PSL201 Week 1

Topics: Homeostasis, cell-cell communication, flow gradients, transport

Physiology

The study of how living organisms function

Anatomy vs physiology

Anatomy: identifying body parts

Physiology: how does the body work?

Describe the 2 ways problems are approached in physiology (e.g. types of questions you can ask)

Functional questions: purpose or significance of event (why)
Process questions: how the event is achieved (how)

Homeostasis

Body’s ability to maintain a relatively stable internal environment despite constant changes occurring inside and outside the body

Error signal

“Quantitative difference between a desired target value (the set point) and the actual measured value of a regulated internal variable”

List and describe the basic components needed to maintain homeostasis

Sensor: detects change in a regulated variable, sends it to the control centre

Control centre: compares current value to desired value/set point; activates effector if change is needed

Effector: produces a response that moves variable back towards its normal range

Negative feedback

Occurs when a change in a regulated variable triggers a response that opposes the original change, moving it back to the set point

Circadian rhythms + relation to physiology

A biological rhythm that repeats approx every 24 hours
Many physiological variables are regulated around this pattern (e.g. cortisol tends to be higher when we wake up, less throughout the day)

Positive feedback (definition, characteristics)

Output of system acts to enhance (amplify) the changes of the input system

The response reinforces the original stimulus
Used to drive process towards a specific end point/goal

Less common than negative feedback

Which is more common?

Describe the positive feedback loop involved in the onset of contractions in child birth

When a contraction occurs, the hormone oxytocin is released into the body, which stimulates further contractions, causing the baby to push even harder against the cervix, leading to the release of more oxytocin and stronger contractions. Cycle continues until baby is delivered, and the stimulus is removed

Describe the positive feedback loop involved in blood clotting

The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot (bleeding stops)

Describe the positive feedback loop involved in lactation

The more the baby suckles, the more milk is produced

How does positive feedback usually come to a stop? Give some examples

Usually, positive feedback loop terminates when the original stimulus is removed, or counter-acting signals are activated to suppress or break down the loop
Examples: chemicals break down the blood clot (signals), lactation stops when the baby no longer nurses (stimulus removed)

What is the (general) role of the endoplasmic reticulum and golgi apparatus?

Protein synthesis and processing

Name the 4 groups of cells in the body

Neurons, muscle cells, epithelial cells, and connective tissue cells

Neurons

Specialized cells that receive, process, and transmit information throughout the nervous system

Muscle cells

Specialized cells that generate force through contraction, enabling movement and mechanical work

Name and describe the 3 major types of muscle cells:

Skeletal muscle: voluntary movement and posture

Cardiac muscle: generate force needed to pump blood through the body

Smooth muscle: found in walls of blood vessels, moving substances through organs and regulating vessel diameter

Epithelial cells

Form protective barriers that line body surfaces, organs, and cavities.

Specialized for protection, absorption, secretion, and transport between the body and the external environment

Cells are tightly packed together, form a continuous layer

In some tissues, epithelial cells surround a hollow space called a lumen

Connective tissue cells

Provide support, protection, transport, and structural integrity throughout the body

Include diverse cell types such as blood cells, bone cells (osteocytes), and fibroblasts

Often separated by extracellular matrix

Why is blood considered a connective tissue cell?

Blood cells are suspended within an extracellular matrix known as plasma

Describe the 2 broad ways in which cells communicate

Direct communication: neighboring cells are physical connected through gap junctions; channels allow ions to move directly between cells
Indirect communication: via chemical messengers, one cell release a signaling molecule that travels to a target cell and binds to a receptor to trigger cell’s response

Name and describe the types of chemical messenger signaling

Paracrine signaling: messenger acts on local nearby cells (signal diffuses through the extracellular fluid)

Autocrine signaling: messenger acts on the cell itself (a cell signals itself)
Neurotransmitter signaling: chemical messengers pass signals between nerve cells across the synapse

Endocrine signaling: release hormones into bloodstream; only cells with appropriate receptors will respond

Describe the body’s 2 major communication systems

Nervous system: NTs are the messengers; travels in synapse targets neurons, muscles, and glands; rapid and specific communication

Endocrine system: hormones are the messengers; travels in bloodstream; targets most cell types; slow, broad, simultaneous communication

Passive transport (definition + characteristics + examples)

Movement down electrochemical gradient, does not require energy

Examples: simple diffusion, facilitated diffusion

Active transport (definition + characteristics + examples)

Movement against electrochemical gradient, requires energy
Examples: primary, secondary

What is the relative concentrations of key molecules inside and outside the cell?

Higher outside cells: Na+, Ca2+, Cl-, HCO3-

Higher inside cells: K, amino acids-, protein-, phosphate-, HCO3-

What does active transport require?

Membrane transport proteins

Describe carriers

They are a type of membrane transport protein. They find a specific molecule and undergoes a conformational change in shape that moves the molecules across the membrane

What types of transport can carriers participate in?

Facilitated diffusion or active transport

Describe pumps

They are a type of membrane transport protein. They use energy (usually ATP) to move substances against their electrochemical gradients. Participate in active transport

Primary active transport (definition + example)

ATP energy used directly
e.g. sodium-potassium pump

Secondary active transport (definition + example)

ATP energy is not used directly; cell takes advantage of an ion gradient that was previously established by a primary active transport

Describe the 2 types of secondary active transport

Co-transport: both move in same direction

Counter-transport: move in opposite directions

For large molecules, cells use _________ transport mechanisms

Vesicular (means: molecules inside the cell are packaged into tiny, membrane-bound sac-like containers called vesicles to move across or around the cell)

Endocytosis (definition + name the 2 types)

Definition: large molecules move into cell
Types: phagocytosis and receptor-mediated endocytosis

Describe phagocytosis

Plasma membrane of cell engulfs large particles (e.g. bacteria)
Once internalized, the particle becomes enclosed with phagosomes (membrane-bound vesicles)
Phagosomes fuse with lysosome (organelles that contains digestive enzymes) to create a phagolysosome, where engulfed material is broken down and digested
Important role in immune defense

Describe receptor-mediated endocytosis process, and what it’s commonly used for

Specific molecules bind to receptors on cell surface and are selectively brought into cell
1. Molecule binds to matching receptor on cell surface

2. Receptor molecule complexes accumulate in coated pits (specialized region on membrane)

3. Membrane folds inwards and pinches off, forming a vesicle that brings bound molecules into cell

Commonly used for uptake of nutrients, hormones, and other signaling molecules

What kind of endocytosis do cells use to take up cholesterol-containing LDL particles from the bloodstream?

Receptor-mediated endocytosis

Describe exocytosis

Definition: large molecules move out of cell
1. Substances packed into membrane-bound secretory vesicles within the cell

2. Intracellular vesicles fuse with plasma membrane, releasing their continents into extracellular space

Every time a neuron communicates with another cell, it releases neurotransmitters through _______

Exocytosis

Describe transcytosis

Combines endocytosis and exocytosis into a single process
1. Substance brought into cell on one side of the membrane through endocytosis

2. Vesicle travels through cell and is released on the opposite side of the cell by exocytosis

Allows cells to act as a transportation bridge

Important in epithelial tissues

Newborns absorb maternal antibodies from breast milk through transcytosis in the intestine, helping provide early immune protection

Describe the 2 major factors that determine flow

1. Size of the energy gradient: larger gradient, greater tendency to move

2. Resistance to flow: opposed movement, makes flow difficult

Flow _________ when gradient increases, ________ when resistance increases

Increases; decreases

Describe concentration gradient. What kind of driving force is it?

H → L conc
The larger the concentration difference, the larger the chemical driving force

Resting membrane potential

Voltage difference across plasma membrane when the cell is at rest
Inside of the plasma membrane negative in comparison to outside

Describe the phospholipid bilayer and what can typically pass through it

Heads: hydrophilic (outer part)
Tails: hydrophobic (inner part/middle)
Molecules that are hydrophobic (non-polar or lipid soluble) as well as water often diffuse directly through the membrane easily
Hydrophilic molecules often require specialized membrane proteins such as channel and transporter to help the move across

Simple diffusion (how it works, types of molecules)

A type of passive transport where molecules pass through the through phospholipid bilayer
Small, nonpolar, lipid soluble molecules

Facilitated diffusion (how it works, types of molecules)

A type of passive transport where molecules requires help of membrane proteins (carriers or channels) to get across the membrane

One-way flux

The rate at which a substance moves across in only one specific direction

Net flux

Difference between the two one-way fluxes; overall rate of diffusion

The larger the concentration difference, the larger the net flux, the faster diffusion occurs

Dynamic equilibrium

The number of molecules moving in each direction is equal

Diffusion increases as the concentration gradient _________

Increases

As concentration gradient increases, the rate of simple diffusion increases in a ______ fashion

Linear

What is required for facilitated diffusion?

Membrane transport proteins

Describe channels

A type of membrane transport protein. Creates a pore/tunnel through the membrane, for appropriate ions or molecules to move down their electrochemical gradient

Describe what a graph looks like for facilitated diffusion

Rapid increase, and then reaches a maximum

Aquaporin

Membrane transport protein for water; most water movement occurs through these

Osmosis

Diffusion of water through a membrane; always passive

Molarity

Concentration of molecules in a 1L solution; measure of how much solute is present in a solution; about molecular weight

True or false: a 1 molar litre of glucose and NaCl contain the same number of molecules per litre of solution

True

Osmolarity

Concentration of particles in 1 L solution

1 Molar solution of NaCl = _ Osmolar solution of NaCl

2 (NaCl dissolves in Na and Cl ions, creating 2 particles from every 1 molecule)

Molarity counts ________, osmolarity counts ________

Molecules; particles

What is more important for predicting water movement, osmolarity or molarity? Why?

Because particles determine water movement, osmolarity is often more useful when discussing osmosis

1 Osmolar (Osm) = how many milliOsmolar (mOsm)?

1000 mOsm

What is the average amount of impermeant solutes in a cell (in mOSm units)?

300mOsm

Is it more important to consider impermeant or permeate solutes when predicting water movement, and why?

When predicting water movement, most important solutes to consider are the impermeant solutes, since permeant ones eventually redistribute across the membrane and lose much of their long term osmotic effect

Tonicity

“Ability of an extracellular solution (a fluid surrounding a cell) to make water move into or out of the cell through osmosis”

Tonicity predicts what happens to cell _______

Volume

Describe isotonic solutions and what they do to a cell (cell shrink, swell, or neither?)

300mOsm of impermeant solutes
Cells neither swell nor shrink when exposed to the solution
Water moves equally, resulting in no net water movement!

Describe hypotonic solutions and what they do to a cell (cell shrink, swell, or neither?)

>300mOsm
Lower concentration of impermeant solutes than the cell
Water moves into cell, causing the cell to swell up

Describe hypertonic solutions and what they do to a cell (cell shrink, swell, or neither?)

<300mOsm
Higher concentration of impermeant solutes than the cell
Water moves out of cell, causing cell to shrink

What substance is constantly moving between the compartments, help with transport and maintain appropriate volume and composition of body fluids?

Water

Absorption

Moving substance from external environment into the body

Secretion

Moving a substance from the body into the lumen/external space

Filtration

Movement of small solutes and fluid from the blood into the kidney tubules where urine can be formed

Reabsorption + example

Moving substances back into the body after they’ve been filtered|
e.g. kidney reabsorbs water and many useful solutes back into blood

Excretion + example

Final removal of substances from the body
e.g. lungs remove carbon dioxide from blood

In the body, water is found in different ___________

Compartments

Intracellular fluid (what is it, describe its characteristics)

Fluid inside the cell
Largest water compartment
Roughly ⅔ of the body’s total water content

What makes up roughly ⅔ of the body’s total water content?

Intracellular fluid

Extracellular fluid (what is it, describe its characteristics)

All the fluid outside of the cells

Roughly ⅓ (14L) of the body’s total water content

Forms the body’s internal environment

Nutrients, oxygen, hormones, waste, must all pass through the ECF as they move between the blood and the cells

What makes up roughly ⅓ of the body’s total water content?

Extracellular fluid

What are the 2 parts of the extracellular fluid?

Plasma and Interstitial fluid (ISF)

Describe plasma

Fluid portion of the blood, found in the blood vessels
Serves as the body’s transportation system (carries oxygen, hormones, etc.)
Makes up 20% of the ECF (3L)
Separated from everything (tissues) by the walls of the blood vessels

Describe interstitial fluid

Fluid that surrounds the cells of your body
Nutrients, oxygen, hormones, waste, must all pass through the ISF before reaching a cell or returning it to the blood
Cell’s "immediate environment” (since cells do not have direct contact with blood vessels)
80% of ECF (11L)

What is the average total body water (TBW) content in L and %?

42L, 60%