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Flashcards in Surviving With Limited Water Deck (53):

Outline the role of water in biological processes.

Life first evolved in aquatic environments and, as a result, water became a key reagent for many metabolic reactions, as well as providing the solvent in which nearly all metabolic reactions take place.


What have some terrestrial organisms developed to remain hydrated and why do all organisms not possess the same adaptation?

A cuticle, an unbroken, waterproof epidermis composed of oily or waxy compounds. An adaptation which allows desert cactus to survive. All organisms don't have one because it prevents the use of evaporation for temperature control & prevents gaseous exchange with the atmosphere.


Define desert from the perspective of biological survival.

Deserts are defined by their low or unpredictable rainfall, but crucially for survival they are defined by the balance between precipitation and evaporation. Cambridgeshire and Jerusalem have the same annual rainfall but cool English climates retain moisture better.


Roughly how much water does the soft tissue of terrestrial plants contain?

90-95% water


When are stomatal pores in plants open?

Usually only in the light, transpiration ceases at night.


Outline the response triggered in desserts after rains & explain why this happens.

Suddenly bright flowers come into bloom creating vast meadows in what was previously arid, bare soil. These plants must complete their entire life cycle in the brief window after rain & attracting pollinators is vital so competition among blooms is high.


Why do cactuses have thorns?

They are precious reservoirs of water in a dessert & must protect themselves from the threat of thirsty animals.


List some of the adaptations plants have made to living in deserts.

Water storage in succulent leaves & swollen trunks, reflective surfaces (bright white), leaves have thick rinds with few pores, or their are no leaves at all, just photosynthetic stems with pores protected in deep ridges (cactus). Some practice self-amputation in excess drought, or live deep underground with 'window leaves'.


What kinds of habitats resemble mini-deserts in the UK?

Habitats that resemble mini-deserts in the UK include habitats where there is little or no soil (e.g. on rocks, walls or very shallow soils) and habitats such as sand dunes which drain rapidly and retain little water.


What are the main two strategies of handling water shortage?

Avoidance & tolerance.


What are the different methods of AVOIDING water shortage?

Maximising water supply, water piracy, dormancy & efficient water use & storage.


Outline the main method plants use to maximise water supply.

Incredibly deep taproots (up to 50m) which can reach the water table & bring water up through hydraulic lift to the plant leaves at the surface.


Outline the method of water piracy, allowing desert plant growth.

Shallow rooted plants, such as grasses, which make us of moisture lifted by the hydraulic lift system of deeper rooted shrubs. Water seeps out of the shrub roots overnight and the shallow rooted plants utilise it.


What are ephemerals?

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


Outline the dormancy strategy as a method of living with water shortage.

A method of water shortage avoidance where plants fit their entire life cycle into the brief period following rains while water is available, then set seeds, which can last many years, then they die. Others shed their leaves & the entire organism enters a dormant state until the next rainfall.


Define dormancy.

Dormancy comes from the Latin dormire, ‘to sleep’. In biology it is used to describe metabolically inactive organisms, which are usually surviving unfavourable conditions by minimising their resource requirements. It is a common strategy in deserts where the availability of one key resource (water) is spasmodic and often unpredictable.


Do desert plants with short life cycles (seed dormancy strategy) favour C3 or c4 pathways and why?

C4 because it allows very rapid short term growth un-inhibited by the excessive light in desserts. C4 is less energy efficient, but more water efficient than C3. Deserts have abundant sunlight but very little water so C4 is favoured.


Define leaf polymorphism.

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


Define 'succulents' and outline which photosynthetic pathway they use.

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


What are resurrection plants?

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.


What are Xerophytes?

plants that remain metabolically active during drought by a combination of restricting water loss, efficient water uptake and high tolerance of low tissue water content. Typical examples are evergreen desert shrubs and grasses.


List some of the adaptations made for water conservation in animals.

Nocturnal activity, lipid coating on skin and hair to minimise evaporation, avoidance of direct solar radiation by living below ground, synchronisation of reproduction with rainfall.


List the three forms of physiological drought with an outline explanation.

Salinity, makes water uptake difficult because of the low water potential at the roots. Clay soils, moisture is held too tightly to the clay minerals for plants to extract it. Waterlogged soil prevents effective water uptake because their is no space for oxygen, reducing root respiration and limiting the energy available.


Define water potential

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


What are the three main factors which influence water potential and what effect do they have?

Hydrostatic pressure - raises water potential
Solutes - lowers water potential
Solid-liquid interface (colloids) - lowers water potential


Define turgor pressure

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


Define plasmolysis

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


At incipient plasmolysis, how would the appearance of a rooted, herbaceous plant (e.g. a tulip) change?

The plant would droop and appear wilted because of the lack of cell turgor.


Define rhizosheath

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


What is the difference between cell secretion and an excretion?

Both terms refer to substances that pass from a cell into its external environment. The difference is that a secretion is produced by the cell to serve a useful purpose once outside, whereas an excretion is simply a waste product that is ejected from the cell.


Outline the cohesion-tension theory

A theory to explain how tall trees can draw up water to a greater height than a perfect vacuum. It proposes that the cohesive forces between molecules and the adhesive forces between water and the xylem walls prevents the continuous columns of water from breaking, even under very negative pressures (tension).


Why do plant pores close at night?

If there is no light, leaves cannot photosynthesise and thus there is no demand for CO2 and therefore there is nothing to gain from keeping the pores open. Some water vapour would leave even at night, so almost all plants close their pores as soon as it gets dark.


Why does increasing solute content affect guard-cell turgor and hence stomatal opening?

The asymmetric thickness of the cell wall and the orientation of cellulose fibres around the guard cell controls how the cell alters its shape as turgor increases.


Why do floating leaves (such as the water Lily) have stomata that never close?

If the leaf is in intimate contact with a whole body of water, then water availability is not an issue; so, because there is no selective pressure to conserve water, the plant has lost the ability to close its stomata.


Define embolism.

A gas bubble that forms within a xylem vessel following cavitation, which then blocks further water flow. The water column can sometimes be re-established when tension in the xylem relaxes, but often embolisms are permanent and the xylem vessel becomes defunct.


What is cavitation?

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


What are aquaporins?

Proteins embedded in the cell membrane that can facilitate water crossing the membrane.


How do plants detect disturbances is water availability?

Water potential gradient is disrupted and ABA (abscisic acid) is produced.


Define hormones

Hormones are chemical messengers that transport signals from one cell to another and are able to convey information from one part of an organism to another.


Define signal cascade

A chain of messages in which each step in the chain elicits a larger response at the next step causing an initially small signal to be greatly amplified.


What are cytokinins?

A group of organic compounds (such as zeatin) that act as plant-growth regulators. They generally act to promote cell division and growth.


What is abscission?

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


What is indole-3-acetic acid?

An organic acid which acts as a plant-growth regulator. It is found naturally across plant types and is a member of the group of regulators known as auxins.


What is the CAM pathway?

Crassulacean acid metabolism. A photosynthetic pathway in which carbon dioxide is initially fixed by the enzyme, phosphoenolpyruvate (PEP) carboxylase, to create malic acid. This fixation tends to occur at night to conserve water. The carbon dioxide is re-released during the day, when it is fixed for a second time by the Rubisco enzyme.


What is PEP carboxylase?

An enzyme that catalyses the reaction between phosphoenolpyruvate (PEP) and carbon dioxide to give malic acid.


Why is it more efficient in terms of water use for CAM plants to open their stomata during the night rather than the day?

The gradient in water vapour pressure is often close to zero at night (the external air is saturated and the leaf is at the same temperature as the surrounding air) whereas during the day in the desert sun, the gradient can be as great as 20 kPa.


What sort of locations are CAM plants found?

Areas of restricted water access, not just in deserts but also areas of shallow roots and limited water access, such as in stone walls or on branches of other trees in rain forests.


What is the second law of thermodynamics?

no energy transformation can be undertaken with 100% efficiency. Movement involves friction which generates heat and likewise many chemical reactions generate heat as a by-product, which is then lost from the system.


What is basal respiration?

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


What is the compensation point in plant respiration?

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.


Name a structure that allows a plant to lie dormant for a period before re-starting growth.

Seeds, bulbs, tubers, buds on deciduous trees are all possible answers.


What are photosynthate?

Organic compounds synthesised by photosynthetic pathways such as phosphoglyceric acid and the larger compounds (glucose, fructose, sucrose, starch) that are made directly from these precursors.


Why do seeds benefit from using lipid as an energy store?

They are able to store the same amount of chemical energy in a smaller, lighter package. In fact, lipids have more than twice the energy density of carbohydrates.