Photosynthesis and

Cellular Respiration

Outline

I. Photosynthesis

A. Introduction

B. Reactions

II. Cellular Respiration

A. Introduction

B. Reactions

Energy Shuttling

Recall ATP: cellular energy-nucleotide based

molecule with 3 phosphate groups bonded to it,

when removing the third phosphate group, lots of

energy liberated= superb molecule for

shuttling energy around within cells.

Other energy shuttles-coenzymes (nucleotide

based molecules): move electrons and protons

around within the cell

NADP+, NADPH NAD+, NADH FAD, FADH2

Photosynthesis Photosynthesis takes place in specialized

structures inside plant cells called chloroplasts

– Light absorbing pigment molecules e.g. chlorophyll

Photosynthesis

Light occurs in photons – packets of energy– White light contains all wavelengths of light in the

visible spectrum

– Certain wavelengths are reflected while others are absorbed to see colors

Pigments absorb specific wavelengths of light energy– Chlorophyll A & B

Green wavelengths of light are reflected

Red & blue wavelengths are absorbed

– Carotenoids –accessory pigments (fall colors)

Photosynthesis

Method of converting sun energy into chemical energy usable by cells

Autotrophs: self feeders, organisms capable of making their own food– Photoautotrophs: use sun energy e.g. plants

photosynthesis-makes organic compounds (glucose) from light

– Chemoautotrophs: use chemical energy e.g. bacteria that use sulfide or methane chemosynthesis-makes organic compounds from chemical energy contained in sulfide or methane

Overall Reaction

6CO2 + 12 H2O + light

energy → C6H12O6 + 6O2+ 6H2O

Carbohydrate made is glucose

Water appears on both sides because 12 H2O molecules

are required and 6 new H2O molecules are made

Water is split as a source of electrons from hydrogen

atoms releasing O2 as a byproduct

Electrons increase potential energy when moved from

water to sugar therefore energy is required (endergonic)

Light-dependent Reactions

Overview: light energy is absorbed by

chlorophyll molecules-this light energy excites

electrons and boosts them to higher energy

levels. They are trapped by electron acceptor

molecules that are poised at the start of a

neighboring transport system. The electrons

“fall” to a lower energy state, releasing energy

that is harnessed to make ATP

Light-dependent ReactionsPhotosystem II

Photosystem: light capturing unit, contains chlorophyll,

the light capturing pigment

– 200-300 chlorophyll molecules & carotenoids grouped together

– 2 parts to a photosystem: an antenna complex & a reaction

center

Antenna complex – captures energy, exciting electrons that are passed to

electrons in a nearby pigment molecule

Reaction center – transfers the electrons to a new compound

Light-dependent ReactionsPhotosystem II

Electron transport system: sequence of electron

carrier molecules that shuttle energized electrons, energy

released to make a proton gradient to make ATP

(photophosphorylation)

Electrons in chlorophyll must be replaced so that cycle

may continue-these electrons come from splitting of

water molecules, Oxygen is liberated from the light

reactions

Light reactions yield ATP and NADPH used to fuel the

reactions of the Calvin cycle (light independent or dark

reactions)

Light-dependent ReactionsPhotosystem I

Light reactions yield ATP and NADPH used to fuel the

reactions of the Calvin cycle (light independent or dark

reactions)

– NADPH is made by transferring 2 electrons and a hydrogen ion to

NADP+

Calvin Cycle (light independent or “dark” reactions)

ATP and NADPH generated in light reactions used to fuel

the reactions which take CO2 and break it apart, then

reassemble the carbons into glucose.

Called carbon fixation: taking carbon from an inorganic

molecule (atmospheric CO2) and making an organic

molecule out of it (glucose)

– Rubisco (ribulose 1,5 bisphosphate

caroxylase/oxygenase

Simplified version of how carbon and energy enter

the food chain

C4 Pathway & CAM Pathway(Crassulacean Acid Metabolism)

This pathway uses PEP carboxylase to add carbon dioxide to a 3 carbon compound – Low oxygen affinity

– Works well for these plants because their stomata are closed frequently giving a high oxygen concentration

Hot, dry habitats (C4 Pathway- sugarcane, corn, CAM Pathway- cacti)

Harvesting Chemical Energy

So we see how energy enters food chains (via autotrophs) we can look at how organisms use that energy to fuel their bodies.

Plants and animals both use products of photosynthesis (glucose) for metabolic fuel

Heterotrophs: must take in energy from outside sources, cannot make their own e.g. animals

When we take in glucose (or other carbs), proteins, and fats-these foods don’t come to us the way our cells can use them

Cellular Respiration Overview

Transformation of chemical energy in food into

chemical energy cells can use: ATP

These reactions proceed the same way in plants

and animals. Process is called cellular

respiration

Overall Reaction:

– C6H12O6 + 6O2 → 6CO2 + 6H2O

Cellular Respiration Overview

Breakdown of glucose begins in the cytoplasm:

the liquid matrix inside the cell

At this point life diverges into two forms and two

pathways

– Anaerobic cellular respiration (aka fermentation)

– Aerobic cellular respiration

C.R. Reactions

Glycolysis

– Series of reactions which break the 6-carbon glucose

molecule down into two 3-carbon molecules called

pyruvate

– Process is an ancient one-all organisms from simple

bacteria to humans perform it the same way

– Yields 2 ATP molecules for every one glucose

molecule broken down

– Yields 2 NADH per glucose molecule

Aerobic Cellular Respiration

Oxygen required=aerobic

2 more sets of reactions which occur in a

specialized structure within the cell called the

mitochondria

– 1. Kreb’s Cycle

– 2. Electron Transport Chain

Kreb’s Cycle

Completes the breakdown of glucose

– Takes the pyruvate (3-carbons) and breaks it down,

the carbon and oxygen atoms end up in CO2 and H2O

– Hydrogens and electrons are stripped and loaded onto

NAD+ and FAD to produce NADH and FADH2

Production of only 2 more ATP but the cycle

loads up the coenzymes with H+ and electrons

which move to the 3rd stage

Electron Transport Chain

Electron carriers loaded with electrons and

protons from the Kreb’s cycle move to this chain-

like a series of steps (staircase).

As electrons are passed from one carrier protein

to the next (drop down stairs) energy is released

to form a total of 32 ATP

Oxygen waits at “bottom of staircase” to pick up

electrons and protons and in doing so becomes

water

Aerobic Respiration

Stage 3 –

The Electron

Transport Chain

(ETC)

Energy Tally

36 ATP for aerobic vs. 2 ATP for anaerobic

– Glycolysis 2 ATP

– Kreb’s 2 ATP

– Electron Transport 32 ATP

36 ATP

Anaerobic organisms can’t be too energetic but are important for global recycling of carbon

Anaerobic Cellular Respiration

Some organisms thrive in environments with little or no

oxygen

– Marshes, bogs, gut of animals, sewage treatment ponds

No oxygen used= ‘an’aerobic

Results in no more ATP, final steps in these pathways

serve ONLY to regenerate NAD+ so it can return to pick

up more electrons and hydrogens in glycolysis.

End products such as ethanol and CO2 (single cell fungi

(yeast) in beer/bread) or lactic acid (muscle cells)