Sunlight is electromagnetic energy. The quantum mechanical model
of electromagnetic radiation describes light as having a
behavior that is both wavelike and particlelike.
AP Biology
Notes:
Photosynthesis
Photosynthesis:
Transforms solar light energy trapped by chloroplasts into
chemical
bond energy stored in sugar and other organic
molecules.
This
process:
* Synthesizes energy-rich organic molecules from the energy-poor
molecules
* CO2 and
H2O
* Uses CO2 as a carbon source and light energy as the energy
source
* Directly or indirectly supplies energy to most living
organisms
Autotrophic nutrition: (Auto = self, trophos = feeds)
Nutritional mode of
synthesizing
organic molecules form inorganic raw
materials
Examples of autotrophic
organisms are plants, which require only CO2 ,H2O,
and
minerals as nutrients.
Because autotrophic organisms produce organic molecules
that
enter an ecosystem's
food store, autotrophs are also known as
producers.
Photoautotrophs : Autotrophic organisms that use light as an
energy
source to synthesize organic molecules. Examples
are
photosynthetic organisms such as plants, algae,
and
some prokaryotes.
Chemoautotrophs: Autotrophic organisms that use the oxidation of
inorganic
substances, such as sulfur or ammonia, as an energy
source
to synthesize organic molecules. Unique to some
bacteria,
this is rare form of autotrophic nutrition.
Heterotrophic
nutrition: (Heteros = other; trophos = feed) nutritional mode of
acquiring
organic molecules form compounds produced by other
organisms.
*
Heterotrophs are unable to synthesize organic molecules from inorganic raw
materials.
* Heterotrophs
are also known as consumers
* Examples are animals that eat plants or other
animals
*
Decomposers, hetrotrophs that decompose and feed on organic
litter.
* Most
hetrotrophs depend on photoautotrophs for food and oxygen
Chloroplasts: (sites of photosynthesis in
plants)
Although all green
plant parts have chloroplasts, leaves are the major sites
of
photosynthesis in most
plants. Chlorophyll is the green pigment in
chloroplasts
that gives a
leaf its color and that absorbs the light energy used to drive
photosynthesis
*Chloroplast are
primarily in cells of mesophyll, green tissue in the leaf's
interior
*CO2 and
O2exits the leaf through microscopic pores called
stomata
* Water
absorbed by the roots is transported to leaves through veins or
vascular
bundles which also export sugar from leaves to nonphotosynthetic parts of
the plant.
1. Intermembrane
space
The chloroplast is
obtund by a double
membrane which partitions its contents
from
the
cytosol. A narrow intermembrane
space
separates the two
membranes.
2. Thylakoid space
Thylakoids form another membranous
system
within the chloroplast. The
thylakoid
membrane segregates the
interior of the chloroplast
into two
compartments: thylakoid space and
stroma
3. Stroma
Reactions that use chemical energy to convert
carbon
dioxide to sugar occur in
the stroma, viscous fluid
outside
the thylakoids.
The nature of
sunlight:
Sunlight is
electromagnetic energy. The quantum mechanical
model
of
electromagnetic radiation describes light as having
a
behavior that is
both wavelike and
particlelike.
Wavelike properties of
light:
Electromagentic
energy is a form of energy that travels in rhythmic
waves
which are disturbances
of electric magnetic fields.
Visible light, which is detectable by the human eye, is only a small
portion of
the
electromagnetic spectrum and ranges from about 380 to 750 nm.
The
wavelengths most
important for photosynthesis are within the range of
visible
light.
Particle like properties of
light:
Light also behaves as
if it consist of discrete particles or quanta called
photons
Each photon has a
fixed quantity of energy which is inversely proportional
to
the wavelength of
light.
Photosynthetic pigments: ( light
receptors)
Pigments:
Substances which absorb visible
light
* Different pigments
absorb different wavelengths of
light
* Wavelengths that are
absorbed disappear, so a pigment that
absorbs
all wavelengths appears black.
* When white light, which contains all the wavelengths of visible light,
illuminates
a
pigment, the color you see is the color most reflected or transmitted by the
pigment.
For example, a leaf appears green because chlorophyll absorbs red and blue light
but
transmits and reflects green
light.
* Each pigment has a
characteristic absorption spectrum or pattern of wavelengths
that
is absorbs. It is expressed as a graph of absorption versus
wavelength.
The absorption spectrum for a pigment in solution can be determined by using
a
spectrophotometer,
*
Since chlorophyll a is the light absorbing pigment that participates
directly in
the
light
reactions, the absorption spectrum of chlorophyll a provides clues as to
which
wavelengths of visible light are most effective for
photosynthesis
* A graph of
wavelengths versus rate of photosynthesis is called an action
spectrum
and profiles the relative effectiveness of different wavelengths of visible
light for
driving
photosynthesis.
The
accessory pigments expand the range of wavelengths available for
photosynthesis.
These pigments
include:
Chlorophyll b, a yellow-green pigment with a structure similar to
chlorophyll a.
This minor structural differences gives the pigments slightly different
absorption spectra.
Carotenoids, yellow and orange hydrocarbons that are built into the
thylakoid
membrane with the
two types of
chlorophyll.
Photoexcitation of chlorophyll
Colors absorbed
wavelengths disappear from the spectrum of transmitted and
reflected light
The absorbed photon boosts one of the pigment of
molecule's electrons in its lowest-
energy state (ground state) to an
orbital of higher energy (excited
state).
Light-harvesting complexes of the thylakoid
membrane:
Antenna complex
Several hundred chlorophyll
a , chlorophyll b and carotenoid molecules are
light-
gathering antennae that
absorb photons and pass the energy from molecule
to
molecule. This
process of resonance energy transfer is called inductive
resonance
Different pigments
within the antennal complex have slightly different
absorption
spectra, so
collectively they can absorb photons from a wider range of the
light
spectrum than would be
possible with only one type of pigment molecule.
Reaction-center
chlorophyll
Only one of the
many chlorophyll a molecules in each complex can actually
transfer
an excited
electron to initiate the light reactions. This specialized
chlorophyll a is
located in the reaction center
Primary electro
acceptor.
Located near the
reaction center, a primary electron acceptor molecule traps
excited
state electrons
released form the reaction center
chlorophyll.
The transfer of
excited state electrons from chlorophyll to primary electron
acceptor
molecules is the
first step of the light reactions. The energy stored in the
trapped
electrons powers the
synthesis of ATP and NADPH in the subsequent steps.