Photosynthesis

Turning sunlight into cellular energy

Photosynthesis only LOOKS like the opposite of Respiration!

6 CO2 + 12 H2O + → C6H12O6 + 6 O2 + 6 H2O

Photosynthetic Organisms Prokaryotes: Plasma Membranes

Eukaryotes: Chloroplasts

Why are plants Green?

Light Chlorophyll Green pigment Absorbs red, violet, blue

light Reflects green light

Where does photosynthesis happen? Leaves Pigments

Chlorophyll a (blue green) Chlorophyll b (olive green) Carotenoids (yellow and

orange)

By absorbing slightly different λs, increase the amount of visible light that can be harvested for energy

Chlorophyll

Absorption and Action Spectra

1883!

What happens to excited electrons when light hits a chlorophyll molecule?

Fluorescence Photosynthesis

What happens in Photosynthesis?

Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

Steps in Photosynthesis

Light Reactions Water e- + H+ + O2

Occurs in thylakoid membranes ATP made by

chemiosmosis and photophosphorylation

Calvin Cycle CO2 sugar

Occurs in the stroma

Also called Carbon

Fixation or the “Dark Reactions”

A. Light Reactions 1: Light Energy Splits Water

A. Light Reactions 2: NADP+ reduced, ATP made, O2 released

Which mechanism is used to make the ATP?

B. Calvin Cycle 1: ATP and NADPH used to fix carbon

B. Calvin Cycle 2: carbon turned into sugar, NADP+ recycled

Sugar exported to rest of plant

Chlorophyll a captures light energy

Photosystems 1 and 2 A photosystem (PS) has a

reaction-center complex surrounded by light-harvesting complexes

Both have chlorophyll a different environments absorb best at different λs

Reaction Center Chlorophyll

Photosystem 2: P680 Photosystem 1: P700

Photosystem structure

Linear Electron Flow (1) Light activates an e-, which eventually

reaches P680 P680 transfers the e- to the primary e- acceptor in the reaction-center complex

Linear Electron Flow (2) P680+ (missing an electron) is

very strong oxidizer H2O is split and P680+, reduced O2 is by-product

Linear Electron Flow (3) The electrons “fall”

down e- transport chain (PS II PS I) H+ pumped across thylakoid membrane ATP synthesized

Linear Electron Flow (4)

P700+ accepts e- OR excited by light

Linear Electron Flow (5)

Another e- transport chain transfers e- to NADP+ to create NADPH

PSII and PSI work together

NADPH carries e- to Calvin Cycle

Making ATP by Chemiosmosis

The Light Reactions: All Together

Carbon Fixation: CO2 from the atmosphere is reduced (fixed) to form sugars

REQUIRES the ATP and NADPH made during light

reactions

Takes three rounds of the cycle (and 3 CO2 molecules) to make 1 sugar

Carbon Fixation

Rubisco adds CO2 to RuBP: most abundant protein in chloroplasts; PROBABLY most abundant on Earth

Reduction

ATP and NADPH from light reactions

Regenerates NADP+

3 CO2 make one new sugar

Regeneration

To make 1 G3P molecule: 9 ATP + 6 NADPH

The energy comes from the light reactions

(i.e., the sun) G3P enters many biosynthetic pathways

to be turned into other organic molecules

Compare Metabolic Cycles

Citric Acid Cycle Catabolic pathway Oxidizes glucose to CO2

and water Synthesizes ATP

Calvin Cycle Anabolic pathway Reduces CO2 to make

complex carbohydrates Uses ATP (made in the

light reactions)

(almost) ALL THE FOOD ON EARTH COMES FROM PHOTOSYNTHESIS Chemical Energy Carbon Skeletons (to make all organic

molecules) Chloroplasts make billions of tons of

carbohydrate each year About 50% is burned in plant mitochondria Rest is transported in plant as sucrose or

turned into cellulose and starch

Cell Communication

Exchanging information between cells

All Cells Communicate

Processes arose early in evolution

Same molecules found in prokaryotes and eukaryotes Recognition Mating Development

Direct communication between cells

Cell-cell recognition

Cell junctions

Communication via secreted signals

Local signaling Long distance signaling

Signal Transduction

Reception Transduction Response Reset

Reception ligand binds receptor

Transduction Signal is amplified by a series of molecules

(second messengers)

Response Cell reacts to signal, then it resets the signal transduction

cascade back to the starting point

Reception requires receptors

Ligands are secreted molecules that bind receptors on or in cells

Ligand – Receptor interaction is specific: Only cells with proper receptor can receive the signal Receptor changes shape when ligand binds - activation

Receptors can be in plasma membrane OR cytoplasm

Common Receptors

Receptors Plasma membrane

G protein-coupled receptors

Tyrosine kinase receptors

Ion channel receptors

Cytosolic

Steroid hormone receptors

Mechanisms Activate another protein

Dimerize; gain P groups

Open or close a channel

Move to nucleus and bind

DNA

G protein-coupled receptors Structure

Function

Tyrosine Kinase Receptors

Ligand-gated Ion Channels

Cytoplasmic Receptors

Signal Transduction Cascades Amplify Signals

1 signal molecule can generate 100,000,000 response molecules in a few seconds or minutes

Phosphorylation Cascades Adding/removing phosphate

groups can activate/inactivate proteins

Kinases: add phosphates (usually on ser or thr)

Phosphatases: remove phosphates

There is a specific kinase and phosphatase for each molecule in the cascade!

Cytoplasmic Organization: Scaffolding proteins

Second Messenger Systems

cAMP Ca++ and IP3

Cellular Responses

Turn genes on or off Activate or inactivate

proteins Stimulate cell division Stimulate apoptosis

Signals interact to fine-tune responses: Same molecule can have different effects in different cells

Review of Cell Communication