Lecture 6

Question 1

What gave rise to land plants?

Answer 1

freshwater algae gave rise to land plants

Question 2

When do the first plant fossils date to?

Answer 2

465 million years ago

Question 3

What did the transition onto land require?

Answer 3

the transition onto land required solving an array of problems, many related to the

• scarcity of water for photosynthesis
• support
• reproduction
• other cellular processes

Question 4

What does land offer?

Answer 4

Land also offered some advantages, including the faster diffusion rate of CO2 in air and, initially, lower competition for space

Question 5

What did land plants inherit?

Answer 5

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

Question 6

What did land plants evolve?

Answer 6

Land plants evolved many new features including…

1) cuticle
2) stomata
3) spores
4) alternation of generations
5) roots
6) xylem
7) phleom

Question 7

Why were plants originally in the water?

Answer 7

not much nutrients on land

Question 8

Describe the qualities of life in water and what it affects

Answer 8

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

Question 9

Describe water molecule

Answer 9

polar molecule with strong hydrogen bonds

• H = net positive change
• O = net negative charge (electronegative)

Question 10

What are some of the challenges for photosynthetic organisms with cell walls of living in air?

Answer 10

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

Question 11

What are the two strategies for dealing with fluctuations in water availability

Answer 11

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

Question 12

Explain the two options of poikilohydric plants

Answer 12

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)

Question 13

What is a boundary layer?

Answer 13

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

Question 14

What is the significance of a boundary layer?

Answer 14

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

Question 15

What does it mean to be desiccation tolerant?

Answer 15

An ability to survive cellular dehydration

- does NOT mean you can grow in dry places

Question 16

Are cacti desiccation tolerant?

Answer 16

Cacti are NOT desiccation tolerant - they avoid rather than tolerate desiccation
- don’t let water get pulled out of them

Question 17

How are cells desiccation tolerant?

Answer 17

cells accumulate appropriate solutes that stabilize macromolecules and membranes at very low water contents
- wall characteristics are also important (less understood)

Question 18

State two plants that are desiccation tolerant

Answer 18

Many mosses and seeds are desiccation tolerant

- NOT every tissue is necessarily desiccation tolerant (ex. leaves are not desiccation tolerant)

Question 19

What are homeohydric plants?

Answer 19

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

Question 20

What structural adaptations are needed to survive outside of the boundary layer?

Answer 20

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

Question 21

What are two adaptations to control water loss?

Answer 21

1) Stomata (turgor controlled valves)

2) Cuticle (water impermeable layer made of waxes and fatty acids)

Question 22

How do stomata work?

Answer 22

open up when they swell with water (water flows in with osmosis) and close when they lose water and turgor

Question 23

Describe the stomata

Answer 23

a nexus of sensory input and response including light, CO2 concentration, water status of tissues

Question 24

What happens when the stomata are open? What does this lead to?

Answer 24

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)

Question 25

List some cells that are in contact with fluid media and are able to take up nutrients. What is this called?

Answer 25

“simple” multicellularity

• choleochaete
• chara and nitella
• zygnematales
• ulva (sea lettuce)
• volvox

Question 26

How does acquisition of nutrients occur on land?

Answer 26

On land, acquisition of nutrients requires contact with hydrated substrates, typically below ground

Question 27

Why is moving around a challenge of living “in air”?

Answer 27

in water, buoyancy and currents are helpful for moving gametes

Question 28

Describe the streptophyte algae life cycle (haplontic).

- taking advantage of the fact that things can float and move around easily

Answer 28

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)

Question 29

What does fertilization require?

Answer 29

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

Question 30

What does dispersal involve?

Answer 30

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

Question 31

Streptophyte green algae have a _______ life cycle

Answer 31

haplontic

Question 32

Ulva has _________ and is therefore what?

Answer 32

Ulva has alternation of generations and is therefore a multicellular chlorophyte with alternation of generations

Question 33

What do all plants have?

Answer 33

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.

Question 34

describe gametophyte generation

Answer 34

good for fertilization: small stature, water more likely to be present

Question 35

describe sporophyte generation

Answer 35

good for dispersal: tall stature, released spores into the wind

Question 36

Where does fertilization occur?

Answer 36

fertilization occurs close to the ground in the boundary layer

Question 37

Where does dispersal occur?

Answer 37

dispersal occurs in the air and requires sporopollenin to protect the tiny cells

Question 38

What are non-motile spores

Answer 38

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

Question 39

What does meiosis result in?

Answer 39

Meiosis results in four non-motile spores that initially remained together in a tetrad

Question 40

What are 3 fossil evidences of the earliest land plants?

Answer 40

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”

Question 41

What is a major theme in plant evolution?

Answer 41

a major theme in plant evolution is an increase in stature and elaboration of the sporophyte generation

Question 42

What type of growth did the earliest land plants have?

Answer 42

earliest land plants likely had parenchymatous growth

Question 43

What does volume provide?

Answer 43

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)

Question 44

What is primary wall speciation?

Answer 44

greater elaboration of cell types - impregnate walls with other materials to strengthen the walls

Question 45

What is lignin?

Answer 45

• Makes wood “woody”
• Slows decomposition which is a problem for paper making and biofuels
• Helps trees support themselves (and transport water)

Question 46

What happens when rubisco uses O2 instead of CO2? What is this called?

Answer 46

leads to a net loss of carbon and energy which is called “photorespiration” because energy is used (ATP, NADPH) and CO2 is given off

Question 47

Where do UV absorbing compounds accumulate?

Answer 47

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)

Question 48

Why did the ancestors of plants leave their aquatic homes?

Answer 48

1) faster rates of CO2 diffusion in air

2) less competition for space leads to more resources

Question 49

What is an epiphyte?

Answer 49

grows on another plant