Transcript Plants & Photosynthesis

Plants &
Photosynthesis
Plant Tissues
 Ground tissue –
 Parenchyma – thin walled, used
for storage, photosynthesis, or
secretion.
 Collenchyma – thick, flexible
walls, used for support
 Sclerenchyma – thickest walls,
used for support.
Dermal •Guard cells – surround stomata,
turgor controls opening
•Epidermis – thin protective layer,
waxy cuticle secretion
•Specialized cells & Cuticle –
secretive, defensive
Vascular –
•Xylem – conduction of water and
minerals; may have both primary &
secondary walls which lends itself to
secondary function as support. Dead
at maturity.
•Tracheids – long,tapered, have
openings at ends called pits (2◦ wall
absent).
•Vessel elements – shorter & wider;
openings at ends have no walls.
•Phloem – conduction of sugars.
Sieve tube elements 
sieve tubes. Lack nuclei &
ribosomes,but are living.
Openings on end form sieve
plates - allows contact between
cells.
Companion cells –
connected by plasmodesmata;
provide physiological support.
Leaf Anatomy
Leaf Anatomy
 Epidermis w/cuticle
 Guard cells - stomata
 Mesophyll – palisade layer,
spongy layer, air spaces
 Vascular bundles – xylem,
phloem
Control of Stomata
 Surrounded by 2 guard cells
which swell w/water. Uneven
construction of cell wall causes
the cells to bulge outward,
creating opening.
 A diffusion of K+ ions creates a
gradient for the movement of
water, which leads to swelling.
Control of Stomata
 Temperature, inner CO2 levels
influence opening.
 Most plants close stomata at
night, probably due to high CO2
levels.
 Water loss, or transpiration is
minimized when stomata are
closed.
Photosynthesis
Light energy + 6H2O + 6CO2  C6H12O6 + 6O2
Overview
 The process begins with lightabsorbing pigments which are
able to absorb the energy from
light. In a series of reactions, this
energy is eventually used to
produce a molecule of glucose.
Chloroplasts
 Similar to the mitochondria,
chloroplasts are composed of an
outer phospholipid membrane
encasing a fluid interior (in this
case stroma).
 Pancakelike membranes called
thylakoids are arranged in stacks
called grana (granum, singular)
 The thylakoids contain the lightabsorbing pigments, such as
chlorophyll a and b, and
carotenoids and various
enzymes.
 The fluid stroma contains several
enzymes involved in
carbohydrate synthesis.
Chemical reactions of
photosynthesis
 The process of photosynthesis
can be divided into two major
series of reactions:
 Light-dependent reactions
 Light-independent or dark
reactions
Light – dependent
reactions
H2O + ADP + Pi + NADP+ + light 
ATP + NADPH + O2 + H+
 This equation sums up what
occurs in this process.
 Notice a photophosphorylation
occurs, making ATP from ADP & Pi
The details of LDR
 Noncyclic photophosphorylation
occurs in two pigment clusters
called Photosystems I and II.
 Different pigments absorb
different ranges of light
wavelengths.
 The absorbed light “excites”
electrons.
 These energized electrons are
unstable and immediately re-emit
the absorbed energy.
 The energy is then reabsorbed by
neighboring pigment molecules in
a chainlike fashion, eventually
being absorbed into a special
chlorophyll a molecule called
P680. (max. amt. of light absorbed nm)
 This is actually photosystem II,
beginning the energy transfer
process.
 A primary electron acceptor than
accepts and passes two
energized electrons through an
electron transport chain of
proteins, including ferredoxin
and cytochrome.
 As the electrons move “down”
the chain, losing energy, the
“lost” energy is used to
phosphorylate ATP. (For every 2
electrons through the system, an average
of 1.5 ATP molecules are made.)
 The pigment cluster,
Photosystem I, is the final
recipient of the electrons, which
are again energized by sunlight.
 Energized electrons are passed
off to P700 and then another
primary electron acceptor.
 Another, shorter, ETC, passes
electrons along, terminating in
the energy-rich coenzyme
NADPH. (NADP+ and H+ are
combined by the absorption of
the electrons)
 This entire process causes the
removal of two electrons from
PS II.
 These electrons are replaced by
splitting water in a process called
photolysis.
H2O  2H+ + ½ O2 + electrons
 Cyclic photophosphorylation
involves PS I and occurs
simultaneously with noncyclic.
 Energized electrons from PS I join
with protein carriers and generate
ATP.
 Since electrons are not passed off to
NADPH, they can be recycled.
 Concentrations of ATP and NADPH
probably influence whether or not
the pathway is cyclic or noncyclic.
Light Dependent
Reactions
 What are the three main
processes that make up LDR?
 What’s needed?
 What’s produced?
 Where does it occur?
Light-independent reactions:
Calvin-Benson cycle
 “Fixes” CO2  organic molecule
 Uses energy in ATP and NADPH
 Six turns of the cycle are needed
to generate 1 glucose molecule,
however, glucose is NOT the
DIRECT result of this cycle.
Calvin-Benson Cycle:
A Closer Look
 Carbon Fixation – CO2 attaches
to a 5C sugar, RuBP (ribulose
biphosphate) via the enzyme
rubisco.
 The 6C intermediary is unstable
and quickly splits to form two
3C PGA (3-phosphoglycerate)
 Reduction – Each PGA receives
an additional phosphate group
from ATP and electrons from
NADPH reducing it to G3P aka
PGAL (also an intermediate of
glycolysis).
Several turns of the cycle are
needed to continue……
 Regeneration – Once three turns
of the cycle are achieved, the
gain of 6 G3P is used to
regenerate 3 molecules of RuBP,
leaving one extra G3P to be used
by the plant to make glucose.
 Carbohydrate synthesis – Once
six turns are achieved, there are
enough molecules of G3P to
make glucose.
Photorespiration
 The enzyme rubisco is capable of “fixing”
oxygen as well as carbon dioxide, thus it
can be very inefficient.
 The products made when oxygen is
“fixed” to RuBP are not useful to make
glucose.
 This reduces the amount of carbon
dioxide that is “fixed”, thus reducing
glucose production and growth.
C4 & Cam Photosynthesis
 Certain plants have evolved a means of
reducing the amount of photorespiration
that can occur.
 C4 plants – carbon dioxide “fixes” to PEP
(phosphoenolpyruvate) with the help of
the enzyme PEP carboxylase to form
OAA (oxaloacetate), a 4 C compound.
C4 & Cam Photosynthesis
 OAA is then converted to malate and
shuttled to special cells within the leaf
which are not in direct contact with
oxygen.
 The malate then breaks down, forming
carbon dioxide (& pyruvate) and the
carbon fixation begins.
C4 & Cam Photosynthesis
 Not only does this increase
photosynthesis efficiency it also helps
these plants conserve water because
they do not need to have their stomata
open as much to take in carbon dioxide.
 These plants are often found thriving in
hot, dry climates.
C4 & Cam Photosynthesis
 Crassulacean acid metabolism –
 Very similar to C4 but for a few
differences.
 Malic acid is made rather than malate.
 Malic acid is shuttled to vacuole of cells.
 Stomata are open at night and closed
during the day to reduce water loss.