Concept 8.1 Photosynthesis uses light energy to make food

Vocab

chloroplast: organelle in plant cells and certain unicellular organisms where photosynthesis takes place

chlorophyll: pigment that gives chloroplast its color and uses light energy to split water molecules during photosynthesis

stroma: thick fluid contained within the inner membrane of chloroplast

thylakoid: disk-shaped sac in the stroma where light reactions of photosynthesis take place

light reactions: the reactions which the sun's energy is converted into chemical energy, takes place in the thykaloid membrane

Calvin's cycle: cycle in plants that makes sugar from carbon dioxide, H+ions, and high energy electrons carried by NADH

Summary

• In most lants, the leave contain the most chloroplasts are the major sites for photosynthesis
  
• Chloroplasts are concentrated in cells of mesophyll, the inner layer of tissuechloroplasts are concentrated in cells of mesophyll, the inner layer of tissue
  
• Tiny pores on the surface of the leaf allow carbon dioxide to enter and oxygen to leave


• Veins carry water and other nutrients from the roots to the leaves, as well as deliver organic molecules produced in the leaves to other parts of the plant
  
• The chloroplasts structure is key to its function

Structuer of chloroplast:

-inner and outer membrane

-inner membrane encloses a thick fluid called stroma

-suspended in the stroma are disk-shaped sacs called thylakoids; each thylakoid is enclosed by a membrane

-thylakoids arranged in stacks called grana (singular, granum)

• Some steps of photosynthesis occur within the thylakoid membranes, while others occur in the stroma


• In contrast to the falling of electrons during cellular respiration, electrons from water are "boosted uphill" by the energy of sunlight


• Chloroplasts use these "excited" electrons along with carbon dioxide and water to produce sugar molecules
  
• Photosynthesis occurs in two main stages: the light reactions, and the Calvin cycle
  
• The light reactions convert energy in sunlight into chemical energy


• These reactions rely on the molecules built into the thylakoid membrane

Steps of light reaction:

1. chlorophyll molecules in the membrane capture energy from the light


2. chlorophyll then remove electrons from water using the captured light (this splits the water into oxygen and hydrogen ions)
   
3. oxygen is a waste product of photosynthesis
   
4. chloroplast uses electrons and hydrogen ions to make an energy-rich molecule called NADPH (electron carrier), similar to NADH in cellular respiration


5. chloroplast also use light energy to produce ATP


6. Overall result of light reactions: the conversion of light energy into chemical energy stored in two compounds: NADPH and ATP.

• The Calvin cycle creates sugar from the atoms in carbon dioxide plus the hyrdogen ions and the high energy electrons carried by NADPH


• Enzymes of the Calvin cycle are outside the thykaloid and dissolve in the stroma
  
• The ATP produced by the light reactions provides energy for making sugar in the Calvin cycle


• The Calvin cycle does not directly need light, but it relies on the light reactions for two inputs: ATP and NADPH

Concept Check

1) Draw and label a simple diagram of a chloroplast that includes the following structures: outer and inner membranes, stroma, thylakoids.

2) What are the reactants for photosynthesis? What are the products?

The reactants for photosynthesis are water and carbon dioxide and the products are sugar and oxygen.

3) Name the two main stages of photosynthesis. How are the two stages related?

The two main stages of photosynthesis are the light reactions and the Calvin cycle. The NADPH and ATP that are produced in the light reactions are used in the Calvin cycle for creating sugar. The Calvin cycle then returns certain substances and some electrons back to the light reactions.