IB Biology: Unit 2.9 Photosynthesis

Explain the relationship between the structure of the chloroplast and its function

*Thylakoids*: Small lumen means small changes in proton concentration have a large effect on the proton motive force. There are small fluid space inside the thylakoids (light dependent reaction occurs here) --> O2, ATP is also produce in thylakoids *Grana*: Thylakoids arranged in stacks to greatly increase the surface area available for light absorption (chlorophyll located in thylakoid membrane) *Stroma*: Contains appropriate enzymes and suitable pH for the light-independent reaction to occur. Starch grains and lipid droplets are also present in stroma, (light independent reaction occurs here)

Limiting Factors

- The law of limiting factors states that when a chemical process depends on more than one essential condition being favourable, the rate of reaction will be limited by the factor that is nearest its minimum value Photosynthesis is dependent on a number of favourable conditions, including: - Temperature - Light intensity - Carbon dioxide concentration

Chromatography

Photosynthetic organisms do not rely on a single pigment to absorb light, but instead, benefit from the combined action of many These pigments include *chlorophylls, xanthophyll, and carotenes* Chromatography is an experimental technique by which mixtures can be separated 1. A mixture is dissolved in a fluid (mobile phase) and passed through a static material (stationary phase) 2. The different components of the mixture travel at different speeds, causing them to separate 3. A retardation factor can then be calculated (Rf value = distance component travels ÷ distance solvent travels) Two of the most common techniques for separating photosynthetic pigments are: 1. Paper chromatography - uses paper (cellulose) as the stationary bed 2. Thin layer chromatography - uses a thin layer of adsorbent (ex. silica gel) which runs faster and has better separation

Photosynthesis Equation

Carbon Dioxide + Water --> Glucose + Oxygen + Water "-->" is in presence of light and chlorophyll 6CO2 + 12H2O --> C6H12O6 + 6O2 + 6H2O

Effect of *Carbon Dioxide* Concentration on Photosynthetic Rate

Carbon dioxide is involved in the *fixation of carbon atoms to form organic molecules* *As carbon dioxide concentration increases reaction rate will increase*, as more organic molecules are being produced At a certain concentration of CO2 photosynthetic rate will plateau, as the enzymes responsible for carbon fixation are saturated

Changes in Oceans (O2)

Earth's oceans initially had high levels of dissolved iron (released from the crust by underwater volcanic vents) When iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide) When the iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere

Changes in Atmosphere (O2)

For the first 2 billion years after the Earth was formed, its atmosphere was anoxic (oxygen-free) The current concentration of oxygen gas within the atmosphere is approximately 20%

Changes in Biological Life (O2)

Free oxygen is toxic to obligate anaerobes and an increase in O2 levels may have wiped out many of these species Conversely, rising O2 levels was a critical determinant to the evolution of aerobically respiring organisms

Chlorophyll

Green pigment found in plants that is responsible for light absorbption --> When chlorophyll absorbs light, it releases electrons which are used to synthesize ATP (chemical energy) Chlorophyll absorbs light *most strongly in the blue portion of the visible spectrum, followed by the red portion* Chlorophyll *reflects* light most strongly in the *green* portion of the visible spectrum (hence the green colour of leaves)

The Effect of *Light Intensity* on Photosynthetic Rate

Light is absorbed by chlorophyll, which converts the radiant energy into chemical energy (ATP) As light intensity increases reaction rate will increase, as more chlorophyll is being photo-activated At a certain light intensity, photosynthetic rate will plateau, as all available chlorophyll are saturated with light Different wavelengths of light will have different effects on the rate of photosynthesis (ex. green light is reflected)

Measuring Biomass

Measuring Biomass (Indirect) Glucose production can be indirectly measured by a change in the plant's biomass (weight) This requires the plant tissue to be completely dehydrated prior to weighing to ensure the change in biomass represents organic matter and not water content An alternative method for measuring glucose production is to determine the change in starch levels (glucose is stored as starch) Starch can be identified via iodine staining (turns starch solution purple) and quantitated using a colorimeter

How to Measure Photosynthesis - CO2 Uptake

Measuring CO2 Uptake Carbon dioxide uptake can be measured by placing leaf tissue in an enclosed space with water Water free of dissolved carbon dioxide can initially be produced by boiling and cooling water Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which changes the pH (↑ acidity) Increased uptake of CO2 by the plant will lower the concentration in solution and increase the alkalinity (measure with probe) Alternatively, carbon dioxide levels may be monitored via a data logger

Measuring O2 Production

Measuring O2 Production Oxygen production can be measured by submerging a plant in an enclosed water-filled space attached to a sealed gas syringe Any oxygen gas produced will bubble out of solution and can be measured by a change in meniscus level on the syringe Alternatively, oxygen production could be measured by the time taken for submerged leaf discs to surface Oxygen levels can also be measured with a data logger if the appropriate probe is available

The Effect of *Temperature* on Photosynthetic Rate

Photosynthesis is controlled by enzymes, which are sensitive to temperature fluctuations As temperature increases reaction rate will increase, as reactants have greater kinetic energy and more collisions result Above a certain temperature the rate of photosynthesis will decrease as essential enzymes begin to denature

Action / Absorption Spectrum

The *absorption* spectrum indicates the *wavelengths* of light absorbed by each pigment (ex. chlorophyll) The action spectrum indicates the *overall rate of photosynthesis* at each wavelength of light There is a strong correlation between the absorption and the action spectrum

Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants

The action spectrum of photosynthesis is a graph showing *the rate of photosynthesis for each wavelength of light* The rate of photosynthesis will not be the same for every wavelength of light The rate of photosynthesis is the least with green or yellow light *(525 nm - 625 nm)* Red-orange light *(625 nm - 700 nm)* shows a good rate of photosynthesis However, the best rate of photosynthesis is seen with violet-blue light (400 nm-525 nm) An *absorption* spectrum is a graph showing the *percentage of light absorbed by pigments within the chloroplasts, for each wavelength of light* there is also good absorption with red orange light. However, most of the green yellow light is reflected and therefore not absorbed. This wavelength of light show the least absoption

Light Spectrum

The colors of the visible spectrum are (from longest to shortest wavelength) Red (700 nm) Orange Yellow Green Blue Indigo Violet (400 nm)

Light Dependent/Independent Reactions

The light-*dependent* reactions convert *light energy from the Sun into chemical energy (ATP)* The light-*independent* reactions use the chemical energy to synthesize organic compounds (ex. carbohydrates) *Light Dependent Reactions* - Light is absorbed by chlorophyll --> the production of ATP (chemical energy) - Light is also absorbed by water, which is split (photolysis) to produce oxygen and hydrogen - The hydrogen and ATP are used in the light-independent reactions, the oxygen is released from stomata as a waste product *Light Independent Reactions*: - ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions - The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.) - The ATP provides the required energy to power these anabolic reactions and fixes the carbon molecules together

Changes in Rock Deposition (O2)

The reaction between dissolved iron and oxygen gas created oceanic deposits called banded iron formations (BIFs) These deposits are not commonly found in oceanic sedimentary rock younger than 1.8 billion years old This likely reflects the time when oxygen levels caused the near complete consumption of dissolved iron levels As BIF deposition slowed in oceans, iron rich layers started to form on land due to the rise in atmospheric O2 levels

Photosynthesis

process by which cells *synthesize* organic compounds (glucose) from inorganic molecules (CO2 and H2O) in the *presence of sunlight* - requires chlorophyll - can only occur in plants and certain bacteria organisms use the light energy from the sun to create chemical energy (ATP) --> This chemical energy can either be used directly by the organism or used to synthesize organic compounds (e.g. glucose) Animals then consume these organic compounds as food and release the stored energy via cell respiration --> Photosynthesis is essentially the reverse of cell respiration (catabolic breakdown) Occurs in green plants, euglena, algae and some bacteria (AUTOTROPHS) they make their own food