it is a physiological process by which green parts of the plant in the presence of light prepare food in the form of carbohydrates using carbon dioxide and water. oxygen is released as a bi-product.

adaptations

• leaves are thing and flat- offering a large surface for photosynthesis
• internally the lose arrangemnet of mesophyll tissues allow for diffusion of gases
• high concentration of chloroplansts help trap solar energy
• have multiple pores called stomata where exchange of gases take place

carotenoids- yellow leaves

anthocyanins- red leaves

• what is the significance of photosynthesis?

mechanism of photosynthesis

polymerisation

interconversion of several glucose molecules to one molecule of starch.

• oxygen is released into the air and the glucose is stored in the mesophyll tissues
• glucose is converted into starch and stored in the leucoplasts

light phase

• hill reaction (photochemical phase)
• photolysis of water- chlorophyll becomes excited in the presence of light initiating the process of photosynthesis. which splits water into H & OH
• production of reducing agent NADP (nicotinamide adenine di nucleotide phosphate) is reduced to NADPH.
• production of molecular oxygen- hydroxide is oxidised to hydrogen peroxide which decomposes to form molecular oxygen
• low grade energy ADP [adenosine di phosphate] is converted to ATP

dark phase

• calvine cycle [biosynthetic phase]
• carbon dioxide is converted to glucose using the NADPH and the ATP
• “light independent phase”
• site of carbon dioxide fixation is in the stroma
• PGA [phosphoglyceric acid is the only stable intermediary product formed]

chloroplast

• indivisual sacks of the grannum is called the thakaloid.
• made up of carbon, hydrogen, oxygen and magnessium
• in the absence of light leaves appear etiolinated

factors affecting photosynthesis

external

• light intensity - in weak light or extremely intense light no starch grains are formed
• carbon dioxide- high conc of carbon dioxide is harmful. but the rate increases with increase in co2
• temperature- optimum temperature is 35 C. with a 10 deg rise in temperature from 25 to 35, the photosynthetic rate almost doubles.

internal

• chlorophyll
• accumulation of photosynthetic products- heavy accumulation will slow down the process
• protoplasmic factor- in absence of o2 photosynthesis comes to a stand still
• in the absence of k starch grains are not formed and guard cells are not turgid

law of limiting factors

when a process is governed by more that one factor the rate of the process is governed by the factor which is in the least amount.

experiments on photosynthesis

destarching [I]

• the plant under study must be destrached prior to the experiment [pre existing starch must be removed from the storage organs]
• plant can be destarched by keeping it in a dark room for 2-3 days

starch test [III]

• the leaf is detached from the plant
• it is dipped in boiling water for 30 seconds [destroys the enzymes and further chemical changes in the plant]
• the leaf is boiled in methylated spirit [dissolves chlorophyll]
• then it is dipped in boiling water [to soften the leaf]
• then the leaf is placed on a white tile and iodine solution is poured over it.

#1 to show that chlorophyll is necessary for photosynthesis to take place

• use a varigated leaf [crotons, geranium]
• destarch the plant and then place the plant in sunlight followed by the starch test

observation & inference-

the parts that were originally green now turn blue black in the presence of iodine. we can interpret that only the green parts of the plant can manufacture food.

#2 to show that sunlight is necessary for photosynthesis

• take a potted plant and cover a strip of a leaf with a black paper.
• destarch the plant and then expose it to light for a few hours
• then perform the starch test

observation & inference-

all the parts of the leaf except the part covered with the paper is now blue black. therefore we can conclude that sunlight is necessary for photosynthesis.

#3 to show that co2 is necessary for photosynthesis

• use a koh pellet to absorb all the co2 present in a glass bell jar [control]
• when starch test is performed on one of these leaves, it will be brown an not blue black

#4 to show that oxygen is evolved during photosynthesis

• take a water plant like hydrilla
• place the apparatus in sunlight
• bubbles are seen