Photosynthesis The Big Picture Photosynthesis and Respiration work

Photosynthesis

The Big Picture Photosynthesis and Respiration work together in plants to make energy for the plant – they are autotrophs Animals only use cellular respiration – why? They are heterotrophs!

What is energy? In cells, energy is stored as ATP (adenosine triphosphate) ATP delivers energy to other cells The energy in ATP is released and used for many chemical processes When the energy is released, ATP becomes ADP – it loses a phosphate atom. Therefore, the energy is stored in the phosphate bond! ATP ADP + energy

ATP 3 phosphate groups Pentose Sugar (ribose) adenine Does this structure look familiar?

Photosynthesis Definition: process by which light energy is converted to chemical energy In English: Plants use light, carbon dioxide and water to make sugar (food) and oxygen. ATP is used as a fuel source to power photosynthesis (like gas powers your car)

Where does photosynthesis occur? Photosynthesis occurs in the leaves of the plants; special pores of the plants called stomata open to take in carbon dioxide and release oxygen

3 Stages of Photosynthesis Stage 1 – Energy is captured from sunlight Stage 2 – Light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH Stage 3 – Chemical energy in ATP and NADPH powers the formation of organic compounds (sugars) using carbon dioxide

Photosynthesis: Stage One The leaves of a plant contain light-absorbing substances called pigments The main pigment of a plant is chlorophyll, which is what makes plants look green. The other pigments are called carotenoids These pigments are located in the chloroplasts of a plant cell

Photosynthesis: Stage One Chloroplasts contain stacks of disk-like structures called thylakoids Light energy hits the thylakoid and the energy is transferred to electrons in chlorophyll; the electrons get “excited” Excited electrons leave chlorophyll and are replaced by electrons in water The water is split by an enzyme and electrons from the hydrogen atom are taken by the chlorophyll. The oxygen atoms from different water molecules combine to make oxygen (O 2)

Electrons Jumping

Photosynthesis: Stage Two The electrons (energy) from stage one is used to make ATP The ATP is made by a process called the Electron Transport Chain The electrons from stage one are passed through several molecules in the membrane of the thylakoid. Another energy molecule called NADPH is also made in the electron transport chain.

Electron Transport Chain Animation

Photosynthesis: Stage Two Stage two makes ATP and NADPH; these are both important because they are needed for stage three. ATP is used to power Stage Three NADPH is used to make bonds in Stage Three Stage Two is also known as the light dependent reactions – they need light to work

Photosynthesis: Stage Three In this stage, carbon dioxide molecules are used to make organic compounds (sugars) where chemical energy is stored. This process is called carbon dioxide fixation The most common method of carbon dioxide fixation is the Calvin cycle

The Calvin Cycle Step one: each carbon dioxide molecule is added to a 5 -carbon compound by an enzyme – this makes a 6 -carbon compound Step two: the 6 -carbon compound splits into two 3 -carbon compounds. Then, phosphate groups from ATP and electrons from NADPH are added to turn the 3 -carbon compounds into 3 -carbon sugars

The Calvin Cycle Step three: one of the 3 -carbon sugars is used to make organic compounds (starch and sucrose) – energy is stored in those compounds for use later Step four: the other 3 -carbon sugars are used to regenerate the 5 -carbon compound, completing the cycle and allowing it to start over again

Factors that affect Photosynthesis Light Carbon dioxide concentration Water See the whole cycle!