Leaves The Food Factory of Plants PDF

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What Is This Module About?

This module is about the leaves, the food producers in plants. The module will describe how the leaf is structurally adapted to perform the function of food production in plants. The discussion has been so organized as to enable you to understand more easily the role of leaves in photosynthesis or food production. There are two lessons in this module: Lesson 1 — The External Parts of the Leaf Lesson 2 — The Internal Parts of the Leaf

What Will You Learn From This Module? After completing this module, you should be able to: describe the external parts of the leaf; describe the internal parts of the leaf; and locate specific parts in the cross section of a leaf. 1

Wait! Before studying this module, make sure that you have read the module—“Cell: The Basis of Life.” This will help you understand this module better.

Let’s See What You Already Know Before you start studying this module, let us first check how much you know about the leaves of a plant. Do not worry if you get a low score. Just go on and do your best! From the choices given after each statement, encircle the correct answer.

1. Which among the following leaf parts is responsible for providing support and strength to the leaf blade? a. b. c. d. 2.

What part of the leaf attaches it to the stem? a. b. c. d.

3.

margins apexes midribs veins

If you examine the cross section of a leaf, you will see many air spaces. What is the primary function of these air spaces? a. b. c. d.

5.

vein midrib petiole base

What do you call the netlike structures visible at the surface of a leaf? a. b. c. d.

4.

margin base apex midrib

to increase the thickness of the leaf to serve as a cushion for the leaf to trap solar energy to accommodate gases such as carbon dioxide and oxygen

In what specific part of the cell does photosynthesis take place? a. b. c. d.

mitochondrium chloroplast ribosome nucleus 2

6.

Just like any other living thing, plants exchange gases with their surroundings. What particular part of the leaf functions like our nostrils? a. b. c. d.

7.

What do you call the “breathing” holes found on the stems of plants? a. b. c. d.

8.

epidermis mesophyll palisade spongy layer

What do you call the part of the leaf that regulates the opening of the stomata? a. b. c. d.

10.

stomata lenticels noses phloems

What do you call the part of the leaf whose primary function is to protect it? a. b. c. d.

9.

hairs veins cuticle stomata

epidermis hair guard cell chlorophyll

In what specific part of the leaf does photosynthesis take place? a. b. c. d.

air space epidermis mesophyll layer hair

Well, how was it? Do you think you fared well? Compare your answers with those in the Answer Key on page 27 to find out. If all your answers are correct, very good! This shows that you already know much about the topics in this module. You may still study the module to review what you already know. Who knows, you might learn a few more new things as well. If you got a low score, don’t feel bad. This means that this module is for you. It will help you understand important concepts that you can apply in your daily life. If you study this module carefully, you will learn the answers to all the items in the test and a lot more. Are you ready? You may go now to the next page to begin Lesson 1.

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LESSON 1

The External Parts of the Leaf Look at your surroundings. Do you see differences among the living things around? Observe the trees, their branches, their flowers and their leaves. Are they different from each other? What about the birds, insects and worms? Do you notice more differences? We call these differences variations. Well, it has been said that variety is the spice of life. If there were no variety, the world would be drab and life would be boring. Now try to concentrate on the leaves, the food manufacturer of the plant. Do they vary in shape, size, color and arrangement? These beautiful and wonderful part of a plant will be the topic of our discussion. By the end of our lesson, you should be able to describe the external parts of the leaf.

Let’s Learn Did you know that the greatest food factories of the world are the green plants? Can you imagine a food factory with a production capacity of more or less one billion tons of organic food per day? Nowhere in the Philippines or in any other country in the world can you find such a factory! Read on and find out how they do this.

Let’s Try This Go outside and find a leaf, preferably a large one. Look at it closely. Draw a picture of the leaf and its visible parts on a separate sheet of paper.

Let’s Learn Have you ever wondered why plants have leaves? Reflect for a few moments on the following questions. 1. Why are leaves important to a plant? 2. What job do they perform? 4

In this lesson we will try to answer these questions and see how they relate to the process called photosynthesis.

The External Parts of the Leaf Plants such as rice, wheat, corn and vegetables are the major sources of food for Filipinos and other people around the world. But how does a plant get the food it needs so that it can grow to become food for people like you and me? Leaves are one of the major means by which a plant gets the food and energy it needs to grow. As food producers, plants have leaves that enable them to capture light energy from the sun and convert this energy into food. Let us first start with the meaning of photosynthesis. This word can be divided into two parts: photo and synthesis. Photo means “light” and synthesis means “to make” or process food by using solar energy. Study your sample leaf. You will see that it has several parts. Each part has its own special function. Let us study these parts in more detail. lamina

petiole

base

apex veinlets

margin

midrib

veins

Petiole—the stalk that attaches a leaf to the stem of a plant Lamina—the flattened part of a leaf Midrib—the rib that runs along the center of a leaf and forms an extension of the petiole Base—the part which supports the blade of a leaf Margin—the side or edge of a leaf

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Apex—the tip of the leaf blade Veins—the netlike structures on the surface of a leaf Veinlets—the smaller netlike extensions of the veins of a leaf All plants have leaves which have a similar structure to the leaf in the picture. Let us see if you can identify the various parts of a gumamela leaf.

Let’s Try This Get a gumamela leaf. Draw and label its parts correctly.

Do not shade your drawings. Check your labels by referring to the picture on page 5. Although plants have leaves with similar parts, their arrangement may vary in different plants. Some leaves may be arranged in a spiral or alternately, others are arranged opposite each other and some are arranged in a whorl or are spread out from a center. Below are more information on the different leaf arrangements.

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The Different Types of Leaf Arrangements 1.

Spiral The plant’s leaves are arranged forming a line winding outwards from a central point in circles or near-circles of regularly increasing size. Examples of plants with this kind of leaf arrangement are the gumamela, santol and atis.

Spiral leaf arrangements

2.

Opposite The plant’s leaves are arranged in pairs on a stem, so that the two members of a pair are exactly opposite each other. Examples of plants with this kind of leaf arrangement are the guava, santan and malunggay.

Opposite leaf arrangement

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3.

Whorl The plant’s leaves are spread out from a common point on the stem. Example of plants with this kind of leaf arrangement are the Adelfa, dita and digman.

Whorl leaf arrangement

Why do you think plants have different leaf arrangements? How might different leaf arrangements help a plant capture light in the process of photosynthesis? We will now try to answer these questions.

Let’s Try This Get three plant samples with different types of leaf arrangements. 1.

Which among the three plants have a leaf arrangement that captures more sunlight? ____________________________________________________________ ___________________________________________________________ __________________________________________________________

2.

Try to find at least three plants with a whorl leaf arrrangement. Will these get enough sunlight? ____________________________________________________________ ___________________________________________________________ __________________________________________________________

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Here are some sample answers: 1.

Spiral leaf arrangement is the most ideal because the leaves do not shade each other.

2.

Examples of plants with a whorl leaf arrangement are the kutsarita, bridal bouquet and santan.

The more exposed the leaves are, the better because they can get more sunlight which hastens photosynthesis. Having expanded, thin and flat leaf blades also hastens photosynthesis. This type of leaf blades tend to get more sunlight compared to narrow leaves. Examples of plants with narrow or needle-shaped leaves are the agoho plant, pine tree and asparagus.

Let’s Try This Go outside and collect ten leaves of different plants you are familiar with. Group the leaves in a table similar to the one below. Two examples have been given for you.

Type of Blade Name of Plant Expanded Banana

Narrow

4

Pine Tree

4

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Let’s Review

1.

Which type of leaf blade is more common? ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

2.

Why is having narrow leaves disadvantageous to plants? ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

Compare your answers with those in the Answer Key on page 27.

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Let’s Learn Did you know that cactus leaves are formed like spines? They are small and narrow. These spines function mainly to protect the plant. The green, fleshy and more or less expanded parts are the stems that take on the task of processing food. Why Are Some Leaves Shiny? Are you familiar with guyabano leaves? Notice that they are shiny. The substance that makes leaves shiny is the cutin on their outermost surface. It is similar to floor wax. Cutin prevents rapid loss of water in plants. The upper surface of the leaves which is more exposed to sunlight has a thicker cutin layer compared to the underpart. This is primarily the reason why the upper surface of a leaf is shinier than the underpart.

Let’s Review 1.

2.

List down 5 common plants with shiny leaf surfaces. a.

_______________________

b.

_______________________

c.

_______________________

d.

_______________________

e.

_______________________

Why is cutin more concentrated on the upper surface than on the undersides of the leaves? ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

Compare your answers with those in the Answer Key on pages 27-28. How Do Plants “Breathe”? Plants “breathe” or exchange gases with their surroundings through their leaves. Try to examine the openings or “breathing” holes on the undersides of leaves. These tiny “breathing” holes are known as stomata. These are where gases enter and exit the leaves. You can only see these with the use of a microscope. Each of these “breathing” holes is bounded by guard cells. The guard cells open and close the stomata.

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Stomata are concentrated more on the undersides of leaves because these are less exposed to dust. Dust might clog the holes, preventing important gases from entering and leaving the leaves. The presence of stomata on the undersides of leaves also prevents loss of too much water in plants. Imagine a leaf 100 times bigger than you can see with your eyes. The stomata would look something like this:

open stoma

closed stoma guard cells

Let’s Try This Try to find a microscope and examine a prepared slide of the stomata of a leaf. Use only the low-power objective to avoid breaking the slide. Draw and label the stomata and guard cells. If you do not have a microscope, study the figure below and label the stoma, guard cells and opening.

To check your drawing and labels, please refer to page 28. 12

Let’s Review Why are stomata concentrated on the undersides of leaves? _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________

Compare your answers with those in the Answer Key on page 28.

Let’s Try This Get a stem of a gumamela plant. Examine it closely. You may use a hand lens if one is available. Lenticels look like tiny hills on the stems. Try other kinds of plants and examine the lenticels on their stems.

Let’s Think About This 1.

Did you find lenticels on the stem? _________________________________________________________________

2.

How do lenticels look? _________________________________________________________________

3.

Among the plants you examined, which has the most lenticels? _________________________________________________________________

Compare your answers with those found in the Answer Key on page 28.

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Let’s Learn In plants, gas exchange does not only take place in the leaves. “Breathing” holes known as lenticels are also found on the stems. These lenticels are very different from the stomata of the leaves. They look like tiny elevated structures on the surface of the stem. No guard cells are found around the lenticels because these are constantly open. Air enters and leaves these “breathing” holes. Lenticels are usually seen as whitish, elevated structures in mature stems. You can usually see them even without using a hand lens.

Challenger Question: Suppose you were going away on a vacation and you could not find somebody to water your house plants. You know that plants lose water through the stomata in their leaves so you decided to cover the leaves with petroleum jelly or pomade to prevent water loss. When you returned home, you found out that your plants died. Why? Were you able to answer the question? See if it is the same as mine: Your plants died because of the jelly that you applied on the leaves. The jelly covered the stomata. Because of this, air was no longer free to move in and out of the leaves. This cut off the oxygen and carbon dioxide supply of your plants. The lack of supply of these gases caused the death of the plants. Well, we are almost through with this lesson. Are you ready for the next lesson?

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Let’s See What You Have Learned Match Column A with Column B. Write the letters only. A

B

_____

1. It is the structure that attaches the leaf to the stem.

a. cutin

_____

2. It gives support and strength to the blade.

b. guard cells

_____

3. These are the breathing organs found on the surface of leaves.

c. petiole

4. It is the waxy substance on leaves that prevents rapid loss of water.

d. midrib

5. They regulate the size of the stomata.

e. stomata

_____

_____

Compare your answers with those in the Answer Key on page 29. Did you get everything right? If you did, that’s very good. You may proceed to the next lesson. If you did not get everything right, study the lesson again, especially the parts you did not understand. Afterwards, proceed to the next lesson.

Let’s Remember Leaves are the part of the plant responsible for the process of food production or photosynthesis. Leaf arrangements differ in plants. Plants may have leaves in a spiral, opposite, or whorl arrangement. Leaf blades vary. Some may be expanded, thin and flat while others may be narrow. The leaf has external and internal parts. The external parts are the petiole, blade, margin, base, apex, midrib, veins, and veinlets. Leaves have cutin, a waxy substance that prevents rapid loss of water. Leaves also have stomata that serve as “breathing” holes.

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LESSON 2

The Internal Parts of the Leaf We have just looked at the external features of a leaf. Now, we are going to examine the internal parts of a leaf. These are the parts that cannot be seen from the outside. You can only see them by cutting a thin section of the leaf and examining it under a microscope. Try to imagine a leaf as a thick bed with springs. The springs are the internal parts of the leaf which can be seen only if we cut the leaf. Have you ever thought why most leaves are green? Why are they not blue, orange, red or violet? Why Am I Green?

The answer to why leaves are green lies in how a leaf absorbs light. Light actually consists of different waves which correspond to different colors. The colors are red, orange, green, yellow, blue, indigo and violet. All these colors are absorbed by the chlorophyll pigment except for the green wave. Since the green wave is not absorbed, it is reflected back to our eyes and this makes us see the leaves as green.

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Let’s Learn Chloroplasts are the chlorophyll-containing bodies in plant cells where photosynthesis takes place. Imagine green ink inside a glass container. The glass container is the chloroplast while the green ink inside is the chloropyll pigment. If we were to cut a leaf in half and look at the cross section magnified 400 times it would look like this: intercellular space

cell wall cytoplasm

cuticle

vacuole chloroplast

upper epidermis palisade mesophyll

spongy mesophyll

lower epidermis

guard cells stoma

vein sclerenchyma

substomatal chamber

parenchyma

Let us examine closely this cross section of the leaf, starting from the top.

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nucle us

intercellular space

cell wall cytoplasm

cuticle

vacuole chloroplast

nucle us

upper epidermis palisade mesophyll

spongy mesophyll

lower epidermis

guard cells stoma substomatal chamber

sclerenchyma parenchyma

vein

The internal parts of the leaf are: Upper epidermis—This is the outermost single layer of cells located at the top. It serves primarily as protection for the leaf in a way similar to human skin. Mesophyll layer—This is the leaf’s main photosynthetic part because it is the engine room of the plant’s food factory. The c...