Chapter 23

Question 1

What characteristic distinguish eukaryotic cells from prokaryotic cells?

Answer 1

The separation of DNA and cytoplasm by a nuclear envelope

The presence in the cytoplasm of membrane-bound compartments with specialized functions: mitochondria, chloroplasts, endoplasmic reticulum (ER), and the Golgi complex, among others.

Highly specialized motor (contractile) proteins that move cells and internal cell parts

Question 2

Endomembrane System

Answer 2

In eukaryotes, a collection of interrelated internal membranous sacs that divide a cell into functional and structural compartments.

Question 3

Vesicle

Answer 3

A small, membrane-bound compartment that transfers substances between parts of the endomembrane system.

Question 4

Smooth ER

Answer 4

Endoplasmic reticulum with no ribosomes attached to its membrane surfaces. Smooth ER has various functions, including synthesis of lipids that become part of cell membranes.

Question 5

Rough ER

Answer 5

Endoplasmic reticulum with many ribosomes studding its outer surface which synthesize proteins.

Question 6

Ribosome

Answer 6

A ribonucleoprotein particle that carries out protein synthesis by translating mRNA into chains of amino acids.

Question 7

Translation

Answer 7

The use of the information encoded in the RNA to assemble amino acids into a polypeptide.

Question 8

Transcription

Answer 8

The mechanism by which the information encoded in DNA is made into a complementary RNA copy.

Question 9

ER Lumen

Answer 9

Each vesicle is formed by a single membrane that surrounds an enclosed space called the lumen of the ER.

Question 10

Endoplasmic Reticulum (ER)

Answer 10

In eukaryotes, an extensive interconnected network of cisternae that is responsible for the synthesis, transport, and initial modification of proteins and lipids.

Question 11

Cisternae (singular, cisterna)

Answer 11

Membranous channels and vesicles that make up the endoplasmic reticulum.

Question 12

Golgi Complex

Answer 12

In eukaryotes, the organelle responsible for the final modification, sorting, and distribution of proteins and lipids.

Question 13

Secretory Vesicle

Answer 13

Vesicle that transports proteins to the plasma membrane.

Question 14

Exocytosis

Answer 14

In eukaryotes, the process by which a secretory vesicle fuses with the plasma membrane and releases the vesicle contents to the exterior.

Question 15

Endocytosis

Answer 15

In eukaryotes, the process by which molecules are brought into the cell from the exterior involving a bulging in of the plasma membrane that pinches off to form an endocytic vesicle.

Question 16

What does a eukaryotic cell look like/contain?

Answer 16

Question 17

Theory of Endosymbiosis

Answer 17

States that the prokaryotic ancestors of modern mitochondria and chloroplasts were engulfed by larger prokaryotic cells, forming a mutually advantageous relationship called a symbiosis, and that slowly, over time, the host cell and the endosymbionts became inseparable parts of the same organism.

The rise in atmospheric O₂ is thought to be a key factor in the occurrence of endosymbiosis. Mitochondria carry out aerobic respiration; thus, it is thought their ancestors were free-living aerobic prokaryotic cells. These cells would have been able to generate far more ATP from the same amount of food as a comparable anaerobic cell. Endosymbiosis of these small aerobic cells would give a larger anaerobic cell a distinct energy advantage compared with other anaerobic cells.

In the same way, the modern chloroplast is thought to be derived from endosymbiotic events involving cyanobacteria. Because cyanobacteria are photosynthetic, the host cell would be able to utilize sunlight as a source of energy. Additionally, because cyanobacteria carry out oxygenic photosynthesis, the host cell could easily supply the water needed to drive photosynthesis.

Whereas virtually all eukaryotic cells contain mitochondria, only plants and algae contain both mitochondria and chloroplasts. This fact indicates that endosymbiosis occurred in stages, with the event leading to the evolution of mitochondria occurring first. Once eukaryotic cells with the ability for aerobic respiration developed, some of these became photosynthetic after taking up cyanobacteria. This lineage developed into the plants and algae of today.

Question 18

What evidence supports the theory of endosymbiosis?

Answer 18

1. Morphology. The form or shape (morphology) of both mitochondria and chloroplasts is similar to that of a prokaryotic cell. Mitochondria resemble aerobic prokaryotes, and chloroplasts resemble cyanobacteria.
2. Reproduction. A cell cannot make a mitochondrion or a chloroplast. Just like free-living prokaryotic cells, mitochondria or chloroplasts are derived only from preexisting mitochondria or chloroplasts. Both chloroplasts and mitochondria divide by binary fission, which is how prokaryotic cells divide.
3. Genetic information. If the ancestors of mitochondria and chloroplasts were free-living cells, then one could predict that these organelles should contain their own DNA. This is indeed the case.
4. Transcription and translation. Both chloroplasts and mitochondria contain a complete transcription and translational machinery, including a variety of enzymes and the ribosomes necessary to synthesize the proteins encoded by their DNA. The ribosomes of prokaryotic cells are distinctly different from those of eukaryotic cells. The ribosomes of mitochondria and chloroplasts are similar to the type found in prokaryotes.
5. Electron transport. Similar to free-living prokaryotic cells, both mitochondria and chloroplasts can generate energy in the form of ATP through the presence of their own electron transport chains.

Question 19

Binary Fission

Answer 19

Prokaryotic cell division—splitting or dividing into two parts.

Question 20

Cytoskeleton

Answer 20

The interconnected system of protein fibres and tubes that extends throughout the cytoplasm of a eukaryotic cell maintaining its shape and internal organization as well as reinforcing the plasma membrane and functioning in movement, both of structures within the cell and of the cell as a whole.

Question 21

Intermediate Filament

Answer 21

A cytoskeletal filament about 10 nm in diameter that provides mechanical strength to cells in tissues.

Question 22

Microfilament

Answer 22

A cytoskeletal filament composed of actin.

Question 23

Microtubule

Answer 23

A cytoskeletal component formed by the polymerization of tubulin into rigid, hollow rods about 25 nm in diameter.

Question 24

Flagellum (plural, flagella)

Answer 24

A long, threadlike, cellular appendage responsible for movement; found in both prokaryotes and eukaryotes, but with different structures and modes of locomotion.

Question 25

Cilium

Answer 25

Motile structure, extending from a cell surface, that moves a cell through fluid or fluid over a cell.

Question 26

Analogous Structures

Answer 26

Structures that perform the same function but do not share a common evolutionary history.

Question 27

Homologous Structures

Answer 27

Structures that are similar because they do share a common evolutionary history.

Question 28

Trophozoite

Answer 28

Motile, feeding stage of Giardia and other single-celled protists.

Question 29

Protist

Answer 29

Protists (of kingdom Protista) are a very heterogeneous collection of about 200 000 eukaryotes that are not actually closely related to each other; that is, they did not all arise from a common ancestor. Most are unicellular and microscopic, but some are large, muticellular organisms. Like their most ancient ancestors, almost all of these eukaryotic species are aquatic.

Question 30

Kingdom Protista aka Protoctista

Answer 30

A diverse and polyphyletic group of single-celled and multicellular eukaryotic species.

Question 31

Describe the importance of endosymbiosis for the evolution of protists.

Answer 31

Protists likely evolved about 1.5 to 2 billion years ago. We don’t fully understand how they evolved, although we know that endosymbiosis played an important role in the process.

As eukaryotes, protists contain mitochondria (although some have very reduced versions of this organelle), and many also contain chloroplasts. As per the theory of endosymbiosis, mitochondria and chloroplasts are the descendants of free-living prokaryotes that, over evolutionary time, became organelles.

All mitochondria are thought to have arisen from a single endosymbiotic event, but the history of chloroplasts is more complex.

The first chloroplasts evolved from free-living photosynthetic prokaryotes (cyanobacteria) ingested by eukaryote cells that had already acquired mitochondria. In some cells, the cyanobacterium was not digested but instead formed a symbiotic relationship with the engulfing host cell—it became an endosymbiont, an independent organism living inside another organism. Over evolutionary time, the prokaryote lost genes no longer required for independent existence and transferred most of its genes to the host’s nuclear genome. Moving some of the genes to the nucleus is thought to have given the host cell better control of overall cell function. The prokaryote had become an organelle, part of the eukaryote cell. Some photosynthetic protists originated from this endosymbiotic event, whereas other protists were formed when a eukaryote engulfed a photosynthetic eukaryote that eventually became a chloroplast.

Question 32

How do protists (which are eukaryotes) differ from prokaryotes?

Answer 32

Because protists are eukaryotes, the boundary between them and prokaryotes is clear and obvious. Unlike prokaryotes, protists have a membrane-bound nucleus, with multiple, linear chromosomes. In addition to cytoplasmic organelles, including mitochondria and chloroplasts (in some species), protists have microtubules and microfilaments, which provide motility and cytoskeletal support. As well, they share characteristics of transcription and translation with other eukaryotes.

Question 33

How do protists differ from fungi?

Answer 33

In contrast to fungi, most protists are motile or have motile stages in their life cycles, and their cell walls are made of cellulose, not chitin.

Question 34

How do photosynthesizing protists differ from plants?

Answer 34

Unlike plants, many photoautotrophic protists can also live as heterotrophs, and some regularly combine both modes of nutrition.

Protists do not retain developing embryos in parental tissue, as plants do, nor do they have highly differentiated structures equivalent to roots, stems, and leaves.

Photosynthetic protists are sometimes referred to as “algae”; these protists are generally aquatic and often unicellular and microscopic (although many are multicellular). However, the different groups of algae are not closely related to each other, so the term “algae” does not indicate any sort of relatedness among organisms referred to by that term.

Question 35

How do protists differ from animals?

Answer 35

Unlike protists, all animals are multicellular and have features such as an internal digestive tract and complex developmental stages. Protists also lack nerve cells, highly differentiated structures such as limbs and a heart, and collagen, an extracellular support protein. These features characterize many animals.

Question 36

Giardia

Answer 36

A diplomonad (subgroup of excavates).

Giardia is a single-celled eukaryote, more specifically a protist, that can exist in two forms: a dormant cyst and a motile feeding stage.

The cysts can survive for months, so it is important to boil or filter water when you are out hiking or camping. If you swallow the cysts they can move from your stomach into your small intestine, the cysts then release the motile feeding stage, trophozoites. Using their multiple flagella, the trophozoites are able to swim about in your intestinal space and attach themselves to the epithelial cells of your intestine.

Infection with Giardia can become chronic, causing inflammation and reduction of the absorptive capacity of the gut. Your immune system does not detect the presence of Giardia and get rid of the parasite because Giardia can alter the proteins on its surface that your immune system relies on to recognize an invader and so escapes recognition; thus, Giardia infections can be persistent or recur.

Question 37

Endosymbiontic Theory

Answer 37

The proposal that the membranous organelles of eukaryotic cells (mitochondria and chloroplasts) may have originated from symbiotic relationships between two prokaryotic cells.

Question 38

Phytoplankton

Answer 38

Microscopic, free-flowing photosynthetic aquatic plants and protists.

Question 39

Zooplankton

Answer 39

Small, usually microscopic, animals that float in aquatic habitats.

Question 40

Where do protist’s live?

Answer 40

Protists live in aqueous habitats, including aquatic or moist terrestrial locations such as oceans, freshwater lakes, ponds, streams, and moist soils and within host organisms.

Question 41

What are the roles of protists in their different habitats?

Answer 41

In the moist soils of terrestrial environments, protists play important roles among the detritus feeders that recycle matter from organic back to inorganic form.

In their roles in phytoplankton, in zooplankton, and as detritus feeders, protists are enormously important in world ecosystems.

Protists that live in host organisms are parasites, obtaining nutrients from the host. Indeed, many of the parasites that have significant effects on human health are protists, causing diseases such as malaria, sleeping sickness, and amoebic dysentery.

Question 42

Colony

Answer 42

Multiple individual organisms of the same species living in a group.

Cells show little or no differentiation and are potentially independent. Within colonies, individuals use cell signalling to cooperate on tasks such as feeding or movement.

Question 43

Contractile Vacuole

Answer 43

A specialized cytoplasmic organelle that pumps fluid in a cyclical manner from within the cell to the outside by alternately filling and then contracting to release its contents at various points on the surface of the cell.

Question 44

Pellicle

Answer 44

A layer of supportive protein fibres located inside the cell, just under the plasma membrane, providing strength and flexibility instead of a cell wall.

Question 45

Pseudopod (plural, pseudopodia)

Answer 45

A temporary cytoplasmic extension of a cell involved in amoeboid motion; the cell extends one or more lobes of cytoplasm called pseudopodia (“false feet”). The rest of the cytoplasm and the nucleus then flow into the pseudopodium, completing the movement.

Question 46

Describe the structures of protists.

Answer 46

1. Single-celled, large and multi-cellular, or colonial.
2. Complex intracellular structures that reflect their habitats, i.e. contractile vacuoles.
3. The cells of some protists are supported by an external cell wall or by an internal or external shell built up from organic or mineral matter; in some, the shell takes on highly elaborate forms. Instead of a cell wall, other protists have a pellicle.
4. At some point in their lives most move either by flagella, cilia, or pseuodopodia.
5. Many protists can exist in more than one form, for example, as a motile form and as a nonmotile cyst that can survive unfavourable conditions. This morphological variability allows the species to live in different habitats at different stages in its life.

Question 47

Describe the metabolism of protists.

Answer 47

Almost all protists are aerobic organisms that live either as heterotrophs—obtaining carbon from organic molecules produced by other organisms—or as photoautotrophs, by producing organic molecules for themselves by photosynthesis.

Some heterotrophic protists obtain organic molecules by engulfing part or all of other organisms (phagocytosis) and digesting them internally.

Others absorb small organic molecules from their environment by diffusion.

Some protists can live as either heterotrophs or autotrophs.

Question 48

How do protists reproduce?

Answer 48

Reproduction may be asexual, by mitosis, or sexual, through meiotic cell division and formation of gametes.

In protists that reproduce by both mitosis and meiosis, the two modes of cell division are often combined into a life cycle that is highly distinctive among the different protist groups.

Question 49

Mitosis

Answer 49

Nuclear division that produces daughter nuclei that are exact genetic copies of the parental nucleus.

Question 50

Meiosis

Answer 50

The division of diploid cells to haploid progeny, consisting of two sequential rounds of nuclear and cellular division.

Meiosis I - The first division of the meiotic cell cycle in which homologous chromosomes pair and undergo an exchange of chromosome segments, and then the homologous chromosomes separate, resulting in two cells, each with the haploid number of chromosomes and with each chromosome still consisting of two chromatids.

Meiosis II - The second division of the meiotic cell cycle in which the sister chromatids in each of the two cells produced by meiosis I separate and segregate into different cells, resulting in four cells each with the haploid number of chromosomes.

Question 51

Haploid

Answer 51

An organism or cell with only one copy of each type of chromosome in its nuclei.

Question 52

Diploid

Answer 52

An organism or cell with two copies of each type of chromosome in its nucleus.

Question 53

What are the major groups of protists?

Answer 53

Excavates

Discicristates

Alveolates

Heterokonts

Cercozoa

Amoebozoa

Arachaeplastida

Opisthokonts

Question 54

Describe the main characters of excavates.

Answer 54

This group takes its name from the hollow (excavated) ventral feeding groove found in most members.

All Excavates are single-celled animal parasites that lack mitochondria and move by means of flagella.

Because they lack mitochondria, they are limited to glycolysis as an ATP source.

The nuclei of Excavates contain genes derived from mitochondria, and they also have organelles that likely evolved from mitochondria. Excavates may have lost their mitochondria as an adaptation to the parasitic way of life, in which oxygen is in short supply.

There are two subgroups: Diplomonadida and the Parabasala.

Question 55

Describe the main characteristics of Diplomonadida.

Answer 55

A subgroup of excavates.

Diplomonad means “double cell,” and these organisms do look like two cells together, with their two apparently identical, functional nuclei and multiple flagella arranged symmetrically around the cell’s longitudinal axis.

The best-known diplomonad is Giardia lamblia.

Some are free-living, but many live in animal intestines; some diplomonads do not cause harm to the host, whereas others, like Giardia, live as parasites.

Question 56

Describe the main characteristics of Parabasala.

Answer 56

A subgroup of excavates.

Parabasalids take their names from cytoplasmic structures associated with their nucleus: parabasal bodies.

Parabasalids are also characterized by a sort of fin called an undulating membrane. The buried flagellum allows parabasalids to move through thick, viscous fluids, such as those lining human reproductive tracts.

Other parabasalids (e.g., Trichonympha) are symbionts that live in the guts of termites and other wood-eating insects, digesting the cellulose in the wood for their hosts. As if this endosymbiotic relationship were not complex enough, biologists recently discovered that the protists themselves cannot produce the enzymes necessary to break down cellu- lose but instead rely on bacterial symbionts to break down the cellulose.

Question 57

Undulating Membrane

Answer 57

In parabasalid protists, a finlike structure formed by a flagellum buried in a fold of the cytoplasm that facilitates movement through thick and viscous fluids. An expansion of the plasma membrane in some flagellates that is usually associated with a flagellum.

Question 58

Endosymbiont

Answer 58

An independent organism living inside another organism.

Question 59

Describe the main characters of Discicristates.

Answer 59

Sometimes referred to as protozoa (proto = first; zoon = animal) because they are similar to animals in that they ingest food and move by themselves.

Named for their disk-shaped mitochondrial cristae (folds of the inner mitochondrial membrane).

The group includes about 1800 species, almost all single-celled, highly motile cells that swim by means of flagella.

Although most are photosynthetic, some can also live as heterotrophs, and some even alternate between photosynthesis and life as a heterotroph.

Other organisms in this group are parasitic.

Two subgroups: Euglenoids and Kinetoplastids.

Question 60

Describe the main characteristics of Euglenoids.

Answer 60

Subgroup of Discictristates.

Important primary producers in freshwater ponds, streams, lakes, and some marine environments.

Most are autotrophs that carry out photosynthesis using the same photosynthetic pigments and mechanisms as plants.

If light is not available, photosynthetic euglenoids can also live as heterotrophs by absorbing organic molecules through the plasma membrane or by engulfing small particles.

Other euglenoids lack chloroplasts and live entirely as hetereotrophs.

Photosynthetic euglenoids have a large eyepot whose internal strucuture stimulates the organism to swim to an area of light optimal for photosynthesis.

Contain numerous organelles including a contractile vacuole.

Rather than an external cell wall, euglenoids have a spirally grooved pellicle formed from strips of transparent, protein-rich material underneath the membrane. In some euglenoids, the strips are arranged in a spiral pattern, allowing the cell to change its shape in a wriggling sort of motion (known as euglenoid movement) that allows the cell to change direction. Euglenoids can also swim by whiplike movements of flagella that extend from one end of the cell. Most have two flagella: one rudimentary and short, the other long.

Question 61

Describe the main characteristics of Kinetoplastids.

Answer 61

Subgroup of Discicristates.

Heterotrophs that live as animal parasites.

Kinetoplastid cells are characterized by a single mitochondrion that contains a large DNA-protein deposit called a kinetoplast.

Most have a leading and a trailing flagellum for movement.

Trailing flagellum sometimes attached along the side of the cell forming an undulating membrane that allows the organism to glide along or attach to surfaces.

Question 62

Trypanosoma brucei

Answer 62

A type of kinetoplastid.

Sleeping sickness is caused by various subspecies of Trypanosoma brucei that are transmitted from one host to another by bites of the tsetse fly. Common in sub-Saharan Africa. Early symptoms include fever, headaches, rashes, and anemia. Untreated, the disease damages the central nervous system, leading to a sleeplike coma and eventual death. The disease has proved difficult to control because the same trypanosomes infect wild mammals, providing an inexhaustible reservoir for the parasite.

Other trypanosomes, also transmitted by insects, cause Chagas disease in Central and South America and leishmaniasis in many tropical countries. Humans with Chagas disease have an enlarged liver and spleen and may experience severe brain and heart damage; leishmaniasis causes skin sores and ulcers, as well as liver and spleen damage.

Question 63

Describe the main characteristics of Alveolates.

Answer 63

This group is named for the small, membrane-bound vesicles called alveoli (alvus = belly) in a layer just under the plasma membrane.

The Alveolates include two motile, primarily free-living groups, the Ciliophora and Dinoflagellata, and a nonmotile, parasitic group, the Apicomplexa.

Question 64

Describe the main characteristics of Ciliophora: the Cillates.

Answer 64

A subgroup of alveolates.

Nearly 10 000 known species.

Primarily single-celled.

Highly complex heterotrophic organisms.

Swim by means of cilia.

Some live individually, others are colonial.

Some are animal parasites, other are symbiotic.

Have many highly-developed organelles including a mouth-like gullet lined with cilia, structures that exude toxins and other defensive materials from the cell surface, contractile vacuoles, and food vacuoles.

A pellicle reinforces the cell’s shape. A complex cytoskeleton anchors the cilia just below the pellicle and coordinates the ciliary beating. The cilia can stop and reverse their beating in synchrony, allowing ciliates to stop, back up, and turn if they encounter negative stimuli.

Only eukaryotes with two nuclei: one or more micronuclei and a macronucleus.

Asexual or sexual.

Abundant in freshwater and marine habitats where they eat algae, bacteria, and each other.

Cilia sweep water and prey into gullet and food vacuoles form. Indigestible material expelled through anal pore. Contractil vacuoles expel excess water.

Question 65

Food Vacuole

Answer 65

A membrane-bound sac used for digestion.

Question 66

Micronucleus

Answer 66

In ciliophorans, one or more diploid nuclei that contains a complete complement of genes, functioning primarily in cellular reproduction.

Question 67

Macronucleus

Answer 67

In ciliophorans, a single large nucleus that develops from a micronucleus but loses all genes except those required for basic “housekeeping” functions (ie.e. feeding, metabolism) of the cell and for ribosomal RNAs.

Question 68

Trichocyst

Answer 68

A dartlike protein thread that can be discharged from a surface organelle for defence or to capture prey.

Question 69

Describe the main charactersitics of Dinoflageletta: the Dinoflagellates.

Answer 69

Subgroup of Alveolates.

Make up a large portion of marine phytoplankton.

Typically have a shell formed of cellulose plates.

The beating of flagella, which fit in grooves of the flates, cause them to spin like a top as they swim.

More than 4,000 species known.

Most are single-celled.

Their abundance in phytoplankton makes them a primary producer of ocean ecosystems.

Some are bioluminescent.

Heterotrophs or autotrophs, many carry out both.

Some are symbionts.

Question 70

Bioluminescent

Answer 70

An organism that glows or releases a flash of light, particularly when disturbed.

Question 71

Describe the main characteristics of Apicomplexa.

Answer 71

Subgroup of Alveolates.

Nonmotile parasites of animals.

Absorb nutrients through their plasma membrane rather than by engulfing food molecules and lack food vacuoles.

Asexual or sexual.

Many use different species for different stages of their life cycles.

Question 72

Plasmodium

Answer 72

A type of Apicomplexa.

Cause of Malaria, one of the most widesperead and debilitating human diseases and is common in tropical regions.

Transmitted by 60 different species of mosquitoes, all members of the genus Anopheles.

Infective cells develop inside the female mosquito, which transfers the cells to human or bird hosts.

The infecting parasites divide repeatedly by asexual reproduction in their hosts, initially in liver cells and then in red blood cells. Their growth causes red blood cells to rupture in regular cycles every 48 or 72 hours, depending on the Plasmodium species. The ruptured red blood cells clog ves- sels and release the parasite’s metabolic wastes, causing cycles of chills and fever.

The victim’s immune system is ineffective because during most of the infective cycle, the parasite is inside body cells and thus “hidden” from antibodies. Furthermore, like Giardia, Plasmodium regularly changes its surface molecules, continually producing new forms that are not recognized by antibodies developed against a previous form. In this way, the parasite keeps one step ahead of the immune system, often making malarial infections essentially permanent.

Question 73

Describe the main characteristics of Heterokonts.

Answer 73

Named for their two different flagella: one smooth and the other covered in bristles giving it a hair appearance.

In most, the flagella only occur on reproductive cells such as sperm and eggs.

Include the Oomycota (water moulds), Bacillariophyta (dia- toms), Chrysophyta (golden algae), and Phaeophyta (brown algae).

Question 74

Describe the main characteristics of Oomycota: water molds and downy mildew.

Answer 74

Subgroup of Heterokonts.

Grow as hyphae forming a mycelium.

Heterotrophs which secrete enzymes that digest the complex molecules of surrounding organic matter or living tissue into simpler molecules that are small enough to be absorbed into their cells.

Live almost exclusively in fresh-water lakes and streams or moist terrestrial habitats, where they are key decomposers.

Others parasitize aquatic animals.

Downy mildews are parasites of land plants.

Asexual or sexual.

Question 75

Hypha (plural, hyphae)

Answer 75

Any of the threadlike filaments that form the mycelium of a fungus.

Question 76

Mycelium

Answer 76

A network of branching hyphae that constitutes the body of a multicellular fungus.

Question 77

Describe the main characteristics of Bacillariophyta: Diatoms.

Answer 77

Subgroup of Heterokonts.

Single-celled organisms with a glassy silica shell which is intricately formed and beautiful in many species.

Substances move in and out of the shell through elaborately patterned perforations in the shell.

Common in fossils.

Shells are either radially or bilaterally symmetrical.

Photoautotrophs that carry out photosynthesis using pathways similars to those of plants.

Among the primary photosynthetic organisms in marine plankton and and are abundant in freshwater habitats as both phytoplankton and bottom-dwelling species.

Most are free-living, some are symbionts inside other marine protists.

Asexual reproduction in diatoms occurs by mitosis followed by a form of cytoplasmic division in which each daughter cell receives either the top or bottom half of the parent shell. The daughter cell then secretes the missing half, which becomes the smaller, inside shell of the box. The daughter cell receiving the larger top half grows to the same size as the parent shell, but the cell receiving the smaller bottom half is limited to the size of this shell. As asexual divisions continue, the cells receiving bottom halves become progressively smaller. When a minimum size is reached, sexual reproduction is triggered. The cells produce flagellated gametes, which fuse to form a zygote. The zygote grows to normal size before secreting a completely new shell with full-sized top and bottom halves.

Although flagella are present only in gametes, many diatoms move by an unusual mechanism in which a secretion released through grooves in the shell propels them in a gliding motion.

Question 78

Describe the main characteristics of Chrysophyta: Golden Algae.

Answer 78

Subgroup of Heterokonts.

Autotophs that carry out photosynthesis using pathways similars to those of plants.

Colour due to brownish carotenoid pigment which masks chlorophyl colour.

Can also live as heterotrophs if there is not sufficient light for photosynthesis. They switch to feeding on dissolved organic molecules or preying on bacteria and diatoms.

Important in freshwater habitats and “nanoplankton” a community of marine phytoplankton composed of huge numbers of extremely small cells.

Colonial and each cell in the colony has a pair of flagella.

Glassy shells in the form of plates or scales.

Question 79

Describe the main characteristics of Phaeophyta: Brown Algae.

Answer 79

Subgroup of Heterokonts.

Include kelp.

All are photoautotrophs.

Nearly all of the 1500 known species inhabit temperate or cool coastal marine waters.

Contain brownish pigment fucoxanthin, which gives them their characteristic colour.

Cell walls contain cellulose and a mucilaginous polysaccharide, alginic acid. This alginic acid, called algin when extracted, is an essentially tasteless substance used to thicken such diverse products as ice cream, salad dressing, jelly- beans, cosmetics, and floor polish. Brown algae are also harvested as food crops and fertilizer.

Question 80

Describe the life cycle of Phaeophyta: Brown Algae.

Answer 80

In many species, consist of alternating haploid and diploid generations.

The large structures that we recognize as kelps and other brown seaweeds are diploid sporophytes, so called because they give rise to haploid spores by meiosis.

The spores, which are flagellated swimming cells, germinate and divide by mitosis to form an independent, haploid gametophyte generation.

The gametophytes give rise to haploid gametes, the egg and sperm cells.

Most brown algal gametophytes are multicellular structures only a few centimetres in diameter.

Cells in the gametophyte, produced by mitosis, differentiate to form nonmotile eggs or flagellated, swimming sperm cells.

The sperm cells have the two different types of flagella characteristic of the heterokont protists.

Fusion of egg and sperm produces a diploid zygote that grows by mitotic divisions into the sporophyte generation.

This complex life cycle is very similar to that of land plants.

Question 81

Sporophyte

Answer 81

An individual of the diploid generation produced through fertilization in organisms that undergo alternation of generations; it produces haploid spores.

Question 82

Gametophyte

Answer 82

An individual of the haploid generation produced when a spore germinates and grows directly by mitotic divisions in organisms that undergo alternation of generations.

Question 83

Describe the main characteristics of Cercozoa: Amoebas.

Answer 83

Subgroup of Heterokonts.

Single-celled protist that moves by means of pseudopodia.

Produce stiff, filamentous pseudopodia, and many produce hard outer shells, also called tests.

Two heterotrophic groups: the Radiolaria and the Foraminifera.

One photosynthesizing group: the Chlorarachniophyta.

Question 84

Describe the main characteristics of Radiolaria.

Answer 84

Group of Cercozoa which is a subgroup of Heterokonts.

Marine organisms characterized by a glassy internal skeleton and axopods.

Heavy but axopods provide buoyancy, as do the numerous vacuoles and lipid droplets in the cytoplasm.

Axopods are also involved in feeding: prey stick to the axopods and are then engulfed, brought into the cell, and digested in food vacuoles.

Question 85

Axopods

Answer 85

Slender, raylike strands of cytoplasm supported internally by long bundles of microtubules.

Question 86

Describe the main characteristics of Foraminifera: Forams.

Answer 86

Group of Cercozoa which is a subgroup of Heterokonts.

These organisms take their name from the perforations in their shells (foramen = little hole), through which extend long, slender strands of cytoplasm supported internally by a network of needlelike spines.

Their shells consist of organic matter reinforced by calcium carbonate.

Most foram shells are chambered, spiral structures that, although microscopic, resemble those of molluscs.

Live in marine environments.

Some species are planktonic, but they are most abundant on sandy bottoms and attached to rocks along the coasts.

Engulf prey that adhere to the strands and conduct them through the holes in the shell into the central cytoplasm, where they are digested in food vacuoles.

Some have algal symbionts that carry out photosynthesis, allowing them to live as both heterotrophs and autotrophs.

Question 87

Describe the main characterists of Chlorarachniophyta.

Answer 87

Group of Cercozoa which is a subgroup of Heterokonts.

Amoebas that contain chloroplasts and thus are photosynthetic. However, they combine this mode of nutrition with heterotrophy, engulfing food with the many filamentous pseudopodia that extend from the cell surface.

Question 88

Describe the main characterists of Amoebozoa.

Answer 88

Use pseudopods for locomotion and feeding for all or part of their life cycles.

Include Amoebas and Slime Moulds.

Question 89

Describe the main characteristcs of Amoebas.

Answer 89

Subgroup of Amoebozoa.

Single-celled organisms.

Abundant in marine and freshwater environments and in the soil.

Most are microscopic.

Some are parasitic.

Most are heterotrophs that feed on bacteria, other protists, and bits of organic material.

Pseudopods of amoebas extend and retract at any point on their body surface and are unsupported by any internal cellular organization.

Reproduce only asexually, via binary fission.

In unfavourable environmental conditions, some amoebas can form a cyst, essentially by rolling up and secreting a protective membrane. They survive as cysts until favourable conditions return.

Question 90

Describe the main characteristics of Slime Moulds.

Answer 90

Subgroup of Amoebozoa.

Heterotrophic.

Exist for part of their lives as individuals that move by amoeboid motion but then come together in a coordinated mass - essentially a large amoeba - that ultimately differentiates into a fruiting body, in which spores are formed.

There are two major evolutionary lineages of slime moulds: the cellular slime moulds and the plasmodial slime moulds, which differ in cellular organization.

Respond to stimuli in their environment, moving away from bright light and toward food.

Live on moist, rotting plant material such as decaying leaves and bark.

The cells engulf particles of dead organic matter, along with bacteria, yeasts, and other microorganisms, and digest them internally.

They can be a range of colours: brown, yellow, green, red, and even violet or blue.

Asexual or sexual.

Question 91

Fruiting Body

Answer 91

In some fungi and slime mould, a stalked, spore-producing structure such as a mushroom.

Question 92

Cellular Slime Mould

Answer 92

Any of a variety of primitive organisms of the phylum Acrasiomycota, especially of the genus Dictyostelium; the life cycle is characterized by a slimelike amoeboid stage and a multicellular reproductive stage.

Question 93

Describe the main characteristics of Plasmodial Slime Moulds.

Answer 93

Subgroup of Amoebozoa.

A slime mould of the class Myxomycetes.

Exist primarily as a multinuclelate plasmodium.

~500 known species.

Feeds by phagocytosis.

Moves by cytoplasmic streaming, driven by actin microfilaments and myosin.

At some point, often in response to unfavourable environmental conditions, fruiting bodies form on the plasmodium. At the tips of the fruiting bodies, nuclei become enclosed in separate cells. These cells undergo meiosis, forming haploid, resistant spores that are released from the fruiting bodies and carried by water or wind. If they reach a favourable environment, the spores germinate to form gametes that fuse to form a diploid zygote. The zygote nucleus then divides repeatedly without an accompanying division of the cytoplasm, forming many diploid nuclei suspended in the common cytoplasm of a new plasmodium.

Question 94

Plasmodium

Answer 94

The composite mass of plasmodial slime moulds consisting of individual nuclei suspended in a common cytoplasm surrounded by a single plasma membrane.

Question 95

Describe the general life cycle of a Slime Mould.

Answer 95

Question 96

Describe the main characteristics of Archaeplastida.

Answer 96

Consists of the red and green algae, which are protists, and the land plants (the Viridaeplantae, or “true plants”), which comprise the kingdom Plantae.

Photoautotrophs.

Question 97

Describe the main characteristics of Rhodophyta: The Red Algae.

Answer 97

4000 known species.

Small seaweeds.

Very few in freshwater lakes and streams or in soils.

Cell walls contain cellulose and mucilaginous pectins that give red algae a slippery texture.

Some species secrete calcium carbonate into their cell walls; these coralline algae are important in building coral reefs.

Typically multicellular organisms, with diverse morphologies, although many have plant-like bodies composed of stalks bearing leaflike blades.

Most are free-living autotrophs, some are parasites that attach to other algae or plants.

Most are reddish in colour, some are greenish purple or black. The colour differences are produced by accessory pigments, phycobilins, which mask the green colour of their chlorophylls. Phycobilins absorb the shorter wavelengths of light (green and blue-green light) that penetrate to the ocean depths, allowing red algae to grow at deeper levels than any other algae. Some red algae live at depths up to 260 m if the water is clear enough to transmit light to these levels.

Have complex reproductive cycles involving alternation between diploid sporophytes and haploid gametophytes. No flagellated cells occur in the red algae; instead, gametes are released into the water to be brought together by random collisions in currents.

Question 98

Carrageenan

Answer 98

A chemical extracted from the red alga Eucheuma that is used to thicken and stabilize paints, dairy products such as pudding and ice cream, and many other creams and emulsions.

Question 99

Agar

Answer 99

A gelatinous product extracted from certain red algae or seaweed used as a culture medium in the laboratory and as a gelling or stabilizing agent in foods.

Question 100

Describe the main characteristics of Chlorophyta: The Green Algae.

Answer 100

Subgroup of Archaeplastida.

Carry out photosynthesis using the same pigments as plants.

Show more diversity than any other algal group.

~16,000 known species.

Single-celled, colonial, and multicellular.

Most live in freshwater aquatic habitats, but some are marine whereas others live on rocks, soil surfaces, tree bark, or even snow.

Other organisms rely on green algae to photosynthesize for them by forming symbiotic relationships.

Many can reproduce either sexually or asexually, and some alternate between haploid and diploid generations. Gametes in different species may be undifferentiated flagellated cells or differentiated as a flagellated sperm cell and a nonmotile egg cell. Most common is a life cycle with a multicellular haploid phase and a single-celled diploid phase.

Among all the algae, the green algae are the most closely related to land plants, based on molecular, biochemical, and morphological data. Evidence of this close relationship includes not only the shared photosynthetic pigments, but also the use of starch as storage reserve, and the same cell wall composition.

Question 101

Describe the most common life cycle of Green Algae.

Answer 101

Question 102

Charophyte

Answer 102

A member of the group of green algae most similar to the algal ancestors of land plants.

Question 103

Describe the main characteristics of Opisthokonts.

Answer 103

Named for the single, posterior flagellum found at some stage in the life cycle of these organisms.

Include choanoflagellates.

Question 104

Describe the main characteristics of Choanoflagellates.

Answer 104

Subgroup of Opisthokonts.

Named for the collar surrounding the flagellum that the protist uses to feed and, in some species, to swim.

The collar resembles an upside-down lampshade and is made up of small, fingerlike projections (microvilli) of the plasma membrane. As the flagellum moves water through the collar, these projections engulf bacteria and particles of organic matter in the water.

~150 species.

Live in either marine or freshwater habitats.

Some species are mobile, with the flagellum pushing the cells along (in the same way that animal sperm are propelled by their flagella), but most are sessile (attached by a stalk to a surface).

A number of species are colonial with a cluster of cells on a single stalk; these colonial species are of great interest to biologists studying the evolution of multicellularity in animals.

Both molecular and morphological data indicate that a choanoflagellate type of protist gave rise to animals: for example, there are many morphological similarities between choanoflagellates and the collar cells (choanocytes) of sponges as well as the cells that act as excretory organisms in flatworms and rotifers. Comparisons of nucleic acid sequences done to date also support the hypothesis that choanoflagellates are the closest living relatives to animals. Molecular data also indicate that a choanoflagellate-like organism was also likely the ancestor of the fungi.

Question 105

Primary Endosymbiosis

Answer 105

In primary endosymbiosis, a eukaryotic cell engulfed a Photosynthetic cyanobacterium but did not digest it. Organisms that originated from this event have chloroplasts with two membranes, one from the plasma membrane of the engulfing eukaryote and the other from the plasma membrane of the cyanobacterium.

The chloroplasts of the Archaeplastida—the red algae, green algae, and land plants—result from evolutionary divergence of the photosynthetic eukaryotes formed via primary endosymbiosis event that happened about 1 billion years ago.

Question 106

Secondary Endosymbiosis

Answer 106

The primary endosymbiotic event, which produced the first eukaryotic photoautorophs, was followed by at least three secondary endosymbiosis events, each time involving different heterotrophic eukaryotes engulfing a photosynthetic eukaryote, producing new evolutionary lineages.

In one of these events, red algal ancestors were engulfed. Over evolutionary time, these became the chloroplasts of the heterokonts and the dinoflagellates. From the same photosynthetic ancestor, loss of chloroplast functions occurred in the lineage of the Apicomplexa, which have a remnant plastid.

In an independent endosymbiotic event, a nonphotosynthetic eukaryote engulfed a green algal ancestor. Subsequent evolution in this case produced the euglenoids.

In yet another event, a similar endosymbiosis involving a green alga led to the chlorarachniophytes. In these protists, the chloroplast is still contained within the remnants of the original symbiont cell.

Organisms that formed via secondary endosymbiosis have chloroplasts surrounded by additional membranes acquired from the new host.

Question 107

Plastids

Answer 107

A family of plant organelles.