Lecture 6 Evolution Of Photosynthesis

Question 1

Anoxygenic photosynthesis

Answer 1

CO2 + 2H2A*+light energy
–> (CH2O)+2A+H2O

*H2A = H2O,H2S, H2 or other electron donor

3400MYA bacteria gained the ability to use water as a source of H+ ions for photosynthesis

Question 2

Oxygenated photosynthesis

Answer 2

6CO2+12H2O+light energy –>
C6H12O6+6O2 +6H2O
(Glucose)

2400MYA start of the great oxidation event when atmospheric O2 levels rise rapidly as O2 sinks filled

Question 3

Cyanobacteria

Answer 3

Capable of oxygenic photosynthesis

RuBisCO - O2 on RuBisCO can cause photorespiration instead of photosynthesis. Hence it is protected by a carboxysome protein shell

Separate thylakoid membranes for photosynthesis but no chloroplasts

Phycobilosome w/physcocyanin pigment, protruding antenna collect light energy focussing it on retention centre for photosynthesis

Very ancient proof: 550MYA stromatolites formed of cyanobacterial debris formed fossil-like land masses

2200 MYA colonial and filamentous cyanobacteria had heterocysts allowing them to fix O2 high SA/vol ratio

Question 4

Primary endosymbiosis

Answer 4

~1500MYA cyanobacterium engulfed by biciliate protozoan. Triple phosphate translocator transit machinery loss of vacuolar membrane host FA synthetase lipolysaccharide lipoprotein

~1300 MYA Glaucophytes - single cell free living algae with peptidoglycan bound cyanobacterium as organelle

Lose peptidoglycans and phycobilosomes, thylakoids stack and green plant cell formed with a and b chlorophyll

Lose peptidoglycans, gain proteobacterial RuBisCo lose chlorophyll b and become red algae - seaweed - containing phycoerythrin (red pigment)

Question 5

Was Cambrian explosion caused by algae?

Answer 5

Occurs post snowball earth
Planktonic algae would feed larger organisms
More ATP > more resp/photosynth > nutrient surge

Question 6

How did land plants evolve?

Answer 6

Thought that all land plants evolved from embryophytes - common ancestor

Stoneworts (charophytes) retain eggs and plasmodesmata connect cells

Land plants evolved from green algae retaining chlorophyll a and b and starch as storage
Cell walls: peptidoglycan to glycoproteins and polysaccharides e.g. carageen and agar to cellulose, hemicellulose and pectin

Question 7

What did plants need to do to succeed on land?

Answer 7

~500 MYA plants appear on land
Adapt to survive dry environment
Structural support
Method to transfer water throughout plant
Method to disperse gametes/progeny

Question 8

Land plant defining features

Answer 8

Protected embryo (separated from green algae 480MYA)
Waxy cuticle to reduce water loss
Gametangia to enclose gametes
Thick walled spores (sporopollenin)

Question 9

Land plant characteristic features

Answer 9

Stomata to regulate gas exchange
UV protective pigment

Vascular tissue: 430MYA, water transport allows plants to grow tall and away from water

Mycorrhizae: interactions with fungi promoting nutrient uptake

Seeds: 420MYA protect embryo and aid dispersal

Flowers/ fruit: 200MYA promote sexual reproduction, attract pollinators

Question 10

Gametophyte/sporophyte lifecycle

Answer 10

1) mitosis - gametophytes produce haploid gametes
2) fertilisation - gametes fuze to form zygote
3) mitosis - zygote develops to diploid multicellular sporophyte
4) meiosis - sporophyte produces haploid spores
5) mitosis - spores germinate and divide to form multicellular haploid gametophyte

Question 11

Gametophyte and sporophyte explained

Answer 11

Gametophyte - multicellular haploid tissue - produces haploid gametes by mitosis

After fertilisation diploid zygote develops into multicellular sporophyte

Sporophyte produces haploid spores by meiosis - may be homo or heterospory

Haploid spores develop into haploid gametophytes ( monoecious or dioecious)

Question 12

Non-vascular plants (bryophytes)

Answer 12

Have no water conducting tissues
Found in damp environments only
Most only a few cm tall
Cuticle usually very thin
Sporophyte depends on gametophyte
Often form mats
Water moves by capillary action
Divisions: Hepatophyta (liverwort) Bryophyta (moss) and Anthoceratophyta ( hornworts)

No true leaves,stems or roots
15000 species of moss

Question 13

Vascular plants (traceophytes) without seeds

Answer 13

Divisions: Lycopodiophyta (club mosses and allies) and monilophyta (horsetails and ferns)

Question 14

Vascular plants (traceophytes) with seeds (spermatophytes)

Answer 14

Gymnosperms
Divisions: cycadophyta (cycads) ginkophyta (ginkos) gnetophyta and coniferophyta (conifers)

Question 15

Angiosperms (flowering plants)

Answer 15

Basal angiosperms
Core angiosperms (mesangiosperms)
Include: Magnolias, monocots and eudicots

Question 16

Moss life cycle

Answer 16

1)Spores germinate bud and grow into mature gametophyte
2) a) sperm develop in antheridium
b)Egg develops in archegonium
3) fertilisation of egg by sperm only possible in the presence of water as non vaac plants require water for sperm to swim to egg
4) fertilised egg divides to produces multicellular diploid sporophyte, while it matures the sporophyte remains attached to and nutritionally dependent on gametophyte
5) sporophyte bears sporangium full of haploid spores. Stomata allow water to leave sporangium and dry out. Once dry it opens and spores are released
6) spores germinate forming a rhizoid (root-hair) and protonema (stem-like chain of cells) - back to 1)

Question 17

Liverwort

Answer 17

Flat green tissue
Produce antheridia (sperm maker)
And archegonia (egg maker) on surface

Gemmae cups contain gemmae small discs of haploid tissue that give rise to new gametes - spread by water

Porous surface must be damp at all times
Gametophytes are nutritionally independent

Stomata-like structure in sporophyte allows spores to dry for dispersal

Question 18

First evidence of terrestrial land plants

Answer 18

430MYA along with the first mycorrhizal affiliation. Rhyniophyte discovered in Aberdeen had fungal sacs in their roots. Club moss have spare capsules amongst leaves.

Question 19

Vascular plants have tracheids

Answer 19

Tracheids have strengthened walls (lignin) connected by pits which increase speed of water flow, add rigidity and allow damaged tracheids to be bypassed - allowing plants to grow taller

Question 20

Monilophyta horsetail

Answer 20

Jumping spores - move by elator tissues contracting and expanding by humidity allowing wide dispersal.

Question 21

Fern lifecycle

Answer 21

1) haploid spore develops into young gametophyte
2) gametophyte antheridia release sperm that travel to egg in archegonia
3) fertilisation, zygote (diploid) develops into sporophyte that grows up out of gametophyte
4) mature sporophyte produces sporangia ( in clusters known as sori) ferns can be identified by their sori (singular sorus)
5) sporangium matures dries out and releases haploid spores - back to 1)

Question 22

Ferns

Answer 22

Very varied: aquatic-azoa, tree fern, tiny moonwort and even epiphytic like stagshorn (grow w/out soil)

Question 23

Overtopping

Answer 23

Growth pattern in which one branch differentiates from and grows beyond the rest

Question 24

Evolution of leaves

Answer 24

Microphylls- sporangiums evolved into simple leaf-like structures as seen in moss fronds

Megaphyll- true leaves, flat plates of photosynthetic tissue develop between branches and branch ends become leaf veins. Branching stem reduced and flattened as seen in ferns. First occured in Devonian era

Question 25

Homospory

Answer 25

1)Gametophyte (n)
2) antheridium produces sperm (n)
archegonium produces egg (n)
3) fertilization forms zygote (2n)
4) sporophyte (2n) forms sporangia (2n)
5) spore mother cell (2n)
6) meiosis forms spores (n)
7) spore becomes gametophyte (n) back to 1)

Question 26

Heterospory

Answer 26

1) a) female megagamete (n)
b) male microgamete (n)
2) a) egg (n)
b) sperm (n)
3) fertilization forms zygote that develops to embryo (2n)
4) sporophyte develops (2n)
5) a) megasporangium
b) microsporangium
Produce spore mother cells (2n)
6) meiosis:
a) megaspore
b) microspore
Which develop to gametes return to 1)

Question 27

Summary

Answer 27

Land plants - heterospory, tracheophytes and vascular system - photosynthetic eukaryotes which develop from embryos protected by parental tissue. Arose from single algal ancestor. Use chlorophyll a & b, store carbs as starch. Life cycle features alterations of multicellular gametophytes (haploid) and sporophyte (diploid) generations. Cuticle, gametangia, protective pigments, stomata, mutualistic relationship with fungi (mycorrhizal)

Question 28

Reproduction in gymnosperms heterospory

Answer 28

Sporophyte (2n)
Develops mega & microsporangium
Which each develop a spore mother cell (2n) which split by meiosis into 4 forming 4 mega spores and 4 microspores
One or more megaspores develop into a megagametophyte : megasporangium+ sporophytic integument =ovule (egg in embryo sac) retained in megasporangium (ovary)
and microspores develop into microgametophytes (pollen) which are released from the anther aka microsporangium