INTRODUCTION
‘Cell’ means a small room or chamber; cells are the structural and functional units of all living organisms. The major parts of a cell are the nucleus and the cytoplasm.
TYPES OF LIVING CELL
The electron microscope allowed classification of cells into two major groups, prokaryotes and eukaryotes based on the presence and absence of the true nucleus.
- Eukaryotes (Greek: Eue = true, karyon = nucleus), which have a membrane enclosed nucleus encapsulating their DNA (deoxyribonucleic acid). Animals, plants and fungi belong to the eukaryotes. Eukaryotic cells are much larger than prokaryotes. Eukaryotes may be multicellular as well as unicellular, are far more complex than prokaryotes and are characterized by having numerous membrane enclosed organelles (subcellular elements) in their cytoplasm, including:
- Mitochondria
- Lysosomes
- Endoplasmic reticulum
- Golgi complexes
- Prokaryotes which comprise the various types of bacteria have relatively small structures and are invariably unicellular. They have no typical nucleus instead consists of nucleoid in which the genome, the complete set of genes, composed of DNA is replicated and stored with its associated proteins. The nucleoid is not separated from the cytoplasm by a membrane (Figure 1.1). Prokaryotes lack membrane enclosed organelles (subcellular elements) in their cytoplasm.
Figure 1.1 and Table 1.1 describe some of the major structural features of the prokaryote and eukaryote cells.
Flash on
- * Viruses are not living organism in the sense that cells are. They are incapable of replicate themselves outside their host cells and have virtually no biochemical activities of their own.
- * However, they possess genomes and have the ability to enter living cells, where they can use the host's molecular machinery to replicate them as a result of which they cause disease.
STRUCTURE AND FUNCTIONS OF A CELL AND ITS SUBCELLULAR COMPONENTS
A cell has three major components:
- Plasma membrane (cell membrane)
- Cytoplasm with its organelles:
- Endoplasmic reticulum
- Golgi apparatus
- Mitochondria
- Lysosomes
- Peroxisomes
- Nucleus.
Plasma Membrane
- The cell is enveloped by a thin membrane called cell membrane or plasma membrane.
- Plasma membranes mainly consist of lipids, proteins (integral and peripheral) and smaller proportion of carbohydrates that are linked to lipids and proteins.
- The plasma membrane is an organized structure consisting of a lipid bilayer primarily of phospholipids and penetrated protein molecules (Figure 1.2).
- The membrane is sometimes referred to as a fluid mosaic (Figure 1.3). Since, it consists of a mosaic (variety) proteins and lipid molecules that can move laterally in the plane of the membrane. The membrane mosaic is fluid because most of the interactions among its components are non-covalent, leaving individual lipid and protein molecules free to move laterally in the plane of the membrane.
Membrane Lipids
- The major classes of membrane lipids are:
- Phospholipids
- Glycolipids
- Cholesterol.
They all are amphipathic molecules, i.e., they have both hydrophobic and hydrophilic ends. - Membrane phospholipids spontaneously form bilayer in aqueous medium, burying their hydrophobic tails and leaving their hydrophilic ends exposed to the water (Figure 1.2).
Membrane Proteins
Proteins of the membrane are classified into two major categories:
- Integral proteins or intrinsic proteins or transmembrane proteins and
- Peripheral or extrinsic proteins.
- Integral proteins are either partially or totally immersed in the lipid bilayer. Many integral membrane proteins span the lipid bilayer from one side to the other and are called transmembrane protein whereas others are partly embedded in either the outer or inner leaflet of the lipid bilayer (Figure 1.2). Transmembrane proteins act as enzymes and transport carriers for ions as well as water soluble substances, such as glucose.
- 3Peripheral proteins are attached to the surface of the lipid bilayer by electrostatic and hydrogen bonds. They bound loosely to the polar head groups of the membrane phospholipid bilayer (Figure 1.2). Peripheral proteins function almost entirely as enzymes and receptors.
Membrane Carbohydrates
Membrane carbohydrate is not free. It occurs in combination with proteins or lipids in the form of glycoproteins or glycolipids. Most of the integral proteins are glycoproteins and about one-tenth of the membrane lipid molecules are glycolipids. The carbohydrate portion of these molecules protrudes to the outside of the cell, dangling outward from the cell surface (Figure 1.2). Many of the carbohydrates act as receptor for hormones.
Functions of Cell Membrane
- The plasma membrane maintains the physical integrity of the cell by preventing the contents of the cell from leaking into the outside fluid environment and at the same time facilitating the entry of nutrients, inorganic ions and most other charged or polar compounds from the outside. It permits only some substances to pass in either direction, and it forms a barrier for other substances.
- The cell membrane protects the cytoplasm and the organelles of the cytoplasm.
- It maintenance of shape and size of the cell.
Cytoplasm and its Organelles
Cytoplasm is the internal volume bounded by the plasma membrane. The clear fluid portion of the cytoplasm in which the particles are suspended is called cytosol. Six important organelles that are suspended in the cytoplasm are:
- Endoplasmic reticulum (ER)
- Golgi apparatus
- Lysosomes
- Peroxisomes
- Mitochondria
- Nucleus.
Endoplasmic Reticulum
- Endoplasmic reticulum is the interconnected network of tubular and flat vesicular structures in the cytoplasm (Figures 1.4A and B).
- Endoplasmic reticulum forms the link between nucleus and cell membrane by connecting the cell membrane at one end and the outer membrane of the nucleus at the other end (see Figure 1.1).
- A large number of minute granular particles called ribosomes are attached to the outer surface of many parts of the endoplasmic reticulum, this part of the ER is known as rough or granular ER.
- During the process of cell fractionation, rough ER is disrupted to form small vesicles known as microsomes. It may be noted that microsomes as such do not occur in the cell.
- Part of the ER, which has no attached ribosomes, is known as smooth endoplasmic reticulum.
Functions of the ER
- Rough ER functions in the biosynthesis of protein.
- The smooth endoplasmic reticulum functions in the synthesis of steroid hormones and cholesterol.
- Smooth endoplasmic reticulum is the site of the metabolism of certain drugs, toxic compounds and carcinogens (cancer producing substances).
Golgi Apparatus
Golgi apparatus is present in all cells except in red blood cells. It is situated near the nucleus and is closely related to the endoplasmic reticulum. It consists of four or more membranous sacs. This apparatus is prominent in secretory cells.
Functions of Golgi Apparatus
The Golgi apparatus functions in association with the endoplasmic reticulum:
- Proteins synthesized in the ER are transported to the Golgi apparatus where these are processed by addition of carbohydrate, lipid or sulfate moieties. These chemical modifications are necessary for the transport of proteins across the plasma membrane.
- Golgi apparatus are also involved in the synthesis of intracellular organelles, e.g., lysosomes and peroxisomes.
Lysosomes
- Lysosomes are vesicular organelles formed from Golgi apparatus and dispersed throughout the cytoplasm.
- Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane to prevent the enclosed hydrolytic enzymes from coming in contact with other substances in the cell and therefore, prevent their digestive actions.
- Many small granules are present in the lysosome. The granules contain more than 40 different hydroxylases (hydrolytic enzymes). All the enzymes are collectively called lysozymes.
Functions of Lysosomes
Lysozymes present in lysosomes digest proteins, carbohydrates, lipids and nucleic acids. Apart from the digestive functions, the enzymes in the lysosomes are responsible for the following activities in the cell:
- Destruction of bacteria and other foreign bodies.
- Removal of excessive secretory products in the cells of the glands.
- Removal of unwanted cells in embryo.
Peroxisomes
- These organelles resemble the lysosomes in their appearance, but they differ both in function and in their synthesis.
- They do not arise from Golgi membranes, but rather from the division of pre-existing peroxisomes or perhaps through budding off from the smooth endoplasmic reticulum.
Functions of Peroxisomes
- Peroxisomes contain enzymes peroxidases and catalase which are concerned with the metabolism of peroxide. Thus, the peroxisomes are involved in the detoxification of peroxide.
- Peroxisomes are also capable of carrying out β-oxidation of fatty acid.
Mitochondria (Powerhouse of Cell)
- Mitochondria are called “Power Plant” of the cell since they convert energy to form ATP that can be used by cell.
- A mitochondrion is a double-membrane organelle (Figure 1.5) that is fundamentally different in composition and function:
- The outer membrane forms a smooth envelope. It is freely permeable for most metabolites.
- The inner membrane is folded to form cristae, which give it a large surface area and are the site of oxidative phosphorylation. The components of the electron transport chain are located on the inner membrane. The inner membrane is impermeable to nearly all ions and polar molecules
- The space within the inner membrane is called the mitochondrial matrix.
Functions of Mitochondria
- The intermembrane space contains several enzymes involved in nucleotide metabolism.
- Whereas, the gel-like matrix (mitosol) consists of high concentration of enzymes required for the metabolic pathways of oxidation of pyruvate produced by glycolysis, fatty acids, and amino acids and some reactions in biosynthesis of urea and heme. The mitochondrial matrix is the site of most of the reactions of the citric acid cycle and fatty acid oxidation.
- Oxidative phosphorylation takes place in the inner mitochondrial membrane. Components of electron transport system and oxidative phosphorylation that are responsible for the synthesis of ATP are embedded in inner membrane.
Flash on
- * Mitochondria contain their own DNA (mtDNA) which encodes a few polypeptides involved in oxidative phosphorylation.
- * Sperms contribute no mitochondria to the fertilized egg, so that mitochondrial DNA is inherited exclusively through the female line. Thus, mitochondria are maternally inherited.
Nucleus
Most of the cells have only one nucleus but cells of skeletal muscles have many nuclei. The matured red blood cell contains no nucleus.
Structure of Nucleus
- The nucleus is spherical in shape and situated near the center of the cell. The nucleus is surrounded by the nuclear envelope.
- The space enclosed by the nuclear envelope is called nucleoplasm; within this the nucleolus is present. Nucleolus is an organized structure of DNA, RNA and protein that is involved in the synthesis of ribosomal RNA. The remaining nuclear DNA is dispersed throughout the nucleoplasm in the form of chromatin fibers. At mitosis, chromatin is condensed into discrete structures called chromosomes.
Functions of Nucleus
The major functional role of the nucleus is that of:
- Replication: Synthesis of new DNA.
- Transcription: The synthesis of the three major types of RNA:
- Ribosomal RNA (rRNA)
- Messenger RNA (mRNA)
- Transfer RNA (tRNA).
CYTOSKELETON
- The cytoplasm of most eukaryotic cells contains network of protein filaments that interact extensively with each other and with the component of the plasma membrane. Such an extensive intracellular network of protein has been called cytoskeleton. The plasma membrane is anchored to the cytoskeleton. The cytoskeleton is not a rigid permanent framework of the cell but is a dynamic, changing structure.
- The cytoskeleton consists of three primary protein filaments:
- Microfilaments are about 5 nm in diameter. They are made up of protein actin. Actin filaments form a meshwork just underlying the plasma membrane of cells and are referred to as cell cortex, which is labile. They disappear as cell motility increases or upon malignant transformation of cells. The function of microfilaments is:
- To help muscle contraction
- To maintain the shape of the cell
- To help cellular movement.
- Microtubules are cylindrical tubes, 20 to 25 nm in diameter. They are made up of protein tubulin. Microtubules are necessary for the formation and function of mitotic spindle. They provide stability to the cell. They prevent tubules of ER from collapsing. These are the major components of axons and dendrites.
- Intermediate filaments are so called as their diameter (10 nm) is intermediate between that of microfilaments (5 nm) and of microtubules (25 nm):
- Intermediate filaments are formed from fibrous protein which varies with different tissue type.
- They play role in cell-to-cell attachment and help to stabilize the epithelium. They provide strength and rigidity to axons.
Functions of Cytoskeleton
- The cytoskeleton gives cells their characteristic shape and form, provides attachment points for organelles, fixing their location in cells and also makes communication between parts of the cell possible.
- It is also responsible for the separation of chromosomes during cell division.
- The internal movement of the cell organelles as well as cell locomotion and muscle fiber contraction could not take place without the cytoskeleton. It acts as “track” on which cells can move organelles, chromosomes and other things.
MEMBRANE TRANSPORT
- One of the functions of the plasma membrane is to regulate the passage of a variety of small molecules across it.
- Biological membranes are semipermeable membranes through which certain molecules freely diffuse across membranes but the movement of the others is restricted because of size, charge or solubility.
- Transport mechanism through cell membrane can be broadly divided into three types (Figure 1.6):
- Passive transport
- Active transport
- Vesicular transport
Passive Transport
In passive transports, the substances pass through the membrane from both sides. The direction of transport of 6molecule is always from a region of higher concentration to lower concentration. It does not require energy in the form of ATP. There are three types of passive transport as follows:
- Simple diffusion
- Facilitated diffusion
- Osmosis
Simple Diffusion
- In simple diffusion, lipid soluble, i.e., lipophilic molecules can pass through cell membrane, through the interstices of the lipid bilayer. Such molecules will pass through membrane along the concentration gradient, i.e., from a region of higher concentration to one of lower concentration.
- For example, oxygen, nitrogen, carbon dioxide and alcohols are lipid soluble, so all these can dissolve directly in lipid bilayer and diffuse through the cell membrane. The rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility (Figure 1.7).
Facilitated Diffusion
- In facilitated diffusion the movement of water soluble molecules and ions across the membrane requires specific transport system. They pass through specific carrier proteins. A carrier protein binds to a specific molecule on one side of the membrane and releases it on the other side. This type of crossing the membrane is called facilitated diffusion or carrier-mediated diffusion.
- An example of facilitated diffusion is the movement of glucose and most of the amino acids across the plasma membrane.
- These diffusion processes are not coupled to the movement of other ions, they are known as uniport transport processes (Figure 1.7).
Osmosis
Osmosis is the process of movement of water (solvent) from the solution (solute + solvent) with the lower concentration of solutes to the solution with higher concentration of solute, when both the solution are separated by a semipermeable (permeable to solvent but not the solute) membrane.
Active Transport
- When a cell membrane transports molecules or ions uphill (towards high concentration) against a concentration gradient or uphill against an electrical or pressure gradient, an external energy source is required; this movement is referred to as active transport.
- Substances that are actively transported through cell membranes include, Na+, K+, Ca++, H+, CI−, several different sugars and most of the amino acids.
- Active transport depends on the carrier proteins; like facilitated diffusion. However, in active transport, the carrier proteins function differently from the carrier in facilitated diffusion. Carrier protein for active transport is capable of transporting substance against the concentration gradient.
- In active transport, the energy is derived from hydrolysis of ATP.
- Active transport of Na+ and K+, pumps Na+ ions out of the cell and at the same time K+ ions pumps from outside to the inside generating an electrochemical gradient.
- The energy liberated by the hydrolysis of ATP leads to conformational change in the carrier protein molecule, extruding the three Na+ ions to the outside and the two K+ ions to the inside (Figure 1.8).
Vesicular Transport
- Vesicular transport is special for macromolecules. Macromolecules cannot be transported by diffusion or active transport process. Therefore they are transferred across the cell membrane mainly by vesicular transport. Amino acids, sugars, waste products of metabolism, cellular secretions, hormones, neurotransmitters and organisms are transported by this mechanism.
- The process by which cells take up large molecules is called endocytosis and the process by which cells release large molecules from the cells to the outside is called exocytosis.
- Fusion of vesicle with the cell membrane occurs in exocytosis and formation of vesicle from cell membrane occurs in endocytosis.
- In vesicular transport, formation and transport of vesicles are facilitated by some vesicular transport proteins. These proteins are calthrin, coating proteins, dynamin and docking proteins.
Endocytosis
Endocytosis is the process of transport in which a substance is taken into the cell by means of vesicle formation. It is the only process by which most macromolecules, such as most proteins, polysaccharides and polynucleotides can enter cells. Endocytosis occurs by two mechanisms: Constitutive and Clathrin-mediated.
Constitutive Endocytosis
Endocytosis by constitutive pathway occurs in almost all cells. It is called “constitutive”, as the process occurs continually and does not require any specific stimulus. The molecule or substance makes contact with the cell membrane that invaginates to form an endocytic vesicle. The non-cytoplasmic side of the membrane then fuses and the vesicle is pinched-off into the cytosol (Figure 1.9).
Clathrin-Mediated
- Clathrin-mediated endocytosis occurs at the specific site of the cell membrane. Clathrin is fibriller protein located in the cell membrane beneath the receptor protein. Clathrin-mediated endocytosis internalizes various organisms, growth factors and lipoproteins (Figure 1.10).
- These molecules first attach to specific receptors on the surface of the membrane.
- The receptors are generally concentrated in small pits on the outer surface of the cell membrane. These receptors are coated on the cytoplasmic side with a fibrillar protein called clathrin and contractile filaments of actin and myosin.
- Once the macromolecules (which are to be absorbed) have bound with the receptors, the entire pit invaginates inward, and the fibrillar protein by surrounding the invaginating pit causes it to close over the attached macromolecule along with a small amount of extracellular fluid.
Digestion Endocyte Vesicles
- Immediately after a endocytotic vesicle appears inside a cell, one or more lysosomes become attached to the vesicle and empty their acid hydrolases to the inside of the vesicles.
- The macromolecules present in vesicle are digested to yield amino acids, simple sugars or nucleotides that can diffuse through the membrane of the vesicle into the cytoplasm and reused by the cell.
- What is left of the digestive vesicle, called the residual body, represent indigestible substances. In most instances, this is finally excreted through the cell membrane by a process called exocytosis, which is opposite of endocytosis.
Exocytosis
Exocytosis is the release of macromolecules from cells to the exterior, which is reverse of endocytosis. By exocytosis, hormones, neurotransmitters, digestive enzymes and undigested foreign particles are released from cells.
- The undigestible substances produced within the cytoplasm may be enclosed in membranes to form vesicles called exocytic vesicles.
- These cytoplasmic exocytic vesicles fuse with the internal surface of the plasma membrane.
- The vesicle then ruptures releasing their contents into the extracellular space and their membranes are retrieved (left behind) and reused (Figure 1.11).
1. Diagrammatic representation of cell with functions of the subcellular organelles.
2. Give structure and function of:
- Mitochondria
- Endoplasmic reticulum
- Golgi apparatus
- Plasma membrane
- Nucleolus
- Lysosomes
- Peroxisomes
1. The following is the metabolic function of ER:
- RNA processing
- Fatty acid oxidation
- Synthesis of plasma protein
- ATP-synthesis
2. In biologic membranes, integral proteins and lipids interact mainly by:
- Covalent bond
- Both hydrophobic and covalent bond
- Hydrogen and electrostatic bond
- None of the above
3. Plasma membrane is:
- Composed entirely of lipids
- Mainly made up of proteins
- Mainly made up of lipid and protein
- Composed of only carbohydrates and lipids
4. Select the subcellular component involved in the formation of ATP:
- Nucleus
- Plasma membrane
- Mitochondria
- Golgi apparatus
- Maternal inherited
- Paternal inherited
- Maternal and paternal inherited
- All of the above
6. All of the following statements about the nucleus are true, except:
- Outer nuclear membrane is connected to ER
- It is the site of storage of genetic material
- Nucleolus is surrounded by a bilayer membrane
- Outer and inner membranes of nucleus are connected at nuclear pores
7. Golgi apparatus is present in all of the following, except:
- RBC
- Parenchymal cells
- Skeletal muscle cells
- Pancreatic cell
8. Peroxisomes arise from:
- Golgi membrane
- Lysosomes
- Mitochondria
- Pre-existing peroxisomes and budding off from the smooth ER
9. Exocytosis:
- Is always employed by cells for secretion
- Is used to deliver material into the extracellular space
- Take up large molecules from the extracellular space
- Allows the salvage of elements of the plasma membrane
10. The cytoskeleton includes all of the following, except:
- Microtubules
- Intermediate filaments
- Myosin filaments
- Actin filaments
11. Ribosomes are found:
- Only in the nucleus
- In the cytoplasm
- Attached to the smooth endoplasmic reticulum
- Both b and c
12. The Golgi apparatus is involved in:
- Packaging proteins into vesicles
- Altering or modifying proteins
- Producing lysosomes
- All of the above
13. All of the following are functions of the cell membrane, except:
- Participating in chemical reactions
- Participating in energy transfer
- Being freely permeable to all substances
- Regulating the passage of materials
14. Lysosomes are produced by the:
- Nucleus
- Mitochondria
- Golgi apparatus
- Ribosomes
1. c | 2. c | 3. c | 4.c | 5. a | 6. c | 7. a | 8. d |
9. b | 10. c | 11. d | 12. d | 13. c | 14. c |