Make a shutterfold
book, leaving a 2-cm
space between the tabs.
Label it as shown. Use
the book as a diagram
of leaf structure.
Plants need food, but they cannot eat as people do.
make their own food, and leaves are the maior
organs of plants. This means that leaves are the sites of
synthesis (foh toh SIHN thuh sus). Photosynthesis is a series
chemical re, actions that convert light energy, water, and carbon ~
ide into the food-energy molecule glucose and give off oxygen.
structure of a leaf is well-suited to its role in photosynthesi~
As shown in Figure 2, ]eaves have many t~pes of ceils, ’
~VORD ~RIGIN ............
from Greek photo-, means
"light"; and synthesis, me~s
Figure 2 Photosynthesis
occurs inside the
chloroplasts of mesophyll
cells in most leaves.
cells that make up the top and bottom layers of a leaf are fI~
irregularly shaped cells called epidermal (eh puh DUR
cells. On the bottom epidermal laye~ of most leaves are
openings called stomata (STOH muh tub). Carbon dioxide
water vapor, and oxygen pass through stomata. Epidermal cells
can produce a waxy covering calle~d the cuticle.
Most photosynthesis Occurs in two types of mesophyll
(ME zuh ill) cells inside a leaf. These cells contain chloroplasts,
the organelle where phdtosynthesis occurs. Near the top su>
face of the leaf are palisade mesophyll cells. They are packed
together. This arrangement exposes the most ceils to light.
Spongy mesophyll ceils have open spaces between them. Gases
needed for photosynthesis flow through the spaces between
the cells.
Palisade
mesophyll Cells
Cuticle
scular
tissue
"Lower
epidermal cells
Cross Section of Leaf
~Visual Check What layer of cells
contains vascular tissue?
272 o
Chapter 8
EXPLAIN
l~ot{
light.
plasts
c~era
chlo~
ment
plan~
refle~
othe*
used
O1
ener{
othel
wate~
oxyg
Figur
1
Figur
enerc.
Upper
epidermal cells
Chloroplast
Stoma
iiesi~
xre
he
lyll
~StS~
sur -
:ked
ght.
ases
As you read about the steps of photosynthesis, refer to Figure 3 to help you understand the process. In the first step of
photosynthesis, plants capture the energy in
light. This occurs in chloroplasts."#Chloroplasts contain plant pigments. Pigments are
chemicals that can absorb and ,effect light.
Chlorophyll, the most common plant pigment, is necessary for photosynthesis. Most
plants appear green because chlorophyll
reflects green light. Chlorophyll absorbs
other colors of light. This light energy is
used du~ing photos~thesis.
Once chlorophyll traps and stores light
energy, this energy can be transferred to
other molecules. During photosynthesis,
water molecules are split apart. This releases
oxygen into the atmosphere, as shown in
Figure 3.
Reading Check How do plants capture
light energy?
Sugars are made in the second step of
photosynthesis. This step can occur without
light. In chloroplasts, carbon dioxide from
the air is converted into sugars by using the
energy stored and trapped by chlorophyll
Carbon dioxide combines with hydrogen
atoms from the splitting of water molecules
and forms sugar molecules. Plants can use
this sugar as an energy source or can store it.
Potatoes and carrots are examples of plant
structures where excess sugar is stored.
Key Concept Check What are the two
steps of photosynthesis?
Why is pho~osyn~hss~s i~p~r~an~?
Try to imagine a world without plants.
How would humans or other animals get the
oxygen that they need? Plants help maintain the atmosphere you breathe.
Photosynthesis produces most of the oxygen
in the atmosphere.
Figure 3 Photosynthesis is a series of complex chemical processes.The first step is capturing light
energy. In the second step, that energy is used for making glucose, a type of sugar,
Carbon dioxide
CO2
st
Water
H20
Photosynthesis
Carbon dioxide + Water ~g> Sugar + Oxygen
Light energy
6C02-F 6H20 ....~ C6H1206 -1- 602
Chlorophyll
EXPLAIN
o 273