The cell is the basic unit of life - plants and animals are made up of cells. Very simple life forms are unicellular (meaning, made up of a single cell). Higher life forms are multicellular (meaning, made up of many cells).
Cells come in a variety of shapes and sizes. Some are microscopic, others are big enough to see with our naked eyes. Some cells are spherical (ball-shaped) others are like ribbons. Cells are functional - this means that cells are made for a specific (special) purpose. Cells come in two flavours - somatic cells (body cells) and germ cells (reproductive cells). Plant cells are different from animal cells in many ways; at the same time they have many common features.
A Simplified Diagram of a Plant Cell
Most cells have the same basic structure - a covering, an interior part, a nucleus, and various other parts. All the structures and parts inside the cell are called cell organelles (meaning, small organs).
The major component (meaning, part) of the cell is the cytoplasm. This is the inside part of the cell. The cytoplasm is made up of cytosol, a gel (meaning, thick fluid, usually very light in colour), and all the other structures inside the cell. The cytoplasm takes up most of the volume and mass of the cell. The cytoplasm is protected and contained by a very fine membrane called the cell membrane (also called cytoplasmic membrane or plasma membrane). The cell membrane is itself covered by a cell wall. The cell wall is much thicker than the cell membrane. The cell membrane is so thin that some substances can easily pass through it. This is how the cell receives nutrients and removes waste material.
A Simplified Diagram of a Cell Nucleus
The most important part of a cell is the nucleus. It is found within the cytoplasm. It is the control center of the cell. The nucleus controls how the cell develops and the functions it performs. The nucleus has a covering called the nuclear membrane. Because of the thick nuclear membrane, the nucleus appears darker and denser than the other structures inside the cytoplasm. The nuclear membrane separates the inner part of the nucleus from the surrounding cytoplasm. However, the nuclear membrane has a number of pores (holes) that actively control the entry and exit of substances and chemicals. The nuclear membrane has two layers. The inner part of the nucleus contains a substance very similar to the cytosol. This substance is called nucleoplasm. The nucleus contains long chains of DNA (deoxyribonucleic acid); the DNA is expressed either as chromatins or as chromosomes (only during cell division). The chromatin (or chromosomes) contain the genes that control the behaviour and development of each cell according to the type of its life form. This is how an individual plant or animal gets the characteristic features from its parent(s) by which we can recognise the species of the life form. This is called heredity. The genes are like code words and the chromosomes are like coded messages that are passed on to new cells to instruct them how to develop and function. If the genes are damaged in any way, the cell will not behave normally. This is very dangerous for the plant or animal. The defective gene can lead to death or, worse still, it can produce defective new life forms. Hence the nucleus is specially protected by the nuclear membrane.
The nucleolus is a dense area of the nucleoplasm. It is the most important part of the nucleus. It is closely linked to the chromosomes/chromatins, and plays a critical (meaning, very important) role in the genetic development process. The nucleolus helps the cell to understand the coded message of the chromosome. A defective nucleolus can lead to disease in the plant or animal.
A Diagram of Endoplasmic Reticulum
Endoplasmic reticulum is a complex structure found in the cytoplasm. Endoplasmic (endo = inside; plasmic = flesh, substance, liquid) reticulum (= network) is made up of layers of membranes that are highly folded; these flat folds form containers called cisternae (= sacs) for liquid substances either produced by the cell or used by the cell.
The endoplasmic reticulum is found around the nucleus and in contact with the nuclear membrane.
The main function of the endoplasmic reticulum is to synthesize (make) and transport proteins and lipids (meaning, fatty substances). Protein synthesis takes place on the outer layer of the endoplasmic reticulum with the help of ribosomes. Ribosomes bind to the outer layer of the endoplasmic reticulum; this combination is called Rough endoplasmic reticulum (RER). Smooth endoplasmic reticulum (SER) synthesizes lipids (fatty substances) and steroids (organic compounds that are important for healthy life). Both types of endoplasmic reticulum are involved in many important chemical process within the cell such as production of enzymes, and carbohydrate metabolism.