BIOL*1080 Biological Concepts of Health
Chapter 1a Humans in the World of Biology
Basic Characteristics of All Living Things
• Living things contain nucleic acids, proteins, carbohydrates, and lipids.
o The nucleic acid DNA is especially important because DNA molecules can make copies of
themselves, an ability that enables organisms to reproduce.
• Living things are composed of cells.
• Living things grow and reproduce.
• Living things use energy and raw materials.
o The term metabolism refers to all chemical reactions that occur within the cells of living things.
• Living things respond to their environment.
• Living things maintain homeostasis.
o Homeostasis is the relatively constant and selfcorrecting internal environment of a living
• Populations of living things evolve and have adaptive traits.
Classification by Evolutionary Relationship
• Organisms with the greatest similarity are grouped together.
• One system recently favored by many biologists recognizes three domains.
• Two of the domains, Bacteria and Archaea, consist of the various kinds of prokaryotes – all very small,
singlecelled organisms that lack a nucleus or other internal compartments.
• All other organisms, including humans, belong to the third domain, Eukarya.
• Eukarya have eukaryotic cells, which contain a nucleus and complex internal compartments called
• Domain Eukarya is subdivided into four kingdoms – protists, fungi, plants, and animals.
• Human characteristics include a large brain size relative to body size and a twolegged gait.
• But nothing distinguishes humans more than culture.
• Culture may be regarded as a set of social influences that produce an integrated pattern of knowledge,
belief, and behaviour.
Levels of Biological Organization
• A population is individuals of the same species (individuals that can interbreed) living in a distinct
• A community is all living species that can potentially interact in a particular geographic area.
• The biosphere is that part of Earth where life is found.
• Is a way of learning about the natural world by applying certain rules of logic to the way information is
gathered and conclusions are drawn.
• Develop a testable hypothesis – an explanation of your observation.
• Make a prediction based on your hypothesis and test it with a controlled experiment.
o A controlled experiment, the research subjects are randomly, divided into two groups.
o One group is designated as the control group and the other is designated as the experimental
o Both groups are treated in the same way except for one factor, called the variable, whose effect
the experiment is designed to reveal.
• Draw a conclusion based on the results of the experiment.
o A conclusion, which is an interpretation of the data. • Statistical significance of the data, which is a measure of the possibility that the results were due to
• Inductive and Deductive Reasoning
o In inductive reasoning, facts are accumulated through observation until the sheer weight of the
evidence allows some logical general statement to be made.
o Deductive reasoning, begins with a general statement that leads logically to one or more
deductions, or conclusions.
o The process can usually be described as an “if then” series of associations.
• Epidemiological Studies
o Researchers look at patterns that occur within large populations.
1d: Body Organization and Homeostasis
From Cells to Organ Systems
• A tissue is a group of cells of similar type that work together to serve a common function
• Human tissues have four primary types
o Epithelial Tissue: Covers body surfaces, lines body cavities and or organs and forms glands
o Connective Tissue: Serves as a storage site for fat, plays an important role in immunity and
provides the body and its organs with protection and support
o Muscle Tissue: Responsible for body movement and the movement of substances throughout the
o Nervous Tissue: Conducts nerve impulses from one part of the body to another
• Epithelial Tissue
o Have a free surface and basement membrane
Free surface: Specialized for protein and secretion/absorption
Basement membrane: Noncellular layer that binds epithelial cells to underlying
connective tissue resists stretching
o Free surface has three basic shapes of cells
Squamous: Flattened, scalelike cells that form linings in the blood vessels of lungs and
allows for O2 and CO2 to diffuse
Cuboidal: Cube shaped cells that are found in many glands and is the lining of kidney
tubules. Provides some protection and are specialized for secretion and absorption.
Columnar: Elongated, columnshaped cells that are specialized for absorption and
secretion and line the small intestine
• A gland is an epithelial tissue that secretes a product
• An exocrine gland secretes their products into ducts leading to body surfaces, cavities or organs
(produce digestive enzymes)
• An endocrine gland lacks ducts. They secrete their products/hormones into spaces just outside the cells
(hormones diffuse into bloodstream)
• Connective Tissue
o Binds tendons and ligaments and supports other tissue (cartilage+bone)
o Most abundant and widely distributed tissue in body
o All connective tissue contain cells embedded in an extracellular matrix
o Matrix consists of protein fibers and a noncellular material called ground substance
o Ground substance may be solid (bone), fluid (blood) or gelatinous (cartilage)
Ground substance is secreted by the connective tissue or other cells near by
o Matrix contains three types of fibers
Collagen: Strong and ropelike fibers that withstand pulling because of their great tensile
strength Elastic: Random coils that can stretch and recoil like a spring (i.e. skin, lung, blood
Reticular: Thin strands of collagen that are interconnecting networks suitable for
supporting soft tissue (support liver+spleen)
• All three types of fibers are produced by cells called fibroblasts in the connective tissue
• Fibroblasts repair tears in body tissue and have two broad categories
o Connective Tissue Proper: Loose and dense connective tissue that contains many cells but has
fewer and more loosely woven fibers
o Specialized Connective Tissue: Cartilage, bone and blood. Provides support and protection and
helps transport CO2, O2 and nutrients, helps fight infection
• Connective Tissue Proper has 3 types
o Areolar Connective Tissue: Functions as a universal packing material between other tissues
(found in muscles) , permits muscles to move freely and anchors the skin to underlying tissues
o Adipose Tissue: Contains cells that are specialized for fat storage, insulation and shock absorber
o Dense Connective Tissue: Forms strong bands because of its amounts of tightly woven fibers,
and is found in ligaments, tendons and dermis
• Specialized Connective Tissue has 3 types
o Cartilage: Strong but flexible, cushioning between certain bones and helps maintain the
structure of certain body parts (ears, nose). Cells in cartilage (chondrocytes) sit within spaces of
the matrix called lacunae. It lacks blood vessels/nerves. Nutrients reach cartilage cells by
diffusion, and the cartilage heals slower than bone
3 types of cartilage
Hyaline: Most abundant/support/flexibility. Known as gristle and is found at the end of
large bones. It forms part of the nose, ribs and contains collagen fibers
Elastic: More flexible that Hyaline. Large amounts of wavy elastic fibers found in the
external ear to provide strength and elasticity
Fibrocartilage: Fever cells than other cartilage and contains collagen fibers. Forms
cushioning layers in the knee joint as well as the outer part of the shockabsorbing disks
between vertebrae of spine. Can withstand pressure
o Bone: Apart of skeletal system and is a living/actively metabolizing tissue with good blood
supply. Protection and support for internal structures, movement and storage of lipids/fats and
produces blood cells. Made from hardened Ca for rigid support, and collagen fibers for strength
o Blood: Consisting of a liquid matrix and fibers in the blood are soluble in proteins. Transports
various substances. Red blood cells transport O2 to cells and carries CO2 away. White blood
cells fight infection and protect the body
• Muscle Tissue
o Composed of muscle cells (muscle fibers) that contract when stimulated
o Three types of muscle tissue
Skeletal: Usually attached to bones and is described as a voluntary muscle. Long,
cylinder shaped cells each containing several nuclei. Actin and Myosin interact to cause
Cardiac: Found only in walls of heart, and contractions are responsible for pumping
blood to rest of body (involuntary muscle). Resembles branching cylinders and have
striations and typically 1 nucleus. Special junctions at the plasma membranes of these
cells that strengthen cardiac tissue and promote rapid conduction of impulses throughout
Smooth: Involuntary muscle found in the walls of blood vessels and airways and also in
walls of organs. Contractions reduce the flow of blood/air. Helps organs in mixing and propelling food through digestive track and eliminating waste. Cells taper at each end of
• Nervous Tissue
o Apart of nervous system: brain, spinal cord, nerves
o Two general types
Neurons: Generate and conduct nerve impulses, which they conduct to other
neurons/muscle cells/glands. 3 main parts (body, dendrites, axon). Body houses nucleus
and most organelles, dendrites branched process that provide a large surface area for the
reception of signals from other neurons and axon is a long extension that conducts
impulses away from cell body.
Neuroglia (accessory cells): Support, insulate and protect neurons. Increase the rate at
which impulses are conducted by neurons and provide neurons with nutrients from
nearby blood vessels. Communicates with one another neurons
• Cell Junctions
o 3 types of junctions between cells
Tight Junction: Creates an impermeable junction that prevents the exchange of materials
between cells. Found between epithelial cells of the digestive tract, where they prevent
digestive enzymes and microorganisms from entering the blood
Adhesion Junction: Holds cells together despite stretching and found in tissues that are
often stretched such as skin, and the opening of the uterus
Gap Junction: Allows cells to communicate by allowing small molecules and ions to
pass from celltocell. Found in epithelia which the movement of ions coordinate
functions, such as beating of cilia; found in excitable tissue such as heart and smooth
Organs and Organ System
• An organ is a structure composed of two or more different tissues that work together to perform a
• An organ system is a group of organs with a common function
Body Cavities Lined with Membranes
• Help protect the vital organs from being damaged when we walk or jump
• Allow organs to slide past one another and change shape
• Two main body cavities
o Ventral (toward abdomen): Pleural cavities house the lungs, Pericardial cavity holds the heart,
all located in the Thoracic (chest). Abdominal holds the digestive system, urinary system and
reproductive system. The diaphragm separates the Thoracic and Abdominal.
o Dorsal (toward the back): Cranial cavity encloses the brain and the Spinal cavity houses the
• Body cavities and organ surfaces are covered with membranes (sheets of epithelium supported by
• Membranes form physical barriers that protect underlying tissue
• 4 types of membranes
o Mucous: Lines passageways that open to the exterior of the body (respiratory, digestive,
reproductive, urinary systems). Some are specialized for absorption, others secrete mucous that
traps bacteria and viruses that can cause illness
o Serous: Line the thoract and abdominal cavities and the organs in them. Secretes fluid that
lubricates the organ in these cavities
o Synovial: Lines the cavity of freely movable joints. Secretes a fluid that lubricates the joint,
o Cutaneous: Covers outside of body, thick and relatively waterproof and relatively dry Skin: An Organ System
• Integumentary System: The skin and its derivatives (hair, nails, sweat glands and oil glands)
• Considered an organ system because they all function together to provide many services for the body
• Major function is protection
• Skin also contains macrophages (more active way of fighting infection)
• Prevents excessive water loss from underlying tissue (keratin protein)
• Temperature regulation
• Two principle layers
o Epidermis (Outer layer): Protective barrier against environmental hazards. Keratin gives the
epidermis its protective properties. Dead skin cells are constantly shedding and are being
replaced by new cells. Is water resistance and lipid soluble (5mm thick)
o Dermis (Inner layer): Much thicker than the epidermis and primarily consists of connective
tissue. Contains blood vessels, hair follicles, oil glands, and ducts for sweat glands, sensory
structures, nerve endings and does not wear away. Lower level consist of collagen + elastic
fibers, which allows the skin to stretch and return to its original shape.
• Hypodermis is a layer of loose connective tissue just below the epidermis and dermis (not part of the
• Has similar functions to skin (cushioning blows + helps to prevent extreme changes in body
• Two factors produce skin color
o Quantity and distribution of pigment
o Blood Flow
• Melanin is a pigment produced by cells called melanocytes at the base of the epidermis
• Melanocytes produce two kinds of melanin
o Yellow to red
o Black to brown
• All people have about the same number of melanocytes
• Albinism ▯melanocytes are unable to produce melanin
• Vitligo ▯melanocytes disappear partially/completely from certain areas of the skin
Hair, Nails and Glands
• Hair in nose/ears prevents entry of bugs
• Eyebrows/eyelashes prevent rain from coming in
• Hair has a sensory role: receptors associated with hair follicles are sensitive to touch
• Consists of shaft + root
o Shaft: Outside skin
o Root: Inside, attaches to hair follicles
• Strand of hair is made up of dead, keratinized cells
• Nails protect the sensitive tips of fingers and toes
• Nail is dead and lacks sensory receptors, it is embedded in the tissue so sensitive that we can detect even
the slightest pressure of an object onto a nail
• Serve as sensory “antennas”
• Modified skin tissue, hardened by keratin
• Three types of glands
o Oil: Found all over body except on palm of hands and soles on feet. Secrete sebum; an oily
substance made of fats, cholesterols, proteins and salts o Sweat: Largely water and some salts, lactic acid, vitamin C and helps regulate body temperature
o Wax: Modified sweat glands found in external ear canal, protects ears by trapping small particles
Negative Feedback Mechanisms
• Maintains homeostasis
• Has 3 components
o Receptor: Detects change in internal/external environment
o Control Center: Determines the factors set point: the level or range that is normal for the factor
o Effector: Muscle/gland that carries out selected response
2a What is Health and Illness?
• A theory is a general belief or beliefs about some aspect of the world we live in or those in it, which
may or may not be supported by evidence
• Bodily factors impact the mind
• Aetiology is the cause of disease
• Dualism is the idea that the mind(nonmaterial) and body(material) are separate entities
• Dualists developed the notion of the body as a machine
o A reductionist approach that reduces behavior to the level of the organ or physical function,
associated with the biomedical model
• The biomedical model is a view that diseases and symptoms have an underlying physiological
o Symptom of illness can be cured through medical intervention
o Described as a reductionist: basic idea that the mind and body can be reduced/explained at the
level of cells/neural activity
• The biopsychosocial model is a view that diseases and symptoms can be explained by a combination of
physical, social, cultural and psychological factors
o Broadening a disease to encompass and emphasis the interaction between biological processes
and psychological/social influences
• Incidence is the number of new cases of disease occurring during a defined time interval not to be
confused with prevalence, which refers to the number of established cases of a disease in a population at
any one time
• Root word of health is wholeness (anglosaxon)
2b Individual, Cultural and Lifespan Perspectives on Health
• Health is related to
o Symptom orientation
• Health behavior: Behavior performed by an individual, regardless of their health status, as a means of
protecting, promoting or maintaining health (eg diet)
• Health is generally viewed as a state of equilibrium across various aspects of the person
Categories of Health defined by survey
• Health as not ill; no symptoms ▯no visits to doctor
• Health as reserve; strong family ▯can recover quickly
• Health as behavior; they are healthy because they look after themselves
• Health as physical fitness and vitality; feeling fit or feeling full of energy • Health as psychosocial wellbeing; defined in terms of mental state
• Health as function; ability to do what you want without being handicapped
World Health Organization Definition of Health
• State of complete physical, mental and social wellbeing and not merely the absence of disease of
• WHO laughed global strategy for health ▯wanted citizens to lead a socially and economically productive
CrossCultural Perspectives on Health
• Idea of “normal” health varies across culture
• Way in which certain behavior is viewed differs across time and between cultures (ie alcohol is legal, but
it used too much its considered deviant)
• If a particular behavior is labeled as a sickness, the consequences will differ greatly from those who are
• Growing evidence that Westernized views on health differ from nonWesternized civilizations
• Westerners divide mind, body, soul between different doctors
• Hindus and Sikhs view disability as a punishment for past sins within the family
• Different belief systems can have effects on living with illness or caring for someone with an illness
• Africans believe that community/family work together for the well being of all
• The collectivist approach is a cultural philosophy that emphasizes the individual as part of a wider unit
and places emphasis on actions motivated by collective, rather than individual needs and wants
• The individualist approach is a cultural philosophy that places responsibility at the feet of the
individual; thus the behavior is often driven by individual needs and wants rather than by community
needs or wants
• The holistic approach is “wholeness” and is concerned with the whole being and its well being, rather
than addressing the purely physical or observable
• Cultures that promote an interdependent self are more likely to view health in terms of social
• Africans▯doctors and herbal medicine
Lifespan, Ageing and Beliefs about Health
• Not only older people who live with chronic illness; can happen to young people too
• Developmental theories: function of the interaction between 3 factors
o Learning: Relatively permanent change in knowledge, skill or ability as a result of experience
o Experience: What we do, see, hear, feel, think
o Maturation: Thought, behavior or physical growth attributed to a genetically determined
sequence of development and ageing rather than experience
Erik Erikson’s different dimensions
• Cognitive and intellectual functioning
• Language and communication skills
• The understanding of illness
• Health care and maintenance behavior
• Piagets maturational framework
o Sensorimotor: Infant lacks symbolic thought, moves into voluntary action
o Preoperational: Around age 2, simple logical/thinking egocentric
o Concrete operational: Age 711, can perform mental operations
o Formal operational: Age 12present, abstract thought, imagination and deductive reasoning
• Illness concept
o Contamination: Recognize that germs or their own behavior can cause illness o Internalization: Illness is within the body, but how symptoms can occur is partially understood
o Physiological: Can define illness in terms of specific bodily organs/functions
o Psychophysiological: Idea that mind and body interact
• Epidemiology is the study of patterns of disease in various populations and the association with other
factors (lifestyle): mortality, morbidity, prevalence, incidence
• Selfconcept are those conscious thoughts and beliefs about yourself that allow you to feel are distinct
from others and that you exist as a separate person
2c: The Seven Dimensions of Health and Wellness
• Physical Health is defined as body shape/size, susceptibility to disease, body functions
• Social Health is defined as satisfying interpersonal relationships/interactions
• Mental Health is defined as the ability to think clearly, reason objectively, analyze critically
• Occupational Health is defined as satisfaction you get from your career/state of career
• Emotional Health is “feeling”▯effectively + appropriately expressing emotions
• Environmental Health is defined as appreciation of the external environment
• Spiritual Health is defined as understanding your role in the world
• Mortality is the death rate
• Morbidity is the illness rate
• Health is defined as dynamic, changing process of trying to achieve individual potential in the 7
• Wellness is defined as a persons attempt to reach his or her potential in each of health’s components
• Health Promotion combines educational, organizational, policy, financial and environmental supports
to enhance health lifestyle choices and to help people change negative health attitudes and behaviors
• Primary Prevention are actions designed to stop problems before they start
• Secondary Prevention is intervention early in the development of a health problem to reduce symptoms
or to half its progression
• Tertiary Prevention is treatment or rehabilitation efforts aimed at limiting the effects of a disease
o More costly and lese effective that primary and secondary
• Four factors that are always biased
o Gender Insensitivity
o Double Standards
Chapter 3 – The Control and Communication Network
3a(i) CellCell Comunication
Mechanisms of Intercellular Communication
• Direct Communication Through Gap Junctions
o Gap junctions link adjacent cells and are formed by plasma membrane proteins, called
connexins, which form structures called connexons.
o These connexons form channels that allow ions and small molecules to pass directly from one
cell to another.
o The movement of ions through gap junctions electrically couples the cells, such that electrical
signals in one cell are directly transmitted to the neighboring cells.
• Indirect Communication Through Chemical Messengers o Most often, cells communicate via chemical messengers, which are all ligands, molecules that
bind to proteins reversibly.
o Communication through chemical messengers occurs when one cell releases a chemical into the
interstitial fluid, usually by a process called secretion, and another cell, called the target cell,
responds to the chemical messenger.
o A target cell responds to the chemical messenger because it has certain proteins, called
receptors, that specifically recognize and bind the messenger.
o Gap junctions composed of membrane protein structures called connexons that link the cytosols
of two adjacent cells, allowing ions and small molecules to move between cells.
o The binding of messengers to receptors produces a response in the target cell through a variety of
mechanisms referred to as signal transduction.
• Functional Classification of Chemical Messengers
o Paracrines are chemicals that communicate with neighboring cells.
The target cell must be close enough that once the paracrine is secreted into the
extracellular fluid, it can reach the target cell by simple diffusion.
Paracrines generally include growth factors, clotting factors, and cytokines.
Growth factors are proteins that stimulate proliferation and differentiation of cells.
Clotting factors are proteins that stimulate formation of a blood clot.
Cytokines are peptides, usually released from immune cells, that function in coordinating
the body’s defense against infections.
o Neurotransmitters are chemicals released into interstitial fluid from nervous system cells called
Are released from a specialized portion of the neuron called the axon terminal, which is
very close to the target cell.
Because the juncture between the two cells is called a synapse, communication by
neurotransmitters is often called a synaptic signaling.
The cell that releases the neurotransmitter is called the presynaptic neuron, whereas the
target cell (which can be another neuron or a gland or muscle cell) is called the
Upon release from the presynaptic neuron, the neurotransmitter quickly diffuses over the
short distance from the axon terminal and binds to receptors on the postsynaptic cell,
triggering a response.
Acetylcholine, which is released by the neuron that trigger contraction of skeletal
o Hormones are chemicals released from endocrine glands (or occasionally other types of tissue)
into the interstitial fluid, where they can then diffuse into the blood.
The hormone then travels in the blood to its target cells, which can be distant from the
site of hormone release.
An example of a hormone is insulin, which is secreted by the pancreas and acts on target
cells throughout the body to regulate energy metabolism.
• Chemical Classification of Messengers
o A messenger’s chemical structure determines its mechanism of synthesis, release, transport, and
o The most important chemical characteristic is whether the messenger can dissolve in water or
cross the lipid bilayer in the plasma membrane of cells.
o Lipophilic (hydrophobic) molecules are lipid soluble and, therefore, readily cross the plasma
membrane – but they do not dissolve in water.
o Hydrophilic (or lipophobic) molecules are watersoluble and do not cross the plasma membrane. o Amino Acid Messengers
Four amino acids are classified as chemical messengers because they function as a
neurotransmitters in the brain and spinal cord: glutamate, aspartate, glycine, and gamma
aminobutyric acid (GABA).
Amino acids are lipophobic; therefore, they dissolve in water but do not cross plasma
o Amine Messengers
Amines, all possess an amine groups (_NH2).
The amines include a group of compounds called catecholamines.
Catecholamines include dopamines, norepinephrine, and epinephrine.
Dopamine and norepinephrine function primarily as neurotransmitters, whereas
epinephrine functions primarily as a hormone.
Other amines include the neurotransmitter serotonin, which is derive from tryptophan;
the thyroid hormones, which are derived from tyrosine; and the paracrine histamines,
which is derived from histidine.
Most of the amines are lipophobic; therefore, they dissolve in water and do not cross
The thyroid hormones are an exception: They are lipophilic and thus do not dissolve in
water, but they readily cross plasma membranes.
o Peptide/Protein Messengers
Most chemical messengers are polypeptides, chains of amino acids linked together by
The term peptide generally refers to chains of fewer than 50 amino acids, whereas
proteins are longer chains of amino acids.
Polypeptides are lipophobic; therefore, they dissolve in water but cannot cross plasma
o Steroid Messengers
Steroids are a class of compounds derived from cholesterol.
They are also lipophilic; they readily cross plasma membranes and are insoluble in water.
o Eidcosanoid Messengers
Are lipids, they readily cross the plasma membrane and are insoluble in water.
Include the following families of chemically regulated compounds: prostaglandins,
leukotrienes, and thromboxanes.
• Synthesis and Release of Chemical Messengers
o Amino Acids
Glutamate and aspartate are synthesized from glucose through a threestep series of
Following their synthesis in the cytosol, amino acid neurotransmitters are transported into
vesicles, where they are stored until they are released by exocytosis.
All amines are derived from amino acids, and all except thyroid hormones are
synthesized n the cytosol by a series of enzymecatalyzed reactions.
Following synthesis, amines are packaged into cytosolic vesicles, where they are stored
until their release is triggered.
o Peptides and Proteins
Cytosolic mRNA serves as the template that cods for the amino acid sequence in the
peptide or protein.
Translation of this mRNA begins on ribosome free in the cytosol.
View page 116 for the synthesis steps of peptides. o Steroids
Steroid messengers are synthesized from cholesterol in a series of reactions catalyzed by
enzymes located in the smooth endoplasmic reticulum or mitochondria.
Consequently, all steroids are capable of crossing the plasma membrane.
Because they are membranepermeant, steroids cannot be stored prior to release and
instead diffused out of the cell into the interstitial fluid as soon as they are synthesized.
Therefore, while cells that secrete peptides or amines can synthesize messengers in
advance and store them in vesicles to be released on demand, steroid hormones are
synthesized on demand and released immediately.
Like steroids, eicosanoids are synthesized on demand and released immediately because
they are lipophilic and able to pass through plasma membranes easily.
The first step in eicosanoid synthesis involves an enzyme called phospholipids.
When active, enzyme catalyzes the release of arachidonic acid from membrane
Because of the eicosanoids’ role in inflammation, many antiinflammatory drugs, such as
aspirin, act by targeting enzymes involved in eicosanoid synthesis.
By inhibiting the activity of the enzyme cyclooxygenase, aspirin decreases not only
inflammation but also blood clotting.
• Transport of Messengers
o Once released, a messenger must first reach and then bind to receptors on the target cell for the
signal to be transmitted.
o In many instances, the messenger is released from cell that is near the target cell, such that the
messenger reaches the receptor by simple diffusion.
o This is true of paracrines and neurotransmitters.
o Typically these messengers are quickly degraded in the interstitial fluid and become inactive,
minimizing the spread of their signaling.
o Hormones can be transported in the blood either in dissolved form or bound to carrier proteins.
o To be transported in dissolved form, the messenger must be a hydrophilic messenger.
o Even though hydrophobic hormones are transported primarily in bound form, a certain fraction
of the hormone molecules dissolve in plasma (generally less than 1%).
o For each such hormone, an equilibrium develops in the bloodstream between the amount of
hormone that is bound to a carrier protein (Pr) in the form of a complex (HPr) and the amount of
free hormone (H) that is dissolved in the plasma:
H – Pr ▯▯H + Pr
o Only free hormone is available to bind to receptors on target cells.
o However, once the hormone binds, it is removed from the blood, and the equilibrium between
bound and free hormone shifts to the right, causing more hormone to be released from the carrier
o Likewise, the secretion of hormones into the blood causes equilibrium to be shifted to the left,
such that more hormones bind to carrier proteins.
o How long a hormone persists in blood is measured in terms of halflife, the time it takes for half
of the hormone in the blood to be degraded.
o Hormones that are present in dissolved form have relatively short halflives.
o However, hormones that are bound to carrier proteins are protected from degradation and have
longer halflives, generally hours.
Signal Transduction Mechanisms
• Chemical messengers transmit their signals by binding to target cell receptors located either on the
plasma membrane, in the cytosol, or in the nucleus. • Properties of Receptors
o The strength of the binding between a messenger and its receptor is termed affinity.
o A single messenger can often bind to more than one type of receptor, and these receptors may
have different affinities for the messenger.
o For example, the catecholamine chemical messengers epinephrine and norepinephrine can both
bind to adrenergic receptors (epinephrine is also called adrenaline, and norepinephrine is also
o A single target cell may have rececptors for more than one type of messenger.
o For example, skeletal muscle cells have receptors for both the neurotransmitter acetylcholine and
the hormone insulin.
o The Relationship Between Receptor Binding and the Magnitude of the Target Cell Response
The magnitude of a target cell’s response to a chemical messenger depends on three
factors: (1) the messenger’s concentration, (2) the number o