what is photosynthesis
= process whereby plants and other photosynthetic organisms use sunlight to synthesise carbohydrates and transform light energy from the sun into chemical energy from inorganic carbon dioxide and water
importance of photosynthesis
-> converts absorbed light energy from sun into chemical energy, which is needed by other organisms for the formation of carbohydrates and their subsequent uses
-> removes carbon dioxide from the atmosphere
-> produces oxygen needed for respiration by other organisms
-> contributes to energy stored in fossil fuels
conditions for photosynthesis
-> light
-> carbon dioxide
-> chlorophyll
-> suitable temperature
-> water
-> suitable pH -> enzyme activity needed for photosynthesis
what are limiting factors
= factors that affect rate of a reaction, rate cannot incr unless value of limiting factor incr
limiting factors - light intensity
-> needed for light-dependent reactions to occur on the thylakoids of chloroplasts
photolysis of water | | excitation of electrons from photosystems
-> as light intensity incr, light-dependent reactions can take place more readily, rate of photosynthesis incr until a constant rate is reached, as light-dependent reactions would already be occurring at a max rate and will not be affected by any further incr in light intensity
-> more light = more energy for photosynthesis
-> light intensity is no longer limiting factor but other factors like CO2 concentration and temp may be the limiting factors now
limiting factors - CO2 concentration
needed for carbon fixation to occur during light-independent reactions of photosynthesis
-> Rubisco catalyses carbon fixation process to produce an unstable 6-carbon compound
-> as CO2 concentration incr as more CO2 is available to bind to active sites of rubisco, rate of photosynthesis incr until a constant rate is reached as all active sites of rubisco are now fully occupied
-> more CO2 = more glucose can be produced
-> from this particular CO2 concentration onward, CO2 is no longer limiting factor but other factors like temp and light intensity may remain as limiting factors
-> temperature
limiting factors - temperature
-> suitable temperature is needed for photosynthetic enzymes such as Rubisco to function
-> as temp incr to optimum temp, rate of photosynthesis incr
-> as temp incr beyond optimum temp, more enzymes are denatured, rate of photosynthesis decr
-> temp is no longer limiting factor but other factors like CO2 concentration and light intensity may be the prevailing limiting factors
what is within a vascular bundle in a stem
in stem:
within a vascular bundle,
-> xylem is located closer inside
-> phloem lies outside xylem
-> tissue called cambium between them
-> stem is covered by a layer of cells called epidermis
|
epidermal cells are protected by a waxy, waterproof cuticle -> greatly reduces evaporation of water from stem
xylem function
-> conduct water and dissolved mineral salts from roots to stems to leaves
xylem features
-> xylem tissue consists of many xylem vessels that provide mechanical support to the plant
|_ = long hollow tube stretching from root to leaf
|_ -> made up of many dead cells without cytoplasm or
cross-walls -> reduces resistance to water flowing
through xylem
|_ -> inner walls are strengthened by deposits of lignin
|
may be deposited in the form of rings, spirals, or whole vessel is
lignified except in regions called pits
phloem function
-> conduct manufactured food substances (sucrose and amino acids) from green parts of plant, especially leaves, to other plants of the plant
phloem features
-> consists mainly of sieve tubes and companion cells
|
each tube consists of a column of elongated, thin-walled living cells called
sieve tube cells/sieve tube elements
| |
| sieve plates separating these cells have a lot of minute pores
* mature cell has only a thin layer of cytoplasm inside the cell that is connected to cells above and below through holes in sieve plates -> allows rapid flow of manufactured food substances through sieve tubes
* each cell has a companion cell beside it that provides nutrients and helpers the cell to transport manufactured food. |
-> carries out metabolic processes needed to keep sieve tube cell alive
-> narrow thin-walled cell with many mitochondria, cytoplasm and a nucleus
|
provide energy needed for companion cells to load sugar from
mesophyll cells into sieve tubes by active transport
what is within the roots
in roots:
-> epidermis of the root is the outermost layer of cells
-> bears root hairs
|_ each root hair is a long and narrow extension growing out of an epidermal cell
|
increases surface area-to-volume ratio of root hair cell
-> increases rate of absorption of water and mineral salts
what is within a vascular bundle in leaves
within a vascular bundle found along spongy mesophyll,
-> xylem is closer to upper surface of leaf
-> phloem is closer to lower surface of leaf
what is translocation
= transport of manufactured food substance, such as sucrose and amino acids, in plants/the phloem tissue
translocation - ‘ringing’ experiment
-> a ring of bark with the phloem is removed from the stem of a woody plant
-> after some time, the stem immediately above the cut region swells up
-> this is because with the phloem removed, food substances from the leaves
cannot reach the stem below the cut region
-> they accumulate above the cut region, causing it to swell up
translocation - using aphids
-> insects such as aphids feed on plant juices
-> long mouthpart, stylet, of each aphid penetrates the leaf or stem
-> the aphid can be anaesthetised with carbon dioxide while it is feeding
-> body of the aphid is then cut off, leaving only the feeding stylet in the plant tissues
-> a liquid will exude from the cut end of the stylet.
|_ contains sucrose and amino acids
-> if the stem is sectioned and examined under the microscope, it can be seen that feeding stylet of the aphid is inserted into the phloem sieve tube
-> shows that that translocation of sugars and amino acids occurs in the phloem
translocation - using isotopes
-> carbon-14 (^14C) is a radioactive carbon isotope
|_ presence can be detected by an X-ray photographic film
-> a leaf is provided with carbon dioxide containing the radioactive carbon, ^14C
-> when photosynthesis takes place, the sugars formed will contain radioactive carbon
-> stem is then cut and a section of it is exposed onto an X-ray photographic film
-> it is found that radioactive substances are present in the phloem, since radioactive substances cause the X-ray film to darken
what is transpiration
= loss of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves
|
so transpiration is linked to gas exchange between
plant and environment and is a consequence of gaseous exchange in plants
process of transpiration
- during photosynthesis, plant releases O2 as waste while simultaneously absorbing CO2
- water vapour more concentrated in intercellular air spaces -> concentration gradient between environment in lead and air around it
- water vapour diffused out of leaf through stomata
*transpiration does not occur when stomata are closed (guard cells control whether stomata is open or closed)
factors of transpiration
- wind/air movement [incr]
-> wind blows away water vapour hat diffused out of leaf
-> steeper concentration gradient between inside and outside leaf - temp [incr]
-> incr evaporation of water from cell surface - light [incr]
-> stomata opens wider for photosynthesis - humidity [decr]
-> affects concentration gradient of water vapour between leaf and surrounding air
rate of transpiration
rate of transpiration = loss in mass (g) / time taken (h)
when measured using: spring balance
rate of transpiration (cm^3/min) = diff in volume (cm^3) / time taken (min)
when measured using: potometer
*assuming rate of absorption is proportional to it
(effect of different environmental conditions can also be determined using potometer)
how root hair cells absorb
- diffusion
-> when concentration of ions in soil solution is HIGHER than in cell sap
-> ions diffuse DOWN concentration gradient - active transport
-> when concentration of ions in soil solution is LOWER than that in cell sap
-> ions are absorbed AGAINST concentration gradient
-> using energy from cellular respiration in root hair cell
root hair cell adaptations
- has a long and narrow extension
-> incr surface area to volume ratio
-> incr rate of absorption of water and mineral salts - has a cell membrane
-> prevents cell sap from leaking out
|_ contains sugars, amino acids, and salts
|_ has lower water potential than soil solution
-> allows water to enter root hair by osmosis - has numerous mitochondria
-> aerobic respiration in mitochondria releases and provides energy for active transport of ions into cell
