- THE STRUCTURE OF A TYPICAL PLANT CELL
A plant cell is bound, i.e. surrounded by a cell wall. Just inside the cell wall is a cell membrane. The cell membrane is a thin, flexible structure, whereas the wall is very rigid and much thicker. The cell wall is made up essentially of two carbon-based sugar polymers. (A Polymer is a group of many smaller molecules, joined like a chain to produce one long molecule); one is pectin and the other is cellulose. The entire contents of the cell are termed the protoplasm. On closer examination one sees a circular ‘shaped’ mass of denser protoplasm, this is called the nucleus.
The nucleus is the control centre of the cell. Everything the cell does is organised by the nucleus. The nucleus also contains small structures that exist in pairs called chromosomes. These chromosomes contain all the records of plans of the organism of which the cell is a part. Information pertaining to shape, size, color, structure and function of a species is contained in the nucleus in the chromosomes. That part of the protoplasm that is not the nucleus is called the Cytoplasm
I.E. CYTOPLASM + NUCLEUS = PROTOPLASM
In the Cytoplasm, other structures can be found and these are called Mitochondria. These structures are little power stations and they burn glucose to produce energy. Also found in cells are small bags containing enzymes (an enzyme is a biological chemical that performs chemical reactions in the cell) called Lysosomes. Certain plant cells contain structures called chloroplasts. These structures use sunlight, carbon dioxide and water to make sugars; hence they are the food factories of the cell. Many plant cells contain a fluid filled vacuole.
See Figure 1 on the next page.
- THE STRUCTURE OF SPECIFIC PLANT CELLS
CELLS THAT CONDUCT WATER AND MINERALS
In the plant stem and other awreas such as the roots and leaves there is a tissue called Xylem tissue. Xylem is responsible for transporting water and dissolved minerals from the roots, where the plant absorbs them, to other regions of the plant. There are a number of cells that make up xylem, the main one is called the vessel element. The vessel element is a barrel-shaped cell that is dead at maturity, it is hollow and open ended. The cell wall is thickened with a very strong substance called lignin. Vessel elements are stacked on top of each other to form long hollow tubes called vessels. The hollow centre of these cells is called the Lumen. See Figure 2, on the next page.
Figure 1: A typical plant cell.
Figure 2 and 3: Shows the structure of xylem and phloem.
CELLS THAT CONDUCT MANUFACTURED FOODSTUFFS:
In the stem there is a type of tissue called Phloem which is responsible for transporting food made by the leaves down to the rest of the plant. The main cells in phloem are the sieve tube elements and their associated companion cells. These cells are, alive however, the sieve tube element does not have a nucleus, only cytoplasm. The companion cell does have a nucleus and this is probably responsible for both cells. The sieve tube element has a sieve plate at the end and, through this, cytoplasm can move from cell to cell. The sieve tube elements are stacked on top of each other to form sieve tubes. These cells do not have any secondary cell wall thickening.
CELLS THAT MANUFACTURE FOODSTUFFS
There are a number of cells that manufacture foodstuffs. The main cells are those in the leaf called Pallisade Cells. In young stems the packing cells of the stem can manufacture foodstuffs for a short period. Pallisade Cells are oblong shaped cells found just below the upper surface of the leaf. They contain hundreds of ‘small structures called Chloroplasts.
Vacuole: a space or vesicle within the cytoplasm of a cell, enclosed by a membrane and typically containing fluid. Lignified: make rigid and woody by the deposition of lignin in cell walls. Chloroplasts contain a green pigment called Chlorophyll and this is responsible for trapping sunlight energy. This is then used to make sugars from Carbon Dioxide and Water.
CELLS THAT GIVE SUPPORT AND STRENGTH:
There are three types of tissue involved – parenchyma, sclerenchyma and collenchyma.
- Parenchyma: This tissue serves two functions, a) as a storage tissue used to store sugars, and other substances, and b) as a filler, a sort of packing that helps give the stem support. These cells are between square and spherical in shape. They are living and possess a large vacuole.
- Sclerenchyma: These cells are dead at maturity and possess thick evenly lignified cell walls. There are two types of sclerenchyma cell: a fiber shown in Figure 3 and
a scleried. Sclerieds do sometimes conduct water in xylem but their function is mainly to give structural support.
- Collenchyma: This is living tissue composed of one type of cell. The cell walls are irregularly lignified and their function is to support and strengthen the stem. See Figure 3 on the next page.
Figure 3: The three types of cell tissues that give support and strength.
- GROWTH OF CELLS
Growth of a plant can be defined as an increase in mass, size or complexity. Growths can occur by one of two methods and both usually occur simultaneously.
THE TWO ARE:
- Cell Division; and
- Cell Enlargement.
This is where cells actually divide to produce more cells by a process called Mitosis. As was mentioned earlier, the nucleus contains many pairs of chromosomes. During Mitosis, these chromosomes shorten and become visible; each individual chromosome splits down the middle longitudinally, the two halves are identical and remain attached to each other. They then migrate to the centre of the dividing cell, the two halves split and one moves to one end of the cell while the other moves to the other end. A cell wall then forms between the two sets of chromosomes and two new cells identical in every detail, are formed. See Figure 4 on the following page.
- Cell before Mitosis; the Chromosomes cannot be distinguished;
- The Chromosomes shorten and can be seen under a microscope;
- The Chromosomes divide but remain joined together;
- Chromosomes lined up along the equator of the cell;
- The Chromosomes split and move to the opposite ends of the cell;
- The new cell wall forms; and
- Two new cells are formed.
As can be seen from Figure 4, after division two smaller cells are formed. These must then become larger. The process by which this occurs is called cell enlargement. More protoplasm is made. The cell wall is added to and an increase in overall size occurs. After enlargement, comes a process called differentiation. This is where the cell turns into a specialised cell with a special function, e.g. Sieve tube element.
Cell division occurs in the growing tips of roots and shoots and cell enlargement occurs just after this. Tissue undergoing constant cell division is called meristematic tissue.