Lecture 4 Ecology Ralph Kirby

Photosynthesis All life on Earth is carbon based CO 2 was the major form of free carbon available in past and still is Only photosynthesis is capable of converting CO 2 into organic molecules Only plants are capable of photosynthesis All other living organisms obtain their carbon via assimilation from plants All organisms also require energy –Energy obtained directly from an energy source by a living organism is called autotrophy (autotroph) Plants are autotrophs So are certain bacteria like Thiobacullus ferrooxidans –Energy obtained indirectly from organic molecules by a living organism is called heterotrophy (heterotroph) All animals are heterotrophs Some organisms can be a a mixture like lichens where you have an alga and a fungus living together

Photsynthesis is a biochemical process that uses light to convert CO 2 into a simple sugar such as glucose –Light of the correct wavelength is absorbed by chlorophyll in the organelle called a chloroplast and converted via the light reactions into ATP and NADPH –H 2 O is split into oxygen and hydrogen –The oxygen is released as O 2 –The hydrogen is linked to CO 2 to form a three carbon organic molecule (C 3 photosynthesis). This is carried out by the enzyme ribulose biphosphate carboxylase oxygenase (Rubisco) –The C3 molecules are then converted into sugar like glucose via the dark reactions –This glucose can then be used to produce energy by respiration in mitochondria or used to produce other organic compounds –Both the chloroplast and the mitochondria are organelles within the cell that contain their own chromosome and represent a very early symbiotic relationship between unicellular algae/Gram –ve bacteria respectively and eukaryote cells.

Obviously the amount of light received by a plant will affect the light reactions of photosynthesis Light Compensation Point –As light declines, it eventually reaches a point where respiration is equal to photosynthesis Light Saturation Point –As light increases, it reaches a point where all chloroplasts are working at a maximum ratePhotoinhibition –In some circumstances, excess light can result in “overloading” and even damage to chlorophyll by bleaching

Photosynthesis takes place in plants in specialized cells in the mesophyll Needs movement of CO 2 and O 2 between cells and atmosphere Diffuses via stomata in land plants –Stomata close when photosynthesis is reduced and keeps up partial pressure of CO 2 Stomata also control transpiration –Reduces water loss –Minimizing water needs from soil –Ratio of carbon fixed to water lost is the water-use efficiency

Temperature is important to a plants –Photosynthesis increases as the temperature increases Energy balance Radiation not used increases internal leaf temperature significantly Some heat can be lost by convection –Needs ate movement Some heat can be lost by radiation –Leaf color –Respiration increases as the temperature increases –Damage to enzymes etc increases with temperature –Water loss increases with temperature Evaporation of water helps to keep the temperature lower Thus relative humidity and available water is important

Plant adaptation to light conditions Plants adapted to a shady environment –Lower levels of rubisco –Higher levels of chlorophyll –Because light is limiting Plants adapted to a full sun environment –Higher levels of rubisco –Lower levels of chlorophyll –Because leaf structure is limiting –Changes in leaf structure evolve

Light also affects whether a plant allocates to leaves or to roots –Acclimatizatio n Shade tolerant species Shade intolerant species

Shade tolerance and intolerance

Remember that land plants are not the only plants on Earth Shade adaptation also occurs in algae

To increase water efficiency in a warm dry environment, plants have modified process of photosynthesis C 3 –Normal in mesophyll with rubisco C 4 –Warm dry environment –Additional step in fixation of CO 2 in the bundle sheath –Phosphoenolpyruvate synthase (PEP) does initial fixation into Malate and aspartate –Malate and aspartate are transported to bundle sheath as an intermediate molecule –Rubisco and CO 2 convert them to glucose

C 3, C 4 and CAM C 4 makes more effective use of CO 2 CO 2 concentration in bundle cell can be 6X that of atmosphere and mesophyll cell As rate limiting aspect of photosynthesis is usually the availability of CO 2, then C 4 is more efficient Also can keep stomata closed longer and therefore better water use But needs large amount of extra enzyme and there only well adapted to high photosynthesis environments But what about deserts with really low water availability and high temperature –Third type – Crassulacean acid pathway – CAM –CO2 fixed converted to malate by PEP during night and stored, while stomata are open –Malate is converted back to CO2 during day and using photosynthesis, light and rubisco changed into sugar –High level of water conservation –Both processes in the mesophyll cells

LIGHTLIGHTLIGHTLIGHT

Plants need to make serious evolutionary adaptations to water availability

Plants need to make serious evolutionary adaptations to temperature

C 3 –<30 o C C 4 –30 o C to 40 o CCAM –>40 o C See also ability to withstand freezing –Ice crystals burst cells –Slow dehydration –Cryoprotective agents –Shedding leaves in deciduous plants

Uptake of a nutrient through the roots depends on its concentration However there is a maximum Some nutrients can be inhibitory Effect of nutrent availability can also reach a maximum

Nutrient availability also needs evolutionary adaptation Nitrogen can limit photosynthesis Need for symbiosis –Rhizobium Peas, beans and a few other plants –Frankia Various woody species in southern Africa

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

Pine species are adapted to live in low nitrogen environments like sandy soils Pines retain their leaves for a long time This saves the recycling of nitrogen through the soil