Lecture 6 Flashcards
What gave rise to land plants?
freshwater algae gave rise to land plants
When do the first plant fossils date to?
465 million years ago
What did the transition onto land require?
the transition onto land required solving an array of problems, many related to the
- scarcity of water for photosynthesis
- support
- reproduction
- other cellular processes
What does land offer?
Land also offered some advantages, including the faster diffusion rate of CO2 in air and, initially, lower competition for space
What did land plants inherit?
Land plants inherited a number of important features from their green algal ancestors, including…
1) sporopollenin
2) oogamy
3) matrotropy
4) the potential for parenchymatous growth
5) glycolate oxidase
What did land plants evolve?
Land plants evolved many new features including…
1) cuticle
2) stomata
3) spores
4) alternation of generations
5) roots
6) xylem
7) phleom
Why were plants originally in the water?
not much nutrients on land
Describe the qualities of life in water and what it affects
1) water is denser
2) water is more viscuous
3) water has a higher specific heat capacity than does air (takes more energy to change the temperature)
- affects buoyancy and fluid dynamic forces, diffusivity, and thermal balance
Describe water molecule
polar molecule with strong hydrogen bonds
- H = net positive change
- O = net negative charge (electronegative)
What are some of the challenges for photosynthetic organisms with cell walls of living in air?
1) staying hydrated
2) acquiring nutrients
3) moving around (dispersal, fertilization)
4) mechanical support (stature for light capture, dispersal)
5) coping with temperature variation
6) protection from ultraviolet radiation
What are the two strategies for dealing with fluctuations in water availability
1) poikilohydric plants
- hydration fluctuates with environment
- must be able to tolerate large swings in hydration
2) homeohydric plants
- maintain a “constant” level of hydration
- construct and sustain the infrastructure needed to supply/regulate water uptake and water loss
- lots of energy to maintain energy levels
Explain the two options of poikilohydric plants
1) grow in only continually wet microsite (swamps, rock in stream…)
TES
2) Tough (desiccation tolerant), Efficient (minimum investment in non-photosynthetic tissues), Small (stay within boundary level)
What is a boundary layer?
the layer of reduced velocity in fluids, such as air and water, that is immediately adjacent to the surface of a solid past which the fluid is flowing
What is the significance of a boundary layer?
if substrate is wet, then the humidity in the boundary layer will be high. This means that surface moisture will persist longer compared to outside the boundary layer
- if a plant is going to be in the air, the boundary layer is where drying will be slower and the moisture is the highest for the longest period of time
What does it mean to be desiccation tolerant?
An ability to survive cellular dehydration
- does NOT mean you can grow in dry places
Are cacti desiccation tolerant?
Cacti are NOT desiccation tolerant - they avoid rather than tolerate desiccation
- don’t let water get pulled out of them
How are cells desiccation tolerant?
cells accumulate appropriate solutes that stabilize macromolecules and membranes at very low water contents
- wall characteristics are also important (less understood)
State two plants that are desiccation tolerant
Many mosses and seeds are desiccation tolerant
- NOT every tissue is necessarily desiccation tolerant (ex. leaves are not desiccation tolerant)
What are homeohydric plants?
1) Ability to control water loss from surfaces (cuticle slows water loss and prevents CO2 from coming in - regulates water loss AND stomata allows CO2 to come in)
2) Ability to access water in the soil with the roots, vascular system (phloem-transport CO2 and xylem-transport water)
3) Lots of infrastructure which is costly but can sustain photosynthesis over a longer time periods and in a wider range of environments/conditions
What structural adaptations are needed to survive outside of the boundary layer?
1) Cuticle which is a mechanism to prevent water loss from surfaces
2) Stomata and internal air spaces to allow CO2 to reach photosynthetic cells
What are two adaptations to control water loss?
1) Stomata (turgor controlled valves)
2) Cuticle (water impermeable layer made of waxes and fatty acids)
How do stomata work?
open up when they swell with water (water flows in with osmosis) and close when they lose water and turgor
Describe the stomata
a nexus of sensory input and response including light, CO2 concentration, water status of tissues
What happens when the stomata are open? What does this lead to?
When the stomata are open, for every CO2 molecule that diffuses in though the stomata and becomes incorporated into carbohydrates via photosynthesis about 400 water molecules diffuse out
THEREFORE, need a system for water uptake from the soil (roots) to transport water to the leaves (xylem) to supply roots with carbohydrates needed for growth and respiration (phloem)
List some cells that are in contact with fluid media and are able to take up nutrients. What is this called?
“simple” multicellularity
- choleochaete
- chara and nitella
- zygnematales
- ulva (sea lettuce)
- volvox
How does acquisition of nutrients occur on land?
On land, acquisition of nutrients requires contact with hydrated substrates, typically below ground
Why is moving around a challenge of living “in air”?
in water, buoyancy and currents are helpful for moving gametes
Describe the streptophyte algae life cycle (haplontic).
- taking advantage of the fact that things can float and move around easily
1) Fertilization: chara and coloechaete have swimming sperm while zygnametales use conjugation (grow together)
2) Dispersal: chara and zygnametales have zygotes that can be carried by currents; in coloechaete meiosis is followed by mitosis resulting in motile “spores” that then disperse by swimming (flagella)
What does fertilization require?
fertilization requires uniting gametes
- first land plants believed to have relied on swimming sperm for fertilization even though swimming on land is a challenge –> future developments
What does dispersal involve?
Dispersal involves moving longer distances and is important because it reduces crowding (competition for resources between close relatives) and disease transmission
–> use sporopollen as a protectant against mechanical damage (including predation), desiccation, and UV exposure
- first land plants dispersed sporopollenin coated haploid spores
Streptophyte green algae have a _______ life cycle
haplontic
Ulva has _________ and is therefore what?
Ulva has alternation of generations and is therefore a multicellular chlorophyte with alternation of generations
What do all plants have?
All plants have alternation of generations in which the two generations are morphologically distinct. Each generation is specialized to enhance different challenges of living on land.
describe gametophyte generation
good for fertilization: small stature, water more likely to be present
describe sporophyte generation
good for dispersal: tall stature, released spores into the wind
Where does fertilization occur?
fertilization occurs close to the ground in the boundary layer
Where does dispersal occur?
dispersal occurs in the air and requires sporopollenin to protect the tiny cells
What are non-motile spores
1) covered by sporopollenin
2) remain tightly packed together in tetrads (can see the 3 other spores it was in contact with)
3) often with trilete mark
What does meiosis result in?
Meiosis results in four non-motile spores that initially remained together in a tetrad
What are 3 fossil evidences of the earliest land plants?
1) spores - no other algae have sporopollenin or triplets marks
2) cuticles - not found in aquatic relatives (NOT found in algae, YES found in plants)
3) “tubes”
What is a major theme in plant evolution?
a major theme in plant evolution is an increase in stature and elaboration of the sporophyte generation
What type of growth did the earliest land plants have?
earliest land plants likely had parenchymatous growth
What does volume provide?
volume provides support but creates problems of access to CO2 and nutrients
- Surface to volume ratio decreases with increasing volume
- Need for internal specialization (ex. air spaces in leaves)
What is primary wall speciation?
greater elaboration of cell types - impregnate walls with other materials to strengthen the walls
What is lignin?
- Makes wood “woody”
- Slows decomposition which is a problem for paper making and biofuels
- Helps trees support themselves (and transport water)
What happens when rubisco uses O2 instead of CO2? What is this called?
leads to a net loss of carbon and energy which is called “photorespiration” because energy is used (ATP, NADPH) and CO2 is given off
Where do UV absorbing compounds accumulate?
in the epidermal cells of leaves and they act as selective sunscreen to reduce the penetration of UVR into the leaf tissue
- do NOT affect the penetration of visible light (essential for photosynthesis)
Why did the ancestors of plants leave their aquatic homes?
1) faster rates of CO2 diffusion in air
2) less competition for space leads to more resources
What is an epiphyte?
grows on another plant
