Cell (Biology) | Biology | Life Science

What is Cell?

Cell word comes from latin word “cella” means “room”.

The burger or tomato you eat is composed of cells, and its contents soon become part of your cells. Cells are so much a part of life that we cannot imagine an organism that is not cellular in nature.

English natural philosopher Robert Hooke was the first to observe cells in 1665, naming the shapes he saw in cork cellulae (Latin, “small rooms”). This is known to us as cells. Another early microscopist, Dutch Anton van Leeuwenhoek, first observed living cells, which he termed “animalcules,” or little animals.

The Cells are of two types

1. Eukaryotic (contains Nucleus)
2. Prokaryotic (lack nucleus) prokaryotic cells do not have an internal membrane system or numerous membrane-bounded organelles.

Cell Structure

Four major features all cells have in common:

(1) A nucleoid or nucleus where genetic material is located: In prokaryotes, the simplest organisms, most of the genetic material lies in a single circular molecule of DNA. It typically resides near the center of the cell in an area called the nucleoid. This area is not segregated, however, from the rest of the cell’s interior by membranes

The nucleus contains most of the genes in the eukaryotic cell. (Some genes are located in mitochondria and chloroplasts.) It is averaging about 5 ìm in diameter.is surrounded
by a double-membrane structure called the nuclear envelope, separating its contents from the cytoplasm. The nuclear envelope is a double membrane.

There is also much evidence for a nuclear matrix, a framework of protein fibers extending throughout the nuclear interior. The nuclear lamina and matrix may help organize the genetic material so it functions efficiently.

Within the nucleus, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule associated with many proteins. Some of the proteins help coil the DNA molecule of each chromosome, reducing its length and allowing it to fit into the nucleus. The complex of DNA and proteins making up chromosomes is called chromatin.
Each eukaryotic species has a characteristic number of chromosomes. For example, a typical human cell has 46 chromosomes in its nucleus; the exceptions are the sex cells (eggs and sperm), which have only 23 chromosomes in humans. A fruit fly cell has 8 chromosomes in most cells and 4 in the sex cells.

Here a type of RNA called ribosomal RNA (rRNA) is synthesized from instructions in the DNA. The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA. The mRNA is then transported to the cytoplasm via the nuclear pores. Once an mRNA molecule reaches the cytoplasm, ribosomes translate the mRNA’s genetic message into the primary structure of a specific polypeptide.

(2) Cytoplasm: A semifluid matrix called the cytoplasm fills the interior of the cell. The cytoplasm contains all of the sugars, amino acids, and proteins the cell uses to carry out its everyday activities. Although it is an aqueous medium, cytoplasm is more like Jell-O than water due to the high concentration of proteins and other macromolecules. We call any discrete macromolecular structure in the cytoplasm specialized for a particular function an organelle. The part of the cytoplasm that contains organic molecules and ions in solution is called the cytosol to distinguish it from the larger organelles suspended in this fluid.

(3) Ribosomes to synthesize proteins: Ribosomes, which are complexes made of ribosomal RNAs and proteins, are the cellular components that carry out protein
synthesis . Ribosomes are not membrane bounded and thus are not considered organelles. Cells that have high rates of protein synthesis have particularly large numbers of ribosomes as well as prominent nucleoli, which makes sense, given the role of nucleoli in ribosome assembly. For example, a human pancreas cell, which makes many digestive enzymes, has a few million ribosomes.

(4) A plasma membrane:The plasma membrane encloses a cell and separates its contents from its surroundings. The plasma membrane is a phospholipid bilayer about 5 to 10 nm (5 to 10 billionths of a meter) thick, with proteins embedded in it. Viewed in cross section with the electron microscope, such membranes appear as two dark lines separated
by a lighter area. The proteins of the plasma membrane are generally responsible for a cell’s ability to interact with the environment. Transport proteins help molecules and ions move across the plasma membrane, either from the environment to the interior of the cell or vice
versa. Receptor proteins induce changes within the cell when they come in contact with specific molecules in the environment, such as hormones, or with molecules on the surface of neighboring cells.

Organelles of Cell

• The Endoplasmic Reticulum: The word endoplasmic means “within the cytoplasm,” and reticulum is Latin for “little net.” It accounts for more than half the total membrane in many eukaryotic cells. The ER consists of a network of membranous tubules and sacs called cisternae (it means researve for liquid). It is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. there are two region of Endoplasmic Reticulum. Rough Endoplasmic reticulum and Smooth Endoplasmic reticulum (lack Ribosomes).

• The Golgi Apparatus: Golgi Apparatus is also called Ware house of the cell. Golgi as a warehouse for receiving, sorting, shipping, and even some manufacturing. Here, products of the ER, such as proteins, are modified and stored and then sent to other destinations.

• Lysosomes: Lysosomes is the digestive compartment of the cell. A lysosome is a membranous sac of hydrolytic enzymes that many eukaryotic cells use to digest (hydrolyze) macromolecules. The environment in this part of the cell is acidic which is suitable for the enzyme to digest the macromolecules, proteins and other engulf viruses or bacteria.
• Vacuoles: Vacuoles perform a variety of functions in different kinds of cells. for eg. Food vacuoles, formed by phagocytosis, many unicellular eukaryotes living in fresh water have contractile vacuoles that pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell
• Mitochondria: It is also called power house of the cell. The function of the mitochondria is energy production from the oxidation of glucose substances and the release of adenosine triphosphate. Some cell having only one mitochondria and some having hundreds of them. Two membranes enclosing the mitochondrion is a phospholipid bilayer with a unique collection of embedded proteins. The outer membrane is smooth, but the inner membrane is convoluted, with infoldings called cristae.
• Chloroplasts: It is mainly present in the plant cell and it contains the green pigment chlorophyll helps in trap the sun light which will be utilized for the photosynthesis. It is also enclosed by double membrane wall.
• Peroxisomes: Peroxisomes contain enzymes that remove hydrogen atoms from various
  substrates and transfer them to oxygen (O2), producing hydrogen peroxide (H2O2) as a by-product (from which the organelle derives its name). The peroxisomes use the oxygen to breakdown the fatty acids into smaller parts and which are then transport to the mitochondria.

Comparison of prokaryotic and eukaryotic cells

Sources: Biology by Peter Raven and Campbell Biology