Biology-Mr.Willets
Lab- Leaf Structure
The leaf is the part of the plant where most photosynthesis occurs . Not all the cells in the leaf photosynthesize. However, all the cells in the leaf are designed to aid the process.
The actual structure of the leaf depends on the type of plant, the amount of light normally available, moisture conditions and other things. You will examine a typical leaf using prepared slides.
Part One- Lower Epidermis
The outer layer of cells of any organism is often called the epidermis. This is true with leaves. A typical leaf has an upper epidermis and a lower epidermis. Both layers serve to protect the cells inside the leaf and to prevent drying out of the leaf. This is particularly true of the upper epidermis which faces the sun. Epidermal cells have a waxy coating to prevent water loss. This can be quite thick if drying out is especially likely.
a) Examine a slide of lower epidermis using low power. Notice the shape of the cells and how they fit together somewhat like a jigsaw puzzle. Most epidermal cells have few, if any, chloroplasts. This is because their job is ____________________ not making _____________.
b) Scattered throughout the epidermal cells are guard cells. They are sort of hot dog shaped and come in pairs. The space between the guard cells is an opening into the leaf. It is called a stoma (plural- stomata). The size of the stoma is controlled by the guard cells. As guard cells gain or lose water, they change shape and cause the stoma to become larger or smaller. (see diagram page 330)
Air containing CO2 enters the leaf through the stomata and oxygen leaves through these openings. Guard cells and stomata are not usually found in the upper epidermis because it faces the sun and water could easily leave through the stomata and the leaf might dry out. There are exceptions to this.
The diagram below is similar to what you are looking at. Use the numbers to label the following parts: 1) epidermal cells, 2) nucleus of epidermal cells, 3) guard cell, 4) stoma, 5) nucleus of guard cell, 6) chloroplasts in guard cells. In the space available on the right, make a diagram of what your slide actually looks like. No labels needed on this second diagram.
Part Two- Leaf Cross Section
Use low power to examine the slide of a leaf cross section. You should be able to see several layers of cells (see page 325)
a) Upper Epidermis- these cells appear somewhat square from the side but are similar to the lower epidermis cells viewed in part one. They serve as protection especially against drying out. In most leaves, these cells face the sun and are thicker than lower epidermal cells. This layer often has a protective waxy coating to prevent water loss. This layer of cells cannot be too thick or it would prevent sunlight from penetrating into the middle part of the leaf.
b) Palisade Mesophyll-this layer contains longer thinner cells where most photosynthesis occurs. These cells contain more chloroplasts than the other cells in the leaf, Being near the upper surface, they receive a lot of sunlight.
c) Spongy Mesophyll- this layer is made up of irregularly shaped cells that are loosely packed leaving many air spaces around them. These cells are arranged to allow free movement of air within the leaf so that the CO2 can get to the palisade cells and the O2 can easily move to the stomata and leave. Some photosynthesis occurs in these cells but not that much since they do not get as much light.
d) Lower Epidermis- described in part one. The guard cells are now seen from the side and their shape is not the same as when seen straight on (in part one). The openings between the guard cells are the stomata.
e) Veins- scattered throughout the spongy layer are veins. Veins are made of cells that carry water from the roots to the leaves and cells that carry food from the leaf to other parts of the plant. The cells in the veins have thicker cell walls and they have been stained to be a different color from the rest of the leaf.
The diagram below is somewhat like what you are looking at. Use the numbers to label the following parts: 1)waxy coating on upper epidermis, 2)upper epidermis, 3)palisade layer, 4)spongy layer, 5)lower epidermis, 6)air spaces, 7)guard cells, 8)stoma, 9)vein.
Questions-
1) What is the general function of epidermal cells?________________________________________
Which layer of epidermis (upper or lower) has a tougher job? _________________ Why?
____________________________________________________________________________
2) What type of coating do epidermal cells often have on them? _____________ What is the purpose of this coating? ____________________________________________________
Plants that live in what environment might have a thicker coating of this than usual? _________________
3) What cells control the size of the stomata? _________________________ These cells change shape by gaining or losing ______________
4) What gas enters the leaf through the stomata? ________________ What gas leaves through the stomata? ___________________
5) If the stomata were in the upper epidermis and faced the sun, what needed substance might leave the leaf due to the heating from the sun?_____________
6) In which layer of epidermis would lily pads have their stomata? _____________________ Why?_________________________________________________________________
(think about the function of the stomata before answering)
7) In what cell layer does most photosynthesis occur? ______________________________
8) What layer of cells creates the air spaces?_____________________________ These air spaces allow ___________________ to get to the palisade cells and allow _______________ to get out of the leaf.
9) Why do spongy mesophyll cells have fewer chloroplasts and photosynthesize less? _________________________________________________________________
10) If upper epidermis cells had very thick cell walls, they could protect better. Why aren't they like that? ___________________________________________________________________________
11)) Through what structures does water enter the leaf from the roots and food leave the leaf to get to the other cells in the plant? ____________________