Homeostasis, Membrane Transport and Excitable Cells
Physiology is the study of the normal functioning of a living organism and its
The cell is the smallest form of life. Cells come together to make tissues. Tissues then
come together to form an organ. Multiple Organs form an organ system. When
multiple organ systems work together, an organism is created.
The cells in a multicellular organism play a critical role in the Homeostasis of the
organism as a whole.
Homeostasis is the ability of the body to maintain a relatively constant internal
environment. Homeostasis is derived from the words homeo= similar and stasic=
Our body’s are constantly being exposed to external environments, such
environments can cause the body’s internal environment to leave homeostasis.
The body uses a system called Negative Feedback to control the changes brought
upon by the external environment. When the body senses a change in pH,
temperature etc. a response is sent to counteract the external environment. An
example is that when the external environment is cold and our body’s become cold,
we start shivering to warm ourselves up.
There is as well Positive Feedback which sends signal to continue certain functions
of the body in the needed time. An example is oxytocin during labor.
67 percent of the total body water is found inside the cell, 7 percent in the plasma of
blood vessels and 26 percent in the area outside cells and blood vessels
If there was 42L of water, 28L would be in the intracellular fluid, 14L in the
extracellular fluid and 3L in the plasma.
Body fluid composition:
Our body fluids act as “exchange sites” with the external environment. Cells
within the body are surrounded by extracellular fluid
Serves not as a transition between the external environment and the
intracellular fluid inside a cell
Extracellular fluid= fluid inside of the cell (cytoplasm)
Extracellular fluid= interstitial fluid (fluid surrounding cell) and plasma
(liquid component of blood) Chemical Component of Cells: (remember salty banana)
Substance Plasma Interstital Intracellular Majority
Fluid Fluid located:
Sodium ion 148 150 15 Outside cell
Potassium ion 4.8 5.0 150 Inside cell
Calcium ion 2.5 2.4 1 Inside cell
Chloride ions 102 125 9 Outside cell
Proteins 1.2 0.2 4 Inside cell
Nucleus: contains DNA
Mitochondria: produces energy (ATP)
Lysosome: Contains enzymes which breaks down uselessness
Cytoskeleton: Structures cell
Rough ER: makes proteins
Smooth ER: makes lipids
Golgi apparatus: adds sugars/ folds proteins (modifies proteins)
Centrioles: Helps with cell division
Phospholipid bilayer formed by hydrophilic phosphate heads facing water while
fatty acid heads try to hid from water. In the cell membrane there are trans
membrane proteins, peripheral proteins, cholesterol, carbohydrate molecules,
glycolipids and glycoprotein molecules.
Long distance communication is essential for maintaining homeostasis
The nervous and endocrine system coordinate many function within body to
Chemical signals release by neuron onto a target call
Has a rapid affect
Affect of cells that has receptors for that chemical
Neuron transmitters travel long distances
Neurotransmitter (the chemical affecting the cell) travels short distance.
Produce by an endocrine cell
Structural difference to neuron Chemical signals secreted into bloodstream
Affect only cells with receptors for specific hormones
Unlike the neuron, the hormones (the chemical) travels, not the cell itself
Slower than the neurotransmitter
Chemical signals made by a neuron and secreted into the blood stream
Only affect cells with the correct receptor.
Cells that are not of endocrine or nervous origin can also communicate with one
another through both chemical and physical means.
Chemical- mediated cell- cell communication:
Autocrine: signals out on the call that made it
Short distance travels
It self motivates the cell.. the cell releases a chemical which interacts with its
Paracrine: signals out on the neighboring cell.
Cytokine: a chemical signal made by any type of cell in response to stimulation
Contact- dependent signals:
Surface molecules on one cell membrane bind to a membrane protein of another
Protein channels that make cytoplasmic “bridges” between adjacent cells.
Can directly transfect chemicals and electrical signals.
Cells are constantly receiving inputs and messages from their environments and
from one another. Frequently, messages that the cell “receives” either begins to be
processed at the cell membrane or need to pass through the cell membrane in order
to be interpreted. Recall that the cell membrane has a highly hydrophobic core and a
number of embedded proteins.
Functions of the protein membranes:
1. Ion Channels
4. Membrane carried
Transportation of glucose
Proteins that can attach and carry molecules (physically move them) Mechanisms of membrane transport:
Cell membrane forms membrane around molecules, pass through
membrane and releases
Pinocytosis of small molecules
2. Diffusion through lipid by layer
3. Diffusion through protein channels
4. Facilitated Diffusion
5. Active transport
Only form of transport that required energy.
Movement of molecules from an area of high concentration to an area
of lower concentration due to a molecules rendon thermal motion
Passive process (no energy is required)
Can reach equilibrium
At equilibrium, concentration are equal but molecules are still moving
(no net movement)
Factors that affect the rate of diffusion:
1. Concentration gradient
Molecules want to move from areas of high concentration to areas of
2. Surface area
More surface area, more places for diffusion to occur
Alveoli in the lungs
3. Size of molecules
Smaller molecules, faster diffusion
4. Membrane thickness
Thicker membrane, slower rate of diffusion
5. Lipid solubility of a molecule
More soluble to fat, faster rate
Diffusion of fat-soluble molecules through the lipid bilayer:
Can pass through the bilayer because fat soluble molecules are hydrophobic and
There is no help needed.
Naturally move from an area of high concentration to an area of low concentration
Example: Oxygen and Carbon Dioxide. Diffusion of water- soluble molecules through protein channels:
Ions cannot pass through gradient because of charge therefore use protein channels
Protein channels block off the bilayer which blocks off the hydrophobic core
allowing hydrophilic molecules to pass through
Protein channels have certain charge:
If channel is negatively charged, positively charged ions will pass
If channel is positively charged, negatively charged ions will pass
The channels have a certain diameter that ions must pass through. Anything bigger
than the diameter of the channel cannot pass through making the channel to be
The greater concentration gradient, the faster the movement is
The greater number of channels, the faster the movement is.
Protein carrier grabs a specific molecule on one side of the membrane and drops it
on the other
The concentration has to be higher on one side than the other
Competitive inhibition can occur if a non- specific molecule tries to bind to it.
Limited in capacity as it can only turn so fast that it can only transport so many
molecules at once
Not all the glucose we need can get into the cell.
Opposing to others where they do not use energy
Must be chemically specific
Moves things against concentration gradient
Other types of transport move with concentration gradient
Limited in capacity
Movement of water across a membrane in response to a solute gradient
Water moves to dilute the more concentrated solution
3 things affect osmosis across a cell membrane
1. Permeability to the membrane
Water must be permeable.
2. Concentration gradient of solution
Must be a concentration gradient for water to diffuse/ t