Theme Plants

Question 1

Endosymbiotic Events

Answer 1

• Engulfing without digestion
• Primary Endosymbiosis resulted in retention of: mitochondrion (alpha proteobacterium), plastid/chloroplast (cyanobacterium)
• Plastids within the eukaryotic cell allowed for photoautotrophs (algae) which reduces the requirement for phagocytosis for nutrient acquisition, increases need for cellular water
• Secondary Endosymbiosis occurred across various lineages of aquatic algae

Question 2

Secondary Endosymbiosis

Answer 2

• Subsequent uptake of a red and/or green alga by other non-photosynthetic eukaryotes gave rise to several aquatic algal lineages and phytoplankton

Question 3

Heterotrophic plants

Answer 3

• Some plants have lost the ability to photosynthesize in favour of alternative nutrient acquisition strategies: mycoheterotrophs: obtain carbon source from fungal associations with other plants, Parasitic plants/heterotrophs: obtain carbon source directly from other plants
• these are and represent <1% of all plant species, all ancestral lineages were photoautotrophs

Question 4

Plant Cell Walls

Answer 4

• Reduced requirement of phagocytosis allows for the development of cell walls
• Primary Cell Walls provide support and protection but allow for flexibility
• Rigidity of cell with a primary cell wall is highly dependent on turgor pressure -> dictated by surrounding environment and maintained by vacuole
• Primary Cell Wall is deposited outside of the plasma membrane
• Composition of the primary cell walls is largely the same among plants

Question 5

Cellulose

Answer 5

• most abundant biopolymer in world/known universe
• long, unbranched microfibrils made up of beta1-4 glucose linkages
• incredibly hard to digest in humans -> major component of dietary fibre
• cell walk is quite porous and allows for nutrients and water to pass through

Question 6

Osmotic Pressure

Answer 6

• Plant cells acquire or lose water through osmosis
• The primary cell wall provides tensile strength and prevents the cell from bursting under hypotonic conditions and from completely collapsing under hypertonic conditions
• The rigidity comes from hydraulic turgor pressure from the vacuole

Question 7

Vacuoles have higher solute concentration and the osmotic flow of water occurs:

Answer 7

from outside the cell (lower solute concentrations) to into the vacuoles

Question 8

Turgid state

Answer 8

• the ideal state for plant cells, helps maintain the structure of the plant and maintains osmotic balance
• The turgidity of the vacuole pushes the plasma membrane outwards and this pushes against the cell wall

Question 9

Effects of Osmotic Pressure

Answer 9

• Rapid and concerted movements of solutes can quickly divert water from the cell and force some cells to contract
• Pressure sensors or mechanosensors can activate ion channels that change the osmotic potential of the cell
• Actually causes an action potential similar to animal cells and can trigger coordinated events in nearby tissues or organs

Question 10

Multicellularity in Eukaryotic Algae

Answer 10

• True multicellularity involves the specialization of cells to perform different functions
• Allows for coordinated behaviour of different tissues to aid in the fitness of the organism as a whole
• Multicellularity arose multiple times in algal lineages, completely separate from the animal/fungal ancestral lineage
• End products are similar but the processes are unique and can be used to distinguish evolutionary trends

Question 11

Land Plant cell division (and the immediate green algal ancestor) progresses through the production of the:

Answer 11

Phragmoplast

Question 12

Critical Importance to the multicellularity of Land Plants

Answer 12

• This arrangement of the microtubules allows for cell wall deposition between the the two separated nuclei prior to the complete division of the cytoplasm
• Deposition of the cell wall materials also involves the fusion of vesicles from the ER and Golgi to form the new plasma membranes

Question 13

Portions of the ER are often trapped within the newly created cell wall, creates:

Answer 13

connections between the two daughter cells known as plasmodesmata

Question 14

Plasmodesmata

Answer 14

intercellular connections between plant cells, analogous to gap junctions in animals

Question 15

These connections are formed cell division

Answer 15

Secondary plasmodesmata can develop through the degradation of the cell wall but does not occur in all cells

Question 16

These connections allow for easier cell-to-cell transport of:

Answer 16

cytoplasmic contents, i.e nutrients, water, minerals, proteins, hormones, nucleic acids

Question 17

Defining Characteristics of Plants:

Answer 17

All plants:

• Are sessile
• Are photoautotrophic (vast majority, >99%)
• Have primary cell walls(largely composed of cellulose and hemicellulose)
• Are multicellular
• Have Cuticles
• Have Stomata for gas exchange
• Have an alternation of generations -> Diphasic life-cycle

Question 18

Why Move Out of the Water?

Answer 18

• No competition
• Unlimited and untapped mineral resources
• Unlimited space for establishment and growth
• Full access to light
• No predators
• Any organisms that could become accustomed to life on land had room to radiate

Question 19

Freshwater algal mats would have been the first to:

Answer 19

• begin photosynthesizing on land, would have to withstand wet-dry cycles but were still largely aquatic.
• Likely formed symbiotic relationships with fungi, established the first soils and allowed for further diversification

Question 20

First truly terrestrial plants were the first of the:

Answer 20

Embryophytes -> Bryophytes

Question 21

Plants established habitable regions, acting as

Answer 21

the primary producers

Question 22

Challenges with Life on Land

Answer 22

• Desiccation (protection from drying out and dealing with scarce water)
• Respiration (now have to exchange atmospheric gases, rather than dissolved ones)
• Reproduction (can no longer release eggs into water)
• Locomotion and support (no longer buoyant, had to develop reinforced tissues)
• Nutrients (minerals now in substrate rather than ions in water)
• Senses (have to adapt to new light, sounds, and smell)

Question 23

Major Early Adaptions

Answer 23

1. Waxy cuticles (to prevent cells from drying out)
2. Stomata (Pores to control gas exchange and water loss)
3. Alternation of generations increasing reliance on “hard” propagules, localized release of gametes)

Question 24

The Waxy Cuticle

Answer 24

• Protects the plants from drying out and also from pathogen attack
• The fossil record shows the presence of cuticle in early land plants and on spores
• Wax is deposited outwards by the epidermal cell layers (upper and lower)
• Thickness varies depending on the perception of heat and light
• Wax is not easily permeable to gases

Question 25

Stromata

Answer 25

• Stomata are pores that allow exchange of gases across photosynthetic surfaces
• Opening and closing of the pore is controlled by specialized cells called Guard cells
• The guard cells themselves are still covered by the cuticle but create a pore when they swell and pull apart
• Regulation of the release of gases only when water is plentiful greatly reduces unnecessary water loss

Question 26

Alternation of Generations

Answer 26

• Instead of one “body”, the plant life cycle progresses through two multicellular growth forms:
  = Diploid (2n) Phase = Sporophyte -> makes spores (n) through meiosis
• Haploid (n) Phase = Gametophyte -> makes gametes (n) through mitosis

Question 27

Sporophyte

Answer 27

Diploid (2n), multicellular body/form, produces spores (n) through meiosis

Question 28

Spores

Answer 28

Haploid, unicellular but will germinate to produce Gametophyte (n) through mitosis

Question 29

Gametophyte

Answer 29

Haploid, multicellular body/form, produces haploid, unicellular gametes (n) through mitosis

Question 30

Gametes

Answer 30

Sperm and egg cells fuse (fertilization) to form a unicellular zygote (2n) that will quickly develop into an embryo through mitosis

Question 31

Embryo

Answer 31

Diploid (2n), multicellular and will develop into a Sporophyte through mitosis

Question 32

Why Do Plants Alternate Generations?

Answer 32

Amplification of gametes and chances of successful fertilization events and offspring