While examining a thin slice of cork, RobertHooke saw that the cork resembled thestructure of a honeycomb consisting of manylittle compartments. Cork is a substancewhich comes from the bark of a tree. Thiswas in the year 1665 when Hooke made thischance observation through a self-designedmicroscope. Robert Hooke called these boxescells. Cell is a Latin word for ‘a little room’.

This may seem to be a very small andinsignificant incident but it is very importantin the history of science. This was the veryfirst time that someone had observed thatliving things appear to consist of separateunits. The use of the word ‘cell’ to describethese units is used till this day in biology.

Let us find out about cells.

5.1 What are Living OrganismsMade Up of ?

Activity ______________ 5.1• Let us take a small piece from an onion

bulb. With the help of a pair of forceps,we can peel of f the skin (calledepidermis) from the concave side (innerlayer) of the onion. This layer can beput immediately in a watch-glasscontaining water. This will prevent thepeel from getting folded or getting dry.What do we do with this peel?

• Let us take a glass slide, put a drop ofwater on it and transfer a small pieceof the peel from the watch glass to theslide. Make sure that the peel isperfectly flat on the slide. A thin camelhair paintbrush might be necessary tohelp transfer the peel. Now we put adrop of iodine solution on this piecefollowed by a cover slip. Take care to

avoid air bubbles while putting thecover slip with the help of a mountingneedle. Ask your teacher for help. Wehave prepared a temporary mount ofonion peel. We can observe this slideunder low power followed by highpowers of a compound microscope.

Fig. 5.1: Compound microscope

What do we observe as we look throughthe lens? Can we draw the structures thatwe are able to see through the microscope,on an observation sheet? Does it look likeFig. 5.2?

Eyepiece

Coarse adjustment

Fine adjustment

ArmObjective lens

StageSwivel

Mirror

Base

Body tube

Clip Microscope slide

Condenser

Fig. 5.2: Cells of an onion peel

55555TTTTTHEHEHEHEHE F F F F FUNDAMENTALUNDAMENTALUNDAMENTALUNDAMENTALUNDAMENTAL U U U U UNITNITNITNITNIT OFOFOFOFOF L L L L LIFEIFEIFEIFEIFE

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Chlamydomonas, Paramoecium and bacteria.These organisms are called unicellularorganisms (uni = single). On the other hand,many cells group together in a single bodyand assume different functions in it to formvarious body parts in multicellular organisms(multi = many) such as some fungi, plantsand animals. Can we find out names of somemore unicellular organisms?

Every multi-cellular organism has comefrom a single cell. How? Cells divide toproduce cells of their own kind. All cells thuscome from pre-existing cells.

Activity ______________ 5.2• We can try preparing temporary

mounts of leaf peels, tip of roots ofonion or even peels of onions of differentsizes.

• After performing the above activity, letus see what the answers to the followingquestions would be:(a) Do all cells look alike in terms of

shape and size?(b) Do all cells look alike in structure?(c) Could we find differences among

cells from different parts of a plantbody?

(d) What similarities could we find?

Some organisms can also have cells ofdifferent kinds. Look at the following picture.It depicts some cells from the human body.

Nerve Cell

Fat cell

Sperm

Bonecell

Smoothmuscle

cell

Bloodcells

Ovum

Fig. 5.3: Various cells from the human body

Mor

e to

kno

w

We can try preparing temporary mountsof peels of onions of different sizes. What dowe observe? Do we see similar structures ordifferent structures?

What are these structures?

These structures look similar to each other.Together they form a big structure like anonion bulb! We find from this activity thatonion bulbs of different sizes have similarsmall structures visible under a microscope.The cells of the onion peel will all look thesame, regardless of the size of the onion theycame from.

These small structures that we see arethe basic building units of the onion bulb.These structures are called cells. Not onlyonions, but all organisms that we observearound are made up of cells. However, thereare also single cells that live on their own.

Cells were first discovered byRobert Hooke in 1665. He observedthe cells in a cork slice with the helpof a primitive microscope.Leeuwenhoek (1674), with theimproved microscope, discovered thefree living cells in pond water for thefirst time. It was Robert Brown in1831 who discovered the nucleus inthe cell. Purkinje in 1839 coined theterm ‘protoplasm’ for the fluidsubstance of the cell. The cell theory,that all the plants and animals arecomposed of cells and that the cell isthe basic unit of life, was presentedby two biologists, Schleiden (1838)and Schwann (1839). The cell theorywas further expanded by Virchow(1855) by suggesting that all cellsarise from pre-existing cells. With thediscovery of the electron microscopein 1940, it was possible to observe andunderstand the complex structure ofthe cell and its various organelles.

The invention of magnifying lenses led tothe discovery of the microscopic world. It isnow known that a single cell may constitutea whole organism as in Amoeba,

THE FUNDAMENTAL UNIT OF LIFE 59

The shape and size of cells are related tothe specific function they perform. Some cellslike Amoeba have changing shapes. In somecases the cell shape could be more or lessfixed and peculiar for a particular type of cell;for example, nerve cells have a typical shape.

Each living cell has the capacity toperform certain basic functions that arecharacteristic of all living forms. How does aliving cell perform these basic functions? Weknow that there is a division of labour inmulticellular organisms such as humanbeings. This means that different parts of thehuman body perform different functions. Thehuman body has a heart to pump blood, astomach to digest food and so on. Similarly,division of labour is also seen within a singlecell in many cases. In fact, each such cellhas got certain specific components within itknown as cell organelles. Each kind of cellorganelle performs a special function, suchas making new material in the cell, clearingup the waste material from the cell and soon. A cell is able to live and perform all itsfunctions because of these organelles. Theseorganelles together constitute the basic unitcalled the cell. It is interesting that all cellsare found to have the same organelles, nomatter what their function is or whatorganism they are found in.

uestions1. Who discovered cells, and how?2. Why is the cell called the

structural and functional unit oflife?

5.2 What is a Cell Made Up of ?What is the StructuralOrganisation of a Cell?

We saw above that the cell has specialcomponents called organelles. How is a cellorganised?

If we study a cell under a microscope, wewould come across three features in almost

every cell; plasma membrane, nucleus andcytoplasm. All activities inside the cell andinteractions of the cell with its environmentare possible due to these features. Let us seehow.

5.2.1 PLASMA MEMBRANE OR CELL

MEMBRANE

This is the outermost covering of the cell thatseparates the contents of the cell from itsexternal environment. The plasma membraneallows or permits the entry and exit of somematerials in and out of the cell. It alsoprevents movement of some other materials.The cell membrane, therefore, is called aselectively permeable membrane.

How does the movement of substancestake place into the cell? How do substancesmove out of the cell?

Some substances like carbon dioxide oroxygen can move across the cell membraneby a process called diffusion. We have studiedthe process of diffusion in earlier chapters.We saw that there is spontaneous movementof a substance from a region of highconcentration to a region where itsconcentration is low.

Something similar to this happens in cellswhen, for example, some substance like CO2

(which is cellular waste and requires to beexcreted out by the cell) accumulates in highconcentrations inside the cell. In the cell’sexternal environment, the concentration ofCO2 is low as compared to that inside thecell. As soon as there is a difference ofconcentration of CO2 inside and outside a cell,CO2 moves out of the cell, from a region ofhigh concentration, to a region of lowconcentration outside the cell by the processof diffusion. Similarly, O2 enters the cell bythe process of diffusion when the level orconcentration of O2 inside the cell decreases.Thus, diffusion plays an important role ingaseous exchange between the cells as wellas the cell and its external environment.

Water also obeys the law of diffusion. Themovement of water molecules through sucha selectively permeable membrane is called

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osmosis. The movement of water across theplasma membrane is also affected by theamount of substance dissolved in water.Thus, osmosis is the passage of water from aregion of high water concentration through asemi-permeable membrane to a region of lowwater concentration.

What will happen if we put an animal cellor a plant cell into a solution of sugar or saltin water?

One of the following three things couldhappen:

1. If the medium surrounding the cell hasa higher water concentration than thecell, meaning that the outside solutionis very dilute, the cell will gain waterby osmosis. Such a solution is knownas a hypotonic solution.

Water molecules are free to passacross the cell membrane in bothdirections, but more water will comeinto the cell than will leave. The net(overall) result is that water enters thecell. The cell is likely to swell up.

2. If the medium has exactly the samewater concentration as the cell, therewill be no net movement of wateracross the cell membrane. Such asolution is known as an isotonicsolution.

Water crosses the cell membranein both directions, but the amountgoing in is the same as the amountgoing out, so there is no overallmovement of water. The cell will staythe same size.

3. If the medium has a lowerconcentration of water than the cell,meaning that it is a very concentratedsolution, the cell will lose water byosmosis. Such a solution is known asa hypertonic solution.

Again, water crosses the cellmembrane in both directions, but thistime more water leaves the cell thanenters it. Therefore the cell will shrink.

Thus, osmosis is a special case of diffusionthrough a selectively permeable membrane.Now let us try out the following activity:

Activity ______________ 5.3Osmosis with an egg

(a) Remove the shell of an egg by dissolvingit in dilute hydrochloric acid. The shellis mostly calcium carbonate. A thinouter skin now encloses the egg. Putthe egg in pure water and observe after5 minutes. What do we observe?The egg swells because water passesinto it by osmosis.

(b) Place a similar de-shelled egg in aconcentrated salt solution and observefor 5 minutes. The egg shrinks. Why?Water passes out of the egg solutioninto the salt solution because the saltsolution is more concentrated.

We can also try a similar activity with driedraisins or apricots.

Activity ______________ 5.4• Put dried raisins or apricots in plain

water and leave them for some time.Then place them into a concentratedsolution of sugar or salt. You willobserve the following:

(a) Each gains water and swells whenplaced in pure water.

(b) However, when placed in theconcentrated solution it loses water,and consequently shrinks.

Unicellular freshwater organisms andmost plant cells tend to gain water throughosmosis. Absorption of water by plant rootsis also an example of osmosis.

Thus, diffusion is important in exhangeof gases and water in the life of a cell. Inadditions to this, the cell also obtainsnutrition from its environment. Differentmolecules move in and out of the cell througha type of transport requiring use of energy inthe form of ATP.

The plasma membrane is flexible and ismade up of organic molecules called lipidsand proteins. However, we can observe thestructure of the plasma membrane onlythrough an electron microscope.

The flexibility of the cell membrane alsoenables the cell to engulf in food and othermaterial from its external environment. Suchprocesses are known as endocytosis. Amoebaacquires its food through such processes.

THE FUNDAMENTAL UNIT OF LIFE 61

Activity ______________ 5.5• Find out about electron microscopes

from resources in the school library orthrough the internet. Discuss it withyour teacher.

uestions1. How do substances like CO

2 and

water move in and out of the cell?Discuss.

2. Why is the plasma membranecalled a selectively permeablemembrane?

5.2.2 CELL WALL

Plant cells, in addition to the plasmamembrane, have another rigid outer coveringcalled the cell wall. The cell wall lies outsidethe plasma membrane. The plant cell wall ismainly composed of cellulose. Cellulose is acomplex substance and provides structuralstrength to plants.

When a living plant cell loses waterthrough osmosis there is shrinkage orcontraction of the contents of the cell awayfrom the cell wall. This phenomenon is knownas plasmolysis. We can observe thisphenomenon by performing the followingactivity:

Activity ______________ 5.6• Mount the peel of a Rheo leaf in water

on a slide and examine cells under thehigh power of a microscope. Note thesmall green granules, calledchloroplasts. They contain a greensubstance called chlorophyll. Put astrong solution of sugar or salt on themounted leaf on the slide. Wait for aminute and observe under amicroscope. What do we see?

• Now place some Rheo leaves in boilingwater for a few minutes. This kills thecells. Then mount one leaf on a slideand observe it under a microscope. Puta strong solution of sugar or salt onthe mounted leaf on the slide. Wait fora minute and observe it again. Whatdo we find? Did plasmolysis occur now?

What do we infer from this activity? Itappears that only living cells, and not deadcells, are able to absorb water by osmosis.

Cell walls permit the cells of plants, fungiand bacteria to withstand very dilute(hypotonic) external media without bursting.In such media the cells tend to take up waterby osmosis. The cell swells, building uppressure against the cell wall. The wall exertsan equal pressure against the swollen cell.Because of their walls, such cells canwithstand much greater changes in thesurrounding medium than animal cells.

5.2.3 NUCLEUS

Remember the temporary mount of onion peelwe prepared? We had put iodine solution onthe peel. Why? What would we see if we triedobserving the peel without putting the iodinesolution? Try it and see what the differenceis. Further, when we put iodine solution onthe peel, did each cell get evenly coloured?

According to their chemical compositiondifferent regions of cells get coloureddifferentially. Some regions appear darkerthan other regions. Apart from iodine solutionwe could also use safranin solution ormethylene blue solution to stain the cells.

We have observed cells from an onion; letus now observe cells from our own body.

Activity ______________ 5.7• Let us take a glass slide with a drop of

water on it. Using an ice-cream spoongently scrape the inside surface of thecheek. Does any material get stuck onthe spoon? With the help of a needlewe can transfer this material andspread it evenly on the glass slide keptready for this. To colour the materialwe can put a drop of methylene bluesolution on it. Now the material is readyfor observation under microscope. Donot forget to put a cover-slip on it!

• What do we observe? What is the shapeof the cells we see? Draw it on theobservation sheet.

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• Was there a darkly coloured, sphericalor oval, dot-like structure near thecentre of each cell? This structure iscalled nucleus. Were there similarstructures in onion peel cells?

The nucleus has a double layered coveringcalled nuclear membrane. The nuclearmembrane has pores which allow the transferof material from inside the nucleus to itsoutside, that is, to the cytoplasm (which wewill talk about in section 5.2.4).

The nucleus contains chromosomes,which are visible as rod-shaped structuresonly when the cell is about to divide.Chromosomes contain information forinheritance of features from parents to nextgeneration in the form of DNA (DeoxyriboNucleic Acid) molecules. Chromosomes arecomposed of DNA and protein. DNA moleculescontain the information necessary forconstructing and organising cells. Functionalsegments of DNA are called genes. In a cellwhich is not dividing, this DNA is present aspart of chromatin material. Chromatinmaterial is visible as entangled mass of threadlike structures. Whenever the cell is about todivide, the chromatin material gets organisedinto chromosomes.

The nucleus plays a central role in cellularreproduction, the process by which a singlecell divides and forms two new cells. It alsoplays a crucial part, along with theenvironment, in determining the way the cellwill develop and what form it will exhibit atmaturity, by directing the chemical activitiesof the cell.

In some organisms like bacteria, thenuclear region of the cell may be poorlydefined due to the absence of a nuclearmembrane. Such an undefined nuclear regioncontaining only nucleic acids is called anucleoid. Such organisms, whose cells lacka nuclear membrane, are called prokaryotes(Pro = primitive or primary; karyote ≈ karyon= nucleus). Organisms with cells having anuclear membrane are called eukaryotes.

Prokaryotic cells (see Fig. 5.4) also lackmost of the other cytoplasmic organelles

present in eukaryotic cells. Many of thefunctions of such organelles are alsoperformed by poorly organised parts of thecytoplasm (see section 5.2.4). The chlorophyllin photosynthetic prokaryotic bacteria isassociated with membranous vesicles (baglike structures) but not with plastids as ineukaryotic cells (see section 5.2.5).

RibosomesPlasmamembrane

Cell wall

Nucleoid

Fig. 5.4: Prokaryotic cell

5.2.4 CYTOPLASM

When we look at the temporary mounts ofonion peel as well as human cheek cells, wecan see a large region of each cell enclosedby the cell membrane. This region takes upvery little stain. It is called the cytoplasm.The cytoplasm is the fluid content inside theplasma membrane. It also contains manyspecialised cell organelles. Each of theseorganelles performs a specific function for thecell.

Cell organelles are enclosed bymembranes. In prokaryotes, beside theabsence of a defined nuclear region, themembrane-bound cell organelles are alsoabsent. On the other hand, the eukaryoticcells have nuclear membrane as well asmembrane-enclosed organelles.

The significance of membranes can beillustrated with the example of viruses.Viruses lack any membranes and hence donot show characteristics of life until they entera living body and use its cell machinery tomultiply.

THE FUNDAMENTAL UNIT OF LIFE 63

uestion1. Fill in the gaps in the following

table illustrating differencesbetween prokaryotic andeukaryotic cells.

Prokaryotic Cell Eukaryotic Cell

1. Size : generally 1. Size: generallysmall ( 1-10 μm) large ( 5-100 μm)1 μm = 10–6 m

2. Nuclear region: 2. Nuclear region:_______________ well defined and_______________ surrounded by aand known as__ nuclear membrane

3. Chromosome: 3. More than onesingle chromosome

4. Membrane-bound 4. _______________cell organelles _______________absent _______________

5.2.5 CELL ORGANELLES

Every cell has a membrane around it to keepits own contents separate from the externalenvironment. Large and complex cells,including cells from multicellular organisms,need a lot of chemical activities to supporttheir complicated structure and function. Tokeep these activities of different kindsseparate from each other, these cells usemembrane-bound little structures (or‘organelles’) within themselves. This is one ofthe features of the eukaryotic cells thatdistinguish them from prokaryotic cells. Someof these organelles are visible only with anelectron microscope.

We have talked about the nucleus in aprevious section. Some important examplesof cell organelles which we will discuss noware: endoplasmic reticulum, Golgi apparatus,lysosomes, mitochondria, plastids andvacuoles. They are important because theycarry out some very crucial functions in cells.

5.2.5 (i) ENDOPLASMIC RETICULUM (ER)The endoplasmic reticulum (ER) is a largenetwork of membrane-bound tubes andsheets. It looks like long tubules or round oroblong bags (vesicles). The ER membrane issimilar in structure to the plasma membrane.There are two types of ER– rough endoplasmicreticulum (RER) and smooth endoplasmicreticulum (SER). RER looks rough under amicroscope because it has particles calledribosomes attached to its surface. Theribosomes, which are present in all activecells, are the sites of protein manufacture.The manufactured proteins are then sent tovarious places in the cell depending on need,using the ER. The SER helps in themanufacture of fat molecules, or lipids,important for cell function. Some of theseproteins and lipids help in building the cellmembrane. This process is known asmembrane biogenesis. Some other proteinsand lipids function as enzymes andhormones. Although the ER varies greatly inappearance in different cells, it always formsa network system.

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Fig. 5.5: Animal cell

Thus, one function of the ER is to serveas channels for the transport of materials(especially proteins) between various regionsof the cytoplasm or between the cytoplasmand the nucleus. The ER also functions as acytoplasmic framework providing a surface

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Camillo Golgi was bornat Corteno near Bresciain 1843. He studiedmedicine at theUniversity of Pavia. Aftergraduating in 1865, hecontinued to work inPavia at the Hospital ofSt. Matteo. At that timemost of his investigationswere concerned with the nervous system,In 1872 he accepted the post of ChiefMedical Officer at the Hospital for theChronically Sick at Abbiategrasso. He firststarted his investigations into the nervoussystem in a little kitchen of this hospital,which he had converted into a laboratory.However, the work of greatest importance,which Golgi carried out was a revolutionarymethod of staining individual nerve and cellstructures. This method is referred to asthe ‘black reaction’. This method uses aweak solution of silver nitrate and isparticularly valuable in tracing theprocesses and most delicate ramificationsof cells. All through his life, he continuedto work on these lines, modifying andimproving this technique. Golgi receivedthe highest honours and awards inrecognition of his work. He shared theNobel prize in 1906 with Santiago RamonyCajal for their work on the structure of thenervous system.

5.2.5 (iii) LYSOSOMES

Lysosomes are a kind of waste disposalsystem of the cell. Lysosomes help to keepthe cell clean by digesting any foreign materialas well as worn-out cell organelles. Foreignmaterials entering the cell, such as bacteriaor food, as well as old organelles end up inthe lysosomes, which break them up intosmall pieces. Lysosomes are able to do thisbecause they contain powerful digestiveenzymes capable of breaking down all organicmaterial. During the disturbance in cellularmetabolism, for example, when the cell gets

Fig. 5.6: Plant cell

for some of the biochemical activities of thecell. In the liver cells of the group of animalscalled vertebrates (see Chapter 7), SER playsa crucial role in detoxifying many poisons anddrugs.

5.2.5 (ii) GOLGI APPARATUS

The Golgi apparatus, first described byCamillo Golgi, consists of a system ofmembrane-bound vesicles arrangedapproximately parallel to each other in stackscalled cisterns. These membranes often haveconnections with the membranes of ER andtherefore constitute another portion of acomplex cellular membrane system.

The material synthesised near the ER ispackaged and dispatched to various targetsinside and outside the cell through the Golgiapparatus. Its functions include the storage,modification and packaging of products invesicles. In some cases, complex sugars maybe made from simple sugars in the Golgiapparatus. The Golgi apparatus is alsoinvolved in the formation of lysosomes [see5.2.5 (iii)].

THE FUNDAMENTAL UNIT OF LIFE 65

damaged, lysosomes may burst and theenzymes digest their own cell. Therefore,lysosomes are also known as the ‘suicidebags’ of a cell. Structurally, lysosomes aremembrane-bound sacs filled with digestiveenzymes. These enzymes are made by RER.

5.2.5 (iv) MITOCHONDRIA

Mitochondria are known as the powerhousesof the cell. The energy required for variouschemical activities needed for life is releasedby mitochondria in the form of ATP(Adenosine triphopshate) molecules. ATP isknown as the energy currency of the cell. Thebody uses energy stored in ATP for makingnew chemical compounds and for mechanicalwork. Mitochondria have two membranecoverings instead of just one. The outermembrane is very porous while the innermembrane is deeply folded. These folds createa large surface area for ATP-generatingchemical reactions.

Mitochondria are strange organelles in thesense that they have their own DNA andribosomes. Therefore, mitochondria are ableto make some of their own proteins.

5.2.5 (V) PLASTIDS

Plastids are present only in plant cells. Thereare two types of plastids – chromoplasts(coloured plastids) and leucoplasts (white orcolourless plastids). Plastids containing thepigment chlorophyll are known aschloroplasts. Chloroplasts are important forphotosynthesis in plants. Chloroplasts alsocontain various yellow or orange pigments inaddition to chlorophyll. Leucoplasts areprimarily organelles in which materials suchas starch, oils and protein granules arestored.

The internal organisation of the plastidsconsists of numerous membrane layersembedded in a material called the stroma.Plastids are similar to mitochondria inexternal structure. Like the mitochondria,plastids also have their own DNA andribosomes.

5.2.5 (vi) VACUOLES

Vacuoles are storage sacs for solid or liquidcontents. Vacuoles are small sized in animalcells while plant cells have very large vacuoles.The central vacuole of some plant cells mayoccupy 50-90% of the cell volume.

In plant cells vacuoles are full of cell sapand provide turgidity and rigidity to the cell.Many substances of importance in the life ofthe plant cell are stored in vacuoles. Theseinclude amino acids, sugars, various organicacids and some proteins. In single-celledorganisms like Amoeba, the food vacuolecontains the food items that the Amoeba hasconsumed. In some unicellular organisms,specialised vacuoles also play important rolesin expelling excess water and some wastesfrom the cell.

uestions1. Can you name the two organelles

we have studied that containtheir own genetic material?

2. If the organisation of a cell isdestroyed due to some physicalor chemical influence, what willhappen?

3. Why are lysosomes known assuicide bags?

4. Where are proteins synthesisedinside the cell?

Each cell thus acquires its structure andability to function because of the organisationof its membrane and organelles in specificways. The cell thus has a basic structuralorganisation. This helps the cells to performfunctions like respiration, obtaining nutrition,and clearing of waste material, or forming newproteins.

Thus, the cell is the fundamentalstructural unit of living organisms. It is alsothe basic functional unit of life.

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Whatyou havelearnt• The fundamental organisational unit of life is the cell.

• Cells are enclosed by a plasma membrane composed of lipidsand proteins.

• The cell membrane is an active part of the cell. It regulates themovement of materials between the ordered interior of the celland the outer environment.

• In plant cells, a cell wall composed mainly of cellulose is locatedoutside the cell membrane.

• The presence of the cell wall enables the cells of plants, fungiand bacteria to exist in hypotonic media without bursting.

• The nucleus in eukaryotes is separated from the cytoplasm bydouble-layered membrane and it directs the life processes ofthe cell.

• The ER functions both as a passageway for intracellulartransport and as a manufacturing surface.

• The Golgi apparatus consists of stacks of membrane-boundvesicles that function in the storage, modification and packagingof substances manufactured in the cell.

• Most plant cells have large membranous organelles calledplastids, which are of two types – chromoplasts and leucoplasts.

• Chromoplasts that contain chlorophyll are called chloroplastsand they perform photosynthesis.

• The primary function of leucoplasts is storage.

• Most mature plant cells have a large central vacuole that helpsto maintain the turgidity of the cell and stores importantsubstances including wastes.

• Prokaryotic cells have no membrane-bound organelles, theirchromosomes are composed of only nucleic acid, and they haveonly very small ribosomes as organelles.

Exercises1. Make a comparison and write down ways in which plant cells

are different from animal cells.

2. How is a prokaryotic cell different from a eukaryotic cell?

3. What would happen if the plasma membrane ruptures or breaksdown?

THE FUNDAMENTAL UNIT OF LIFE 67

4. What would happen to the life of a cell if there was no Golgiapparatus?

5. Which organelle is known as the powerhouse of the cell? Why?

6. Where do the lipids and proteins constituting the cell membraneget synthesised?

7. How does an Amoeba obtain its food?

8. What is osmosis?

9. Carry out the following osmosis experiment:

Take four peeled potato halves and scoos each one out to makepotato cups. One of these potato cups should be made from aboiled potato. Put each potato cup in a trough containing water.Now,

(a) Keep cup A empty

(b) Put one teaspoon sugar in cup B

(c) Put one teaspoon salt in cup C

(d) Put one teaspoon sugar in the boiled potato cup D.

Keep these for two hours. Then observe the four potato cupsand answer the following:

(i) Explain why water gathers in the hollowed portion ofB and C.

(ii) Why is potato A necessary for this experiment?

(iii) Explain why water does not gather in the hollowed outportions of A and D.