Unit 6 – Protists• In this unit, we study the precursors to animals,

plants and fungi.

Kingdom Protista• The protists belong to the Kingdom Protista,

which falls under the Eukarya Domain.

• Protists are a very diverse group, made up of any eukaryotes that don’t fit into the other kingdoms (plants, animals, fungi).

• Due to this, there are very few common traits to all protists other than being eukaryotic.

Origins of Eukaryotes• The origins of eukaryotic cells comes later than

prokaryotic cells. • Protists (or protist-like creatures) were the very

first eukaryotes. • They would go on to become the precursors for

each of the other eukaryotic kingdoms. • The word “protist” in fact comes from the same

root as “prototype”: the Greek word protos, meaning first.

• By studying protists we can see the transition between single-cell prokaryotes and modern multicellular eukaryotes.

Origins of Eukaryotes• The organelles of eukaryotes are not all of

common origin.

• The endomembrane system (ER, Golgi, lysosomes, etc) is simply made of membranes and enzymes like those found in the cell otherwise.

• Mitochondria and chloroplasts (along with some other organelles) in fact are like mini-creatures – they have their own DNA, ribosomes, etc. They are like a miniature cell living in the full cell.

Autogenous Hypothesis

• The beginnings of the eukaryotic cell type began with the formation of the endomembrane system.

• This began by cells beginning to create folds and valleys into the membrane to increase surface area.

Autogenous Hypothesis

• These small pockets eventually fully internalized, creating a series of internal membranes.

• In this way, the internal structures of the cell got their beginning. This mechanism for creation of the endomembrane system is called the autogenous hypothesis.

Autogenous Hypothesis

• As time went on, these membrane-compartments would begin to become associated with enzymes that allowed them to specialize into the modern endomembrane system: the ER, neucleus, , Golgi, vesicles, lysosomes, etc.

Endosymbiosis• The establishment of

mitochondria and chloroplasts (and others) occurred by endosymbiosis.

• Through this process, other prokaryotes were taken into the cell, but not destroyed: they came to live in the cell in a mutually beneficial relationship.

Endosymbiosis• Endosymbiosis is

supported by the fact that much of the biochemical makeup and processes in these organelles very closely resemble specific types of existing prokaryotes.

Endosymbiosis• Some features these organelles share with

prokaryotes:o Organelles replicate in a process similar to binary fissiono Organelle DNA is circular and similar in size to

prokaryotic DNAo Organelles ribosomes resemble prokaryotic ribosomes.o Mitochondrial enzymes and transport systems resemble

bacterial versions.o These organelles are similar in size to prokaryotes.o When an organelles is completely destroyed in a cell,

the cell cannot re-create them itself.

Endosymbiosis• Most evidence shows

that an aerobic heterotroph (uses oxygen and organics to make fuel) was first engulfed to become a mitochondria.

• Over time, the enzymes and membranes would become so specialized that the mitochondria could power the entire cell.

Endosymbiosis• Some time later, some

of these cells would take up a second partner – a phototrophic prokaryote. This creatures would use light energy to create food for the host, becoming what we now call chloroplasts.

Origins of Eukaryotes• It is not certain if autogenous creation of the

endomembrane system or endosymbiosis creating the other organelles came first.

• There are conflicting pieces of evidence for both cases.

Types of Protists• As protists are very diverse, the easiest groupings

to use are those based on which type line up with the other eukaryotic kingdoms.

• We will look at:o Protozoa – animal-likeo Slime Molds – fungi-likeo Algae – plant-like (unicellular)o Seaweed – plant-like (multi-cellular)

Protozoans• Protozoa are the most animal-like of the protists.

• Protozoa are chemoheterotrophs, like animals. They both obtain their energy and build their organic compounds from the organic compounds found in their ingested food.

• These creatures come in many shapes and sizes, and get their organics from differing food sources.

Protozoans• Some protozoa actively hunt other

microorganisms and engulf them to absorb their nutrients.

• Some others will simply scavenge stray organic material in the environment.

• Others are parasites who live within other, larger organisms and feed off them in some way.

Protozoans• Many protozoans move using flagella. Among

these are the trypanosomes, a family containing the parasite responsible for African sleeping sickness.

• Others move using a temporary membrane extension known as pseudopodia (fake-foot). Amoebae move in this way to hunt prey.

• Still others use large numbers of cilia to walk/swim, similar to a millipede.

Slime Molds• Slime molds resemble fungi but are a product of

unrelated ancestors. This is convergent evolution, where two unrelated species create similar adaptations. The way both birds and insects have wings is another example of convergent evolution.

• Slime molds come in two varieties: plasmodial and cellular.

Slime Molds• Plasmodial slime molds appear as large,

multicellular colonies but in fact are one large fused cell with many nuclei. This fusing and networking of cells resembles fungal structures.

• Cellular slime molds are unique in that their individualism varies greatly in their life cycle. They exist as individual amoeba-like cells but come together to form a large colony in times of crisis and will even make complex fungi-like reproductive structures to reproduce.

Slime Molds• Plasmodial

• Cellular

Algae• Algae (sing. alga) are photoautotrophic protists.

• Some have hard cellulose cell walls similar to plants.

• Unlike plants, many algae are quite capable of movement using flagella.

• Green algae in particular give off a very similar appearance to plants, as they use the same pigments to appear green as do plants.

Algae

Seaweed• Seaweed is a sub-group of the algae family. They

are large multicellular structures similar to plants. However, they are not in fact plants and their plant-like structures are another example of convergent evolution.

• Seaweeds are an important resource for humans. They can be consumed as food, although many of the complex sugars are not digestible. Many of the gelling components in algae are used to make foods like ice cream and pudding. As well, agar can be extracted from algae. Agar is the gel-like material used to grow microorganisms on in petri dishes.

Origins of Multicellular Life

• Multicellularity has come about from certain species beginning to live as small colonies. In these cases, some of the cells on one surface may have become the primary movers and thus further developed their flagella while other cells may have focused on digesting food or photosynthesis. Over time, these colonies became dependent on each other’s specialities that they could no long live apart.

• Some protists, like cellular slime molds, walk the line between unicellular and multicellular.

Video• http://youtu.be/vAR47-g6tlA