Photosynthesis

• ATP (adenosine triphosphate)- basic energy source of all cells, chemical compound that cells use to store and release energy– Adenine, 5- carbon sugar (ribose), and 3 phosphate groups

• ADP (adenosine diphosphate)-chemical compound similar to ATP, but has two phosphate groups– Key for storing energy

• How is the energy that is stored in ATP released?– Chemical bond between second and third phosphate group is

broken• Because of the characteristics of ATP, it is an exceptional

source of energy

Sec 2- Photosynthesis: An Overview

• Photosynthesis- series of reactions that uses light energy from sun to convert water and carbon dioxide into sugars and oxygen

• 6 CO₂ + 6 H₂O→ C₆H₁₂O₆+ 6 O₂• In addition to water and carbon dioxide, it requires light and

chlorophyll, molecule in chloroplasts• Plants gather the sun’s energy with light absorbing molecules called

pigments• Chlorophyll- principal light absorbing pigment

– Chlorophyll a and chlorophyll b– Absorbs light very well in certain regions of visible spectrum,

green light is reflected

Sec 3- Reactions in Photosynthesis

• Photosynthesis takes place in chloroplasts• Thylakoids- sac-like photosynthetic membranes inside

chloroplasts – Arrange in stacks known as grana– Proteins in thylakoids organize chlorophyll into photosystems

– 2 types of photosystems- light collecting units of the chloroplast

– 2 types of reactions in photosystems- light dependent reactions and light independent reactions= Calvin cycle

– Light dependent reactions-thylakoid membranes– Calvin cycle= stroma, a region outside thylakoid membranes

Electron Carriers

• Sunlight excites electrons in chlorophyll= great deal of energy

• Electron carriers transport high energy electrons from chlorophyll to other molecules

• NADPᶧ- electron carrier• High energy electrons used to build molecules for

cell

Light Dependent Reactions

• Requires light• Converts ADP and NADPᶧ into ATP and NADPH and produces

oxygen• Steps of light dependent reaction

– Light is absorbed by electrons in photosystem II Are electrons ever used up? Where do the electrons come from?

– High energy electrons move through electron transport chain to photosystem I

– Photosystem I reenergizes electrons released. Electrons are used to form NADPH

– Positively charged hydrogen ions fill up thylakoid membrane– Hydrogen ions pass through ATP synthase, ADP is converted into ATP

Calvin Cycle• Plants use energy that ATP and NADPH contain to build high

energy compounds• Does not require light, known as light- independent reactions• Steps of Calvin cycle

– CO₂ enters the cycle, 6 carbon dioxide molecules combined with six 5 carbon molecules to form twelve 3- carbon molecules

– ATP and NADPH are used to convert twelve carbon molecules into higher energy forms

– Two 3 carbon molecules removed from cycle to form sugars, lipids, amino acids

– 10 remaining 3- carbon molecules converted into six 5- carbon molecules- used in next cycle

• Plants use sugars to build cellulose