Cells and Energy

sunlightMAIN IDEAPhotosynthesis in plants occurs in chloroplasts.Chloroplasts are the membrane-bound organelles where photosynthesis takesplace in plants. Most of the chloroplasts are in leaf cells that are specialized forphotosynthesis, which has two main stages as shown in FIGURE 4.5. The twomain parts of chloroplasts needed for photosynthesis are the grana and thestroma. Grana (singular, granum) are stacks of coin-shaped, membraneenclosedcompartments called thylakoids (THY-luh-KOYDZ). The membranesof the thylakoids contain chlorophyll, other light-absorbing molecules, andproteins. The stroma is the fluid that surrounds the grana inside a chloroplast.FIGURE 4.5 Photosynthesis OverviewChloroplasts absorb energy from sunlight and produce sugarsthrough the process of photosynthesis.STAGE 1: Light-Dependent Reactions1Energy from sunlight isabsorbed. Water moleculesare broken downand oxygen is released.chloroplastgranum (stack of thylakoids)BIOLOGYView an animationof photosynthesisat ClassZone.com.6H 2 O 6O 2thylakoidSTAGE 2: Light-Independent Reactions2energyEnergy-carryingmolecules, includingATP, transfer energy.6CO 2stroma (fluid outside the thylakoids)1 six-carbon sugarC 6 H 12 O 63 Carbon dioxide4molecules are usedto build sugars.Identify What are the reactants and the productsin photosynthesis?Six-carbon simple sugarsare produced. The sugarsare often used to buildstarches and cellulose.104 Unit 2: Cells