The cell is a primary constituent that comprises an organism or a biological entity. Based on the number of cells an organism consists of it can be grouped into 2 classes: unicellular and multicellular.
Within a single cell itself, there is a division of labour in time and space, as different localized reactions take place in different compartments or cell organelles. Some of the important cell organelles have been described here.
Mitochondrion: It helps in cellular respiration by oxidation of respiratory substrates and thus, generates ATP.
Cytoplasm: This semi-fluid surrounds the cell organelles and other content and is enclosed by a plasma membrane.
Golgi Apparatus: Proteins synthesized in ER are processed, modified and packaged inform of vesicles.
Endoplasmic Reticulum: It is involved in protein and lipid synthesis.
Vesicles: They constitute packages enclosed by a membrane meant for transportation or secretion.
Nucleus: The command center of the cell that regulates all the metabolic reaction happening in the cell and contains the genetic material.
Cell Membrane: It encloses all the cell contents and is comprised of proteins and lipids. It performs the structural, functional role as in transportation and cell signaling or communication and protective role.
Lysosomes: These membranous sacs enclose hydrolytic digestive enzymes.
Cell Wall: It is present only in the case of plant cells and prokaryotes and provides protection against pathogen invasion and also provides structural support. A structure that characteristically is found in plants and prokaryotes and not animals that plays a structural and protective role.
Cell Specialization: Cells differentiate and gain particular functions in different specialized tissues. Similar cells are grouped into tissues that form an organ and then they form an organ system.
Cell Differentiation and Function
Nerve cells function to relay messages across their synapses from different receptors or from the peripheral nervous system to the central nervous system to pass the messages back and forth.
Epidermal cells may have a waxy layer called cuticle for preventing water loss and they form a barrier against pathogens.
Xylem vessels play an important role in water transport and conduction to different parts of the plant and also provide structural support.
Cells are organised in a tissue such as to perform a common specialized function or task. The function of the cell will impact its specialization and its structure.
For instance, muscle cells have a smooth and long structure that aids to perform their function that is movement by allowing for flexibility and elasticity.
Leukocytes are specialized to eliminate and degrade pathogens by different mechanisms.
Photoreceptor cells in the eyes can differentiate the light intensities as they are sensitive to light stimuli and can help relay this to nervous sytem.
In the process of being specialized cells not only gain some functions, they also loss the ones that they do not require in order to be efficient.
This semi-permeable bi-lipid membrane ios constituted by phospholipids, proteins and carbohydrates. The lipids form the bi-lipid membrane and are responsible for fluidity while the proteins are embedded in this membrane and give a mosaic view. Fuid mosaic model is widely accepted.
This barrier prevents leakage of cell contents and prevent the entry of pathogens. As it is semi-permeable it allows only certain molecules to pass through it. Transportation or exchange of materials can occur across this permeable membrane by the following ways.
The 3 mechanisms by which molecules are transported across the bi-lipid memebrane are:
Active Transport: This transport requires energy as molecules are not transported against the concentration gradient and requires carriers for transport.
Passive Transport: It refers to diffusion of molecules across a concentration gradient with the help of a carrier but does not require energy expenditure.
Simple Diffusion: The movement here is along a concentration gradient without any carriers or the need for energy. Simple diffusion of water is referred to as osmosis.
The degradation of Materials in a Cell
Catabolic reaction need to occur to maintain homeostatis, to derive energy from complex food materials and to provide raw materials for various pathways. Phagocytosis (cell eating), pinocytosis (cell drinking) and receptor mediated endocytosis are ways through which a cell can take cells or other substances from their extracellular space into their cytoplasm.
Absorption and Secretion
Absorption refers to the process in which simple materials and substances are absorbed or uptaken into the cells as in intestinal cells. Whereas, secretion refers to ejection of materials from the cell.