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Cell physiology
P
hysiology : is the science that deal with the study of the function of the
healthy living organisms and the changes which occurs during activity .
The goal of physiology is to explain the physical and chemical factors
that are responsible for the origin, development, and progression of life.
Each type of life, from the simple virus to the largest tree or the
complicated human being, has its own functional characteristics. Therefore, the vast
field of physiology can be divided into viral physiology, bacterial physiology,
cellular physiology, plant physiology, human physiology, and many more
subdivisions.
Human Physiology: In human physiology, we attempt to explain the
specific characteristics and mechanisms of the human body that make it a living
being. The very fact that we remain alive is almost beyond our control, for hunger
makes us seek food and fear makes us seek refuge. Sensations of cold make us look
for warmth. Other forces cause us to seek fellowship and to reproduce. Thus, the
human being is actually an automaton, and the fact that we are sensing, feeling, and
knowledgeable beings is part of this automatic sequence of life; these special
attributes allow us to exist under widely varying conditions.
Cells as the Living Units of the Body
The basic living unit of the body is the cell. Each organ is an aggregate
of many different cells held together by intercellular supporting structures. Each
type of cell is specially adapted to perform one or a few particular functions. For
instance, the red blood cells, numbering 25 trillion in each human being, transport
oxygen from the lungs to the tissues. Although the red cells are the most abundant
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Cell physiology
of any single type of cell in the body, there are about 75 trillion additional cells of
other types that perform functions different from those of the red cell. Size of the
different cells varies depending on the function . The cell with the largest diameter
is an Oocyte with a diameter of about 140 µm.
Although the many cells of the body often differ markedly from one
another, all of them have certain basic characteristics that are alike. For instance, 1in all cells, oxygen reacts with carbohydrate, fat, and protein to release the energy
required for cell function.
2- Further, the general chemical mechanisms for changing nutrients into energy are
basically the same in all cells .
3- all cells deliver end products of their chemical reactions into the surrounding
fluids.
4- Almost all cells also have the ability to reproduce additional cells of their
own kind except neurons .
The Extracellular Fluid—The “Internal Environment”
About 60 per cent of the adult human body is fluid, mainly a water solution of
ions and other substances. Although most of this fluid is inside the cells and is
called intracellular fluid, about one third is in the spaces outside the cells and is
called extracellular fluid. This extracellular fluid is in constant motion throughout
the body. It is transported rapidly in the circulating blood and then mixed between
the blood and the tissue fluids by diffusion through the capillary walls.
In the extracellular fluid are the ions and nutrients needed by the cells to
maintain cell life. Thus, all cells live in essentially the same environment—the
extracellular fluid. For this reason, the extracellular fluid is also called the internal
environment of the body, or the milieu intérieur, a term introduced more than 100
years ago by the great 19th-century French physiologist Claude Bernard.
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Cell physiology
Cells are capable of living, growing, and performing their special functions
as long as the proper concentrations of oxygen, glucose, different ions, amino acids,
fatty substances, and other constituents are available in this internal environment.
Differences Between Extracellular and Intracellular Fluids.
The extracellular fluid contains large amounts of Na+, Cl-, and Co3- plus
nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also
contains Co2 that is being transported from the cells to the lungs to be excreted, plus
other cellular waste products that are being transported to the kidneys for excretion.
The intracellular fluid differs significantly from the extracellular fluid;
specifically, it contains large amounts of K+, Mg+, and Po4-3 instead of the Na+and
Cl- found in the extracellular fluid. Special mechanisms for transporting ions
through the cell membranes maintain these differences .
“Homeostatic” Mechanisms of the Major Functional Systems
The term homeostasis is used by physiologists to mean maintenance of
nearly constant conditions in the internal environment.
Essentially all organs and tissues of the body perform functions that help
maintain these constant conditions. For instance, * the lungs provide oxygen to the
extracellular fluid to replenish the oxygen used by the cells, *the kidneys maintain
constant ion concentrations, and* the gastrointestinal system provides nutrients.
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Cell physiology
A- Extracellular Fluid Transport System
Extracellular fluid is transported through all parts of the body in two stages:
1- The first stage is movement of blood around and around the circulatory system.
2-The second is movement of fluid between the blood capillaries and the cells.
B-Origin of Nutrients in the Extracellular Fluid
1- The Respiratory System: during circulation , each time the blood passes
through the body, it also flows through the lungs. The blood picks up O2 in
the alveoli, thus acquiring the O2 needed by the cells.
2- The gastrointestinal tract.:. A large portion of the blood pumped by the heart
passes through the walls of gastrointestinal organs. Here, different dissolved
nutrients, including carbohydrates, fatty acids, and amino acids, are absorbed
from the ingested food into the extracellular fluid of the blood.
3- Liver and Other Organs That Perform Primarily Metabolic Functions :Not all
substances absorbed from the gastrointestinal tract can be used in their
absorbed form by thecells. The liver changes the chemical compositions of
many of these substances to more usable forms, and other tissues of the body—fat
cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the
absorbed substances or store them until they are needed.
4- Musculoskeletal System.: Sometimes the question is asked, “How does the
musculoskeletal system fit into the homeostatic functions of the body?”
The answer is: This system provides motility for a- obtaining the food
requiring for nutrition . b- protection against adverse surroundings .
c- Removal of Metabolic End Products
1- Removal of Carbon Dioxide by the Lungs.: At the same time that blood picks
up oxygen in the lungs, carbon dioxide is released from the blood into the
lung alveoli; the respiratory movement of air into and out of the lungs carries
the carbon dioxide to the atmosphere.
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Cell physiology
2- Kidneys.: Passage of the blood through the kidneys removes most substances
from the plasma that are not needed by the cells. These substances include
different end products of cellular metabolism, such as urea and uric acid; they
also include excesses of ions and water from the food that might have
accumulated in the extracellular fluid.
The kidneys perform their functions by :
a- Filtering large quantities of plasma through the glomeruli into the tubules.
b- reabsorbing into the blood those substances needed by the body, such as
glucose, amino acids, appropriate amounts of water, and many of the ions.
However , substances that are not needed by the body are not reabsorbed but ,
instead , pass through the renal tubules into the urine.
D- Regulation of Body Functions
1- Nervous System: The nervous system is composed of three major parts:
a- the sensory input portion b- the central nervous system (or integrative
portion), c- the motor output portion.
 Sensory receptors detect the state of the body or the state of the surroundings.
(skin, eyes, ears , etc.) .
 The CNS is composed of the brain and spinal cord. The brain can store
information, generate thoughts, create ambition ‫ابتداء الطموح‬, and determine
reactions that the body should performs in response to the sensations.

Appropriate signals ‫ االيعازاا المازبا‬are then transmitted through the motor output
portion of the nervous system to carry out one’s desires.
2- Hormonal System of Regulation : Located in the body are eight major
endocrine glands that secrete chemical substances called hormones.
Hormones are transported in the extracellular fluid to all parts of the body to
help regulate cellular function. For instance,
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Cell physiology
 thyroid hormone increases the rates of most chemical reactions in all cells, thus
helping to set the tempo of bodily activity.
 Insulin controls glucose metabolism.
 adrenocortical hormones control sodium ion, potassium ion, and protein
metabolism.
 parathyroid hormone controls bone calcium and phosphate.
Thus, the hormones are a system of regulation that complements the nervous
system. The nervous system regulates mainly muscular and secretory activities of
the body,whereas the hormonal system regulates many metabolic functions.
E- Reproduction
Sometimes reproduction is not considered a homeostatic function. It does, however,
help maintain homeostasis by generating new beings to take the place of those that
are dying .This may sound like a permissive usage of the term homeostasis, but it
illustrates that, in the final analysis, essentially all body structures are organized
such that they help maintain the automaticity and continuity of life.
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Cell physiology
Organization and general function f cells :
The cells is considered the basic functional unit of living organisms because :
1- most of the basic life processes are carried out at the cellular level .
2- the human body contains many different kinds of cells , each of which has
characteristic shape and size and a particular function to perform .
CELL MEMBRANE
The plasma membrane ( often called the cell membrane ) defines the
boundaries of a cell . similar membranes form the boundaries of the organelles
within cells . All membranes are composed of the same structural components .
Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and in the nucleus.
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Cell physiology
The plasma membrane consists of a phospholipids bilayer, which is a thin,
double-layered film of lipids—each layer only one molecule thick—that is
continuous over the entire cell surface. One end of each phospholipid molecule is
soluble in water; that is, it is hydrophilic. The other end is soluble only in fats; that
is, it is hydrophobic. The phosphate end of the phospholipid is hydrophilic, and the
fatty acid portion is hydrophobic.
The polar (charged) phosphate end of the molecules are oriented toward the
inner & outer surface , while non-polar ( fatty acid ) ends point toward each other in
the interior of the membrane .
The cholesterol molecules in the membrane are also lipid in nature . These
molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly
help determine the degree of permeability (or impermeability) of the bilayer to
water-soluble constituents of body fluids. Cholesterol controls much of the fluidity
of the membrane as well.
Cell Membrane Proteins : globular masses floating in the lipid bilayer. These
are membrane proteins, most of which are glycoproteins. Two types of proteins
occur: integral proteins that protrude all the way through the membrane, and
peripheral proteins that are attached only to one surface of the membrane and do not
penetrate all the way through. The proteins are held to the cell surface by glycosyl
phosphotadylinositol anchors . when the protein extends throughout the thickness of
the membrane it is called “ trans membrane protein channel “ .
Functions of Cell Membrane Proteins :
1- Structural proteins contribute to the structure of cell membrane .
2- Some cell membrane proteins are “ cell adhesion molecule” that anchor cells to
their neighbors or to the basal lamina .
3- Some proteins function as a “ pumps” for active transport of substances .
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Cell physiology
4- “ Carrier proteins” transport substances down their electrochemical grad
gradient.
5- “Ion channels” permit passage of ions in to or out of cell when activated .
6- “Aquaporins” are membrane proteins present in most cells , which act as water
channels permitting high rate of water flow through the membrane .
7- The peripheral proteins function almost entirely as “enzymes” or as controllers
of transport of substances through the cell membrane “pores.”
Carbohydrates.: Carbohydrates have little structural function in the cell
except as parts of glycoprotein molecules, but they play a major role in nutrition of
the cell. Sugar combine with proteins to form glycoproteins or with lipids to form
glycolipids . Some of Carbohydrates serve as recognition sites that allow cells to
recognize other cells in cell- to- cell interactions .
Structure of the cell membrane, showing that it is composed mainly of a lipid bilayer of phospholipid molecules,
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Cell physiology
 The primary function of cell membrane :
1- To regulate the internal environment of the cell & thus maintain homeostasis
by controlling the passage of substances in to and out of the cell .
2- Cell membrane also play a part in transmitting information .
Cytoplasm and Its Organelles
The cytoplasm is filled with both minute and large dispersed particles and
organelles .The clear fluid portion of the cytoplasm in which the particles are
dispersed is called cytosol; this contains mainly dissolved proteins electrolytes, and
glucose.
1- The endoplasmic reticulum (ER) : is an extensive network of interconnected
membrane – bound vesicles that transports proteins and possibly other substances
synthesized by the cell . The surface of smooth ER is thought to be the site of lipid
synthesis and the rough ER ( with its ribosomes ) the site of protein synthesis .
2- Ribosomes : small bodies that lack membranes , are found on the surface of
the rough ER and in the cytoplasm . They serve as sites for protein synthesis .
3- The Golgi apparatus : consists of a stack of membrane- bound vesicles . its
processes proteins synthesized by the cell .
4- Mitochondria : consist of an outer smooth membrane & an inner folded
membrane (which surrounded the matrix) . mitochondria contain the enzymes that
carry out oxidative energy – producing reactions .
5- The nucleus : is the control center of the cell . it is surrounded by a nuclear
envelope and contains chromosomes & one or more nucleoli .
6- Cell inclusions : are not true organelles , but rather consist of ingested
substances such as fat or glycogen .
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Cell physiology
7- Lysosomes : formed from Golgi apparatus , its contain digestive enzymes
(hydrolases) that are released when a cell engulfs particles , is injured or dies ; thus
, its help in intracellular digestion of food, bacteria, damaged cell structures etc.
8- Peroxisomes : formed by self- replication or budding from smooth ER, they
are similar to lysosomes , but smaller in size . they contain enzymes(oxidases) that
oxidize various organic substances producing H2O2 ;thus , helps in the etoxification
of injurious substances .
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Cell physiology
Transport of Substances Through the Cell Membrane
A living cell is a dynamic entity , with substances constantly moving into &
out of it . Understanding how these movements occur is essential to understanding
how a cell functions .
 Some substances diffuse into & out of membranes through the pores ( through
electrical charges on the pores exert some control over such diffusion ) .
 Lipid soluble substances dissolved in the lipid & diffuse through it .
 Still other substances are moved across the membrane by carrier molecules .
Thus, the movement of an ion or molecule across a membrane is determined
by a combination of factors :
1- Particle size .
2- Electrical charge .
3- Lipid solubility .
4- The availability of carrier substances in the membrane .
In considering the mechanisms by which substances move into and out of
cells , we will consider passive & Active processes . Passive transport involves
movement of a substances a long a gradient ( from higher to lower concentration or
from higher to lower pressure ) . The cell expends no energy to move substances
along a gradient ; thus the substances are said to move passively . Active transport
involves movement of a substances against the gradient ( from lower to higher
concentration or from lower to higher pressure ) .
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Cell physiology
I. Diffusion :
 Movement occur in all directions .
 Movement occur along the gradient .
Thus , the diffusion is the net movement of similar molecules from their area
of higher concentration to their area of lower concentration.
Substances that diffuse through membranes do so by :
1- Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer :
Lipid soluble substances will diffuse through the lipid layer of the membrane
at greater rates than they will diffuse through the protein part of the membrane .
However ; diffusion through the lipid layer is affected by :
1- The solubility of substance in lipid .
2- Temperature .
3- Concentration gradient .
2- Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein
Channels :
In this type of movement , substances move from higher to lower
concentration also , but their movement is affected by :
1- Electrical charge along the surface of the pore .
2- The size of the pore .
3- The specific nature of the pore .
II. Facilitated diffusion
Glucose and other sugar molecules are : 1- too large to pass through the pores
and 2- not sufficiently soluble in the membrane lipids to diffuse through them .
these substances are transported by facilitated diffusion or by active transport . In
facilitated diffusion ,the sugar molecule combine with a carrier substance (a protein
molecule) that is embedded in the membrane .
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Cell physiology
This type of diffusion is like other types of diffusion except for the carrier’s
ability to help the sugar molecule move across the membrane .[ its move from
higher to lower concentration . However , it does so at a faster rate than that of
simple diffusion ] . When the sugar concentration is particularly high , the rate of
movement is limited by the number of carrier molecules available .
III. Osmosis :“Net Diffusion” of Water
Water passes through pores in cell membrane rapidly in both directions . If a
concentration gradient exist , water will move from its own area of higher
concentration to its own area of lower concentration at a greater rate than it move in
the opposite direction . This net movement of water across a selectively permeable
membrane is Osmosis . Osmosis occur particularly if the membrane is permeable
only to water molecules .
 During osmosis water molecules passes through the plasma membrane
in two ways :
1- By moving through lipid bilayer because of their small size and high
kinetic energy .
2- By moving through aquaporins .
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Cell physiology
 Osmotic pressure : is the force under which water moves from an area
of low solute concentration to an area of high solute concentration .
Cells normally are in osmotic balance with the fluids in their environment,
that is , water is moving in & out of the cell at the same rate, so that no net
movement occurs . Both the cells 7 the fluids around them contain proteins & other
molecules and ions that exert osmotic pressure . If a concentration gradient develops
between the cells and their environment , the net movement of water will be toward
the area of higher concentration of solute ( lower concentration of water ) until
osmotic balance restored .
 Osmol: A gram molecular weight of a substance divided by the number of
ions each molecule produces when it dissociates .
 Osmolarity : the osmotic concentration of a solution determined by the
number of osmotically active particles it contain . [ is the number of osmoles per
liter of solution ] .
 Osmolality : is the number of osmoles per kilogram of solvent .

Osmoreceptor : Hypothalamic receptors that respond to change in the
osmotic pressure of the blood .
 Oncotic pressure : the colloid osmotic pressure due to the plasma colloids
in the blood is called oncotic pressure .
 Tonicity : the term tonicity is used to describe the osmolality of a solution
relative to plasma . Tonicity is a measure of the ability of the solution to change the
volume of cell by altering their water content .
 Isotonic : Solution that have the same osmolality as that of plasma are
said to be isotonic to that of plasma , e.g. 0.9 % sodium chloride , 5%
dextrose in water etc.
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Cell physiology
 Hypertonic : a solution is hypertonic when it causes a net movement of
water out of the cell ; that is , when the volume of the cell decreases &
the volume of the solution increases .
 Hypotonic : a solution is hypotonic when it causes a net movement of
water in to the cell ; that is , when the volume of the cell increases &
the volume of the solution decreases .
Vomiting ; diarrhea , & a variety of other symptomatic of illness can cause
imbalance in the distribution of water between cells & their environment .
Understanding what happens to the cells when they contain too little or too much
water is important in treating individuals with such conditions .
When cells are surrounded by hypotonic fluids , the cell will gain water
(swell) & their membranes may even burst . If cells surrounded by hypertonic fluids
the cell will lose water (shrink) & become dehydrated .
Note : Pure water is very hypotonic .
Q// discuss [[ the hypotonic solutions can be used to treat patients with
dehydration ]] . ??
IV. Filtration :
In some situations in the body substances are pushed through membranes by
pressure , such as the pressure the pressure of flowing blood . this phenomenon ,
called filtration , is essentially the same as the movement of substances through
filter paper in a funnel because those substances are pushed through the filter paper
by the pressure of the solution standing in the funnel . This pressure is called
hydrostatic pressure . Filtration occurs as blood flows through the capillaries . Blood
pressure pushing blood along the inside of capillaries also pushes nutrients & other
substances through the walls of the capillaries out in to the tissue fluids .
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Cell physiology
V. Bulk flow :
In the capillaries and in certain other locations in the body , water molecules
move across membranes at more rapid rates that can be accounted for by diffusion .
In all causes water is moving along a gradient created by osmotic pressure or
hydrostatic (filtration) pressure . But, the rapid movement of water appears to be
due to a particular property of water molecules :- Once moving in a given direction
, water molecules tend to flow in streams through the pores of a membrane . This
streaming property of water molecules is called bulk flow . Bulk flow is an
important factor in the movement of fluids between the circulatory system & the
cells of the body .
VI. Active transport :
The transport of a substances against a gradient using an enzyme , a carrier ,
& ATP . Active transport causes molecules to move against concentration gradients
from areas of lower to areas of higher concentration .
e.g.// Na+ are pumped out of cells in to extracellular fluid where the
concentration of Na+ is higher than inside the cells . this process require energy from
ATP .
The substances to be actively transported :
1- Attaches to a carrier molecule that causes the substance to move across
the cell membrane .
2- An enzyme that is usually part of the carrier molecule release energy from
ATP . this energy is used to a-) activate the carrier , b-) separate the
substance being transported from the carrier , c-) & to discharge the
substance into or outside the cell .
 The energy is required because the substances is being moved against the
concentration gradient .
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Cell physiology
 The carrier molecule is a protein or glycoprotein & it has on its surface a
specific carrier site for the substance it transports .
 Rates of movement by active transport depend primarily on the amount of
carrier molecule- enzyme available to bring about the movement .
 Actively transported substances include :1- Ions such as Na+ , K+, Ca++, Fe++ , H+& I- .
2- Some sugar , A.A , & other organic substances are moved across
membranes with the energy made available by the active transport of Na+
in the same direction .
 The transport of Na+ is so important in the function of cells that its active
transport mechanism has been specifically named the sodium pump . It has
been estimated that 40% of a cell’s ATP is used for active transport of Na+ &
K+ . This sodium pump is constantly transporting Na + out of cells against the
concentration gradient ( the gradient allows ions to diffuse back in to cells).
K+ are actively transported in to cells by the same carrier system . This might
be called a coupled active transport system because two substances are
transported by the same carrier .
 Glucose moves by facilitated diffusion where an appropriate concentration
gradient exists . When movement is against the gradient , glucose is moved
by active transport . e.g.// In the intestine , where glucose is being absorbed
from the digested food , the concentration of glucose in the blood vessels
receiving it may be higher than in the intestinal tract . Thus , active transport
is essential if glucose is to be absorbed .
 In all cells that have been studied amino acid also cross membranes by active
transport . It is thought that at least four different carrier systems exist to
transport A.As . Each carrier transports a group of A.As. with similar
chemical properties .
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Cell physiology
 Both A.A transport & glucose transport are regulated by hormones .
 Active transport is essential to normal cell function because passive
processes such as diffusion , osmosis , filtration , bulk flow cannot increase the
concentration of substances on either side of a membrane against a concentration
gradient . Active transport can establish concentration gradients on which several
other process depend in one way or another . These processes include :1- The initiation & transmission of nerve impulse .
2- The contraction of muscles .
3- The concentration of nutrients in the cells .
4- The removal of wastes from cells .
 It appears that ( even within cells) for certain organelles such as the
mitochondria & the endoplasmic reticulum , active transport along with passive
transport is involved in the movement of substances across their membranes .
VII. Endocytosis ( Pinocytosis & Phagocytosis ) :
Pinocytosis : also called cell drinking , the process by which the cell take in
small particles including water droplets .
Phagocytosis : also called cell eating , the process by which cells take in large
particles .
 Protein molecules , water , & ions can be taken in by pinocytosis .
 Debris from dead cells & microorganisms are taken in by phagocytosis.
References : Text book of medical physiology(Guyton)
Text book of medical physiology(N Geetha)