1. Introduction
All living things can be grouped according to the type of building blocks they are made up of. Most organisms are made up of one or more cells. Cells can be divided into two basic sorts: prokaryotes, which are simple cells and eukaryotes, which are more complex in their structure. Eukaryotes can be further divided into protests, fungi, plants and animals. Viruses are non cellular and have no cell machinery of their own, All cells must secure a source of energy if they are to survive and to carry out their metabolic processes. Autotrophs are able to obtain energy from a source that is from the physical environment. Heterotrophs obtain their energy from other living organisms or their dead remains.
Bacteria cells are extremely small. They have no distinct nucleus; thus within the cytoplasm is a ring of DNA, normally attached to the plasma membrane. They have no membranous organelles.
The cell contains all the structures and molecular constituents needed for life. The cell can take in raw materials and from these, extract useful energy. It can synthesize its own molecules, grow into an organized manner, respond to stimuli from its surroundings and very significantly reproduce itself.
2. The Size of Cells
Most are not seen with the naked eye. Apart form specialized cells like the nerve cells, most fall within a small size range. In general, the size of a plant cell is greater than an animal cell because of its large vacuole. However, the amount of cytoplasm is the same as an animal cell.
There is an evolutionary trend to keep the cell size small and simply increase the number to accommodate he increases in size. Larger animals have more cells than smaller ones. Structural differences between groups of cells are related to the specialized functions preformed. Where cells carry out specialized functions more efficiently, the results are referred to as division of labour. Division of labour among cells leads to increased efficiency by avoiding duplication of effort. This division also causes greater dependence of specialized cells on the activities of other cells. Furthermore, the division of labour leads to highly specialized cells to carry out particular tasks. Hence, in multicellular organisms, similar cells with the same function cluster into tissues which, when grouped together, become organs that carry out specific tasks. Groups of organs form systems, or organ systems, which are responsible for major body functions.
2.1 Surface Area-Volume Relationship
Cells remain small due to problems of area and volume. This is called the surface area-volume relationship. As the cell increase in size, its volume increases disproportionately to its surface area. Thus, growth continues till a point is reached at which the membrane can no longer serve the needs of the active cytoplasm and nucleus. The growth stops and the cell divides into two. Large cells have more surface area than small cells do, but large cells have much less surface area relative to their volumes than do small cells of the same shape.
This relationship has important implications for processes involving transport into and out of cells across membranes. For activities such as gas exchange, the surface area available for diffusion is a major factor limiting the rate at which oxygen can be supplied to tissues. The size of a cell is determined by its metabolic requirements and these in turn are largely dependant on the ability of the plasma membrane to serve the cytoplasmic volume within the cell. There are lower and upper limits to cell size. At minimum, a cell must be able to house DNA, protein molecules and internal structures to survive and reproduce. The maximum size of a cell is limited by its requirement for enough surface area to obtain adequate nutrients from the environment and dispose of wastes.
3. Characteristics of a Eukaryotic Cell
They are found in protest, plants and animals. They can be unicellular or multicellular. Membranes partition the cytoplasm into a maze of compartments thus forming membranous organelles. They have a membrane-enclosed nucleus and organelles. Cellular metabolism can occur within membranous organelles. Such internal membranes increase a eukaryote’s total surface area. This aid in meeting the cell’s metabolic needs.
4. Cytoplasm
Protoplasm refers to the living contents of a cell i.e. the cytoplasm and nucleus. The whole cell can thus be called the protoplast. Cytoplasm is a general term for the material located inside the cell membrane and outside the membrane surrounding the nucleus. It is referred to as the aqueous ground substance. The cytosol contains dissolved nutrients, ions, soluble and insoluble proteins and waste products. The cell membrane separates the cytosol from the surrounding extra cellular fluid. It is located between the cell organelles and appears transparent and structureless in the EM. It is about 90% water and forms a solution which contains all the fundamental biochemicals of life. Some of these are ions and small molecules forming true solutions, such as salts, sugars, amino acids, fatty acids, nucleotides, vitamins and dissolved gasses. Others are large molecules which form colloidal solutions, notably proteins and to a lesser extent RNA. The ground substance is the site of certain metabolic pathways, an important example being glycolysis. Synthesis of fatty acids, nucleotides and some amino acids also occurs. Organelles, which are structures that perform specific functions within the cell, are suspended within an aqueous environment.
5. Cell Organelles
Membranous organelles are as follows: nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, microbodies, mitochondria and chloroplasts.
Non-membranous organelles
are listed as such: ribosomes, microtubules, microfilaments, intermediate
filaments, plant cell wall, intercellular junctions.
