photosynthesis Flashcards
structure of the leaf
The leaf is the main photosynthetic structure in eukaryotic plants. Chloroplasts are the cellular organelles within the leaf where photosynthesis takes place
The leaves are adapted for photosynthesis with:
- a large surface area to absorb as much sunlight as possible
- arrangement of leaves that minimises overlapping so avoids shadowing of one leaf with another
- thin as most light is absorbed within the first few micrometres of the leaf and the diffusion distance for gases is kept short
- a transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells beneath
- long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight
- numerous stomata for gaseous exchange so that all mesophyll cells are only a short diffusion pathway from one
- stomata that open and close in response to changes in light intensity
- many air spaces in the lower mesophyll layer to allow rapid diffusion in the gas phase of CO2 and O2
- a network of xylem that brings water to the leaf cells, and phloem that carries away the sugars produced during photosynthesis
structure and role of chloroplasts in photosynthesis
- the grana. Stacks of up to 100 disk-like structures called thylakoids, where the light-dependant stage of photosynthesis takes place. Within thylakoids is the photosynthetic pigment called chlorophyll. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana. These are called inter-granal lamellae
- the stroma. Is a fluid-filled matrix where the light-independent stage of photosynthesis takes place. Within the stroma are a number of other structures such as starch grains
photosynthesis definition
A series of chemical reactions where light energy is used to fix carbon from inorganic into organic, so it is in a more useful form
- plants are photoautotrophic- produce their own food
- photosynthetic pigment- a coloured chemical that can absorb light to be used in photosynthesis- plants contain different photosynthetic pigments that absorb light at different wavelengths. This is beneficial to the plant because it means that a large range of light wavelengths can be absorbed, to allow for maximum light absorption and therefore maximum photosynthesis rate
efficiency of photosynthesis
Efficiency is what proportion of the light energy going in to the system is transferred to chemical energy in the form of glucose
- some light energy is reflected
- some light misses the chlorophyll
- some light energy is the wrong wavelength of light
- some of the heat energy will be used for transpiration stream and therefore supporting the movement of water up the xylem, this is due to the heat energy that has been transferred to the surroundings, instead of used in the production of glucose
- some energy also lost as heat during respiration
production
Generation of new, organic molecules within an organism
- plants are producers and we therefore refer to primary production
- gross primary production relates to all the glucose produced in photosynthesis. Net primary production relates to the organic molecules remaining after a proportion of the glucose is transferred through respiration
net primary production= gross primary production- respiration
photophosphorylation by chemiosmosis
- each thylakoid is an enclosed chamber into which protons, H+, are pumped from the stroma using protein carriers in the thylakoid membrane called proton pumps
- the energy to drive this process comes from electrons released when water molecules are split by light- photolysis of water
- the photolysis of water also produces protons which further increases their concentration inside the thylakoid space
- overall this creates and maintains a concentration gradient of protons across the thylakoid membrane with a high concentration inside the thylakoid space and a low concentration inside the stomata
- the protons can only cross the thylakoid membrane through ATP synthase protein channels- the rest of the membrane is impermeable to H+. These channels form small granules on the membrane surface and so are known as stalked granules
- as the protons pass through these ATP synthase channels they cause changes to the structure of the enzyme which then catalyses the combination of ADP with Pi to form ATP
- IT IS CHLOROPHYLL THAT ABSORB THE LIGHT ENERGY AND THERFORE UNDERGO PHOTOIONISATION
site of the light-dependent reaction
- thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the light-dependent reaction
- a network of proteins in the grana hold the chlorophyll in a very precise manner to allow for maximum absorption of light
- the granal membranes have ATP synthase channels within them, which catalyse the production of ATP. They are also selectively permeable which allows establishment of a proton gradient
- chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in the light-dependant reaction
calvins method
Called the lollipop experiment. Algae are grown in thin ‘lollipop’- at 5 second intervals, samples of the photosynthesising algae are dropped into hot methanol to stop chemical reactions instantly- then separated and those radioactive identified
-Calvin introduced radioactive NaHCO3 to photosynthesising algae, by using C14- radioactive isotope. Then allowed a sample of algae to move through the tap into boiling ethanol. This will kill/denature the algae and enzymes within it immediately. Calvin then used chromatography to separate the chemicals
He then used autoradiography and photographic film to compare the chemicals with the spots of a pure sample of each chemicals. He identified the chemicals within the standards
- Calvin knew it was a cycle because he then stopped the sodium hydrogen carbonate entering being radioactive. The RuBP and glycerate phosphate continued to be radioactive after a long period of time, so carbon continuing to be used to form glycerate phosphate so the reaction is a cycle
limiting factors
At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value
- when light is limiting factor, the rate of photosynthesis is directly proportional to light intensity
- as light intensity is increased, the volume of oxygen produced and carbon dioxide absorbed due to photosynthesis will increase to a point at which it is exactly balanced by the oxygen absorbed and the carbon dioxide produced by cellular respiration. No net exchange of gases into or out of the plant. Known as light compensation point
- further increases in light intensity will cause a proportional increase in the rate of photosynthesis, until another factor, eg temperature or CO2 conc will become limiting
- limiting factors can be controlled in agriculture as want maximum photosynthesis and therefore plant growth. These range from growing plants under artificial lighting to maximise the light intensity, heating a greenhouse to increase the temperature. Burning fuel, such as paraffin heaters/ burners to release more carbon dioxide.
The extent that each technique is used needs to consider in terms of profit. If the extra growth from photosynthesis is minimal, it wont be cost effective to pay for heating/lighting/fuel
required practical- DCPIP
- remove midrip and leaf stalks as don’t contain many chloroplasts and are too tough to blend
-DCPIP turns from blue to colourless, as reduced- so gains electrons - then filter using muslin cloth. Muslin cloth is used over filter paper as has finer separations/ smaller gaps so allows for only chloroplasts to be filtered into solution
- homogenise leaf suspension- cold, pH, buffered- then remove supernatant- only chloroplasts remain- isolation median contains sucrose to prevent osmosis
- dont want to damage chloroplasts before centrifugation, but afterwards want chloroplasts to open to allow for thylakoid membrane to be revealed, as that is where light-dependant stage takes place
- then set up lots of test tubes with control- foil for darkness, no DCPIP, boiling to denature proteins
