The Cell PDF

`0  The cell Definition Structural unit of the human body. Formed of Protoplasm (cytoplasm, nucleus, organelles) Surrounded by cell membrane. Cell membrane Very thin Elastic Semiperm...

`0  The cell Definition Structural unit of the human body. Formed of Protoplasm (cytoplasm, nucleus, organelles) Surrounded by cell membrane. Cell membrane Very thin Elastic Semipermeable Dynamic (renewed at different rates). Structure: formed mainly of Lipid bilayer (2 molecules of phospholipids) Globular proteins Lipid bilayer For flexibility & selective permeability Cholesterol Affect permeability & give toughness to membrane Proteins Act as channels, carriers, pump, receptor, enzymes, structural protein Carbohydrates Act as recognition sites (glycoprotein & Attach cells together glycolipid) `1  Transport through the cell membrane Diffusion / Active transport / Vesicular All molecules in constant motion I. Diffusion: Movement of molecules from high to low concentration (down conc. gradient) (Simple, facilitated, osmosis) A- Simple diffusion: passive Movement of molecules from high to low concentration, without carrier Conc. Gradient X Surface area of the membrane X Temp. Diffusion rate 𝜶 √Molecular Weight X Distance ❖ Simple diffusion through 3 ways: 1. lipid bilayer A. Lipid soluble substances: as O2, nitrogen (Rate of diffusion directly proportional with lipid solubility) B. Water molecules: transported through lipid bilayer or protein channels As water molecules are very small, have high kinetic energy can penetrate like bullets C. Lipid insoluble substances: if (small & uncharged) as urea 2. Protein channels: have ability to change shape to allow passage of ions and water Small Ions cannot transported via lipid bilayer due to: Electrical charges interact with multiple H2O molecules (bonded)→form big hydrated ions (hydrated Na+ ions > hydrated K+ ions). Characters of channels: A. Selective permeability: each channel has diameter, shape, charge. B. Gating: o Non-gated = opened o Gated = closed by gate at outer or inner surface Opened by conformational change in the shape of protein molecule Voltage gating; opened or closed by changes of electric potential: E.g. Na+ channels. Ligand gating: opened or closed when bound to a ligand as acetylcholine 3. Gap Junctions (channel junctions) Channels connecting cells: allow o Rapid passage of ions and molecules < 1000 between cells, without entering intercellular space o Rapid propagation of electrical activity from one cell to another. Hexagonal arrangement of protein unit to form connexon surrounding a channel→ in line with the channel in corresponding connexon of the adjacent cell o Diameter of channel: regulated by intracellular Ca++, pH, hormones, drugs `2 B. Facilitated Diffusion Definition Passive, Carrier mediated → diffusion of large molecules (glucose, amino acid) Mechanism Carrier protein has large channel →transport molecule part- way (not all the way) Molecule enters channel, bind specific receptor→ cause conformational change → channel opens to opposite side Characters of facilitated diffusion Specific Competition between similar substances (glucose and galactose) for the same carrier. Saturation: Rate of diffusion α concentration gradient up to maximum rate. Sensitivity to temperature > simple diffusion C. OSMOSIS Definition diffusion of water (from high to low conc.) across semipermeable membrane to water (not solutes) Osmotic pressure pressure necessary to stop osmosis ✓ Determined by numbers of particles (molecules or ions) per unit volume of fluid, not mass ✓ Measured in mm Hg. Osmolality: number of osmoles in one kg of water. Osmolarity: number of osmoles in one liter one kg of water= one L of water Osmolarity of ICF = ECF = 300 mosmol/L 280: due to Na+, Cl-, HCO3, 20: due to other substances Water diffuse in or out of cells to keep equal osmolarity of ECF & ICF Tonicity: is the osmolality of a solution relative to osmolality of plasma. Isotonic solution Osmolality = plasma (300 mOsm /L). e.g. (0.9 % NaCI solution or 5% glucose) Hypertonic osmolality > plasma Hypotonic osmolality < plasma `3  II. Active transport Definition transport of substance against electrochemical gradient. Depends on: Specific carrier, has ATPase activity Energy (ATP) Types of active proteins: 3 types Uniport Symport = cotransport Antiport= counter transport Carrier one substance 2 substances One substance in one direction transports in one direction simultaneously Another substance in in one direction opposite direction. Example Ca++ pump. Carriers of glucose & Na+ Na+ - K+ pump from intestinal lumen to inside of the cells. Types of active transport: 1. Primary active transport: e.g. Na+ / K+ pump Structure: A.  Subunit has; 2 K+ binding sites outside, 3 Na+ binding sites inside, ATP binding site. B.  subunit: has ATPase activity break ATP Characters: activity of pump is energy dependent, energy from ATP hydrolysis →(3 Na+ pumped out, 2 K+ into the cell) Importance: Maintain high extracellular Na+ and high intracellular K+ Control cell volume→ prevent cell rupture Electrogenic (has role in RMP) 2. Secondary active transport: e.g. carriage of glucose secondary to active transport of Na+ Na+ is carried 1st actively outside the cell → to maintain Na+ concentration gradient, then Na+ and glucose bind carrier→ carrier change shape→ carry glucose inside the cell `4 III. Vesicular transport: Definition: movement of macromolecules (enclosed inside cell membrane bound vesicles). 2 types A. Endocytosis: movement from outside to inside Pinocytosis (cell drinking) For very large: water soluble macromolecules (protein) Mechanism: substance enter cells as small invaginating vesicles containing ECF→ then the portion of the membrane breaks away from the surface → forming pinocytic vesicle Phagocytosis (cell eating): the same way as pinocytosis except if involves large matter e.g. bacteria and dead tissue. Occur only in certain cells e.g. some white blood cells. B. Exocytosis: movement from inside to outside N.B. balance between exo- and endocytosis →constant surface area of membrane Neither decreases (if endocytosis is more) nor it becomes redundant (if exocytosis is more) `5  Intercellular communications A] Cells communicate with each other via chemical messengers, reach cells via (3 types) Gap Junctions Neural Hormonal communication Chemicals passes directly Neurotransmitters are released Endocrine: Hormone from cell to cell from one neuron to another reach distant cells via blood across synaptic clefts between (systemic) contiguous nerve cells Paracrine: Hormone reach nearby cells via diffusion in ISF Autocrine: Hormone act directly on cell produces it B] Chemical messengers bind specific receptors (in cell membrane or cytoplasm or nucleus) Exert effects via Opening of channels ↑cyclic AMP) ↑cytoplasmic Ca+2 Regulation of body function: ability of an organism to maintain stable internal environment in constantly changing environment depends on Nervous regulation Endocrine regulation Fast response Slow response Lasts for short duration Lasts for long duration `6