Cell Structure and Division PDF

CELL STRUCTURE AND CELL DIVISION NOTES BY, MRS. NISHA KEFIYA ASST. LECTURER DEPT. OF ANATOMY AND PHYSIOLOGY  INTRODUCTION Cell is the fundamental structural and functional unit...

CELL STRUCTURE AND CELL DIVISION NOTES BY, MRS. NISHA KEFIYA ASST. LECTURER DEPT. OF ANATOMY AND PHYSIOLOGY  INTRODUCTION Cell is the fundamental structural and functional unit of the living matter and is capable of carrying on the processes of the life independently. The main function of these cells is to maintain a proper homeostasis in the organism. Functions of cell : They provide structure for the body. Takes nutrients from food , convert those nutrients into energy , and carry out specialized functions. Division of cell : Cell can be divided as prokaryotic and eukaryotic kingdom.  Cell Prokaryotic Eukaryotic Plants Animals Structure of cell : This Photo by Unknown Author is licensed under CC BY This Photo by Unknown Author is licensed under CC BY-NC-ND A cell consists of : Cell membrane Nucleus Cytoplasm The nucleus is separated from the cytoplasm by a nuclear membrane and the cytoplasm is separated from the surrounding by a cell membrane. Cell membrane : cell membrane is also known as plasma membrane is the outer covering of the cell. The membrane isolates the individual cell and takes part in the maintenance of the internal environment by active transport of ions and nutrients. Due to high permeability properties and flexibility , under the electron microscope the membrane is thin , invisible and some time folded to form a brush border. The average thickness is about 7.5nm. This Photo by Unknown Author is licensed under CC BY-SA This Photo by Unknown Author is licensed under CC BY-NC Composition of cell membrane : Li 50-55% pi d Pro 40-45% tein s Carbohydrat 5-10% es Lipids in cell membrane : the membrane that surrounds the cell consists of a phospholipids bilayer, a double layer of phospholipids molecules. Interspersed within and embedded in the phospholipids bilayer of the cell membrane are the integral membrane proteins and peripheral membrane protein , which make up almost half of the total mass of the membrane. It has two ends. This Photo by Unknown Author is licensed under CC BY 1. Polar / hydrophilic (head end) : water – soluble and positively charged phosphate group. 2. Non – polar /hydrophobic (tail end ) water insoluble and negatively charged lipid group. Phospholipid molecules of the lipid layer are arranged in two rows where as the hydrophobic ends line up the side by side in same row but abutting on the hydrophobic ends. Thus the non polar groups of lipid molecules face each other but the protein molecule that form the inner and outer layer of the unit membrane are absorbed on the polar group. This Photo by Unknown Author is licensed under CC BY-NC Functions of lipid layer : The lipid layer is semi permeable in nature and allows only the fat soluble substances , such as oxygen , carbon dioxide and alcohol to pass through it. It does not allow the water - soluble materials , such as glucose , urea and electrolytes to pass through it. Proteins in cell membrane : the proteins are present at different locations in the membrane as : I. Integral membrane proteins : traversing the membrane , often forming transmembrane channels providing passage to the hydrophilic molecules. These molecules include water , ions , nutrients such as monosaccharides, amino acids etc. II. Outer or inner membrane proteins : it is embedded in respective membrane layer – forming anchor for receptors (on the outside) or intracellular signaling molecules ( on the outside ). III. Peripheral proteins : located outside the lipid bilayer but attached to the polar heads. On the inside of the cell these maintain the connections with the intracellular cytoskeleton and on the outside with the extracellular matrix. Cholesterol provides lipid density and fluidity to the membrane. Carbohydrates in cell membrane : Carbohydrates along with other glycoproteins and glycolipids form the cell surface receptor enabling communication between outside and inside of the cell. The messages of the neurotransmitters and protein hormones such as insulin , oxytocin , glucagon etc. are received on the outside by the receptors and conveyed to the inside of the cell. The blood group antigens are constituted of glycoproteins. Membrane transport : the cell membrane is selectively permeable for the transport of molecules. Only gases or steroid hormones can pass through the membrane unhindered , rest all the molecules require specific mechanisms. Endocytosis : in endocytosis , a large molecule enters the cell by the invagination of the plasma membrane which gradually grows around the molecule and enters the cell by forming an endocytic vesicle. Endocytosis is often receptor mediated and is guided by clathrin molecules present on the inner side of the membrane. Phagocytosis : is a type of endocytosis in which phagosome is formed instead of endocytic vesicle and large molecules usually cells like bacteria or other cells are engulfed inside. Usually phagocytosis is undertaken by immune cells that help fight infection , cancer or aid in apoptosis.  Pinocytosis : in pinocytosis , uptake of fluids or other smaller molecules is involved via the formation of pinocytic vesicles. The export from the cell is mediated via secretary or exocytic vesicles most often originating from Golgi complex and fusing with the plasma membrane. The process is thus called exocytosis or reverse pinocytosis. This Photo by Unknown Author is licensed under CC BY The transport channels formed by the transmembrane proteins are involved in passive transport or active transport. This Photo by Unknown Author is licensed under CC BY-NC  Passive transport : it involves movement of ingredients down the concentration gradient not requiring external energy. It is further of two types – diffusion (free movement across the membrane like gases ) and - Facilitated diffusion ( via a trans-membrane protein channel for example water ). Active transport : it involves movement of ingredients against the concentration gradient driven by energy released by ATP hydrolysis. Eg, glucose , amino acids and electrolytes (sodium-potassium pump ). Nucleus : is the most easily spotted , membrane bound organelle and a characteristic feature of a eukaryotic cell. It houses the cell’s genetic material – DNA condensed in the form of chromosomes. Chromosomes : can be defined as the thread like structure in which one double – helical DNA molecule is packaged along with proteins.  A typical chromosome has centromere at the center with the terminal ends of the rest of the DNA molecule called as telomeres. During cell division, a large proteinaceous structure forms around the centromere known as kinetochore. It attaches the chromosome to the mitotic spindle fibers. Histones are basic proteins that form a nucleosome chore around which the DNA molecule is wrapped and further supercoiled a number of times to form a final chromatin fiber.  DNA replication at the time of cell division takes place inside the nucleus. RNA synthesis or transcription from the active genes also takes place within the nucleus and the RNA molecules transported to the cytoplasm. A spherical specialized structure within the nucleus free from chromosomes constituting of RNA is termed as nucleolus. A nucleolus essentially harbours ribosomal machinery synthesizing and shuttling ribosome and ribosomal RNAs to the cytoplasm.  Nucleoplasm is the fluid in which the DNA , RNA, nucleoli , associated proteins and enzymes of related pathways are suspended. The whole of the nucleoplasm is held together as a cellular compartment separate from cytoplasm by a double layered membrane called nuclear membrane. The membrane has nuclear pores that allow the communication and transport of molecules between nucleus and cytoplasm. Cytoplasm It is the gel like , clear coloured solution made up of proteins ,ions , enzymes and organelles all surrounded by plasma membrane. Each cytoplasm contains numerous organelles , each of which performs a specialized metabolic function that is essential for maintaining cellular homeostasis and cell life such as mitochondria , endoplasmic reticulum , Golgi apparatus or Golgi complex , lysosomes , ribosomes , centrioles , centrosomes and peroxisomes.  Intermediated filaments : are hollow tubes that connect the nuclear and plasma membrane , provide the tracts for the movement of chromosomes , mitochondria and other secretary granules. Microfilaments : are contractile proteins present in the periphery of cell , provide cell its shape and aid in cellular motion. Microtubules are larger proteins providing rigidity to the cell. Centrosomes : is an area in the cytoplasm which is near the nucleus. This Photo by Unknown Author is licensed under CC BY  It houses a pair of centrioles which are important for cell division. The centrioles are barrel – shaped organelles made up of microtubules. During the mitosis , centrioles form the region from where the mitotic spindle arises. It is the centrioles that move to the opposite poles and the two chromosomes are also pulled in their direction. It is the location of the centriole after a telophase in the dividing cell that defines the nuclear region of the future daughter cells.  Endoplasmic reticulum : This Photo by Unknown Author is licensed under CC BY  Endoplasmic reticulum (ER) is an interconnected membrane system in the cytoplasm in continuity with the nucleus connected through nuclear pore. It forms flattened sacs on the outside of the nucleus constituting more than half of the membranous content of a cell. The major role of ER is : Synthesis and movement of molecules within.  Types of endoplasmic reticulum : 1. Rough ER 2. Smooth ER 1. Rough ER : has ribosomes attached to the membranes providing anchor and seat for the protein synthesis. This gives it rough or granular appearance. The synthesized proteins are either secreted outside the cell secretary vesicles via Golgi apparatus or sent to other organelles such as lysosomes. 2. The smooth ER : is also attached to nuclear envelope consists of tubules and vesicles. The reticular pattern contributes to increased surface area. While it lacks ribosomes unlike rough ER , it consists of several enzymes. Function of smooth ER is : Synthesis of lipids , cholesterol , steroid hormone, calcium storage (in muscle cells) and detoxification (in liver cells). Differences between rough and smooth endoplasmic reticulum : Rough endoplasmic Smooth endoplasmic reticulum reticulum Has ribosomes Does not have ribosomes Found near cytoplasm Found near cell membrane Mainly composed of Mainly composed of cisternae tubules Function : protein synthesis Function : lipid synthesis Golgi complex or Golgi apparatus : is a membranous cytoplasmic organelle constituting of flattened stacks of various types of membranes. Some are called cisternae and other in the form of tubules and vesicles. Perinuclear in location , these are located right next to endoplasmic reticulum. The vesicles leaving ER enter the Golgi complex as cis-cisternae (cisternae nearest to ER ) and leave at the trans – cisternae ( cisternae farthest of ER).  Golgi bodies are the seats for processing of proteins and lipids (most commonly addition of carbohydrate moieties forming glycoproteins and glycolipids ) , packaging into vesicles and further responsible for transportation to respective destinations. The destinations include other cytoplasm or organelles (respective protein or enzymes ), lysosomes (lysosomal proteins) cell membranes (cell surface receptors like blood group antigens ) or outside of the cells(secretory proteins ).  Mitochondria Mitochondria are the oval or spherical cellular organelles that generate cell’s energy ATP. Present in almost all the cells with exception of mature red blood cells. The mitochondria possess two separate membrane systems (composed of proteins and phospholipids ) , multiply by fission and possess their own DNA.  The mitochondria is termed powerhouse of the cell. The outer membrane is relatively permeable for most of the molecules as has porin protein in it forming transmembrane channels. The intermembrane space between outer and inner membrane constitutes many proteins and ions either located there or moving in and out of the mitochondria.  The inner mitochondrial membrane is highly selective in its permeability. The inner membrane has dense infoldings towards the matrix side known as cristae increasing the surface area by manyfold. Most important and well-known function of the inner membrane is : Participation in Kreb’s cycle ATP generation (by oxidative phosphorylation ).  The protein unit synthesizing ATP is called as FOF1 ATPase complex.  The inside of the mitochondria is termed as mitochondrial matrix and is filled up with gel like fluid. It carries various enzymes , acts as calcium storage mitochondrial DNA and ribosomes. The matrix also plays a crucial role in initiation of programmed cell death or apoptosis.  Membrane bound vesicles Lipid bilayer bound small sacs involved in various functions of storage and transportation within the cell is termed as membrane bound vesicle. These originates either from the plasma membrane or the ER /Golgi complex. They contain enzymes , proteins , toxins , or other molecules.  Based on their function these are of four types: Transport vesicles : These transport molecules from one location to its desired destination. Eg , vesicles carrying newly synthesized and modified proteins from Golgi complex to other cellular locations. Secretary vesicles : These vesicles play role in secreting proteins and other substances outside the cell. The secretion is carried out by fusing the secretary vesicle with the plasma membrane. Eg., cell surface receptors , hormones or glycoproteins targeted for outside of the cell. Storage vesicles : These vesicles store particular molecules after synthesis until these are required. Eg , neurotransmitter acetylcholine. Upon receiving a signal multiple acetylcholine vesicles fuse with plasma membrane releasing substantial amount of the neurotransmitter into the synapse. Lysosomes : Vesicles that act as cell’s digestive system degrading both external polymers such as carbohydrates , lipids , nucleic acids and proteins and for autophagy (digesting cell’s worn-out molecules /organelles ) are termed lysosomes. Lysosomes also mediate phagocytosis of bacteria and help fight infection. Act as cell’s recycle bins, giving back the cell’s building blocks to the cytoplasm for reuse.  Lysosomes act at very low pH (around 4.5) which is maintained by pumping hydrogen ions into the lumen. The specific acid hydrolase enzymes mediate digestion and the smaller molecules are released into the cytoplasm. Applied aspect : Lysosomal storage disorders : are identified either as defect in hydrolases or transport of smaller molecules out to the cytoplasm. Eg , Gaucher’s disease in which there is accumulation of the glycolipids and Hurler’s syndrome which is associated with defective metabolism of mucopolysaccharides. Peroxisomes : these are membrane bound small subcellular organelles involved in a variety of metabolic reactions. The most prominent of these are oxidation reactions giving rise to hydrogen peroxide. Since hydrogen peroxide is highly toxic for the cell, it is rapidly neutralized by the enzyme catalase present in the organelle by converting it to water and oxygen.  The most common biomolecules subjected to breakdown in the peroxisomes are uric acid , amino acids and fatty acids. Some tissues do not have peroxisomes while others do have. Other important reactions associated with peroxisomes are : synthesis of cholesterol , dolichol and plasmalogens. Plasmalogens are a type of phospholipids present in the plasma membrane of heart and brain cells.  Mutations associated with peroxisomes give rise to peroxisomal disorders. These are usually associated with cognitive impairment , developmental issues , intellectual disabilities , impaired vision and hearing and liver failure. *************