Cells: The Living Units Introduction Life in general occurs in an aqueous environment All chemical processes essential to life occur within the aqueous environment of the cell and surrounding fluids contained in the body To survive humans must maintain a normal volume and composition in ICF and ECF TBW = 50 to 60% BW (Approx. 40L) ICF ~ 25L (Cytoplasm) ECF ~ 15L (Interstitial fluid, CSF, Blood Plasma & Lymph) Figure 3.2 Structure of the generalized cell. Plasma Membrane: Structure Structure that forms the boundary between the cell and its surroundings Cellular membranes surround cell organelles Made from lipids that form a lipid bilayer Materials enclosed within the plasma membrane may be loosely referred to as cytoplasm Plasma Membrane: Structure Singer and Nicolson (1972) published the fluid mosaic model of the cell membrane Primary structure of the cell membrane is the lipid bilayer Phospholipids: Major constituent of lipid bilayer Move freely within the membrane Cholesterol: About 20% of lipids Stabilizes neighboring phospholipids and decrease flexibility of cell membrane
Plasma Membrane: Structure Proteins: Make up approx. half the mass of the cell membrane Associated with the cell membrane either by insertion or by association with one surface Types of proteins Integral Proteins (Transmembrane Proteins) Peripheral proteins Integral Proteins Tightly associated with the membrane Part extends into lipid bilayer Extended portion has hydrophobic amino acids held by hydrophobic interactions Transmembrane proteins extend all the way through the membrane Peripheral Proteins Bound to the membrane surface Held in place by weak, non-covalent bonds. Plasma Membrane: Structure Glycoproteins Coat outer surface of the membrane Forms sticky layer called glycocalyx Different cell types within the body have different CHO in the glycocalyx thus cells are able to recognize each other Flexibility of membranes allow changes in shape of cells and organelles
Fluidity allows membranes to fuse and break apart Most body cells are attached to other cells (RBCs are exceptions) Specialized Membrane Junctions Tight Junctions An impermeable junction formed by the fusion of a series of integral protein molecules in the plasma membrane of adjacent cells Desmosomes (Anchoring junctions) Mechanical couplings scattered like rivets along the sides of abutting cells that prevent their separation Gap Junctions (Nexus) Communicating junction that allows chemical substances to pass between adjacent cells Figure 3.5 Cell junctions. Diffusion Tendency of molecules or ions to disperse and be evenly distributed in an environment Recall that all molecules possess kinetic energy Net movement occurs along a concentration gradient, from higher concentration to lower concentration Cellular membranes restrict and regulate the movement of particles within the body Figure 3.6 Diffusion. Membranes that allow movement across are described as semi-permeable membranes Simple (Passive) Diffusion Direct movement of particles through the lipid bilayer along a concentration gradient
No energy is provided by the cell (passive) Such molecules are usually small and nonpolar (0 2, CO 2, CH 4 ) Some larger nonpolar molecules can also cross in this manner (steroid hormones) Factors That Affect The Rate of Diffusion Particle size Charge Non-polar molecules move easily Polar molecules do not move easily Charged particles cannot move across (all in reference to the lipid bilayer) Temperature Concentration gradient Number of carrier proteins or channels Facilitated Diffusion Assisted movement of particles across cellular membranes along a concentration gradient Assist is provided by carrier (transport) proteins, or channels (both transmembrane proteins) Carriers attach to the given particle and shuttles it across the membrane Channels are made up of proteins that form a hole through the lipid bilayer Open Channels Gated channels Figure 3.7 Diffusion through the plasma membrane. Osmosis
Passive movement of solvent molecules along a concentration gradient Recall solute, solvent, and solution Two solutions are isotonic if they have the same concentration Hypertonic solution:higher concentration (more solute; less solvent; high osmotic pressure) Hypotonic solution: Lower concentration (less solute, more solvent, low osmotic pressure) Figure 3.9 The effect of solutions of varying tonicities on living red blood cells. Filtration A process that forces water and solutes through a membrane or capillary wall by fluid or hydrostatic pressure Passive process, not selective and involves a pressure gradient Examples Active Transport Fluid movement out of capillaries Fluid excreted by kidneys as urine Movement of particles against a concentration gradient Energy (generally in the form of ATP) is required and is provided by the cell Example: Active pumping of two K + and three Na + when Na + /K + ATPase hydrolyses a molecule of ATP Active transport is NOT diffusion Vesicular Transport Two main categories Endocytosis (bringing substances into the cell)
Exocytosis (expelling substances from within the cell) Molecules are usually too large to cross membrane even with the help of a carrier Bulk-Phase endocytosis Plasma membrane folds inward trapping substances from the surrounding Invaginated region of membrane pinches off from plasma membrane and enters cell A vesicle (phagosome), containing the trapped substances, is formed Phagocytosis Describes a situation where one cell engulfs another cell Receptor-mediated endocytosis Small particles attach to receptor on the external surface of cell membrane Receptor and particle aggregates at one region of cell membrane Region of cell membrane invaginates to form a vesicle Figure 3.12 Events of endocytosis mediated by protein-coated pits. Figure 3.14a Exocytosis. Active or Passive? Osmosis Exocytosis Filtration Simple diffusion Endocytosis Facilitated diffusion
Match the following White blood cell engulfing bacteria Neurotransmitter release at neuron terminal Gas exchange at lung Exchange at capillaries due to pressure gradient Calcium movement through voltage-gated channels Osmosis Exocytosis Filtration Simple diffusion Endocytosis Facilitated diffusion Movement of a solvent along its concentration gradient