Lecture 4: Cellular Biology I

I. Cell Theory

Concepts:

1. Cells are the functional and structural units of living organisms

2. The activity of an organism is dependent on both the individual and collective activity of the cells that comprise that organism

3. Subcellular structures determine the biochemical activities of cells (i.e., principle of complementarity)

4. Continuity of life has a cellular basis

 

II. Generalized Cell

 

A. Major parts

1. Nucleus

2. Cytoplasm

            a. Contain organelles (see below)

3. Plasma membrane

 

III. Plasma Membrane

 

A. Fluid mosaic model

1. Membranes are a mosaic of different proteins are embedded in a phospholipid bilayer

2. Hydrophilic portions of both proteins and phospholipids are maximally exposed to water

3. Hydrophobic portions are in the nonaqueous environment inside the membrane

 

B. Types of proteins:

1. Integral: transmembrane proteins; span the hydrophobic interior        

            a. Channels

            b. Carriers

2. Peripheral: not embedded; attached to surface

            a. Enzymatic activity

            b. Structure

 

C. Carbohydrates associated with the exterior surface of the membrane

1. Glycolipids

2. Glycocalyx: attached to proteins in contact with extracellular matrix

 

D. Function of membrane proteins:

1. Transport

2. Enzyme

3. Receptor sites

4. Intercellular junctions

5. Cell-cell recognition

6. Cytoskeletal and extracellular matrix attachment

 

 

E. Specialized Structures and Functions

1. Microvilli: increase surface area; absorption

2. Membrane junctions

            a. Tight junctions—impermeable junctions

            b. Desmosomes—anchoring junctions; plaques and linkers; intermediate filaments

            c. Gap junctions—movement of chemicals between adjacent cells; connexons

 

F. Membrane transport

1. Membranes are selectively permeable

2. Movement across membranes

            a. Passive and active processes

3. Passive movement: Diffusion

            a. Net movement of a substance down a concentration gradient

i. Concentration gradient—graded concentration change over a distance in a particular direction

            b. Results from intrinsic kinetic energy

                        i. Affected by temperature and molecular size

            c. Random molecular movement

            d. Continues until a dynamic equilibrium is reached

4. Types of diffusion

a. Simple—nonpolar substances that are lipid soluble pass directly through lipid bilayer

            i. Polar and charged particles can diffuse if they can fit through pores

b. Osmosis—diffusion of a solvent through a selectively permeable membrane

i. Hypertonic solution—solution with a greater solute concentration than inside a cell

ii. Hypotonic solution—solution with a lower solute concentration than inside a cell

iii. Isotinic solution

iv. Osmolarity—total concentration of all solutes in a solution

v. Osmotic pressure—amount of pressure required to prevent net movement of water into a solution

c. Facilitated diffusion—lipid insoluble molecules too large to diffuse through membrane pores can move passively with carrier molecules

            i. Selective—not just size and lipid solubility (specific)

            ii. Limited by number of carrier molecules present (saturated)

5. Passive movement: Filtration—water and solutes are forced through a membrane or capillary by hydrostatic pressure

a. Pressure gradient pushes solute-containing fluid out

6. Active Processes: Active Transport

            a. Cell uses energy to move substances across the membrane

b. Transport molecules harvest energy from ATP to pump molecules against concentration gradients

c. Coupled systems—move more than one substance

            i. Symport—same direction

            ii. Antiport—opposite direction

d. Sodium-Potassium Pump

            i. Na+ binding (cytoplasm) stimulates ATP formation

            ii. Phosphorylation causes conformational change

            iii. Shape change releases Na+ to outside

            iv. K+ binding causes phosphate release

            v. Pump returns to original conformation

            vi. K+ is released to inside

7. Bulk Transport (Active)

 a. Exocytosis—substance is released from vesicle (membranous sac)

            i. Fuses with membrane and releases contents to outside

b. Endocytosis—large substances progressively enclosed by membrane and taken into cell

 

            i. Phagocytosis

            ii. Pinocytosis

iii. Receptor-mediated endocytosis

            a. Coated pit

            b. Clathrin

 

IV. Cell-Environment Interactions

A. Cell-adhesion molecules (CAMS)

1. Anchor cell to extracellular matrix and to each other

2. Cell migration

3. Cell signaling

B. Membrane receptors

1. Contact signaling

2. Electrical signaling

            a. Voltage-gated channels

3. Chemical signaling

            a. Neurotransmitter and hormone receptors act as ligands

            b. Binding of ligand causes conformational change

                        i. Creates force (muscle)

                        ii. Opens and closes ion channel

                        iii. Activates an enzyme

            c. G protein-linked receptors

                        i. Ligand binds to receptor

                        ii. G protein affects a second membrane system (enzyme or ion channel)

                        iii. Second messenger is created

                        iv. Signaling cascade is initiated

 

V. Cytoplasmic Organelles

A. Cytoplasm—cellular material inside cell

1. Most cellular activities occur here

2. Comprised of:

a. Cytosol—fluid in which other components are suspended

b. Organelles (see below)

c. Inclusions—non-functioning chemicals substances that may be unique to a given cell type

B. Mitochondria—transduce energy into useable cellular work

 

1. Double membrane—structure similar to plasma membrane

            a. Outer membrane—permeable to small solutes

            b. Inner membrane—contains embedded proteins involved in cellular work

                        i. Cristae—folds of the inner membrane

            c. Inner membrane space

d. Mitochondrial matrix—contains enzymes for metabolic steps of cellular respiration

C. Ribosomes—site of protein synthesis

1. Complexes of RNA and protein

2. Free in cytosol or bound to endoplasmic reticulum

D. Endomembrane System—interactive system of membranes that interact either directly through physical contact or indirectly through vesicles

1. Vesicle—membrane-enclosed sacs that are pinched off portions of membranes moving from one membrane to another

2. Endomembrane system includes:

            a. Nuclear envelope

            b. Endoplasmic reticulum

            c. Golgi apparatus

            d. Lysosomes

            e. Vacuoles

3. Endoplasmic reticulum—network of membranous tubules and sacs (cisternae) within the cytoplasm

 

1. Continuous with the nuclear envelope

2. Two regions—smooth ER and rough ER

3. Smooth ER—lacks ribosomes

            a. Synthesis of lipids, phospholipids and steroids

            b. Carbohydrate metabolism

            c. Detoxifies drugs

            d. Calcium storage

4. Rough ER—protein synthesis (has ribosomes)

            a. Ribosomes attached to ER synthesize secretory proteins

b. Growing polypeptide is threaded through ER membrane (receptor site) into cisternal space

            c. Protein folds into native conformation

d. If a glycoprotein, oligosaccharides are enzymatically added to the secretory protein

e. Protein departs in a transport vesicle pinched off from the ER

5. Golgi apparatus—modifies, concentrates and packages rough ER products

 

            a. Organelle of stacked, flattened membranous sacs (cisternae)

            b. Has polarity

                        i. Cis face—receives transport vesicles from rough ER

                        ii. Trans face—pinches off vesicles

            c. The rough ER products are modified as they move through Golgi apparatus

 

 

6. Lysomes—digestive compartments; membranous sac containing hydrolytic enzymes

a. Digest all major classes of macromolecules

b. Acidic

                        i. Pump H+ in from cytosol

c. Sequesters destructive enzymes from the cytosol

            d. Functions:

                        i. Intracellular digestion—phagocytosis

                        ii. Recycle cellular organic material

                        iii. Programmed cell death

            e. Role in disease—storage diseases

                        i. Lack specific lysosomal enzymes which causes substrate accumulation

                        ii. Pompe’s—glycogen in liver

                        iii. Tay-Sachs—lipid accumulation in brain

 

VI. Cytoskeleton—network of fibers throughout the cytoplasm that form a framework for support and movement and regulation

A. Functions:

1. Mechanical support to maintain shape

2. Allows cell to change shape when environment changes

3. Associated with motility (has motor molecules—see later lectures)

4. Regulatory role in transmitting signals from cell’s surface to its interior

B. Constructed from three types of fibers:

 

1. Microtubules (thickest)

a. Radiate from cell’s center

b. Determine cell shape

c. Provide tracks for organelle movement

d. Invovled in separation of chromosomes during cell division

2. Microfilaments (thinnest)

            a. Made up of contractile protein (actin)

            b. Attach to cytoplasmic side of plasma membrane

            c. Participate in muscle contraction

            d. Localized cell contraction

3. Intermediate filaments—most stable and permanent cytoskeletal element

            a. Act as guy wires to resist pulling forces on the cell

            b. Fix organelle position