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Chlorarachniophytes, which belong to the phylum Cercozoa, contain a small nucleomorph, which is a relict of the algae's nucleus.

Euglenids, which belong to the phylum Euglenozoa, live primarily in fresh water and have chloroplasts with only three membranes.

The shape varies from plant to plant; they may be of discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon shaped.

However, the exact origin of the chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events.

The table below describes the composition of the three major groups of algae.

The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with red algae suggest a relationship there.

In the first three of these groups (Chromista), the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether the heterokonts, Haptophyta, and cryptomonads are in fact more closely related to each other than to other groups.

Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria.

Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago.

This is the first use of a biochemical criterion in plant systematics.

Harvey's four divisions are: red algae (Rhodospermae), brown algae (Melanospermae), green algae (Chlorospermae), and Diatomaceae.

Some retain plastids, but not chloroplasts, while others have lost plastids entirely.

Phylogeny based on plastid These algae have 'primary' chloroplasts, i.e.

The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and the stoneworts. One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria.

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