·Positive and negative feedback – what are they? Understand basics of what they do and
how they affect physiology
-Remember homeostasis is the tendency to resist change in order to maintain a stable,
relatively constant internal environment. In the maintenance of homeostasis, the body uses two
feedback mechanisms: Positive and negative. Homeostasis typically involves negative feedback
loops that counteract changes of various properties from the target values, which are known as
set points.
-Negative feedback loops act to oppose a stimulus or cue that triggers them. For
example, when our body temperature is too high, a negative feedback loop will act to bring it
back down toward the set point of 37C. This works through a stimulus triggering sensors in the
body detecting a change, which then relay the stimulus to a control center. The control center till
then activate effectors whose job it is to oppose the stimulus in the body. This feedback loop will
continue until the body has reached homeostasis once again.
-Positive feedback loops, unlike negative, amplify the starting signal or stimulus. Positive
feedback loops are usually found in processes that need to be pushed to completion. A good
example of a positive feedback loop is childbirth. In childbirth, the baby’s head presses on the
cervix, which activate neurons in the brain. The neurons send a signal that lead to release of the
hormone oxytocin from the pituitary gland. Oxytocin increases uterine contractions, and thus
increases pressure on the cervix. This continues to build and amplify releasing more oxytocin,
which produces stronger and stronger contractions until the baby is born.
-Homeostasis depends on feedback loops. So, anything that interferes with the feedback
mechanism can and usually will disrupt homeostasis. This is what leads to disease in the body.
-The body must have both positive and negative feedback mechanisms
-Remember that negative feedback leads to a positive outcome
-Positive feedback typically leads to instability and disease
·Transport of molecules through the membrane; diffusion and what affects it; facilitated
diffusion, active transport; and osmosis and what it is
-Remember, the cell wall is a phospholipid bilayer. Each layer is one molecule thick and
each molecule consists of a hydrophilic head and a hydrophobic tail.
-In the cellular membrane are a number of protein channels. Proteins are the workhorse
of the cell. They’re so important because they facilitate movement of substance across the
cellular membrane.
, - The majority of molecule transfer depends on specialized membrane transport proteins
that span the lipid bilayer. These are thoroughfares for select molecules.
-Mediated transport: term used to describe cell membrane transport facilitated by
proteins.
-Each type of cell tissue has its own types of transport proteins that determine which
solute can pass in and out of the cell or cell organelle
-There are two main classes of transport proteins: transporters and channels. A
transporter is specific and allows only those ions that fit into the unique binding site to pass
through. Channels, when open, form a pore across the membrane, which allows ions and
selective polar organic molecules to diffuse across.
·Membrane permeability and ion permeability
-The diffusion rate is influenced by differences in electrical potential across the
membrane. Because the pores in the lipid bilayer are often linked with Ca++, other cations (Na+
and K+) diffuse slowly. Consider, when trying to place the similar poles of a magnet together
they repel each other.
- Remember substances with a lower permeability coefficient (i.e. 10-2), have a higher
permeability across the cell membrane. For example, water has a high permeability across the
cell membrane and has a lower permeability coefficient. Remember also, that charged
molecules have a lower permeability across the cell membrane and therefore a higher
coefficient. According to the textbook, the permeability coefficient relates to the size of the
molecule.
·Diffusion potentials
Khan video explains this and resting membrane potential very well.
https://www.khanacademy.org/test-prep/mcat/cells/transport-across-a-cell-
membrane/v/membrane-potentials-part-1
To summarize what’s explained in the video- using K as an example- A cell has higher levels of
K intracellularly vs in the extracellular fluid. Therefore a diffusion gradient exists, and K wants to
equalize and move outside the cell to balance the concentration inside and outside the cell.
·Resting membrane potential
Concentration gradient, permeability (to a specific ion)
·Understand the process of nerve action potentials and steps involved
·Myelinated vs. unmyelinated fibers. Know the basic differences purpose of myelin
·Acetylcholine role in skeletal muscle excitation
·Neuromuscular junction physiology; know basic process in order
·Calcium role in skeletal muscle contraction; tropomyosin function and Troponin
·Excitation and contraction of smooth muscle, calcium, and calmodulin
·Atrophy, hypertrophy, dysplasia; know what they are and what effect they have on the
cells
·Reperfusion injury physiology
·Apoptosis vs. necrosis
, ·Cellular aging, frailty, somatic death
·Cellular injury basic patho hypoxia; infiltrations, etc
·Understand basics of common toxins and how their patho behind how they cause cellular alterations
(lead, radiation, carbon monoxide; etc)
Module 2 Genetics; Cancer
·Transcription and Translation. Understand basics of each process
- Transcription
o RNA is synthesized from the DNA template via RNA polymerase (actually binds to a
specific site on the DNA which signifies the beginning of the gene. This site is the
promotor site. RNA polymerase transcribes it by pulling apart DNA strand apart and
allows unattached DNA bases to be exposed. One of the DNA strands then provide the
coding template for the mRNA with binding of those complementary base pairs from rna
to dna/ but remember rna does not have thymine. Instead of A binding to T, it binds to
U. and C binds to G.
▪ The end product is mRNA.
▪ RNA polymerase binds to the promoter site on DNA.
o DNA specifies a sequence of mRNA.
o Transcription continues until the termination sequence is reached.
o mRNA then moves out of the nucleus and into the cytoplasm.
o Gene splicing occurs.
▪ Introns and extrons
• A lot of the RNA sequences are removed. And a few of the RNA
sequences are spliced together to form together to form functional
mRNA that will move to cytoplasm and serve to code proteins. The ones
that are sliced and removed are known as introns.
▪ Introns are removed - non coding sequences
▪ Exons litigate together once introns are removed and go on to code for proteins.
(and to form the functional RNA)
- RNA Splicing
o Example:
▪ BOB THE BIG TAN CAT
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