Chapter 8 Cellular Energy

8.1 How organisms obtain energy  Main Idea: all living organisms use energy to carry out all biological processes

Transformation of Energy  You are using energy even when you don’t know it!  How???  Energy = ability to do work

1 st law of thermodynamics  Conservation of energy: energy cannot be created or destroyed just converted from one form to another

Autotrophs and heterotrophs  Nearly all the energy for life comes from the sun  Autotrophs – organisms that make their own food  Chemoautotrophs – use inorganic substances as source of energy  Heterotrophs – organisms that need to ingest food to obtain energy

Metabolism  All of the chemical reactions in a cell  Metabolic pathway – series of reactions where products of one become reactants of another  Catabolic – break stuff down, release energy  Anabolic – put stuff together, absorb energy

Photosynthesis  Anabolic  Light  chemical energy  6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2

ATP  In living things chemical energy is stored in biological molecules and can be converted to other forms of energy when needed  Adenosine triphosphate = ATP: most important biological molecule that provides chemical energy  Found in all types of organisms  What if there was none?

ATP structure  Nucleotide  Adenine base, ribose sugar, 3 phosphate groups

ATP Function  Lots of energy is stored in chemical bonds  ATP releases energy when bond between 2 nd & 3 rd phosphate group is broken  Left with ADP + free phosphate group

Learning Goal  Describe Photosynthesis as a chemical process

8.2 Photosynthesis  Main Idea: Light energy is trapped and converted into chemical energy during photosynthesis

Overview of photosynthesis  Done by most autotrophs  6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2  Occurs in 2 phases  Light reactions – light energy absorbed & converted into ATP and NADPH  Dark reactions – energy from ATP & NADPH makes glucose

a A look inside the leaf b One of the photosynthetic cells inside leaf leaf’s upper epidermis photosynthetic cell in leaf leaf vein leaf’s lower epidermis Leaf Structure 16

Chloroplasts  Site of photosynthesis in plant cells  Capture light energy  Found mainly in the cells of leaves  Why?

Important structures in Chloroplasts Thylakoids: Disk shaped membranes Contain photosynthetic pigments. Site of light dependent reactions. Grana: Stacks of thylakoids. Stroma: Fluid filled space surrounding grana. Site of light independent reactions. 18

Properties of Light  White light from the sun is composed of a range of wavelengths. 20

Pigments  Light absorbing colored molecules  In thylakoid membranes  Different pigments absorb specific wavelengths of light  Major = chlorophylls  Absorb in blue region & a little in red region  Reflect in green region  Why are most plants green?

Why do leaves change color?

 Suppose a large meteor hit the earth. How could smoke and soot in the atmosphere wipe out life far beyond the area of direct impact?

Photosynthesis Equation 6H 2 O + 6CO 2 6O 2 + C 6 H 12 O 6 watercarbon dioxide oxygenglucose LIGHT ENERGY 24

Photosynthesis  Two stages:  light-dependent reactions require light to work (in thylakoids)  light-independent reactions do not require light (but still happen during the day) (In stroma) 25

sunlight Where the two stages of photosynthesis occur inside the chloroplast light- dependent reactions light- independent reactions CO 2 sugars NADPH, ATP NADP +, ADP O2O2 H2OH2O Two Steps in Photosynthesis 26

Light Reactions  In THYLAKOIDS  Large surface area for ETC  Membranes have photosystems in them  Proteins that contain light absorbing pigments (chlorophyll)  Electron transport chain

Photosystem II – where it all begins  Photosystem II – a protein in the membrane of the thylakoid that contains the pigment chlorophyll

 Light energy excites electrons in photosystem II

 Energy from excited electron is transferred to a chlorophyll molecule where it is used to excite chlorophyll’s electrons  Chlorophyll molecule gives excited electron to primary electron acceptor  Transfers electron to ETC

Meanwhile…  As electrons keep getting taken away from the chlorophyll molecule, the electrons must be replaced. The replacement electrons come from water.  Water splitting equation

 When water is split oxygen is released as a waste product and H + (protons) are released into thylakoid space

What happened to that electron?  Primary electron acceptor transfers electron to photosystem I  Another protein in thylakoid membrane  Photosystem I absorbs light to re-energize electron then transfers electron to ferrodoxin (final electron acceptor)

 Ferrodoxin transfers electron to NADP + forming NADPH  Energy storage molecule

Light reactions summary ferrodoxin Photosystem I Primary electron acceptor Photosystem II

1. Light energy excites electrons in photosystem II 1. Water splits releasing 1 electron, H + ion, & Oxygen 2. Electron travels to primary electron acceptor 3. Primary electron acceptor transfers electron to photosystem I 4. With light energy photosystem 1 transfers electron to protein (ferrodoxin) 5. Ferrodoxin transfers electron to NADP + forming NADPH 6. ETC ETC

Chemiosmosis  Produces ATP  H + released during ETC accumulate inside the thylakoid  H + diffuse into stroma through ion channels  ATP synthases convert ADP to ATP  ATP formed in stroma

 xp17rY4

Products of light-dependent reactions  O 2 (waste)  NADPH (used in dark reactions)  ATP (used in dark reactions)

Light Independent Reactions  What are the products of light reactions?  Not stable enough to store energy for a long time  Calvin cycle:  ATP + NADPH + CO 2  Glucose  Takes place in stroma

1. Carbon fixation: Rubisco(enzyme) takes C atoms from CO 2 in air to create organic molecule 2. Organic molecule uses energy from ATP and NADPH to form G3P some of which then form glucose (C 6 H 12 O 6 ) 3. Remaining G3P use energy from ATP to regenerate the original organic molecule

Summary  Light reactions:  Reactants  Products  Dark Reactions:  Reactants  Products

Factors that Affect Rates  3 factors can limit the speed of photosynthesis: light intensity, CO 2 concentration, temperature  Without enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and CO 2. 47

48  Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air.  Even if there is plenty of light, a plant cannot photosynthesize if there is insufficient carbon dioxide.

 If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesize if it gets too hot. 49

How does CO 2 enter plant?

Stomata  Primary sites for gas exchange in plants  CO 2 enters for photosynthesis  O 2 exits  water vapor evaporates  Closed stomata:  prevents water evaporation  O 2 cannot be released from the chloroplasts   CO 2 cannot enter them   No carbon = no Calvin cycle  plant growth comes to a standstill; crops fail 52

Alternative pathways  Plants in extreme climates have alternative photosynthesis pathways to maximize energy conversion because stomata are closed during the day to prevent water loss

 C4  Sugarcane, corn  Fix CO 2 into 4-carbon compounds instead of 3-Carbon compounds  photosynthesizes faster than C3 plants under high light intensity and high temperatures  better water use efficiency because PEP Carboxylase brings in CO 2 faster  doesn’t need to keep stomata open as much

 CAM  Plants that live in deserts, salt marshes, other places water is limited  Cacti, orchids, pineapple  Stomata closed during the day  CO 2 enters leaves only at night  Calvin cycle only occurs during the day