References:
Learning Objectives:Plants and People, Chapter 2, pages 19-39.
On Food and Cooking, pages 140-143.
Like all other living things, plants are composed of cells. The explanation stating that all living things are composed of cells is called CELL THEORY, and is one of the basic theoretical foundations of the science of biology. Cells are highly specific complexes of organic molecules. We will look at some of the more basic structures that characterize the plant cell.What is the cell wall?
What are membranes?what is the endoplasmic reticulum?What is the nucleus?
what is the Golgi apparatus?what are chromosomes?
Starting with the outermost part of the cell, we find the CELL WALL. The cell wall is composed of polysaccharides, mainly cellulose. There are usually two layers to a plant cell wall, a thinner PRIMARY CELL WALL that is formed as the plant cell is growing. The primary cell wall must be able to expand as the cell grows. When the cell reaches its full size, a much stronger SECONDARY CELL WALL may form. Secondary cell walls contain a chemically very complex polysaccharide called LIGNIN that is much more resistant to degradation (only two types of organisms, a bacterium found in the stomach of a single cells organism called Mastigophora that is found in the stomachs of termites and some fungi can degrade lignin). Cellulose and lignin are the two most common organic molecules on Earth.
Inside the cell wall will be a PLASMA MEMBRANE that surrounds the cell. The basic components of all membranes in the cell are phosopholipids. Phospholipids naturally form a BILAYER when placed in water, with the polar HEAD part of the phospholipid sticking to water molecules and the fatty acid TAIL part of the one phospholipid sticking to the fatty acid tail part of another phospholipid.
The current model of cell membranes, called the FLUID MOSAIC MODEL, indicates that the cell membrane is liquid, and that the parts of the membrane float around within the membrane. Phospholipid bylaters in membranes act as barriers, preventing large polar molecules and ions from passing through. Small polar molecules like oxygen gas and water can pass through easily, as well as rather large non-polar molecules like steriods. The phospholipid bilayer part of the plasma membrane thus isolates the contents of the cell from the rest of the environment, allowing the cell to have a distinct chemical identity.
The distinct chemical identity of each cell is the result of proteins present in the cell membrane. Proteins allow for the transport of large polar molecules and ions across the pasma membrane. Protein activity in the cell is highly regulated, and the regulation of protein facilitated transport of materials across the plasma membrane defines the biological role of cells.
Plant cells, like our cells, contain membrane bound structures within the plasma membrane. These membrane bound structures, often called ORGANELLES, carry out specific metabolic tasks within the cell. The membrane bound structures that we will look at are; the chloroplast, the mitochondrion, the nucleus, and the endomembrane system. The liquid contents of the cell inside the plasma membrane but outside the organelles is called the CYTOSOL.
The Endomembrane System
The membranes throughout the cell, except for the membranes in the chloroplast and the mitochondrion form the endomenbrane system. The innermost component of the endomembrane system is called the NUCLEAR MEMBRANE, and forms the nucleus. The next innermost component of the endomembrane system is the ENDOPLASMIC RETICULUM (ER). Proteins associated with the membrane are made and processed within the ER. Lipids are also made in the ER. The ER is a system of membranous tubules and flat sacs rqadiating out from the center of the cell. Small round sacs, called VESICLES, bud off from the ER to transport specific proteins to other parts of the cell, such as the plasma membrane or the GOLGI APPARATUS. Proteins transported to the plasma mambrane in vesicles will be secreted from the cell. Proteins transported from the ER to the Golgi are processed further in the Golgi. Vesicles from the Golgi then transport proteins to other parts of the cell or to the plasma membrane.
Proteins are made by large molecular "machines" called RIBOSOMES. Ribosomes are made in the nucleus and exported from the nucleus into the cytosol. Proteins that function in the cytosol are made in the cytosol. Proteins that are made in the ER are made by ribosomes that attach to the ER. ER where proteins are made have many ribosomes attached, and is thus called the "ROUGH ER". Rough ER is usually in the form of flat sacs. ER without ribosomes is where lipids are made. ER where lipids are made is not covered with ribosomes, and is thus called the "SMOOTH ER". The lipids used to make the membrane are made in the smooth ER. Thus the part purpose of the ER can be thought of as the place where membrance components are manufactured. The ER in general manufactures molecules to be transported and used in specific parts of the cell, including the exterior of the cell.
The Chloroplast
The chloroplast is an organelle that consists of 2 outer membranes and an inner set of membranous sacs called the GRANA. Photosynthesis occurs in chloroplasts. The liquid portion contained within the chloroplast is called the STROMA. Photosynthesis produces high energy sugar which plant cells, and our cells, degrade to produce energy.
The chloroplast is a type of PLASTID. Other plastids are LEUCOPLASTS, which store starch. Leucoplasts are the main compoent of cells in things like potatoes and bannanas. ANother type of plastid is a CHROMOPLAST, which store pigment molecules. Chromoplasts contain the red pigment that gives red tomatoes their color.
The Mitochondrion
Mitochondrai consist of an outer smooth membrane surrounding an inner highly folded membrane. The folds of the inner membrane are called CRISTAE. The liquid portion contained within the mitochondrion is called the MATRIX. The mitochondia degrade the 3 carbon sugar PYRUVATE to carbon dioxide, consuming oxygen and producing energy in the form of a ribonucleotide called ADENOSINE TRIPHOSPHATE (ATP) occurs in the mitochondrion. Oxygen is toixic to cells such as those in plants, as well as those in ourselves. Without mitochondria, cells such as those found in plants and in ourselves would not be able to exist in an oxygen atmosphere. In removing oxygen, mitochondria also produce tremendous amounts of energy in the form of the ATP. Without the ATP energy source, neither plants, nor humans could exist.
The Nucleus
The nucleus contains structures called CHROMOSOMES. Chromosomes consist of DNA and protein. Genes are made of the DNA located within chromosomes. Chromosomes in the typical cell form material that when stained takes on a granular appearance, and the chromosomes are clooectivey called CHROMATIN. Chromosomes are linear structures that coil up (CONDENSE) such that they can be seen under a light microscope. Chromosomes condense prior to the division of a cell into two cells. Condensation of the chromosomes during cell division allows for individual copies of each chromosomes to be separated and segregates into each new cell.
A region of the nucleus that stains quite darkly is the NUCLEOLUS. The nucleolus is located at the place where major genes associated with ribosomes are located, and thus the nucleolus is the place where ribosomes are assembled.
Another major feature of plant cells is the CENTRAL VACUOLE. The central vacuole can take up to 90% of the cell. The central vacuole is filled with a liuid that often contains molecules such as pigment molecules, like those that give purple onions their color. The central vacuole is also a means to store water. Sometime poisonous materials are produced and stored in the central vacuole.
Assignment:
Read "A Closer Look" 2.2 on page 24 of your Plants and People and "A Closer Look" 2.3 on page 25 of Plants and People.