Cell Biology Lecture Notes (BIOCHEM LC 2A) PDF

B. CELLS COURSE OUTLINE I. CELL STRUCTURE AND ORGANELLES A. Living Things B. Cells C. Microscope D. The Cell Theory E. Eukaryotes Figure 1: (a) Nasal sinus cells (viewed...

B. CELLS COURSE OUTLINE I. CELL STRUCTURE AND ORGANELLES A. Living Things B. Cells C. Microscope D. The Cell Theory E. Eukaryotes Figure 1: (a) Nasal sinus cells (viewed with a light II. CELL MOLECULAR COMPONENTS microscope), (b) onion cells (viewed with a light microscope, A. Endosymbiosis and Evolution plant cell), and (c) A microorganism Vibrio tasmaniensis bacterial cells (viewed using a scanning electron B. Outside the cells microscope) are from very different organism, yet all share C. Cell connections certain characteristics of basic cell structure. (credit a: III. PLASMA MEMBRANE modification of work by Ed Uthman, MD; credit b: A. Functions modification of work by Umberto Salvagnin; credit c: B. Composition modification of work by Anthony D'Onofrio; scale-bar data from Matt Russell) C. Structure D. Membrane Transport IV. OSMOTIC PROPERTY OF CELLS C. MICROSCOPE A. Osmosis Anton van Leeuwenhoek B. Donnan Equilibrium Invented the microscope with the purpose C. Ionic steady state of looking into small details of fibers D. Molecules related to cell First to observe a microorganism and permeability termed it animalcules V. CELL MOLECULE TRANSPORT A. Carrier-mediated transport Robert Hooke B. Channel mediated transport First to describe the cell by using the C. Coupled transport microscope Leuwenhoek invented D. Active transport He even coined the term cell from E. Na+/K+ transport “cellula,” which is associated with the F. Bulk transport rooms of monastery monks, after VI. REFERENCES observing the cork of wine bottles under a microscope INTRODUCTION TO CELL BIOLOGY Microscope to Study Cells Studying cells using microscopes ○ Electron- 10,000,000x magnification I. CELL STRUCTURE AND ORGANELLES ○ Light – 1000x magnification ○ Dissecting – 100x magnification A. LIVING THINGS Importance of microscope in medicine: for All living things have in common, such as: direct observe cells and pathogens, for dx ○ Cellular organization purposes ○ Ability to reproduce ○ Growth and development ○ Energy – ability to transform and utilize one form of energy to another ○ Homeostasis ○ Response to their environment ○ Ability to adapt ○ Ability to evolve in a long run by growing evidences of evolution Figure 2: (a) Most light microscopes used in a college biology lab can magnify cells up to approximately 400 times. (b) Dissecting microscopes have a lower magnification than light microscopes and are used to examine larger objects, such as tissues. PREPARED BY: BATCH 2028 1D BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 Some bacteria have capsule (for staining and identification) Has pili and flagella for locomotion B. Eukaryotic cells: found in plants and animals 10x larger Have membrane bound organelles, cell membrane Figure 3: (a) Salmonella bacteria are viewed with a light microscope. (b) This scanning electron micrograph shows Cell wall in plants and yeast Salmonella bacteria (in red) invading human cells. (credit a: Animal cells modification of work by CDC, Armed Forces Institute of Pathology, Plant cells – has chloroplasts Charles N. Farmer; credit b: modification of work by Rocky Mountain Laboratories, NIAID, NIH; scale-bar data from Matt Russell). Figure 5: Prokaryotic Cell vs. Eukaryotic Cell Figure 4: These uterine cervix cells, viewed through a light microscope, were obtained from a Pap smear. Normal cells are on the left. The cells on the right are infected with human papillomavirus. (credit: modification of work by Ed Uthman; scale-bar data from Matt Russell). D. THE CELL THEORY CELL Basic unit of life; all chemical reactions that makes us alive happens in the cell All cells arrives from existing cells Is a building block for all living organisms: ○ Unicellular ○ Multicellular - Each type of cell with a specific Figure 6: Generalized structure of a prokaryotic cell. function e.g. immune cells fight against pathogens CELL COMPONENTS Membrane – boundary between the ECF and ICF Cytoplasm – contains intracellular compartment which contain organelles like small organs in the cell that has different function Nucleus – contains DNA/genetic material CELL TYPES A. Prokaryotic cells: bacteria and archaea Figure 7: Relative sizes of different cells and cellular components; Do not have nucleus an adult human is shown for comparison. 10x smaller Have DNA and is not membrane bound Have cell membrane and ribosomes Have cell wall but have different types and components PREPARED BY: BATCH 2028 1D 2 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 E. EUKARYOTIC CELL PLASMA MEMBRANE 1. Plasma Membrane 1. Components Cell wall in plants and yeast Lipids 2. Nucleus ○ Double layer of phospholipids and 3. Cytoplasm cholesterol & other lipids Organelles: Proteins Mitochondria Carbohydrates Ribosomes ○ Attached to either lipids or proteins Endoplasmic reticulum 2. Function Golgi apparatus Allows selective passage in & out the Lysosomes cell Peroxisomes Cytoskeleton Plants: Vacuoles and chloroplast Figure 10: The plasma membrane is a phospholipid bilayer with embedded proteins, such as the peripheral membrane proteins (only attached at the periphery) and integral membrane proteins (embedded across the lipid bilayer that acts as a channel, allowing the proteins to pass through). There are other components, such as cholesterol and carbohydrates, which can be found in the membrane in addition to phospholipids and protein. CYTOSKELETON A network of fibrillar proteins organized into filaments or tubules. It is composed of proteins and assembled into 3 different types: 1. Microtubules – made of tubulin proteins and helps with: ○ Supports and acts as a framework of the skeletal system. ○ Transporting vesicles/ organelles within the cell ○ mitotic spindle (in cell division) ○ movement of cells (see flagellum, cilia) 2. Intermediate filaments – helps with the shape of the cells (e.g. Keratin) Figure 8: Comparison between (a) a typical animal cell and (b) a typical plant cell. 3. Microfilaments – made up of actin proteins and helps with: ○ movement of cells (pseudopods) ○ contraction in the muscle cells ○ Cleavage furrow during cell division Figure 11. Microfilaments, intermediate filaments, and microtubules comprise a cell’s cytoskeleton. Figure 9: Structure of Animal Cells PREPARED BY: BATCH 2028 1D 3 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 CILIA AND FLAGELLA Types: Cilia – many in number; size: short, hair-like ○ Rough ER (RER) or Granular ER Helps in movement of cells or substances - has ribosomes attached to their (mucus) surfaces on the membrane - Made of flattened vesicle and Flagellum – 1 or 2 on cell surface; very long attached to the nuclear Helps in movement of cells envelope Example is the flagellum of a sperm, and - Role in modifying proteins is similar to a cilium. In fact, it has much - Ribosomes are composed of a the same type of structure and the same mixture of RNA and proteins. type of contractile mechanism. The ○ Smooth ER (SER) or Agranular ER flagellum, however, is much longer and - shaped as tubules moves in quasi-sinusoidal waves instead - Role in storage of calcium ions of whiplike movements. (Ca+), synthesize lipids, steroid hormones, carbohydrates NUCLEUS - It has no ribosome The control center of the cell, sends messages to the cell to grow and mature, to replicate, or to die. Contains large quantities of DNA, which compromise the genes. Genes determine the characteristics of the cell’s proteins, including the structural proteins, as well as the intracellular enzymes that control cytoplasmic and nuclear activities. Structure Nuclear membrane – the envelope surrounding the nuclear membrane compartmentalizing the cytoplasm and the intranuclear space. Figure 13. ER: SER & RER o Nuclear pores – allows passage in & out, particularly the mRNA o Surrounded by rough endoplasmic GOLGI APPARATUS reticulum Structure Nucleoplasm – inside the nucleus, it Flatten big vesicles or sacs contains: Have two faces o nucleolus – where rRNA is made o Cis face – receiving face towards the o DNA – under the form of chromatin ER and nucleus o Trans face – releasing face towards the cell plasma membrane Function Modifies proteins and lipids Figure 12: Parts of the nucleus. The nucleus is surrounded by RER and the outermost boundary of the nucleus is the nuclear envelope. Notice that the nuclear envelope consists of two phospholipid bilayers, an outer membrane and an inner Figure 14: The Golgi apparatus (GA) under transmission electron membrane. In contrast to the plasma membrane (Figure 10), which micrograph of a white blood cell. GA is visible as a stack of consists of only one phospholipid bilayer. semicircular flattened rings; several vesicles can be seen near the organelle. ENDOPLASMIC RETICULUM (ER) Packs and prepares the proteins outside LYSOSOMES of the cell. Structure ER structure is made up of membranes as Membranous vesicle boundaries. Containing enzymes and low pH PREPARED BY: BATCH 2028 1D 4 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 Role as “garbage disposal” by removing any MITOCHONDRIA broken organelles, pathogens etc Powerhouse” or “energy factories” - o In macrophages (a type of white function in making ATP (chemical energy) blood cells) that is responsible in from food degrading pathogens and toxins.. Structure: oval-shaped Has enzymes that catalyzes the breakdown vesicles of unwanted biomolecules of the cell. Made of two membranes The interior is kept acidic by the action of a o Outer membrane proton pump, or H+. ATPase. o Inner membrane – with folds called cristae o Intermembrane space o Matrix It has the ability to form the energy-rich compound ATP by oxidative phosphorylation. Figure 15: Phagocytosis of a macrophage. The pathogenic bacterium is enveloped into a vesicle, which then fuses with a lysosome within the cell. ENDOMEMBRANE SYSTEM A system of structures of different shapes made of membranes Figure 16: Transmission electron micrograph of a mitochondrion. Role in making and modifying proteins Notice the inner and outer membranes, the cristae, and the mitochondrial matrix. and lipids It includes: o Plasma membrane, nuclear envelope, What is ATP? RER, SER, Golgi, lysosomes Energy currency of the cell Can combine to form coenzymes (coenzyme A (CoA) Used as signaling molecules (cyclic AMP) Nucleotides also “carry” chemical energy from easily hydrolyzed phosphoanhydride bonds Figure 17: General structure of a nucleic acid showing the Figure 16: Endomembrane system availability of an energy-rich phosphate group. RIBOSOMES Organelles not made of membranes Made of rRNA and proteins Attached to RER or free in the cytoplasm Roundly shape Free ribosomes: Protein synthesis/translation The ribosomes that become attached to the endoplasmic reticulum synthesize all transmembrane proteins and will be excreted by the cell in the intracellular space. PREPARED BY: BATCH 2028 1D 5 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 CHLOROPLAST (PLANT CELL) C. CELLS CONNECTIONS Vesicle with two membranes: Holds and connects cells together, also inner and outer allows the exchange of material. Stroma: inside the chloroplast o Thylakoids: flatten vesicles inside the stroma o Organized in stacks called grana o Made of: - Membrane boundary: contains green pigments or chlorophylls - Lumen Role in photosynthesis – transforming solar energy into food (chemical energy) Figure 18: Simplified diagram of a chloroplast II. CELL MOLECULAR COMPONENTS A. ENDOSYMBIOSIS & EVOLUTION Symbiosis = living together Mitochondria and chloroplasts are organelles that in the past were independent organisms (i.e. bacteria) living outside the cells. Proofs: 1 circular DNA, ribosomes, binary division B. OUTSIDE THE CELLS Protection against tension and compression Extracellular matrix (ECM) o animals: made of proteins like collagen and glycoproteins (sugar + protein) Cell wall o plant cells: made of carbohydrates like pectin and cellulose Figure 20: Intercellular connections (a) Plasmodesmata: channel between the cell walls of plant cells (b) Tight junction: prevents leaking at the intercellular space (e.g., lining of the digestive tract) (c) Desmosome: joins adjacent animal cells and spot welds (e.g., desmosomes in tissue found in skin and muscle) (d) Gap junction: allows ions and small molecules for intercellular communication (e.g., electrical excitable tissues found in heart and smooth muscle to synchronize movement) Figure 19: ECF is a network of secreted molecules PREPARED BY: BATCH 2028 1D 6 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 III. PLASMA MEMBRANE viral/bacterial infection (immunophenotyping: nametag or the Components: identity from different cells) Lipids Proteins Importance: Prevents the degradation from Glycoproteins the unwanted normal cells o Change in shape – immune cells can squeeze through blood vessels to quickly locate pathogens o Change in fluidity that allows to pass through to the openings in the blood vessels. Clinical Significance ○ Gross Alterations - disturbance of waer balance and ion influx ○ Specific Component Deficience or Alteration - disease states B. PLASMA MEMBRANE COMPOSITION FIgure 21: Visualization of a phospholipid bilayer Fluid Mosaic model – explains the organization of the plasma membrane and its components A “mosaic” of many types of molecules that are in constant (wavelike) movement, making the membrane look like a fluid Figure 23: Components of a phospholipid bilayer Approximate composition: 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, Figure 22a: A phospholipid unit and 3% carbohydrates (Guyton & Hall, 2021). Integral proteins interact with “lipid bilayer”. ○ Passive transport pores and channels o Active transport pumps and carriers o Membrane-linked enzymes, receptors and transducers Glycocalyx (glycoproteins and glycolipids) are responsible for the Figure 22b. Basic Structure of an animal cell membrane overall negative surface charge of the cell, allows cell attachment, and receptor A. PLASMA MEMBRANE FUNCTIONS functionality. It is a boundary of the cell with many Sterols stabilize the lipid bilayer functions ○ Cholesterol ○ Selective permeability – allows some substances in and out the cell ○ Immunity – distinguish between “self” and “non-self”. Important in blood transfusion, organ transplant, PREPARED BY: BATCH 2028 1D 7 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 a. b. b. Figure 24. (a) 3D structure of cholesterol (b) Structural formula of cholesterol c. Figure 26. The parts of a phosphoglyceride molecule. This example is a phosphatidylcholine, represented, (a) schematically, (b) by a formula, (c) as a space-filling model. Figure 25. Phospholipid bilayer showing the presence of cholesterol Figure 27. Packing arrangements of lipid molecules in an aqueous environment. (a) Cone-shaped lipid molecules (above) form micelles, whereas cylinder-shaped phospholipid molecules (below) form bilayers. (b) A lipid micelle and a lipid bilayer. a. PREPARED BY: BATCH 2028 1D 8 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 1. PLASMA MEMBRANE LIPIDS Exist as a sheet Undergo lateral diffusion (side-to-side or translational movement, and flexing) a. Major Lipids i. PHOSPHOLIPIDS - Do not flip-flop (pass from one side of the bilayer to the other) - Symmetrically distributed - Arranged with their hydrophilic head groups facing the aqueous medium and their fatty acyl tails forming a hydrophobic membrane core (that is buried) GLYCEROL LIPIDS: Figure 28. HIV docks and binds to the CD4 receptor, a Phosphatidylcholine glycoprotein on the surface of T cells, before entering, or infecting, - Constitute the outer face of the cell. the bilayer - Major plasma membrane lipid in most cell types eg. bacterial toxin secreted by Clostridium perfringens, is a lipase that hydrolyzes the phosphocholine from phosphatidylcholine. The resulting lysis of the cell membrane releases intracellular contents that provide the bacterial nutrients for rapid growth. Amino phospholipids - Constitute the inner or cytoplasmic face of the bilayer Phosphatidylethanolamine Phosphatidylserine - Contains a net negative charge that contributes to the Figure 29. The spontaneous closure of a phospholipid bilayer to membrane potential form a sealed compartment. The closed structure is stable - Might be important for binding because it avoids the exposure of the hydrophobic carbon tails to positively charged molecules water, which would be energetically unfavorable. within the cell Phosphatidylinositol - Found only in the inner C. PLASMA MEMBRANE STRUCTURE membrane Composed of a lipid bilayer containing - Transfer of information from embedded proteins hormones and Continuous and sealed so that the neurotransmitters across the hydrophobic lipid bilayer selectively cell membrane into the cell restricts the exchange of polar - Molecule that is compounds between the external and the associated with intracellular fluid secondary messengers Referred to as a fluid mosaic because it (eg.hormones) consists of a mosaic of proteins and lipid molecules that can, for the most part, ii. SPHINGOLIPID move laterally in the plane of the Most variable membrane. Ex: sphingomyelin (contains sphingosine backbone) PREPARED BY: BATCH 2028 1D 9 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 - Hydrophilic regions of the proteins protrude into the aqueous medium on b. Lipids are Amphipathic Molecules both sides of the membrane have both hydrophobic and hydrophilic - Many of these function as either channels ends or transporters for the movement of c. Permeability compounds across the membrane, as Impermeable to most water-soluble receptors for the binding of hormones and molecules since they would be neurotransmitters, or as structural proteins insoluble in the hydrophobic core Lipids Soluble Compounds Variation in the Way in which Proteins are ○ Gases inserted into Membranes - Oxygen - CO2 LDL Receptor ○ Nitrogen ○ crosses the membrane once ○ Lipid-Derivative Molecules (eg: ○ amino terminal on the exterior hormones) ○ type I transmembrane protein ○ Organic Non-electrolyte Molecules Asialoglycoprotein Receptor d. Detergents ○ crosses the membrane once Amphipathic molecules ○ carboxyl terminal on the exterior Used to solubilise membrane proteins ○ type II transmembrane protein during purification Cytochrome P450 Hydrophobic ends vind to hydrophobic ○ type III transmembrane protein region of the protein displacing bound ○ its deposition is similar to type I lipids proteins Polar is free → bringing proteins into ○ does not contain a cleavable solution signal sequence Various Transporters 2. PLASMA MEMBRANE PROTEINS ○ cross the membrane a number of Plasma membrane proteins consists of times Structural proteins ○ type IV transmembrane proteins Antigens ○ also referred to as polytopic Transport proteins membrane proteins Receptors Enzymes - Comprise most of membrane proteins - Usually globular - Amphipathic Membrane Enzyme - Asymmetrically distributed Plasma 5’-Nucleotidase Transmembrane Proteins Adenylyl cyclase ○ Exposed to both on the outer and Na+, K+, ATPase cytoplasmic faces of the membrane Endoplasmic Glucose-6-phosphatase Other examples of Integral Proteins Reticulum ○ Immunoglobulin molecules of Golgi Apparatus lymphocytes’ plasma membrane Cis GlcNAc transferase I ○ Many hormone receptors Medial Glogi mannosidase II - Two of the prominent integral proteins in Trans Galactosyl transferase the RBC membrane are glycophorin, TGN Sialyl transferase which provides an external negative charge that repels other cells, and band Inner mitochondrial ATP Synthase 3, which is a channel for bicarbonate and membrane chloride exchange. The transport of Table 1. Plasma Membrane Proteins with Enzymatic Activity. bicarbonate into RBC in exchange for chloride helps to carry the bicarbonate to a. Integral proteins the lungs, where it is expired as CO2. - Contain transmembrane (span the lipid bilayer) domains with hydrophobic amino b. Peripheral proteins acid side chains that interact with the Attached to the membrane surface hydrophobic portions of the lipids to seal through hydrogen bonding or the membrane electrostatic interactions to lipids or to the exposed surface of integral proteins PREPARED BY: BATCH 2028 1D 10 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 Ankyrin Rate of movement depends on the fluidity ○ Bound to integral protein band 3 of the membrane of RBC membrane Membrane Fluidity Spectrin Family of Proteins Depends on the nature of the packing and ○ Bound to the intracellular interaction of the fatty acyl chains in membrane surface membrane phospholipids ○ Provide mechanical support for ○ Fatty acids are aligned or ordered -> the membrane stiff structure ○ Spectrin- bound to actin to form ○ Long chain saturated fatty acids inner membrane skeleton or the pack closely and interact strongly -> cortical skeleton rigid structure - All cells contain an inner membrane skeleton of spectrin-like proteins. RBC Transition Temperature (Tm) spectrin was the first member of the - Temperature at which the structure spectrin family described. The protein undergo transition from ordered to dystrophin present in skeletal muscle cells disordered (melting) is a member of the spectrin family. - Increased temperature -> hydrophobic Genetic defects in the dystrophin are side chain undergo transition from ordered responsible for Duchenne’s and Becker’s to disordered muscular dystrophies. - Temperatures above melting point, Tm -> change in fluidity c. Lipid-Anchored Proteins - The longer the chain length and more Bound to the inner or outer saturated the fatty acids -> higher the Tm surface of the membrane - Unsaturated fatty acids with cis double Many integral proteins also bonds do not pack closely -> more fluid contain attached lipid groups to Tm is lowered increase their stability in the - The greater the double bonds -> the lower membrane the Tm -> the greater the fluidity Glycophosphatidylinositol Glycan CHOLESTEROL (GPI) Anchor Interspersed between the phospholipids ○ Covalently attached lipid that In the phosphoacylglycerols, unsaturated anchors proteins to the external fatty acid chains bent into the cis surface of the membrane conformation form a pocket cholesterol, - The prion protein, present which binds with its hydroxyl group in the in neuronal membranes, external hydrophilic region of the provides an example of a membrane and its hydrophobic steroid protein attached to the nucleus in the hydrophobic membrane membrane through GPI core anchor. This is the protein that develops an altered Function pathogenic conformation Maintains membrane fluidity in both mad cow disease ○ Cholesterol reduces membrane and Creutzfeldt-Jakob fluidity by preventing the movement disease. of fatty acyl chains ○ Decreases the ability of Other Anchors phospholipids to translate and flex ○ A number of proteins involved in hormonal regulation are Tm anchored to the internal surface At temperatures (C14) Fatty Acyl Groups increased fluidity - Geranyl (C20) or At temperatures >™. Cholesterol limits the Farnesyl (c15) Isoprenyl disorder because it is more rigid than the fatty acid hydrocarbon tail -> decreased d. Movement of Proteins in Plasma Membranes fluidity High cholesterol: phospholipid ratio -> Tm is abolished FLUID-MOSAIC MODEL BY SINGER AND NICHOLSON Membrane proteins can move laterally in the matrix of the lipid bilayer PREPARED BY: BATCH 2028 1D 11 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 Cholesterol and Cis Unsaturated Fatty Acids a. Membrane Glycoproteins in the Membrane - generally contain branched Prevent the hydrophobic chains from oligosaccharide chains of packing too closely together approximately 15 sugar residues ○ lipid and protein molecules that that are attached through: are not bound to external of i. N-Glycosidic Bonds - to the internal structural proteins can amide nitrogen of an asparagine rotate and move laterally in the side chain plane of the leaflet -> enables the ii. Glycosidic Bond - to the plasma membrane to: oxygen of serine a. Partition (O-Glycoproteins) Between daughter cells during - carbohydrate moiety of cell division glycoproteins are in contact with b. Deform the environment limiting As cell pass through capillaries movement of glycoproteins c. Form and Fuse with Vesicle membranes b. Membrane Glycolipids Between daughter cells during - usually galactosides or cell division cerebrosides - specific carbohydrate chains on MEMBRANE FLUIDITY VS FUNCTION the glycolipids serve as cell Increased fluidity recognition molecules ○ Increased water permeability ○ Increased lateral mobility of The variable carbohydrate components of the integral proteins glycolipids on the cell surface function as cell If the function of the protein resides in the recognition markers. For example, the A,B, or O hydrophilic segment, fluidity change will blood groups are determined by the carbohydrate have little effect composition of the glycolipids. If the function of the protein resides in the hydrophobic segment, fluidity change Cell surface glycolipids may also serve as binding may have significant effect sites for viruses and bacterial toxins before Protein-protein interactions may restrict penetrating the cell. For example, the cholera AB integral protein mobility within the toxin binds to Gm1-gangliosides on the surface of membrane the intestinal epithelial cells. The toxin is then endocytosed in caveolae (invaginations or “caves” A patient is suffering from both short-term and that can form in specific regions of the membrane). long-term effects of ethanol on his central nervous system. Data support the theory that the short term D. MEMBRANE TRANSPORT effects of ethanol on the brain partially arise from Membrane transport types: an increased membrane fluidity caused when a. Passive ethanol intercalates between the membrane lipids. Doesn’t use energy The changes in membrane fluidity may affect Molecules are moving from high proteins that span the membrane (integral concentration to low proteins), such as ion channels and receptors for neurotransmitters involved in conducting the nerve b. Active impulse. Requires energy Molecules are moving from low THE GLYCOCALYX OF THE PLASMA concentration to high MEMBRANE one with the help of proteins called pumps short chains of carbohydrates (oligosaccharides) that extend into the PASSIVE TRANSPORT aqueous medium contained by some of Diffusion – transport of substances (or the proteins and lipids and on the external solute when in solution) from higher surface of the membrane concentration to lower concentration hydrophilic carbohydrate layer Osmosis or Diffusion of water – transport protects the cell against digestion of solvent molecules (water) restricts the uptake of hydrophobic Facilitated diffusion – transport of solute compounds with help from membrane proteins: complex oligosaccharide play a role in ○ Channels e.g. Integral proteins - cell-to-cell interactions a special gate for specific constitute 2-10% of the weight of plasma substances membranes ○ Transporters PREPARED BY: BATCH 2028 1D 12 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 Solution = solvent + solutes Solutions with higher concentrations of solute are called hypertonic Solutions with lower concentrations of solute are called hypotonic Solutions with the equal concentrations of solute are called isotonic Figure 30: Diffusion through a permeable membrane follows the concentration gradient: solutes move from high to low concentration. (credit: modification of work by Mariana Ruiz Villarreal) OSMOSIS Figure 32a. Osmotic pressure changes the shape of red blood The diffusion of water molecules (solvent) cells in hypertonic, isotonic, and hypotonic solutions until the concentration of the solute is equal on both sides of the membrane; special type of diffusion to maintain homeostasis CELL MOLECULE TRANSPORTATION Direction of water flow is: ○ From higher concentration of free water A. DONNAN EQUILIBRIUM molecules to low ○ Or from low concentration of solute molecules to high - Only water flows through the semipermeable membrane and it is being moved by the electromotive force of water moving to an area with higher concentration IV. OSMOTIC PROPERTIES OF CELLS A. OSMOSIS Figure 32a. Donnan Equilibrium Osmotic properties of cells Osmosis (Greek, osmos “to push”) ○ Movement of water down its concentration gradient Hydrostatic pressure ○ Movement of water causes fluid mechanical pressure ○ Pressure gradient across a semipermeable membrane Figure 32b. Donnan Equilibrium B. IONIC STEADY STATES Potassium cations most abundant inside the cell Chloride anions ions most abundant outside the cell Sodium cations most abundant outside the cell Figure 31. Osmosis - In osmosis, water always moves from an area of higher concentration (of water) to one of lower concentration (of water). In this system, the solute cannot pass through the selectively permeable membrane. PREPARED BY: BATCH 2028 1D 13 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 Figure 35. Permeability coefficient for the passage of various molecules through synthetic lipid bilayers CELL PERMEABILITY Passive transport is carrier mediated. Figure 33. Muscle cell interior. Facilitated diffusion Solute molecule combines with a “carrier” or transporter Electrochemical gradients determines the direction Integral membrane proteins form channels CROSSING THE MEMBRANE Simple or passive diffusion Passive transport ○ Channels or pores Facilitated transport ○ Assisted by membrane-floating proteins Active transport pumps & carriers ○ ATP is required Figure 34. Electrochemical gradients arise from the ○ Enzymes and reactions may be combined effects of concentration gradients and required electrical gradients. V. CELL MOLECULE TRANSPORT C. MOLECULES RELATED TO CELL PERMEABILITY Dependent on Molecules size (electrolytes more permeable) Polarity (hydrophilic vs hydrophobic) Charge (anion vs. cation) Water vs. lipid solubility Figure 36. Different modes of transport PREPARED BY: BATCH 2028 1D 14 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 A. CARRIER-MEDIATED TRANSPORT D. ACTIVE TRANSPORT Integral protein binds to the solute and undergo a - Energy is required conformational change to transport the solute Three main mechanisms: across the membrane Coupled carriers: a solute is driven uphill compensated by a different solute being transported downhill (secondary) ATP-driven pump: uphill transport is powered by ATP hydrolysis (primary) Light-driven pump: uphill transport is powered by energy from photons (bacteriorhodopsin) Figure 37. Carrier-mediated transport Figure 40. Coupled carriers, ATP-Driven pump, Light-driven pump B. CHANNEL MEDIATED TRANSPORT Proteins form aqueous pores allowing specific E. NA+/K+ PUMP solutes to pass across the membrane Actively transport Na+ out of the cell and Allow much faster transport than carrier proteins K+ into the cell Against their electrochemical gradients For every 3 ATP, 3 Na+ out, 2 K+ in Figure 38. Channel mediated transport Figure 41. Na+/K+ pump NA+/K+ SYMPORT C. COUPLED TRANSPORT Na+ exchange (symport) is also used in Some solutes “go along for the ride” with a carrier epithelial cells in the gut to drive the protein or an ionophore absorption of glucose from the lumen, and eventually into the bloodstream (by passive transport). Figure 39. Coupled transport Figure 42. Transcellular transport of glucose PREPARED BY: BATCH 2028 1D 15 BIOCHEMISTRY LC 2A: INTRODUCTION TO CELL BIOLOGY DR. ESPIRITU, A. DATE: 08/14/2024 F. BULK TRANSPORT Moving big particles, fragments of cells, is called bulk transport. Endocytosis – bringing them into the cells ○ Phagocytosis – when big molecules/particles are brought inside ○ Pinocytosis – when large amount of liquid in brought in ○ Receptor-mediated endocytosis – when only specific molecules enter the cell Exocytosis – taking them out the cell Figure 44. In exocytosis, a vesicle migrates to the plasma Figure 43. Endocytosis membrane, binds, and releases its contents to the outside of the cell. Three variations of endocytosis are shown. 1. In one form of endocytosis, phagocytosis, the cell membrane surrounds the particle Reference(s): 1. Dr. A. Espiritu (2024). Lecture and Powerpoint and pinches off to form an intracellular Presentation. vacuole. 2. In another type of endocytosis, pinocytosis, the cell membrane surrounds a small volume of fluid and pinches off, forming a vesicle. 3. In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds at the receptor on the external cell membrane. (credit: modification of work by Mariana Ruiz Villarreal) PREPARED BY: BATCH 2028 1D 16