Biology Today - May 2018

Table : Characteristics of different classes of algae

Classes Structure Occurrence Major

pigments

Reserve

food

material

Reproduction

Vegetative Asexual Sexual

Chlorophyceae,

(Green algae)

e.g., Spirogyra,

Ulothrix

Unicellular to

heterotrichous

filaments. Cellulosic

cell wall, starch

sheathed pyrenoids,

motile cells with

2-4 equal flagella.

Mostly fresh

water a few are

marine. A marked

tendency towards

terrestrial

habitat.

Chl. a and b,

carotenes and

xanthophyll

True starch

and sugar

Fragmentation

or fission

Zoospores Isogamous

to advanced

oogamous

Phaeophyceae

(Brown algae)

e.g., Fucus,

Sargassum

Simple filamentous

to bulky

parenchymatous

with giant size,

external and internal

differentiation,

motile cells with 2

lateral flagella.

Mostly marine Fucoxanthin,

flavoxanthin,

b-carotenes,

Chl. a and c.

Laminarin,

mannitol

Fragmentation

is most

common

Zoospores,

tetraspores

etc.

Isogamous

to oogamous

Rhodophyceae

(Red algae) e.g.,

Polysiphonia,

Porphyra

Simple filamentous

to complex forms,

motile cells are not

known.

Few freshwater,

others are

marine.

Phycoerythrins,

phycocyanin,

allophycocyanin,

carotenoids, Chl.

a and d.

Floridean

starch

Uncommon,

except

unicellular

ones

Monospores,

carpospores,

polyspores

etc.

Advanced

oogamous

type

producing

special

carpospores.

Economic Importance of Algae

• Marine algae such as Laminaria, Sargassum are used as food.


• Algae are primary producers of food in large bodies of water and thus form
  the basis of food cycles of all aquatic animals.


• Agar obtained from Gelidium and Gracilaria is used as a culture medium.


• Algae such as Chlamydomonas, Chlorella are used in sewage oxidation tanks
  and provide aerobic conditions for disposal of sewage by decomposers.

BRYOPHYTES

• Bryophytes are most primitive non-vascular terrestrial plants of moist habitats
  in which a multicellular diploid sporophyte is completely dependent on an
  independent multicellular haploid gametophyte.


• They are called “amphibians of plant kingdom” because although they are terrestrial plants fundamentally but require
  presence of water to complete their life cycle.


• Bryophytes are divided into three classes - hepaticopsida (liverworts), anthocerotopsida (hornworts) and bryopsida (mosses).


• The dominant phase of the plant is a free living gametophyte.


• The gametophytes are either thalloid (not differentiated into true roots, stem and leaves) or leafy shoot having stem-like central
  axis and leaf-like appendages.


• The vascular tissues are completely absent.


• Rhizoids are present instead of roots which may be unicellular or multicellular.


• Vegetative reproduction occurs through fragmentation, tubers, gemmae, e.g., Marchantia, adventitious branches, e.g., Riccia.


• Two types of sex organs are present male antheridium and female archegonium. Both are multicellular and jacketed.


• Antheridium produces a number of flagellate male gametes called sperms or antherozoids.


• Archegonium is flask-shaped with tubular neck and a swollen venter.


• The venter encloses a venter cavity having a sterile venter canal cell and a fertile egg or oosphere.


• An external layer of water is essential for the swimming of male gametes to the archegonia.

2013

1

2

3

4

5

2014 2015 2016 2017

AIPMT/NEET

AIIMS

JIPMER

Analysis of various PMTs from 2013-2017