section 12 leaf structure photosynthesis
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Leaf Structure and
Photosynthesis
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DESIGN FEATURES FOR A LEAF
Exposed to sunlight
Large surface area
Capable of exchanging gasesCO2in, O2out
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DESIGN FEATURES FOR A LEAF
Import minerals and water
Not obtained from atmosphere
Export fixed carbon to sinks
Control water loss
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A TYPICAL LEAF
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THE LEAF BLADE
Broad expanded part of the leaf frequently has
the following features:
Large surface area
Thin with a small distance between upper and
lower surfaces
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THE LEAF BLADE
These anatomical features:
Maximize the surface area while minimizing volume
Reduce the distance that gases must diffuse
through the leaf
Leaf structure varies to allow plants to survive and
grow under diverse conditions.
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LEAF TISSUES - EPIDERMIS
Epidermis
epi - upon
dermis - skin
The outer layer of cells
Comprised of a number of different cell types
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LEAF EPIDERMIS
Epidermal cells usually lack chloroplasts
Epidermis is also covered by a waxy cuticle
Impermeable to water
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STOMATA AND GUARD CELLS
Stomata are pores in the epidermis that lead to
intercellular spaces in the leaf (from the Greek
stoma, meaning mouth)
Found on both upper and lower surfaces of the
leaf
Formed by specialized guard cells
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STOMATA AND GUARD CELLS
Crescent shaped cells
When guard cells are turgid, stomata are open;
pores close when cells are not turgid
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WHY ARE STOMATA IMPORTANT FOR
PHOTOSYNTHESIS?
Some plants open stomata during the day,
close them at night
Some plants can fix CO2 at night (stomata
open) in order to minimize water loss during the
day (CAM)
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MESOPHYLL TISSUE
Occupies most of the internal tissue of the leaf
Comprised of two cell types
Palisade parenchyma cells
Spongy parenchyma cells
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MESOPHYLL TISSUE
Palisade parenchyma cells
Elongated cells
Contain many chloroplasts
Primary site for photosynthesis
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MESOPHYLL TISSUE
Spongy parenchyma cells
More randomly arranged
Air spaces between cells
Fewer chloroplasts
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VASCULAR TISSUE
Visible as veins distributed throughout the leaf
Required for transport of material to and from
the leaf
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VASCULAR TISSUE
Xylem
Distribution of water and minerals transported from
root
Phloem
Transport of fixed carbon compounds from the leaf
to the rest of the plant
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VASCULAR TISSUE
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PowerPoint Lectures for
Biolog y: Concepts and Connect ions , Fif th Edi t ionCampbel l, Reece, Taylor, and Simon
Lectures by Chris Romero
Photosynthesis: Using Light to Make FoodMAKIPHO
PHOTOSYNTHESIS
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Plant Power
Photosynthesis makes sugar and oxygen gas from
carbon dioxide and water
Carried out by plants, algae, and some bacteria
The ultimate source of all food eaten by animals
A major source of heat, light, and fuel
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AN OVERVIEW OF PHOTOSYNTHESIS
Autotrophs are the producers of the biosphere
Autotrophs produce their own food
Photoautotrophs produce organic molecules using
light energy
Plants, algae, and photosynthetic bacteria
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Photosynthesis occurs in chloroplasts
In plants, photosynthesis occurs primarily in leaves
CO2enters and O2exits
Chloroplasts are the site of photosynthesis
Concentrated in mesophyll cells of leaves
A protein molecules with a magnesium compound linked
to it. (important to know)
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Chloroplast structure
Stroma
Location of sugar synthesis
Thylakoids
Interconnected sacs
Contain chlorophyll in membranes
LE 7-2
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Leaf
Leaf Cross Section Mesophyll Cell
Vein
Mesophyll
Chloroplast
Stoma
Stroma
Chloroplast
CO2 O2
Granum
Grana Stroma
TEM9,750
Thylakoid Thylakoid space
Intermembrane
space
Inner
membrane
Outer
membrane
LM2,600
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Plants produce O2gas by splitting water
Photolysis
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ADP &
NADP
ATP &NADPH
CO2& H2O
(CH2O)
O2
H2O
Light Chlorophyll
LIGHT
REACTION
CALVIN
CYCLE
PHOTOSYNTHESIS
Photosynthesis can be described in this series
of coupled reactions:
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LIGHT REACTION
Occur in thylakoid membranes
Convert light energy to chemical energy as ATP and
NADPH
Produce O2as a waste product
LE 7-6A
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THE LIGHT REACTIONS: CONVERTING SOLAR
ENERGY TO CHEMICAL ENERGY
Visible radiation drives the light reactions
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gammarays X-rays UV Infrared
Micro-waves
Radiowaves
Visible light
Wavelength (nm)
650nm
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Several pigments are built into the thylakoids of
chloroplasts
Chlorophyll a
Absorbs blue-violet and red light, reflects green light
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Photosystems capture solar power
When a pigment molecule absorbs a photon
A H+electron is raised in energy
The electron releases excess energy as heat
Some pigments also emit light (bioluminescence)
LE 7-8B
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Electron transport
chain
NADPH
ee
e
e
Photosystem II Photosystem I
e
e
e
ATP
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CALVIN CYCLE
Occurs in stroma
Assembles sugar molecules from CO2using ATP
and NADPH from light reactions
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THE CALVIN CYCLE: CONVERTING CO2 TO
SUGARS
ATP and NADPH power sugar synthesis in the
Calvin cycle
LE 7-10A
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Input
CO2
ATP
NADPH
CALVINCYCLE
Output: G3Pglucose
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PHOTOSYNTHESIS REVIEWED AND EXTENDED
Review: Photosynthesis uses light energy tomake food molecules
The light reactions
Capture light energyGenerate NADPH and ATP
Release O2and water
The Calvin cycle
Manufactures sugarCells use many of the same mechanisms in
photosynthesis and cellular respiration
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PLANT USE OF GLUCOSE - ASSIMILATION
Glucose
Sucrose for
transport
Starch for
storage
Cellulose
For cell walls
Proteins
And DNA
Lipids-
Oils in seeds
chlorophyll
Magnesium
Ions from soilMineral ions
from soil
(nitrogen & phosphates
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PLANT USE OF GLUCOSE
Glucose is moved from the leaves to sinks
(growing areas)
This movement of glucose is referred to as
Translocation
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FACTORS AFFECTING PHOTOSYNTHESIS
Limiting factors are components of a reaction
that is in shortest supply
This can be used to help or hinder plant growth
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FACTORS AFFECTING PHOTOSYNTHESIS
Deciduous trees loose their leaves during the
cooler parts of the year.
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FACTORS AFFECTING PHOTOSYNTHESIS
The loose of leaves can sometimes be
described as an ex retory pro ess
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FACTORS AFFECTING PHOTOSYNTHESIS
Mineral Nutrients
- Nitrate
- Phosphate
- Potassium
- Magnesium
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FACTORS AFFECTING PHOTOSYNTHESIS
Nutrient ions are critical for plant growth and
development
A shortage of a particular mineral results in
particular symptoms called mineral deficiency
disease
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NITRATES
Used in amino acids, proteins, chlorophyll, DNA
and other compounds
Deficiency symptoms are stunted growth of
plant, older leaves turn yellow
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PHOSPHATES
Use in making DNA and many other
compounds; part of the cell
membrane
Deficiency symptoms are poor root
growth; young leaves turn purple
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POTASSIUM
Needed for enzymes of respiration and
photosynthesis to work
Deficiency symptoms leaves turn yellow with
dead spots
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MAGNESIUM
Used as part of the chlorophyll molecule
Deficiency symptoms leaves turn yellow
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Questions 37 and 38 on your own