Name: Pengjun Duan
Date: 03/25/2015
Course: ENGL 202C
Assignment 4: Definition and Description Final Draft
Process of Photosynthesis
Introduction
Trees are everywhere around us, and they don’t seem to do anything, they stay in
one place and growing slowly and not moving at all. But trees and other plants have a
secret, under the calm exterior, they are busily working inside leaves, by their hard
work, leaves can absorb water and carbon dioxide to release oxygen and glucose, and
we call this chemical reaction is photosynthesis.
What is photosynthesis?
Photosynthesis is a process used by plants and other
organisms to convert light energy, normally from the Sun, into
chemical energy that can be later released to fuel the organisms'
activities.
Photosynthesis can also be described by the word equation:
carbon dioxide + water
glucose + oxygen
Photosynthesis in chemical equation:
6CO2 + 6H2O + [light energy] → 6O2 + C6H12O6
From the equation we can know that in order to produce oxygen and glucose by
photosynthesis, we need the carbon dioxide and water. At first is carbon dioxide:
carbon dioxide enters through pores called stomata located on the underside of the
leaf. The stomata can close at night when no photosynthesis is taking place, or during
the heat of the day when the plant is in danger of too much water evaporating from its
leaves. Then is water, the water is absorbed by the roots and sent up to the leaves
through the xylem part of the plant's vascular tissue. In addition to this, we also need
sunlight and chlorophyll as the catalysis, sunlight: the sun provides the energy that
makes the process run, and then the most important catalysis is chlorophyll: a green
pigment, present in all green plants and in cyanobacteria, responsible for the
absorption of light to provide energy for photosynthesis. Because of chlorophyll is
vital for photosynthesis, only green plants and leaves can do photosynthesis to
produce glucose and oxygen.
Leaves and Leaf Structure
Plants are the only photosynthetic organisms to have leaves (such as, tree, bracket
plant, bamboo and so on. But not all plants have leaves, for those plants which are not
have leaves cannot do photosynthetic). A leaf can be viewed as a solar collector fill
with full of photosynthetic cells.
The reactants of photosynthesis which enter the cells of the leaf are water and
carbon dioxide, and the products of photosynthesis which leave the leaf are sugar and
oxygen.
Water enters the root and is transported up to the leaves through xylem vessels
(the specialized plant cells). Land plants must guard against drying out and so have
developed specialized structures known as stomata to allow gas to enter and leave the
leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf,
but it can enter the leaf through the stoma (the singular of stomata), located in the side
of two guard cells. In the same way, oxygen produced during photosynthesis can only
pass out of the leaf through the opened stomata. But the bad news is, while these
gases are moving between the inside and outside of the leaf, a great deal of water is
also lost. For example, cottonwood trees will lose 100 gallons (about 450 dm3) of
water per hour during hot desert days.
The structure of the chloroplast
The thylakoid is the one of the most
important structural units of photosynthesis. Both
photosynthetic prokaryotes and eukaryotes have
these
flattened
sacs/vesicles
containing
photosynthetic chemicals. Only eukaryotes have
chloroplasts with a surrounding membrane.
Thylakoids are piled up like pancakes in
stacks known collectively as grana. The areas
between grana are referred to as stroma. While the
mitochondrion has two membrane systems, the
chloroplast
has
three,
forming
three
compartments.
Stages of photosynthesis
When the chlorophyll absorb light energy, an electron gains energy and become
excited. This excited electron is transferred to another molecule like a primary
electron acceptor. The chlorophyll molecule will lose the electron and has a positive
charge. Photo activation of chlorophyll reacts in the splitting of water molecules and
the transfer of energy to Adenosine Triphosphate (ATP) and reduced nicotinamide
adenine dinucleotide phosphate (NADP).
The chemical reactions involved include two parts:
1. Condensation reactions – responsible for water molecules splitting out,
including phosphorylation (the addition of a phosphate group to an organic
compound)
2. Oxidation/reduction (redox) reactions involving electron transfer.
Photosynthesis is a two stage process, which include the light dependent reactions and
the light independent reactions.
The light dependent reactions
The photoionization is happened when the leaves or plants are under the sun shine,
the light energy from the sun is absorbed by a chlorophyll molecule, its electrons gain
energy and move to higher energy levels in the molecule under the photoexcitation.
Sufficient energy ionizes the molecule, with the electron being 'freed' leaving a
positively charged chlorophyll ion.
In whole chloroplasts each chlorophyll molecule is associated with an electron
acceptor and an electron donor. These three molecules make up the core of
a photosystem. Two electrons from a photo ionized chlorophyll molecule are
transferred to the electron acceptor. The positively charged chlorophyll ion then takes a
pair of electrons from a neighboring electron donor such as water.
The light independent reactions
In the Light-Independent Process, the reactions are happened inside the leaves
without light; carbon dioxide from the atmosphere is captured and modified by the
addition of hydrogen to form carbohydrates. The incorporation of carbon dioxide into
organic compounds is known as carbon fixation. The energy for this comes from the
first phase (the light dependent reactions) of the photosynthetic process. Living
systems cannot directly take advantage of light energy, but it can utilized through a
complicated series of reactions, convert it into C-C bond energy that can be released
by glycolysis and other metabolic processes.
Conclusion
Chlorophyll is the important roll act in process of Photosynthesis, all the plants
with chlorophyll can do photosynthesis to absorb carbon dioxide and produce oxygen
for us. Everyone breathe in oxygen and breathe out carbon dioxide, so that plant more
green plants will improve our living environment.