biology - unit 2

Organisms that do oxygen-producing photosynthesis:

Cyanobacteria, seven groups of algae, and all land plants

Starting materials of photosynthesis:

CO₂ and H₂O (the waste products of respiration)

Byproduct of photosynthesis:

Oxygen (O₂)

What gets oxidized in photosynthesis:

Water (it gets split)

What gets reduced in photosynthesis:

CO₂ (it becomes sugar)

Chloroplasts and mitochondria relationship:

Evolutionary cousins with related proteins that work in opposite directions

Chloroplast membranes:

Triple-membrane structure (outer, inner, and thylakoid membranes)

Stroma:

The fluid inside the chloroplast; similar to the mitochondrial matrix

Thylakoids:

Hollow, membrane-enclosed disks floating in the stroma

Grana:

Stacks of thylakoids

Thylakoid space (lumen):

The fluid inside a thylakoid disk

Where chlorophyll and light-trapping proteins are found:

Thylakoid membranes

Two stages of photosynthesis:

Light-dependent reactions and the Calvin Cycle (light-independent reactions)

Light-dependent reactions:

Capture light energy and store it in ATP and NADPH

Where the light-dependent reactions happen:

Thylakoids

Inputs of the light-dependent reactions:

Sunlight and water

Outputs of the light-dependent reactions:

ATP, NADPH, and O₂

NADPH:

Electron carrier used in photosynthesis; similar to NADH

Where O₂ comes from in photosynthesis:

Splitting water

Calvin Cycle:

Uses ATP and NADPH to turn CO₂ into sugar

Other names for the Calvin Cycle:

Light-independent reactions, dark reactions, C3 photosynthesis

Where the Calvin Cycle happens:

Stroma

Inputs of the Calvin Cycle:

CO₂, ATP, NADPH

Output of the Calvin Cycle:

Organic molecules / sugar

Carbon fixation:

Turning inorganic CO₂ into organic molecules

Photons:

Discrete bundles of light energy

Wavelength and energy relationship:

Inversely proportional (shorter wavelength = more energy)

Blue light (~400 nm):

Short wavelength, more energy

Red light (~740 nm):

Long wavelength, less energy

Photoelectric effect:

A photon with the right energy excites an electron in a pigment to a high-energy state

Why pigments have color:

They reflect the wavelengths they don’t absorb

Chlorophyll a: Main pigment;

absorbs violet-blue and red light; reflects green

Only pigment that directly converts light into chemical energy:

Chlorophyll a

Chlorophyll b:

Accessory pigment; absorbs blue and red-orange light; reflects green

Purpose of accessory pigments:

Broaden the range of light the plant can absorb

Carotenoids:

Accessory pigments that absorb blue and blue-green light and appear orange

Why carotenoids look orange:

They can’t absorb orange or red light

Why leaves turn orange in autumn:

Chlorophyll breaks down, revealing carotenoids

Bonus function of carotenoids:

Antioxidants that protect the cell from chemical damage

Photosystem:

A complex of pigments and proteins on a photosynthetic membrane

Antenna complex:

Hundreds of accessory pigments that funnel light energy to the reaction center

Reaction center:

Chlorophyll a molecules at the center of a photosystem whose electrons get excited and passed to an acceptor

Electron donor in plants:

Water

Why water is split in photosynthesis:

To replace the electrons lost by the reaction center

Z Scheme:

The zigzag path electrons take through Photosystem II and Photosystem I

Photophosphorylation:

Making ATP using light energy

Vertical position in the Z diagram:

Energy level (higher = more energy)

First photosystem in the Z scheme:

Photosystem II

Photons absorbed by Photosystem II per electron pair:

2 photons

Plastoquinone (PQ):

Carrier that takes electrons from Photosystem II

Where water is split:

At Photosystem II

b6-f complex:

Uses electron energy to pump protons across the thylakoid membrane

Second photosystem in the Z scheme:

Photosystem I

What Photosystem I does:

Re-energizes electrons with light to a very high energy state

Ferredoxin:

Carrier that receives electrons from Photosystem I

Final electron acceptor in photosynthesis:

NADP+

What NADP+ becomes:

NADPH

Output per water molecule split:

1 NADPH and about 1 ATP

Where protons are concentrated in the chloroplast:

Thylakoid space (lumen)

Three things that build the proton gradient:

b6-f pumping protons in, water splitting in the lumen, and protons removed from the stroma to make NADPH

How protons leave the thylakoid space:

Through ATP synthase into the stroma

Where ATP is made in the chloroplast:

Stroma

ATP is made from:

ADP and phosphate

Non-cyclic photophosphorylation:

Standard straight-line electron flow through the Z scheme

Non-cyclic route

Water → Photosystem II → b6-f complex → Photosystem I → NADPH

Non-cyclic output:

Equal amounts of ATP and NADPH

Cyclic photophosphorylation:

Electrons from Photosystem I loop back to the b6-f complex instead of going to NADP+

Why plants use cyclic photophosphorylation:

The Calvin Cycle needs more ATP than non-cyclic flow provides

Cyclic output:

Extra ATP, zero NADPH

Why it’s called C3 photosynthesis:

The first molecule made (PGA) has 3 carbons

Three phases of the Calvin Cycle:

Carbon fixation, reduction, regeneration

RuBP:

The 5-carbon sugar that CO₂ is attached to

Carbon fixation step:

CO₂ + RuBP → two 3-carbon PGA molecules

Rubisco:

Enzyme that runs carbon fixation; the most abundant enzyme on Earth

PGA:

The first 3-carbon molecule made in the Calvin Cycle

Reduction step:

ATP and NADPH are used to turn PGA into G3P

G3P:

The sugar directly produced by the Calvin Cycle

Regeneration step:

Most G3P is converted back into RuBP so the cycle continues

Turns needed for 1 G3P:

3 turns (3 CO₂)

Turns needed for 1 glucose:

6 turns (6 CO₂)

How glucose is made from the Calvin Cycle:

2 G3P leave to the cytoplasm and combine into 1 glucose

Sucrose:

What glucose becomes for transport

Starch:

what glucose becomes for long-term storage

Carboxylation:

Rubisco grabbing CO₂ (the normal reaction)

Oxidation (by Rubisco):

Rubisco accidentally grabbing O₂ instead of CO₂

Photorespiration:

Rubisco grabbing O₂, breaking down RuBP and releasing CO₂, undoing carbon fixation

Carbon lost to photorespiration at 25°C:

About 20% of fixed carbon

Stomata:

Leaf pores that let gases in and out

Why plants close stomata in hot weather:

To stop water loss

Why closing stomata causes photorespiration:

O₂ builds up and CO₂ runs low inside the leaf

PEP carboxylase:

Enzyme with high affinity for CO₂ and no affinity for O₂

What PEP carboxylase makes

A 4-carbon molecule

C4 plants:

eeparate carbon fixation and the Calvin Cycle into different cells (spatial separation)

C4 step 1 location:

Mesophyll cells

C4 step 1:

PEP carboxylase fixes CO₂ into oxaloacetate, then malate

C4 step 2 location:

Bundle sheath cells

C4 step 2:

Malate is broken into pyruvate and CO₂, flooding Rubisco with CO₂

Cost of C4 photosynthesis:

12 extra ATP per glucose

C4 plant examples:

Corn, sugarcane, sorghum, grasses

CAM plants:

Separate carbon fixation and the Calvin Cycle by time (temporal separation)