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CHAPTER 3
CELLULAR LEVEL OF
ORGANIZATION
Cell Overview
•
Plasma Membrane – phospholipid
bilayer
Cytoplasm
•
-
•
Cytosol = intracellular fluid
Organelles = “cell’s organs”
Nucleus – contains chromatin
TYPICAL EUKARYOTIC CELL
PLASMA MEMBRANE:
Fluid Mosaic Model
ECF
ICF
Figure 3.2
PLASMA MEMBRANE:
Fluid Mosaic Model
• Plasma membrane is about as viscous as olive
oil
“Fluid”
• Molecules move freely in two dimensions
-
Phospholipids
Proteins
Cholesterol
Glycolipids
“Mosaic”
• Components naturally reorient themselves
when disturbed, torn or punctured
PLASMA MEMBRANE:
Structural Components
• LIPIDS
- ~ 50 lipid molecules for
each protein molecule
ECF
• PROTEINS
ICF
PLASMA MEMBRANE:
Structural Components
• LIPIDS: barrier to exit and entry of most
polar substances
- Phospholipids: 75% of membrane lipids
• Amphipathic – having both polar and nonpolar
regions
- Cholesterol: Hydroxyl groups H-bond with
polar heads of other lipids
• Imparts some rigidity (but too much makes cells too
stiff)
- Glycolipids: on extracellular surface
Figure: 2.18
PLASMA MEMBRANE:
Structural Components
• PROTEINS – Integral & Peripheral
- Integral proteins are amphipathic
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html
PLASMA MEMBRANE:
Structural Components
• PROTEINS – Integral &
Peripheral
- Mostly “gatekeepers”
(regulate solute passage)
- Functions:
• Channels – transmembrane
pores
• Transporters – facilitate
solute crossing
• Receptors – bind with
specific ligands, initiate
cellular response
Fig 3.3
• PROTEINS
- Functions, cont.
• Enzymes – embedded
for metabolic action on
PM
• Identity Markers –
glycoproteins
• Linkers – add strength
to cell or cell connection
Fig 3.3
PLASMA MEMBRANE:
Selective Permeability
• PM permits passage only of select
molecules
• Molecules that can pass through the PM:
- Nonpolar & Uncharged Substances:
O2, other gases, lipids
- Special Case: H2O
• Can pass through spaces formed as phospholipids
move
PLASMA MEMBRANE:
Selective Permeability
• Molecules that can not pass through the PM:
- Polar and/or charged substances:
Ions, glucose, amino acids, some vitamins
• To cross the PM, they rely on other
mechanisms:
Channels, transporters, endocytosis/exocytosis
PLASMA MEMBRANE:
Gradients
Figure: 3.4
ECF
ICF
ECF
ICF
TRANSPORT MECHANISMS:
Movement Across the Plasma Membrane
• Passive transport
- No ATP input involved
- Substance moves down
its gradient
- Diffusion
- Osmosis
- Facilitated diffusion
(requires a transporter
protein)
• Active transport
- Cellular energy (ATP)
input required
- Substance moves up
(against) its gradient
PASSIVE TRANSPORT MECHANISMS:
Diffusion
• Diffusion –
• Simple diffusion through the Phospholipid
Bilayer
• Diffusion through Protein Channels –
• Open channels
• Gated channels (typically for ions flowing
down electrical gradient)
Gated Ion Channel
Figure: 3.9
PASSIVE TRANSPORT MECHANISMS:
Diffusion
• Diffusion –
• Simple diffusion
• Diffusion through protein channels
• Facilitated Diffusion - protein changes
shape to allow molecular access
(glucose)
Figure:
3.10
Facilitated
Diffusion
Figure: 3.5
Passive Transport Processes:
Factors Affecting Diffusion Rate
• Steepness of gradient
• Temperature
• Size of the molecule
• Surface area
• Diffusion distance
PASSIVE TRANSPORT MECHANISMS:
Osmosis
• Osmosis: Diffusion of water down its
own concentration gradient
Figure: 3.6
PASSIVE TRANSPORT MECHANISMS:
Osmosis
• Effects of H2O concentration on cells:
• Isotonic solution – No effect
• Hypotonic solution – (Osmotic) Lysis
• Hypertonic solution – Crenation
Figure: 3.7
ACTIVE TRANSPORT MECHANISMS
Moves molecules against their concentration
gradient
•
-
Uses transport proteins
Requires energy
Two Types:
• Primary Active Transport
• Secondary Active Transport
ACTIVE TRANSPORT MECHANISMS:
Primary Active Transport
• Primary Active Transport: fueled by ATP
hydrolysis
• Example: Na+/K+ Pump - keeps intracellular
concentrations of Na+ low
Figure: 3.8
ACTIVE TRANSPORT MECHANISMS:
Secondary Active Transport
• Secondary active transport: takes advantage of
stored (potential) energy of steep ionic gradients
- Often Na+ gradient
- Stored energy is released as Na+ leaks back into the cell
(i.e. down its concentration gradient)
- Another substance travels against its own gradient by
using this released energy
• Takes advantage of Na+ gradient
• Established by 1E Active Transport
• Symporters vs. Antiporters
SECONDARY ACTIVE TRANSPORT:
Symporters vs. Antiporters
Move in opposite
directions
Move in same
direction
Figure: 3.9
ACTIVE TRANSPORT MECHANISMS:
Vesicle Transport
• Vesicles: small membrane-enclosed sacs
• Two types of vesicle transport:
- Exocytosis (substances exit the cell)
- Endocytosis (substances enter the cell)
- Both require ATP  Active transport
VESICLE TRANSPORT:
Exocytosis
• Bulk removal of cellular
contents
Exocytosis
- Vesicle attaches to inside
of PM
- Vesicle bilayer fuses with
PM
- Contents are released into
interstitial fluid/space
http://www.biology.washington.edu/bsa/IonTransport/exocytosis.html
VESICLE TRANSPORT:
Endocytosis
•
Intake of substances by the cell
-
Invagination of the PM
Extracellular contents brought into the cell by
means of vesicle formation
Specific types:
• Pinocytosis (bulk-phase endocytosis) – endocytosis
of extracellular fluid (H20 plus solutes)
• Phagocytosis – endocytosis of large extracellular
particles (microbes, debris, dead self cells)
• Receptor-mediated endocytosis – endocytosis of
receptor-specific ligands
Figure:
3.15
Pinocytosis
Figure: 3.12
Figure: 3.14
Phagocytosis
Figure: 3.11
Figure: 3.13
Receptormediated
endocytosis
Figure: 3.10
Examples of Ligands for ReceptorMediated Endocytosis
•
Toxins
Diptheria Toxin
Pseudomonas toxin
Cholera toxin
•
Viruses
Rous sarcoma virus
Semliki forest virus
Vesicular stomatitis virus
Adenovirus
•
Serum transport proteins
and antibodies
Transferrin
Low density lipoprotein
•
Hormones and Growth
Factors
Insulin Epidermal
Growth Factor
Growth Hormone
Thyroid stimulating hormone
Nerve Growth Factor
Calcitonin
Glucagon
Prolactin
Luteinizing Hormone
Thyroid hormone
Platelet Derived Growth Factor
Interferon
Cell Overview
•
•
Plasma Membrane – phospholipid
bilayer
Cytoplasm
•
•
•
Cytosol = intracellular fluid
Organelles = “cell’s organs”
Nucleus – contains chromatin
CYTOPLASM
CYTOSOL
• Liquid component of cytoplasm
• 75-90% Water - depending on cell type
• Other contents = various ions & molecules
• Many chemical reactions occur here
CYTOPLASM
ORGANELLES (“Cell’s organs”)
• Intracellular structures with specific purposes
• Possess a recognizable shape
• Membrane bound or non-membrane bound
• Different concentrations in different cells
• Same organelles can make different products
in different cells
Figure 3.1
ORGANELLES:
CYTOSKELETON
• Fibrous framework of
the cell
- Composed of various
protein fiber types
• Microfilament movement/support
• Intermediate filament support
• Microtubule movement/transport
Figure 3.16