Lecture 6: Chlorophyta (Green Algae) PDF

lecture 6 Chlorophyta (green algae)  6-Chlorophyta (green algae) They are unicellular, colonial or multicellular. The pigments of green algae are chlorophyll a, b and carotenoids. Most of the green algae have flagella. The cell walls of green algae have cellulose. Green a...

lecture 6 Chlorophyta (green algae)  6-Chlorophyta (green algae) They are unicellular, colonial or multicellular. The pigments of green algae are chlorophyll a, b and carotenoids. Most of the green algae have flagella. The cell walls of green algae have cellulose. Green algae have many characters common to plant. They have similar pigments, reserve products, and cell walls. Due to these similarities with plants, it is generally believed that plant have evolved from ancestral green algae. The RNA sequencing of plants and green algae also supports this idea. It indicates that the green algae and plants form a monophyletic lineage. Chlamydomonas (unicellular), Desmids, Volvox (colonial algae), Spirogyra (filamentous algae) Ulva (sheet like multicellular body) and Chlorella.  Division; Chlorophyta Class: Chlorophyta Class: Chlorophyceae Order: Volvocales 1- Chlamydomonas Family: Chlamydomonacea It is a minute single-celled, unicellular, flagellated green alga, Genus: Chlamydomonas sp. Shape of the thallus: round, oval, or pear-shaped. they have narrow anterior and a broad posterior end. The cell wall: is thin, firm and is made up of rigid cellulose, The chloroplast is cup-shaped and has a red pigmented eye spot or stigma, considered to be light sensitive\ Photoreceptive organ. The alga has 2 equal anterior flagella, 2 anterior contractile vacuoles that perform osmoregulatory and excretory functions, and a single nucleus placed in the anterior part. The pyrenoid body functions as a center for the formation of starch then changes into an oil. Although pyrenoids are widely distributed in algae, they do not occur in most land plants. While Chlamydomonas are green, photosynthetic cells, they may get some of their nutrients readymade from the surrounding water. 1- Chlamydomonas cell structure: Cilium: A pair of flagella are on the anterior of the Chlamydomonas. They are typically 10- 12 micrometers long. They are a greater length than the thallus/body and originated from a blepharoplast which comes through a small opening in the cell wall. Cell Wall: The exterior of the wall is smooth, thin, and contains cellulose. At the anterior end of the cell, the cell wall extends itself to make an apical papilla. The cell wall has many layers and is complex in structure. Plasma Membrane: Just inside the cell wall is where you’ll find the plasma membrane. In Chlamydomonas, the plasma membrane is divided by an opaque area. Its role is to move nutrients throughout the cell and get rid of toxic substances from the cell. The membrane also provides rigidity and protection. Cytoplasm: Located in the middle of the cell wall and chloroplast, it’s a structure that houses the nucleus, mitochondria, endoplasmic reticulum, dictyosomes, ribosomes, and more. The dictyosomes and Golgi apparatus are located around the nuclear and have small vesicles. Golgi Body: Also known as the Golgi apparatus, is an organelle that assists with packaging proteins and lipids. It has a stacked appearance of its membranes.  Nucleus: The singular nucleus is big and dark. It stores all of the genetic information of the cell. Contractile Vacuoles: Chlamydomonas have two contractile vacuoles, one at each flagellum. They assist with excretion and osmoregulation. Chloroplast: The Chlamydomonas’ chloroplast is big in size and cup-shaped. They can be other sizes and shapes depending on the species. The chloroplast is the organelle that helps convert sunlight into usable energy and sugars through photosynthesis. Pyrenoids: These help with the synthesis of starch in the Chlamydomonas. There are approximately two to six thylakoids that join within the chloroplast. The pyrenoid can be thought of as a small compartment inside of the chloroplasts that is unique to algae. Eyespot: Located at the anterior end of the Chlamydomonas, the eyespot (also known as the stigma) is uniquely colored orange. It is photoreceptive and helps direct the movement of the flagella. Two to three pigmented plates curve and help orientate the Chlamydomonas, which also contain carotenoids. Reproduction and Life Cycle of Chlamydomonas Reproduction in Chlamydomonas sp. Sexual reproduction Asexual reproduction  Asexual Reproduction in Chlamydomonas In asexual reproduction, the cell undergoes mitotic divisions within the cell wall. There may be as few as two divisions or as many as four. The daughter cells are at first contained within the parent cell wall; after a short time, however, the wall breaks down, and the miniature Chlamydomonads escape and grow to full size.  Sexual reproduction is isogamous, anisogamous, or oogamous as ❖the gametes are walled and liberated from the mother cell fused together then form a zygote (2n) with smooth or ornamented walls. ❖The zygote divided by meiosis to rise from 2-8 zoospores (n) as new individuals.  Sexual reproduction in Chlamydomonas sp. 1-Isogamous 2-Anisogamous 3- Oogamous  Sexual Reproduction in Chlamydomonas Isogamy: In isogamy, Chlamydomonas produces gametes that are morphologically similar (same in size and shape) but physiologically different. The gametes of the opposite strains come in contact, and a fusion of their cytoplasm and nucleus takes place. A quadriflagellate zygote (4 flagella) is formed, which later on loses the extra flagella, secretes a thick wall, and is known as a zygospore. Anisogamy: In anisogamy, gametes of two different sizes are produced. The two gametes come together to form a diploid zygote (2n) which is later converted into zygospores.  Sexual Reproduction in Chlamydomonas Oogamy: In oogamy, the vegetative thallus of the Chlamydomonas becomes non-motile and acts as a female gamete or macrogamete or egg. The male gamete is formed by the division of protoplasts and is known as microgametes. The microgamete and macrogamete come together, and a zygote is formed. Zygote or Zygospores The zygote is a resting diploid spore (2n) that secretes thick, ornamented walls around itself. It accumulates a large amount of starches and oils. The zygospores tolerate a long stretch of unfavorable conditions and start to germinate as soon as favorable conditions are restored. It divides by meiosis to form four haploid nuclei that develop into separate individuals. The Life cycle of Chlamydomonas  2-Volvox sp. Classification of Volvox sp. Kingdom: Protista The Volvox is a mobile colony of green algae, Division: Chlorophyta between 500 and 50,000 cells within the colony, Class: Chlorophyceae Order: Volvocales and all cells are contained within a gelatinous wall. Family: Volvocaceae Genus: Volvox sp. These colonies are spherical or oval hollow in nature and may be larger than a pinhead. For movement, each cell contains two flagella, which allows the organism to move through the water. For food, the Volvox cells contain chlorophyll and are able to make their own food through photosynthesis.  Reproduction in Volvox: Volvox reproduces Asexually and Sexually. 1- Asexual reproduction takes place under favorable conditions by forming gonidial cells during spring and early summer. In Volvox mostly the cells of the posterior part of the colony take part in reproduction. These reproductive cells can be recognized by their larger size, prominent nuclei, dense granular cytoplasm, more pyrenoids, and absence of flagella. 2- The Sexual reproduction by oogamous type forming antheridia and oogonia Asexual Reproduction: During asexual reproduction some cells of the posterior part of colony become reproductive. These cells enlarge up to ten times, become rounded and lose flagella. These cells are called gonidia (Sing, gonidium). The gonidia lose eye spot. Pyrenoids increase in number. The gonidia are pushed towards the interior of the colony. The first division of gonidium is longitudinal to the plane of coenobium and this forms 2 cells. The second division is also longitudinal and at a right angle to the first, forming 4 cells. By the third longitudinal division, all four cells divide to make 8 cells of which 4 cells are central and 4 are peripheral. These 8 cells are arranged in curved plate-like structures and are called plakea stages. Each of these 8 cells divides by longitudinal division forming 16 cells arranged in the form of a hollow- sphere. The sphere is open on the exterior side as a small aperture called a phialopore. The cells at this stage continue to divide till the number of cells reaches the characteristic of that species. The cells at this stage are naked and in close contact with each other. The pointed anterior end of cells is directed towards the inside. The next step is called the inversion of the colony. As cells become opposite in direction, their anterior pointed end has to face the periphery of the colony. Sexual reproduction The sexual reproduction in Volvox is oogamous type. Reproductive cells mostly differentiate in the posterior part of the colony. These cells enlarge, lose flagella, and are called gametangia. The male reproductive cells are called antheridia or androgonidia and female reproductive cells are called oogonia or gynogonidia. Formation of Antherizoids in Volvox sp. Oogonium fertilization forming Asexual reproduction in Volvox sp. the zygote in Volvox sp. Fertilization of Volvox: After liberation from antheridium, the antherozoids swim freely on surface of water. Due to chemotactic response the antherozoids reach the oogonia. The Volvox life cycle  Division: Chlorophyta 3-Spirogyra sp. Class III : Charophyceae 1-Spirogyrs occur in freshwater bodies such as ditches, ponds, and Order: Zygnematales lakes. It is commonly called water silk or pond scum because of the Family: Zygnemataceae slippery touch of its thread-like filaments. Genus: Spirogyra sp. 2- It is a multicellular filamentous green alga covered by a mucilagenous sheath. 3- Each filament is unbranched and consists of cylindrical cells placed end to end. 4- The cell wall is two-layered and is made up of cellulose and pectin. 5- Cytoplasm lies in the periphery of the cell enclosing a vacuole in the center. 6- One or more ribbon-shaped spirally arranged chloroplasts are present in the cytoplasm and each chloroplast bears pyrenoids. 7- A single nucleus is stranded in the center of the vacuole by cytoplasmic strands.  Spirogyra reproduction 1-Asexual reproduction The cells undergo simple mitotic division to produce new cells. They need favorable conditions for Asexual reproduction to take place. The zygospore formation in Spirogyra during Asexual reproduction is an interesting feature. These usually are dormant for some while and then they further divide and give rise to new spirogyral cells.  Spirogyra reproduction 2-Sexual reproduction Spirogyra conjugates, a form of sexual reproduction. Spirogyra and its relatives can be found during summer as pond scum, floating mats in ponds. Two filaments of Spirogyra form conjugation tubes. The contents of one cell pass through the tube and fuse with a cell from the other filament. Conjugation is of 2 types lateral (between adjacent cells in the same filament) or scalariform (between cells of 2 different filaments). After fertilization, the zygote is formed with resistance thick walls around it forming zygospore. Under favorable conditions, the zygospore germinates its nucleus divided by meiosis and form 4 haploid nuclei 3 degenerate and one remain to form the nucleus of the first cell of the new filament and then divides by mitosis giving a new filament. Fertilization occurs and a zygote is formed. This develops into a thick-walled resistant zygospore. These zygospores can withstand harsh conditions. They can survive the cold winter or when a pond dries up. It may take a long time before new filaments start to grow. Conjugation: Lateral sexual reproduction Conjugation: Scalariform sexual reproduction ‫االقتران الجنسي السلمي في طحلب السبايروجيرا‬ Lateral conjugation