Physiology Lecture 1

Cell physiology

• structure of cells and their interaction

Cell

• Structural and functional unit

• Smallest unit that can carry on life processes

Cell components - Eukaryotic cells

• Plasma membrane

• Cell organelles

• Interior of the cell: nucleus (genetic material) and cytoplasm (organelles suspended in cytosol)

Biological Membrane Composition

• Double layer of lipid molecules with embedded proteins

• May have different ratios of lipids and proteins based on function

Lipids

• Primarily carbon and hydrogens, some oxygen and phosphates

• lipids are amphipathic: have polar and nonpolar regions

• Phospholipids, steroids, and glycolipids

Phospholipids

• Polar heads face out, nonpolar tails face eachother

• Polar head group: phosphate attached to glycerol, nitrogen containing R group, glycerol backbone

  • Hydrophilic (loving)

• Nonpolar tail: 2 fatty acids chains attached to flycerol backbone, made up of carbon and hydrogen

  • may be saturated or unsaturated (causes kinks/bends)

  • Hydrophobic (fearing)

Steroids

• Cholesterol

• Amphipathic (single OH group)

• Sits between FA chains to maintain membrane fluidity

Glycolipids

• lipids with short CHO chains attached

• Part of the outer leaflet of the plasma membrane

• Amphipathic

• Short chain of monosaccharides extend from cell surface in ECF forming glycocalyx

Membrane proteins

• Distributed throughout the membrane unequally, are responsible for selective permeability

• Can be:

  • Integral/intrinsic

  • Peripheral/extrinsic

  • Glycoproteins

Integral/intrinsic membrane proteins

• Amphipathic

• May partially span membrane or fully span (transmembrane)

Peripheral/extrinsic membrane proteins

• Not amphipathic

• Only associate with polar regions on inner or outer surface of membrane

• Influence cell shape and motility

Membrane proteins - glycoproteins

• proteins with attached carbs

• Found on extracellular surface forming glycocalyx

Specialized junctions

stabilize interaction and promote communication between cells

Desmosomes

• adhering, anchor cells together

• found in tissues subject to stretching

• Formed from

  • plaques: proteins on cytoplasmic membrane that are anchoring points for cadherins

  • cadherins: transmembrane proteins that link cells

  • intermediate filaments: linked to plaque and anchor cytoplasmic surface to interior of cell for support

Tight junctions

• link cells together and limit movement of molecules through spaces between cells, forces substance to cross plasma membrane to move through tissues

• Composed of: occludins which attach to adjacent cells forming impermeable junctions

• Found in: epithelial tissues specialized for molecular transport like the kidneys or intestines

Gap junctions

• used for fast communication, transmembrane channels link cytoplasm of adjacent cells

• Composed of: protein channels called connexons that link to connexons of other cells

• Function: electrically and metabolically couple cells allowing small molecules and ions to move between

• Found in: heart muscle to allow simultaneous contraction of cells

Nucleus

• Function: store/transmit genetic information

• Nuclear envelope: double layered membrane (2 phospholipid bilayers) with pores to allow selective movement

• Nucleolus: site of ribosomal RNA synthesis; dense region, no membrane

• Chromatin: consists of DNA and associated proteins found in the nucleus and condensed to chromosomes during division

Ribosomes

• Non-membrane bound

• Function: protein synthesis; AAs → proteins

• Consist of 2 subunits (small and large) composed of protein and ribosomal RNA

  • separate in cytoplasm when not in use, functional when joined together

• 2 types:

  • Free in cytoplasm

  • Bound to ER

Endoplasmic Reticulum

• Fluid filled membranous system; distributed throughout cytoplasm

• 2 types:

  • Rough ER: appears granular due to bound ribosomes; flattened sac in layers

    • Function: synthesis of proteins with bound functional ribosomes and post-translation modification in the RER

  • Smooth ER: appears agranular; branched tubular structure

    • Function: synthesizes lipids (FAs and steroids), has additional functions in specialized cells (ex. stores calcium in muscle cells, drug detox in liver cells)

Golgi Apparatus

• Composed of 3-8 cisternae - sets of flattened and slightly curved membrane bound sacs stacked in layers

• Associated with many vesicles

• Function: further post-translational modification of proteins form RER

  • Sorts and packages proteins into vesicles

Lysosomes

• Small oval sphere

• Contain digestive enzymes: hydrolyze enzymes and breakdown polymners into small subunits

• Can only function at pH of 5 (phospholipid membrane allows acidity without changing pH of entire cell)

• Function: degrade extra/intracellular debris which are then removed by exocytosis or recycled

Peroxisomes

• Small spherical

• Contain oxidative enxymes: use oxygen to remove hydrogen from molecules and breakdown FAs, alcohol, and drugs (mainly liver and kidney)

• Produces H2O2 which can be toxic so they also contain catalase which can breaks down H2O2 into water and oxygen products

Mitochondria

• Converts energy from chemical bonds to ATP (usable form)

• Double membrane: inner is folded into cristae to increase SA

• Matrix contains products for cellular respiration

• Have their own DNA - double stranded, circular, and have instructions for making proteins for cellular respiration

Cytoskeleton

• Non-membrane bound

• Made of protein filaments that are anchored to membrane and dispersed through cytoplasm; dynamic and can dissemble/assemble

• Functions: maintain cell shape, organelle position, cell/organelle motility

Cytoskeletal filaments: Actin / Microfilaments

• Protein subunit: G-actin (globular)

• Support, cell shape, and movement

Cytoskeletal filaments: Intermediate filaments

• Protein subunit: several (ex. keratin)

• Support, mechanical strength

• Desmosomes anchor intermediate filaments of neighbouring cells together

Cytoskeletal filaments: Microtubules

• Protein subunit: tubulin, forms large hollow tubes

• Support, acts as a track for organelles and vesicles to move along