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BTNY LS1203 - Plant Biology


The Components of a Plant Cell

A eukaryotic cell contains many membrane bound compartments called organelles. Each organelle has several functions that it is responsible for. Key organelles found in a "typical" plant cell: 

1. nucleus = where genetic material (DNA) is stored in the form of chromosomes.  The initial steps of gene expression (DNA --->  RNA) occur here.

2. plastids = a group of organelles which include chloroplasts (photosynthesis), chromoplasts (lack chlorophylls but have carotenoids), and amyloplasts (store starch).  

3. mitochondria = convert stored energy in sugars (mostly) to useable cell energy in the form of ATP by the process of cellular respiration

4. vacuole = large, water-filled area that can occupy as much as 90% of the volume of a mature plant cell. It provides a place for water soluble pigments (anthocyanins), hydrolases, toxins (like alkaloids), wastes, storage proteins, sugars, and minerals. The water in vacuoles provides turgor pressure, which supports young, flexible plant structures.  The pH is usually acidic.

5. endoplasmic reticulum (ER) and Golgi apparatus = packaging system for export of materials to the cell exterior (including the cell wall) or to vacuoles.   

Other important cell structures that are not membrane bound:

6. plasma membrane = the selective membrane that surrounds a cell and controls entry and exit of solutes like sugars and mineral ions.  It defines the cell interior and exterior.  Some molecules, like water and carbon dioxide, pass freely through membranes.

7. ribosomes = follow instructions from nucleic acids to link amino acids together to make proteins. Found free in cytosol (see below) or attached to the ER.

8. cell wall = rigid support structure located to the outside of the plasma membrane. It provides a mechanical barrier to large things (like bacteria) but is very porous to small things (like soil mineral ions).  Cell specialization often involves elaborations to the cell wall, such as thickenings or addition of lignin or suberin.  The primary cell wall contains cellulose, hemicellulose, and pectin.  It tends to be more flexible than the secondary wall.  The secondary wall, if made, has more cellulose than the primary wall and is often further reinforced by the addition of lignin.  These additional materials make the secondary wall rigid.  For many of the cells that are dead at functional maturity (such as the water conducting cells of the xylem), the cell wall is the only cellular structure left.

Other cell-related terms:
protoplast = a cell whose wall has been removed
protoplasm = the entire cell contents inside the plasma membrane
cytoplasm = protoplasm minus the nucleus
cytosol = "the ground substance of the cytoplasm," which means everything inside a plasma membrane except the organelles.


The Cell Cycle (cell division)

cell cycle = interphase + mitosis + cytokinesis
interphase = a period of cell growth and DNA replication that precedes mitosis
mitosis = a nuclear division process that results in two identical nuclei; Four stages:  prophase, metaphase, anaphase, telophase
cytokinesis = division of the cytoplasm

The nucleus houses genetic material in the form of chromosomes.  The chromosomes are made of a material called chromatin.  The chemical composition of chromatin is roughly half DNA and half protein.  The DNA, which is the portion of the chromatin that codes for genetic characteristics, is a long, thin molecule that wraps around the more globular protein portions of chromatin.  The proteins are called histones.  So each individual chromosome consists of a very long DNA molecule wrapped around histone beads.

As a result of cell division, one cell is split in half to form two genetically identical cells.  Although the resulting cells might differ functionally and morphologically at maturity, they contain the same amount and type of DNA.  

Interphase

Interphase lasts from the end of one nuclear division to the start of another.  The majority of cells in an organism are in interphase.  Interphase itself can be divided into three steps:  G1, S and G2.  

During  G1, or the first gap, the cell enlarges to mature size.  General metabolic activity occurs.  There is a lot of RNA and protein synthesis.  This is the step of the cell cycle that most cells are arrested in.  If this cell is no longer dividing, it is usually described as being in G0.  If this is a meristematic cell, toward the end of  G1  you will see synthesis of the enzymes needed for DNA replication as well synthesis of proteins needed to form the spindle apparatus.

During the S phase, the histones are synthesized and DNA is replicated (or duplicated).  The details of DNA structure and replication will be covered later.  This synthetic step causes each chromosome to go from a single strand of chromatin to two connected strands of chromatin.  The two chromatin strands of one chromosome are called chromatids, and they connect at the centromere.

During G2, the second gap, RNA and protein synthesis continues as the cell finished its preparations to divide the nucleus.

    If you look at an interphase cell with a microscope, the nucleus will be clearly visible and apparently just sitting there.  When the cell cycle was first studied, the only tool available was the light microscope.  As the intense activity of G1, S, and G2 cannot be seen with a light microscope, researchers assumed the nucleus was resting, and interphase quite mistakenly became known as the resting phase.  

    During mitosis, the nuclear membrane disperses and the chromosomes become visible with a light microscope.  Mitosis literally means thread-like disease or condition.  A cell in interphase, with its nucleus intact, was viewed as the normal or healthy condition, while a mitotic cell, with its nucleus disintegrated to release the thread-like chromosomes, was abnormal.  As a result, early researchers spent many hours watching the events of mitosis and named four steps of this portion of the cell cycle.  These steps are prophase, metaphase, anaphase, and telophase.  Also, the cell was pictured as rather globe-like in appearance, and so they also described the cell as having an equator and two poles. 

Where can mitosis occur in a plant?  Meristems, located at the tips of stems and roots or in the outer cell layers of woody plants. 

Mitosis

Prophase
During prophase the chromosomes become visible as the chromatin packs together tighter and tighter.  As the chromosomes become visible, you can now see the two chromatids.  They appear to be held together at a pinched region of the chromosome, the centromere.  A kinetochore attaches to each chromatid at the centromere.  At the end of prophase, the nuclear membrane disrupts.

Metaphase
In metaphase, the chromosomes move to the center of the cell and align themselves with the centromeres along the equator of the cell and the chromatids of each chromosome point toward opposite poles.  Microtubules attach to the kinetochores and extend to the poles.  This arrangement of microtubules forms what is called the spindle apparatus.  The action of the microtubules is such that the chromatids are separated as the microtubules pull at the kinetochores.  

Anaphase
The separation of the chromatids marks the start of anaphase.  Once the two chromatids have separated, they are now each a chromosome.  The chromosomes move to opposite poles as they are pulled away from the equator.  

Telophase
Once all of the chromosomes have reached a pole, telophase begins.  During telophase, two nuclei form, one at each pole.  The nuclear membrane forms, the spindle apparatus disappears, the chromosomes uncoil, and nucleoli reappear. 

Cytokinesis

Cytokinesis is the division of the cytoplasm and generally starts during telophase of mitosis.  The visible indicator of the start of cytokinesis is the formation of the cell plate along the equator of the dividing cell.  Cell plate formation starts when vesicles from the Golgi apparatus deposit material that will form the middle lamella between the two new cells.  The membranes surrounding these vesicles will form the plasma membranes for the two new cells.  ER channels that become surrounded as the middle lamella forms will form plasmodesmata.  Finally, cellulose for the cell walls will be secreted by the two new cells.  This cellulose will be trapped between the plasma membranes and the common middle lamella.  Eventually, the cytoplasm is cut in two, and two cells are formed.  What happens to the cell that started this whole process?  It no longer exists.  That one cell, as a result of the cell cycle, has become two genetically identical cells.  If that one cell had four chromosomes, each of the two new cells has four chromosomes.  The new cells can then differentiate to become specialized cells or remain as meristematic cells.  One final word about cytokinesis.  It does not always happen, nor does it have to happen at telophase.  For example, coconut milk, the liquid endosperm of the coconut seed, results from mitosis without cytokinesis.


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10 January 2011