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.
 Plasma Membrane: Functions
 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
 Membranes that allow movement across are described as semi-permeable
membranes
 Figure 3.6 Diffusion.
 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 (02, CO2, CH4)
 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.
 Plasma Membrane: Functions
 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
 Fluid movement out of capillaries
 Fluid excreted by kidneys as urine
 Active Transport
 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
 Plasma Membrane: Functions
 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
 Plasma Membrane: Functions
 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
 Plasma Membrane: Functions
 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
 Osmosis
 Neurotransmitter release at neuron
terminal
 Exocytosis
 Gas exchange at lung
 Exchange at capillaries due to
pressure gradient
 Calcium movement through
voltage-gated channels
 Filtration
 Simple diffusion
 Endocytosis
 Facilitated diffusion
 Movement of a solvent along its concentration gradient