BIOL 600 Lecture Notes - Lecture 1: Autotroph, Heterotroph, Chlorophyll
26 May 2018
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AP Bio Chapter 10 Photosynthesis
Lecture Outline
Overview: The Process That Feeds the Biosphere
Life on Earth is solar powered.
The chloroplasts of plants use a process called photosynthesis to capture light energy
from the sun and convert it to chemical energy stored in sugars and other organic
molecules.
Plants and other autotrophs are the producers of the biosphere.
Photosynthesis nourishes almost all the living world directly or indirectly.
o All organisms use organic compounds for energy and for carbon skeletons.
o Organisms obtain organic compounds by one of two major modes: autotrophic
nutrition or heterotrophic nutrition.
Autotrophs produce their organic molecules from CO2 and other inorganic raw materials
obtained from the environment.
o Autotrophs are the ultimate sources of organic compounds for all heterotrophic
organisms.
o Autotrophs are the producers of the biosphere.
Autotrophs can be separated by the source of energy that drives their metabolism.
o Photoautotrophs use light as a source of energy to synthesize organic compounds.
Photosynthesis occurs in plants, algae, some other protists, and some
prokaryotes.
Chemoautotrophs harvest energy from oxidizing inorganic substances,
such as sulfur and ammonia.
Chemoautotrophy is unique to prokaryotes.
Heterotrophs live on organic compounds produced by other organisms.
o These organisms are the consumers of the biosphere.
o The most obvious type of heterotrophs feeds on other organisms.
Animals feed this way.
o Other heterotrophs decompose and feed on dead organisms or on organic litter,
like feces and fallen leaves.
Most fungi and many prokaryotes get their nourishment this way.
o Almost all heterotrophs are completely dependent on photoautotrophs for food
and for oxygen, a by-product of photosynthesis.
Concept 10.1 Photosynthesis converts light energy to the chemical energy of food
All green parts of a plant have chloroplasts.
However, the leaves are the major site of photosynthesis for most plants.
o There are about half a million chloroplasts per square millimeter of leaf surface.
The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.
o Chlorophyll plays an important role in the absorption of light energy during
photosynthesis.
Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the
leaf.
O2 exits and CO2 enters the leaf through microscopic pores called stomata in the leaf.
Veins deliver water from the roots and carry off sugar from mesophyll cells to
nonphotosynthetic areas of the plant.
A typical mesophyll cell has 30–40 chloroplasts, each about 2–4 microns by 4–7 microns
long.
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma is an elaborate system of interconnected membranous sacs, the thylakoids.
o The interior of the thylakoids forms another compartment, the thylakoid space.
o Thylakoids may be stacked into columns called grana.
Chlorophyll is located in the thylakoids.
o Photosynthetic prokaryotes lack chloroplasts.
o Their photosynthetic membranes arise from infolded regions of the plasma
membranes, folded in a manner similar to the thylakoid membranes of
chloroplasts.
Evidence that chloroplasts split water molecules enabled researchers to track atoms
through photosynthesis.
Powered by light, the green parts of plants produce organic compounds and O2 from
CO2 and H2O.
The equation describing the process of photosynthesis is:
o 6CO2 + 12H2O + light energy --> C6H12O6 + 6O2+ 6H2O
o C6H12O6 is glucose.
Water appears on both sides of the equation because 12 molecules of water are
consumed, and 6 molecules are newly formed during photosynthesis.
We can simplify the equation by showing only the net consumption of water:
o 6CO2 + 6H2O + light energy --> C6H12O6 + 6O2
The overall chemical change during photosynthesis is the reverse of cellular respiration.
In its simplest possible form: CO2 + H2O + light energy --> [CH2O] + O2
o [CH2O] represents the general formula for a sugar.
One of the first clues to the mechanism of photosynthesis came from the discovery that
the O2 given off by plants comes from H2O, not CO2.
o Before the 1930s, the prevailing hypothesis was that photosynthesis split carbon
dioxide and then added water to the carbon:
Step 1: CO2 --> C + O2
Step 2: C + H2O --> CH2O
o C. B. van Niel challenged this hypothesis.
o In the bacteria that he was studying, hydrogen sulfide (H2S), not water, is used in
photosynthesis.
o These bacteria produce yellow globules of sulfur as a waste, rather than oxygen.
o Van Niel proposed this chemical equation for photosynthesis in sulfur bacteria:
CO2 + 2H2S --> [CH2O] + H2O + 2S
He generalized this idea and applied it to plants, proposing this reaction for their
photosynthesis:
o CO2 + 2H2O --> [CH2O] + H2O + O2
Thus, van Niel hypothesized that plants split water as a source of electrons from
hydrogen atoms, releasing oxygen as a byproduct.
Other scientists confirmed van Niel’s hypothesis twenty years later.
o They used 18O, a heavy isotope, as a tracer.
o They could label either C18O2 or H218O.
o They found that the 18O label only appeared in the oxygen produced in
photosynthesis when water was the source of the tracer.
Hydrogen extracted from water is incorporated into sugar, and oxygen is released to the
atmosphere (where it can be used in respiration).
Photosynthesis is a redox reaction.
o It reverses the direction of electron flow in respiration.
Water is split and electrons transferred with H+ from water to CO2, reducing it to sugar.
o Because the electrons increase in potential energy as they move from water to
sugar, the process requires energy.
o The energy boost is provided by light.
Here is a preview of the two stages of photosynthesis.
Photosynthesis is two processes, each with multiple stages.
The light reactions (photo) convert solar energy to chemical energy.
The Calvin cycle (synthesis) uses energy from the light reactions to incorporate CO2
from the atmosphere into sugar.
In the light reactions, light energy absorbed by chlorophyll in the thylakoids drives the
transfer of electrons and hydrogen from water to NADP+ (nicotinamide adenine
dinucleotide phosphate), forming NADPH.
o NADPH, an electron acceptor, provides reducing power via energized electrons to
the Calvin cycle.
o Water is split in the process, and O2 is released as a by-product.
The light reaction also generates ATP using chemiosmosis, in a process called
photophosphorylation.
Thus light energy is initially converted to chemical energy in the form of two
compounds: NADPH and ATP.
The Calvin cycle is named for Melvin Calvin who, with his colleagues, worked out many
of its steps in the 1940s.
The cycle begins with the incorporation of CO2 into organic molecules, a process known
as carbon fixation.
The fixed carbon is reduced with electrons provided by NADPH.
ATP from the light reactions also powers parts of the Calvin cycle.
Thus, it is the Calvin cycle that makes sugar, but only with the help of ATP and NADPH
from the light reactions.
The metabolic steps of the Calvin cycle are sometimes referred to as the light-
independent reactions, because none of the steps requires light directly.
Nevertheless, the Calvin cycle in most plants occurs during daylight, because that is
when the light reactions can provide the NADPH and ATP the Calvin cycle requires.
While the light reactions occur at the thylakoids, the Calvin cycle occurs in the stroma.
Document Summary
The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules. Plants and other autotrophs are the producers of the biosphere. Photosynthesis nourishes almost all the living world directly or indirectly: all organisms use organic compounds for energy and for carbon skeletons, organisms obtain organic compounds by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs produce their organic molecules from co2 and other inorganic raw materials obtained from the environment: autotrophs are the ultimate sources of organic compounds for all heterotrophic organisms, autotrophs are the producers of the biosphere. Autotrophs can be separated by the source of energy that drives their metabolism: photoautotrophs use light as a source of energy to synthesize organic compounds. Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes. Chemoautotrophs harvest energy from oxidizing inorganic substances, such as sulfur and ammonia.
