The Cellular Level The CELL THEORY 3 Main Parts of a Cell Plasma Membrane cell is the basic structure and functional unit of all organisms principle of complementarity of structure and function biochemical reactions are dictated by the subcellular structures present in the cell reproduction has a cellular basis plasma membrane defines boundary of a cell cytoplasm the interior of the cell between the membrane and the nucleus contains cytoplasmic organelles nucleus contains the genes which control activities of the cell Forms the boundary of a cell Thickness 7 10nm very thin Composed of 2 layers of phospholipids lipid bilayer arranged tail to tail with the polar hydrophilic heads exposed to the aqueous extracellular fluid and the intracellular fluid arrangement is predictable Embedded in this lipid bilayer are Membrane Proteins Cholesterol Lipid bilayer exhibits fluidity and the membrane proteins are in constant flux their shapes constantly change as in a kaleidoscope or a mosaic pattern Hence the plasma membrane is said to be of the FLUID MOSAIC MODEL Cholesterol inserts between the phospholipids tails to stabilize the plasma membrane cholesterol therefore maintains the integrity of the plasma membrane Proteins Integral span membrane exposed on one surface or both surfaces transmembrane Peripheral attached to integral or the phospholipids on cytoplasmic face Functions Transport transporters or carrier molecules to transport hydrophilic polar substances across membrane Enzyme Activity like adenylate cyclase that mediate the cAMP dependent signal transduction mechanism Receptors bind hormones and NTs hormones NTs are chemical mediators that maintain homeostasis via binding to the specific receptors polar hormones NTs cannot cross the hydrophobic lipid bilayer of membrane thus receptors expressed on surface Intercellular Joining act as adhesive cells form tissues and if cells are closely packed they exhibit junctions that are formed by membrane proteins Cell Cell Recognition membrane proteins attached by carbohydrates to form glycoproteins act as signal molecules most of the glycoprotein will be part of the glycocalyx consists of glycoprotein glycolipids carbohydrates attached to the polar phospholipid heads on the extracellular face of the membrane Glycocalyx has many functions on surface of cells pattern of carbohydrates is unique for each cell type changes in the pattern indicates the cell is turning cancerous important role in reproduction sperm recognizes the glycocalyx on surface of the ovum to cause fertilization important role in immunity specific immune cells in the body B and T lymphocytes recognize glycocalyx on surface of pathogens to mount an immune attack to destroy the pathogen virus hides in helper T cells and destroys Attachment to the Cytoskeleton skeleton of the cell which maintains shape of cell and allows for movement of substances within the cell Tight most common type in tissues fusion of integral proteins in plasma membrane of adjacent cells forming impermeable junction Desmosome linker proteins extending from plaques on the cytoplasmic surface of the plasma membrane of adjacent cells interdigitate to hold the cells together and prevent their separation anchoring junction epidermis cardiac etc Gap formed by hollow cylinder called connexons allows for rapid transfer of ions between cells communicating junction gap junctions couple cells electrically can be depolarized almost at the same time by allowing the rapid transfer of ions muscle contraction preceded by depolarization of muscle cells hence a muscle tissue with cells connected via gap junctions depolarize almost the same time and contract together a phenomenon known as function syncytium in cardiac tissue and single unit smooth muscle Membrane Junctions Membrane Transport as the outer boundary of a cell membrane regulates what substances get into out of the cell exchange between interstitial and intracellular fluid across the membrane Passive Process cross without energy input Diffusion movement from high to low concentration down concentration gradient concentration is driving force type based on size and polarity of substance Simple nonpolar hydrophobic lipid soluble substances diffuse through phospholipid bilayer directly O CO2 etc Facilitated large polar hydrophilic substances cannot cross directly moved via integral proteins exhibits saturation and specificity carrier mediated via protein carrier specific for one chemical binding changes shape change of transport protein finite number of transporters for each type of substance thus exhibit saturation where all specific transporters are engaged and max transport is reached typically never reached except maybe in diabetes or something compromising the saturation membrane infection i e glucose transporters wont transport amino acids channel mediated via channel protein mostly ions selected on basis of size and charge Osmosis movement of water through a semipermeable membrane via specific channel protein aquaporin or through lipid bilayer semi permeable membrane from lower to higher solute concentration down water concentration do not exhibit saturation but exhibit specificity Tonicity Isotonic concentration of solution inside and outside of cell is the same same amount of water moving in and out shape of cell is unchanged 0 9 NaCl 5 glucose Hypotonic concentration of solution outside of cells is lower that inside of cells water moves via osmosis into the cell cell swells and bursts lyse Hypertonic concentration of solution outside of cells is higher than inside water moves out of the cell cells shrink crenate Filtration no energy input movement of solution from area of higher pressure to lower pressure down a pressure gradient pressure is the driving force Active Process cell provides energy ATP required to move substances across membrane Active Transport solute pumping movement of solutes from area of lower concentration to higher across the lipid bilayer of the plasma membrane exhibit specificity and saturation carrier proteins used are called pumps pump energy required Operation of the Na K pump an antiport pump Vesicular Transport energy required is to move vesicle in or out of the cell 2 types based on direction vesicles are moved Exocytosis movement of substances enclosed in vesicles from the interior of cells to the exterior Endocytosis movement of substances from the exterior of the cell to the interior phagocytosis movement of solid particles
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