The Structure of the Leaf
and the Process of
Photosynthesis
Unit 4- Part 2
Mrs. Stahl
Leaves
• Major site of photosynthesis / food production.
• Minimize water loss by collecting water and
transpiration.
• Take in carbon dioxide and produce oxygen
through the stomata.
• Stomas are tiny pores in the leaf.
• Protects stems and roots with shade and shelter.
Basic Structure
• Blade- usually broad and flat; collects the
sunlight
• Petiole- stem that holds the leaf blade up.
4 Types of Plant Tissues
• 1. Ground Tissue- most common
• 2. Dermal Tissue
• 3. Vascular Tissue
• 4. Meristematic Tissue- division
of new cells.
Covers the outside.
Live parenchymal cells cover
the outside, and have a
cuticle (guard cells)
Bark= dead cells
Makes up much of the
inside.
Provides support and stores
materials in roots and stems.
Packed with chloroplasts.
Transport water, mineral
nutrients, and organic
compounds to all parts of
the plant. Xylem and
phloem.
Ground Tissue
most common and they differ
based on their cell walls- 3 Types
–1. Parenchyma
–2. Collenchyma
–3. Sclerenchyma
Parenchyma Cells
• The most common type of plant cell
• Store starch, oils and water
• Help heal wounds to
the plant
• Found throughout the plant
Collenchyma Cells
–Provide support to a growing
plant
–They are strong and flexible.
–They have unevenly thick cell
walls.
Sclerenchyma cells
–Strongest, support, very thick cell
walls
–Second cell wall hardened by lignin
–Die when they reach maturity
–Used by humans to make linen and
rope
Meristematic Tissue
•
•
•
•
Growth tissue
Where cell division occurs
Turns into ground, dermal, or vascular
Apical Meristems- tips of roots and stems->
primary growth occurs here.
• Lateral Meristems- secondary growth. Increase
the thickness of roots and stems.
Leaf Structure
Let’s go inside!!!
Use your foldable of the leaf and the
chloroplast for this portion of the
notes.
• 1. Cuticle: waxy coating, prevents water loss,
decreases microbial penetration, and the amount of
wax increases with light intensity exposure.
• 2. Upper Epidermis: no chloroplasts, provides a
clear passageway for light to penetrate into the leaf.
• 3. Palisade Mesophyll: Filled with chloroplasts,
performs more photosynthesis because it is closer
to the top of the leaf.
• 4. Spongy Mesophyll: Contain chloroplasts and
performs photosynthesis. Not as efficient as the
palisade mesophyll.
• 5. Air Spaces: allow gases to be exchanged between
the inside and outside of the leaf. Oxygen exits, CO2
enters.
• 6. Lower Epidermis: coats the bottom of the leaf
(cuticle). Contains stomata for gas exchange. More
stomata at the bottom of the leaf than at the top.
• 7. Stoma: allows for gas exchange. The opening and
closing is regulated by guard cells.
• 8. Guard cells: help form the stoma. Inflate to open
the stoma, or deflate to close the stoma.
• 9. Vascular Bundle (Vein): xylem & phloem
surrounded by the bundle sheath. Passageways in
the leaf, stem, and root of the plant.
• 10. Bundle Sheath Cells: surround the xylem and
phloem, strengthen the veins, and protect the
conductive tissues.
• 11. Xylem: transports water and minerals from
the roots to the leaves.
• 12. Phloem: “food,” transports glucose and other
products from the leaf to other parts of the plant
for use or storage. Ex- sap.
The Chloroplast Diagram
• 13. Outer Membrane: semi-permeable, smooth,
phospholipid membrane.
• 14. Inner Membrane: semi- permeable,
phospholipid bilayer under the outer membrane.
• 15. Intermembrane Space: region between the
outer and inner membranes.
• 16. Stroma: liquid in the inner chloroplast. Contains
enzymes needed to catalyze the LDR (light
dependent reactions) in photosynthesis.
• 17. Granum: “stack of pancakes,” stack of thylakoids
• 18. Thylakoid: houses the chlorophyll (pigment),
which helps with photosynthesis. Light
absorption (LDR). Contains accessory pigments,
enzymes, and electron transport systems.
• 19. Thylakoid Lumen: the region located within
each thylakoid.
• 20. Lamella: connect two or more grana to each
other. Makes sure the grana are evenly spaced to
maximize the ability to intercept sunlight.
The Importance of Guard Cells and
Stomata
• The stomata is the site of transpiration and gas
exchange.
• Guard cells surround each stomata, and open and
close by changing shape.
• Day- stoma is open, allowing the carbon dioxide to
enter and water to evaporate.
• Night- close
Guard Cells
Open Stoma
• Allows CO2 necessary for
photosynthesis to enter.
• Open due to potassium ions from
neighboring cells accumulating in
the guard cells, causing water to
also enter the guard cells.
• Water evaporates from the leaves.
• Potassium ions accumulate in the
guard cells and when there is a
high concentration of K+ it causes
water to flow into the cells. When
the plant is full of water, the guard
cells plump up and open the
stomata.
Closed Stoma
•
•
•
When the plant is losing water
from leaves faster than it is
gaining water at its roots, the
guard cells deflate and close their
stomata.
May run low on CO2 and slow or
stop photosynthesis.
Stomata close at night.
Factors that cause the guard cells to
open and close
• Temperature, humidity, hormones, and the
amount of carbon dioxide in the leaves tells
the guard cells to open and close.
Physiological Process of
Transpiration,
Photosynthesis, and
Cellular Respiration
Transpiration
• Evaporation of water from leaves
• Water is pushed up through the xylem by root
pressure created from water moving up the soil to
the plants root system and into the xylem-> results
in small droplets of sap-> called guttation.
• Water is also pulled up through cohesion through
the xylem tissue-> creates a negative pressure or
tension from roots to leaves.
Rate of Transpiration
•
•
•
•
Slows in high humidity
Accelerates or speeds up in low humidity
Increases with wind
Increases with intense light= increased
photosynthesis and water vapor
Photosynthesis
• Defined as the process that captures
energy from sunlight to make sugars that
store chemical energy.
• Location- Chloroplast of plant cells.
Chloroplast
Leaf Cell
Leaf
Photosynthesis
• Chloro= Green
• Phyll= Leaf
• Plast = Molded
chloroplast
leaf cell
leaf
Two Processes
• Light dependent reactions= NEED SUNLIGHT
• Light independent reactions= OCCUR IN THE
DARK
Equation
Chloroplast- refer to your
foldable
• Three main parts are:
–Grana- stacks of coined shaped
membranes.
Thylakoid
– Inside the grana and they are the little disks.
They contain chlorophyll and other light
absorbing pigments.
– Photosystems- light collecting units. They are
proteins that organize chlorophyll and other
pigments into clusters.
Add this onto
your foldable.
Stroma
– Fluid that surrounds the grana inside the
chloroplast.
• Chlorophyll- the molecule in the
chloroplast that absorbs the energy from
the sunlight. There are two main types,
chlorophyll a and b, that absorb mostly
red and blue light. Other pigments absorb
the green.
• Green color in plants comes from the
reflection of the green wavelengths by
chlorophyll.
You do not
have to put
this in your
notes!!!
Just a little
fun fact!
Carotenoids are yellow-orange
pigments which absorb light in violet,
blue, and green regions.
When chlorophyll breaks down in fall,
the yellow-orange pigments in leaves
show through.
Fall Foliage
So let’s begin
• The sunlight hits the leaves and
CO2 is let in through the stomata
(little pores) while H2O is let in
through the roots.
Photosynthesis is broken down into two different reactions!!!
1st
Light Dependent
Reactions or Light
Reactions
– Requires sunlight
– Take place in the thylakoids
– Water and sunlight are needed
– Chlorophyll absorbs energy
– Energy is transferred along the thylakoid
membrane, and then to light-independent
reactions
– Oxygen is released as a waste product
2nd
Light Independent Reactions
• Uses the energy transferred from the light
dependent reactions to make sugars.
• Reactions occur in the stroma
• Does NOT require sunlight
• Carbon dioxide is absorbed and used at this stage.
• Calvin Cycle- metabolic pathway found in the stroma
of the chloroplast; where carbon enters in the form
of CO2, and leaves in the form of glucose.
• ATP is produced as a final step. The enzyme ATP
synthase is responsible for making ATP by adding
phosphate groups to ADP.
Chloroplast
Sunlight entering
Thylakoid
Chlorophyll
Energy is
transferred to
electrons
Oxygen
Energy carrying molecules
ATP & NADPH
transferred to the LIR
H2O
Glucose
CO2 from the
atmosphere
Calvin Cycle in the
stroma
Questions to review
• 1. Where do the light dependent reactions
occur?
• 2. Where do the light independent reactions
occur?
• 3. What two reactants are shown entering the
chloroplast?
• 4. What two products are shown leaving the
chloroplast?
• 5. What does the Calvin Cycle produce?
Answers
•
•
•
•
•
1. Thylakoid membrane
2. Stroma
3. Water and carbon dioxide
4. Oxygen and sugar
5. Glucose
Now that we have a brief overview
let’s look at it in a little more detail.
Light Dependent Reactions
• 1. Energy absorbed from sunlight and transferred to electrons
(electrons = energy) that enter the ETC.
• 2. Water molecules are broken down; electrons enter
chlorophyll.
• 3. Electrons jump from protein to protein down the ETC, and
their energy is used to pump the H+ ions from outside to
inside the thylakoid membrane (against the concentration
gradient = ACTIVE TRANSPORT)
• 4. Energy from sunlight continues to be absorbed, energizing
electrons and pushing them along the ETC.
• 5. Electrons are then added to the molecule NADP+ (functions
like ADP) to produce NADPH (functions like ATP).
• 6. H+ ions flow (diffusion) through a channel in the thylakoid
membrane.
• 7. ATP is produced. ADP is changed into ATP when hydrogen
ions flow through ATP synthase (enzyme).
Light Dependent Cont.
• Electron Transport Chain (ETC)- series of proteins in the
membrane of the thylakoid.
• Energy-> electrons->ATP and NADPH (transferred to
the later stages)
• Arrows represent energy and enzymes!
• NADP= coenzyme that can accept hydrogen and acts as
an enzyme
http://www.biologyonline.org/dictionary/Nicotinamide_adenine_dinucleotide_phospha
te
Photosystem II captures and transfers
energy.
Photosystem I captures energy and
produces energy-carrying molecules.
Light Independent / Calvin Cycle
• Uses the ATP from light dependent reactions. ATP is crucial
because without it the reaction would not happen.
• Does not need sunlight
• Occurs in the stroma and produces sugars
• Energy sources are ATP and NADPH
• Energy that is needed for a series of chemical reaction is
called the Calvin Cycle, named after the scientist- Melvin
Calvin.
• Rubisco is the enzyme used to set up the Calvin Cycle. It’s
said to be the most abundant protein on Earth, but is much
slower than most enzymes.
Light Independent Reactions
• 1. Carbon dioxide enters the Calvin Cycle, and
are added to the already five carbons
molecules that are there.
• 2. Energy is added. The six carbon molecules
split to form three- carbon molecules. More
energy is added (ATP & NADPH), and the
molecules are rearranged into higher energy
molecules.
• 3. A high energy three-carbon molecule exits
for every 3 CO2 molecules that enter. After 2
three-carbon molecules have exited, they
bond to form 1 six-carbon sugar.
• 4. Three carbon-molecules are changed back
to five carbon molecules by energy from ATP.
Videos
https://vimeo.com/7316737
http://www.mhhe.com/biosci/bio_animations/02_MH_Photosynthesis_Web/
http://www.youtube.com/watch?v=lDwUVpOEoE4
Review Questions
•
•
•
•
•
•
1. Where do the light reactions occur?
2. Where do the electrons come from in the ETC?
3. What role do these electrons play?
4. What two energy carriers are produced?
5. When does active transport take place?
6. What enzyme speeds up the process?
• 7. Where in the chloroplast do light independent
reactions occur?
• 8. Where do ATP and NADPH come from for the
light independent reactions?
• 9. What does the LDR make? What does the LIR
make?
• 10. How many cycles or turns does it take to make
one glucose molecule?
• 11. What enzyme sets up the Calvin Cycle?
Answers
• 1. Thylakoid membrane
• 2. Chlororphyll
• 3. Provide energy to move hydrogen ions into the
thylakoid and to produce molecules of NADPH
• 4. NADPH and ATP
• 5. Step 3 when hydrogen ions are transported
• 6. ATP synthase
• 7. Stroma
• 8. LDR
• 9. LDR= makes ATP, LIR= makes sugars
• 10. 2
Let’s Summarize- Bellwork
Write the Equation for Photosynthesis
Process
Light Dependent
Reactions
Where the
photosystems take
place.
Light Independent
Reactions.
Where the Calvin
Cycle takes place
Location
Reactants
Ending Products
Let’s Summarize
6CO2 + 6H2O -> C6H12O6 + 6O2
Process
Location
Reactants
Ending Products
Light Dependent
Reactions
Thylakoid Membrane
Sunlight
H2O
ATP
NADPH
O2
Stroma
ATP
NADPH
CO2
Glucose
Where the
photosystems take
place.
Light Independent
Reactions.
Where the Calvin
Cycle takes place