Topics 11&12

The Protists & the Origin of Eukaryotes

Biology 1001

October 24-28, 2005

11.1 Introduction to the Protists Protists is the informal name given to a

diverse collection of mostly unicellular (some colonial or multicellular) eukaryotes

Protists are exceedingly complex at the cellular level, and exhibit more structural and functional diversity than any other group of organisms

Formerly the Kingdom Protista in the 5K system of classification, now about 20 different kingdoms in the Domain Eukarya

The different lineages of protists are not a monophyletic group; in fact some protists are more closely related to plants, animals or fungi than they are to other protists

A Tentative Phylogeny of the Eukaryotes

A Sampling of Protistan Diversity

Giardia intestinalis, a diplomonad Trichomonas vaginalis, a parabasalid

Euglena acus, a euglenid

Trypanosoma sp., a kinetoplastid

Paramecium sp., a ciliatePfiesteria shumwayae,

a dinoflagellate Diatoms

Kelp, giant brown algae seaweedsPhysarum polychalum,

a slime moldAmoeba sp., a gymnamoeba

A radiolarian

The Functional & Structural Diversity of Protists Nutrition Protists can be photoautotrophic, chemoheterotrophic, or mixotrophic Mode of nutrition not phylogenetically informative but ecologically useful

Ingestive heterotrophs (animal-like) = protozoa Absorptive heterotrophs (fungus-like) = no general name Autotrophs (plant-like) = algae

Habitat Most are aquatic, preferring moist environments

Seas, ponds, lakes, moist soil, the human body… Are important constituents of plankton

Phytoplankton contains algae and cyanobacteria Many protists are symbiotic and some are parasitic/pathogenic

Life Cycle Some are strictly asexual Others can also reproduce sexually (meiosis & fertilization)

All three basic types of sexual life cycle are employed1

12.0 Examples of Autotrophic Protists 12.1 Euglena sp.

Members of the Euglenid group of the clade Euglenozoa Characterized by an anterior pocket from which one or two flagella

emerge, and the storage polysaccharide paramylon

Figure 28.8

The eyespot functions as a light shield allowing only certain light rays to strike the light detector

The pellicle is constructed of protein bands beneath the plasma membrane and provides strength and flexibility

More about Euglena sp.NutritionEuglena are mixotrophic –Perform photosynthesis in the lightLose chlorophyll in the dark & absorb organic molecules via the

plasma membrane

LocomotionLocomotion is either swimming (flagellar motion), gliding, or

euglenoid movement1

Euglena exhibit positive phototaxis – the light detector senses light, the flagellum propels the Euglena toward it

OsmoregulationEuglena are hypertonic to their freshwater environmentWater enters by osmosis and needs to be removedThe contractile vacuole fills with water and then fuses with the

gullet to release it

12.0 Examples of Autotrophic Protists 12.2 Laminaria sp.

A brown algae species of the Stramenopila clade, characterized by “hairy flagella” (only flagellated stage is a motile reproductive cell)

A seaweed - a large, complex, multicellular, marine alga The thallus body consists of a rootlike holdfast, a stemlike stipe, and

leaflike blades

More about Laminaria sp. Exhibits a life cycle called

alternation of generations1

Two multicellular stages that differ in ploidy

The sporophyte is diploid; the gametophyte is haploid

The gametophyte produces haploid gametes by mitosis

The gametes unite by fertilization to form a zygote that develops into a sporophyte

The sporophyte produces haploid spores by meiosis

The spores grow up into male or female gametophytes

The main form is the sporophyte, the gametophytes are short, branched filaments – the two generations are heteromorphic

Figure 28.21

Eukaryotic CellS Topics 11.2-11.6

The cells of all protists, plants, animals, and fungi

Eukaryotic cells are structurally more complex and larger than prokaryotic cells They have a true membrane-bound

nucleus and other membrane bound organelles

Metabolic requirements impose theoretical lower and upper limits on cell size1

Eukaryotic cells are 10-100 µm and the smallest bacteria are 0.1-1 µm

Fig. 6.7

Exploring Plant and Animal CellsFigure 6.9

• Eukaryote cells have extensively and elaborately arranged internal membranes that divide the cell into compartments and house enzymes for various metabolic functions

Components of the Eukaryotic Cell The nucleus and its envelope, Figure 6.10

The nucleus contains the chromosomes, made of chromatin (DNA & proteins)

The nuclear envelope is a double membrane (two phospholipid bilayers with associated proteins) perforated by pores

A prominent structure in the nucleus is the nucleolus where the ribosomal subunits are assembled

The Endomembrane System A network of membranes with diverse functions, connected to

each other physically or by vesicles

Contains the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various vacuoles, and the plasma membrane

A lysosome is a membranous sac of hydrolytic enzymes that an animal cell uses to digest all kinds of macromolecules

Mature plant cells have a large central vacuole enclosed by a membrane called the tonoplast. This vacuole has numerous functions: it stores nutrients and defense compounds, acts as a disposal site for metabolic wastes, increases the membrane available to the cytosol, and enables the cell to increase in size by taking in water.

The Endoplasmic ReticulumFigure 6.12

The ER is an extensive network of membranes that accounts for half of the total membrane in a cell

A network of membranous tubules and sacs called cisternae, it has an interior lumen continuous with the gap between the two membranes of the nuclear envelope

ER can be smooth or rough (studded with ribosomes). Smooth ER functions include synthesis of lipids, metabolism of carbohydrates, and detoxification

Rough ER is involved in the synthesis of proteins that are destined for secretion, and also the synthesis of new membrane

The Golgi ApparatusFigure 6.13

A series of flattened membranous sacs called cisternae

The Golgi receives products from the ER, modifies and sorts them, and transports them to other parts of the cell

It also synthesizes macromolecules such as polysaccharides

Products are received from the ER at the cis face of the Golgi, and transported away from the trans face, in transport vesicles

Mitochondria & Chloroplasts Mitochondria and chloroplasts are the energy transformers

of the eukaryotic cell

Mitochondria, present in all eukaryotes, are the sites of cellular respiration, where ATP is generated using energy from the anabolism of macromolecules – C6H12O6 + O2 CO2 + H2O + energy

Chloroplasts, present only in plants and algae, are the sites of photosynthesis, where solar energy is converted to chemical energy in the form of macromolecules - CO2 + H2O + energy C6H12O6 + O2

Although membrane-bound, neither mitochondria nor chloroplasts are part of the endomembrane system

Each of these organelles has its own ribosomes and DNA

Figure 6.17 – The mitochondrion, site of cellular respiration

Figure 6.18 – The chloroplast, site of photosynthesis

Other Differences Between Prokaryotes and Eukaryotes Type of cell division: binary fission in prokaryotes, meiosis or

mitosis in eukaryotes Many linear chromosomes instead of one circular one Presence of a cytoskeleton to support the cell, maintain its

shape, transport vesicles and chromosomes around the cell, and give the cell motility

The cytoskeleton consists of three types of subunits, microtubules, microfilaments, and intermediate filaments, each with several functions

11.7 The Origin of Eukaryotes The first eukaryotes were predators: the cytoskeleton

allows a eukaryotic cell to engulf other cells

The endosymbiosis theory explains how complex eukaryotic cells likely arose from a prokaryotic ancestor

The theory proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes called endosymbionts living within larger host cells

An aerobic heterotrophic alpha proteobacteria was the mitochondrial ancestor, and a gram negative cyanobacteria was the chloroplast ancestor

Serial endosymbiosis accounts for the fact that all eukaryotes have mitochondria, but only some have chloroplasts

Figure 26.13

Endosymbiosis in Eukaryote Evolution The evidence for endosymbiosis is

overwhelming Mitochondria and chloroplasts have

their own DNA and ribosomes, and their ribosomes are more similar to prokaryotes

They reproduce by binary fission They have enzymes similar to

prokaryotes

Secondary endosymbiosis accounts for some of the diversity of protists Red and green algae are thought to

have been engulfed in the past by other eukaryotes, leading to some of the contemporary protistan forms

Figure 28.3 Secondary Endosymbiosis