Transcript Physiology of Cells

Physiology of Cells
Passive Transport
1. Diffusion
–
–
Tendency of small particles to spread
out evenly within a given space
Occurs down a concentration gradient
until equilibrium is reached
• Measurable difference between one area to
another
–
–
Diffusion or permeability dependent
upon presence of specified channels
Selectively permeable – channels open
and close based on cell’s needs
Diffusion
Passive Transport
2. Dialysis (fig 4-4, pg. 94)
– Selectively permeable membrane
separates smaller and larger particles
Passive Transport
3. Osmosis (fig 4-5, pg. 95)
– Diffusion of water through a selectively
permeable membrane
Osmotic Pressure
• Osmotic pressure – pressure that develops
in a solution as a result of osmosis
– Important concept for maintaining homeostasis
• Isotonic – two fluids have the same osmotic
pressure
• Hypotonic – lower concentration of solute
outside of the cell (ECF)
• Hypertonic – higher concentration of solute
outside of cell (ECF)
Effects of Osmosis on a Cell
Passive Transport
4. Facilitated
Transport
– (also carrier-mediated
passive transport)
– Movement of a
molecule is facilitated
by a carrier
mechanism in the cell
membrane
– Still requires
concentration
gradient
Passive Transport
5. Filtration
– Passing of water and permeable solutes
through a membrane by the force of
hydrostatic pressure
– Hydrostatic pressure: force or weight of a
fluid pushing against a surface
– Occurs down a hydrostatic pressure
gradient
– Most often associated with movement
across a sheet of cells (ex: capillaries)
Active Transport
• Carrier-mediate process
• Moves molecules “uphill” (against
concentration gradients)
• Ex: Sodium-potassium pump
– Operates in all human cells
Sodium-Potassium Pump
• Transports Na+ out of the cell and K+
into the cell
• Requires energy (sodium-potassium
ATPase) – made in mitochondria
• 3 Na+ bind on the intracellular side
and are exchanged for 2 K+ from the
ECF
Sodium-Potassium Pump
Endocytosis
1. Receptors in the plasma membrane bind
to molecules in the ECF
2. Cytoskeleton pulls a portion of the
membrane inward, creating a pocket
3. Edges of the “pocket” fuse forming a
vesicle
4. Vesicle is pulled inside the cell by the
cytoskeleton
5. Vesicle fuses with the membrane walls of
a lysosome
Exocytosis
Process by which large molecules
(proteins) exit the cell
1. Wastes/proteins enclosed by a
membrane vesicle
2. Cytoskeleton moves vesicle to
plasma membrane
3. Fuses with membrane & releases
contents to ECF
Cell Life Cycle
• Two main
processes:
– Growth
– Reproduction
Cell Growth
• During cell growth additional
cytoplasm and plasma membrane are
produce through protein synthesis
– Includes structural proteins and enzymes
– Anabolic process
DNA Replication
• Prior to cell reproduction, DNA must
be replicated:
1. DNA unzips
2. Nucleotides attached to exposed
base pairs according to base pair rule
(A-T; G-C)
3. DNA polymerase binds nucleotides
4. Two identical DNA strands are
formed
DNA Replication
Cell Reproduction
• One parent cell (diploid) produces two
identical daughter cells (also diploid)
• Two steps:
– Mitosis
– Cytokinesis
Mitosis
• Cell organizes replicated DNA
into two identical sets and
distributes one set to each
daughter cells
• 4 phases of mitosis (PMAT)
1. Prophase
–
–
–
–
Chromosomes shorten & thicken
Centrioles move to poles of cell
Spindle fibers appear
Nuclear membrane disappears
Mitosis
2. Metaphase
–
–
Chromosomes lined up at the equator
(middle)
Spindle fibers attached to centromere
3. Anaphase
–
Centromeres break apart separating
sister chromatids
Mitosis
4. Telophase
– Spindle fibers disappear
– Nuclear envelope reappears
Cytokinesis:
• Completes cellular reproduction by
splitting the plasma membrane and
cytoplasm into two
Meiosis
• Cell division which occurs in primitive
sex cells
• The end result is mature gametes
– Males  sperm
– Females  ova (egg)
• Gametes are haploid (contain half the
number of chromosomes of a somatic
cell)
– Reduction division
Deoxyribonucleic Acid (DNA)
• Double Helix (twisted ladder)
• Monomer: nucleotide
1. 5 Carbon sugar (deoxyribose)
2. Phosphate group
3. Nitrogen base
• Nitrogen bases:
• Purines: adenine, guanine
• Pyrimidines: cytosine, thymine
• Chargaff’s rule
Deoxyribonucleic Acid (DNA)
Deoxyribonucleic Acid (DNA)
• Three base pairs =
codon
• Sequence of base pairs
determines each gene
• Genes control the
production of proteins
(therefore determine
individual traits
Ribonucleic Acid (RNA)
• Single helix
• Smaller than DNA
• 5 carbon sugar = ribose
• Uracil replaces thymine
which binds with adenine
Protein Synthesis
• Occurs in 2 steps:
– Translation: DNA 
RNA
– Transcription: RNA
 Proteins
Translation
• Occurs in the nucleus
• DNA “unzips” exposing base pairs
• RNA nucleotides (already present in the
nucleus) attach themselves to the exposed
bases along one side of the DNA molecule
• These RNA nucleotides bind to each other
with the help of RNA polymerase
• The chain that results is called messenger
RNA (mRNA)
Preparation of mRNA
• Occurs in the nucleus
• The mRNA pulles away from the DNA
strand
• The result is a copy or “transcript” of a gene
• Non-coding portions of mRNA (introns) are
removed
• Remaining portions (exons) are spliced back
together
• mRNA travels to the nucleus via nuclear
pores
Translation
• Occurs in the cytoplasm
• Ribosomal subunits attach to one of the
mRNA
• Transfer RNA (tRNA) bring specific amino
acids to the ribosome site
– Which amino acid is based upon each mRNA
codon
– The tRNA must have a complementary anticodon
to fit onto the binding site of each condon
Translation
• Amino acids are joined by peptide
bonds
• Forms long molecules called
polypeptides
• Several polypeptides are needed to
make a protein
After Translation…
• Enzymes within the endoplasmic
reticulum, Golgi apparatus and/or
cytosol line polypeptides to form
larger structures
– May also form lipoproteins or
glycoproteins
Translation Video
http://www.dnatube.com/video/1947/ProteinSynthesis-Translation-1