Cell Walls and Surfaces, Reproduction, Photosynthesis


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Those that were eaten fresh were harvested locally and consumed in short order. As seaweeds can be dried and, in that form, kept for a long time and transported easily, they were recognized early on as a valuable foodstuff and became a trading commodity. Over time, the demand for seaweeds, for a multiplicity of purposes, grew so great that for many centuries they have been actively cultivated, especially in the Far East.

The life history of algae is complicated, and this is what really differentiates them from plants. In fact, macroalgae can pass through life stages so distinct that, in the past, they have been mistaken for separate species. Seaweed reproduction can involve either exclusively sexual or asexual phases, while some species display an alternation of generations that involves both in succession.

In the former, the seaweed produces gametes egg and sperm cells with a single set of chromosomes and, in the latter, spores containing two sets of chromosomes. Some species can also reproduce asexually by fragmentation—that is, the blades shed small pieces that develop into completely independent organisms. The the red alga Porphyra , used for making Japanese nori, has a highly complex life cycle. Asexual reproduction allows for fast propagation of the species but carries with it an inherent danger of limited genetic variation.

Sexual reproduction ensures better genetic variation, but it leaves the species that depend on this method of reproduction with an enormous match-making problem, as the egg and sperm cells need to find each other in water that is often turbulent. Some species solve the match-making problem by equipping the reproductive cells with light-sensitive eyespots or with flagella so that they can swim. Others make use of chemical substances, known as pheromones or sex attractants. These are secreted and released by egg cells and serve to attract the sperm. Some species for example, the large seaweed masses in the Sargasso Sea secrete enormous quantities of slime, which ensures that the egg and sperm cells stick close to each other and do not go astray.

A newly discovered species of red seaweed is now named Porphyra migitae. The red alga Porphyra has an especially complicated life cycle, with a fascinating aspect that merits further discussion because of the interesting history associated with its discovery. It relates directly to the cultivation of Porphyra for the production of nori, which is especially widely used in Japanese cuisine—most familiarly, as for the wrapping for maki rolls See the recipe in the caption for the nori roll image below.

The blades used in nori production grow while the seaweed is in the generation that reproduces sexually, although the organism itself can actually develop asexually from spores. The blades produce egg cells and sperm cells. The egg cells remain on the blades, where they are fertilized by the sperm cells. The fertilized eggs can then form a new type of spores, which are released.

These spores germinate into a calcium-boring filament stage that can grow in the shells of dead bivalves, such as oysters and clams, in the process developing spots that give the organism a pinkish sheen. Until the s it was thought that this sexual stage was actually an entirely separate species of alga, given the name Conchocelis rosea.

Without an understanding of the true life cycle, it was not possible to grow Porphyra effectively in aquaculture. No one knew where the spores for the fully grown Porphyra originated. This was the main reason for the recurring problems experienced by the Japanese seaweed fishers in their attempts to cultivate Porphyra in a predictable manner. It was an English alga researcher, Dr.

Kathleen Mary Drew-Baker, who discovered the secret of the sexual segment of the Porphyra life cycle. Drew-Baker was unaware of the difficulties of the seaweed fishers. Instead, she was preoccupied with shedding light on the mystery of why the species of laver Porphyra umbilicalis that grew around the coast of England seemed to disappear during the summer, reappearing again only toward the end of autumn.

She tried without success to germinate spores that she had collected. They would even grow on an eggshell. A few months later, the resulting small, roseate sprouts produced their own spores that, in turn, could germinate and develop into the well-known large purple laver. Drew-Baker published her results in Shortly thereafter the Japanese phycologist Sokichi Segawa repeated her experiments using local varieties of Porphyra and found that they behaved in the same way as the English species. The mystery was solved and the results were quickly put to use in Japan.

Drew-Baker died at a relatively young age in , apparently unaware that her curiosity and seminal research had laid the foundations for the development of the most valuable aquaculture industry in the world.

Cells to systems - Revision 5 - KS3 Biology - BBC Bitesize

As in green plants, photosynthesis enables seaweeds to convert sunlight into chemical energy, which is then bound by the formation of the sugar glucose. The photosynthetic process uses up carbon dioxide, which is thereby removed from the water. In addition, phosphorous, a variety of minerals, and especially nitrogen are required. Oxygen is formed as a by-product, dissolved in the water, and then released into the atmosphere.

This by-product is of fundamental importance for those organisms that must, like humans, have oxygen to be able to breathe. Photosynthesis can even, to a certain extent, be carried out when seaweeds are exposed to air and partially dehydrated. They now run Maine Coast Sea Vegetables, a company which has its own building and 20 employees who transform the locally harvested seaweeds into more than 20 different products. The raw material for this business is delivered by about 60 seaweed harvesters who work along the coasts of Maine and Nova Scotia, where algae are found in abundance.

Shep trains the harvesters himself. It is of utmost importance to him that they understand the principles of collecting the different types of marine algae sustainably so that they do the least harm to the environment. Maine Coast Sea Vegetables processes about 50 tons of dried seaweeds annually, of which about 60 percent is the dulse for which the company is especially famous. Eating dulse is an old tradition in Maine, brought to its shores by settlers from Wales, Ireland, and Scotland. I have become a great fan of their applewood smoked dulse; I eat it as if it were candy.

1. Introduction

When dried dulse is brought to the factory, it is sorted by hand, and epiphytes, small crustaceans, and bivalves are picked off. The bone-dry dulse is placed in a sealed room to reabsorb some moisture and then left to ripen for a couple of weeks. In tightly sealed packages, the chewy blades have a shelf life of about a year. During the night, when the light level is low, photosynthesis stops and the seaweeds begin to take in oxygen, burn glucose, and give off carbon dioxide. Under normal conditions, photosynthesis is the dominant process, allowing the seaweeds to build up their carbohydrate content.

To the extent that they have access to light in the water, seaweeds actually utilize sunlight more efficiently than terrestrial plants. Marine algae are a much better source of iron than foods such as spinach and egg yolks. The red macroalgae normally grow at the greatest depths, typically as far as 30 meters down, the green macroalgae thrive in shallow water, and the brown algae in between.

This distribution of species according to the depth of the water is somewhat imprecise, however; a given species can be found at a location where there are optimal conditions with respect to substrate, nutritional elements, temperature, and light. In exceptionally clear water, one can find seaweeds growing as far as meters below the surface of the sea. It is said that the record is held by a calcareous red alga that was found at a depth of meters, where only 0. Even though the waters at that depth may appear pitch-dark to human eyes, there is still sufficient light to allow the alga to photosynthesize.

In turbid waters, seaweeds grow only in the top, well-lit layers of water, if at all. Formerly it was thought that seaweed species had adapted to their habitat by having pigments that were sensitive to the different wavelengths of the light spectrum. In this way they could take advantage of precisely that part of the spectrum that penetrated to the depths at which they lived. For example, the blue and violet wavelengths reach greater depths. The red algae that live in these waters must contain pigments that absorb blue and violet light and, as a consequence, appear to have the complementary color red.

Experiments have since shown that this otherwise elegant relationship does not always hold true. Given that all the substances that seaweeds need in order to survive are dissolved in the water, macroalgae, unlike plants, have no need of roots, stems, or real leaves.

Prokaryote structure

Nutrients and gases are exchanged directly across the surface of the seaweed by diffusion and active transport. In some species there is no meaningful differentiation, and each cell draws its supply of nutrients from the surrounding water. On the other hand, specialized cell types and tissues that assist in the distribution of nutrition within the organism can be found in a number of brown macroalgae. Access to nitrogen is an important limiting factor in seaweed growth, particularly for green algae. The increasing runoff into the oceans of fertilizer-related nitrogen from fields and streams has created favorable conditions for the growth of algae, especially during the summer when it is warm and the days are long.

Omelette tamago-yaki with Nori 1 sheet of nori seaweed 3 eggs mirin sweet rice wine salt and sugar 1. Crack the eggs into a bowl. Add a little salt, sugar, and mirin optional and whisk everything together lightly with a fork. Heat a pan that has been greased with a tiny amount of fat, preferably one that has virtually no flavor of its own. Pour the egg mixture into the pan a little at a time over low heat.

Place the nori sheet on the wooden surface and, using chopsticks or a wooden spatula, fold the set egg mixture together on itself several times to create a flat, layered omelette tamago. Remove the omelette from the pan and press itinto shape with a bamboo rolling mat, which will imprint a nice surface texture on it. Different species of seaweeds avail themselves of a variety of strategies in order to grow. In sea lettuce Ulva lactuca , the cells all undergo division more or less randomly throughout the organism. Other species, among them several types of brown algae, have a growth zone at the end of the stipe and at the bottom of the blade; this is where an existing blade grows and new blades are formed.

The oldest blades are outermost, eventually wearing down and falling off as the seaweed ages. As a result, the stipe can be several years old, while the blades are annuals. This growth mechanism allows the seaweed to protect itself from becoming overgrown by smaller algae, called epiphytes, which fasten on to it.


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On certain seaweed species, the epiphytes are found overwhelmingly on the stipes, which can become covered with them, while the blades retain a smooth surface as long as they are young and still growing. Finally, some types of seaweeds, such as bladder wrack Fucus vesiculosus and the majority of the red algae, grow at the extremities of the blades. The overall effect of seaweeds on the global ecosystem is enormous.

It is estimated that all algae, including the phytoplankton, are jointly responsible for producing 90 percent of the oxygen in the atmosphere and up to 80 percent of the organic matter on Earth.

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We can compare their output with that of plants by looking at the amount of organic carbon generated per square meter on an annual basis. Macroalgae can produce between 2 and 14 kilograms, whereas terrestrial plants, such as trees and grasses in temperate climates, and microalgae can generate only about 1 kilogram. The vast productive capacity of macroalgae can possibly be best illustrated by the fact that the largest brown algae can grow up to half a meter a day.

That amounts to a couple of centimeters an hour! Seaweeds are made up of a special combination of substances, which are very different from the ones typically found in terrestrial plants and which allow them to play a distinctive role in human nutrition. Most notably, the mineral content of seaweeds is 10 times as great as that found in plants grown in soil; as a consequence, people who regularly eat seaweeds seldom suffer from mineral deficiencies. In addition, marine algae are endowed with a wide range of trace elements and vitamins. Because they contain a large volume of soluble and insoluble dietary fiber, which are either slightly, or else completely, indigestible, seaweeds also have a low calorie count.

A wild strain of Chondrus crispus , or Hana-Tsunomata in Japanese, appeals to both the eye and the palate. This seaweed has a distinct crunchy texture and a milder taste than most other sea vegetables. Its flamboyant colors—pink, green, and yellow—are completely natural. Marine algae possess a fantastic ability to take up and concentrate certain substances from seawater. Reproduction for most dinoflagellates is asexual, through simple division of cells following mitosis. The dinoflagellates are important constituents of plankton, and as such are primary food sources in warmer oceans.

Many forms are phosphorescent; they are largely responsible for the phosphorescence visible at night in tropical seas. There are approximately species of dinoflagellates.

Introduction

Chrysophyta : large group of eukariotyes algae commonly called golden algae , found mostly in freshwater. Originally they were taken to include all such forms except the diatoms and multicellular brown algae, but since then they have been divided into several different groups based on pigmentation and cell structure.

In many chrysophytes the cell walls are composed of cellulose with large quantities of silica. Formerly classified as plants, they contain the photosynthetic pigments chlorophyll a and c. Under some circumstances they will reproduce sexually, but the usual form of reproduction is cell division.

Phaeophyta : phylum of the kingdom protista consisting of those organisms commonly called brown algae. Many of the world's familiar seaweeds are members of phaeophyta. Like the chrysophytes brown algae derive their color from the presence, in the cell chloroplasts, of several brownish carotenoid pigments, as fucoxathin. With only a few exceptions, brown algae are marine, growing in the colder oceans of the world, many in the tidal zone, where they are subjected to great stress from wave action; others grow in deep water.

There are approximately species of phaeophyta. Rhodophyta : phylum of the kingdom protista consisting of the photosynthetic organisms commonly known as red algae. Members of the division have a characteristic clear red or purplish color imparted by accessory pigments called phycobilins.

The red algae are multicellular and are characterized by a great deal of branching, but without differentiation into complex tissues. Most of the world's seaweeds belong to this group. Although red algae are found in all oceans, they are most common in warm-temperate and tropical climates, where they may occur at greater depths than any other photosynthetic organisms. Most of the coralline algae, which secrete calcium carbonate and play a major role in building reefs, belong here.

Red algae are a traditional part of oriental cuisine. There are known marine species of red algae; a few species occur in freshwater. C yanobacteria : phylum of prokaryotic aguatic bacteria that obtain their energy through photosynthesis. They are often referred to as blue-green algae , even though it is now known that they are not related to any of the other algal groups, which are all eukaryotes.

Cyanobacteria may be single-celled or colonial. Depending upon the species and environmental conditions, colonies may form filaments, sheets or even hollow balls.

Some filamentous colonies show the ability to differentiate into three different cell types. Despite their name, different species can be red, brown, or yellow; blooms dense masses on the surface of a body of water of a red species are said to have given the Red Sea its name. There are two main sorts of pigmentation. Most cyanobacteria contain chlorophyll a , together with various proteins called phycobilins, which give the cells a typical blue-green to grayish-brown colour.

A few genera, however, lack phycobilins and have chlorophyll b as well as a , giving them a bright green colour. Unlike bacteria, which are heterotrophic decomposers of the wastes and bodies of other organisms, cyanobacteria contain the green pigment chlorophyll as well as other pigments , which traps the energy of sunlight and enables these organisms to carry on photosynthesis. Cyanobacteria are thus autotrophic producers of their own food from simple raw materials.

Nitrogen-fixing cyanobacteria need only nitrogen and carbon dioxide to live: they are able to fix nitrogen gas, which cannot be absorbed by plants, into ammonia NH 3 , nitrites NO 2 or nitrates NO 3 , which can be absorbed by plants and converted to protein and nucleic acids. Cyanobacteria are found in almost every conceivable habitat, from oceans to fresh water to bare rock to soil.

Cyanobacteria produce the compounds responsible for earthy odors we detect in soil and some bodies of water. The greenish slime on the side of your damp flowerpot, the wall of your house or the trunk of that big tree is more likely to be cyanobacteria than anything else. Cyanobacteria have even been found on the fur of polar bears, to which they impart a greenish tinge. In short, Cyanobacteria have no one habitat because you can find them almost anywhere in the world. Related topics. Toggle navigation.

Cell Walls and Surfaces, Reproduction, Photosynthesis Cell Walls and Surfaces, Reproduction, Photosynthesis
Cell Walls and Surfaces, Reproduction, Photosynthesis Cell Walls and Surfaces, Reproduction, Photosynthesis
Cell Walls and Surfaces, Reproduction, Photosynthesis Cell Walls and Surfaces, Reproduction, Photosynthesis
Cell Walls and Surfaces, Reproduction, Photosynthesis Cell Walls and Surfaces, Reproduction, Photosynthesis
Cell Walls and Surfaces, Reproduction, Photosynthesis Cell Walls and Surfaces, Reproduction, Photosynthesis

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