Chapter 8: Cellular Transport and the Cell Cycle
Plasma Membrane
maintain proper concentration of materials inside of cell
selective permeability: maintains homeostasis
Only molecules of water, oxygen, nitrogen, carbon dioxide
and a few other small nonpolar molc can diffuse
directly across the lipid bilayer proteins determine
which particles pass thru membrane (diffusion thru
proteins)
Osmosis
Plasma memb. limits diffusion of water soluble molecules, but
ALLOWS water thru
Osmosis: diffusion of water across a selectively permeable
barrier
high water conc (low solute) to low water conc. (high solute)
high [water] -------> low [water]
Regulation of osmosis very important to homeostasis
Concentration gradient controls osmosis
Isotonic Solutions
[solutes]inside cell = [solutes]outside cell
Cells in such a solution do not experience osmosis- retain
their shape
Hypotonic Solutions
[solutes]inside cell > [solutes]outside cell
Therefore, more water outside cell
Water rushes in, cell swells, animal cells may burst
Plant cells don't burst b/c they have cell walls, rather they
become more firm
Turgor pressure: the internal pressure in a cell that results
from water diffusing into a cell--- gives shape to plant
cells
some organisms have contractile vacuoles to remove excess
water
Hypertonic Solutions
[solutes]inside cell < [solutes]outside cell
Therefore, more water inside cell
Water flows out
Plasmolysis-loss of water from a cell resulting in a drop of
turgor pressure
animal cells shrivel, plants wilt
Membrane Transport
Passive Transport (thru membrane)
water, lipids, lipid soluble molc. diffuse thru membrane
therefore, requires no energy = "passive transport"
Passive Transport (via channel proteins)
"facilitated diffusion"
sugar, amino acids
driven by concentration gradient
Active Transport
moves solutes against concentration gradient
from [low] -----> [high]
Requires energy
Carrier protein binds substance to be transported
(each carrier protein has shape that matches specific molecule or ion)
Solute binds, chemical energy allows cell to change protein shape so that solute
is released on the other side of the membrane
After particle released, protein resumes original shape
Endocytosis
Lg molecules, groups of molecules, other cells too large to go thru membrane
Cell surrounds, engulfs, internalizes material
Exocytosis
reverse of endocytosis
expel wastes, secretion
Cell Growth: Limitations on Cell Size
Most cells are between 2 and 200 mm
1. Surface Area to Volume Ratio
Area2: Volume3
Surface area = #sides x length x width
Volume = length x width x height
1mm x 1mm x 1mm 2mm x 2mm x 2mm
S.A. 6mm2 24mm2 Vol 1mm3 8mm3
If cell size doubles, 8x as much nutrients necessary, but only 4x
as much membrane to allow diffusion
2. Diffusion
fast and efficient as long as cell is small
slow and less effective as cell gets larger (surface area to volume ratio)
3. DNA
must support production of proteins- the larger the cell,
the larger the demand for proteins
Cell Reproduction
Cells divide before they get too big
Cell division = 1 cell ---------> 2 identical cells
Chromatin- long strands of DNA wrapped loosely around proteins
Chromosomes- densely packed chromatin, organized for cell division
The Cell Cycle: Read p211
Sequence of growth and division in a cell
Interphase: growth period
most of time spent here
size increases, metabolism proceeds, chromosomes
duplicate
Mitosis: 2 daughter cells form with complete complement
of chromosomes
Cytokinesis: actual division
1. Interphase:
G1: cell growth and protein production
S phase: DNA synthesis and replication
G2: growth, mitochondria and organelles manufactured
2. Prophase (longest phase of mitosis
chromatin coils to form chromosomes
identical sister chromatids pair at centromere
nuclear envelope disappears
nucleolus disintegrates
in animals, centrioles appear (made of microtubules, just
outside nucleus), move to opposite ends of cells
spindle forms between (cage like structure) (in plants,
spindle forms w/o centrioles)
3. Metaphase
Sister chromatids become attached by centromeres line up on midline
4. Anaphase
Separation of chromatids, b/c spindle fibers shorten
5. Telophase
chromatids reach poles
chromosomes loosen ------> chromatin
spindle breaks down
nucleolus reappears
nuclear envelope forms around chromosomes
cell membrane begins to form
6. Cytokinesis: Cell division
In animals, equator tightens and pinches
In plants- cell plate along equator ---> membrane ---> cell wall
In unicellular organisms, mitosis = 2 new cells
In multicellular organisms, mitosis --> tissues --> organs --->
organ systems
Control of the Cell Cycle
Cell cycle controlled by proteins and enzymes produced
cyclically (called cyclins)
Genes- segment of DNA that controls the production of a
protein
Cancer- loss of control of the cell cycle, often as a result of a
change (mutation) in a gene that directs production of
a protein/enzyme involved in cell cycle
- too much or too little of specific protein, or wrong timing
Tumor- masses of cancerous cells form tissue that deprive normal cells of nutrients
Benign: not harmful
Malignant: cancerous and harmful
Second leading cause of death in US (after heart
disease)
Metastasis- cancer cells detach from tumor and enter the
circulatory system and spreads throughout the body
Causes of Cancer:
Genetic (mutation) Environmental (free radical)
Inherited DNA damage
Cigarette smoke
Air and water pollution
UV exposure (sunburn)
Some viruses cause DNA damage
Cancer prevention:
Heathful lifestyle
Low fat, high fiber
Vitamins and minerals -----> antioxidants A, C, E, calcium 
