Cell-The Basic Unit of Life
- Cell theory
- Size, shape and cell number
- Types of cells
- Cell structure
- Reference books
1. INTRODUCTION :
A cell is considered to be the structural and functional unit of all organisms. Tiny organisms like bacteria, fungi, higher plants and animals including the most evolved types are basically made up of independent units called 'cells'. The term 'cell' was coined by Robert Hooke while observing cork piece under primitive microscope. The study of cells is called 'Cytology', with the availability of various cytological techniques and method and most sophisticated tools, the intricate details of number of biological phenomena occurring inside the cells came to be understood. In recent times, the study of cells deals with a broad range of aspects including structural, functional and hereditary characters of cells. Hence in modern context the study of cells not only deals with cell structure and types but also concerns various cell organelles, and their functions, the inheritance of characters etc., therefore the study of cells in broad perspective is referred as "Cell Biology".
2. CELL THEORY :
MatiasSchleiden, a German botanist and Theodore Schwann, a British zoologist together formulated the cell theory. This theory however did not explain as to how new cells were formed. Rudolf Virchow first explained that cells divide and new cells are formed from pre-existing cells (Omnis - Cellula - e - Cellula). He modified the hypothesis of Schleiden & Schwann to give the cell theory a final shape. Cell theory as understood today is:
i. All living organisms are composed of cells and products of cells
ii. All cells arise from pre-existing cells.
3. SIZE, SHAPE & NUMBER OF CELLS :
Cells differ greatly in size, for example : Mycoplasmas the smallest cells, are only 0.3 um in length while bacteria could be 3 to 5 um. The largest isolated single cell is the egg of an Ostrich. Among multicellular organisms, human red blood cells are about 7.0 um in diameter. Nerve cells are the longest cells.
Cells also vary greatly in their shape. They may be disc - like, polygonal, columnar, cuboid, thread like or even irregular. The shape of the cell may vary with the function they perform.
The cell number in an organism also varies from one to many and even billions in the higher plants and animals.
4. TYPES OF CELLS :
Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, a semi fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cell. Various chemical reactions occur in it to keep the cell in 'living state'.
Beside the nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic recticulum (ER), the golgi complex, lysosomes, mitochondria, plastids (in plants), microbodies and vacuoles. The prokaryotic cells lack such membrane bound organells.
Ribosomes are non - membrane bound organelles found in all cells - both eukaryotic as well as prokaryotic within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles - chloroplasts (in plants) and mitochondria and on rough ER.
Animal cells contain another non - membrane bound organelle called centriole which helps in cell division.
5. CELL STRUCTURE :
The basic appearance of a cells is normally evident at the light microscopic level where as the details are seen only under an electron microscope.
At light microscopy level :
Excepting in a few cases, where the organisms or its cells are very large, in others, the cell shows very few internal details when viewed under a light microscope. Each cell has its outer boundary of cell wall (in plants) followed by a thin membrane, the plasma membrane, that is common to both plants and animals. The entire volume of the cell is occupied by a homogeneous, uniform, granular matrix called the cytoplasm (also called the cytosol). Located usually, at the centre, is a prominent, often more deeply stained structure called the nucleus; only at the time of cell division (mitosis or meiosis), the structural details of chromosomes, chromatin and other secondary constrictions are evident.
Ultra structure under Electron Microscope (EM) :
The ultra structure of a cell is more visible under an electron microscope revealing all the internal structure; however it needs special treatment to observe certain of the organelles and their substructures even at EM level. For example, the mitochondria are specifically stained by Janus green. Similarly the various membranes and the internal components are known only by appropriate modifications in staining or fixation procedures. Under an electron microscope, a number of internal structures are identified in most cells and these are grouped under the cell inclusions distributed differently in the cytoplasm. Some of them have specialized, fixed structures to perform fixed functions. These are all called as the cell organelles. Some of them are prominent in most of the cells performing very important functions in the cells and are termed as the major cell organelles, while others may be few or confined only to specific cell types in the organisms and were termed as minor cell organelles. A very interesting point is that all of these cell inclusions are not present in all of the cells. Therefore, here we will consider a typical plant and animal cell each to look into the basic structure of each organelle, its location in the cell and the main functions, it performs in a cell. The following account deals with various organelles of a typical plant cell, their location / distribution in the cell and the one or two important functions they perform in the cell.
Figure :Ultrasturcture diagrams of an animal and a plant cell. The diagrams are of typical cells showing different structures as visible under electron microscope drawn to scale.
Cell wall :This is the outer most boundary of each cell in plants and bacteria. There is a further distinction into primary, secondary and tertiary wall. The main components are celluloses and hemicelluloses. It gives shape and rigidity to the cell.
Plasma membrane: This is a thin membrane present inner to the cell wall in plant cells and below the extra cellular matrix (ECM) in animal cells. It is a continuous sheet of bilayered phospholipids (about 4-5 nm thick) with proteins on either side. The various proteins in the membrane have a wide variety of functions such as transport of materials
Endoplasmic reticulum :The cytoplasm has a net work of tubules and flattened sacs called vesicular system. Each vesicle is a part of the organelle that also has the tubular membranous system with a space in side called ER lumen. It main function is transport in the cell.
Golgi apparatus :It is located usually nearer to the nucleus and has a stack of plates; each plate like structure in a stack is called the cisterna; there are 4 - 6 such cisternae in each Golgi apparatus. It carries vesicles of different substances from E R to the secretary system, plasma membrane and other membranous systems.
Lysosomes: Spherical or avoid bodies containing hydrolytic enzymes that help in intracellular digestion.
Cytoskeleton :In the cytosol of every cell, an array of protein filaments are present throughout forming a net work and help in mechanical strength as well as transport. There are called the cytoskeletal elements. Three main types of cytoskeletal elements are known - microtubules, actin filaments and intermediate filaments.
Peroxisomes :There are membrane bound vesicles containing oxidative enzymes, more so the catalases. Their function is the degradation of hydrogen peroxide and in photorespiration.
Vacuoles :Organelle with a single membrane, each enclosing a space inside and contain different chemical components. They help in cell expansion and growth and also in transport of solutes.
Mitochondria :These are one of the very important organelles in a cell. Each mitochondrion is bilayered enclosing a matrix space. The inner membrane is very much convoluted inside to form the cristae; the elementary particles present on their surface play a crucial role in photophosphorylation. These organelles break down the complex organic compounds during cell respiration and store the energy in the ATP molecules and make available to the cell when needed. Hence they are called the power houses of the cells.
Chloroplasts :These are another kind of essential organelles. Each chloroplast is formed from an undifferentiated proplastid with the synthesis of green pigment, chlorophyll, in the mature chloroplasts. Its membrane is also bilayered and encloses a protenaceous space inside called the stroma space with a net work of well organized lamellar system differentiated into the stacked lamellae and unstacked lamellae. The organelle plays a universal role in photosynthesis and in photorespiration.
Nucleus :With a rare exception, this important organelle is present in all of the cells in the organisms. It is visible even under light microscope as a deeply stained body present located in the centre of most of the cells. In a few cases, it is localized to the boundaries of the cell. Its space is occupied by the genetic material DNA / RNA which is present in the form of the uniformly dense, well stained chromatin that is visible even in the ordinary microscope. A more close observation shows a further distinction into deep stained areas (heterochromatin) and the less stained regions (euchromatin) It also contains inside a single spherical structure called the nucleolus. The main function of the nucleus is in possessing the genetic information and its transmission to the daughter cells through a well defined machinery involving in DNA replication and cell division.