photosynthesis notes
TRANSCRIPT
Photosynthesis
Chapter 6
The Light Reactions
• Almost all of the energy in living systems comes from the sun
• Sunlight energy enters living systems when plants and some other organisms absorb light and use it to make sugars
Obtaining Energy
• Organisms can be classified by how they get energy
• Autotrophs use energy from sunlight to make sugars (their own food)– Most use photosynthesis to convert sunlight
energy into chemical energy• Heterotrophs get energy from food
Obtaining Energy
Overview of Photosynthesis
• The products of photosynthesis are the reactants for cellular respiration
• The products of cellular respiration are the reactants for photosynthesis
Overview of Photosynthesis
• Photosynthesis can be divided into 2 stages– Light Reactions – light energy is converted to
chemical energy, which is temporarily stored in ATP and NADPH
– Calvin Cycle – Sugars are formed using CO2 and the chemical energy stored in ATP and NADPH
Overview of Photosynthesis
• Overall photosynthesis equation:
6CO2 + 6H2O C6H12O6 + 6O2
Carbon dioxide + water sugars + oxygen
Light energy
Light energy
Capturing Light Energy
• The first stage of photosynthesis is called the light reactions because they require light
• Photosynthesis begins with the absorption of light in the chloroplast
Parts of the chloroplast
• Chloroplasts – organelles found in the cells of plants and algae
• Thylakoids – membranes arranged as flattened sacs
• Grana – stacks of thylakoids
• Stroma – solution surrounding the grana
Light and Pigments
• Light from the sun appears white, but it is made of a variety of colors
Light and Pigments
• Pigments are compounds that absorb light• Many objects contain pigments that absorb
some colors of light and reflect others• The colors that are reflected are the ones you
see
Chloroplast Pigments
• There are several pigments in the thylakoid membranes– Most important are chlorophylls• Chlorophyll a absorbs mostly red and blue light and
reflects mostly green light
– Accessory pigments• Chlorophyll b assists chlorophyll a in capturing light
energy• Carotenoids are the yellow, orange, and brown
pigments
Chloroplast Pigments
Chloroplast Pigments
• In plant leaves, chlorophylls are the most abundant pigments and therefore mask the colors of the other pigments
• During the fall, many plants lose their chlorophylls and become the color of the carotenoids
Light Reactions
• The first set of reactions – the light reactions take place in the thylakoid membranes
Light dependent reactions
1. Light energy is absorbed by chlorophyll in photosystem II
2. Electrons in chlorophyll are energized, or “excited” and leave the chlorophyll
3. These electrons are passed to the primary electron acceptor
Light Dependent Reactions
4. The electrons move down the electron transport chain (series of molecules that transport electrons)a. As the electrons move down the chain, the
energy is used to move H+ ions into the thylakoid
Light-Dependent Reactions
5. Light is absorbed by chlorophyll in Photosystem I
6. Electrons are energized and passed on to a different electron transport chain
7. NADP+ accepts these electrons and is converted to NADPH
Replacing Electrons in Light Reactions
• To replace the electrons lost from chlorophyll in photosystem II, an enzyme splits water molecules
• 2H2O 4H+ + 4e- + O2
Making ATP in Light Reactions
• Hydrogen ions move from the inside of the thylakoid to the outside through ATP Synthase, an enzyme
• As this happens, ATP is produced
Summary
• The light-dependent reactions use:– Sunlight– Water
• And produce:– NADPH– ATP
The Calvin Cycle
• Second set of reactions in photosynthesis• Uses the energy stored in ATP and NADPH to
produce sugars
The Calvin Cycle
1. Carbon atoms from carbon dioxide in the air enter the plant
2. This carbon joins with sugars in the plant3. Energy from ATP and NADPH converts these
compounds into sugar for the plants to use4. Using 3 ATP the remaining sugars are
converted back to the original carbon compounds to begin the cycle again
A Summary of Photosynthesis
• Happens in two stages1. The light reactions – Energy is absorbed from
sunlight and converted into chemical energy, which is temporarily stored in ATP and NADPH
2. The Calvin cycle – Carbon dioxide and the chemical energy stored in ATP and NADPH are used to form sugars
Summary of Photosynthesis
Factors That Affect Photosynthesis
• Light Intensity• Carbon Dioxide Levels
Factors that Affect Photosynthesis
• Temperature