Essential Academic Learning Requirements:

Reading:                  1. The student understands and uses different skills and strategies to read.

1.2 build vocabulary through reading

 Communication:   1. The student uses listening and observation skills to gain understanding.

1.1 focus attention

1.2 listen and observe to gain and interpret information

1.3 check for understanding by asking questions and paraphrasing

Science:                1. The student understands and uses scientific concepts and principles. To meet this standard, the student will:

1.2 recognize the components, structure, and organization of systems and the interconnections within and among them

1.3 understand how interactions within and among systems cause changes in matter and energy  

VOCABULARY:

abscission: the dropping or shedding of a leaf

anthocyanin: a water-soluble red to blue plant pigment

carotene: orange-yellow pigment located in the chloroplasts

chlorophyll: green pigment in green plants

chloroplasts: specialized cellular body where photosynthesis occurs

conifer: cone bearing plants

cuticle: waxy covering or layer on outsides of leaves

deciduous: broadleaved woody plants that that drop their leaves in the autumn

epidermis: outer layer of cells of leaves, roots, stems

grana: the stacks of plate-like structures in the chloroplasts

mesophyll: the layer of cells between the upper and lower epidermis

mitochondria: minute cellular bodies where cellular respiration occurs

palisade cells: mesophyll cells directly under upper epidermis

petiole: the stem of the leaf blade

photosynthesis: the plant process by which plants take water and carbon dioxide and form carbohydrates and oxygen

pigment: any substance that absorbs light, the color of the pigment coming from the the wavelengths of light reflected

respiration: the plant process by which plants oxidize carbohydrates for the release of energy

stomata: the pores in the leaf and stem epidermis through which carbon dioxide enters the leaf and water vapor is lost

xanthophyll: a yellow to colorless photosynthetic plant pigment

  Leaves Are Food Factories

Tree leaves can be called the “food factories” or perhaps even the “power plants” for trees. It’s in the leaves that most photosynthesis takes place. Photosynthesis is the process by which the tree is able to use light (“photo”) energy to make (“synthesis”) food in the form of carbohydrates. Carbohydrates can be considered as stored energy and are used to provide the energy for growth and the synthesis of other compounds and plant processes.

Green plants are the only living things that create their own food. Animals, including humans, depend on green plants as the source of all food energy. (How can this be? I eat meat. Cows certainly aren’t green plants. No they’re not green plants, but they do eat green plants and seeds (grain) produced by green plants.)  

Where Photosynthesis Takes Place

Let’s take a look at these food factories that drive all of life. Looking at a leaf through a microscope you’ll see chloroplasts moving around the cells. Chloroplasts are specialized cell structures that contain chlorophylls, the green pigments that absorb light energy for photosynthesis.  Other pigments are also important in this light energy transfer. They include carotene and xanthophylls. Chloroplasts are made up of several layers of short membranes called grana. It is in the grana that photosynthesis takes place. The chlorophylls intercept the blue and red spectrum of light. The carotene and xanthophylls absorb the blue-green spectrum of light. 

The chemical process of photosynthesis:

Building a Leaf

Let’s look at the structure of a leaf. The epidermis is the “skin” or outer protective layer of cells on a leaf. The epidermis is covered with a waxy coating. It’s called the cutin or the cuticle. The cuticle may also contain “hairs” as part of the cuticle. The cuticle keeps the leaf tissues from drying out and protects them from insects and disease. In many of our shade trees the layer of cutin thickens gradually as the weather warms in the spring and summer.

 Considering that sunlight comes from overhead, where would you expect to find the cells containing the greatest amounts of chloroplasts in the leaf? You’re right... they’re in a layer of cells just under the top surface of the leaves. You’ll find the most chloroplasts in the palisade layer of cells just below the upper epidermis. 

If there is a place for gas exchange to occur in the spongy mesophyll there must be a place for air to get into the leaf. Remember that carbon dioxide is required for photosynthesis and oxygen is a product of this process.  The openings in a leaf are called stomata (stoma, singular). They open or close depending on environmental conditions The opening and closing of the stomata is regulated by the guard cells. Stomata are closed during hot temperatures, dry weather, and darkness.  Water vapor is also lost through the stomate openings.

Most stomata are on the undersides of leaves. Why do you think that might be? Probably to protect from excess loss of moisture through leaves, to keep stomata from becoming plugged with dust, and to prevent easy entry by fungal spores.

Veins running in the mesophyll are part of the tree’s plumbing system. They carry water and nutrients to the leaves and transport away the photosynthates or the products of photosynthesis.  Keep in mind that these are the tree’s food or chemical energy stored in the form of carbohydrates.

Getting Energy from the Food Factories

How does a plant get its stored energy to use for plant processes and the building of other plant compounds, such as vitamins, fats, and proteins? It’s able to obtain energy  by the means of process of respiration by which a plant uses oxygen to convert the carbohydrate energy into energy and carbon dioxide and water.

The chemical process of respiration:

Photosynthetic energy is stored in cells in the form of carbohydrates, starch, fats, and proteins. The is energy becomes available through the respiration process within the cells. The site of respiration is in the mitochondria. The released energy is used for other plant processes and the synthesis of plant compounds needed for growth.

Leaf Fall - The Autumn Miracle

 As we’ve already mentioned tree leaves contain different types of pigments.  The predominant one is usually green and it comes from the chlorophyll needed for photosynthesis... the process by which the leaf captures sunlight and uses that energy to make sugars out of water and carbon dioxide.  In the fall as the leaf begins the process of senescence or dying and falling off the tree, photosynthesis stops and chlorophyll breaks down, revealing the underlying yellow and orange pigments in the leaf...these are carotene (orange-yellow pigment), and xanthophyll (yellow). Red and purple colors come from anthocyanin pigments. Anthocyanins are not masked but actually start to build up in the leaves of certain trees as the chlorophyll breaks down. 

The weather that leads to the best fall colors are those which promote the highest levels of sugars in the leaves.  Bright, sunny warm days and cool nights will lead to the most brilliant hues of oranges through reds and purples. Heavy frosts and overcast days can diminish fall color, while a mild drought can favor anthocyanin production and fall red color.&

Its important to point out that some trees such as red maple, dogwood, sweetgum, and dogwood are capable of exhibiting fall color under the right conditions and other plants, such as sycamore, black locust, black walnut, linden, catalpa, and elm will never provide an attractive autumnal display.

Conifers are cone bearing trees. Most conifers are needled evergreens. While most conifers don’t lose their leaves in the fall, they do lose some of their oldest needles each year. This is often a more gradual, unnoticed process, but in some years needles may turn bright yellow in the fall and drop over a short period of time. There are a few deciduous conifers, such as larch and dawn redwood, that lose all their needles each fall.

Necessary Materials:

 Teaching Procedure:

Discuss the function and structure of leaves. Use diagrams to illustrate photosynthesis, respiration, and leaf structure. Provide several leaves with a magnifying glass to look at stomata. Have leaf cross section and leaf surface slides available for students to look at through a microscope and see cell structure. Talk about leaf pigments and changes within leaves that lead to fall coloring immediately before activity session on leaf pigmentation and leaf drying.