Biol 115 Lecture 2:

Cell Structure and Function

T. Irving 01/07/97; revised A. Howard 01/16/00

Cell Doctrine

• All living things come from cells
• All cells come from other cells

What about things like viruses and prions? Are they alive? They need cells too!

Microscopy as a probe of cellular structure

• Conventional Light microscopy
  • Micron scale structures (200 nm up)
  • Many variants polarizing, phase, nomarsky all
  • increase contrast of unstained specimens
  • stained specimens - histology
• Fluorescence microscopy
  • label with dyes that absorb UV and emit visible light
  • can see things much smaller than resolution limit of microscope
  • much of what we know about cytoskeleton discovered this way.
• scanning electron microscopy
  • nm's -100's of nm
  • typically need to metal coat specimens
• Transmission electron microscopy
  • .2 nm - 100's of nm
  • cell ultrastructure
  • thin section, cryo, negative stain
• Scanning probe microscopies

Atomic resolution (.01 nm) up

Cellular Organelles

Organelle	Function
Cell membranes	Selective barrier inside & outside of cell
Nucleus	"Control of cell"
Nucleolus	Ribosome formation (mainly RNA)
Chromatin	Direct protein synthesis (DNA+protein)
ER (rough)	Protein synthesis
ER(smooth)	Lipid & CHO synthesis
Ribosomes	Protein synthesis (RNA+protein)
Golgi Apparatus	Processing & packaging of secretory proteins
Vacuoles & vesicles	Contain materials want to keep separate from cytoplasm
Lysosomes	Contain hydrolytic enzymes for intracellular digestion
Mitochondrion	Aerobic cellular respiration, i.e. ATP synthesis
Cytoskeleton	Cell skeleton, cellular motility
Centrioles	Microtubule organizing centers, cell division
Cilia and flagellae	Cell movement, sperm cells, respiratory tract

Cell Membranes

membrane components:

• phospholipids
• proteins
• glycoproteins
• glycolipids

Trans-membrane transport

• A lipid bilayer is a very effective barrier to most substances.
• Cell membranes are called semi-permeable
• Cells needed to evolve elaborate systems to import nutrients into cells and to expel wastes

Mechanism	Direction	Requirement
Diffusion: No channels	High to low	[ ] gradient
Diffusion: Channels	High to low	Conc. e.g. gases water
Facilitated Transport	High to low	carrier molecule [ ] gradient, e.g. sugar, AA's
Active transport	Low to high	ATP & Pumps
Exocytosis	Low to high	Vesicle release, e.g. secretion, neurotransmitters
Endocytosis	Low to high	Vesicle formation,e.g. phagocytosis

Osmosis, Water Channels and Regulation of Cell Volume

• Osmosis is the movement of water from a lower to a higher solute concentration.
• Osmotic pressure is a colligative property,
  i.e. depends of the number of particles
• In hypotonic solutions cells tend to swell
• In hypertonic solutions cell tend to shrink
• In isotonic media cells neither shrink nor swell i.e. physiological

The Cytoskeleton

• Consists of:
  • 7-9 nm diameter microfilaments
  • 10 nm diameter intermediate filaments
  • 24 nm diameter microtubules
    
• All are involved in giving shape and rigidity to the cell
  
• Cell motility involves microfilaments and microtubules either by:
  1. interacting with motor proteins
     (specialized mechano-enzymes which convert ATP into force)
  2. dynamic assembly and disassembly monomers from polymers
     
• Microfilaments Contain Actin
  • Actin filaments dynamically polymerize & depolymerize in the cell
  • The actin based cystoskeleton is commonly found either in bundles (parallel) or networks (non-parallel)
  • Motility in some systems seems to arise from actin polymerization/ depolymerization alone. e.g. sea urchin sperm.
  • Most actin based motility involves the motor protein myosin
    
• Microtubules made of tubulin
  • Microtubules may be purely structural
    but often microtubules are also responsible for cell movements
  • They form cilia and flagella
  • involved in vesicle transport esp. in neurons
  • involved in mitosis
  • Like actin, motility may be due to polymerization/depolymerization of subunits or due to the action of microtubule specific motors such as dynein or kinesin.
  • Cilia are found on surface of many epithelial cells
    in e.g. the oviduct , respiratory passages to move particles along.
  • Microtubules are organized by structures called centrioles
    
• Intermediate filaments have a strictly structural role
  • Intermediate filaments (10 nm diameter) are extremely stable
  • keratins in epithelial cells become hair, nails and wool.
  • A desmin based skeleton surrounds sarcomeres in striated muscle
  • Prevents breakage of myofibrils in case of defective sarcomeres ("safety-rope")

Transcription and Translation

• DNA is stored in nucleus
• Process of reading out DNA to form messenger RNA called transcription
• This occurs in the nucleus
• The process of translation takes
  messenger RNA + transfer RNA + amino acids => protein
• This occurs at ribosomes in the cytoplasm and a the surface of the rough ER

Protein Synthesis & Secretion

• Golgi stack of membranous sacks
• Modify proteins primarily by adding carbohydrate;
  proteins become glycoproteins
• Proteins move through the golgi and are packaged in secretory vesicles
• Vesicles move to plasma a membrane and exocytosed
• If proteins are incorporated into vesicle membrane,
  become incorporated in PM

Mitochondria

• May have evolved from endosymbionic bacteria - own DNA
• Inner and outer membrane, cristae, matrix
• Sites of aerobic cellular respiration
• CHO + O₂ -----> CO₂ + H₂O
• ADP ------> ATP

Cellular metabolism

• Conversion of foodstuffs require the participation of many enzymes
• Enzymes are catalysts that increase the rates
  of otherwise energetically favorable reactions
• Enzymes are specific to certain substrates
  because of the shapes & chemistry of their active sites
• Many enzymes require coenzymes,
  non-protein organic molecules necessary for the enzymes action
• NAD is a coenzyme (a dehydrogenase)

Aerobic cellular respiration

• Requires oxygen
• Glycolysis occurs in cytoplasm
• Krebs Cycle occurs in mitochondrial matrix
• Electron transport occurs in cristae of mitochondria

Anaerobic Cellular Metabolism

• If not enough oxygen available,
  ATP is generated by the process of fermentation
• Metabolism stops at the end of glycolyis
• pyruvate --> lactate
• "oxygen debt"
• When oxygen becomes available again,
  lactate can be broken down and cellular respiration resumes