Download Homeostasis

AHS C&T2
11/1/12
C&T2 — Homeostasis/Cellular Function
Outline — Homeostasis
Definition(s)
Significance — key concept
External and internal environments
Physiological variables
–  regulated
Outline — Cells
Organelles
Plasma membrane
Nucleus
Endoplasmic reticulum
Golgi apparatus
Mitochondrion
Cytoskeleton
Feedback mechanisms
Protein synthesis.
Homeostasis
KEY CONCEPT"
Defined as maintenance of a relatively stable internal environment
It does not mean that composition, temperature, and other characteristics
are absolutely unchanging
Essential for survival and function of all cells
Each cell contributes to maintenance of a relatively stable internal
environment
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AHS C&T2
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Homeostasis
Definitions
‘Homeostasis is essential for the
survival of each cell, and each cell,
through its specialized activities,
contributes as a part of a body
system to the maintenance of the
internal environment shared by all
such cells.’ [Sherwood]
‘dynamic constancy of interstitial fluid
composition’
Physiological Variables
Mean arterial BP (systolic)
120 mmHg (range 105-150)
Red blood cell concentration
Male 5.4 x 106/microlitre (4.5-6.5)
Female 4.8 x 106/microlitre (3.9-5.6)
Variation by physiological state
Gender, age, fasted/fed, exercise etc.
In other words:
The composition of the body fluids
is a key element of homeostasis
Key concept
Negative Feedback
Acts to counteract the change in the controlled variable
Components
1.  Controlled variable
2.  Sensor
3.  Integrator
4.  Effector
5.  Compensatory response.
Sherwood 1-7"
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Negative Feedback
NB ‘set point’
Sherwood Fig. 1-7
1. 
Controlled variable — body temperature
2. 
Sensor — nerve cells
3.  Integrator — temperature control centre
4.  Effectors — skeletal muscle (etc)
5.  Compensatory mechanism — heat production.
Positive Feedback
reinforces the change in the controlled variable
Contraction of the uterus at birth
cervix
Oxytocin —> contraction of uterine
smooth muscle —> moves baby
lower —> pressure on cervix —>
oxytocin release
Other examples
Clotting
Propagation of action potential in
a neuron
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Pathophysiology
‘ … a state in which normal function at
any organizational level is disrupted to
the extent that normal regulatory and
compensatory mechanisms cannot
maintain homeostasis.’ [Davis et al.]
Cells
Exchange materials (eg. nutrients,
oxygen, carbon dioxide, wastes) with
surrounding environment
Perform chemical reactions that provide
energy for the cell
Synthesize cellular components
Sense and respond to changes in
surrounding environment (receptors)
Reproduce (divide)
Cell physiology, biochemistry and
molecular biology allows us to further
assess function of subcellular
organelles, proteins etc.
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Cells are structurally specialised for function
Humans are very different from single-celled or simple
multicellular organisms!
Neuron
Cardiac muscle cell
Gamete
Plasma Membrane
Controls passage of materials
Has embedded proteins and molecules (not shown here)
Receptors, carriers, recognition, etc etc.
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Nucleus
DNA (genetic)
Pores (selective)
Nucleolus
Ribosomal RNA synthesis
A crucial stage in protein
synthesis.
Endoplasmic reticulum (ER)
Membrane "
encloses space (lumen)"
Continuous with nuclear
envelope"
‘Rough’ ER — ribosomes"
Protein synthesis"
Smooth ER"
Lipids (NB steroids)"
Ca2+ store (muscle)"
Detox (liver)."
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Golgi Apparatus
Flattened sacs
(green in picture)
Between ER and
cell membrane
Processes and
packages
molecules into
vesicles for
transport.
Stanfield & Germann 2.19"
Proteins — roles
Within cell
S&G 2.9
"
e.g. enzymes,
signalling, structure
Cell membrane
e.g. transporters,
structure
Proteins are chains of amino acids (AA) & may
take very complex forms eg haemoglobin"
Outside cell
e.g. digestive
enzymes, hormones.
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Transcription/Genetic Code
A triplet codes for one amino
acid (AA)
Or start/stop
Bases (GTCA) are
complementary
Forms mRNA
mRNA to cytoplasm
Translation by ribosomes
Addition of AA — peptide.
Read more: Germann & Stanfield pp46-50"
Protein Synthesis & Sorting
Pathway (radiolabel expt)
Uptake of amino acids in RER
(synthesis)
Golgi (packaging)
Vacuoles (storage)
Cell membrane (release)
Further packaging into vesicles
takes place in Golgi and ER
Sorting
"
AGA Inst. www.gastroslides.org
AGA Inst. www.gastroslides.org"
Leader sequence determines fate.
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The Mitochondrion
Energy production
From glucose
Generates ATP
ADP + Pi + energy
!
ATP
1 mole glucose yields ~38 moles of ATP
(40% efficient; rest is heat)"
Most ATP synthesis is in the cristae."
Cytoskeleton
Dynamic array of
filaments
Confer shape
Allow movement
1. Microfilaments
2. Intermediate filaments
3. Microtubules.
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Cytoskeleton
Microfilaments (8 nm)
Two helices of F-actin (fibrous)
Formed from G-actin (globular)
Actin is most common
Movement: amoeboid; cell division
Support for microvilli.
Intermediate filaments (10 nm)
Stronger & more stable
Keratin (skin & hair)
Myosin (muscle)
Microtubules (25nm)
Strength
Cell division
Movement
Cilia and flagellae e.g. respiratory tract, spermatozoa.
Cilium"
Learning Outcomes
Homeostasis
To be able to:
Cells
1.  outline the structure and function of
mammalian cell organelles (i.e.
1.  give clear physiological examples of
nucleus, membrane, mitochondria,
homeostasis
ribosomes, endoplasmic reticulum,
cytoskeleton, and golgi)
2.  give a physiological example of
positive and negative feedback
2.  the cellular events involved in
3.  give an example of a change
exceeding the body’s capacity to
rectify it (pathophysiology)
protein synthesis.
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