy Mitochondriay Endoplasmic reticulumy 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:

y Phospholipids is the lipid which contains phosphate groupy Glycolipids - is the lipid which contains carbohydratesy 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.

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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 1940s 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 FluidMosaicModel 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.

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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 recognizeother 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

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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 ofcharacteristics from one generation to another.

2. It controls the metabolic activities since DNA is organized into genes which control all theactivities of the cell.

3. Formation of the ribosomal RNA by nucleolus.4. Nuclear division gives rise to cell division hence reproduction.

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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 its 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 moleculesbefore 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

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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 (70s ribosomes) than those of eukaryotic cells (80s 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 peroxisomeare 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).

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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.

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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 knownas 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.

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Examples of leucoplasts include:

y Amyloplast store starch.y Lipidoplast store lipid, eg. In sunflower seedsy 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.

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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 ordamaged 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.

Autophagyis 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.

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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.

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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

krebs 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.

to be continued......................................