Download Owen Photosynthesis

Unit: 4
Photosynthesis:
Autotrophs make their own food, and most use
photosynthesis to do it.
* Plants are the most common, but algae and some
bacteria use it.
* All life depends on autotrophs (directly or indirectly) for
food.
* Nearly all living things obtain energy either directly
(like plants) or indirectly (like heterotrophs that eat
plants) from the sun’s energy captured during
photosynthesis.
Photosynthesis (Simply Summarized)
Leaves of plants have chloroplasts that are filled with
chlorophyll, which capture the energy from the
sunlight).
* Water enters through the roots.
* CO2 enters through stomata openings on the
underside of the leaves.
* Water and CO2 move to the chloroplasts.
* Chemical reactions there, produce (O2) and sugars
like glucose (C6H12O6).
* Cells then use the energy in the sugars
(carbohydrates) to function.
Photosynthesis

Plant chloroplast capture light energy and
converts it to chemical energy, which is
stored in the bonds of sugar and other
organic molecules synthesized from
carbon dioxide and water

This captured light energy is converted
and stored as chemical energy know as
photosynthesis
Water
Sun Light
Carbon Dioxide
Photosynthesis
Sugar
Oxygen
Cellular Respiration
Energy - ATP
(Lab) Chloroplasts in Elodea
www.soulcare.orgSid Galloway
Chloroplast

Chloroplasts are the site of photosynthesis

Are found in all green plant parts

The leaves are the major site of photosynthesis in most
plants

Chlorophyll is green pigment in chloroplast that gives
leaf its color

Chloroplast are found primarily in cells of the mesophyll
( of the leaf)

Each mesophyll contains 30-40 chloroplast
Parts of Chloroplast: 3
1.
2.
3.
Stroma: fluid filled space outside
thylakoid, Calvin cycle occurs here
Thylakoid: flattened membranous sac
inside chloroplast, light reaction occurs
here.
Grana: whole stack, while thylakoid is
each individual
Intermembrane space: a double
membrane that partitions its contents
from cytosol
Formula for Photosynthesis
ATP Molecule

The overall photosynthetic equation
has been known since the early
1880s

Glucose is the major product of
photosynthesis

Water appears on both sides of the
equation because 12 molecules are
consumed and 6 newly formed
during photosynthesis

The discovery in 1930s that oxygen
given off by plants is derived from
water and not from carbon dioxide
was one of the earliest clues to the
mechanisms of photosynthesis and
indicates that chloroplasts split
water into hydrogen and oxygen

Products of photosynthesis are:
glucose, oxygen, water and ATP
2 Stages of Photosynthesis
Light Reaction: occurs in the
thylakoid

1.

2. Calvin Cycle: occurs in stoma,
also called dark reaction
Light Reaction

Occurs in Thylakoid

Involves the conversion of light
energy into chemical energy
(photolysis)

Water is split during this process
and oxygen is released as a by
product…reason plants make oxygen

The coenzyme NADP+ picks up hydrogen and electrons
from the split water molecules and stores them for use
in the Calvin cycle (dark reaction)

ATP is also generated and stoed for energy for the dark
reaction
The Light Reaction in the Thylakoid Membrane
The Light Reaction in the Thylakoid Membrane
Primary electron acceptor Primary electron acceptor
Photosystem II
Electron Transport Chain
Photosystem I
Electron Transport Chain
www.soulcare.orgSid Galloway
www.soulcare.orgSid Galloway
Electron flow

2 possible routes for electrons to
flow during light reaction:

1. Cyclic flow: simplest pathway,
generates ATP only

2. Noncyclic flow: light reaction
usually happens this way NADP is
formed
Calvin Cycle ( dark reaction)

During this cycle, carbon dioxide from the
air is fixed to a 5 carbon sugar (ribulose
bisphosphate ) by the addition of
electrons from NADP+ from the light
reaction ( this is called carbon fixation)
resulting in a new unstable 6 carbon sugar

This unstable 6 carbon sugar begins going
through various chemical reactions

The 2 necessary components for carbon
fixation to occur here are: an electron
source (NADP+) and an energy source
(ATP)…both supplied by the light reaction

Water is released as a by-product

Glucose is made as the plant energy/food
supply

Ribulose bisphosphate is the 5 carbon
sugar made at the end of the cycle and is
again used to combine with carbon
dioxide for carbon fixation

9 molecules of ATP and 6 molecules of
NADPH are needed to synthesis one
glucose molecule

Also called dark reaction…does not require light
directly, but need the products of light reaction to
occur.
Three Steps of the Calvin Cycle:
a. CO2 combines with RuBP to form two molecules of PGA.
b. Each molecule of PGA is converted into a molecule of PGAL.
c. Most of the PGAL is converted back into RuBP, but some
PGAL can be used later to make different organic compounds.
* RuBP = five-carbon carbohydrate.
* PGA and PGAL are both three-carbon molecule
* C3 Plants (produce the 3-Carbon PGA),
and use only the Calvin Cycle for carbon fixation.
The Two Processes Visualized
www.soulcare.orgSid Galloway
www.soulcare.orgSid Galloway
Calvin
Cycle
www.soulcare.orgSid Galloway
Calvin
Cycle
Calvin
Cycle
Photosynthetic pigments

Pigments: substances that absorb visible
light

Different pigments absorb light of
different wavelengths
Types of pigments include:

Chlorophyll A ( bright green)

Chlorophyll B ( yellow green)

Chlorophyll contains the ion magnesium

Carotene: faint yellow

Xanthrophyll: yellow

Anthrocyanin: red
*
- Accessory Pigments :
-
indirectly assist other pigments.
( Chlorophyll “b” is an accessory pigment assisting “a”.)
Carotenoids are other accessory pigments
(yellow, orange, brown, etc.)
(They absorb blue and green light.)
(In the fall, plant leaves turn color because they lose
chlorophylls, which reflect green light.)
Photosystems:

Found in the thylakoid membrane of
chloroplast

Only 1 chlorophyll a (
photosynthetic pigment) is needed
to start the light reaction

All of the other chlorophyll a
molecules, chlorophyll b molecules
are carotenes function as light
gathering antenaes

The entire complex of light gathering molecules is
called a photosystem.

There are 2 types of photosystems:

1. photosystem I: called P700..absorbs light up to
700nm wavelength

2. Photosystem II: called P600…absorbs light up to
600 nm wavelength
Electron Transport System of the light reactions.
* Photosystem = cluster of pigment molecules grouped
in the thylakoid membrane.
Two types:
Photosystem II – Actually begins the process
Photosystem I – Called “1” is believed to have
evolved first.
* Accessory pigment molecules start the light reactions
by absorbing light energy.
(It is passed to other pigment molecules until it
reaches chlorophyll a molecules.)
*
Alternative Pathways
* First, remember that C3 Plants are those that only
use the Calvin Cycle to fix carbon.
- They are called C3 plants, since they fix CO2 into a
compound with 3 carbons (PGAL).
* Others in hot, dry climates supplement the Calvin
cycle with alternatives (C4 or CAM), because their
stomata openings in the leaves must close to preserve
moisture, so the amount of CO2 they absorb is
reduced.
C4 Pathway – use an enzyme which fixes CO2 into
compounds with 4 carbons, which are then transported
to other cells where CO2 is available to then use the
Calvin Cycle.
(corn, sugar cane, are examples)
CAM Pathway – These plants open the stomata only at
night to reduce water loss.
They take in and fix it into compounds, which then
release it during the day for use in the Calvin Cycle.
(cactuses, pineapples, etc.)
Rate of Photosynthesis
* Increases as either light intensity or CO2
increase, but eventually plateaus at a maximum.
* Increases as the temperature increases, up to
a certain temperature.
(Beyond a certain high temperature, the rate of
photosynthesis decreases.)