All full grown leaves share a basic anatomy, due to their

specialized function in photosynthesis.

are mostly thick walled

they form an unbroken cover which serves to ◦protect the leaf from dehydration and intense UV sunlight

they do not contain chloroplasts

The tissue specialized in photosynthesis is the mesophyll

2 types◦palisade mesophyll ◦spongy mesophyll

palisade mesophyll is the main photosynthetic tissue of the leaf

cylindrical and elongated cells at right angles to the epidermis

they contain chloroplasts.

an open and net-like structure with large inter-cellular spaces that facilitate gas diffusion

major function of the spongy parenchyma is the transport of oxygen, carbon-dioxide and water vapor

contains some chloroplast also is involved in the transport of

water and the products of photosynthesis, the sugars

A special adaptation to land-plants can be actively opened or closed and thus allow a controlled gas exchange with the environment

Submerged plants do not possess stomata

guard cells controls the gas exchange rate by controlling the opening and closing of the stomata

All photosynthesis occurs in the chloroplast

The innermost membrane of the chloroplast is called the thylakoid membrane. ◦The light-dependent reactions occur on the thylakoid membranes

The thylakoid membrane is folded upon itself forming many disks called grana

The inner matrix of the chloroplast not including the grana is called the stroma ◦The light-independent reactions occur in the stroma

 

Photosynthesis uses the energy of sunlight to

convert water and carbon-dioxide into

high energy sugars and oxygen

6CO2 + 6H2O C6H12O6 + 6O2

Light from the sun travels in waves of energy and appears as white light.

In actuality it is comprised of many different wave-lengths and is reflected back as the visible spectrum of light. ROY G BIV

Light can be absorbed, reflected, or transmitted as it strikes an object.

Pigments absorb light. ◦Some absorb different wavelengths more

than others◦ What is not absorbed is reflected back as

the color we see

Chloropylls are pigments found in plant cells, particularly in the chloroplasts

Most common types of chlorophyll are chlorophyll-a and chlorophyll-b.

Chlorophyll-a is directly involved in the light reactions of photosynthesis.

Chlorophyll-b and other pigments such as carotenoids also aid in capturing light. These are all recognized as accessory pigments.

Chlorophylls and other pigments are contained in the thylakoid membranes of the chloroplast in clusters called photosystems.

There are 2 kinds of photosystems

photosystem-I photosystem-II

The absorbance of light by these photosystems excites electrons in the thylakoid membranes

These high-energy e- are are carried by special carrier molecules in the thylakiod membrane.

When working together in a series these molecules are referred to as the electron transport chain.

Step 1◦Light from the sun is absorbed by photosystem-II

◦The photons of light excite the e-of chlorophyll-a.

◦These e- are so highly charged they leave the chlorophyll

◦They are picked up by the primary e- acceptor and carried along by the electron transport chain.

Step 2

◦As these high-energy e- are moved along the ETC from photosystem-II they lose some energy

◦This energy is used by the carrier proteins of the ETC to move H+ from the stroma to the inner thylakoid space.

Step 3

◦Pigments in photosystem-I absorb light also and use that light energy to energize e-.

Step 4◦These high-energy e- are then passed to a different ETC which ends at the outer edge of the thylakoid membrane

◦In the stroma of the chloroplast is NADP+ a key e- acceptor.

Step 5

◦In the stroma the NADP+ charged with e- combines with H+ to form NADPH a key e- transporter.

the e- from P-II essentially replace the e- released in P-I.

If the e- lost in P-II are not replaced both chains would essentially come to a halt.

An enzyme on the interior surface of the thylakoid membrane splits water into 2H+ an O2.

The H electrons replace those lost by chlorophyll in P-II and the “waste” product of O2 is released into the air.

This is the source of nearly all of the oxygen in our atmosphere.

This is the process by which ATP is synthesized in the light

reaction of photosynthesis.

Inside the thylakoid membrane there is a build up of protons from H+ ions. these H+ ions come from:

◦break down of H2O inside the thylakoid to replace e- into P-II.

◦the pumping of H+ from the stroma of the chloroplast to the interior of the thylakoid between P-II and P-I.

This concentration gradient of protons represents potential energy.

That energy is harnessed by ATP synthase, a protein located in the thylakoid membrane.

As H+ diffuse across the thylakoid membrane, ATP synthase converts the potential energy into energy stored in the bonds formed by adding a phosphate group to ADP forming ATP.

The diffusion of H+ to the stroma is the source of H+ ions to form ATP and NADPH, both of which harness energy for the Calvin Cycle .

The Calvin Cycle uses NADPH and ATP from the light reactions of

photosynthesis to produce high-energy sugars

The “dark” reactions occur in the stroma of the chloroplast.

1. 6 molecules of CO2 combine with 6 5-Carbon molecules (Ribulose Biphosphate or RuBP) resulting in 6 6-carbon molecules which immediately split into 12 3-carbon molecules.

6C + 6(5C)

6(6C)

12(3C)

2. The 12 3-carbon are converted into high-energy forms due to the donation of energy by ATP and NADPH in the cycle.

ATP / NADPH 12(3C) *12(3C)

3. 2 of these high-energy 3-carbon molecules are ejected from the cycle and used to produce needed biomolecules, primarily sugars.

2(3C) 6-C sugar

4. The remaining 10 3-carbon molecules are converted by the addition of more ATP into 6 5-Carbon molecules to begin the cylcle again.

6(5C)12(3C)

2(3C) 10(3C)

Other biomolecules such as lipids and proteins might also result from the conversion of those freed 3C molecules depending on the needs of the cell and the plant.