Skip to main content
Biology LibreTexts


  • Page ID
  • Eukaryotic Cells

    Eukaryote cells have membranous organelles, including a nucleus.

    Eukarya is a domain; it descended from prokaryotic ancestors.

    Eukaryotes may have arisen 2.1 billion years ago. The oldest fossils of prokaryotes are approximately 3.5 billion years old.

    Endosymbiotic Theory

    Earth’s early atmosphere did not contain oxygen. The earliest organisms were anaerobic. Photosynthesis produced oxygen.

    \[\mathrm{6CO_2 + 6 H_2O + Energy \rightarrow C_6H_{12}O_6 + 6O_2}\]

    Aerobic organisms evolved because oxygen enables organisms to obtain much more energy from their food then they could without oxygen.

    The endosymbiotic theory states that some eukaryotic organelles evolved from prokaryotes. For example, the mitochondrion may have arisen from aerobic prokaryotic cells that came to reside within an anaerobic host cell. This enabled the host cell to use aerobic cellular respiration. In return, the host cell provided the necessary nutrients, a stable chemical environment, and protection. Evolutionary change eventually resulted in this relationship being obligate.

    For example, the mitochondrion, chloroplast, and other plastids originated as prokaryotic cells that came to reside within a host cell. They enabled the host cell to use sunlight as an energy source (the chloroplast) and use aerobic cellular respiration (the mitochondrion). In return, the host cell provided the necessary nutrients, a stable chemical environment, and protection. Evolutionary change eventually resulted in this relationship being obligate.

    Below: Mitochondria are eukaryotic organelles that function in aerobic cellular respiration. They may have arisen by anaerobicprokaryotes engulfing aerobic prokaryotes. The anaerobic cell benefited by the extra energy available from aerobic respiration. The engulfed cell benefited by nutrients provided by its host.

    The mitochondrion may have arisen early in the evolution of eukaryotes because all known eukaryotes have them.

    Base sequences of DNA taken from the plastids of red and green algae resemble those of cyanobacteria. This suggests that the chloroplast and other plastids of these two groups of algae may have arisen from a symbiotic relationship between a heterotrophic cell and a cyanobacterium. The relationship enabled the host cell to use sunlight as an energy source. Eventually the relationship became obligate.

    Below: Heterotrophic prokaryotes that engulfed (or became infected by) photosynthetic prokaryotes were able to benefit by using the sugars produced using solar energy. The prokaryotes benefited by receiving necessary nutrients from their host cell. The chloroplast and other plastids evolved from this photosynthetic endosymbiont.

    The origin of the double membrane surrounding plastids can be seen in the diagram above. The outer membrane originated as part of the plasma membrane of the heterotrophic cell when the photosynthetic cell was engulfed. The inner membrane originated as the plasma membrane of the photosynthetic cell.

    Secondary endosymbiosis may have occurred when red and green algae became symbionts within other eukaryotic cells.

    Evidence linking organelles to prokaryotes

    Examples of this type of symbiotic relationship can be found today. For example, some sponges harbor photosynthetic algae within their tissues. Sponges that are infected with algae have the ability to photosynthesize.

    The organelles (chloroplasts and mitochondria) resemble bacteria in size and structure.

    These organelles each contain a small amount of DNA (prokaryote) but lack a nuclear membrane.

    Each has the capability of self-replication. They reproduce by binary fission.

    They make their own proteins.

    During protein synthesis, these organelles use the same control codes and initial amino acid as prokaryotes.

    They contain their own ribosomes, which resemble prokaryote ribosomes. They make their own ribosomes independent of the synthesis of ribosomes in the rest of the cell.

    Eukaryotic organelles and prokaryotes both employ negative gene-control mechanisms. Eukaryotic cells, however, usually regulate protein synthesis by positive control mechanisms.

    The enzymes that replicate DNA and RNA (called RNA polymerase and DNA polymerase) of the organelles are similar to those in prokaryotes but different from those of eukaryotes.

    The organelles have a double membrane that might be derived from a prokaryote’s plasma membrane and the membrane of a vesicle (see diagram above).

    Like prokaryotic cells, the organelles do not contain endoplasmic reticulum, golgi bodies, lysosomes, microtubules, and other membrane-bound organelles typical of eukaryotes.


    The eukaryotic flagellum may have arisen as a result of a spirochete prokaryote becoming attached to a host cell.

    Other Organelles

    The nuclear membrane, endoplasmic reticulum, and golgi apparatus may have arisen as a result of invagination (folding inward) of the plasma membrane.

    Life Cycles

    Review of Cell Division

    Mitosis results in two cells that have the same number of chromosomes as the parent cell.

    Meiosis results in four cells that have half as many chromosomes as the parent cell.

    Activity: Introduction to Life Cycles

    The diagram below shows a typical animal life cycle. It is a diploid-dominant life cycle because the dominant phase is diploid.

    1. Draw a life cycle that has haploid adults (called haploid-dominant).

    To do this, first, draw a dotted line to separate haploid stages from diploid stages.

    Next, label the area above the dotted line "diploid" (or "2N") and the area below the line "haploid" (or "N").

    Add the words "fertilization" and "meiosis" as shown in the diagram above.

    In order to get from above the line to below the line (from diploid to haploid), a cell must undergo meiosis. In order to get from below the line to above the line (from haploid to diploid) fertilization must occur.

    After this is done, add the following terms:

    • adult
    • sperm, eggs
    • meiosis
    • mitosis
    • fertilization
    • zygote
    • 2N (diploid)
    • N (haploid)
    • spore - Spores are haploid cells that are capable of growing into haploid adults. They may be resistant to environmental extremes and thus allow overwinter survival or survival during dry periods. They may also be a mechanism to allow dispersal of the species. For example, windborne spores may be carried for many miles before they are deposited.

    2. Draw a life cycle that has diploid adults (called diploid-dominant).

    3. Draw a life cycle that alternates haploid and diploid adults (called alternation of generations).

    Types of Life Cycles

    The three types of life cycles listed below occur in protists.


    The multicellular form (adult) is haploid.


    The multicellular form (adult) is diploid.

    Alternation of Generations

    A multicellular haploid generation alternates with a multicellular diploid generation.

    Diversity of Protists


    Protists are Eukaryotes that are not fungi, plants, or animals. This polyphyletic group includes a wide variety of organisms. Most groups of protists are unicellular but some are multicellular.

    Molecular evidence suggests that protists include several different lineages and therefore is not a kingdom. Some lineages are more closely related to either Fungi, plants, or animals than they are to other protist groups.

    Plants,fungi,and animals evolved from protist ancestors.

    Protists are a very diverse group and include organisms that range in size from single cells to complex structures more than 100 meters long. They show a variety of reproductive and nutritional strategies.

    Some protists are photoautotrophs, others ingest food (heterotrophs) or they release digestive enzymes into the environment and absorb organic molecules (saprotrophs). Some protists are both autotrophs and heterotrophs (mixotrophs).

    Most protists are aquatic but they are also found in moist terrestrial environments. They are important components of plankton in many aquatic food chains.

    Some groups of photoautotrophic protists are referred to as algae (green algae, red algae, brown algae, golden algae). The word algae is not used as a taxonomic category.



    Have modified mitochondria; respiration is anaerobic.

    Contain two nuclei. Swim by flagella.

    Some species are parasitic, living in the gut of animal hosts. Other species live in stagnant water.

    Example - Giardia is a common intestinal parasite in humans.

    Reproduction is asexual.


    All species in this group are parasites.

    Have modified mitochondria; function anaerobically

    This group includes Trichomonas vaginalis, commonly found in the vagina. Trichomoniasis is a sexually transmitted disease caused by Trichomonas.

    Several species live in the gut of termites. They produce enzymes that digest cellulose. Termites eat wood (mostly cellulose) but do not produce cellulose-digesting enzymes.

    Reproduction is asexual.



    Flagellated, unicellular.

    Have a dark staining region of mitochondria called a kinetoplast.

    Some kinetoplastids are symbiotic (close) relationships with other organisms.

    Trypanosomes are Kinetoplastids that cause African sleeping sickness. They are transmitted to their human hosts by the bite of a tsetse fly.



    Many Euglenids feed by phagocytosis. Many species of Euglenids are photosynthetic but can become heterotrophic when sunlight is unavailable (mixotrophs).

    Euglena use flagella for moving. The outer covering called a pellicle, is flexible and assists in moving.

    Some have an eyespot with a photoreceptor is capable of detecting the presence of light.

    Reproduction is asexual.


    Diatoms are the most numerous unicellular algae in the oceans and as such are an important source of food and oxygen. They are also important in freshwater environments. They capture 20 to 25% of solar energy captured by living organisms.

    The cell walls of diatoms contain silica (a component of glass) and are formed in 2 halves like a pillbox.

    Their remains form diatomaceous earth. It is used for filtering agents, and abrasives such as scouring powders.

    Diatoms are a major component of phytoplankton in freshwater and marine environments.

    Golden Algae

    Golden algae are photoautotrophs but some are mixotrophs.

    The golden color is due to carotenoid pigments.

    The cells are usually biflagelate. Many are unicellular, planktonic but some are colonial.

    Brown Algae

    Brown algae are autotrophs (photosynthetic).

    The characteristic brown color is due to carotenoid pigments.

    They are multicellular and range in size from small to very large. Some are 50 m to 100 m long.

    They are often found along rocky shores in temperate climates.

    The body (thallus) contains holdfasts for attachment, blades, and a stem-like structure that holds the blades is called a stipe. Many species have floats that function in floatation.

    Some have gas-filled floats.

    Mucilaginous (slimy) material in the cell walls retards drying in exposed individuals when the tide goes out.

    Most species have a life cycle with alternation of generations.


    Fucus is a common "seaweed" found along the rocky coast.

    Some species of Fucus have a diploid-dominant life cycle.

    Below: Fucus. Gametes are produced in the receptacles.

    Click on the image to view an enlargement.

    Macrocystis and Nereocystis

    Macrocystis and nereocystis are deep-water kelps.


    Sargassam sometimes breaks off to form floating masses. Other marine organisms congregate around these masses.


    Laminaria is a brown alga that is usually found attached just below the intertidal zone. It has a life cycle with alternation of generations.


    Protective cellulose plates cover dinoflagellates and two flagella enable them to move. One of the flagella lies in a transverse groove that causes cell to spin as it moves.

    Most are found in marine or freshwater environments and many are photosynthetic. They are important components of phytoplankton and thus are important in aquatic food chains. This group also includes many heterotrophic and many mixotrophic species.

    Some species are responsible for red tides that kill fish and shellfish (Gymnodinium, Gonyaulax, Pfiesteria).

    Some live as symbiants within some invertebrates. For example, some corals grow faster with dinoflagellates living within their cells.

    Some species are capable of bioluminescence (they produce light).

    Both sexual and asexual reproduction occur. Sexual reproduction produces cysts which are resistant to unfavorable environmental conditions. Cysts are dormant and become active when environmental conditions improve.


    Apicomplexans are mainly parasitic with a complicated life cycle that usually involves the formation of infective spores.

    The apex of the cell contains a complex of organelles used to penetrate the plasma membrane of the host cell.


    The genus Plasmodium has four species that cause malaria.

    Sporozoites are transferred to human hosts by Anopheles mosquitos. Within a human host, sporozoites invade liver cells and reproduce asexually to form merozoites. After several days, the infected cells rupture, releasing the merozoites, which then infect red blood cells. They reproduce asexually within the red blood cells. Periodically, large numbers of red cells rupture and release merozoites. The merozoites may infect other red blood cells. Some merozoites become gametocytes which are ingested by Anopheles mosquitoes. Gametocytes differentiate into gametes in the gut of the mosquito. Fertilization occurs within the mosquito, producing a diploid zygote. Meiosis followed by mitosis result in the production of sporozoites within the mosquito host.

    Approximately 1/2 to 1 million people die each year from malaria. The disease is curable if diagnosed and treated properly.


    Example - Paramecium

    The pellicle (outer covering) of paramecium is covered with hundreds of cilia.

    They have numerous organelles including a gullet (oral groove) and an anal pore.

    Below: Paramecium caudatum X 100

    Ciliates have a large macronucleus and a smaller micronucleus.

    The micronucleus is involved in sexual and asexual reproduction. Other nuclear activities are handled by the macronucleus.

    The macronucleus is polyploid (approximately 860 N in Paramecium aurelia) and the micronucleus is diploid..

    During reproduction, the macronucleus disintegrates. Later, a micronucleus will develop into a macronucleus.

    Most reproduction is asexual (mitosis). Sexual reproduction is by conjugation.

    The micronucleus will divide by meiosis; 3 of the 4 resulting nuclei will disintegrate as will the macronucleus. The remaining haploid nucleus will divide by mitosis producing an individual with two haploid nuclei. Two conjugating individuals will each exchange one of the nuclei. The two haploid nuclei will then fuse producing a diploid nucleus.

    Below: Conjugation in Paramecium X 200


    They have a shell (test) composed of organic material hardened with calcium carbonate. The tests have many tiny holes through which pseudopods extend.

    Foraminiferans are marine; most live in the sediments but some are planktonic.

    Some species have an endosymbiotic relationship with algae. The algae contribute sugars produced by photosynthesis. Some species prey upon and partially digest algae but retain the chloroplasts (called Kleptoplasty). The chloroplasts remain functional for a period of time.

    The number of foraminiferans may be so great, that their shells accumulate in the sediments. The white cliffs of Dover are composed of limestone produced from shells that accumulated in the sediment and eventually formed rock.


    Cercozoans are amoeboid or flagellate.

    Many are heterotrophs, including many parasites or predators. They feed using pseudopods.

    They are comon in marine, freshwater, and soil ecosystems.

    They are typically the most abundant eukaryotes in soil ecosystems and may be the most numerous predators on earth.


    Radiolarians are mostly marine plankton with an internal skeleton composed of silica, and numerous needle-like pseudopodia. The pseudopodia are used for feeding on small microorganisms.

    Red Algae

    Red algae are mostly multicellular and are found mainly in warmer, tropical oceans.

    The red color is due to an accessory photosynthetic pigment called phycoerythrin. The accessory pigments of red algae are able to absorb blue and green light. This allows some species to survive in deep waters where blue and green light predominates.

    Some species are filamentous but most have a complex pattern of branching.

    Some coralline forms deposit calcium carbonate in their cell walls, which contributes to the development of coral reefs.

    Green Algae

    Four common forms of green algae are single-celled, colonial, filamentous, and multicellular.

    Green algae are thought to be ancestors of the first plants. Both kinds of organisms have the following characteristics in common:

    They have a cell wall that contains cellulose.

    They have chlorophyll’s a and b.

    They store their food as starch inside the chloroplast.

    Most species are freshwater but there are many marine species. Some live in damp soil.


    Chlamydomonas is a single-celled organism with 2 flagella.

    Although this organism is a single cell, the life cycle is similar to a haploid-dominant life cycle.

    It reproduces asexually (by mitosis) when conditions are favorable.

    Sexual reproduction occurs when conditions become unfavorable. The zygote forms a thick-walled zygospore that is resistant to environmental extremes and divides by meiosis when environmental conditions become favorable.

    Most species of Chlamydomonas are isogamous (both gametes are the same size; they are isogametes), some are oogamous (gametes are two sizes; the larger gametes are eggs, the smaller ones are sperm).


    Volvox is a colonial green algae.

    The cells are arranged in a gelatinous sphere with two flagella directed to the outside.

    They divide asexually to produce a daughter colony.

    Some cells are specialized to produce sperm and eggs for sexual reproduction.

    Specialization of cells as seen in the reproductive cells is a characteristic of multicellular organisms. Volvox is considered to be a colony because it appears to be intermediate between a group of individual cells and a multicellular organism.


    Spirogyra is a filamentous form.

    It has a ribbonlike spiral-shaped chloroplast.

    The life cycle is haploid-dominant.

    Sexual reproduction occurs by conjugation. Conjugation refers to the process where gametes are transferred from one individual to another by a connection between the two.

    The zygote is resistant and overwinters. In the spring, it divides by meiosis to produce haploid filaments.


    Ulva is multicellular with a leaflike body that is two cells thick but up to one meter long.

    The life cycle is alternation of generations. Both the haploid and the diploid generations look alike (isomorphic).

    Plasmodial (Acellular) Slime Molds

    Acellular slime molds are diploid, multinucleate masses that creep along the substrate and phagocytize dead organic material and microorganisms. The mass is one large cell referred to as a plasmodium. Note- Do not confuse the use of the word "plasmodium" here with the genus Plasmodium discussed under Apicomplexans above.

    Slime molds play an ecological role similar to that of fungi. They are decomposers, feeding on dead organic material. They differ from fungi in that slime molds ingest their food.

    Below: Physarum polycephalum. Click to view an enlargement.

    When environmental conditions are unfavorable such as when sufficient food or moisture are unavailable, sporangia form, and spores are produced by meiosis. Spores are resistant to environmental extremes and germinate when environmental conditions become favorable. They germinate to produce haploid cells that are either biflagellate (two flagella) or amoeboid. These cells can act as gametes, fusing to produce a diploid zygote that matures into the plasmodium.

    Cellular Slime Molds

    Cellular slime molds exist as individual amoeboid cells that phagocytize bacteria and yeast.

    When food becomes scarce, the cells aggregate to produce a mass that resembles the plasmodium of a plasmodial slime mold. This mass of cells may continue to move about but eventually will settle down and cells within the mass will produce fruiting bodies (reproductive structures). The cells at the tips of the fruiting bodies become spores. The spores germinate when conditions become favorable.

    The amoeboid cells are haploid. In the sexual phase of the life cycle, two amoeboid cells fuse to form a zygote. New amoeboid cells are produced by meiosis.


    Tubulinids move by cytoplasmic extensions called pseudopodia.

    They feed by phagocytizing (engulfing) their prey.

    Click on the image below to view movement in Amoeba.


    Tubulinids are found in soil, marine, and freshwater environments. Amoeba proteus (below) is found in freshwater.

    Amoeba proteus X 100

    VIDEO - Amoeba movement and phagocytosis


    Entamoebas are parasites. Entamoeba histolytica causes amoebic dysentery in humans.