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Introduction
The different kinds of micromolecules and macromolecules that we have studied so far occur in the cell dissolved in solution or as suspensions in a colloidal state. Alternatively, they may be assembled into subcellular components like cell membrane, cytoplasm and cell organelles. In addition to those, some cells may show the presence of a cell wall or a cell coat.
Cell Wall
A cell wall is a characteristic feature of plant cells, bacteria and fungi. The composition however varies in different groups.
Plasma Membrane
The plasma membrane or cell membrane surrounds the cell contents in all types of cells. It is a multipurpose covering composed of phospholipids and proteins.
Cytoplasm
The term cytoplasm can be used to describe the matrix of a cell found between its plasma membrane and the nucleus. It encloses various cell organelles and inclusions.
Cell Organelles
The word organelles (Latin: organas=instrument) is used to describe the living structures inside the cytoplasm, having definite shape, structure and functions.
Membranous Cell Organelles
Most of the organelles in the cell are bound by unit membranes similar to the cell membrane.
Non-Membranous Cell Organelles
Ribosomes are granule-like structures found in both prokaryotic and eukaryotic cells. They are found distributed all over the cytoplasm. In a eukaryotic cell ribosomes occur freely in the cytoplasm, attached to the surface of branches of endoplasmic reticulum (RER), in the matrix of organelles like mitochondria and chloroplasts. It is also found attached to the nuclear membrane and inside the nucleolus.
Vacuoles
Vacuoles are sac-like structures bound by a single unit membrane called tonoplast. A cell may have one or two, small or large vacuoles. In animal cells, vacuoles, when present are smaller in size. In plant, cells vacuoles are large and prominent.
Cell Inclusions
The cell inclusions are also known as ergastic substances. They are represented by various organic and inorganic substances that can be classified under the following three categories
Nucleus
Nucleus is the most conspicuous and the largest organelle in the cell. Most cells have a single nucleus and hence they are described as uninucleate. Some cells have two nuclei (binucleate) while a few may be even multinucleate.
Summary of Structures found in a Cell
Largest cell organelle, enclosed by an envelope of two membranes that is perforated by nuclear pores. It contains chromatin which is the extended form taken by chromosomes during interphase. It also contains a nucleolus.
Chronology of Significant Events in Cell Biology
Electron microscopes were widely used revealing the ultrastructure of cells.
Summary
Protoplasm is represented by cytoplasm and nucleus. Cytoplasm is jelly-like homogenous ground substance called cytosol. It encloses living inclusions called cell organelles and nonliving inclusions called ergastic substances.