47455770 Biology Sem1 Chap1

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LEARNING OUTCOME 1.1 CELL THEORY

**Describe the cells as the smallest independent unit of life and form the basis of living organisms. 1.2 PROKARYOTIC AND EUKARYOTIC CELLS

**Compare the structure of prokaryotic & eukaryotic cells 1.3 PLANT AND ANIMAL CELLS **Detailed structure ( plant and animal ) **Compare plant and animal cells 1.4 CELLS AS BASIC UNIT OF LIVING ORGANISMS **Specialized cells. a) plant ± meristem,par meristem,parenchym enchyma,collenc a,collenchym hyma,scler a,sclerenchy enchyma ma b) ani anima mall ± epit epithe heli lium um,, ner nerve ves, s, mus muscl cle, e, con conne nect ctiv ive e tis tissu sue e (DESCRIPTION OF STRUCTURE FUNTION AND DISTRIBUTION)

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LEARNING OUTCOME 1.1 CELL THEORY

**Describe the cells as the smallest independent unit of life and form the basis of living organisms. 1.2 PROKARYOTIC AND EUKARYOTIC CELLS

**Compare the structure of prokaryotic & eukaryotic cells 1.3 PLANT AND ANIMAL CELLS **Detailed structure ( plant and animal ) **Compare plant and animal cells 1.4 CELLS AS BASIC UNIT OF LIVING ORGANISMS **Specialized cells. a) plant ± meristem,par meristem,parenchym enchyma,collenc a,collenchym hyma,scler a,sclerenchy enchyma ma b) ani anima mall ± epit epithe heli lium um,, ner nerve ves, s, mus muscl cle, e, con conne nect ctiv ive e tis tissu sue e (DESCRIPTION OF STRUCTURE FUNTION AND DISTRIBUTION)

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CELL THEORY. THEORY. THEORY;  ± All living organisms are composed of one or more cells.  ± Cells- basic units units of structure and function function in an organism.  ± cells have the same chemical composition.  ± Cells come only from reproduction of existing cell cellss- mito mitosi sis s  ± Genetic Genetic materia materialsls- pass to daughte daughterr cells during during cell division.  ± P/S ± virus/chloroplast/mitocho virus/chloroplast/mitochondria ndria have have their  own DNA 3

1.2 Prokaryotic And Eukaryotic Cells TYPES OF CELLS  ± Prokaryotic (prokaryote)  ± Eukaryotic (eukaryote) Cells

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Prokaryotes 

Pro = before; karyon = nucleus

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Single celled organism

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relatively small ± 0.5 to 10 um

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lack membrane-bound organelles

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earliest cell type

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No nucleus/ no nuclear envelope

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E.g.; bacteria/yanobacteria(blue ±green algae)

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Surrounded by cell wall

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Eukaryotes    

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Eu = true; karyon = nucleus 10-100 micrometer  contain membrane-bound organelles Evolved from prokaryotes by endosymbiotic association of two or more prokaryotes Include Protists, Fungi, Animals, and Plants ( cell wall made of  cellulose) E.g.; Protist/fungi/animal and plant

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Features of Prokaryotic Cells

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Features of Prokaryotic Cells 

Cytoplasm  ±  ±  ±  ±

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semi-fluid cell interior  no membrane-bound organelles location for metabolic enzymes location of ribosome for protein synthesis

Ribosome.  ± The smaller (70 S) type.

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Nucleoid  ± region where naked DNA is found

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Nucleoid (or Nuclear Zone)  ± The region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane.

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Features of Prokaryotic Cells 

DNA.  ± Always circular, and not associated with any proteins to form chromatin ± naked DNA

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Plasmid.  ± Small circles of DNA, used to exchange DNA between bacterial cells, and very useful for genetic engineering.

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Cell membrane.  ± made of phospholipids and proteins, like eukaryotic membranes- separates the cell from the environment 9

Features of Prokaryotic Cells 

Mesosome.  ± A tightly-folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis.  ± Can also be associated with the nucleoid.

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Cell Wall.  ± Made of murein (not cellulose), which is a glycoprotein (i.e. a protein/carbohydrate complex, also called peptidoglycan).  ± There are two kinds of cell wall, which can be distinguished by a Gram stain: Gram positive bacteria have a thick cell wall and stain purple, while Gram negative bacteria have a thin cell wall with an outer lipid layer and stain pink. 10

Features of Prokaryotic Cells 

Cell Wall  ± rigid structure - maintain its shape (made of  peptidoglycan)

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Capsule (or Slime Layer ).  ± A thick polysaccharide layer outside of the cell wall, like the glycocalyx of eukaryotes.  ± Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis. 11

Features of Prokaryotic Cells 

Capsule  ± outer sticky protective layer   ± this is in NO way the same as the cell wall of a plant cell

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Flagellum.  ± A rigid rotating helical-shaped tail used for  propulsion(movement)  ± The motor is embedded in the cell membrane and is driven by a H+ gradient across the membrane.  ± Clockwise rotation drives the cell forwards, while anticlockwise rotation causes a chaotic spin.  ± This is the only known example of a rotating motor in nature.

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Features of Eukaryote Cells

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Features of Eukaryote Cells 

Cytoplasm (or Cytosol).

 ± This is the solution within the cell membrane.  ± contains enzymes -glycolysis (part of respiration) and other  metabolic reactions together with sugars, salts, amino acids, nucleotides and everything else needed for the cell to function.  ± All living content excluding nucleus and plasma membrane

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Features of Eukaryote Cells 

Nucleus.  ± largest organelle.  ± Surrounded by a nuclear envelope-double membrane with nuclear pores - large holes containing proteins that control the exit of substances (RNA /ribosomes)from the nucleus.  ± The interior is called the nucleoplasm, which is full of  chromatin- a DNA/protein complex in a 1:2 ratio containing the genes.  ± During cell division the chromatin becomes condensed into discrete observable chromosomes.  ± The nucleolus is a dark region of chromatin, involved in making ribosome. 15

Features of Eukaryote Cells 

Mitochondrion (pl. Mitochondria).  ± sausage-shaped organelle ( 8µm long)-where aerobic respiration takes place in all eukaryotic cells.  ± Mitochondria are surrounded by a double membrane: the outer membrane is simple and quite permeable, while the inner membrane is highly folded into cristaelarge surface area.  ± The space enclosed by the inner membrane is called the mitochondrial matrix, and contains small circular  strands of DNA.  ± The inner membrane is studded with stalked particles, which are the site of ATP synthesis. 16

Features of Eukaryote Cells 

Chloroplast.  ± Bigger/fatter than mitochondria, chloroplasts-site of  photosynthesis,-only found in photosynthetic organisms (plants and algae).  ± enclosed by a double membrane, also have a third membrane (thylakoid membrane)-folded into thylakoid disks which then stacked into piles( grana)  ± The space between the inner membrane and the thylakoid is called the stroma.  ± The thylakoid membrane contains chlorophyll and other photosynthetic pigments arranged in photosystems, together with stalked particles, and is the site of photosynthesis and ATP synthesis.  ± Chloroplasts also contain starch grains, ribosomes and circular DNA. 17

Features of Eukaryote Cells 

Ribosomes.  ± These are the smallest and most numerous of the cell organelles, and are the sites of protein synthesis.  ± They are composed of protein and RNA, and are manufactured in the nucleolus of the nucleus.  ± Ribosomes are either found free in the cytoplasm-site of proteins synthesis for the cell's own use, or they are found attached to the rough endoplasmic reticulum, where they make proteins for export from the cell.  ± They are often found in groups called polysomes.  ± All eukaryotic ribosomes are of the larger, " 80S", type. 18

Features of Eukaryote Cells 

Smooth Endoplasmic Reticulum (SER).  ± Series of membrane channels involved in synthesizing and transporting materials, mainly lipids, needed by the cell.

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Rough Endoplasmic Reticulum (RER).  ± Similar to the SER, but studded with numerous ribosomes, which give it its rough appearance.  ± The ribosomes synthesis proteins, which are processed in the RER (e.g. by enzymatically modifying the polypeptide chain, or adding carbohydrates), before being exported from the cell via the Golgi Body. 19

Features of Eukaryote Cells 

Golgi Body (or Golgi Apparatus).  ± Another series of flattened membrane vesicles, formed from the endoplasmic reticulum.  ± F(x)transport proteins from the RER to the cell membrane for export.  ± Parts of the RER containing proteins fuse with one side of the Golgi body membranes, while at the other  side small vesicles bud off and move towards the cell membrane, where they fuse, releasing their contents by exocytosis.

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Features of Eukaryote Cells 

Vacuoles.  ± These are membrane-bound sacs containing water or  dilute solutions of salts and other solutes.  ± Most cells can have small vacuoles that are formed as required, but plant cells usually have one very large permanent vacuole that fills most of the cell, so that the cytoplasm (and everything else) forms a thin layer round the outside.  ± Plant cell vacuoles are filled with cell sap, and are very important in keeping the cell rigid, or turgid.  ± Some unicellular protoctists(protist) have feeding vacuoles for digesting food, or contractile vacuoles for  expelling water. 21

Features of Eukaryote Cells 

Lysosomes.  ± These are small membrane-bound vesicles formed from the RER containing a cocktail of  digestive enzymes.  ± They are used to break down unwanted chemicals, toxins, organelles or even whole cells, so that the materials may be recycled.  ± They can also fuse with a feeding vacuole to digest its contents. 22

Features of Eukaryote Cells 

Cytoskeleton.  ± This is a network of protein fibres extending throughout all eukaryotic cells, used for support, transport and motility.  ± The cytoskeleton is attached to the cell membrane and gives the cell its shape, as well as holding all the organelles in position.  ± There are three types of protein fibres (microfilaments, intermediate filaments and microtubules), and each has a corresponding motor protein that can move along the fibre carrying a cargo such as organelles, chromosomes or other  cytoskeleton fibres.  ± These motor proteins are responsible for such actions as: chromosome movement in mitosis, cytoplasm cleavage in cell division, cytoplasmic streaming in plant cells, cilia and flagella movements, cell crawling and even muscle contraction in animals. 23

Features of Eukaryote Cells 

Centriole.  ± This is a pair of short microtubules involved in cell division.  ± Before each division the centriole replicates itself and the two centrioles move to opposite ends of the cell, where they initiate the spindle that organises and separates the chromosomes.

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Features of Eukaryote Cells 

Undulipodium (Cilium and Flagellum).  ± This is a long flexible tail present in some cells and used for motility.  ± It is an extension of the cytoplasm, surrounded by the cell membrane, and is full of microtubules and motor  proteins so is capable of complex swimming movements.  ± There are two kinds: flagella (no relation of the bacterial flagellum) are longer than the cell, and there are usually only one or two of them, while cilia are identical in structure, but are much smaller and there are usually very many of them. 25

Features of Eukaryote Cells 

Microvillus (MICROVILLI)  ± These are small finger-like extensions of the cell membrane found in certain cells such as in the epithelial cells of the intestine and kidney, where they increase the surface area for  absorption of materials.  ± They are just visible under the light microscope as a brush border.

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Cell Membrane (or Plasma Membrane).  ± This is a thin, flexible layer round the outside of all cells made of  phospholipids and proteins.  ± It separates the contents of the cell from the outside environment, and controls the entry and exit of materials.  ± The membrane is examined in detail later. 26

Features of Eukaryote Cells 

Cell Wall.  ± This is a thick layer outside the cell membrane used to give a cell strength and rigidity.  ± Cell walls consist of a network of fibers, which give strength but are freely permeable to solutes (unlike membranes).  ± A wickerwork basket is a good analogy.  ± made mainly of cellulose, also contain hemicelluloses, pectin, lignin and other polysaccharides. -built up in three layers > primary cell wall, the secondary cell wall and the middle lamella.  ± There are often channels through plant cell walls called plasmodesmata, which link the cytoplasms of adjacent cells.  ± Fungal cell walls are made of chitin (poly-glucosamine).  ± Animal cells do not have a cell wall, though they do have a layer  of carbohydrate outside the cell membrane called the cell coat, 27 or glycocalyx.

Microscopic structures of plant and animal cells

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COMPARISON- PROKARYOTIC& EUKARYOTIC CELLS CHARATERISTICS

PROKARYOTES

EUKARYOTES

1. SIZE

Small cells ,diameter-0.5-5 micrometer

Large cells, up to 40 micrometer 

2.DNA

Circular & free in cytoplasm-naked DNA

DNA is associated with histone protein to form chromosome

3.NUCLEUS

Not enclosed by membrane-naked

Bounded by membrane( nucleus envelope)

4.ORGANELLES

No membrane bound organelles/ no centriols and microtubules

Has membrane bound organelles; mitochondria,chloroplast,centri ols.etc.

5.RIBOSOMES

Small ( 70 s)

large (80 s)

6.CELL WALL

Present- made up from peptidolican

Only plant(cellulose), fungi and chitin

7.FLAGELLA

Present- movement

Absent

8.CELL DIVISION

Simple fission

Mitosis

9.REPRODUCTION

No sexual systems

Meiosis and fertilization 29

PLANT CELL

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Tissue and Organs in plant:  ± Dermal Tissue System (epidermis)-forms the outer layer of a plant.  ± Ground Tissue System (parenchyma/collenchyma/sclerenchyma)(makes up the bulk of roots and stems  ± Vascular Tissue ( xylem& phloem)-transports water and food throughout the plant. 

The FOUR Plant Organs are ROOTS, STEMS, LEAVES AND FLOWERS. 31

Specialized cell : Plant Meristem

Meristems- cells that undergo repeated division. These are the cells from which all other cells in the plant originate. No undergoes differentiation. 2 types of meristem : 1) apical>shoot and root 2) lateral>cambia-vascular cambium -cork cambium three types : primary, secondary, and pericycle . The primary meristems give rise to primary tissues in the plant, which elongate the plant by either shoots or roots. Secondary meristems form the basis of secondary tissues that enlarge the diameter of the stem or root. the pericycle is a specialized meristem-found only in the root system-gives rise to branch roots. 32

Meristem, parenchyma, collenchyma, sclerenchyma, xylem, and phloem tissues

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Specialized cell : Plant Parenchyma  ± Unspecialized and are found in all organs of plant.        

Can divide and give rise to more specialized cells. Living at maturity Isodiametrical/ polyhedral(many sided)-capable 4 cell division Thin 1st cell wall and without 2 nd cell walls Loosely packed Involved in food storage. Involved in apoplast pathway 4 mvment of water and minerals. Divide to 4 type: 1) epidermis 2) mesophyll 3) endodermis 4) pericycle

Collenchyma - thicker primary walls. - Polygonal shape - f(x) : mechanical support 

Form bundles underneath epidermis.  ± Flexible support to immature regions of the plant.

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Sclerenchyma  ± Sclerenchyma cells have thick secondary walls impregnated with lignin.  ± Most are non living.  ± More rigid than collenchyma cells  ± Most cells death at functional maturity  ± Can be divided into 2 types: a) fibres- elongated b) scelereids( stone cells ) - spherical - can be found in the cortex, pith and phloem * Function ± supporting element of the plant

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Specialized cell : Plant Xylem

 Xylem

transports water and minerals from the roots to the leaves.  ± Tracheids  

elongated, with tapered ends and have lignified walls Matured cells death with empty lumen  ± Pits in end walls.  ± Vascular rays  ± Fibers

 ± Vessel Elements  

Larger, with perforated plates in their end walls. Long ,narrow tube formed

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Xylem

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Specialized cell : Plant Phloem 

Phloem transports sucrose and other organic compounds from the leaves to the roots.  ± Sieve-tube members are conducting cells. 

Contain cytoplasm but no nuclei.

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Channels in end walls.

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Plasmodesmata extend from one cell to another through sieve plate.

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Companion cells- contain nucleus, vacuoles, ribosomes and mitochondria

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Phloem

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SUMMARY OF SPECIALISED PLANT CELLS: FUNCTION & DISTRIBUTION PARENCHYMA CELLS

SPECIALIZED PARENCHYMA

Characteristic : >Living and unspecialized

1) Epidermis : single layer covering plant body, elongated and flattened, protection against desiccation and infection 2) Mesophyll : between upper and lower epidermis of  leaves

>isodiametrical,sometimes elongated >Composition :cellulose, pectins and hemicellulose, covering of cutin >Large central vacuole with cytoplasm pushed to side of wall >Thin cell wall >Loosely packed, large intercellular air space Function : > Provided support in herbaceous plant

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Pericyle : between central vascular tissue and endodermis in roots,isodiametrical,retain meristematic activity giving rise to lateral root and 2nd growth 4) Endodermis :singled celled betwn outer cortex and inner pericycle tissue; impregnated suberin forming Casparian strip which prevent passage of water via apoplast pathway

> Gaseous exchange > Food storage > Replacement of injured and damage cells Distribution : >Cortex ,pith , medullary rays ,packing tissue in xylem and phloem

COLLENCHYMA CELLS Characteristic : > Living , unevenly thickened (extra cellulose at corners of he cells) > Polygonal and elongated with tapering ends > Composition: cellulose , pectins and hemicelluloses > small intercellular air spaces@ non exist Function: > support And mechanical strength Distribution : Outer region of cortex: below epidermis

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SCLERENCHYMA CELLS Characteristic : > Polygonal and elongated with tapering ends, tightly packed with no intercellular air  spaces > Deposits of lignin on 1 st cell wall resulting in death matured cells ; incapable of  elongation at maturity Function : > support and mechanical strength :provided great tensile and compressional strength as a result of deposits of lignin at cellulose cell wall

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types of sclerenchyma cells : fibre and sclereids Sclerenchyma

fibre : elongated, polygonal with tapering ends, death at maturity, no protoplasm ,narrow empty lumen, pit present, tapering ends overlapping and interlocked with one another thus provided mechanical strength Scelereids(stone

cells): shorter  than fibres , dead cells with thicker  lignified cell wall, simple pit present , found in stems,leaves, fruits and provide strength and support plant structures and organs

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Specialized cell : Animal Epithelial cell/ epithelium 

One of the simplest tissues.

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However, it can be built up into tissues of  varying complexity.

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Lining tissues : in it simplest form, it consists of a single layer of cells covering the surface of the body and the organs within it. 43

Specialized cell : Animal Epithelial cell   Also

lines various spaces and tubes ( in which situation it is usually referred to as endothelium).

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Typically the individual cells : firmly attached to each other.

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Specialized cell : Animal Epithelial cell 

rest on a basement membrane.

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have a free surface

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The main function of epithelial tissues is protection.

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Specialized cell : Animal Epithelial cell 

Other functions: To

increase the surface area for the reabsorption of materials/which absorption can take place.

Exchange  As

materials by diffusion.

a gland ; secrete mucus or enzyme.

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Types of Epithelial Tissues Epithelial Tissues

Simple Epithelial Tissues

Stratified Epithelial Tissues

Pseudostratified Epithelial Tissues

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Simple Epithelial Tissues 1. Simple squamous epithelial tissues. 2. Simple cuboidal epithelial tissues. 3.

Simple columnar epithelial tissues.

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Simple squamous epithelial tissues

  

Cells are flattened. Disc- shape nucleus The sheet of cell is delicate, thin and leaky.

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Simple squamous epithelial tissues 

Functions: 

exchange of material/protect underlying tissues  lubricates the movement between adjacent surface./facilitated diffusion of gaseous across alveoli 

Distribution:  the linings of blood vessels.  alveoli  mouth cavity  Bowmann capsule

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Simple cuboidal epithelial tissues 

Cells are cubical in shape.

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View from free surface are polygonal.

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Specialized for  secretion.

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Make up the epithelia of kidney tubules and many glands ± thyroid and salivary glands. 51

Simple columnar epithelial tissues 

Cells are elongated at right angle to the basement membrane.

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Relatively large cytoplasmic volume.

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Often located where secretion or active absorption is an important function.

 Distribution: 

linings of small intestine( stomach)

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oviduct/ ventricles of brain

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nasal cavity.

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Stratified Epithelial Tissues 

Several layer of cells.

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With only the lower ones columnar and metabolically active.

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Division of lower cells causes older one to pushed upward forward surface, becoming flattened as they move. 53

Stratified Epithelial Tissues 

Functions ± only for protection.

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Main location ± skin; mouth and vaginal lining.

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Pseudostratified Epithelial Tissues

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Specialized cell : Animal Nerve cell  Neurons  ± Cell body contains nucleus.  ± Dendrites receive signals from sensory receptors.  ± Axon conducts nerve impulses. 

 Any long axon is also called a nerve fiber.  ± Covered by myelin sheath.

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Types of Neurons: 1) Motor Neurons take nerve impulses from the CNS to muscles or glands. 2) Sensory Neurons take impulses from sensory receptors to CNS. 3)

Interneuron convey nerve impulses between various parts of  the CNS. 56

Neurons 

The pathway of communication between the brain and the body.

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Electrical impulses pass along the neurons from stimuli receivers to the effectors .

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Neurons differ considerably in structure but all neurons have three things in common;  ±  ±  ±

Cell body Fine cytoplasmic fibre ± dendron or dendrite Axon. 57

Neurons 

The cytoplasm of neurons cell body is densely packed with mitochondria, ribosomes, golgi body and rough endoplasmic reticulum.

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Neurons can be classified by their function.

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On the basic, there are three types of  neuron; 58

Types of Neurons

Sensory

neuron

Intermediary/ Relay neuron

Motor  neuron

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A Nerve Cell (Neuron) Dendrites

Cell Body Myelin Sheath

Axon of another  Axon neuron

Dendrites of another 60 neuron

Neuroglia 

Supporting cells in the central nervous system.

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There are several types of glial cell in the brain and spinal cord.

  As

a group, they do far more than simply glue neurons together.

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Specialized cell : Animal Muscle cell 

In the invertebrate body there are three types of muscle cell; a) Skeletal muscle. b) Smooth muscle. c) Cardiac muscle.

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Skeletal muscle   

 Attached to the bones by tendons. Responsible for voluntary movement.  Also called striated muscle.

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Smooth muscle 

Found in the walls of internal organs ± digestive tract, urinary bladder etc.

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Spindel shaped.

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Responsible in involuntary activities.

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Contract more slowly than skeletal muscle.

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Cardiac muscle 

Form the contraction wall of the heart.

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Striated like skeletal muscle but have branched.

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The ends of the cells are joined by intercalated disc.

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Specialized cell : Animal

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Connective tissues 

The major types of connective tissues in vertebrates are: a) Bone b) Cartilage c) Blood

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Bone Tissue 

The skeleton supporting the body of most vertebrates.

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Bone is a mineralized connective tissues.

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Bone-forming cells called osteoblasts; deposit a matrix of  collagen. 68

Bone Tissue 

The combination of hard mineral and flexible collagen makes bone harder than cartilage.

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Hard mamalian bone consists repeating units called Harvesian systems.

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Each system has concentrate layers of the mineralized matrix, deposited around a central canal containing blood vessels and nerves that service the bone.

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Once osteoblast become trapped in their own secretion , called osteocytes.

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The osteocytes are located in lacunae, space surrounded by the hard matrix and connected to each other by long thin extensions. 69

Cartilage Tissue 

Has an abundance of collagenous fibers embedded in a rubbery matrix called chondroitin sulphate.

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Chondroitin sulphate and collagen are secreted by chondrocytes

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Cartilage Tissue 

Cells confined to scattered spaces called lacunae in the matrix.

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The composite of collagenous fibers and chondroitin sulphate make cartilage a strong yet somewhat flexible support material.

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Main locations ; nose, ears, the rings that reinforce the windpipe,caps on the ends of some bones.

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Blood  Although

blood functions differently from other  connective tissues, it does meet the criterion of having an extracellular matrix called plasma.

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The plasma consists of water, salts and a variety of  dissolved proteins

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Suspended in plasma are two classes of blood cells, erythrocytes (red blood cells), leukocytes (white blood cells) and cell fragments called platelets. 72

Erythrocytes o the the mos mostt numer numerou ous s bloo blood d cell cells s (5 to 6 million per cubic millimeter) o the structure structure of of the the red red blood blood cell cell is is anothe anotherr excelle excellent nt of  structure fitting function. o a huma human n ery eryth thro rocy cyte tes s is a biconcave disk, thinner  in the center than at its edges. o mamma mammali lian an ery erythro throcy cyte tes s lack nuclei an unusual characteristic for living cells 73

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Erythrocytes

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the major function of erythrocytes is to carry oxygen.. oxygen

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the small size of erythrocytes erythrocytes also suits their  function ± the smaller the cells, cells , the greeter the total area of plasma membrane in a given volume of blood.

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the biconcave shape also adds to its surface area .

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erythrocytes productio production n occurs in the red marrow of bone 7

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Leukocytes

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major types:

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Leukocytes their collective function is to fight infections in various ways monocytes and neutrophils are phagocytes which engulf  and digest bacteria and debris from our own dead cells. lymphocytes become specialized as B cells and T cells which produce the immune response against foreign substances 76

Platelets the third cellular element of blood, platelets are fragments of cells about 2 ± 3 µm in diameter. they have no nuclei. originate as pinched ± off cytoplasmic fragments of large cells in the bone marrow. the function of platelets is in the important process of blood clotting.

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