Cell A&P PDF

# **Cell** All the living things are composed of cells. A single cell is the smallest unit that has all the characteristics of life. Cell is defined as the structural and functional unit of the living body. ## **General Characteristics of Cell** Each cell in the body: 1. Needs nutrition and oxygen 2. Produces its own energy necessary for its growth, repair and other activities 3. Eliminates carbon dioxide and other metabolic wastes 4. Maintains the medium, i.e. the environment for its survival. Shows immediate response to the entry of invaders like bacteria or toxic substances into the body 6. Reproduces by division. There are some exceptions like neurons, which do not reproduce ## **Structure of the Cell** Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane. Cell body has two parts, namely nucleus and cytoplasm surrounding the nucleus. **Structure of the Cell** | Structure | Description | | ---------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **Secretory vesicle** | Small organelles that store and transport cellular products, often proteins or hormones. | | **Lysosome** | Membrane-bound organelles that contain enzymes to break down various cellular waste products. | | **Rough endoplasmic reticulum** | Network of interconnected membranous sacs studded with ribosomes. Involved in protein synthesis, folding, and modification. | | **Ribosomes** | Small, non-membranous organelles responsible for protein synthesis. Can be found free in the cytoplasm or attached to the rough endoplasmic reticulum. | | **Mitochondrion** | Powerhouse of the cell, responsible for generating ATP (cellular energy) through cellular respiration. Contains its own DNA and ribosomes. | | **Cell Membrane** | Thin, flexible barrier that encloses the cell, regulating the passage of molecules in and out of the cell. Composed mainly of phospholipids, proteins, and carbohydrates. | | **Microfilament** | Thin, solid fibers composed of the protein actin. Involved in cell movement, shape, and contraction. | | **Golgi apparatus** | Stack of flattened, membrane-bound sacs involved in processing, packaging, and sorting proteins and lipids for secretion or delivery to other organelles. | | **Nucleus** | Control center of the cell, containing the genetic material (DNA) and directing cellular activities. Surrounded by a double membrane called the nuclear envelope. | | **Nucleolus** | Dense region within the nucleus, responsible for the synthesis and assembly of ribosomes, which are essential for protein synthesis. | | **Centrioles** | Small, cylindrical structures found in pairs near the nucleus. Play a critical role in organizing microtubules during cell division. | | **Smooth endoplasmic reticulum** | Network of interconnected membranous sacs lacking ribosomes. Involved in lipid synthesis, detoxification, and calcium storage. | | **Microtubule** | Hollow, cylindrical structures made of the protein tubulin. Involved in maintaining cell shape, intracellular transport, and cell division. Form the spindle fibers during mitosis. | | **Cytoplasm** | The gel-like substance enclosed by the cell membrane, which houses the various organelles and is the site of many cellular processes. | ## **Cell Membrane** Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane or plasmalemma. This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF). ### **Composition of Cell Membrane** Cell membrane is composed of three types of substances: 1. Proteins (55%) 2. Lipids (40%) 3. Carbohydrates (5%). ### **Structure of Cell Membrane** **Lipid Layers of the Cell Membrane** The central lipid layer is a bilayered structure. This is formed by a thin film of lipids. The characteristic feature of the lipid layer is that it is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell. The materials dissolved in the lipid layer also move to all areas of the cell membrane. **Major lipids are:** 1. Phospholipids 2. Cholesterol **1. Phospholipids** Phospholipids are the lipid substances containing phosphorus and fatty acids. Aminophospholipids, sphingo-myelins, phosphatidylcholine, phosphatidyletholamine, phosphatidylglycerol, phosphatidylserine and phosphatidylinositol are the phospholipids present in the lipid layer of the cell membrane. Phospholipid molecules are arranged in two layers. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail portion. Head portion is the polar end and it is soluble in water and has strong affinity for water (hydrophilic). Tail portion is the non-polar end. It is insoluble in water and repelled by water (hydrophobic). Two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of the membrane. Hydrophilic head portions of the outer layer face the ECF and those of the inner layer face ICF (cytoplasm). **2. Cholesterol** Cholesterol molecules are arranged in between the phospholipid molecules. Phospholipids are soft and oily structures and cholesterol helps to 'pack' the phospholipids in the membrane. So, cholesterol is responsible for the structural integrity of the lipid layer of the cell membrane. ### **Functions of Lipid Layer in Cell Membrane** Lipid layer of the cell membrane is a semipermeable membrane and allows only the fat-soluble substances to pass through it. Thus, the fat-soluble substances like oxygen, carbon dioxide, and alcohol can pass through this lipid layer. The water-soluble substances such as glucose, urea, and electrolytes cannot pass through this layer. ### **Protein Layers of the Cell Membrane** Protein layers of the cell membrane are electron-dense layers. These layers cover the two surfaces of the central lipid layer. Protein layers give protection to the central lipid layer. The protein substances present in these layers are mostly glycoproteins. Protein molecules are classified into two categories: 1. Integral proteins or transmembrane proteins. 2. Peripheral proteins or peripheral membrane proteins. **1. Integral proteins** Integral or transmembrane proteins are the proteins that pass through the entire thickness of the cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane. Examples of integral protein: i. Cell adhesion proteins ii. Cell junction proteins iii. Some carrier (transport) proteins iv. Channel proteins v. Some hormone receptors vi. Antigens vii. Some enzymes. **2. Peripheral proteins** Peripheral proteins or peripheral membrane proteins are the proteins which are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane. Peripheral proteins are loosely bound with integral proteins or the lipid layer of the cell membrane. So, these protein molecules dissociate readily from the cell membrane. Examples of peripheral proteins: i. Proteins of cytoskeleton ii. Some carrier (transport) proteins iii. Some enzymes. ### **Functions of Proteins in Cell Membrane** 1. Integral proteins provide the structural integrity of the cell membrane 2. Channel proteins help in the diffusion of water-soluble substances like glucose and electrolytes 3. Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or passive transport 4. Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane 5. Receptor proteins serve as the receptor sites for hormones and neurotransmitters 6. Enzymes: Some of the protein molecules form the enzymes and control chemical (metabolic) reactions within the cell membrane 7. Antigens: Some proteins act as antigens and induce the process of antibody formation 8. Cell adhesion molecules or the integral proteins are responsible for attachment of cells to their neighbors or to the basal lamina. ### **Carbohydrates of the Cell Membrane** Some of the carbohydrate molecules present in the cell membrane are attached to proteins and form glycoproteins (proteoglycans). Some carbohydrate molecules are attached to lipids and form glycolipids. Carbohydrate molecules form a thin and loose covering over the entire surface of the cell membrane called glycocalyx. ### **Functions of Carbohydrates in Cell Membrane** 1. Carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell 2. Glycocalyx from the neighboring cells helps in the tight fixation of cells with one another 3. Some carbohydrate molecules function as the receptors for some hormones. ## **Functions of Cell Membrane** 1. Protective function 2. Selective permeability 3. Absorptive function 4. Excretory function 5. Exchange of gases 6. Maintenance of shape and size of the cell ## **Cytoplasm** Cytoplasm of the cell is the jelly-like material formed by 80% of water. It contains a clear liquid portion called cytosol and various particles of different shape and size. Cytoplasm is made up of two zones: 1. Ectoplasm: Peripheral part of the cytoplasm, situated just beneath the cell membrane 2. Endoplasm: Inner part of the cytoplasm, interposed between the ectoplasm and the nucleus. ## **Endoplasmic Reticulum** Endoplasmic reticulum is a network of tubular and microsomal vesicular structures which are interconnected with one another. It is covered by a limiting membrane which is formed by proteins and bilayered lipids. The lumen of the endoplasmic reticulum contains a fluid medium called endoplasmic matrix. ### **Types of Endoplasmic Reticulum** Endoplasmic reticulum is of two types, namely rough endoplasmic reticulum and smooth endoplasmic reticulum. ### **Rough Endoplasmic Reticulum** It is the endoplasmic reticulum with rough, bumpy or bead-like appearance. Rough appearance is due to the attachment of granular ribosomes to its outer surface. Hence, it is also called the granular endoplasmic reticulum. Rough endoplasmic reticulum is vesicular or tubular in structure. ### **Function of rough endoplasmic reticulum** 1. Synthesis of proteins 2. Degradation of wornout organelles ### **Smooth Endoplasmic Reticulum** It is the endoplasmic reticulum with smooth appearance. It is also called agranular reticulum. It is formed by many interconnected tubules. So, it is also called tubular endoplasmic reticulum. ### **Functions of Smooth Endoplasmic Reticulum** 1. Synthesis of lipids and steroids 2. Role in cellular metabolism 3. Storage and metabolism of calcium 4. Catabolism and detoxification of toxic substances ## **Golgi Apparatus** Golgi apparatus or Golgi body or Golgi complex is a membrane-bound organelle, involved in the processing of proteins. It is present in all the cells except red blood cells. ### **Functions of the Golgi Apparatus** 1. Processing, packaging, labeling, and delivery of proteins and lipids 2. Formation of lysosomes ## **Lysosomes** Lysosomes are the membrane-bound vesicular organelles found throughout the cytoplasm. The lysosomes are formed by the Golgi apparatus. ### **Types of Lysosomes** Lysosomes are of two types: 1. Primary lysosome, which is pinched off from the Golgi apparatus. It is inactive in spite of havinghydrolytic enzymes 2. Secondary lysosome, which is the active lysosome. It is formed by the fusion of a primary lysosome with a phagosome or an endosome ### **Functions of Lysosomes** 1. Degradation of macromolecules 2. Degradation of wornout organelles 3. Removal of excess of secretory products 4. Secretion of perforin, granzymes, melanin, and serotonin **Important lysosomal enzymes** 1. Proteases, which hydrolyze the proteins into amino acids ## **Mitochondrion** Mitochondrion (plural - mitochondria) is a membrane-bound cytoplasmic organelle concerned with the production of energy. It is a rod-shaped or oval-shaped structure with a diameter of 0.5 to 1 μ. ### **Functions of Mitochondria** 1. Production of energy 2. Synthesis of ATP 3. Initiation of apoptosis ## **Ribosomes** Ribosomes are the organelles without limiting membrane. These organelles are granular and small dot-like structures with a diameter of 15 nm. Ribosomes are made up of 35% of proteins and 65% of ribonucleic acid (RNA). RNA present in ribosomes is called ribosomal RNA (rRNA). Ribosomes are concerned with protein synthesis in the cell. ### **Types of Ribosomes** Ribosomes are of two types: i. Ribosomes that are attached to rough endoplasmic reticulum ii. Free ribosomes that are distributed in the cytoplasm. ### **Functions of Ribosomes** Ribosomes are called 'protein factories' because of their role in the synthesis of proteins. Messenger RNA (mRNA) carries the genetic code for protein synthesis from nucleus to the ribosomes. The ribosomes, in turn, arrange the amino acids into small units of proteins. Ribosomes attached to rough endoplasmic reticulum are involved in the synthesis of proteins such as the enzymatic proteins, hormonal proteins, lysosomal proteins, and the proteins of the cell membrane. Free ribosomes are responsible for the synthesis of proteins in hemoglobin, peroxisome, and mitochondria. ## **Cytoskeleton** Cytoskeleton is the cellular organelle present throughout the cytoplasm. It determines the shape of the cell and gives support to the cell. It is a complex network of structures with varying sizes. In addition to determining the shape of the cell, it is also essential for the cellular movements and the response of the cell to external stimuli. Cytoskeleton consists of three major protein components: 1. Microtubule 2. Intermediate filaments 3. Microfilaments. ### **Microtubules** Microtubules are the straight, hollow and tubular structures of the cytoskeleton. These organelles without the limiting membrane are arranged in different bundles. Each tubule has a diameter of 20 to 30 nm. The length of the microtubule varies and it can be 1000 times more than the thickness. Structurally, the microtubules are formed by bundles of globular protein called tubulin. Tubulin has two subunits, namely αsubunit and βsubunit. ### **Functions of microtubules** Microtubules may function alone or join with other proteins to form more complex structures like cilia, flagella or centrioles and perform various functions. Microtubules: i. Determine the shape of the cell ii. Give structural strength to the cell iii. Act like conveyer belts which allow the movement of granules, vesicles, protein molecules, and some organelles like mitochondria to different parts of the cell iv. Form the spindle fibers which separate the chromosomes during mitosis v. Are responsible for the movement of centrioles and the complex cellular structures like cilia. ### **Intermediate Filaments** Intermediate filaments are the structures that form a network around the nucleus and extend to the periphery of the cell. The diameter of each filament is about 10 nm. The intermediate filaments are formed by rope-like polymers, which are made up of fibrous proteins. Intermediate filaments are divided into five subclasses: i. Keratins (in epithelial cells) ii. Glial filaments (in astrocytes) iii. Neurofilaments (in nerve cells) iv. Vimentin (in many types of cells) v. Desmin (in muscle fibers). ### **Functions of Intermediate Filaments** Intermediate filaments help to maintain the shape of the cell. These filaments also connect the adjacent cells through desmosomes. ### **Microfilaments** Microfilaments are long and thin threadlike structures with a diameter of about 3 to 6 nm. These filaments are made up of non-tubular contractile proteins called actin and myosin. Actin is more abundant than myosin. Microfilaments are present throughout the cytoplasm. The microfilaments present in the ectoplasm contain only actin molecules and those present in the endoplasm contain both actin and myosin molecules. ### **Functions of Microfilaments** Microfilaments: i. Give structural strength to the cell ii. Provide resistance to the cell against the pulling forces iii. Are responsible for cellular movements like contraction, gliding, and cytokinesis (partition of the cytoplasm during cell division). ## **Nucleus** The nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 µ to 22 µ and occupies about 10% of the total volume of the cell. The nucleus is present in all the cells in the body except the red blood cells. The cells with a nucleus are called eukaryotes and those without a nucleus are known as prokaryotes. The presence of a nucleus is necessary for cell division. Most of the cells have only one nucleus (uninucleated cells). Few types of cells like skeletal muscle cells have many nuclei (multinucleated cells). Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of the nucleus vary in some cells. ### **Structure of Nucleus** The nucleus is covered by a membrane called the nuclear membrane and contains many components. The major components of the nucleus are nucleoplasm, chromatin, and the nucleolus. ### **Nuclear Membrane** The nuclear membrane is double-layered and porous in nature. This allows nucleoplasm to communicate with the cytoplasm. The outer layer of the nuclear membrane is continuous with the membrane of the endoplasmic reticulum. The space between the two layers of the nuclear membrane is continuous with the lumen of the endoplasmic reticulum. Pores of the nuclear membrane are guarded by protein molecules. The diameter of the pores is about 80 to 100 nm. However, it is decreased to about 7 to 9 nm because of the attachment of protein molecules with the periphery of the pores. Exchange of materials between nucleoplasm and cytoplasm occurs through these pores. ### **Nucleoplasm** Nucleoplasm is a highly viscous fluid that forms the ground substance of the nucleus. It is similar to cytoplasm present outside the nucleus. Nucleoplasm surrounds chromatin and nucleolus. It contains dense fibrillar networks of proteins called the nuclear matrix and many substances such as nucleotides and enzymes. The nuclear matrix forms the structural framework for organizing chromatin. The soluble liquid part of the nucleoplasm is known as nuclear hyaloplasm. ### **Chromatin** Chromatin is a thread-like material made up of large molecules of DNA. The DNA molecules are compactly packed with the help of a specialized basic protein called histone. So, chromatin is referred to as DNA-histone complex. It forms the major bulk of nuclear material. DNA is a double helix which wraps around the central core of eight histone molecules to form the fundamental packing unit of chromatin called a nucleosome. Nucleosomes are packed together tightly with the help of a histone molecule to form a chromatin fiber. Just before cell division, the chromatin condenses to form a chromosome. ### **Chromosomes** A chromosome is the rod-shaped nuclear structure that carries a complete blueprint of all the hereditary characteristics of that species. A chromosome is formed from a single DNA molecule coiled around histone molecules. Each DNA contains many genes. Normally, the chromosomes are not visible in the nucleus under a microscope. Only during cell division, the chromosomes are visible under a microscope. This is because DNA becomes more tightly packed just before cell division, which makes the chromosome visible during cell division. All the dividing cells of the body except reproductive cells contain 23 pairs of chromosomes. Each pair consists of one chromosome inherited from the mother and one from the father. The cells with 23 pairs of chromosomes are called diploid cells. The reproductive cells called gametes or sex cells contain only 23 single chromosomes. These cells are called haploid cells. ### **Nucleolus** The nucleolus is a small, round granular structure of the nucleus. Each nucleus contains one, or more, nucleoli. The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. The RNA is synthesized by five different pairs of chromosomes and stored in the nucleolus. Later, it is condensed to form the subunits of ribosomes. All the subunits formed in the nucleolus are transported to the cytoplasm through the pores of the nuclear membrane. In the cytoplasm, these subunits fuse to form ribosomes, which play an essential role in the formation of proteins. ## **Functions of Nucleus** The major functions of the nucleus are the control of cellular activities and storage of hereditary material. Several processes are involved in the nuclear functions. **Functions of nucleus:** 1. Control of all the cell activities that include metabolism, protein synthesis, growth, and reproduction (cell division) 2. Synthesis of RNA 3. Formation of subunits of ribosomes 4. Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA) 5. Control of the cell division through genes 6. Storage of hereditary information (in genes) and transformation of this information from one generation of the species to the next. ## **Simple Diffusion Through Protein Layer** The protein layer of the cell membrane is permeable to water-soluble substances. Mainly, electrolytes diffuse through the protein layer. ### **Protein Channels or Ion Channels** Throughout the central lipid layer of the cell membrane, there are some pores. Integral protein molecules of the protein layer invaginate into these pores from either surface of the cell membrane. Thus, the pores present in the central lipid layer are entirely lined up by the integral protein molecules. These pores are the hypothetical pores and form the channels for the diffusion of water, electrolytes, and other substances, which cannot pass through the lipid layer. As the channels are lined by protein molecules, these are called protein channels for water-soluble substances. ### **Types of Protein Channels or Ion Channels** The characteristic feature of the protein channels is the selective permeability. That is, each channel can permit only one type of ion to pass through it. Accordingly, the channels are named after the ions which diffuse through these channels such as the sodium channels, potassium channels, etc. ### **Regulation of the Channels** Some of the protein channels are continuously opened, and most of the channels are always closed. Continuously opened channels are called ungated channels. Closed channels are called gated channels. These channels are opened only when required ### **Gated Channels** Gated channels are divided into three categories: i. Voltage-gated channels ii. Ligand-gated channels iii. Mechanically gated channels. **i. Voltage-gated channels** Voltage-gated channels are the channels, which open whenever there is a change in the electrical potential. For example, in the neuromuscular junction, when the action potential reaches the axon terminal, the calcium channels are opened and calcium ions diffuse into the interior of the axon terminal from ECF. Similarly, in the muscle during the excitation-contraction coupling, the action potential spreads through the transverse tubules of the sarcotubular system. When the action potential reaches the cisternae, a large number of calcium ions diffuses from the cisternae into the sarcoplasm. **ii. Ligand-gated channels** Ligand-gated channels are the type of channels which open in the presence of some hormonal substances. The hormonal substances are called ligands and the channels are called ligand-gated channels. During the transmission of impulse through the neuromuscular junction, acetylcholine is released from the vesicles. The acetylcholine moves through the presynaptic Membrane (membrane of the axon terminal) and reaches the synaptic cleft. Then, the acetylcholine molecules cause opening of the sodium channels in the postsynaptic membrane and sodium ions diffuse into the neuromuscular junction from ECF. **iii. Mechanically gated channels** Mechanically gated channels are the channels which are opened by some mechanical factors. Examples are, channels present in the pressure receptors (Pacinian corpuscles) and the receptor cells (hair cells) of the organ of Corti and vestibular apparatus. When a Pacinian corpuscle is subjected to pressure, it is compressed, resulting in deformation of its core fiber. This deformation causes opening of the sodium channel and development of the receptor potential. ## **Facilitated or Carrier-Mediated Diffusion** Facilitated or carrier-mediated diffusion is the type of diffusion by which the water-soluble substances having larger molecules are transported through the cell membrane with the help of a carrier protein. By this process, the substances are transported across the cell membrane faster than the transport by simple diffusion. Glucose and amino acids are transported by facilitated diffusion. Glucose or amino acid molecules cannot diffuse through the channels because the diameter of these molecules is larger than the diameter of the channels. The molecule of these substances binds with the carrier protein. Now, some conformational change occurs in the carrier protein. Due to this change, the molecule reaches the other side of the cell membrane. ## **Special Types of Passive Transport** In addition to diffusion, there are some special types of passive transport, viz. 1. Bulk flow 2. Filtration 3. Osmosis ### **Bulk Flow** Bulk flow is the diffusion of large quantities of substances from a region of high pressure to the region of low pressure. It is due to the pressure gradient of the substance across the cell membrane. The best example for bulk flow is the exchange of gases across the respiratory membrane in lungs. Partial pressure of oxygen is greater in the alveolar air than in the alveolar capillary blood. So, oxygen moves from alveolar air into the blood through the respiratory membrane. ### **Osmosis** Osmosis is the special type of diffusion. It is defined as the movement of water or any other solvent from an area of lower concentration to an area of higher concentration of a solute, through a semipermeable membrane. The semipermeable membrane permits the passage of only water or other solvents, but not the solutes. ## **Active Transport** Active transport is the movement of substances against the chemical or electrical or electrochemical gradient. It is like swimming against the water tide in a river. It is also called uphill transport. Active transport requires energy, which is obtained mainly by the breakdown of high-energy compounds like adenosine triphosphate (ATP). ### **Active Transport vs Facilitated Diffusion** The active transport mechanism is different from facilitated diffusion in two ways: 1. The carrier protein of active transport needs energy, whereas the carrier protein of facilitated diffusion does not need energy 2. In active transport, the substances are transported against the concentration, or electrical, or electrochemical gradient. In facilitated diffusion, the substances are transported along the concentration or electrical or electrochemical gradient. ### **Carrier Proteins of Active Transport** Carrier proteins involved in active transport are of two types: 1. Uniport 2. Symport or antiport. **1. Uniport** The carrier protein that carries only one substance in a single direction is called uniport. It is also known as a uniport pump. **2. Symport or Antiport** Symport or antiport is the carrier protein that transports two substances at a time. The carrier protein that transports two different substances in the same direction is called symport or symport pump. The carrier protein that transports two different substances in opposite directions is called antiport or antiport pump. ### **Mechanism of Active Transport** When a substance to be transported across the cell membrane comes near the cell, it combines with the carrier protein of the cell membrane and forms a substance-protein complex. This complex moves towards the inner surface of the cell membrane. Now, the substance is released from the carrier proteins. The same carrier protein moves back to the outer surface of the cell membrane to transport another molecule of the substance. ### **Substances Transported by Active Transport** Substances, which are transported actively, are in ionic form and non-ionic form. Substances in ionic form are sodium, potassium, calcium, hydrogen, chloride, and iodide. Substances in non-ionic form are glucose, amino acids, and urea. ### **Types of Active Transport** Active transport is of two types: 1. Primary active transport 2. Secondary active transport. ### **Primary Active Transport** Primary active transport is the type of transport mechanism in which the energy is liberated directly from the breakdown of ATP. By this method, the substances like sodium, potassium, calcium, hydrogen, and chloride are transported across the cell membrane. ### **Primary Active Transport of Sodium and Potassium: Sodium-Potassium Pump** Sodium and potassium ions are transported across the cell membrane by means of a common carrier protein called the sodium-potassium (Na+-K+) pump. It is also called the Na+-K+ ATPase pump or Na+-K+ ATPase. This pump transports sodium from inside to outside the cell and potassium from outside to inside the cell. This pump is present in all the cells of the body. The Na+-K+ pump is responsible for the distribution of sodium and potassium ions across the cell membrane and the development of resting membrane potential. ### **Structure of Na+-K+ Pump** The carrier protein that constitutes the Na+-K+ pump is made up of two protein subunit molecules, an α-subunit with a molecular weight of 100,000 and a β-subunit with a molecular weight of 55,000. Transport of Na+ and K occurs only by the α-subunit. The β-subunit is a glycoprotein, the function of which is not clear. The α-subunit of the Na+-K+ pump has got six sites: i. Three receptor sites for sodium ions on the inner (towards cytoplasm) surface of the protein molecule ii. Two receptor sites for potassium ions on the outer (towards ECF) surface of the protein molecule iii. One site for enzyme adenosine triphosphatase (ATPase), which is near the sites for sodium. ### **Mechanism of action of Na+-K+ Pump** Three sodium ions from the cell get attached to the receptor sites of sodium ions on the inner surface of the carrier protein. Two potassium ions outside the cell bind to the receptor sites of potassium ions located on the outer surface of the carrier protein. Binding of sodium and potassium ions to the carrier protein activates the enzyme ATPase. ATPase causes breakdown of ATP into adenosine diphosphate (ADP) with the release of one high energy phosphate. Now, the energy liberated causes some sort of conformational change in the molecule of the carrier protein. Because of this, the outer surface of the molecule (with potassium ions) now faces the inner side of the cell. And, the inner surface of the protein molecule (with sodium ions) faces the outer side of the cell. Now, dissociation and release of the ions take place so that the sodium ions are released outside the cell (ECF) and the potassium ions are released inside the cell (ICF). The exact mechanisms involved in the dissociation and release of ions are not yet known. ### **Electrogenic activity of Na+-K+ Pump** The Na+-K+ pump moves three sodium ions outside the cell and two potassium ions inside the cell. Thus, when the pump works once, there is a net loss of one positively charged ion from the cell. Continuous activity of the sodium-potassium pumps causes reduction in the number of positively charged ions inside the cell, leading to an increase in the negativity inside the cell. This is called the electrogenic activity of the Na+-K+ pump. ### **Transport of Calcium Ions** Calcium is actively transported from inside to outside the cell by a calcium pump. The calcium pump is operated by a separate carrier protein. Energy is obtained from ATP. ### **Transport of Hydrogen Ions** Hydrogen ion is actively transported across the cell membrane by the carrier protein called hydrogen pump. It also obtains energy from ATP by the activity of ATPase. The hydrogen pumps that are present in two important organs have some functional significance. 1. **Stomach:** Hydrogen pumps in the parietal cells of the gastric glands are involved in the formation of hydrochloric acid. 2. **Kidney:** Hydrogen pumps in the epithelial cells of the distal convoluted tubules and collecting ducts are involved in the secretion of hydrogen ions from blood into urine. ### **Secondary Active Transport** Secondary active transport is the transport of a substance with sodium ion, by means of a common carrier protein. When sodium is transported by a carrier protein, another substance is also transported by the same protein simultaneously, either in the same direction (of sodium movement) or in the opposite direction. Thus, the transport of sodium is coupled with transport of another substance. Secondary active transport is of two types: 1. Cotransport 2. Counter transport. ### **Sodium Cotransport** Sodium cotransport is the process in which, along with sodium, another substance is transported by a carrier protein called symport. Energy for movement of sodium is obtained by breakdown of ATP, and the energy released by the movement of sodium is utilized for movement of another substance. Substances carried by sodium cotransport are glucose, amino acids, chloride, iodine, iron, and urate. ### **Carrier protein for sodium cotransport** The carrier protein for the sodium cotransport has two receptor sites on the outer surface. Among the two sites, one is for binding of sodium and another site is for binding of the other substance. ### **Sodium cotransport of glucose** One sodium ion and one glucose molecule from the ECF bind with the respective receptor sites of the carrier protein of the cell membrane. Now, the carrier protein is activated. It causes conformational changes in the carrier protein, so that sodium and glucose are released into the cell. The sodium cotransport of glucose occurs during absorption of glucose from the intestine and reabsorption of glucose from the renal tubule. ### **Sodium cotransport of amino acids** The carrier proteins for the transport of amino acids are different from the carrier proteins for the transport of glucose. For the transport of amino acids, there are five sets of carrier proteins in the cell membrane. Each one carries different amino acids depending upon the molecular weight of the amino acids. Sodium cotransport of amino acids also occurs during the absorption of amino acids from the intestine and reabsorption from the renal tubule. ### **Sodium Counter Transport** Sodium counter transport is the process by which the substances are transported across the cell membrane in exchange for sodium ions by a carrier protein called antiport. Various counter transport systems are: i. **Sodium-calcium counter transport:** In this, sodium and calcium ions move in opposite directions with the help of a carrier protein. This type of transport of sodium and calcium ions is present in all the cells. ii. **Sodium-hydrogen counter transport:** In this system, the hydrogen ions are exchanged for sodium ions and this occurs in the renal tubular cells. The sodium ions move from the tubular lumen into the tubular cells and the hydrogen ions move from the tubular cell into the lumen. iii. **Other counter transport systems:** Other counter transport systems are sodium-magnesium counter-transport, sodium-potassium counter transport, calcium-magnesium counter transport, calcium-potassium counter transport, chloride bicarbonate counter transport, and chloride sulfate counter transport.

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