(37-A): Photosynthesis

INTRODUCTION

Photosynthesis is the process by which plants, some bacteria, and some protistans use the

energy from sunlight to produce sugar, which cellular respiration converts into ATP, the “fuel”

used by all living things. The conversion of unusable sunlight energy (solar energy) into

usable chemical energy, is associated with the actions of the green pigment chlorophyll. Most

of the time, the photosynthetic process uses water and releases the oxygen .We can write the

overall reaction of this process as :

The above chemical equation translates as: Six molecules of water plus six molecules of

carbon dioxide produce one molecule of sugar plus six molecules of oxygen.

STRUCTURE OF LEAF

Plants are the only photosynthetic organisms to have leaves (and not all plants have

leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic

cells.

The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the

leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf.

Water enters the root and is transported up to the leaves through specialized plant

cells known as xylem.

Land plants must guard against drying out (desiccation) and so have evolved

specialized structures known as stomata to allow gas to enter and leave the leaf.

Carbon dioxide cannot pass through the protective waxy layer covering the leaf

(cuticle), but it can enter the leaf through an opening (the stoma; plural = stomata;

Greek for hole) flanked by two guard cells.

Likewise, oxygen produced during photosynthesis can only pass out of the leaf

through the opened stomata.

Unfortunately for the plant, while these gases are moving between the inside and

outside of the leaf, a great deal water is also lost.

Cottonwood trees, for example, will lose 100 gallons of water per hour during hot

desert days. Carbon dioxide enters single-celled and aquatic autotrophs through no

specialized structures.

CHLOROPHYLL AND ACCESSORY PIGMENTS

A pigment is any substance that absorbs light. The color of the pigment comes from

the wavelengths of light reflected (in other words, those not absorbed).

Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all

wavelengths of visible light except green, which it reflects to be detected by our eyes.

Black pigments absorb all of the wavelengths that strike them.

White pigments/lighter colors reflect all or almost all of the energy striking them.

Pigments have their own characteristic absorption spectra, the absorption pattern of a

given pigment.

Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among

plants and other photosynthetic organisms. All photosynthetic organisms (plants,

certain protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a.

Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory

pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls,

and carotenoids (such as beta-carotene). Chlorophyll absorbs its energy from the

Violet- Blue and Reddish orange-Red wavelengths, and little from the intermediate

(Green-Yellow-Orange) wavelengths.

STAGES OF PHOTOSYNTHESIS

Photosynthesis is a two stage process.

The first process is the Light Dependent Process (Light Reactions), requires the

direct energy of light to make energy carrier molecules that are used in the second

process. The Light Independent Process (or Dark Reactions) occurs when the

products of the Light Reaction are used to form C-C covalent bonds of carbohydrates.

The Dark Reactions can usually occur in the dark, if the energy carriers from the light

process are present. Recent evidence suggests that a major enzyme of the Dark

Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of

a misnomer. The Light Reactions occur in the grana and the Dark Reactions take

place in the stroma of the chloroplasts.

THE CARBON CYCLE

Plants may be viewed as carbon sinks, removing carbon dioxide from the atmosphere

and oceans by fixing it into organic chemicals. Plants also produce some carbon

dioxide by their respiration, but this is quickly used by photosynthesis. Plants also

convert energy from light into chemical energy of C-C covalent bonds. Animals are

carbon dioxide producers that derive their energy from carbohydrates and other

chemicals produced by plants by the process of photosynthesis.

The balance between the plant carbon dioxide removal and animal carbon dioxide

generation is equalized also by the formation of carbonates in the oceans. This

removes excess carbon dioxide from the air and water (both of which are in

equilibrium with regard to carbon dioxide). Fossil fuels, such as petroleum and coal,

as well as more recent fuels such as peat and wood generate carbon dioxide when

burned. Fossil fuels are formed ultimately by organic processes, and represent also a

tremendous carbon sink. Human activity has greatly increased the concentration of

carbon dioxide in air.

(37-B): Digestion in Plants

FACTS FROM NCERT: The leaves have a green pigment called chlorophyll. It helps leaves to capture the

energy of the sunlight.

Besides leaves, photosynthesis also takes place in green stems and green

branches. Desert plants have green stems which carry out photosynthesis.

Algae are nitrogenous substance which contains nitrogen.

A plant like cascuta (amarbell) does not have chlorophyll. It takes readymade food

from the plant on which it is climbing. The plant on which it climbs is called a host.

Since it deprives the host of valuable nutrients. It is called a parasite.

Insectivorous plants:- these are insect eating plants. The insects is digested by the

digestive juices secreted in the pitcher, the pitcher like structure is the modified part of

the leaf.

Fungi have a different mode of nutrition. They secrete digestive juices on the dead

conert it into a solution. Then they absorb the nutrient from it. This mode is called

saprotrophic nutrition. Plants which use saprotrophic mode of nutrition are called

saprotrophs.  Eg: A mushroom as a saprotrophs.

Some organisms live together and share shelter and nutrients. This is called

symbiotic relationship. Organism like algae and fungus live together. The fungus

provides shelter. Water and minerals to the algae and in return the algae provides

food which is prepared by photosynthesis.

Rhizobium cannot make its own food. So it lives in the roots of gram, peas, moong

beans and other legumes and provides them nitrogen in return , the plants provide

food and shelter to the bacteria, they have a symbiotic relationship.

(37-C): Plant Reproduction

INTRODUCTION

Animal life cycles have meiosis followed immediately by gametogenesis. Gametes are

produced directly by meiosis. Male gametes are sperm. Female gametes are eggs or ova.

The plant life cycle has mitosis occurring in spores, produced by meiosis, that germinate into

the gametophyte hase. Gametophyte size ranges from three cells (in pollen) to several million

(in a “lower plant” such as moss). Alternation of generations occurs in plants, where the

sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces

spores by meiosis within a sporangium. The gametophyte phase produces gametes bymitosis

within an antheridium (producing sperm) and or archegonia (producing eggs). Within the plant

kingdom the dominance of phases varies. Nonvascular plants, the mosses and liverworts,

have the gametophyte phase dominant. Vascular plants show a progression of increasing

sporophyte dominance from the ferns and “fern allies” to angiosperms.

FLOWERING PLANTS

Flowering plants, the angiosperms, were the last of the seed plant groups to evolve,

appearing over 100million years ago during the middle of the Age of Dinosaurs (late

Jurassic). All flowering plants produce flowers and if they are sexually reproductive, they

produce a diploid zygote and triploid endosperm.

FLOWERS

Flowers are collections of reproductive and sterile tissue arranged in a tight whorled array

having very short internodes. Sterile parts of flowers are the sepals and petals. When these

are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the

stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to

as the pistil, the female parts collectively termed the gynoecium).

STAMENS

The individual units of the androecium are the stamens, which consist of a filament which

supports the anther. The anther contains four microsporangia within which microspores

(pollen) are produced by meiosis. Stamens are thought to represent modified sporophylls

(leaves with sporangia on their upper surface

POLLEN

Pollen grains (fromthe greek palynos for dust or pollen) contain the male gametophyte

(microgametophyte) phase of the plant. Pollen grains are produced by meiosis of

microsporemother cells that are located along the inner edge of the anther sacs

(microsporangia). The outer part of the pollen is the exine, which is composed of a complex

polysaccharide, sporopollenin. Inside the pollen are two (or, at most, three) cells that

comprise the male gametophyte. The tube cell (also referred to as the tube nucleus) develops

into the pollen tube. The germ cell divides by mitosis to produce two spermcells. Division of

the germcell can occur before or after pollination.

POLLINATION

The transfer of pollen from the anther to the female stigma is termed pollination. This is

accomplished by a variety of methods. Entomo-phyly is the transfer of pollen by an insect.

Anemophyly is the transfer of pollen by wind. Other pollinators include birds, bats, water, and

humans. Some flowers (for example garden peas) develop in such a way as to pollinate

themselves. Others have mechanisms to ensure pollination with another flower. Flower color

is thought to indicate the nature of pollinator: red petals are thought to attract birds, yellow for

bees, and white for moths. Wind pollinated flowers have reduced petals, suchas oaks and

grasses.

GYNOECIUM

The gynoecium consists of the stigma, style, and ovary containing one or more ovules. These

three structures are often termed a pistil or carpel. In many plants, the pistils will fuse for all or

part of their length.

THE STIGMA AND STYLE

 The stigma functions as a receptive surface on which pollen lands and germinates its pollen

tube. Corn silk is part stigma, part style. The style serves to move the stigma some distance

from

the ovary. This distance is species specific.

THE OVARY

The ovary contains one or more ovules, which in turn contain one female gametophyte, also

referred to in angiosperms as the embryo sac. Some plants, such as cherry, have only a

single ovary which produces two ovules. Only one ovule will develop into a seed.

THE GAMETOPHYTES

The male gametophyte develops inside the pollen grain. The female gametophyte develops

inside the ovule. In flowering plants, gametophyte phases are reduced to a few cells

dependant for their nutrition on the sporophyte phase. This is the reverse of the pattern seen

in the nonvascular plant groups liverworts, mosses, and hornworts (the Bryophyta).

Angiosperm male gametophytes have two haploid nuclei (the germnucleus and tube nucleus)

contained within the exine of the pollen grain (or microspore). Female gametophytes of

flowering plants develop within the ovule (megaspore) contained within an ovary at the base

of the pistil of the flower. There are usually eight (haploid) cells in the female gametophyte: a)

one egg, two synergids flanking the egg (located at the micropyle end of the embryo sac) b)

two polar nuclei in the center of the embryo sac; and three antipodal cells (at the opposite

end  of the embryo sac from the egg).

DOUBLE FERTILIZATION

The process of pollination being accomplished, the pollen tube grows through the stigma and

style toward the ovules in the ovary. The germ cell in the pollen grain divides and releases

two sperm cells which move down the pollen tube. Once the tip of the tube reaches the

micropyle end of the embryo sac, the tube grows through into the embryo sac through one of

the synergids which flank the egg. One sperm cell fuses with the egg, producing the zygote

which will later develop into the next-generation sporophyte. The second sperm fuses with the

two polar bodies located in the center of the sac, producing the nutritive triploid endosperm

tissue that will provide energy for the embryo’s growth and development. FRUIT The ovary

wall, after fertilization has occurred, develops into a fruit. Fruits may be fleshy, hard, multiple

or single. Note:- View the Seeds of Life site for illustrations and information about fruits and

seeds. Seeds germinate, and the embryo grows into the next generation sporophyte.

VEGETATIVE PROPAGATION

Many plants also have an asexual method of reproduction. Often some species, such as

many orchids, are more frequently propagated vegetative than via seeds. Tubers are fleshy

underground stems, as in the Irish potato. Leaflets are sections of leaf will develop roots and

drop off the plant, effectively cloning the plant. Runners are shoots running along or over the

surface of the ground that will sprout a plantlet, which upon settling to the ground develop into

a new independent 

FACTS FROM NCERT Flowers are the reproductive parts of a plant.

In asexual reproduction plants can give rise to new plants without seeds.

In sexual reproduction, new plants are obtained from seeds.

In asexual reproduction the reproduction is happen through the negative parts of the

plant, it is known as negative propagation.

Yeast reproduces through budding.

Algae grow and multiply rapidly by fragmentation. An algae break up into two or more

fragments. These fragments or pieces grow into new individuals.

Fungi on bread pieces grow from spores which are present in the air, spores are

asexual     reproductive bodies

Under favourable condition a spore germinates and develops into a new individual.

 Plants such as mass and farness also reproduce by means of spores.

In sexual reproduction the stamens are the male reproductive part. The pistil is the

female reproduction part.

The flowers which contain either only the pistil or only the stamens are called

unisexual flowers

The flowers which both stamens and pistil are called bisexual flowers.

Corn, papaya and cucumber produce unisexual flowers.

Spores are covered by a hard protective coat to with stand unfavorable conditions

such as high temperature and low humidity.

Mustard, rose and petunia have bisexual flowers.

Another contains pollen grains which produce male gametes.

The female gamete or the egg is formed in an ovule.

The transfer of pollen from the other to the stigma of a flower is called pollination.

When the pollen of a flower lands on the stigma of another flower of the same plant or

that of a different plant of the same kind is called crosspollination.

After fertilization the ovary grows into a fruit, the fruit is the ripened ovary.