Prokaryotes and Eukaryotes
Characteristic
Prokaryotes:
Bacteria, Archaea
Eukaryotes:
Protists, Plants, Fungi, Animals
1-10 um
- circular DNA, not bound by a membrane
-genome made up of a single chromosome
-not by mitosis and meiosis
-asexual reproduction common
-unicellular
-mitochondria and other membrane-bound
organelles absent
-many are anaerobic (do not require
oxygen to carry out cellular respiration)
100-1000 um
-DNA in nucleus bounded by membrane
-genome made up of several chromosomes
-by mitosis and meiosis
-sexual reproduction common
-most forms are multicellular
-mitochondria and other membrane-bound
organelles present
-most are aerobic (require oxygen to carry
out cellular respiration)
Size
Genetic Material
Cell Division
Reproduction
Number of Cells
Organelles
Metabolism
Viruses
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A VIRUS is a structure that contains strands of DNA or RNA surrounded by a protective protein coat. It cannot live
independently outside of cells.
o Viruses differ from both prokaryotic and eukaryotic cells in many ways. Viruses are functionally
dependent on the internal workings of cells (either prokaryotic or eukaryotic). Viruses are not
capable of living independently outside of cells.
o They must invade cells and use the host cell’s machinery to survive and reproduce.
o Outside a cell, a virus is dormant.
o Viruses are not cellular, so they do not have cytoplasm, membrane-bound organelles, or cell
membranes. This is why scientists do not believe that viruses are living organisms.
o Viruses cause disease in plants and animals – which can affect populations, species, and
ecosystems.
o Uncontrolled viruses can affect plants, such as wheat, oats, or barley. This can lead to food
shortages.
o Other viruses, such as polio, HIV and H1N1, infect humans, leading to severe illness.
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o Classifying Viruses:
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Scientists use size and shape of the capsid to classify viruses.
A CAPSID is the protein coat that surrounds the genetic material, either DNA or RNA, of a virus.
Polio resembles small crystals, and has as many as 20 sides. HIV has a spherical shape
Viruses are also classified by the types of diseases they cause. Viruses that infect humans are
currently classified into 21 groups. These groups differ in their genomes, or sets of genes, and
their method of replication.
o Reproduction in Viruses:
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Since viruses are not cellular, the do not reproduce by cell division. Instead, they undergo
REPLICATION which is the fundamental process of all cells, in which the genetic material is
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copied before the cell produces within a host cell. The host cell can be either a prokaryote or a
eukaryote. Viruses use the host cell to produce multiple copies of themselves. Then, the copies
are assembled by the host cell inside it.
The typical replication cycle of viruses is called the lytic cycle.
The LYTIC CYCLE is the replication process in viruses in which the virus’s genetic material uses
the copying machinery of the host cell to make new viruses.
In the lytic cycle, the entire replication process occurs in the cytoplasm of the host cell. The
virus’s genetic material enters the host cell, and the cell replicates the viral DNA or RNA. The
host cell makes new capsids and assembles new viral particles. The host cell lyses, or breaks
open, and the new viruses leave the cell.
Sometimes, the genetic material of the virus enters the nucleus of the host cell. This occurs
during the lysogenic cycle.
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The LYSOGENIC CYCLE is the replication process in viruses, in which the viral DNA enters and
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becomes part of the host cell’s chromosome. Once this occurs, the infected cell has viral genes
permanently. The viral DNA may remain dormant and later activate and instruct the host cells
to produce more viruses. Activation results in a continuation of the lytic cycle.
The viral DNA that has become part of the host chromosome is then referred to as a
PROVIRUS. A provirus can invade a cell, but does not kill it.
The Lytic and Lysogenic Cycles
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The Lytic Cycle
VIRUSES AND DISEASE
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In the lytic cycle of a virus, newly formed viruses burst from the host cell. This usually kills the host cell.
o In multicellular hosts, these new viruses then infect neighbouring cells, causing damage to their host.
In viruses that undergo the lysogenic cycle, effects on the host may not be immediate.
o For example) HIV is a type of virus called a retrovirus. RETROVIRUSES contain an enzyme called
reverse transcriptase. This enzyme causes the host cell to copy the viral RNA into DNA. In this form, the
viral DNA enters the chromosomes of the host cell, so it is a provirus. When the host cell divides by
mitosis, it replicates the provirus along with its own DNA. This process can continue for years, with no
harm to the host. Because the virus is part of the host chromosomes, it cannot be easily detected by
medical tests. At any time, however, the provirus can separate from the host chromosomes and
complete the more damaging lytic cycle.
o Patterns of Disease:
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For example) The herpes simplex virus causes cold sores in humans. The sores will appear and
disappear on the skin of an infected person throughout their lifetime. The sores will appear when the
viral cycle destroys cells, and they disappear when the virus is in its provirus stage.
For example) HIV forms a provirus in the host cell chromosomes, but it also produces small numbers of
new viruses while the cell continues to function normally. This is why people may test positive for HIV
and still remain healthy for many years. Only once the infection spreads to more and more cells do the
symptoms of AIDS eventually appear. HIV destroys the body’s T-lymphocytes, which help the immune
system fight off other diseases.
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o Prions: Non-viral Disease-causing Agents:
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A PRION is an infectious particle that causes damage to nerve cells in the brain, and that appears to
consist mostly or entirely of a single protein. It is a new type of disease-causing agent.
Prions are proteins that are found normally in the body. Prions are the only disease-causing agents that
lack RNA or DNA.
Disease results when prions convert from their normal form into harmful particles that have the same
chemical composition, but a different molecular shape.
Prions cause several deadly brain diseases, such as Creutzfeldt-Jacob disease (CJD) and Bovine
spongiform encephalopathy (BSE) or “mad cow disease” in cows.
VIRUSES AND BIOTECHNOLOGY
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Since viruses enter host cells and direct the activity of the host cell’s DNA, they can be useful tools for genetic
engineers.
For example) If researchers want to make a copy of a gene, they first insert the gene into the genetic material of
a virus. The virus will then enter a host cell and directs the cell to make multiple copies of the virus. Each new
virus in each new cell contains the added gene that the researchers wanted copied.
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COMPARING BACTERIA AND ARCHAEA
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Prokaryotes are represented by two domains: Bacteria and Archaea.
Since they are in different domains, biologists are saying that the two groups are more different from each other
than any two groups within a domain.
BACTERIA is an individual prokaryotic cell or a single species that is in the domain Bacteria.
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ARCHAEA is an individual prokaryotic cell or a single species that is in the domain Archaea.
Comparing Morphology
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The most common forms in both bacteria and archaea are spheres and rods.
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The spherical forms are known as COCCI (singular: coccus), and the rod forms are called BACILLI (singular:
bacillus). A third form present in both is a spiral shape.
o Aggregations: Cells Grouped Together:
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Aggregations are when individual cells group together. For example) Streptococcus bacteria are found
in chains of spheres. Streptobacillus are rod-shaped bacteria that form similar chains.
Comparing Nutrition
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Bacteria and Archaea species obtain energy in many ways: some carry out photosynthesis, some consume other
organisms, and other get energy from inorganic compounds such as hydrogen sulfide or iron.
One metabolism that is unique to Archaea is METHANOGENESIS which produces methane gas as a by-product.
Methane (CH4) is the simplest organic compound. It is a useful fuel and a potent greenhouse gas.
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Methanogenesis is an anaerobic process that occurs in environments that lack oxygen, and it is often one of the
final stages of decomposition. Methane-producing archaea live in the digestive tracts of animals such as cattle,
making these animals a source of methane gas.
Another key difference between Archaea and Bacteria is photosynthesis. Some bacteria are photosynthetic.
The most common of these is called cyanobacteria. Cyanobacteria use solar energy to convert carbon dioxide
and water into sugar. In the process, they produce oxygen. These bacteria are abundant in both fresh and salt
water, and account for much of the atmospheric oxygen on Earth.
Comparing Habitats
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Both archaea and bacteria occupy environments with oxygen (aerobic) and without oxygen (anaerobic).
Archaea have the ability to live in extreme environments. This is why they are referred to as EXTREMOPHILE
which are an organism that lives in habitats characterized by extreme conditions.
Habitat
Deep Sea Vents and
Hot Springs
Type of
Extremophile
Thermophile
(“heat- lover”)
-both have extreme
temperatures of over
100°C
-there is an absence of
sunlight.
-the most heat tolerant
species is in the genus
Methanopyrus. They can
withstand temperatures as
high as 120°C.
Volcanic Crater Lakes
and Mine Drainage
Lakes
Acidophile
(“acid- lover”)
-they are extremely
acidic, with a pH of less
than 3 due to sulfur.
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Habitats of Extremophiles
Example
-Picrophilus can live at a
pH of 0, which is the
acidity of car battery acid.
Salt Lakes and Inland
Seas
Halophile
(“salt-lover”)
-have concentrations of
salt higher than 20%
(ocean water has a salt
concentration of 3.5%)
-At 20% salt
concentration, only
halophilic archaea (such as
Halococcus) can thrive.
Most bacteria are MESOPHILES which are organisms that occupy environments with moderate (less extreme)
conditions.
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Comparing Reproduction
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Since bacteria and archaea lack nuclei, they do not reproduce by mitosis or meiosis.
The genetic material in prokaryote is contained in a single chromosome within the cell.
Prokaryotes reproduce through the asexual process of BINARY FISSION in which a cell divides into two
genetically identical cells (or organelles).
In binary fission, as a cell grows, it makes a copy of its original, single chromosome. When the cell reaches a
certain size, it elongates, separating the original chromosome and its copy. The cell then builds a partition
between them, called a septum, and eventually the original cell splits into two smaller, genetically identical cells.
The Process of Binary Fission
o Conjugation: New Genetic Content:
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In less favourable conditions, some bacteria and archaea are able to exchange DNA by CONJUGATION
which is the process in which there is a transfer of genetic material involving two cells. This process
produces cells with new genetic combinations, which provides it with a chance to be better adapted to
changing conditions.
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During conjugation, one cell links to another cell through a bridging structure and transfers all or part of
its chromosomes to the other cell.
Unlike asexual reproduction, conjugation results in cells with new genetic content. The receiving cell
then undergoes binary fission to produce more cells with the same, new, genetic make-up.
o Plasmids: Small Loops of DNA:
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Plasmids are small loops of DNA that are separate from the main chromosome and contain genes.
However, these genes are different from those found in the chromosome.
Plasmids can split from the chromosome and rejoin it. They may be transferred from one cell to another
during conjugation. This makes plasmids an important means of genetic recombination in prokaryotes.
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o Endospores: Protecting Genetic Material:
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Some species of bacteria can form endospores when environmental conditions threaten their survival.
ENDOSPORES are dormant bacterial cells that are able to survive for long periods during extreme
conditions.
Endospores are hard-walled structures that protect and store the organism’s genetic material. They are
resistant to high temperatures, drying out, freezing, radiation, and toxic chemicals.
So far, endospores have not been found archae.
When suitable conditions return, the endospore germinates back into an active bacterium.
Classifying and Identifying Bacteria and Archaea
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One method to identify and classify bacteria and archaea is to use the GRAM STAIN, which is a stain that
separates bacteria into two major divisions (Gram positive and Gram negative) based on the cell wall’s response
to the stain.
Gram positive bacteria have a thick protein layer on their cell wall and stain purple.
Gram negative bacteria have a thin protein layer on their cell wall and stain pink.
Other ways to identify and classify prokaryotes include size and shape, nutrition, movement, and genetic
components. However, DNA comparisons are the preferred technique.
Bacteria and Human Health
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For example) Botulism is a type of food poisoning caused by the species of anaerobic bacteria, Clostridium
botulinum. It is commonly found in the soil, and forms endospores that are very resistant to heat and that
germinate in anaerobic conditions. The metabolism of these bacteria produces toxic products that can cause
nausea or even death in humans. This can occur when people can, or seal, food into bottles or jars at home
without proper care. To ensure that bacterial endospores are killed, food must be heated under high pressure
at temperatures above the boiling point of water.
For example) Bacteria can cause illnesses such as strep throat, an ear infection, or a cavity in your tooth.
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Bacteria and the Environment
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As decomposers, bacteria break down organic materials and release carbon, hydrogen, and other elements into
the environment for use by other organisms. These links join different types of bacteria into microscopic food
webs.
Cyanobacteria are major producers of oxygen through the process of photosynthesis. They were probably the
first organisms on Earth to carry out this process.
Other species of cyanobacteria are the only organisms that are able to convert atmospheric nitrogen into a form
that is usable by most organisms.
EUKARYOTIC EVOLUTION AND DIVERSITY
Endosymbiosis
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ENDOSYMBIOSIS is a theory that explains how eukaryotic cells evolved from the symbiotic relationship between
two or more prokaryotic cells. In endosymbiosis, one cell engulfs a different type of cell. However, the engulfed
survives and becomes an internal part of the engulfing cell.
Two eukaryotic organelles present evidence of endosymbiosis – one is the chloroplast, and the other is the
mitochondrion.
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o Chloroplasts and Mitochondria
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There are two eukaryotic organelles that provide evidence of endosymbiosis: One is the chloroplast (the
organelle found in photosynthetic eukaryotes that converts solar energy into sugar). The other is the
mitochondrion (an organelle that extracts energy stored in sugar so that the cell can do work).
It is believed that both of these organelles were engulfed by other, larger cells. Rather than being
digested, the remained intact. They continued to do inside these cells what they had previously done
outside the cell – convert solar energy into molecular energy in the case of the chloroplast, and convert
molecular energy into work in the case of the mitochondrion.
The organelle that is engulfed by another cell is called an ENDOSYMBIONT. The engulfing cell is called
the HOST CELL.
The ribosomes (structures used to assemble proteins) in chloroplasts and mitochondria are much more
similar to ribosomes in prokaryotic cells instead of ribosomes in eukaryotic cells.
Both of these organelles reproduce by binary fission within the cell.
Both of these organelles contain a circular chromosome.
o Multicellularity
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It is estimated that the first multicellular organisms existed 1.2 to 1.5 billion years ago (half the time
that unicellular organisms have been on Earth).
The oldest fossils are of red algae found in rocks in arctic Canada.
Scientists believe that the first multicellular organisms arose from colonies created by dividing
individual cells. Genes from within these cells contained instructions from some of the cells to
become specialized for different functions. For example, some cells became specialized to absorb
nutrients, while other became specialized to gather information from the environment.
Life Cycles and Reproduction
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Not only are eukaryotes more structurally diverse than prokaryotes, but they also have more reproductive
diversity as well.
In prokaryotes, cell division and reproduction are usually the same process: asexual reproduction.
Unicellular eukaryotes also sometimes use asexual reproduction.
However, most eukaryotes use more complicated methods, such as multiple fission. This is when there are
multiple copies made of a cell at one time.
In multicellular individuals, cell division is not the same as reproduction. For example) Human cells are being
reproduced all the time. However, reproduction of individual organisms is only through sexual reproduction.
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In sexual reproduction, two individuals make egg and sperm (known as GAMETES, which are haploids).
HAPLOID cells contain only one set of chromosomes, compared to the two sets of chromosomes in other cells.
When an egg and a sperm fuse, they form a ZYGOTE (a cell that is a diploid). A DIPLOID cell contains two sets
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of chromosomes, one set inherited from each parent cell.
Sexual reproduction is only possible through meiosis, which is unique to eukaryotes.
Organisms that reproduce through sexual reproduction alternate between meiosis (which makes sperm and
egg) and fertilization (which merges sperm and egg).
Asexual life Cycle
Sexual Life Cycle
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Sexual Life Cycle: Meiosis and Asexual Life Cycle: Mitosis
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PROTISTS: THE UNICELLULAR EUKARYOTES
Characteristics of Protists
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PROTISTS are eukaryotic organisms, usually unicellular, that are not fungus, plant, or animal.
When compared to prokaryotic cells, protists are large and diversely shaped.
Most protists are unicellular.
They are grouped as protists because they do not fit into the other kingdoms.
The most common unicellular eukaryotes (protists) are:
Group
Animal-like Protists
(Protozoans)
Fungus-like Protists
Plant-like Protists
Amoebas, ciliates, and
flagellates
Slime moulds, and water moulds
Euglenoids, diatoms, and
dinoflagellates
-They are animal-like
because they consume
other organisms for food.
-Some species are
parasites.
-They are fungus-like because
they absorb nutrients from other
organisms, living or dead.
-Some slime moulds consume
other organisms.
-Some water moulds are
parasites.
-They are plant-like because they
make their own food by
photosynthesis.
-Some consume other organisms
when light is unavailable.
-Some live as symbionts within
other organisms.
Example
Distinguishing
Feature
Animal-like Protists
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Animal-like protists, often called protozoans, are heterotrophs.
The commonly consume other organisms for food, especially prokaryotes and other protozoans, or organic
waste.
A number of species are PARASITES which are organisms that benefit by living in or on another organism at the
expense of that organism. Many parasites take nutrients from the organism in which they reside.
o The Cercozoans: Phylum Cercozoa
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The most familiar of the cercozoans are the amoebas.
Their surface is a cell membrane without a cell wall. This means that they change shape, using their
internal cytoskeleton to move and create different forms.
Temporary extensions of the cytoplasm that are used for both feeding and locomotion are called
PSEUDOPODS (“false feet”).
Amoebas live in salt water, fresh water, and mud. A few are parasites living inside an animal host.
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o The Ciliates: Phylum Ciliophora
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CILIA are short, hair-like projections that function in cell movement and particle manipulation when
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coordinated with other cilia. Their dual purpose is locomotion and sweeping food particles along the
cell surface to move them into the cell.
One of the best known ciliates is the paramecia.
o Flagellates: Phylum Zoomastigina
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Protists in this phylum are called flagellates because they have one or more FLAGELLA (long hair-like
projection extending from the cell membrane that propels the cell using a whip-like motion).
Flagellates have a hard protective covering over their outer membrane.
Some mutualistic flagellates are the species that live in the digestive tract of animals, and help the host
animal digest plant material. For example, the flagellates that live in termites to help them break down
the cellulose of wood.
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o The Sporozoans: Phylum Sporozoa
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Sporozoans are parasites of animals, taking in nutrients they need from their hosts.
Most of these members have life cycles that alternate between sexual and asexual reproduction – and
often alternate between two hosts.
For example) Malaria. It is a protest that is transferred from mosquitoes to humans.
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Fungus-like Protists
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Members of this group are heterotrophs, but instead of ingesting other organisms, they absorb nutrients from
living organisms, dead organisms, and wastes.
Like fungi, fungus-like protists produce spores.
The cell wall of fungus-like protists is different from the cell wall of fungi.
Type
Plasmodial slime
moulds
Cellular slime
moulds
Water moulds
Description
Example
-Are visible to the unaided eye as tiny
slug-like organisms that creep over damp,
decaying plant material in forests and
fields.
-This streaming blob contains many
nuclei.
-they fed by engulfing small particles of
food into their cytoplasm.
-Some of the cytoplasm is concentrated
to form a skeleton-like structure through
which the liquid cytoplasm flows.
-Exists as individual amoeboid cells with
one nucleus each.
-The cells feed by ingesting tiny bacteria
or yeast cells.
-When food becomes scarce, the cells
release a chemical that causes them to
gather together to form a
pseudoplamodium.
-Are filamentous organisms that resemble
fungi.
-They live on dead organic matter.
-Some are parasites on fish, insects, and
plants.
-They extend fungus-like threads into
their host’s tissues, where they release
digestive enzymes and absorb the
resulting nutrients.
Plant-like Protists
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Plant-like protists contain pigments in their chloroplasts to carry out photosynthesis. The most common pigment is
chlorophyll, which gives plants a green colour.
Unicellular plant-like protists include diatoms, dinoflagellates, and eugenoids.
o Diatoms: Phylum Chrysophyta
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Phytoplankton are single-celled, free-floating aquatic organisms.
Diatoms are the most diverse and abundant phytoplankton and are an important source of food for
larger marine organisms.
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Diatoms have rigid cell walls with an outer layer of silica (a common ingredient in sand and glass). The
walls are made up of two unequal parts. The smaller part fits neatly inside the other, like a box with a
lid. Normally, diatoms reproduce asexually through mitosis.
o Dinoflagellates: Phylum Pyrrophyta
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Most dinoflagellates are phytoplankton.
Dinoflagellates have two flagella at right angles to each other. As the flagella beat, a twirling motion is
produced, so that these organisms move by spinning through the water.
When nutrients are plentiful, dinoflagellates reproduce very quickly. The resulting population explosion
is called a bloom or algal bloom.
In species that have red photosynthetic pigments, the bloom is referred to as a RED TIDE (a coastal
phenomenon in which dinoflagellates that contain red pigments are so concentrated that the seawater
has a distinct red colour). This can produce a toxin that becomes concentrated in the tissues of
plankton-eating shellfish. If humans eat those shellfish, they can become seriously ill or die.
o Euglenoids
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Most species of euglenoids are found in shallow, fresh water. They have chloroplasts and conduct
photosynthesis. They also have flagella and can absorb nutrients.
With both plant-like, and animal-like characteristics, euglenoids tend to be autotrophs in sunlight and
heterotrophs in the dark.
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