SUMMARY NOTES—PHOTOSYNTHESIS
I.
Chloroplasts function in energy transformations.
Chloroplasts are double membrane.
Innermost membrane consists of thylakoids in stacks called grana that hold chlorophyll
and enzymes for photosynthesis.
 Stroma is the liquid that contains ribosomes and DNA.
 Chloroplasts are site of photosynthesis where light energy is converted to chemical
energy.
II.
The photosynthetic production of oxygen by green plants is necessary for most
organisms to obtain the energy for life.
 Photosynthesis uses carbon dioxide, water, and light to produce carbohydrate and 6CO 2
+ 12H20 - C6H12O6 + 6O2 + 6H2O
 Water must be acquired by roots and transported to leaves, where photosynthesis takes
place in the mesophyll. Carbon dioxide, water, and oxygen are exchanged through
openings called stomata in the leaf’s surface.
 Photosynthesis occurs in a two-pathway process. The light-dependent reactions produce
ATP from light energy via photophosphorylation. The Calvin Cycle uses ATP and
NADPH + H2 from the light reactions to trap CO2 and produces sugars that act as an
energy storage.
III.
The interaction of light and pigment is fundamental to photoshynthesis.
 Discrete packets of light are called photons that can be scattered, transmitted, or
absorbed. Properties of light depend on wavelength. When a photon is absorbed it is
boosted into a higher energy level.
 Pigments absorb wavelengths in the visible spectrum. Chlorophyll appears green because
it absorbs blue and red light and reflects green.
 In plants, photosynthesis relies on chlorophyll a and b and accessory pigments.
Chlorophylls are the most abundant. But they only absorb photons with blue or orangered wavelengths. Accessory pigments, such as carotenoids and phycobilins, absorb
photons that chlorophylls cannot and transfer energy to the chlorophylls.
IV.
In photosynthesis, a pigment molecule in the excite state passes the absorbed
energy along.
 Absorbed light can either be released as fluorescence, or it can be passed to
another pigment.
 Photosynthetic antennae function to pass the energy from light from one pigment to
another until the reaction center is reached, where light energy is converted to
chemical energy. In plants, the reactions center is chlorophyll a.
 Chlorophyll acts as a reducing agent and begins a process that eventually drives ATP
synthesis.
 Noncyclic electron flow produces ATP and NADPH + H 2. Water is oxidized to form
O2, H+, and electrons. Electrons move from water to chlorophyll, through electron
carriers, and ultimately to NADP+. Release of free energy drives ATP synthesis
through chemiosmosis.
 Noncyclic electron flow requires two photosystems to continuously absorb light.
Photosystem II takes slightly higher energy (680 nm) than photosystem I (700nm).
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The electron transport pumps protons actively into the thylakoid compartment. This
proton gradient powers the formation of ATP.
 ATP synthesis is produced via chemiosmosis by changing ADP to ATP during
photophosphorylation.
The Calvin cycle incorporates CO2 into sugars.
 In the Calvin cycle, ATP and NADPH from the light reactions are used to convert
CO2 to sugars. This process occurs in the stroma of the chloroplasts.
 ATP and NADPH from the light reactions are used to convert the fixed CO 2 from
PGA to two 3-C molecules called PGAL
 Finally, ATP is used to regenerate RuBP so that additional CO 2 can be fixed.
 The Calvin cycle takes 6 turns to create one sugar molecule. Each time one carbon
is removed to form sugar.
Plant cells are specifically suited for their functions.
 Cell walls play important roles in plants. They provide turgor pressure, are
chemically active, allow for transport, and become modified for specific functions.
 Palisade parenchyma cells are the main site of photosynthesis. They are regular
shaped, closely packed cells located in the upper part of the mesophyll. The spongy
parenchyma cells are located in the lower part of the mesophyll and are where water
and carbon dioxide are stored.
 The leaf veins are composed of xylem and phloem which make up the vascular tissue.
Xylem transports water from the roots upward to the leaves. Phloem transports
food downward from the leaves to the roots.
 The entire leaf is covered with a protective epidermis and is waterproofed by the
waxy cuticle. Gas exchange occurs through the stomata which are regulated by the
guard cells to limit water loss.
Water and nutrients are taken up in the roots of plants.
 The movement of water across a membrane is called osmosis. Water moves from
high water content (hypotonic solute) to low water content (hypertonic solute).
 The central vacuole in plants is the storage location for water. When the plant cell
is in a hypotonic environment, water will move into the cell. This creates osmotic
pressure (pressure from the movement of water going toward the hypertonic side)
that causes the central vacuole to fill. This pushes the cell membrane against the
cell wall creating turgor pressure (osmotic pressure).
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VI.
VII.