Transcript Lecture3Protozoa

Protozoans

Protozoans include a wide diversity of taxa that
do not form a monophyletic group but all are
unicellular eukaryotes.

Protozoa lack a cell wall, have at least one
motile stage in their life cycle and most ingest
their food.

Protozoan cell is much larger and more complex
than prokaryotic cell and contains a variety of
organelles (see pages 90-92 of text).
Protozoans

Eukaryotic cell was developed through
endosymbiosis.

In distant past aerobic bacteria appear to have
been engulfed by anaerobic bacteria, but not
digested. Ultimately developed symbiotic
relationship with engulfed aerobic bacteria
becoming mitochondria and eukaryotes
developed. In a similar fashion, ancestors of
chloroplasts formed symbiotic union with other
prokaryotes.
Protozoans
 Protozoans
include both autotrophs and
heterotrophs.They include free-living and
parasitic forms.
 Reproduction
can be asexual by fission or
budding or sexual by conjugation or
syngamy (fusion of gametes).
Protozoans
 The
protozoa were once considered a
single phylum, now at least 7 phyla are
recognized.
 Were
also once grouped with unicellular
algae into the Protista an even larger
paraphyletic group.
Figure 11.01
Movement in Protozoa
 Protozoa
move mainly using cilia or
flagella and by using pseudopodia
 Cilia
also used for feeding in many small
metazoans.
Cilia and flagella
 No
real morphological distinction between
the two structures although cilia are
usually shorter and more abundant and
flagella fewer and longer.
 Each
flagellum or cilium is composed of 9
pairs of longitudinal microtubules arranged
in a circle around a central pair.
Cilia and flagella
 The
collection of tubules is referred to as
the axoneme and it is covered with a
membrane continuous with the rest of the
organism’s cell membrane.
 Axoneme
anchors where it inserts into the
main body of the cell with a basal body.
Figure 11.09a
Protein spoke
Dynein motor
Basal body
Cilia and flagella
 The
outer microtubules are connected to
the central pair by protein spokes.
 Neighboring pairs of outer microtubules
(doublets) are connected to each other by
an elastic protein.
Figure 11.09a
Protein spoke
Dynein motor
Cilia and flagella

Cilium is powered by dynein motors on the outer
doublets. As these motors crawl up the adjacent
doublet (movement is powered by ATP) they
cause the entire axoneme to bend.

The dynein motors do not cause the doublets to
slide past each other because the doublets are
attached to each other by the elastic proteins
and the radial spokes and have little freedom of
movement up and down. Instead the walking
motion causes the doublets to bend.
Flagella, “intelligent design” and
irreducible complexity
 Oddly,
the humble flagellum has been
dragged into the evolution culture wars!
Flagella, “intelligent design” and
irreducible complexity
 The
U.S. Supreme Court has prohibited
the teaching of creationism in public
schools as a violation of the
“establishment of religion” clause of the
constitution.
 Latest
attempt to insert creationism into
schools is the idea of “Intelligent Design.”
Flagella, “intelligent design” and
irreducible complexity

The concept of “intelligent design” is outlined
most clearly in Michael Behe’s book “Darwin’s
Black Box.”

The central idea in “intelligent design” is that
some structures in the body are so complex that
they could not possibly have evolved by a
gradual process of natural selection. These
structures are said to “irreducibly complex.”
Flagella, “intelligent design” and
irreducible complexity
 By
“irreducibly complex” Behe means that
a complex structure cannot be broken
down into components that are
themselves functional and that the
structure must have come into existence in
its complete form.
Flagella, “intelligent design” and
irreducible complexity
 If
structures are “irreducibly complex”
Behe claims that they cannot have
evolved. Thus, their existence implies
they must have been created by a
designer (i.e. God, although the designer
is not explicitly referred to as such).
Flagella, “intelligent design” and
irreducible complexity

One of Behe’s main examples is flagella/cilia.

Behe claims that because cilia are composed of
at least half a dozen proteins, which combine to
perform one task, and that all of the proteins
must be present for a cilium to work that cilia
could not have evolved in a step-by step process
of gradual improvement.
Flagella, “intelligent design” and
irreducible complexity

Since the publication of Behe’s book, it has been
demonstrated repeatedly that things he has
claimed to be irreducibly complex are not in fact
so.

The flagellum is not, in fact, irreducibly complex.
For example, the flagellum in eel sperm lacks
several of the components found in other flagella
(including the central pair of microtubules, radial
spokes, and outer row of dynein motors), yet the
flagellum functions well.
Movement in Protozoa:
Pseudopodia
 Pseudopodia
are chief means of
locomotion of amebas but are also formed
by other protozoa and ameboid cells of
many invertebrates.
 In
ameboid movement the organism
extends a pseudopodium in the direction it
wishes to travel and then flows into it.
Pseudopodia

Ameboid movement involves endoplasm and
ectoplasm. Endoplasm is more fluid than
ectoplasm which is gel-like.

When a pseudopodium forms an extension of
ectoplasm (the hyaline cap) appears and
endoplasm flows into it and fountains to the
periphery where it becomes ectoplasm. Thus, a
tube of ectoplasm forms that the endoplasm
flows through. The pseudopodium anchors to
the substrate and the organism moves forward.
Figure 11.10
Feeding in amebas
 Feeding
in amebas involves using
pseudpodia to surround and engulf a
particle in the process of phagocytosis.
 The
particle is surrounded and a food
vacuole forms into which digestive
enzymes are poured and the digested
remains are absorbed across the cell
membrane.
Phagocytosis
Reproduction in protozoa
 The
commonest form of reproduction is
binary fission in which two essentially
identical individuals result.
 In
some ciliates budding occurs in which
a smaller progeny cell is budded off which
later grows to adult size.
Binary fission
in various taxa
Sexual reproduction in protozoa
 All
protozoa reproduce asexually, but sex
is widespread in the protozoa too.
 In
ciliates such as Paramecium a type of
sexual reproduction called conjugation
takes place in which two Paramecia join
together and exchange genetic material
Figure 11.28
Malaria

Malaria is one of the most important diseases in the
world.

About 500 million cases and an estimated 700,000 to
2.7 million deaths occur worldwide each year (CDC).

Malaria was well known to the Ancient Greeks and
Romans. The Romans thought the disease was caused
by bad air (in Latin mal-aria) from swamps, which they
drained to prevent the disease.
Malaria symptoms

The severity of an infection may range from
asymptomatic (no apparent sign of illness) to the
classic symptoms of malaria (fever, chills,
sweating, headaches, muscle pains), to severe
complications (cerebral malaria, anemia, kidney
failure) that can result in death.

Factors such as the species of Plasmodium and
the victims genetic background and acquired
immunity affect the severity of symptoms.
Malaria
 Despite
humans long history with malaria
its cause, a sporozoan parasite called
Plasmodium, was not discovered until
1889 when Charles Louis Alphonse
Laveran a French army physician
identified it, a discovery for which he won
the Nobel Prize in 1907.
Malaria
 In
1897 an equally important discovery,
the mode of transmission of malaria, was
discovered by Ronald Ross.
 His
identification of the Anopheles
mosquito as the transmitting agent earned
him the 1902 Nobel Prize and a
knighthood in 1911.
Plasmodium
 There
are four species of Plasmodium: P.
falciparum, P. vivax, P.ovale and P.
malariae.
 P.
falciparum causes severe often fatal
malaria and is responsible for most
deaths, with most victims being children.
Plasmodium

Both Plasmodium vivax and P. ovale can go
dormant, hiding out in the liver. The parasites
can reactivate and cause malaria months or
years after the initial infection.

P. malariae causes a long-lasting infection. If
the infection is untreated it can persist
asymptomatically for the lifetime of the host.
Life cycle of malaria
 Plasmodium
has two hosts: mosquitoes
and humans.
 Sexual
reproduction takes place in the
mosquito and the parasite is transmitted to
humans when the mosquito takes a blood
meal
Life cycle of malaria: humans

The mosquito injects Plasmodium into a human in the
form of sporozoites.

The sporozoites first invade liver cells and asexually
reproduce to produce huge numbers of merozoites
which spread to red blood cells where more merozoites
are produced through more asexual reproduction.

Some parasites transform into sexually reproducing
gametocytes and these if ingested by a mosquito
continue the cycle.
Plasmodium gametocyte
Life cycle of malaria: mosquitoes

Gametocytes ingested by a mosquito combine in
the mosquito’s stomach to produce zygotes.

These zygotes develop into motile elongated
ookinites.

The ookinites invade the mosquito’s midgut wall
where they ultimately produce sporozoites,
which make their way to the salivary glands
where they can be injected into a new human
host.
How Plasmodium enhances
transmission rates
 The
Plasmodium parasite engages in a
number of manipulative behaviors to
enhance its chances of being transmitted
between hosts.
 Such
manipulations are a common feature
of parasite behavior in general as we will
see throughout the semester.
How Plasmodium enhances
transmission rates
 Mosquitoes
risk death when feeding and
attempt to minimize risk and maximize
reward when doing so.
 To
obtain blood a mosquito must insert its
proboscis through the skin and then locate
a blood vessel. The longer this takes, the
greater the risk.
How Plasmodium enhances
transmission rates
 As
soon as the mosquito hits a blood
vessel the host’s body responds by
clotting the wound.
 Platelets
clump around the proboscis and
release chemicals which cause the
platelets to clot together.
How Plasmodium enhances
transmission rates

To slow clotting and speed feeding, mosquitoes
inject anticoagulants including one called
apyrase that unglues the platelets. They also
inject other chemicals that expand the blood
vessels.

Plasmodium in the host helps the mosquito feed
by releasing chemicals that also slow clotting.
The parasite’s help increases the chances of the
mosquito feeding successfully and sucking up
the parasite.
How Plasmodium enhances
transmission rates

Once in the mosquito Plasmodium needs about
10 days to produce sporozoites that are ready to
be injected into a human.

During this time to reduce the chances of the
mosquito dying, Plasmodium apparently
discourages its host from eating. Although how
the parasite does this is not clear, mosquitoes
containing ookinites abandon feeding attempts
sooner than parasite-free mosquitoes.
How Plasmodium enhances
transmission rates
 Once
sporozoites are in the salivary
glands, however, Plasmodium wants the
mosquito to bite and bite often.
 In
the salivary gland the parasite cuts off
the mosquito’s anticoagulant apyrase
supply. This makes it harder for the
mosquito to feed so it is hungrier and bites
more hosts.
How Plasmodium enhances
transmission rates
 As
a result, an infected mosquito is twice
as likely to bite two people in a single night
as an uninfected mosquito is.
 As
a result, the parasite is spread more
widely.
Behavior of Plasmodium in humans

Plasmodium enters the blood steam through a
mosquito bite.

The parasite must avoid the host’s immune
system. To do so while in the body it moves
from one hiding place to another.

The parasite moves first to the liver. Can get
there in about 30 minutes, which is usually fast
enough to avoid triggering the immune system.
Behavior of Plasmodium in humans
 At
the liver Plasmodium enters a liver cell.
 The
cell responds by grabbing
Plasmodium proteins and displaying the
antigens on its cell surface in a special cup
the major histocompatibility complex or
MHC.
Behavior of Plasmodium in humans
 The
immune system recognizes the
Plasmodium antigens and mounts an
immune response.
 However,
in a week before the immune
system has mounted its full response the
parasite has produced about 40,000
copies of itself and these burst out of the
liver to seek red blood cells.
Behavior of Plasmodium in humans
 The
parasites leave the liver, reenter the
bloodstream, and find a red blood cell to
enter.
 Each
parasite spends two days in a red
blood cell consuming the hemoglobin and
reproducing.
Plasmodium in red blood cell
Red blood cells
 Red
blood cells (strictly red blood
corpuscles) are a challenging environment
to live in.
 They
lack a nucleus and have little
metabolic activity. As a result, they have
few proteins for generating energy and
also lack most of a normal cell’s channels
for transporting fuel in and wastes out.
Red blood cells
 Red
blood cells are specialized to
transport oxygen, which they carry by
binding and wrapping in hemoglobin
molecules.
 A red
blood cell is pumped around the
body by the heart and travels about 300
miles over its lifetime.
Red blood cells
 Red
blood cells are squeezed through
slender capillaries and compressed to one
fifth of their normal diameter before
rebounding.
 To survive this squeezing, red blood cells
have a network of proteins under their
membrane that can fold like a concertina
and allow the cell to stretch and squeeze
as needed.
Red blood cells
 Old
red blood cells eventually lose their
elasticity and become stiff.
 Those
that show signs of such aging are
filtered out as they pass through the
spleen and destroyed.
Behavior of Plasmodium in humans
 Plasmodium
cannot swim but uses hooks
to move along the blood vessels.
 At
the parasite’s tip are sensors that
respond only to young red blood cells and
clasp on to proteins on the cell’s surface.
Behavior of Plasmodium in humans
 The
parasite uses a set of organelles
concentrated at its apical end to gain
entry. A suite of proteins are produced
that cause the red blood cell’s membrane
to open and let the parasite squeeze in.
 It
takes only about 15 seconds for the
parasite to get in.
Figure 11.30
Plasmodium Sporozoite
Behavior of Plasmodium in humans
 Inside
in the red blood cell the
Plasmodium consumes the hemoglobin. It
takes in a small amount of hemoglobin,
slices it apart with enzymes and harvests
the energy released.
 The
toxic core of the hemoglobin molecule
is processed into an inert molecule called
hemozoin.
Behavior of Plasmodium in humans

In order to reproduce, Plasmodium needs more
than hemoglobin.

It sets about modifying the red blood corpuscle
so it can obtain amino acids and make proteins.

The parasite builds a series of tubes that
connect it to the surface of the cell and uses
these to bring in materials from the blood steam
and to pump out wastes.
Behavior of Plasmodium in humans

The parasite also produces proteins that help to
maintain the red blood cell’s springiness for as
long as possible so it not eliminated by the
spleen.

After a few hours, however, the red blood cell
has been too modified by the parasite to fool the
spleen. The parasite now produces sticky latch
proteins that glue the cell to blood vessel walls.
Behavior of Plasmodium in humans

Infected cells clump up in capillaries.

After another day the contents of the cell have
been used up. The cell ruptures and 16 new
parasites burst out to infect other red blood cells.

Some of these parasites transform into sexually
reproducing gametocytes and, as mentioned
previously, these if ingested by a mosquito will
continue the cycle.
Behavior of Plasmodium in humans
 While
in the red blood cells Plasmodium is
invisible to the immune system because
the red blood cells have no MHC and
cannot alert the immune system.
 The
latch proteins however do stimulate
the immune system.
Behavior of Plasmodium in humans
 The
latch protein is made by a single
gene, but Plasmodium has over 100 such
genes each of which produces a unique
latch.
 In
each generation some of the new
parasites switch on a new latch gene and
so the immune system is always playing
catch up.
Effects of malaria on human
evolution
 The
intense selection pressure imposed
by malaria has resulted in a large number
of mutations that provide protection
against the parasite being selected for in
humans.
 The
best known is sickle cell anemia.
Anti-malaria mutations: Sickle cell
anemia
 Sickle
cell anemia is a condition common
in West Africans (and African Americans of
West African ancestry).
 In
sickle cell anemia red blood cells are
sickle shaped as a result of a mutation
which causes hemoglobin chains to stick
together.
Anti-malaria mutations: Sickle cell
anemia

People with the sickle cell allele are protected
against Plasmodium because their hemoglobin
under low oxygen conditions contracts into
needle-shaped clumps.

This contraction not only causes the sickling of
the cell, but harms the parasite. Parasites are
impaled on the clumps and the cell loses its
ability to pump potassium, which the parasite
needs.
Anti-malaria mutations: Sickle cell
allele
 People
with two copies of the sickle cell
allele usually die young, but heterozygotes
are protected against malaria.
 As
a result the geographic distribution of
the allele and malaria in Africa match quite
closely.
Anti-malaria mutations: (G6PD)
deficiency
 Glucose-6-phosphate
dehydrogenase
(G6PD) deficiency. There are hundreds
of alleles known and with more than 400
million people affected G6PD deficiency is
the commonest enzyme deficiency known.
Anti-malaria mutations:
Thalassemia
 Geographic
distribution suggests it
protects against malaria and
epidemiological evidence also supports
this.
 People
with G6PD-202A a reduced activity
variant common in Africa have a
significantly reduced risk of suffering
severe malaria.
Anti-malaria mutations:
Thalassemia
 Thalassemia:
People with thalassemia
make the ingredients of hemoglobin in the
wrong amounts.
many or too few α or ß hemoglobin
chains are produced and when they are
assembled into hemoglobin molecules
spare chains are left over.
 Too
Other anti-malaria mutations:
Thalassemia

Extra chains clump together and cause major
problems in the cell. These clumps grab oxygen,
but don’t enclose it and the oxygen often
escapes and because it is strongly charged the
oxygen damages other molecules in the cell.

Severe thalassemia is fatal, but mild forms
protect against malaria because the loose
oxygen severely damages the parasite and
renders it unable to invade new cells.
Anti-malaria mutations:
Ovalocytosis

Ovalocytosis: Occurs in South east Asia and
has same genetic rules and consequences as
sickle cell anemia.

People with ovalocytosis have blood cell walls
that are so rigid they can’t slip through
capillaries. The rigid cell walls make it hard for
the parasite to enter the cell and the cell’s rigidity
appears to prevent the parasite pumping in
phosphates and sulphates it needs to survive.
Anti-malaria mutations:

One major advantage of these various antimalarial mutations appears to that they provide
a natural vaccination program for children.

By slowing the development of the parasite
these mutations give a child’s naïve immune
system time to overcome Plasmodium’s
attempts to elude the immune system and mount
an immune response. Mild cases of malaria
thus immunize children to malaria and allow
them to survive to adulthood.
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