Photosynthesis: An Overview Section 8.2

Inside Chloroplasts Chloroplasts contain saclike photosynthetic membranes called thylakoids where chlorophyll is located. The thylakoids are interconnected and arranged in stacks called grana. The fluid in chloroplasts is called the stroma.

Photosynthesis Overview Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high energy sugars and oxygen.

Light Sunlight is white light which is a mixture of colors in the visible spectrum of varying wavelengths. Plants gather the sun’s energy using light-absorbing molecules called pigments.

Chlorophyll The principle pigment used by plants to collect light energy is chlorophyll. Two types of chlorophyll are found in plants: chlorophyll a and chlorophyll b. These pigments absorb light in the blue-violet and red regions and reflect light in the green region. This is why plants are green!

Chlorophyll Molecular Structure

Accessory Pigments Plants also contain red and orange pigments such as carotene that absorb light in other regions. The intense color of chlorophyll overwhelms these accessory pigments. Chlorophyll breaks down first in the fall, leaving orange and red pigments.

Chromatography A separation technique that utilizes the differing polarities of the substances in a mixture. Chromatography can be used to separate the pigments in plant leaves.

High Energy Electrons Once chlorophyll absorbs light energy, the energy is transferred to electrons. The plant uses electron carriers to transport the high energy electrons. One of the electron carriers is NADP+.

Nicotinamide Adenine Dinucleotide Phosphate NADP+ accepts two high energy electrons and a hydrogen ion to become NADPH. This enables some of the light energy to be trapped in chemical form. The electron carrier can move throughout the cell and use the energy they carry to help build molecules the cell needs.

Light Dependent Reactions Overview Require the direct involvement of light and light-absorbing pigments. Uses energy from sunlight to produce ATP and other energy-rich compounds (NADPH). Water is required and oxygen is a byproduct.

Photosystems Takes place in thylakoid membranes of the chloroplast. Thylakoids contain clusters of chlorophyll and proteins known as photosystems. The photosystems absorb sunlight and generate high-energy electrons that are passed to a series of electron carriers in the thylakoid membrane.

Electron Transport Chain A series of electron carrier proteins that shuttle high-energy electrons during ATP-generating reactions.

Light Dependent Reactions Details Step 1: PS II absorbs light energy and uses it to energize electrons made by splitting water molecules.

Step 2: High-energy electrons move down the transport chain and are used to pump H+ across the membrane against its concentration gradient.

Step 3: PS I uses light energy to produce NADPH by reenergizing electrons.

Step 4: The H+ ions move back across the membrane through ATP synthase, which generates ATP.

Helpful Videos http://www.youtube.com/watch?v=joZ1EsA5_NY http://www.youtube.com/watch?v=ww33L0lD37I

Light Independent Reactions Overview No light is required. ATP and other energy-rich compounds (NADPH) from the light dependent reactions are used to produce sugars from carbon dioxide. Takes place in the stroma of the chloroplast.

Calvin Cycle Details Step 1: Carbon dioxide is added Carbon dioxide is added to five-carbon molecules already in the cycle to form three-carbon molecules. Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)

Step 2: High-energy three-carbon molecules formed Energy from light dependent reactions is used by enzymes to form high-energy three-carbon molecules.

Step 3: Three-carbon molecules exit Three-carbon molecules leave the cycle to form sugars. Plants form starch to store the sugars.

Step 4: Three-carbon molecules recycled Energy from light dependent reactions is used to change the three-carbon molecules left in the cycle back to five-carbon molecules that are then used to restart the cycle.

Factors Affecting Photosynthesis Temperature Light Intensity Water

Temperature Enzymes needed to catalyze the reactions of photosynthesis work best between 0 and 35 degrees Celsius.

Light Intensity High intensity of light increases the rate of photosynthesis, but there is a maximum value for this rate.

Water Shortages of water can slow or stop photosynthesis and/or damage plant tissues. Some plants that live in dry areas have waxy coatings on their leave to prevent water loss.

C4 Photosynthesis Have a specialized chemical pathway that allows them to capture carbon dioxide in extreme conditions. Examples: corn, sugar cane, sorghum

CAM Plants Like C4 plants, have a specialized chemical pathway that allows them to capture carbon dioxide in extreme conditions. Stomata are only open at night and are sealed tightly during the day. Examples: pineapple trees, desert cacti