Photosynthesis and Cellular Respiration Chapter 5

5.1 Energy and Living Things  Remember: Metabolism is either the energy used to build molecules or the release of energy by breaking down molecules

Photosynthesis  Turn light energy into chemical energy  Autotrophs – make their own food  Most are photosynthetic  Some are chemosynthetic (get energy from inorganic compounds)

Cellular Respiration  Get energy from food  Heterotrophs – must ingest energy  Produces ATP

Transfer of Energy to ATP  Energy from food released gradually in a series of enzyme-assisted chemical reactions  Product of 1 reaction becomes the reactant in the next reaction  Some energy is released as heat  Most energy is stored in ATP which can deliver it where it is needed

How ATP Works (Yes, again)  ATP = Adenosine Triphosphate  3 negative phosphates repel each other  Energy released when bonds break  Produces ADP = adenosine diphosphate

 Can be reversed back into ATP  If a 2 nd phosphate is broken off = irreversible  Water is used in breaking  H 2 O + ATP = ADP + P + ENERGY

Photosynthesis  Stages of Photosynthesis  First 2 are called Light Dependent Reactions  Stage 1: Energy is captured from sunlight  Stage 2: Light energy is changed to chemical energy  Chemical energy stored temporarily in ATP and a carrier molecule called NADPH  Stage 3 is called Light Independent or Dark Reactions  Energy in ATP and NADPH powers the formation of organic compounds using CO 2

Where does all of this take place?  In the chloroplasts of plant cells and algae and in the cell membrane of certain prokaryotes  6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2  Carbon dioxide and water make sugars and oxygen with the help of light energy  Some sugars form starch and are stored in stems and roots  These can be broken down later to make ATP  Sugars break down and make proteins, nucleic acids, and others

Stage 1 Absorption of Light Energy (Light Dependent)  Light is a form of radiation  Pigments are light absorbing substances  But they only absorb certain wavelengths  Chlorophyll is the primary pigment  Absorbs mostly blue and red  Reflects green and yellow which is why we see them as these colors

A few different kinds  Chlorophyll a  Chlorophyll b  Carotenoids  Yellow and orange reflecting pigments  Absorb different wavelengths than chlorophyll

Stage 1: Oxygen Production  Pigments in chloroplasts of leaf cells  Found in part of chloroplast called Thylakoid which is a disk shaped structure  Light hits thylakoid and energy is transferred to electrons in chlorophyll  Electrons jump to higher energy and are considered “excited”  Some electrons jump from chlorophyll to other nearby molecules in the thylakoid membrane where they power 2 nd stage

 Other electrons from H 2 O replace those that leave  H 2 O is split by an enzyme in the thylakoid  Chlorophyll takes e- from H leaving H+  The O left behind combines with others to form O 2

Stage 2: Conversion of Light Energy  e- that jump from thylakoid membrane  This stage uses electron transport chains  Chain 1  e- move through a protein  Some energy lost as they power protein which pumps H+ into thylakoid  H+ become concentrated in thylakoid (concentration gradient causes diffusion out)  Carried by carrier protein, - ion channel, and enzyme  As H+ passes through it catalyzes a reaction so ADP + P make ATP

 Chain 2  Makes NADPH which provides high energy electrons needed to form carbon-hydrogen bonds in stage 3  e- combine with H+ and NADP+ forming NADPH

Summary of Light Dependent Reactions  Light absorbed by pigments in thylakoid of chloroplast  E- in pigments are excited and move through e- transport chains  E- replaced by e- from water molecules split by an enzyme  O in water forms O2 gas  H+ accumulate inside thylakoids, concentration gradient occurs powering ATP production

Stage 3: Storage of Energy (Light-Independent or Dark Reactions)  Carbon atoms from CO 2 make organic compounds to store chemical energy  This is called Carbon Dioxide Fixation  Several ways this can be done

Calvin Cycle is most common way  Produces 3-carbon sugars  Step 1  3 CO 2 needed  Each CO 2 is added to a 5-carbon compound by an enzyme