3.2a plant nutrition (photosynthesis)

define autotrophic nutrition

mode of nutrition where organisms make their own food from simple inorganic substances like carbon dioxide and water using energy from sunlight or chemical reactions. (eg plants — photosynthesis)

define heterotrophic nutrition

mode of nutrition where organisms obtain ready-made food by consuming other plants or animals, as they cannot make their own food. (humans)

what is chlorophyll for?

it absorbs light energy and converts it into chemical energy for the formation of carbohydrates and their subsequent uses

briefly explain why most forms of life are completely dependent on photosynthesis

• produces food (glucose) that serves as the primary source of energy for almost all organisms, and releases oxygen needed for respiration.

• without it, there would be no energy source or oxygen to support most living things on Earth

state the equation, in words and symbols, for photosynthesis

carbon dioxide (6CO₂) + water (6H₂O) [+ light + chlorophyll] → glucose (C₆H₁₂O₆) + oxygen (6O₂)

outline how light energy is harnessed and converted into chemical energy during the light-dependent reactions of photosynthesis

• occurs at photosystems (PS I and PS II) in the chloroplast in the grana (pl.) (stack of thylakoids which contain chlorophyll) in presence of light

photochemical step:

• chlorophyll in PS I and PS II absorb light → triggers release of high energy photo-excited electrons (photo-activation)

• ATP is generated from ADP and phosphate when photo-excited electrons pass from PS II to PS I down the electron transport chain (ETC)

• light energy (LE) harnessed to split water molecules into oxygen and hydrogen atoms (ie photolysis of water)

  • in PS I, hydrogen is removed by a hydrogen receptor, NADP via a short ETC to form NADPH (reduced NADP)

  • oxygen released as by-product

  • electrons lost from PS II are replaced by electrons released from water via photolysis

outline the role of Calvin cycle / light-independent reactions in photosynthesis for the synthesis of sugar

• occurs in stroma (of chloroplast) (ie fluid-space surrounding grana; contains enzymes)

1. carbon fixation

• CO₂ diffuses into stroma

  • combines with RuBP (5-carbon sugar compound)

  • in the presence of RuBisCO (CO₂ fixation enzyme)

• product: 2 x GP (3-carbon glycerate-3-phosphate compound)d

2. reduction

• GP is reduced to triose phosphate (3-carbon sugar compound)

  • by NADPH and ATP (from light-dependent stage)

    • energy from ATP and hydrogen from NADPH (oxidation of NADPH to NADP) are used to reduce GP to triose phosphate

• from triose phosphate, carbohydrates (other sugars and starch, and sucrose for translocation), lipids and amino acids can be synthesised

3. reduction of RuBP

• RuBP is regenerated using energy from ATP, so that more CO₂ can be fixed

what are some external factors affecting rate of photosynthesis and how do they affect?

1. light intensity

• as LI increases, ROP increases until a constant rate is reached

• more light = more energy for photosynthesis

2. CO₂ concentration

• as CO₂ conc. increases, ROP increases until a constant rate is reached

• more CO₂ = more glucose produced

3. temperature

• as temperature increases to the optimum temperature, ROP increases

• as temperature increases beyond the optimum temperature, ROP decreases

discuss the effects of light intensity on the rate of photosynthesis

• needed during the light-dependent stages of photosynthesis:

  • for the excitation of electrons from the photosystems

  • for the photolysis of water

• if light intensity increases, excitation of electrons in the photosystems and the photolysis of water can take place more readily

• however, from a certain light intensity onwards, light-dependent reactions would already be occurring at a maximal rate and will not be affected by further increase in light intensity

• other factors (eg CO₂ concentration / temperature) may be the limiting factor now

discuss the effects of carbon dioxide concentration on the rate of photosynthesis

• CO₂ taken in during carbon fixation stage (in Calvin cycle) for the production of useful biomolecules (eg glucose) which is used for starch formation

  • it is an important raw material for the light-independent stage of photosynthesis

• RuBisCO (enzyme) catalyses the carbon fixation process to produce an unstable 6-carbon compound (2×3C → GP). If concentration of CO₂ substrate continues to increases, ROP will increase bcos more CO₂ (ie substrate molecules) are available to bind to the active sites of RuBisCO

• however, if CO₂ concentration continues to increase, ROP will not rise but will remain at a constant level; bcos all the active sites of RuBisCO are now fully occupied at this point (ie enzymes are saturated)

• from this particular CO₂ concentration onward, other factors (eg light intensity / temperature) may remain as the limiting factor of photosynthesis

discuss the effects of temperature on the rate of photosynthesis

• as many enzymes are involved in the photosynthesis eg RuBisCO, ROP is temperature-dependent

• however, beyond a certain temperature, as more enzymes are denatured, ROP will drop

• other factors (eg CO₂ concentration / light intensity) may be the limiting factor now

what is a limiting factor?

a factor that affects the rate of a reaction. the rate cannot increase unless the value of the limiting factor is increased

discuss light intensity as limiting factor on the rate of photosynthesis

• as light intensity increases, ROP increases. light intensity is the limiting factor

• beyond a certain point, other factors such as temperature / CO₂ concentration becomes the limiting factor

discuss carbon dioxide concentration as limiting factor on the rate of photosynthesis

• when ROP stops increasing as light intensity keeps on increasing, carbon dioxide is the limiting factor

• when carbon dioxide is increased (in %), ROP increases

discuss temperature as limiting factor on the rate of photosynthesis

• when ROP stops increasing as light intensity keeps on increasing, temperature is the limiting factor

• when temperature is increased, ROP increases