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2 Process: the evolution of photosynthesis

2 Process: the evolution of photosynthesis

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PROCESS, FORM AND PATTERN



likely that pigments evolved in a purely protective role, providing

protection from UV. The amount of UV radiation was considerably

higher then because of the lack of UV-absorbing oxygen in the atmosphere. The radiation reaching the surface of the Earth included

the potentially highly damaging short wavelengths (UV-C, wavelength

190--280 nm) that are now completely shielded out, as well as slightly

less-damaging longer wavelengths (UV-B, 280--320 nm). Even today

cyanobacteria produce a pigment in their sheath called scytonemin,

which strongly absorbs UV-C radiation. The presence of this pigment

may explain their ability to have colonised shallow marine environments prior to 2.5 billion years ago.

Absorption of a photon of light energy in a chromophore elevates

electrons to an excited state. The energy must then be dissipated in a

way that does not produce toxic photoproducts. It can occur in one

of four different ways:



r by emission of infra-red radiation, i.e. heat;

r by fluorescence;

r by transferring the excited electron state to a neighbouring

molecule;

r by the receptor molecule becoming an electron donor.



Figure 1.6. The pigment

phycocyanobilin (ball and stick

model: grey represents the

hydrocarbon backbone, blue –

nitrogen, red – oxygen).



Figure 1.7. The pigment

β-carotene.



For example the phycobilin pigments found in cyanobacteria and red

algae (Rhodophyta) absorb strong light at different wavelengths and

release it by fluorescing at a very narrow range of wavelengths.

Phycobiliproteins (= phycobilins) have a tetrapyrrole-based structure like haemoglobin. One kind is the bluish pigment phycocyanin

that gives the cyanobacteria or blue-green algae their name. Another

phycobilin called phycoerythrin makes the red algae, Rhodophyta,

red. The absorbance spectra of phycocyanin and phycoerythrin pigments are shown in Figure 1.8.

Another class of pigments is the carotenoids of which β-carotene,

the carrot pigment, is one. It absorbs blue light strongly and so looks

orange. Others are red. Different carotenoid pigments absorb wavelengths between 400 and 550 nm. The carotenoids also have a protective role in plants though not only by shielding the cell. They

seem to have gained another way of protecting the cell from damage

because they scavenge toxic products such as superoxide (O2 − ) and

singlet oxygen (1 O2 *) that are created by absorbing light. Like many

pigments, carotenoids have a ring-based structure but here with two

six-carbon rings attached to either end of a long carbon chain. The

carotene found in some green photosynthetic bacteria has a carbon

ring at only one end. Carotenoids are soluble in lipids and are normally attached to the cell membrane or found in specialised vesicles

(plastids) called chromoplasts.

Another interesting class of compounds that absorb light are

the phytochromes. They are used by green plants as photoreceptors, signal-receiving molecules, directing their development depending on the quality of light. Phytochrome-like proteins may have



1.2 PROCESS: THE EVOLUTION OF PHOTOSYNTHESIS



Figure 1.8. Absorption spectra

of pigments involved in photosynthesis in various organisms, and

the level of excitation achieved.



an ancient history pre-dating the origin of plants. For example

they have been detected in non-photosynthetic bacteria, such as

Deinococcus radiodurans, where they protect the bacterium from visible

light. Deinococcus has a close evolutionary relationship with the

cyanobacteria.

The most important photosynthetic pigments are chlorophylls but

carotenoids and other pigments are also usually present and act to

extend the light harvesting capabilities of the organism. They garner

these different wavelengths and pass on the trapped energy to chlorophyll. Several types of chlorophyll have been identified and they all

have a complex multiple ring structure, a porphyrin, like a tetrapyrrole but with magnesium at its centre. What makes chlorophylls such

powerful photosynthetic pigments is the stable ring structure, around

which electrons can move freely and be lost and gained easily. Different chlorophylls differ either in the form of one of the rings,

as in bacteriochlorophyll compared to chlorophyll, or in the side

chains, as in the different forms of chlorophyll called a, b, c, cs d, e

and g.



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PROCESS, FORM AND PATTERN



H



H

N



N

Mg

N



N



H

H

H

O



O

O



O



O



Phtyl



O



N

N

gM



1.2.2 Harvesting light and transferring energy



N



N



H

H

H

O

O



O



O



O

lythP



O



N

N

gM

N



N



H

H

H

O



O

O



O



These differences in chemical structure have the effect of modifying the wavelength at which different pigments, including the chlorophylls, absorb light (Figure 1.8) and the level of excitation achieved.

This is particularly important in water or in shade because different

wavelengths penetrate to different degrees. Water normally absorbs

longer wavelength red light faster than the shorter blue wavelengths.

The deepest living seaweeds are species of coralline red algae. Their

ability to live and photosynthesise in only 0.05%--0.1% of surface irradiance is attributable to the pigment phycoerythrin, which is able to

absorb in the middle ranges of the visible spectrum and then pass

on the energy to chlorophyll. In shallower coastal water organic compounds from decomposing materials or released by vegetation absorb

the blue wavelengths preferentially and therefore a different range

of pigments are required.



O



lythP



Figure 1.9. Chlorophyll

pigments. Molecular structure

showing how the pigments differ in

the presence and position of

oxygen, resulting in subtle changes

in the absorption spectra of the

molecules: chlorophyll a,

chlorophyll b and

bacteriochlorophyll.



The first steps in the evolution of photosynthesis may have occurred

by the photoreduction of carbon dioxide by iron rich clays to form

the simple organic compounds, oxalate and formate. Iron remains

an important component of the electron transport processes of living cells as part of cytochromes, which contain iron atoms held in

place by a haem group; the iron atoms alternate between an oxidised ferric state Fe3+ and a reduced ferrous state Fe2+ as they lose or

gain electrons. An earlier stage of the evolution of electron transport

systems is indicated by the continued presence of non-haem bound

iron--sulphur proteins. Ferredoxin, a small water-soluble iron--sulphur

protein, passes reducing power from another iron--sulphur protein,

the Rieske protein, to NADH, and is also an important elsewhere in

electron transport. Pheophytin is another molecule involved in electron transport. It is a form of chlorophyll a in which magnesium is

replaced by two hydrogen atoms.

Sulphur-containing (thio-) compounds were also important precursors in synthesis. For example acetyl thioesters polymerise to

form the important electron acceptor molecule, quinone. Pheophytin

passes electrons on to a quinone. Quinone is a molecule with a sixcarbon ring. It is reduced to hydroxyquinone, but oxidised back to

quinone when it passes these electrons on to the next part of the

electron transport system. Molecular data indicate that the mechanism of photosynthesis in purple sulphur bacteria is the earliest

evolved surviving type of photosynthesis. Light capture evolved from

photoreduction in iron-rich clays through the use of phycobilins

and carotenoids to chlorophyll pigments. Photosynthesis began in

the UV and evolved through the absorption of blue, yellow, orange

and red light as a consequence of bacteria colonising more productive upper layers of microbial mats where the sunlight intensity was greater. When pigments acting as sunblock did not just

dissipate the energy they absorbed from sunlight, but utilised it,



1.2 PROCESS: THE EVOLUTION OF PHOTOSYNTHESIS



photosynthesis had originated. By this hypothesis, photosynthesis

is one of the primary metabolic processes in the evolution of

life.

In photosynthesis efficiency is gained by pigments being arranged

in an antenna-like complex that funnels captured light energy to

a reaction centre. In the first stages of transfer some energy is

lost as heat. Different organisms have different antennae. Most

cyanobacteria and Rhodophyta have phycobilins (phycobiliproteins)

feeding electrons to chlorophyll a. Phycobilins are found aggregated

together in a particular arrangement; one, called allophycocyanin, is

attached to the photosynthetic membrane and surrounded by phycocyanin and phycoerythrin molecules. Plant chloroplasts have a photosynthetic antenna system with carotenoids instead of phycobilins

feeding electrons to chlorophyll b, then to chlorophyll a, and then

finally to another chlorophyll a molecule in the reaction centre. Some

derived members of the cyanobacteria, called the Prochlorophytes,

possess a plant-like pattern, including the possession of both chlorophyll a and b, and are without phycobilins. There is a greater diversity

of chlorophyll(ide) pigments among groups of small planktonic algae

than large sedentary algae. This diversity may be related to the lower

degree of self-shading in the free-floating smaller organisms. They can

use a wider range of pigments to exploit a spectrally more-diverse

environment.

In photosynthesis electrons boosted to an excited state by absorbing light are transferred from the chromophore to neighbouring

molecules and with their transport down a chain of electron acceptors produce power, stabilised in forms utilisable by the cell. Photosynthesis is a process that can drive other chemical reactions. The

transfer of energy by the transport of electrons permits, for example,

the fixation of carbon dioxide into energy storing sugars, or the production of the energy storing compound ATP. An important electron

carrier, the target molecule of the light reactions of photosynthesis, is

the molecule nicotinamide-adenine dinucleotide phosphate (NADP+ ).

It is freely diffusible and when reduced by the gain of an electron

to make NADPH it carries reduction potential to where it can be

utilised.

There are two distinct kinds of reaction centres that differ in the

form of their electron transport. They are so distinct that they may

have evolved separately, although cyanobacteria and plants have both

kinds. In one kind pheophytins and quinones act as intermediates

and terminal electron acceptors, whereas the other kind uses iron-sulphur centres as terminal acceptors. The first kind is present in

Photosystem II of plants, algae and cyanobacteria, and is the only

one found in purple bacteria and green non-sulphur bacteria. The

other kind is present in Photosystem I of plants, algae and cyanobacteria, and is also present in green sulphur bacteria and heliobacteria. Halophilic (salt loving) archaebacteria in anaerobic conditions

carry out a different, and relatively inefficient, kind of photosynthesis



Figure 1.10. Absorption of light

energy in a phycobilisome and

Photosystem II of a chloroplast,

showing how light is first absorbed

by subsidiary pigments and the

excitation passed on to

chlorophyll.



e



Cytochrme

reduc



o xidse

b/f comple



x



e

Figure 1.11. Electron transport

by cytochrome.



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14



PROCESS, FORM AND PATTERN



utilising a purple pigment called bacteriorhodopsin. It may have

evolved separately from other kinds of photosynthesis.



1.2.3 Anoxygenic photosynthesis



Figure 1.12. FeS iron cluster

type reaction centre.



Bacteria, including photosynthetic bacteria, can be divided into two

main kinds depending on their staining reaction to a procedure called

Gram staining. Differences in staining are a measure of a fundamental difference in their cell walls. Heliobacteriaceae are the only Grampositive photosynthetic bacteria. They are one of several kinds of photosynthetic bacteria that are anoxygenic, that is, they do not split

water to provide electrons for photosynthesis but use other sources

of electrons. The study of the Heliobacteriaceae and filamentous photosynthetic green bacteria is particularly useful for understanding

the earliest stages in the evolution of photosynthesis. The Heliobacteriaceae include the genus Heliobacter. They have bacteriochlorophyll

g, which closely resembles chlorophyll a, but absorbs wavelengths of

light that can penetrate deep water. A photosynthetic reaction centre

(RC-1) is embedded in the cytoplasmic membrane and contains only

a core FeS (iron--sulphur cluster) and lacks an extensive peripheral

antenna system. Heliobacteriaceae are strict anaerobes and reside in

places like stagnant rice paddy fields and alkaline soils. Their cells

are red-brown owing to the presence of a carotenoid pigment neurosporene. Although they are photosynthetic, gaining energy from

light, they are heterotrophic because they cannot fix carbon dioxide

but must utilise simple carbon compounds such as pyruvate, acetate

and lactate as a carbon source. These simple carbon compounds are

also their primary source of electrons. In the dark they can live by

fermentation of pyruvate.

A similar kind of reaction centre (RC-1) is found in the green sulphur bacteria such as Chlorobium, though here the primary source of

electrons is hydrogen sulphide. By oxidising hydrogen sulphide they

produce sulphur and release electrons and hydrogen ions.

Green sulphur bacteria probably evolved in deep water where light

levels are low and filtered by the organisms above, and where reduced

sulphur compounds are also available. They have exceptionally large

antenna arrays of 1000--1500 bacteriochlorophyll c molecules to each

bacteriochlorophyll a molecule at the reaction centre. The pigments

are packed into vesicles called chlorosomes attached to the cytoplasmic membrane. The green sulphur bacteria can make organic compounds through the fixation of CO2 by a reductive tricarboxylic acid

cycle.

Another type of reaction centre, called RC-2, which probably

evolved from RC-1, is present in green filamentous bacteria such as

Chloroflexus. It has pheophytin and a pair of quinones as early electron acceptors. Chloroflexus forms thick microbial mats in neutral

or alkaline hot springs. It has bacteriochlorophyll a located in the

chlorosomes. The early oceans were rich in sulphide and the use of

hydrogen gas or hydrogen sulphide (H2 S) as the initial electron donor

in Chloroflexus may date from that time. Chloroflexus is also sometimes



1.2 PROCESS: THE EVOLUTION OF PHOTOSYNTHESIS



called a green non-sulphur bacterium to contrast it with the green

sulphur bacteria like Chlorobium from which it differs in many ways,

not least in that it has a unique chemical pathway, the hydroxypropionate pathway, for carbon dioxide fixation.

Photosynthetic reaction centre RC-2 is also found in the two

groups of purple bacteria: the purple sulphur bacteria (including the

genus Chromatium) and the purple non-sulphur bacteria (the genus

Rhodospirillum). The former utilise sulphide as a primary electron

source and the latter utilise hydrogen. Two novel features are of particular interest in these organisms. Firstly carbon dioxide is fixed by

the Calvin cycle, as it is in plants. Secondly these organisms are versatile. They can grow autotrophically by photosynthesis, and will do

so in the light in anaerobic conditions, but they can also grow heterotrophically in aerobic conditions. In fact many of the components

of the energy metabolism, the electron transport system, are the same

or are very similar for both these activities. Paradoxically in the context of the evolution of plants, it is not the photosynthesis of purple

bacteria, which is most interesting, but their aerobic metabolism.

Purple non-sulphur bacteria are the probable ancestors of the mitochondria of eukaryotic organisms including plants.

The activities of these primitive anoxygenic photosynthetic bacteria are recorded in rocks of a great age because, by their activity,

soluble ferrous iron was oxidised to the insoluble ferric state. The

brown precipitate was preserved in rocks as ‘banded iron formations’

(BIF) that formed extensively in ocean sediments in the Archaean

eon (Precambrian pre-2500 million years ago) and early Proterozoic

eon (Precambrian 2500--590 million years ago). The banded iron formations are composed of silica-rich layers of fine grained quartz or

chert interspersed by Fe3 O4 (ferrous oxide) and Fe2 O3 (ferric oxide)

with about 30% iron content. Later the production of ferric precipitates was enhanced because of the greater concentration of oxygen in

the atmosphere from the evolution of oxygenic photosynthesis along

with the burying of organic carbon in sediments ‘freeing’ existing

oxygen from CO2 .



1.2.4 Oxygenic photosynthesis

The cyanobacteria are the most important oxygenic photosynthetic

bacteria. They have two photosystems: Photosystem I is related to the

RC-1 containing photosynthesis most primitively seen in the heliobacteria, and Photosystem II is related to the RC-2 containing photosynthesis seen most primitively in the green filamentous bacteria.

Probably this conjunction of photosystems occurred by gene transfer

between distinct Heliobacter and Chloroflexus type organisms. It was a

coupling that was to prove enormously successful, transforming the

world because it permitted oxygenic photosynthesis.

It worked because the two photosystems acting in concert provide

a double hit, boosting the energy level of electrons and thereby providing sufficient oxidising power to split the inexhaustible supply of

water to provide a reductant without sacrificing the ability to use



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PROCESS, FORM AND PATTERN



Figure 1.13. Oxygenic photosynthesis. Simplified diagram showing the light reactions

of photosynthesis starting with the oxidation of water. Through the absorption of light in

Photosystem II the electrons gained from water are elevated to an excited state.

Subsequently they are transferred to Photosystem I. Here they are excited again by

absorption of light and transferred ultimately to NADPH. The action of Photosystems II

and I together is called non-cyclic photophosphorylation. The ATP and NADPH

produced as a result are input into the Calvin cycle. The dashed line shows cyclic

phosphorylation involving Photosystem I only and without the production of NADPH.

The diagram does not attempt to be chemically balanced. Not all the details are shown.

For example the cytochrome complex contains two cytochrome b and one cytochrome

f molecules as well as a Rieske iron–sulphur protein.



photons in the red region of the spectrum. Oxgen is produced as a

side-product. The first hit is from Photosystem II and results in noncyclic photophosphorylation, the flow of electrons to Photosystem I

with the production of ATP, and the splitting (oxidisation) of water

(Figure 1.13). The second hit is from Photosystem I where electrons

are excited again, and now they are transported to ultimately produce reducing power in the shape of NADPH. However, when sufficient reducing power is already present Photosystem I can carry out

cyclic photophosphorylation to produce ATP.

The structure of fossil microbes from the Warrwoona Group

in Western Australia from about 3500 million years ago is comparable to living cyanobacteria and is taken as evidence that oxygen producing photosynthesis had evolved by then. However, the

close relationship between oxygenic and anoxygenic photosynthesis

is emphasised by the activity of some cyanobacteria. Although they

possess both photosystems, they are able to carry out anoxygenic

photosynthesis by utilising Photosystem I alone to carry out cyclic

photophosphorylation. In this case they oxidise H2 S to gain electrons



1.2 PROCESS: THE EVOLUTION OF PHOTOSYNTHESIS



Figure 1.14. The Calvin cycle.

The reducing power of NADPH

and the energy from ATP is used

to build sugars.



and produce sulphur in the same way as the sulphur bacteria. For

example the cyanobacterium Oscillatoria limnetica lives in sulphiderich saline ponds along with sulphur bacteria. Rather than giving

off oxygen, globules of sulphur accumulate on the outside of its

filaments.



1.2.5 Carbon fixation

Carbon dioxide is found in the atmosphere and dissolved in water.

There are a number of different chemical pathways by which it is

utilised or fixed to make organic compounds. The most important

is a cyclical process called the Calvin cycle or Calvin--Benson cycle

after the workers who discovered it (Figure 1.14). The first step is

the covalent linking of the carbon in the carbon dioxide to a fivecarbon compound, ribulose-1,5-bisphosphate (RuBP). This process is

catalysed by the enzyme rubisco (d-ribulose-1,5-bisphosphate carboxylase/oxygenase). Rubisco makes up more than 15% of the protein in

chloroplasts and may be the most abundant protein on Earth. It is

also found in purple bacteria, cyanobacteria, chemolithotrophic bacteria and even some archaebacteria as well as plants. In many algae

and in hornworts it is particularly associated with the pyrenoid, a

region inside the chloroplast that forms part of a CO2 -concentrating

mechanism.

The widespread occurrence of rubisco hints at an alternative function of rubisco at early stages of life on Earth when it may have

acted as an oxygen detoxifier; in low CO2 concentrations it catalyses a reaction in which oxygen is taken up, causing what is called



17



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PROCESS, FORM AND PATTERN



photorespiration. This can be very wasteful in plants, because in normal atmospheric conditions up to 50% of carbon fixed in photosynthesis may be reoxidised to CO2 , but it is an important capacity in

anaerobic organisms.



1.2.6 The cyanobacteria and Prochlorophytes



(b)

(a)

(d)



(c)

Figure 1.15. The diversity of

Cyanobacteria. (a,b) Anabaena: the

enlarged cell in (a) is an akinete

(a cell that forms a resting stage);

(b) a heterocyst (a cell associated

with nitrogen fixation). (c)

Chlorococcus. (d) Spirulina (drawn

from http://vis-pc.plantbio.ohiou.

edu/algaeindex.htm).



Figure 1.16. Prochloron, a

representative of the small number

of genera in the ’Prochlorophytes’,

Cyanobacteria that, like green

algae and plants, have both

chlorophyll a and b (from

http://www-cyanosite.bio.

purdue.edu/images/images.html).



The cyanobacteria are photosynthetic Gram-negative eubacteria that

traditionally have been referred to as ‘blue-green algae’. They are quite

diverse, especially morphologically, which is unusual for Eubacteria.

Over 150 genera and 1000 species have been described. The cyanobacteria have chlorophyll a and phycoerythrin (a phycobiliprotein) as

primary pigments. Like chloroplasts, which are derived from them,

they have a complex system of thylakoid membranes with associated

spherical phycobilisomes to which the photosynthetic pigments are

attached.

Cyanobacteria occupy a diverse range of extreme environments.

Some species photosynthesise and grow in the high temperatures of

hot springs and hyper-saline pools. They can also be found in the

polar regions and at high altitude, surviving in snow and ice or in

cracks in transparent rocks like quartz. They survive desiccation in

deserts. Cyanobacteria of the order Chamaesiphonales (Chamaesiphon)

occur in terrestrial and fresh-water habitats and are also epiphytic

on mosses. In marine and fresh-water habitats they are important

components of the plankton (e.g. Trichodesmium and Microcystis) and

are often responsible for algal blooms. The Nostocales, exemplified

by the genera Nostoc and Scytonema are found in soils, rocks and on

tree trunks. As well as their importance as photosynthetic organisms,

cyanobacteria are important ecologically because many can fix atmospheric nitrogen, and they are often symbiotically associated with

plants.

Some workers recognise a group of planktonic photosynthetic

bacteria called the ‘Prochlorophytes’, but DNA sequence data indicate that the three Prochlorophyte genera, Prochloron, Prochlorococcus and Prochlorthrix, have evolved separately and are not a single

group distinguishable from other cyanobacteria. However, they are

interesting because they are similar to plants as they have divinylchlorophylls a and b, which are very similar to plant chlorophyll a and

b, and they lack phycoerythrin. However, although they have plantlike stacked thylakoid membranes, their own light-harvesting complex probably evolved as a response to the permanent iron-depleted

conditions found in inter-tropical oceanic waters. ‘Prochlorophytes’

are very widespread in oceans and constitute up to 40% of the chlorophyll present in some regions.

Oxygen producing photosynthesis by the cyanobacteria and

Prochlorophytes gradually enriched the atmosphere with oxygen. As a

shielding ozone layer formed in the upper atmosphere, UV exposure

declined. Paradoxically, although the damaging effects of UV light

were reduced, the presence of highly reactive oxygen provided a different kind of challenge to living organisms. Some organisms were



1.3 FORM: THE ORIGIN OF COMPLEX CELLS



poisoned by it and survived only in the remaining anaerobic areas of

deep stagnant water and waterlogged soil. Meanwhile many opportunities were created for aerobic organisms that had the mechanisms to

mitigate the toxic effect of oxygen. An ecological transition was established in microbial mats between aerobic organisms growing on the

surface of the soil, to more and more strictly anaerobic organisms,

growing in deeper and deeper layers. It was the evolution of oxygen

liberating photosynthesis by cyanobacteria and Prochlorophytes that

provided an environment for dramatically increased rates of organic

molecule production.



1.3 Form: the origin of complex cells

The evolution of cells permitted the localisation and isolation of

potentially competing metabolic processes and a much more energy

efficient metabolism. Increasingly complex metabolism evolved with

the development of distinct membrane systems and intra-cellular

compartmentalisation. For example membranes could become energy

transducing by the location of electron transporters in separate places

in them. Photosynthesis is only one activity that drives electrons

across a membrane to establish an electrochemical potential. This

potential is then used as the motive power for other activities.

It is especially in the boundary of the cell, in the cell membrane

and any cell wall exterior to it, that a profound difference between

three kinds or domains of living organism can be recognised: the

Archaebacteria, the Eubacteria and the Eukarya.



1.3.1 Cell membranes and cell walls

The cell membrane is normally called the cytoplasmic membrane

because it separates the living cytoplasm of the cell from the exterior

environment. It is a phospholipid bilayer. The interior is hydrophobic and composed of long-chain fatty acids, and linked to it by an

ester link is the outer part that has relatively hydrophilic glycerol

and phosphate components. Embedded in the membrane, and sometimes passing right through it, are proteins that carry out many of

the activities of the cell.

The cytoplasmic membrane does not just surround the cell: it is

often highly folded inwardly, providing a greatly increased area for

the localisation of other components of the cell. The chlorosomes of

green sulphur Eubacteria are a good example of this but intra-cellular

membrane systems (endomembrane systems), including intra-cellular

membrane bound organelles are a particular feature of the eukaryote

grade of living organism (Table 1.1).

Vesicles bud off the cytoplasmic membrane by endocytosis, capturing materials from outside the cell, or are part of an excretory system by carrying out exocytosis. The Golgi apparatus and

the rough endoplasmic reticulum (rough ER) are important examples of an endomembrane system in eukaryotes. Another example



19



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PROCESS, FORM AND PATTERN



Table 1.1 Fundamental grades of organisation

Prokaryotes (Monera)

(includes both Archaebacteria and Eubacteria)



Eukaryotes (Eukarya)

(includes the four kingdoms: protists, plants,

fungi and animals)



r No membrane-bound nucleus

r DNA in circular chromosomes and without histones



r Membrane-bound nucleus

r DNA complexed with histones in



r Cell fission

r No cytoplasmic membrane-bound organelles (but



chromosomes

r Mitosis and cytokinesis

r Cytoplasmic membrane-bound organelles



mesosomes and membrane systems may be present)



Figure 1.17. Drawing of a

transmission electron micrograph

of the cell of the green alga

Chlamydomonas showing organelles

and membranes systems: (Ch)

chloroplast, (CV) contractile

vacuole, (Er) endoplasmic

reticulum, (G) Golgi body, (F)

flagellum, (M) mitochondrion, (V)

vesicle, (N) nucleus, (Py) pyrenoid,

(Cm) cell membrane, (Cw) cell

wall (from Lee, 1999).



(mitochondria, chloroplasts, Golgi

apparatus, endoplasmic reticulum)



is the extensive thylakoid membrane system of cyanobacteria and

chloroplasts: thylakoids are stacks of flat membranes in which the

photosynthetic pigments are located. In addition, most plant cells

have one or more vacuoles; liquid-filled sacs surrounded by a membrane called the tonoplast. An example of the complex internal structure of a eukaryote is shown in Figure 1.17.

The cytoplasmic membrane is quite fluid but is stabilised in the

Eukarya and methanotrophic Eubacteria by the presence of rigid flat

sterol molecules that are absent from most prokaryotes, although

some of these have similar molecules called hopanoids. The Archaebacteria have slightly different membranes from other organisms,

perhaps because they have a tendency to occupy high-temperature

environments that would disrupt a fluid cytoplasmic membrane:

the interior fatty acids are linked to the glycerol part of the membrane by ether links and, in addition, some Archaebacteria have a

membrane in which the interior hydrophobic part is stabilised as a

monolayer.

The cytoplasmic membrane is also stabilised, in many organisms

by the presence of a cell wall exterior to the membrane. The simplest

cell walls are found in Gram-positive Eubacteria and Archaebacteria.

They are called Gram-positive because during a particular staining

regime, devised by the microbiologist Christian Gram, they retain

a stain called crystal violet even when washed with ethanol. Their

cell wall is thick and composed of 90% peptidoglycan. Gram-negative

prokaryotes have a more complex multi-layered cell wall in which

peptidoglycan makes up only 10%. Different kinds of cell walls are

found in protists, fungi and plants. Animal cells and some protists

are normally naked. Fungi have a cell wall in which chitin is a major

component. The cell walls of protists are very diverse in chemical

composition and structure and many planktonic protists such as the

diatoms, dinoflagellates and desmids have remarkably sculptured cell

walls.

Plants and some kinds of protists have a type of cell wall where

cellulose is a major structural component. The evolutionary origin of



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