Chapter 7 Photosynthesis: using light to make food

Plants, algae and certain prokaryotes convert light energy to chemical energy and store it in sugar

Autotrophs:

  • make their own food through the process of photosynthesis.

  • sustain themselves

  • don’t usually consume organic molecules from other organisms

Photoautotrophs- use the energy of light to produce organic molecules

Chemoautotrophs- prokaryotes that use inorganic molecules as their energy source

Heterotrophs- are consumers that feed on plants, animals. they decompose organic material

Photosynthesis in plants:

  • takes place in chloroplast

  • converts CO2 and water into organic molecules

  • releases O2

Chloroplast are the major sites of photosynthesis in green plants

Chlorophyll

  • and important light absorbing pigment in chloroplast

  • makes plants green

  • plays a major role in converting solar energy to chemical energy

Chloroplast are concentrated in the cells of the mesophyll- the green tissue in the interior of the leaf

stomata- tiny pores in the leaf that allow the CO2 to move in and O2 to exit

Veins in the leaf deliver H2O absorbed by the roots

Chloroplast consists of an envelope of two membranes that:

  • enclose the inner compartment with a thick fluid called stroma

  • contain a system of interconnected membranous sacs called thylakoids

Thylakoids

  • are often concentrated into stacks called grana

  • have an internal space called thylakoid space

  • thylakoid membrane also have most of the machinery that converts light energy to chemical energy

Chlorophyll molecules are:

  • built into the thylakoid membrane

  • capture light energy

Photosynthesis is a redox (oxidation- reduction) process

  • CO2 becomes reduced to sugar as electrons from H2O are added to it

  • Water molecules are oxidized when they lose electrons along with hydrogen ions

6CO2 + 6H2O → C6H12O6 + 6O2

In photosynthesis

  1. light energy is captured by chlorophyll molecules to boost the energy of electrons

  2. light energy is converted to chemo energy

  3. chemical energy is stored in the chemical bonds of sugars

Photosynthesis occurs in two stages

  1. Light reactions- occurs i. the thylakoid membranes

  • H2O→ O2

  • ADP + P → ATP

  • NADP+ → NADPH by using light to excite the electrons

the NADPH produced by the light reactions provides the electrons for reducing carbon in the carbon cycle

  1. Calvin cycle- occurs in the stroma of the chloroplast

  • Uses CO2 and ATP to assemble sugar molecules

  • Carbon fixation- a process that incorporates CO2 into organic compounds

  • After carbon fixation enzymes of this cycle make sugars by further reducing the carbon compounds

  • AKA light independent reaction

Sunlight contains energy called electromagnetic energy

Visible light is only a small part of the electromagnetic spectrum

Electromagnetic energy travels in waves

Wave length- the distance between the crests of two adjacent waves

Light behaves as discrete packets of energy called photons

Photon- a fixed quantity of light energy

*the shorter the wavelength the greater the energy

Pigments absorb light and are built into the thylakoid membrane

Plant pigments absorb some wavelengths of light and reflect or transmit others

*we see the color of wavelengths that are transmitted

Chloroplast contains several different pigments that absorb different wavelengths

chlorophyll a - absorbs blue, violet, red and reflects green

chlorophyll b - absorbs blue and orange and reflects yellow and green

carotenoids -

  • broaden the spectrum of colors used for photosynthesis

  • provide photoprotection- absorbing or getting rid of excess light energy that would have damaged the chlorophyll or interacted with O2 to create reactive oxidative molecules

Most plants use CO2 directly from the air and carbon fixation occurs when the enzyme rubisco adds CO2 to RuBP. These plants are called C3 plants.

they are called C3 plants because the first product of carbon fixation is a 3 carbon compound, 3- PGA

in hot and dry weather C3 plants:

  • close their stomata to reduce water loss

  • prevent CO2 from entering the leaf and O2 from leaving

  • as O2 builds up in a leaf, rubisco adds O2 instead of CO2 to RuBP, and a two-carbon product of this reaction is then broken down in the cell. This process is called Photorespiration because it occurs in the light, consumes O2 and releases CO2

  • Photorespiration uses ATP instead of producing it

C4 plants evolved in the means of:

  • carbon fixation that saves water during photosynthesis and

  • optimizes the Calvin cycle

C4 plants are called C4 plants because they fix CO2 into a four carbon compound

In hot and dry weather C4 plants keep their stomata mostly closed to conserve water

CAM plants- plants like pineapples and cacti

Conserve water by opening their stomata and admitting CO2 only at night

CO2 is fixed into a four carbon compound that:

  • banks CO2 at night and

  • released it into the Calvin cycle during the day