Photosynthesis

The Biology of Photosynthesis

An Overview of Photosynthesis
This presentation will focus on:
The importance of autotrophs
An overview of photosynthesis
Leaf and chloroplast structure and function
The electromagnetic spectrum
Pigment function and variety
The Light Reactions
The Calvin Cycle

The Importance of Autotrophs

All organisms require a source of energy
All organisms require a source of carbon compounds
Provides molecular oxygen for the atmosphere

2 Types of Autotrophs:
Chemotrophs
Photoautotrophs

An Overview of Photosynthesis 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2

An Overview of Photosynthesis

Leaf Structure and Function
Plants are composed of three major organ systems:
The Root System
anchors the plant in place
stores excess sugars
absorbs water and mineral nutrients
The Shoot System
supports the plant body
provides passageway for nutrients and minerals
Leaves
site of photosynthesis (food production)

Plant organs are made up of 3 types of tissues:
Dermal
cells for structure and protection
secretion of cutin to prevent water loss
Ground
photosynthesis
storage of food and water
Vascular
specialized for transport of water and nutrients

Chloroplast Structure and Function

The Electromagnetic Spectrum
The ultimate source of energy is the sun
Solar energy is called electromagnetic radiation
Travels in waves of photons
The Electromagnetic spectrum describes the range of energy in solar radiation
The EM spectrum is measured by wavelength

The Electromagnetic Spectrum

Pigments Pigments are organic molecules that absorb visible light Wavelengths not absorbed are reflected, creating the visible color of an object Wavelengths absorbed cause the excitations of electrons Plants contain a major pigment, chlorophyll a, and accessory pigments like carotenoids and anthocyanins

Pigments

Chlorophyll Structure and Function

Review of Major Concepts So Far! Go to http://www.wiley.com/legacy/college/boyer/0470003790/animations/photosynthesis/photosynthesis.htm Go through the sections called “Overview” and “Strategy/Players”

The Reactions of Photosynthesis
The Light Reactions
Light Dependent
“ Photo” Division
Energy Capturing
The Calvin Cycle
Light Independent
“ Synthesis” Division
Energy Storage

The Light Reactions
Occur on thylakoid membranes
Use 2 Photosystems
PSI (p700) and PSII (p680)

Chlorophyll Structure and Function

Click image to see a membrane view of the light reactions

Events of the Light Reactions: PSII
Sunlight strikes p680 boosting electron to excited state
High energy electrons passed from primary electron acceptor down electron transport system via oxidation-reduction. These reactions power the active transport of H + from the stroma into the thylakoid space
H + diffuse back into stroma through ATP synthetase converting ADP + P into ATP
Electrons from p680 end up at p700. Hole at p680 filled by oxidation of H 2 O into O 2

Events of the Light Reactions: PSI
Sunlight hits p700 boosting electron to excited state
High energy electrons passed from primary electron acceptor down electron transport system via oxidation-reduction.
High energy electrons transferred by NADP + reductase to NADP + to form NADPH
Hole at p700 filled by electrons from p680

Click image for link to animated overview of noncyclic photophosphorylation

Light Reactions Summary Sunlight + H 2 O O 2 + ATP + NADPH ADP, P, and NADP + from Inorganic Nutrient Pool are raw materials Study the following tutorials: You control the light reactions! Biology Project Light RXNs Tutorial Photosynthesis Light RXNs Interactive

The Calvin Cycle Occurs in the Stroma Uses ATP and NADPH from Light Rxns CO 2 is Raw Material

The Calvin Cycle Phase 1: Carbon Fixation CO 2 comes into the stroma of the chloroplast via the stomata of the leaves. Rubisco catalyzes the bonding of CO 2 to RuBP to create an unstable 6-carbon molecule that instantly splits into two 3-carbon molecules of 3-PGA. Back to diagram

The Calvin Cycle Phase 2: Reduction ATP phosphorylates each 3-PGA molecule and creates 1,3-bisphosphoglycerate (1,3 DPGA). NADPH reduces 1,3-bisphosphoglycerate which causes the molecule to become glyceraldehyde-3-phosphate (PGAL). NADPH is oxidized by this process and becomes NADP+. Back to diagram

The Calvin Cycle Phase 3: Regeneration For every six molecules of PGAL created, five molecules continue on to phase 3 while one leaves to be used for organic compounds. ATP is once again needed. However, this time it phosphorylates G3P to regenerate RuBP after some rearrangement. Back to diagram

The Calvin Cycle
Summary of Calvin Cycle
CO 2 + ATP + NADPH ADP + NADP + + PGAL
PGAL is rearranged to produce:
Glucose for cellular respiration
Fructose in Fruits
Sucrose for Transport Throughout Plant
Starch for Storage
Study the following tutorials: The 3 Phases Animated Calvin Cycle Tutorial at Biology Project Photosynthesis Dark RXNs Interactive

Alternate Mechanisms of Photosynthesis
Plants fight water loss and dehydration
Close stoma to prevent water loss
Closed stoma mean no input of CO 2 into leaf
Light reactions continue to produce O 2
Photorespiration
When O 2 concentration increases, rubisco adds O 2 to the Calvin Cycle instead of CO 2

Photorespiration
Occurs on bright, hot, dry days when stoma close
Consumes O 2
Releases CO 2
Unlike cellular respiration, generates no ATP
Unlike photosynthesis, generates no food
Actually decrease rate of photosynthetic output

Alternate Mechanisms of Photosynthesis Mechanisms to fight photorespiration: C4 and CAM pathways Incorporate CO 2 into organic acids first, then release it into the Calvin Cycle

PEP Carboxylase accepts CO 2 in mesophyll cells
A 4C intermediate carries the CO 2 into the bundle-sheath cells
This maintains a high concentration of CO2 in the bundle sheath to avoid photorespiration

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Photosynthesis

by cgales on Jan 10, 2007

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Photosynthesis — Presentation Transcript