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S295 The Biology of Survival > Topic 5 Surviving with limited water > Flashcards

Topic 5 Surviving with limited water Flashcards

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1
Q

When would the capacity for air to hold water increase?

A

When air temperature rises.

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2
Q

What is the cuticle of a plant? What does it prevent?

A

The outer, waxy, water-impermeable layer on the surface of leaves - prevents evaporative loss.

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3
Q

What are two major drawbacks of being waterproof?

A
  1. The organism would not be able to use evaporation as a means of controlling its internal temperature.
  2. The organism would not be able to exhange gases with the atmosphere so if heterotrophic, lack of O2 from the atomosphere would limit aerobic respiration, and if photosynthetic, lack of CO2 from the atmosphere would limit growth.
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4
Q

What is the name of the specialised pores within the surface layer of leaves that permit gaseous exchange between air spaces within the lead and the atmosphere?

A

Stomata

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5
Q

What is transpiration?

A

The process of water movement through a plant and its evaporation from aerial parts e.g., leaves, stems, flowers.

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6
Q

What are the beneficial uses of transpiration for a plant?

A
  • Can cool leaves because evaporating water absorbs its latent heat of evaporation from surrounding tissue.
  • By producing a flow of water through the plant (transpiration stream) it can promote the transport of mineral nutrients absorbed by roots and carried in solution to the shoots.
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7
Q

Why do many plants have reflective surfaces?

A

Reflecting solar radiation prevents plants from heating up and thereby reduces their water use.

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8
Q

What are the two strategies by which desert plants survive water shortage?

A
  • Avoidance
  • Tolerance
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9
Q

What are the features of the avoidance strategy used by some desert plants to survive water shortage?

A
  • Maximising supply through water tapping
  • Water piracy
  • Dormancy
  • Water use efficiency and storage
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10
Q

What are the features of the tolerance strategy used by some desert plants to survive water shortage?

A
  • Dessication (dry out and recover later)
  • Xerophytes (continue to photosynthesise during drought without major structural changes)
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11
Q

What is hydraulic lift?

A

The process by which water moves from wet layers deep in the soil profile to drier shallow layers via a plant’s root system.

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12
Q

What are ephemerals?

A

Short-lived plants that reproduce quickly by seed and then die.

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13
Q

What the the advantages of the C4 pathway for desert plants?

A
  • C4 plants can grow faster than C3 plants where light is not limiting and therefore can complete their life cycle faster.
  • C4 plants use water more efficiently than C3 counterparts.
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14
Q

What is leaf polymorphism?

A

The ability of an individual plant to produce more than one type of leaf.

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15
Q

What are succulents?

A

Plants with thick fleshy leaves and stems that store much more water than a non-succulent plant. Generally use CAM photosynthesis that requires large cells containing large vacuoles to store the organic acid made overnight.

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16
Q

How do succulents avoid water shortage?

A

By storing water, when available, in specialised cells in their stems or leaves, and have very thick cuticles to prevent the loss of this water.

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17
Q

What are resurrection plants?

A

Plants whose tissues are able to dehydrate while remaining alive. The plant appears dead until water is added, when it rehydrates its tissues and appears to come back to life.

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18
Q

What does the term ‘epiphyte’ mean?

A

Dscribes a plant that grows on another plant, not as a parasite, but just using it for support.

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19
Q

What are xerophytes?

A

Plants adapted to living in very dry environments and remain metabolically active during drought by a combination of restricting water loss, efficient water uptake, and high tolerance of low tissue water content.

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20
Q

Give three characteristics of xerophytes.

A
  • Relatively small leaves which have vertical orientation
  • Stomata that do not close until leaf water potential is much lower than in non-xerophytes
  • Biochemical adaptations that allow tissue water content to fall to very low values without damaging the cells.
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21
Q

What adaptations do kangaroo rats have to conserve water?

A
  • Nocturnal activity to minimise vapour pressure gradients
  • Lipid coating on skin and hair to minimise evaporative losses direct from the surface
  • Avoidance of direct solar radiation by staying below ground in long burrorws during the day
  • Synchronisation of their reproductive cycle with heavy rain events.
  • Efficient kidneys that recover almost all the water from their urine before it is exreted, by means of kidney tubules proportionally longer than other mammals.
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22
Q

Describe limited water of saline soils.

A

Salts build up when surface evaporation exceeds rainfall (evaporites). Dissolved salts make water uptake more difficult for plants because water potential at the root surface is low.

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23
Q

Describe limited water of clay soils.

A

Clay soil can contain 30% water and be so dry no plant can get moisture from it. This is due to the water being held so tightly to clay minerals.

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24
Q

Describe limited water of waterlogged soils.

A

Waterlogged soil has no air spaces so oxygen can only diffuse into it very slowly from the atmosphere, meaning plants are unable to efficiently respire and have produce enough energy to enable them to take up sufficient water.

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25
Q

How have some plants become adapted to waterlogged soil?

A

Their root systems contain air-filled pipes as tissue (aerenchyma) that allow oxygen to diffuse from the atmosphere to the roots.

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26
Q

Define water potential (ψ)

A

A measure using units of pressure to denote the availability of water compared with a reference state of pure water at atmospheric pressure / the acpacity of water to move

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27
Q

In terms of water potential, which direction does water move?

A

Water moves down gradients of water potential from regions of higher ψ to regions of lower ψ.

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28
Q

What unit is water potential (ψ) measured in?

A

kilopascals (kPa) or megapascals (mPa) - measured relative to pure liquid water at ambient temperature and pressure which is set as 0.

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29
Q

What is Δψ?

A

Difference in water potential - the driving force for water movement.

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30
Q

What is the equation to calculate water potential (ψ)?

A

ψ = P − π − m

ψ - water potential
P - hydrostatic pressure (turgot pressure)
π - osmotic pressure
m - matric pressure

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31
Q

What is turgor pressure?

A

Equivalent to turgot potential - a component of water potential reflecting the pressure a cell is exerting on its cell walls.

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32
Q

What is plasmolysis?

A

The state of a cell when its turgot pressure is zero due to dehydration. The cytoplasm of the cell shrinks and begins to pull the cell membrane away from the cell wall.

33
Q

What is osmoregulation?

A

The process by which a cell alters its solute content to modulate its solute potential or osmotic pressure.

34
Q

What is a rhizosheath?

A

A zone external to the root but immediately surrounding it, where the physical, chemical, and chemical and biological properties and heavily influenced by mucilage and other secretions from the root.

35
Q

What does the cohesion-tension theory argue?

A
  1. Transpiration from leaves generates a gradient of water potential through leaf cells to veins and causes water to move out of the xylem vessels.
  2. The result is a lowering of water potential in the xylen vessels within the small veins of the leaf resulting in a suction force to be created which is the driving force for the transpiration stream.
  3. Tension in the leaf xylem vessels is transmitted all the way down to the roots, provided that continious columns of water are maintained within the xylem.
36
Q

Why are continuous water columns under tension possible?

A
  1. The strong, lignified cell walls of xylem vessels do not collapse under tension.
  2. Water molecules adhere strongly by hydrogen bonding to the walls of the xylem tubes, so the water column does not pull away from the walls
  3. Hydrogen bonding ensures the cohesion of water molecules to each other, holding the water column together and preventing water from vaporising at the low pressure experienced.
37
Q

What is the equation to calculate flux (the rate at which water is lost to the atmosphere)?

A

Flux = driving force x conductance

38
Q

What does the symbol gs denote?

A

Stomatal conductance.

39
Q

What does stomatal conductance mainly determine?

A

The rate at which plants lose water.

40
Q

What is stomatal conductance determined by?

A

The width of the stomatal pores.

41
Q

What are the the environmental variables to which stomata can respond, and what is the stomatal response to each?

A
  • Light - open when light is sufficient to drive photosynthesis
  • Internal CO2 concentration - Close when CO2 concentration is high enough to supply photosynthesis.
  • Humidity - Close when humidity falls to a level that lowers water-use efficiency below an acceptable threshold.
  • Temperature - Open when termperature is within a suitable range for photosynthesis
  • Hydration - close when the water potential of the leaf falls
  • Damage - Electrical signals trigger closure when neighbouring tissue is damages, eaten or burned
  • Plant growth regulators (hormones) - Some plant growth regulators trigger opening, other closure
42
Q

Describe the structure of a stoma and the surrounding tissue.

A

Each stoma consists of a pore surrounded by two guard cells. Adjacent epidermal cells intimately involved in guard cell functioning are known as subsidary cells. Guard cells and subsidary cells are known as the stomatal complex.

43
Q

What are two key properties of guard cells?

A
  1. They can rapidly and reversibly alter their turgor, which causes the volume of the cells to change, as water uptake occurs and turgot increases above that of adjacent cells.
  2. Guards cells change shape at high and low turgot.
44
Q

Why are solutes actively pumped into guard cells by surrounding subsidary cells to open the stoma?

A

Increasing solute content raises osmotic pressure and lowers water potential, causing water to move into guard cells from surrounding cells.

45
Q

What characteristics of guard cells controls how the cell alters in shape as turgor increases?

A
  1. The asymmetric thickness of the cell wall
  2. The orientation of cellulose fibres.
46
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

47
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

48
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

49
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

50
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

51
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

52
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

53
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

54
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creastt

55
Q

What is cavitation in plants?

A

The event of a water column snapping within a xylem vessel when the tension becomes such that a gas bubble forms, creating an embolism that blocks the flow of water.

56
Q

What hormone is released by plant cells whose turgor is falling? What cells is it released from and what does it trigger?

A

Abscisic acid (ABA); mesophyll cells into cell walls; triggers release of ions from the guard cells leading to a loss of turgot and stomatal closure.

57
Q

What does isohydric mean?

A

Having approximately constant water potential irrespective of external conditions.

58
Q

Guard cells tend to respond to any condition that causes a fall in the rate of what process?

A

Carbon fixation.

59
Q

What are aquaporins?

A

Proteins embedded in the cell membrane that can faciliate water corssing the membrane.

60
Q

What hormone will animals respond to during water stress? What does it do?

A

Vasopressin, made in the base of the brain in response to low water potential of the blood. Stimulates the activity of the kidney tubules, which are able to recover water from urine.

61
Q

How does ABA achieve informing shoots of frying soils?

3 steps

A
  1. Biosynthesis triggers by stress.
  2. Transport to receptors faciliated by rise in pH of xylem sap (water shortage leads to root loading fewer hydrogen ions into the xylem). ABA becomes trapped in compartments of high pH so its concentration increases. The guard cell’s ABA receptors face outward on its cell membrane and are therefore sensitive to the apopplastic concentration of ABA.
  3. Its catabolism in the leaf is maintained at a constant rate, avoiding build up and allowing signal to be removed when stress disappears.
62
Q

What events occur when ABA arrives at its receptor at the guard cells?

A
  • Detected by receptor in cell membrane of guard cells and binds
  • Receptor creates pore for calcium ions to cross the cell membrane, entering the cell.
  • Large efflux of potassium ions triggers.
  • Loss of ions lowers the osmotic pressure within the cell
  • Water leaves the cell
  • Cell turgot falls
  • Stoma closes.
63
Q

What is the difference between positive and negative hormone signals?

A
  • Positive signals trigger a reaction by their presence.
  • Negative signals trigger a response by their absence.
64
Q

What is abscission?

A

The process by which an organism sheds one or more of its parts e.g., leaves, fruitm flowers, seeds.

65
Q

What hormone promotes leaf retention?

A

indole-3-acetic acid (IAA)

66
Q

The timing of abcission in plants is based on a balance between which hormones?

A

ABA (abscasic acid)
IAA (indole-3-acetic acid)

67
Q

What does CAM stand for?

A

Crassulacean acid metabolism

68
Q

What are all species that use the CAM phosynthetic pathway also known as?

A

Succulents

69
Q

Name three characteristics of the CAM phosotsynthetic pathway?

A
  • Diurnal fluctuation in malic acid content.
  • CO2 fixation from the atmosphere occuring mainly at night
  • Stomata open at night but closed for most of the day.
70
Q

Outline the process of CAM photosynthesis.

A

CAm plants open their stomata and fix CO2 at night using PEP carboxylase to produce OAA, and then malic acid. Malic acid is stored in the vacuoles. During the day, the stomata are closed. The malic acid is gradually released into the cytoplasm and is decarboxylated to pyruvate.The carbon dioxide produced then enters the C3 cycle, combining with RuBP to create two molecules of PGA, catalysed by rubisco. A proportion of the PGA leaves the cycle by conversion to carbohydrate, and the remainder is converted back to RuBP. The other carboxylation pyruvate is converted to PEP, the substrate of PEP carboxylase.

71
Q

What is the main difference between C4 and CAM photosynthesis?

A

In C4, CO2 capture and CO2 fixation are separated in space (different cells), whilst in CAM they are separated by time.

72
Q

What are facultative CAM species?

A

Species that switch from CAM to C3 metabolism or a mixture of both, depending on water supply and temperature.

73
Q

What is CAM idling?

A

A survival strategy in CAM plants where stomata remain permanantly closed, but repiratory CO2 is recycled internally via the CAM pathway. No growth opccurs during this phase.

74
Q

What is the second law of thermodynamics?

A

No energy transformation can be undertaken with 100% efficiency

75
Q

What is Basal respiration?

A

The rate of respiration required for maintenance of an organism. It is the resting rate when the organism is undertaking minimal activity.

76
Q

What is meant by ‘compensation point’?

A

The level of radiation at which the rate of respiration within a plant organ is exactly matched by its rate of photosynthesis such that the net exchange of carbon dioxide with the environment is zero.

77
Q

What does Pmax denote?

A

The maximum rate of photosynthesis

78
Q

Name some plant storage organs.

A
  • Root
  • Bulb
  • Seeds
  • Tubers
79
Q

Is CAM energy-efficient?

A

No, its maximum growth rates are often limited by its capacity to store CO2.

S295 The Biology of Survival (13 decks)

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