PHOTOSYNTHESIS The Basics BIO11McIntyre

Photosynthesis Photosynthesis is the process by which carbohydrates (an organic nutrient) are synthesized from inorganic sources using the energy of light. Photosynthesis is the process by which carbohydrates (an organic nutrient) are synthesized from inorganic sources using the energy of light.

PHOTOSYNTHESIS OVERVIEW 6CO 2 + 6H 2 O + energy (sun)  C 6 H 12 O 6 + 6O 2

In order for photosynthesis to take place the following conditions must be met: In order for photosynthesis to take place the following conditions must be met: 1. Light – plants use the visible light spectrum 2. Photosynthetic pigments – there are various photosynthetic pigments found in chloroplasts including: chlorophyll A, chlorophyll B, chlorophyll C, carotene, and xanthophyll (the most important being chlorophyll A) 3. Thylakoid membranes – specialized membranes of the chloroplasts that are embedded with chlorophyll. These are in stacks called grana, surrounded by a gel-like substance called the stroma.

Chloroplast

Photosynthesis overview Photosynthesis occurs in two distinct phases: Photosynthesis occurs in two distinct phases: 1. The light reactions – known as the photophase because it requires light energy. 2. The dark reactions (Calvin cycle) – during this phase, light is not required; however, the ATP and NADPH produced by the light reactions are.

STRUCTURE OF A LEAF Leaves are a plant's main photosynthetic organs. Leaves are a plant's main photosynthetic organs. Leaves must permit carbon dioxide access to the photosynthetic cells but impede water from diffusing out. Leaves must permit carbon dioxide access to the photosynthetic cells but impede water from diffusing out. The oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf. The oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf.

STRUCTURE OF A LEAF In which cells does photosynthesis take place?

STRUCTURE OF A LEAF Mesophyll cells are specialized for photosynthesis. Mesophyll cells are specialized for photosynthesis. These cells in the middle of the leaf contain many chloroplasts, the organelles that perform photosynthesis. These cells in the middle of the leaf contain many chloroplasts, the organelles that perform photosynthesis.

STOMA The stoma allows for the plant to exchange gasses with its environment. The stoma allows for the plant to exchange gasses with its environment. Stoma Guard cells

PIGMENTS & CHLOROPHYLL Pigment: any substance that absorbs light of a specific wavelength. Pigment: any substance that absorbs light of a specific wavelength. Photosynthetic Pigments absorb light (solar) energy and convert it to chemical energy. Photosynthetic Pigments absorb light (solar) energy and convert it to chemical energy. Chlorophyll by itself can’t carry out photosynthesis - it will absorb light but radiate it back at a different wavelength. Chlorophyll by itself can’t carry out photosynthesis - it will absorb light but radiate it back at a different wavelength.

PIGMENTS & CHLOROPHYLL Electron Acceptors must also be present for photosynthesis. Electron Acceptors must also be present for photosynthesis. Chlorophyll is found embedded in flattened membranes called Thylakoids in the Chloroplast. Chlorophyll is found embedded in flattened membranes called Thylakoids in the Chloroplast. Thylakoids are arranged I interconnected stacks called Granna. Thylakoids are arranged I interconnected stacks called Granna.

Photosynthesis: An Overview of the Light and ‘Dark’ Reactions Occurs in Photoautotrophs (organisms that can make their own using energy from the sun). Photosynthesis takes place in the chloroplasts. Photosynthesis includes two processes… LIGHT REACTIONS Requires sunlight Occurs in the granna of chloroplasts Produces ATP and NADPH (used to power the Calvin cycle) ‘DARK’ REACTIONS (a misnomer…aka Calvin cycle) Doesn’t require sunlight (happens 24/7). Occurs in the stroma of chloroplasts Produces PGAL (which can later be used to make glucose) animationsimple animation

Light reactions Chlorophyll absorbs light energy into the thylakoid membrane and is said to be ‘energized’. The energized chlorophyll causes the following reactions to occur: Chlorophyll absorbs light energy into the thylakoid membrane and is said to be ‘energized’. The energized chlorophyll causes the following reactions to occur: A water molecule is split into hydrogen ions and oxygen atoms. This split occurs due to chlorophyll’s strong attraction for electrons. The hydrogen ions are then held by the coenzyme NADP +. The oxygen atoms are released to the atmosphere as a byproduct. A water molecule is split into hydrogen ions and oxygen atoms. This split occurs due to chlorophyll’s strong attraction for electrons. The hydrogen ions are then held by the coenzyme NADP +. The oxygen atoms are released to the atmosphere as a byproduct. ADP adds a phosphate group to become ATP (phosphorylation). The energy stored in ATP is used during the Calvin cycle. ADP adds a phosphate group to become ATP (phosphorylation). The energy stored in ATP is used during the Calvin cycle.

Overview of light dependent reactions

The Calvin Cycle In Photosynthesis, ATP and NADPH are produced in photophosphorylation, aka the Light Reactions. This happens in the thylakoid but notice that the products are actually produced in the stroma. This sets up the next series of reactions, the Calvin cycle which happens completely in the stroma. This is where sugars are manufactured. Melvin Calvin discovered this cycle in In Photosynthesis, ATP and NADPH are produced in photophosphorylation, aka the Light Reactions. This happens in the thylakoid but notice that the products are actually produced in the stroma. This sets up the next series of reactions, the Calvin cycle which happens completely in the stroma. This is where sugars are manufactured. Melvin Calvin discovered this cycle in 1940.

THE CALVIN CYCLE The reactions that convert carbon dioxide into carbohydrate molecules occur in the stroma of chloroplasts. The reactions that convert carbon dioxide into carbohydrate molecules occur in the stroma of chloroplasts. CO2 is converted to glucose (a carbohydrate) and H2O is giving off to the atmosphere. CO2 is converted to glucose (a carbohydrate) and H2O is giving off to the atmosphere. The process occurs by a cyclic series of reactions. The process occurs by a cyclic series of reactions.

… The Calvin Cycle The end product of photosysnthesis isn’t really glucose; it’s PGAL (phosphoglyceraldehyde). PGAL can be used to manufacture glucose, or other sugars, fatty acids or amino acids and as an energy source for cellular respiration. The end product of photosysnthesis isn’t really glucose; it’s PGAL (phosphoglyceraldehyde). PGAL can be used to manufacture glucose, or other sugars, fatty acids or amino acids and as an energy source for cellular respiration. During a series of reactions, ATP and NADPH produced by the light reactions help convert CO 2 into PGAL. During a series of reactions, ATP and NADPH produced by the light reactions help convert CO 2 into PGAL. The Calvin Cycle has three phases: The Calvin Cycle has three phases: 1st phase: Carbon Fixation 1st phase: Carbon Fixation 2nd phase: Reduction 2nd phase: Reduction 3rd phase: Regeneration of the Carbon acceptor molecule 3rd phase: Regeneration of the Carbon acceptor molecule

3 x CO 2 6 x (3-C) 3 x (5-C) 1 Rubisco 2 1 st Phase: Carbon Fixation 1. Three five-carbon sugar molecules are the acceptors that bind 3 CO 2 molecules (dissolved in the stroma). This reaction is catalyzed by the enzyme rubisco. 2. Three unstable 6-C molecules are produced (not shown) which quickly break down to give six 3-carbon molecules of the Phase 1: CARBON FIXATION Animation: Calvin cycle Phosphate carbon

3 x CO 2 3 x RuBP (5-C) 6 x ATP 6 x ADP 6 x NADPH 6 x NADP 6 x P i 6 x PGAL (3-C) 1 x PGAL (3-C) 1 Rubisco nd Phase: Reduction 3. The six 3-C molecules are phosphorylated and reduced to PGAL (phosphoglyceraldehyde), a three-carbon sugar. This phosphate bond is then broken and hydrogen is added from NADPH. 4. Six molecules of PGAL are produced. However, only one of the six molecules exits the cycle as an output (to make sugar, etc.) while... Phase 2: REDUCTION REACTIONS Animation: Calvin cycle 6 x 1,3 BPG NOTE: PGAL is also referred to as G3P 6 x 3-PGA (3-C)

3 x CO 2 3 x RuBP (5-C) 6 x ATP 6 x ADP 6 x NADPH 6 x NADP 6 x P i 6 x PGAL (3-C) 1 x PGAL (3-C) 5 x PGAL (3 C) 3 x ADP 3 x ATP 1 Rubisco rd Phase: Regeneration of the Carbon acceptor molecule the remaining five enter a complex process that regenerates more five-carbon sugar molecules to continue the cycle In this process, ATP is used to convert the five PGAL’s to three 5-C molecules. 7. Summary... 9 ATP used 6 NADPH used 1 PGAL produced 5-C regenerated Phase 3: REGENERATION Animation: Calvin cycle 6 x 1,3 BPG 6 x 3-PGA (3-C)

Glucose formation The sixth PGAL will be used to form a glucose molecule. The sixth PGAL will be used to form a glucose molecule. Since each glucose molecule (C 6 H 12 O 6 ) is formed from two PGAL’s, two complete turns of the cycle are required. Since each glucose molecule (C 6 H 12 O 6 ) is formed from two PGAL’s, two complete turns of the cycle are required. NADP+, ADP and Pi are then released by the Calvin cycle and are recycled back to the thylakoid membrane to be transformed yet again by the light reactions. NADP+, ADP and Pi are then released by the Calvin cycle and are recycled back to the thylakoid membrane to be transformed yet again by the light reactions.