1 UNIT 2 PART 4: PLANT NUTRITION Photosynthesis is a process where plants transform the energy from light into chemical bond energy. Green plants are autotrophs: they manufacture food from inorganic raw materials. A heterotroph eats food.

2 Chlorophyll: A pigment A substance that absorbs light is called a pigment. Plants have green pigments, chlorophylls a & b. They look green because green light is reflected. Other plant pigments include – carotene (orange) & –xanthophyll (yellow). Together, these pigments absorb the red & blue light used in photosynthesis.

3 Chloroplast: Site of Photosynthesis The chloroplasts in the palisade & spongy cells consist of 2 membrane layers surrounding a space called the stroma. Within the space are the grana, made of thylakoid membranes that contain the chlorophylls and other pigments.

4 Photosynthesis: The overall reaction 6CO H 2 O  C 6 H 12 O 6 + 6O 2 + 6H 2 O

5 Step 1: The light- dependent reaction Requires light Light energy captured by chlorophyll is used to split water into H 2 and O. Occurs in thylakoid membranes. Produces oxygen, O 2. Photosynthesis occurs in two steps:

6 Step 2: the light- independent reactions Do not need light, but do need some of the products of the light- dependent reactions. Uses CO 2 (carbon fixation) to produce glucose in the Calvin cycle Occurs in the stroma. Glucose is stored as starch in leaves.

7 The rate of photosynthesis is affected by: –Light intensity More light  increased photosynthesis –Temperature Too high or low  decreased photosynthesis –Amount of water available –Amount of minerals available A shortage of either water or minerals will slow or stop photosynthesis Light intensity Rate of photosynthesis Temperature

8 Respiration and Photosynthesis are reverse reactions Respiration occurs in the cytoplasm and in mitochondria. Converts organic molecules to inorganic C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 0 Releases energy from chemical bonds in glucose making ATP (energy) Photosynthesis occurs in chloroplasts. Converts inorganic molecules to organic 6CO 2 + 6H 2 0  C 6 H 12 O 6 + 6O 2 Stores energy from the sun in chemical bonds in glucose