Lecture 4Lecture 4EcologyEcology

Ralph KirbyRalph Kirby

PhotosynthesisPhotosynthesisAll life on Earth is carbon basedAll life on Earth is carbon basedCOCO22 was the major form of free carbon available in past and was the major form of free carbon available in past and still isstill isOnly photosynthesis is capable of converting COOnly photosynthesis is capable of converting CO22 into into organic moleculesorganic moleculesOnly plants are capable of photosynthesisOnly plants are capable of photosynthesisAll other living organisms obtain their carbon via assimilation All other living organisms obtain their carbon via assimilation from plantsfrom plantsAll organisms also require energyAll organisms also require energy– Energy obtained directly from an energy source by a living organism Energy obtained directly from an energy source by a living organism

is called autotrophy (autotroph)is called autotrophy (autotroph)Plants are autotrophsPlants are autotrophsSo are certain bacteria like So are certain bacteria like Thiobacullus ferrooxidansThiobacullus ferrooxidans

– Energy obtained indirectly from organic molecules by a living Energy obtained indirectly from organic molecules by a living organism is called heterotrophy (heterotroph)organism is called heterotrophy (heterotroph)

All animals are heterotrophsAll animals are heterotrophsSome organisms can be a a mixture like lichens where you have an alga Some organisms can be a a mixture like lichens where you have an alga and a fungus living togetherand a fungus living together

Photsynthesis is a biochemical process that uses light to Photsynthesis is a biochemical process that uses light to convert COconvert CO22 into a simple sugar such as glucose into a simple sugar such as glucose– Light of the correct wavelength is absorbed by chlorophyll in the Light of the correct wavelength is absorbed by chlorophyll in the

organelle called a organelle called a chloroplastchloroplast and converted via the light and converted via the light reactions into ATP and NADPHreactions into ATP and NADPH

– HH22O is split into oxygen and hydrogenO is split into oxygen and hydrogen– The oxygen is released as OThe oxygen is released as O22

– The hydrogen is linked to COThe hydrogen is linked to CO22 to form a three carbon organic to form a three carbon organic molecule (Cmolecule (C33 photosynthesis). This is carried out by the enzyme photosynthesis). This is carried out by the enzyme ribulose biphosphate carboxylase oxygenase ribulose biphosphate carboxylase oxygenase (Rubisco)(Rubisco)

– The C3 molecules are then converted into sugar like glucose via The C3 molecules are then converted into sugar like glucose via the dark reactionsthe dark reactions

– This glucose can then be used to produce energy by respiration in This glucose can then be used to produce energy by respiration in mitochondriamitochondria or used to produce other organic compounds or used to produce other organic compounds

– Both the chloroplast and the mitochondria are organelles within the Both the chloroplast and the mitochondria are organelles within the cell that contain their own chromosome and represent a very early cell that contain their own chromosome and represent a very early symbiotic relationship between unicellular algae/Gram –ve symbiotic relationship between unicellular algae/Gram –ve bacteria respectively and eukaryote cells.bacteria respectively and eukaryote cells.

Obviously the amount of Obviously the amount of light received by a plant light received by a plant will affect the light will affect the light reactions of reactions of photosynthesisphotosynthesisLight Compensation PointLight Compensation Point– As light declines, it As light declines, it

eventually reaches a point eventually reaches a point where respiration is equal where respiration is equal to photosynthesisto photosynthesis

Light Saturation PointLight Saturation Point– As light increases, it As light increases, it

reaches a point where all reaches a point where all chloroplasts are working at chloroplasts are working at a maximum ratea maximum rate

PhotoinhibitionPhotoinhibition– In some circumstances, In some circumstances,

excess light can result in excess light can result in “overloading” and even “overloading” and even damage to chlorophyll by damage to chlorophyll by bleachingbleaching

Photosynthesis takes place in Photosynthesis takes place in plants in specialized cells in plants in specialized cells in the mesophyllthe mesophyll

Needs movement of CONeeds movement of CO22 and and OO22 between cells and between cells and atmosphereatmosphereDiffuses via stomata in land Diffuses via stomata in land plantsplants– Stomata close when Stomata close when

photosynthesis is reduced and photosynthesis is reduced and keeps up partial pressure of COkeeps up partial pressure of CO22

Stomata also control Stomata also control transpirationtranspiration– Reduces water lossReduces water loss– Minimizing water needs from soilMinimizing water needs from soil– Ratio of carbon fixed to water Ratio of carbon fixed to water

lost is the water-use efficiencylost is the water-use efficiency

Temperature is important to a Temperature is important to a plantsplants– Photosynthesis increases as Photosynthesis increases as

the temperature increasesthe temperature increasesEnergy balanceEnergy balanceRadiation not used increases Radiation not used increases internal leaf temperature internal leaf temperature significantlysignificantlySome heat can be lost by Some heat can be lost by convectionconvection

– Needs ate movementNeeds ate movement

Some heat can be lost by Some heat can be lost by radiationradiation

– Leaf colorLeaf color

– Respiration increases as the Respiration increases as the temperature increasestemperature increases

– Damage to enzymes etc Damage to enzymes etc increases with temperatureincreases with temperature

– Water loss increases with Water loss increases with temperaturetemperature

Evaporation of water helps to Evaporation of water helps to keep the temperature lowerkeep the temperature lowerThus relative humidity and Thus relative humidity and available water is importantavailable water is important

Plant adaptation Plant adaptation to light conditionsto light conditions

Plants adapted to a Plants adapted to a shady environmentshady environment– Lower levels of rubiscoLower levels of rubisco– Higher levels of Higher levels of

chlorophyllchlorophyll– Because light is limitingBecause light is limiting

Plants adapted to a full Plants adapted to a full sun environmentsun environment– Higher levels of rubiscoHigher levels of rubisco– Lower levels of Lower levels of

chlorophyllchlorophyll– Because leaf structure Because leaf structure

is limitingis limiting– Changes in leaf Changes in leaf

structure evolvestructure evolve

Light also Light also affects affects whether a whether a plant allocates plant allocates to leaves or to to leaves or to rootsroots– AcclimatizatioAcclimatizatio

nn

Shade tolerant Shade tolerant speciesspecies

Shade Shade intolerant intolerant speciesspecies

Shade tolerance and intoleranceShade tolerance and intolerance

Remember that land plants are not Remember that land plants are not the only plants on Earththe only plants on Earth

Shade Shade adaptation adaptation also occurs in also occurs in algaealgae

To increase water To increase water efficiency in a warm dry efficiency in a warm dry environment, plants have environment, plants have modified process of modified process of photosynthesisphotosynthesisCC33– Normal in mesophyll with Normal in mesophyll with

rubiscorubisco

CC44– Warm dry environmentWarm dry environment– Additional step in fixation Additional step in fixation

of COof CO2 2 in the bundle in the bundle sheathsheath

– Phosphoenolpyruvate Phosphoenolpyruvate synthasesynthase (PEP) does (PEP) does initial fixation into Malate initial fixation into Malate and aspartate and aspartate

– Malate and aspartate are Malate and aspartate are transported to bundle transported to bundle sheath as an intermediate sheath as an intermediate moleculemolecule

– Rubisco and CORubisco and CO22 convert convert them to glucosethem to glucose

CC33, C, C4 4 and CAMand CAMCC44 makes more effective use of CO makes more effective use of CO22

COCO22 concentration in bundle cell can be 6X that of concentration in bundle cell can be 6X that of atmosphere and mesophyll cellatmosphere and mesophyll cellAs rate limiting aspect of photosynthesis is usually the As rate limiting aspect of photosynthesis is usually the availability of COavailability of CO22, then C, then C44 is more efficient is more efficientAlso can keep stomata closed longer and therefore Also can keep stomata closed longer and therefore better water usebetter water useBut needs large amount of extra enzyme and there only But needs large amount of extra enzyme and there only well adapted to high photosynthesis environmentswell adapted to high photosynthesis environmentsBut what about deserts with really low water availability But what about deserts with really low water availability and high temperatureand high temperature– Third type – Crassulacean acid pathway – CAMThird type – Crassulacean acid pathway – CAM– CO2 fixed converted to malate by PEP during night and stored, CO2 fixed converted to malate by PEP during night and stored,

while stomata are openwhile stomata are open– Malate is converted back to CO2 during day and using Malate is converted back to CO2 during day and using

photosynthesis, light and rubisco changed into sugarphotosynthesis, light and rubisco changed into sugar– High level of water conservationHigh level of water conservation– Both processes in the mesophyll cellsBoth processes in the mesophyll cells

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Plants need to make serious evolutionary adaptations to water availabilityPlants need to make serious evolutionary adaptations to water availability

Plants need to make serious Plants need to make serious evolutionary adaptations to temperatureevolutionary adaptations to temperature

CC33

– <30<30ooCC

CC44

– 3030ooC to 40C to 40ooCC

CAMCAM– >40>40ooCC

See also ability to See also ability to withstand freezingwithstand freezing– Ice crystals burst cellsIce crystals burst cells– Slow dehydrationSlow dehydration– Cryoprotective agentsCryoprotective agents– Shedding leaves in Shedding leaves in

deciduous plantsdeciduous plants

Uptake of a nutrient Uptake of a nutrient through the roots through the roots depends on its depends on its concentrationconcentration

However there is a However there is a maximummaximum

Some nutrients can Some nutrients can be inhibitorybe inhibitory

Effect of nutrent Effect of nutrent availability can also availability can also reach a maximumreach a maximum

Nutrient availability also needs evolutionary Nutrient availability also needs evolutionary adaptationadaptation

Nitrogen can limit Nitrogen can limit photosynthesisphotosynthesis

Need for symbiosisNeed for symbiosis– RhizobiumRhizobium

Peas, beans and a few Peas, beans and a few other plantsother plants

– FrankiaFrankiaVarious woody species Various woody species in southern Africain southern Africa

Plants respond Plants respond differently to differently to etra nitrogen etra nitrogen depending on depending on their natural their natural environment’s environment’s level of nitrogen level of nitrogen or other nutrientor other nutrient

Pine species are adapted to live in low nitrogen Pine species are adapted to live in low nitrogen environments like sandy soilsenvironments like sandy soilsPines retain their leaves for a long timePines retain their leaves for a long timeThis saves the recycling of nitrogen through the soilThis saves the recycling of nitrogen through the soil