Cell Structure and Function PDF

Cell Structure and Function MED 1103:Histology 1 Lecturer: Dr Christopher Heywood Quotable quote “The whole is greater than the sum of its parts” - Aristotle  Outline 1. Introduction 1. Energy production and storage 2. Cell component...

Cell Structure and Function MED 1103:Histology 1 Lecturer: Dr Christopher Heywood Quotable quote “The whole is greater than the sum of its parts” - Aristotle  Outline 1. Introduction 1. Energy production and storage 2. Cell components: 2. Ribosomes - Cell membrane 3. Protein Biosynthesis and degradation - Nucleus 4. Lipid Biosynthesis - Endoplasmic Reticulum 5. Cytoskeleton - Golgi apparatus 6. Intra and intercellular transport - Lysosomes 7. References - Peroxisomes - Mitochondria Introduction: The Cell Functional unit of living organisms Comprises Cytoplasm and Nucleus Cytoplasm: Cytosol + Organelles – Nucleus Membrane-bound organelles provide ideal microenvironments Membranes participate in reactions based on associated enzymes Cell Components- Cell Membrane Aka Plasma Membrane or Plasmalemma Structure: - Amphiphilic phospholipid bilayer with cholesterol - Unsaturated and Saturated tails usually present - Intrinsic membrane proteins/ carbohydrates/ lipids - Glycolipids and glycoproteins form glycocalyx - Fluid mosaic model  Cell components- Cell Membrane Functions: - Separation of Internal contents from exterior (cells, extracellular matrix) - Nutrient transfer e.g. aquaporins - Glycocalyx: i. Cell adhesion ii. Intercellular signaling iii. Mechanical and chemical protection Cell components- Nucleus Structure: - Largest organelle - Nucleoplasm bound by nuclear membrane/ envelope - Found in all cells except erythrocytes Cell components- Nucleus - Contains DNA, RNA, nucleoproteins - Nucleoproteins: o synthesized in cytoplasm: histone (structural), non-histone (enzymes) o Nucleosome= chromosome + histone - Nucleosomes seen as euchromatin (electro-lucent, active) and heterochromatin (electrodense, inactive) Cell components- Nucleus - RNA: o transfer, messenger and ribosomal o micro RNA, small nuclear RNA and small interfering RNA - Barr body: inactivated X chromosome in females Cell components- Nucleus - Variations in shape and polarity of nucleus help in cell identification: o Round/ Oval: epithelial o Spindle-shaped: smooth muscle o Bi-lobed: Eosinophils o Flattened: endothelial cells Cell components- Nucleus - Nuclear envelope comprises: o Two lipid bilayers separated by intermembranous/ perinuclear space o Outer membrane continuous with E.R. o Intermembranous space continuous with E.R. lumen o Nucleoporins in complex forming pores o Lamins which link membrane proteins to chromatin Cell components- Nucleus - Nucleolus: o Site of rRNA synthesis and assembly; tRNA processing o Controls cell cycle and stress responses o Comprises ribosomal genes, rRNA, ribosomal proteins and ribonuclear proteins o Changes size with cell activity  Cell components- Nucleus - Nucleolus: o Includes: i. dense fibrillar components (rRNA synthesis) ii. fibrillar centres iii. granular component (ribosome assembly) Cell components- Nucleus Function: - Storage and replication of genetic material - Allows for differentiation via variations in histone physical and chemical structures Cell components- Endoplasmic Reticulum Structure: - Flattened membrane-bound saccules, tubules, cisternae - May be smooth or rough (associated with ribosomes)  Cell components- Endoplasmic Reticulum Function: - Rough ER: o Protein synthesis o Modification and folding of proteins i. creating tertiary structure ii. forming disulphide bonds iii. starting glycosylation Cell components- Endoplasmic Reticulum Function: - Smooth ER: o Lipid synthesis (cholesterol, phospholipids, steroids), membrane synthesis and repair o Metabolism and detoxification via cytochrome p450 in liver o Storage and release of Ca 2+ ions Cell components- Golgi Apparatus Structure: - Located near nucleus - Comprises membrane- bound cisternae/saccules - Each cisternae has specific enzyme to add specific sugar - Cis (convex) and trans Golgi networks Cell components- Golgi Apparatus Function: - Receive uncoated (COP 2) or coated (COP 1) vesicles from rER - Final assembly and glycosylation of proteins and lipids - Synthesis of glycosaminoglycans - Dispatching of proteins to their ultimate destination (SNARE proteins allow docking at particular destination) Cell components- Lysosomes Structure: - Contains hydrolytic enzymes (protease, lipase, nuclease) at low pH - May be associated with: o Secondary/ phagolysosomes o Late endosomes/ multivesicular bodies Cell components- Lysosomes Functions: - degrade old or unnecessary cellular cells (autophagy) - degrade phagocytosed or endocytosed material Cell components- Peroxisomes (microbodies) Structure: - Membrane-bound vesicle containing oxidases and catalase Functions: - Production of hydrogen peroxide for killing pathogens - detoxification of certain toxic materials - β-oxidation of long chain fatty acids - synthesis of bile acids (in liver) Cell components- Mitochondria Structure: - elongated, sausage-shaped; mobile (via microtubules) - variable in number. Division, Fusion and autophagy influence numbers - Smooth outer membrane, convoluted inner membrane - self-replicate Cell components- Mitochondria Structure comprises: o Outer membrane- includes porins, enzymes o Inner membrane- folded into cristae o Inner cavity- has matrix with mitochondrial DNA, granules, ribosomes, enzymes o Intermembranous space- contains enzymes, cytochromes Energy production and storage Cellular respiration (anaerobic or aerobic) leads to provision of energy Mitochondrial respiration is strictly aerobic Mitochondrial respiration occurs within matrix and on inner membrane Energy is usually stored in the form of ATP Glycolysis: -begins in cytosol producing pyruvate (and small quantity of ATP). -Pyruvate migrates to mitochondria where most ATP is produced ATP production from fatty acids occurs entirely in mitochondria Ribosomes Structure: - Comprises two rRNA subunits bound to proteins - May be attached to rER or free - Contain ribozymes - May be attached to mRNA (polyribosomes) Ribosomes Functions: - To align mRNA so tRNA can bring respective amino acids in sequence - catalyse peptide bond formation between amino acids - Attached to rER: Produce and modify proteins for export, lysosomes, integral membrane proteins - Free: Produce modify proteins for cytoplasm, mitochondria, nucleus; Proteins- Biosynthesis and Degradation Proteins may be: - Structural e.g. collagen, elastin - Enzymes - Transport molecules - Regulatory molecules All cells types produce protein with some having a more dominant role Differentiation allows for specific expression by individual cells Protein Biosynthesis- Procedure 1. Transcription: Pre-mRNA is formed as a complementary copy of a gene 2. Post-transcription processing: Spliceosome (small nuclear RNA)+proteins remove introns (non-coding regions of genes) to form mRNA 3. Translation mRNA leaves nucleus and binds to ribosomes and mRNA is read  Protein Degradation 1. Damaged or useless proteins bind to Ubiquitin 2. Proteasomes (proteolytic enzymes closely associated but without an enclosing membrane) take up bound proteins OR 3. Proteolytic enzymes within lysosomes degrade proteins  Lipid Biosynthesis Occurs in all cells Fatty acids, triglycerides are synthesised in the cytosol Cholesterol and phospholipids are synthesised in the sER May be done to: i. replenish membranes ii. Store energy iii. Transport lipids iv. Transfer information to other cells Cell components- Cytoskeleton Structure: - Includes: microfilaments, intermediate filaments and microtubules - Made of protein subunits (monomers) bound into filaments (polymers) - Include motor proteins (actin, myosin kinesin) Cell components- Cytoskeleton Functions: - maintain cell shape and orientation - cell movement - movement of organelles around the cell - movement of chromosomes during cell division Cell components- Cytoskeleton Microfilaments - F-actin: 5-7 nm strands with two protofilaments twisted together - G-actin: monomers making up protofilaments, associated with ATP - These structures are then arranged into 3D structures - Sliding of actin and myosin (in muscle) cause contraction Cell components- Cytoskeleton Intermediate filaments -~10 nm in diameter - Self assemble and bind intracellular structures together - Many varieties exist, which aid in identification Cell components- Cytoskeleton Microtubules -~25nm in diameter - Form from polymerisation of subunits (alpha and beta tubulin) -Centrosome: 2 centrioles (diplosome) from which tubulin molecules originate (microtubule organizing centre)  Intra and Intercellular transport 1. Passive Diffusion - Movement down an electrochemical gradient in the absence of energy - Influenced by size, polarity and charge of molecule - Includes: o Lipids and lipid soluble (non-polar) molecules e.g. hormones o Gases (CO2, H2O, N2) o Small polar molecules- Water, urea Intra and Intercellular transport 2. Facilitated Diffusion - Use of special proteins to move hydrophilic molecules - Carriers may be pores/ channels or transporters - Some channels are gated -Includes transport of: water, ions, glucose, amino acids Intra and Intercellular transport 3. Active transport - Pumping of substances against electrochemical gradient using energy generated by ATP 4. Bulk transport - Occurs vis use of vesicles - Vesicles form from addition of membrane to a protein coat - Includes: endocytosis, exocytosis, intracellular transport Intra and Intercellular transport 5. Transmembrane signaling - Occurs via: o hormones entering cell and binding to receptor (oestrogen) o Receptor binding leading to intracellular signaling (insulin) o Binding to ion channels causing depolarization (neurotransmitters) Intra and Intercellular transport Exocytosis - Can be constitutive (all cells) or regulated - Regulated exocytosis is signal-dependent - Procedure: o Coated vesicles leave Golgi and fuse with others to form larger vesicles o Microtubules aid movement of vesicles o As they approach lumen they become more concentrated as they lose excess membrane o Vesicle merges with membrane and release contents via porosome Intra and Intercellular transport Endocytosis -Includes phagocytosis (> 0.5 µm), pinocytosis, transcytosis - Pinocytosis may be: o Clathrin-mediated (involves clathrin, receptors, ligands) o Caveolae- mediated o Macropinocytosis S.E.- Sorting endosome; R.E.- Recycling endosome; CV- coated vesicle; MVB- multivesicular body; LE- late endosome; EL- endolysosome Intra and Intercellular transport -Phagocytosis: o Includes WBC’s such as neutrophils, macrophages o Pseudopodia form around substrate after binding with receptors o Phagosome forms then fuses with lysosome, becoming phagolysosome o Residual bodies may remain, accumulating with time to form lipofuscin granules Cell components- Cytoskeleton Microtubules - Motor proteins (dynein and kinesin) move along tubules - Addition or removal of subunits cause movement - Alpha tubulin ring complexes form area for polymerization - Microtubule associated proteins stabilize structure  References 1. Young, B., Woodford, P., & O’Dowd, G. (2013). Wheater’s functional histology (6th ed.). Churchill Livingstone. 2. www.youtube.com. (n.d.). Endosome, Lysozome and phagosome. [online] Available at: https://www.youtube.com/watch?v=6MZN24Il5Sc [Accessed 3 Oct. 2022]. 3. www.youtube.com. (n.d.). Cytoskeleton Structure and Function - YouTube. [online] Available at: https://www.youtube.com/watch?v=YTv9ItGd050.