Minimod 2 - Plant Physiology Flashcards
(90 cards)
1
Q
What are some of the issues with crops?
A
- big plants
- Long generation time
- Often complex genomes
- Difficultly to genetically manipulate
- Impractical
2
Q
Why is arabidopsis thaliana the plant model species?
A
The plant model species:
* small
* Short generation time
* Easy to genetically manipulate
* Small genome
* The first multicellular eukaryote to have its genome sequenced
3
Q
What does germination require?
A
Requires:
* water
* Oxygen
* Light
4
Q
What is meant by a seed being etiolated?
A
If seed germinates and doesn’t detect light it will become etiolated
* extended hypocotyl
* Cotyledons don’t unfold
Being able to detect light is critical, seed reserves will eventually run out
5
Q
What is a photochrome?
A
- photoreceptor - a protein that detects light
- Mediates many aspects of plant growth
- Including germination
(There are other plant photoreceptor proteins eg cryptochromes, phototropins and ZTLs) - absorbs red and far red light (600-750nm) most strongly
- Can also absorb blue light (350-500nm) and UV-A radiation (320-400nm)
6
Q
What is phytochromobilin?
A
- a multi domain protein
- A chromosphere called phytochromobilin is attached to the photo sensory domain
- phutochromobilin can convert between 2 forms - Pr and Pfr
- Pr and Pfr are cis-trans isomers
- Red and far red light mediates this conversion
7
Q
What does isomerism of phytochromobilin cause?
A
- cis-trans isomerism of phytochromobilin causes changes to structure of the protein
- enables Phytochrome to move to the nucleus
8
Q
What is a phyla mutant?
A
- phyla is a plant with a mutated Phytochrome
- plants with mutated PHYA (Phytochrome) can’t detect light
- Looks like wild type seedling grown in the dark (cotyledons not expand, hypocotyls continue to extend upwards)
- As if still under soil
9
Q
How can plants see eachother?
A
Light available at top of canopy compared to understory is very different. Chlorophyll absorbs red but not far-red light. Unproductive to germinate until there is a gap in the canopy
10
Q
Why is living biologically stressful for a plant?
A
Living on a rotating planet is biologically stressful
Over a 24hr period there is large variation in environmental conditions including temperature, light intensity and humidity.
Typical day light fluctuation - 10c
Extreme day night temperature difference: 57 (-48* to 9*, Montana 1972)
11
Q
What are circadian rhythms?
A
- plants and animals have rhythmic behavior associated with these changes
- Circadian clocks are biological oscillators with a 24 hour period
12
Q
Why have a circadian clock?
A
- a circadian clock runs without the need for external stimuli
- Enables the organism to predict time
- Can hit the ground running
13
Q
What are the 3 parameters of a circadian clock?
A
- period: time between comparable points
- Amplitude: height of peaks/trophs
- Phase: any point in cycle that is recognisable from one day to the next
14
Q
What is the simplified clock mechanism?
A
Night = gene z is active, producing Z protein
Dawn = when the amount of Z protein reaches a certain level, it activates expression from gene x and y
Day = x and y proteins accumulate
- x and y proteins affect expression of clock controlled genes
- x and y proteins inhibit the expression of gene Z
15
Q
How can gene expression be controlled by the clock? How can we see this?
A
- transcript levels of clock-controlled genes change throughout day
- Eg genes encoding components of photosynthetic apparatus
- Can be seen with northern blots
16
Q
How can we identify clock components?
A
*enables you to see the
rhythm of the clock
* randomly mutate these plants
* Thousands of mutants screened
for changes in circadian oscillations in luciferase
- promoter controlled by the clock
- luminescence reporter protein from fireflies
17
Q
What is entrainment?
A
- in constant dark, the clock starts to get out of sync
- In natural conditions the clock checks the natural environment to reset itself
- This is called entrainment
18
Q
How does the phytochrome entrain the clock?
A
- Phytochrome senses the start of the day
- Feeds this information into the clock
- Induces expression from x and y
19
Q
What are the 3 parts to a circadian clock?
A
- central oscillator - a negative feedback loop
- Input for entrainment to local conditions
- Output to effect responses
20
Q
Why do plants need water?
A
- metabolism - eg photosynthesis
- Turgor (mechanical stability, physiological processes, eg cell growth, stomata opening)
- Nutrient acquisition
21
Q
What is osmosis?
A
Osmosis is a process by which molecules of a solvent (usually water) tend to pass through a semi permeable memrbane from a less concentrated soiultion into a more concentrated one.
There are still the same number of solute molecules on each side of the membrane -
22
Q
What is a semi-permeable membrane?
A
- water and other small uncharged molecules can cross
- Large solutes and charged molecules cannot
- Cell membranes are semi permeable
23
Q
What is water potential?
A
- helps to describe the tendency of water to move from one area to another
- Particularly between cells
- Water always moves from high water potential to low
- The water potential of a solution can be described by -
solute potential + pressure potential + gravitational potential
24
Q
What is gravitational potential?
A
- gravity causes water to move downward
- Difference between neighbouring cells is negligible
- Generally omitted when considering water transport at the cellular level
25
What is solute potential?
* effect of dissolved solutes on water potential
* Pure water has a solute potential of 0
* Add solute and solute potential decreases
* Solute potential is always negative
* The more solute molecules present, the more negative the solute potential is
26
What is pressure potential?
* effect of pressure on water potential
* Positive (eg turgid cell) and negative (eg xylem) pressures
* Pure water at atmospheric pressure has pressure potential of 0
27
What is turgid cell state?
Cell fully inflated with water
28
What is flaccid cell state?
No pressure against cell wall
29
What is plamolysed state?
Cell membrane pulled away from wall
30
What is meant by hypertonic?
Concentrated solutes outside
31
What is meant by isotonic?
Equivalent solute concentration
32
What is meant by hypotonic?
Dilute solutes outside
33
What are the levels of water transport in plants?
* into/out of cells
* Between tissues
* Bulk flow - long distance
34
What is the soil plant air continuum?
* 97% water absorbed through roots is lost through transpiration
It is a balancing act
35
How are plants waterproof?
* cells are full of water
* Doesn’t evaporate straight out of cells in leaf because of cut in in the epidermis forming a waxy cuticle
36
What are stomatal pores?
* but, must have gas exchange for photosynthesis
* Stomata regulate gas exchange
* Water evaporates out of stomatal pores as CO2 enters
37
What are stomata? Mutants?
* stomata usually present on both upper and lower surfaces of leaf, but more on the lower
* Open during the day, closed during the night
* Aperture dependent on temperature and water availability
- mutants in stomatal patterning include lack of stomatal openings and too many stomatal openings
38
What are the 2 things transpiration rate depends on?
Transpiration rate depends on -
1) the difference in water vapor concentration between leaf and air spaces and atmosphere
* massively influenced by temperature
2) diffusion al resistance of the pathway from leaf air spaces to atmosphere
* leaf stomatal resistance (are they open or closed)
* Boundary layer resistance
39
What is the boundary layer?
* layer of unstirred air next to the leaf surface
* Water vapor must diffuse through this
* Thickness of a layer determined by wind speed and leaf size
* Still air, boundary layer gets thicker, transpiration slower
40
What is bulk flow through xylem and phloem?
* water evaporating out of leaf through stomata provides the energy to pull a column of water through the whole plant under negative tension
* negative hydrostatic pressure
* Pressure driven bulk flow
41
What is cavitation?
* negative pressure becomes too great
* Water column snaps
* Air cavity forms
* Called a cavitation or embolism
* Water flow stops
* vessels and tracheids are connected by pits
* So water can bypass the cavitation
* But if too many cavitations form, the entire
xylem could become blocked
42
Difference between vessels and tracheids?
Vessels are wider
* can achieve faster flow
* More vulnerable to cavitation
There is always a trade off between fast flow of water and safety from cavitation
Most gymnosperms do not have vessels, only tracheids
43
How can cavitations be repaired?
* vessels can be seen with MRI
* The number of full vessels (white spot on image) increases
when stem is given water and put in the dark
* positive root pressure
* At night - roots pump solutes from soil into xylem
* As solutes accumulate, psi is lowered
* Water moves from high psi to low psi so moves from soil to root
44
Where is carbon ideally concentrated?
* concentrate CO2 around RuBisCO
* Reduced oxygenate activity
* Reduce energy wasted in photorespiration
45
What is C4 photosynthesis?
C4 photosynthesis - spatial separation of initial carbon fixation and carbon fixation into the Calvin cycle. Has specialist anatomy and metabolism.
46
Aim of C4 photosynthesis? 2 specialised cell types?
Aim: to concentrate Co2 around RuBisCO to
reduce photorespiration
2 specialised cell types key to C4 photosynthesis:
* mesophyll
* Bundle sheath (BS)
47
What are the 5 stages to the C4 mechanism?
1. Co2 is fixed by PEPC into a C4 carbon (C4) acid in the mesophyll cells
2. The C4 acid is moved into the bundle sheath cells
3. The C4 acid is decarboxylated to produce Co2
4. The Co2 is used by RuBisCO in the usual Calvin cycle reactions
5. The C3 acid is recycled back to the mesophyll cell to start the cycle again
48
What is PEPC?
* PEPC can discriminate between Co2 and O2
* First molecule produced has 4 carbons
* Hence C4 photosynthesis
49
What is Kranz anatomy?
* kranz means wreath in German
* RuBisCO is only in BS cells
* So need to maximise BS compared to C3 plants ( closer vein spacing, BS cells much bigger)
50
Examples of C4 plants?
- maize
- sugarcane
- miscanthus
- sorghum
about 4.5% of dicots and 40% monocots
51
Why don’t all plants use C4 photosynthesis?
The regeneration of PEP 9C30 costs 2 ATP per Co2, assimilated
* at low temperatures there is little photorespiration, so C4 is too expensive to run
* At 20* energetic costs of running C4 = costs of photorespiration
52
How does temperatures affect C4 and C3 photosynthesis?
Low temperatures -
* cost of c4 > cost of photorespiration
* C3 has higher rate of photosynthesis
High temperatures -
* cost of C4< cost of photorespiration
* C4 has a higher rate of photosynthesis
53
Habitats where C4 plants live?
* places with hot summers
* Tropical regions
* Only 4 species native to the UK
54
What is CAM photosynthesis?
* crassulacean acid metabolism
* Temporal separation of initial carbon fixation by PEPC and refixation by RuBisCO
55
Where is CAM photosynthesis distributed?
* found in 5% of all plant species
* Associated with succulent plants in semi-arid environments
* But also found in tropical rainforests, particularly among epiphytes such as orchids and bromeliads
56
Examples of CAM plants?
Pineapple
Agave (tequila)
Vanilla
All cacti
57
What are the 6 stages of CAM photosynthesis?
1. Stomata open at night when humidity is high and temperate low
2. PEPC converts CO2 into a C4 acid (malic acid)(same as C4 ps)
3. The C4 acid is stored in the vacuole until dawn
4. During the day, the stomata close to con serve water
5. The C4 acid is decarboxylated, releasing CO2 and a C3 acid
6. The CO2 is used by RuBisCO in the Calvin cycle
7. The C3 acid is recycled back to the start
58
What is CAM physiology?
* other traits to minimise water loss (thick cuticles, large vacuoles, stomata with small apertures
59
What are CAM switchers?
CAM switchers
* some species are obligate CAM plants (ie use CAM all the time)
* Some species can switch between C3 photosynthesis and CAM
(Use CAM when water is scarce, switch to C3 when water is abundant, transition can also be induced by environmental conditions such as salinity, temperature and light)
60
What is mesembryanthemum crystallinum (the ice plant)
- an inducible CAM plant that responds to salinity and drought
- great tool to use for the study of CAM photosynthesis
61
C4 photosynthesis summary?
- spatial separation of CO2 fixation between mesophyll and bundle sheath cells
- species generally grow in hot climates
62
CAM photosynthesis summary?
- temporal separation of CO2 fixation
- species generally grow in arid conditions or as epiphytes
63
Where do plant nutrients come from?
* photosynthesis provides carbon
* But carbon isn’t enough
* Plants also need nutrients for growth
* These come from mineral ions dissolved in water in the soil
64
Importantance of plant nutrient uptake to humans?
* entry point of minerals into the food chain
* It’s vital to eat the right balance of plants for adequate nutrient provision
* Cereal diets - fe and Zn deficient
* Leads to miscarriage stillbirth and foetal abnormality
65
What are the 3 key macronutrient requirements?
Nitrogen -
* organic compounds eg amino acids, nucliec acids
Phosphorus -
* sugar phosphates, nucleic acids, phospholipids, energy (ATP)
Potassium
* osmotic relations, signalling, enzyme cofactors
*hence NPK fertiliser*
66
What are the secondary macronutrients?
Sulphur - certain amino acids
Calcium - cell signalling, enzyme cofactors
Magnesium - chlorophyll
67
What are the 3 key micronutrients?
Iron - cytochromes
Boron - holds cell walls together
Zinc, copper, manganese, molybdenum - enzyme cofactors
*essential for life but you don’t need much of them*
68
Wha are deficiencies?
* inadequate supply of nutrients affects growth of plant
* Deficiencies of several nutrients may occur simulataneously
* Viruses induced diseases often give very similar symptoms
69
How does a phosphate deficiency manifest in plants?
Marks leaves with reddish purple, particularly on young plants
70
what does a potash deficiency show as on plants?
Deficiency appears as a firing or drying along the tips and edges of lowest leaves
71
What is nitrogen deficiency manifested as in plants?
Sign is yellowing that starts at the tip and moves along middle of leaf
72
What does magnesium deficiency manifest as in plants?
Deficiency causes whitish strips along the veins and often a purplish colour on the undersides of the lower leaves
73
Describe root structure in dicots and monocots
Dicot - tap root with branching laterals and ephemeral feeder roots
Monocot - fibrous root system
74
What features make up the root?
Epidermis
Cortex
Endodermis
Stele:
- pericycle
- xylem
- phloem
75
What are the 3 main compartments in vascuolated cells?
- cell wall
- cytosol
- vacuole
*specific transport membranes and tonoplast traffic molecules between 3 compartments
76
What is meant by apoplast?
Cell wall (between the fibres)
77
What is meant by symplast?
Cytoplasmic (linked by plasmodesmata)
78
What is meant by symplastic transport route?
* through cytoplasm
* Cells connected by plasmodesmata
79
What is meant by apoplastic transport route?
* through the cell walls, without crossing any membranes
* Apoplast forms continuous phase from soil through the cortex
80
Why is ion uptake highly regulated in roots?
* minerals absorbed in the root are transported to the rest of the plant in the xylem
* Uptake of nutrients must be highly regulated to prevent absorption of toxic ions
* Must protect against pathogen invasion from the soil
81
What is the casparian strip? What does it do?
* band of Suberin running through the walls of endodermis - hydrophobic barrier
* Prevents water, ions and pathogens diffusing into the vascular system through cell walls
* Enables plant to control what goes into the xylem (hence to the rest of the plant)
- xylem cells are dead
- forces nutrients to reenter apoplast to enter xylem
82
What is xylem loading?
- second point of regulation
- specialised transporters expressed in the pericycle and xylem in parenchyma
83
What is the mechanism behind the casparian strip?
- ions pumped into symplast from low concentration to high to cross casparian strip
- casparian strip prevents nutrients diffusing back into soil through apoplast
- casparian strip allows xylem to have higher nutrient concentration than is found in the soil
84
What are ion transporters?
* plants absorb minerals in the ionic form
* Most transport proteins show specificity for the ions they transport - so huge diversity
* In arabidopsis there are 48 gene families encoding transporters
85
What is meant by membrane permeability?
* composition of the membrane
* Chemical nature of the solute
* Most ions cannot diffuse straight through the membrane
86
What is passive transport through channels?
* enhance diffusion of septic molecules across membranes
* Specificity determined by - size of pore, density and nature of surface charges on interior lining
* Sometimes gated - pore opens and closes in response to signals
87
What is passive transport - facilitated diffusion?
Carrier proteins
* facilitated difffusion
* do not have pores extending completely across
the membrane
* Highly selective
* Much slower than channels
88
What is active transport?
* for nutrients that need transporting against their electrochemical gradient
- primary (pump mediated)(uniport and cotransport)
- secondary (carrier mediated)(antiport and symptom)
89
How do we measure the different activities of transporters?
* measure how much of a radioactive tracer is absorbed in a set length of time over different ion concentrations
* 2 phases to the curve
* Represents the activity of 2 different types of transporter
90
How is nitrogen uptake coupled to the clock?
Night - metabolic activity low, small needed for nutrients
Day - photosynthesis high - needs nutrients
- stomata open - high level of transpiration, so able to transport nutrients
- nitrogen typically enters the root as nitrate via nitrate transporters
- mRNA transcripts of nitrate transporter genes peak just before the start of the day
