3. Plant Gas Exchange Flashcards
What happens to CO2 in photosynthesis?
Carbon is “fixed” from CO2 and used to produce glucose
What happens to water in photosynthesis?
It is split: the H is used to help in the production of glucose, but the oxygen is excreted as a waste gas
Energy transfer in photosynthesis
Light energy is transferred to chemical energy stored in the glucose molecule
What is glucose used for (as a product of photosynthesis)?
- Respiration
- Stored as starch
- Building cell walls out of cellulose
Function of waxy cuticle
Reduces water loss via transpiration
Main feature & function of upper epidermis
Transparent: allows light through
Adaptations/features of palisade cells
- Lots of chloroplasts: high rate of photosynthesis
- Cells arranged vertically: more light energy being trapped, more cells exposed, more efficient photosynthesis
- Cells are very compact
Adaptations/features of spongy mesophyll cells
Cells loosely packed with lots of air spaces: efficient gas exchange, gases can reach all the cells, more space for diffusion
Function of xylem
Brings water & mineral ions from the soil
Photosynthesis & ensures cells are turgid
Function of phloem
Takes away products of photosynthesis (sucrose)
What maintains the concentration gradient of gases for photosynthesis?
Wind!
Oxygen gets swept away so that there is a high CO2 concentration outside the leaf
What do cotyledons contain?
Food reserves for the developing embryo, usually in the form of starch
What are monocotyledons/dicotyledons?
Monocotyledons have one embryo leaf, dicotyledons have 2
Leaf adaptations for efficient gas exchange
- Large SA: wide & flat leaves, long & thin palisade cells, spongy mesophyll spaced apart
- Short diffusion distance: flat leaves, air spaces in spongy mesophyll layer, stomata
- Steep conc gradient: CO2 used in photosynthesis
How does evaporation in the leaf tissue happen?
Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll. This vapour diffuses out of the leaf via the stomata, creating a negative pressure gradient within the leaf.
How does evaporation within the leaf cause water to be drawn up from the xylem?
The negative pressure gradient creates a tension force in leaf cell walls, drawing water from the xylem (transpiration pul). The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls.
What is transpiration rate, and what is it regulated by?
Amount of water lost from the leaves
Regulated by the opening & closing of the stomata
What happens when guard cells become flaccid?
Block the opening/stomata, so stomatal pore closes due to loss of turgor
Why do levels of photosynthesis affect transpiration?
Because stomatal pores are responsible for gas exchange in the leaf
Factors affecting transpiration: humidity
High humidity = shallow diffusion gradient inside & outside of leaf = less transpiration
Factors affecting transpiration: temperature
High temp = higher rate of evaporation = faster transpiration
Factors affecting transpiration: light intensity
Higher light intensity = open stomata = higher rate of transpiration
Xerophytes
Plants that have adapted to thrive under dry conditions
Halophytes
Plants that have adapted to cope with high-salt environments (e.g. living in or near the sea)
How do xerophytes survive (in a nutshell)?
By reducing transpiration!
Environments that xerophytes live in
Deserts, sand dunes
Xerophytes life cycle adaptations
- Perennial plants bloom in wet seasons
2. Dormant seeds can survive for many years until conditions are ideal for growth
Xerophytes metabolic adaptations
CO2 absorbed at night & stored as a C4 compound. During the day, photosynthesis can occur with the stomata closed by using these carbon stores.
Xerophytes physical adaptations
- Fewer leaves/stomata
- Rolled leaves/spines
- Stomata in pits with hairs
- Deeper roots to reach water
- Waxy cuticle reduces evaporation
Cross section of marram grass adaptations
- Waxy cuticle
- Rolled leaf protects boundary layer from wind & reduces outward SA (like a little humid pocket!)
- Hairs trap a layer of water vapour
- Stomata in pits maintain boundary layer
Explaining general adaptations of xerophytes: thick cuticle
Stops uncontrolled evaporation from leaf cells
Explaining general adaptations of xerophytes: small leaf SA to volume ratio
Reduces SA for evaporation
Explaining general adaptations of xerophytes: stomata sunken in pits
Maintains humid air around stomata so shallower concentration gradient
Explaining general adaptations of xerophytes: leaf hairs
Maintains humid air around stomata so shallower concentration gradient
Explaining general adaptations of xerophytes: rolled leaves
Maintains humid air around stomata so shallower concentration gradient
Explaining general adaptations of xerophytes: deep roots
Access deep water sources
Explaining general adaptations of xerophytes: widespread shallow roots
As soon as it rains water can be quickly absorbed into the roots (rapid access to rainwater)
Explain how the waxy cuticle reduces water loss
Impermeable to water/waterproof
