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CSIR Unit 2- Full Notes
CSIR Preparation (Amrita Vishwa Vidyapeetham)
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A1. MEMBRANES-STUCTURE
Cell membranes are crucial to the life of the cell. The plasma MEMBRANE ULTRASTRUCTURE AND FLUID MOSAIC
membrane encloses the cell, defines its boundaries, and MODEL
maintains the essential differences between the cytosol and
the extracellular environment. Inside the cell the This model, most forcefully proposed by S.J. Singer (A.R.
membranes of the endoplasmic reticulum, Golgi apparatus, Biochem. 1974, 43: 805‐826), is the currently most favored
mitochondria, and other membrane‐bounded organelles in model. Some consider it the 6 most current modification of
eucaryotic cells maintain the characteristic differences the original Danielli‐Davson model. It proposes:
between the contents of each organelle and the cytosol. Ion 1. A phospholipid bilayer with the hydrophobic (nonpolar)
gradients across membranes, established by the activities of fatty acid chains directed inwards and the polar hydrophilic
specialized membrane proteins, can be used to synthesize heads lining the surfaces.
ATP, to drive the transmembrane movement of selected
solutes, or, in nerve and muscle cells, to produce and 2. Proteins, glycoproteins, and lipoproteins, associate with
transmit electrical signals. In all cells the plasma membrane the inner and outer surfaces of the membranes. The polar
also contains proteins that act as sensors of external signals, portions of the proteins associate with the polar surface and
allowing the cell to change its behavior in response to may protrude from it while portions of the proteins may be
environmental cues; these protein sensors, or receptors, embedded in the bilayer of lipid. It is proposed that the
transfer information rather than ions or molecules across nonpolar portions of these proteins associate with the
the membrane. nonpolar fatty acid chains. Proteins having both polar and
nonpolar areas (residues) are termed amphipathic.
Despite their differing functions, all biological membranes 3. Under physiological conditions the membrane
have a common general structure: each is a very thin film of components are not rigidly fixed, but exist in a fluid and
lipid and protein molecules, held together mainly by non‐ dynamic state. Lipids of the membrane are fluid at
covalent interactions. Cell membranes are dynamic, fluid physiological temperatures, thus there may be fluid
structures, and most of their molecules are able to move movement of the lipid molecules and lateral movement
about in the plane of the membrane. The lipid molecules are (diffusion) of the proteins that are embedded in the
arranged as a continuous double layer about 5 nm thick. membrane. "Protein icebergs in a sea of lipid." (Singer) This
This lipid bilayer provides the basic structure of the forms a heterogeneous mosaic of proteins which may be
membrane and serves as a relatively impermeable barrier to continuously rearranged (but often with specificity.)
the passage of most water‐soluble molecules. Protein
molecules "dissolved" in the lipid bilayer mediate most of 4. Not only is there lateral mobility of lipids but there is also
the other functions of the membrane, transporting specific exchange from one monolayer of the bi‐leaflet to the other
molecules across it, for example, or catalyzing membrane‐ monolayer. This exchange (flip‐flop) is much slower than the
associated reactions, such as ATP synthesis. In the plasma lateral exchange, detected by nuclear resonance studies.
membrane some proteins serve as structural links that This process is facilitated by specific proteins (enzymes)
connect the membrane to the cytoskeleton and/or to either called flippases.
the extracellular matrix or an adjacent cell, while others
serve as receptors to detect and transduce chemical signals 5. Proteins may be integral or peripheral, probably most
in the cell's environment. As would be expected, cell being integral and displaying various different secondary
membranes are asymmetrical structures: the lipid and and tertiary forms. The proteins, being capable of secondary,
protein compositions of the outside and inside faces differ tertiary, and quaternary structure changes in response to
from one another in ways that reflect the different functions stimuli, may reorient in relation to the phospholipids
performed at the two surfaces of the membrane.
Downloaded bilayer.
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INSTITUTE FOR ADVANCED STUDIES, JODHPUR 1
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6. In a phospholipid bilayer, the long fatty acyl side chains in 10. As a phospholipid bilayer is heated, it undergoes a phase
each leaflet are oriented toward one another, forming a transition from a gel‐like to a more fluid state over a short
hydrophobic core; the polar head groups line both surfaces. temperature range.
7. The phospholipid bilayer forms the basic structure of all 11. Cholesterol is a major determinant of bilayer fluidity,
biomembranes, which also contain proteins, glycoproteins, although its effect depends on the composition of a
cholesterol and other steroids, and glycolipids. The presence membrane. Natural biomembranes generally have a fluid‐
of specific sets of membrane proteins permits each type of like consistency, and cells adjust their phospholipid
membrane to carry out distinctive functions. composition to maintain bilayer fluidity.
8. All cellular membranes line closed compartments and
have a cytosolic and an exoplasmic face. The asymmetry of 12. In all cells, proteins in the plasma membrane selectively
biological membranes is reflected in the specific orientation absorb nutrients, expel wastes, and maintain the proper
of each type of integral and peripheral membrane protein intracellular ionic composition. Proteins in the plasma
with respect to the cytosolic and exoplasmic faces. The membrane anchor the membrane to intracellular
presence of glycolipids exclusively in the exoplasmic leaflet cytoskeletal fibers and the extracellular matrix or cell wall.
also contributes to membrane asymmetry. In multicellular organisms, plasma membrane proteins also
act in the interactions and communication between cells,
9. Most integral proteins and lipids are laterally mobile in which are critical for proper functioning of multicellular
biomembranes. According to the fluid mosaic model, the tissues.
membrane is viewed as a two‐dimensional mosaic of
phospholipid and protein molecules. 13. In plants, the cell wall, which is built mainly of cellulose,
is the major determinant of cell shape and imparts rigidity to
cells. Animal cells, which lack a wall, are surrounded by an
extracellular matrix consisting of collagen, glycoproteins
A2. MEMBRANE PROTEINS
1. Transmembrane Proteins: lipid bilayer,these proteins are not free‐floating and cannot
be isolated and purified biochemically without first
Membrane proteins are either extrinsic or intrinsic. dissolving away the lipid bilayer with detergents. (Indeed,
Extrinsic membrane proteins are entirely outside of the much of the washing we do in our lives is necessitated by the
membrane, but are bound to it by weak molecular need to solubilize proteins that are embedded in lipid
attractions (ionic, hydrogen, and/or Van der Waals bonds). membranes using detergents!)
Intrinsic membrane proteins, the class we are mainly
interested in, are embedded in the membrane. Many of them 3. Glycoproteins:
extend from one side of the membrane to the other and are
referred to as transmembrane proteins. Cells are constantly For reasons that are not well understood, many
pumping ions in and out through their plasma membranes. transmembrane proteins are glycoproteins in the sense that
In fact, more than half the energy that are bodies consume is sugar side chains are covalently attached to their
used by cells to drive the protein pumps in the brain that do hydrophilic domains that protrude into the extracellular
nothing else but transport ions across plasma membranes of membrane. A typical mammalian cell may have several
nerve cells. How can ions be transported across membranes hundred distinct types of glycoprotein studding its plasma
that are effectively impermeable to them? membrane. Each of these glycoproteins will have its
extracellular domain glycosylated with a complex branching
Cells contain proteins that are embedded in the lipid bilayer bush of sugar residues covalently linked to the asparagine
of their plasma membranes and extend from one side of the side chains. Some glycoproteins may have 2 or 3 asparagine‐
membrane through to the other. Such transmembrane linked sugar side chains, others may have dozens.
proteins can function to effect ion transport in several ways.
But how can they cope with the energetically highly 4. Multimembranespanning proteins:
unfavorable situation in which an ion must pass through the
hydrophobic inner layers of the plasma membrane? An elaboration of this scheme depicting membrane proteins
having single transmembrane domains involves certain
2. Domains: membrane proteins that have multiple transmembrane
domains. As one scans along the amino acid sequence of
If we examine the detailed structures of many these proteins, it becomes apparent that hydrophilic
transmembrane proteins, we see that they often have three domains (i.e. having hydrophilic amino acids) alternate with
different domains, two hydrophilic and one hydrophobic. A hydrophobic domains. The protein chain as a whole when
hydrophilic domain (consisting of hydrophilic amino acids) embedded in the plasma membrane actually weaves back
at the N‐terminus is poking out in the extracellular medium, and forth between opposite sides of the plasma membrane.
a hydrophobic domain in the middle of the amino acid chain, Some think such proteins have the configuration of snakes
often only 20‐30 amino acids long, is threaded through the and hence term them serpentine membrane proteins. A
plasma membrane, and a hydrophilic domain at the C‐ commonly used type of structure seen in many hundreds of
terminus protrudes into the cytoplasm. The transmembrane serpentine transmembrane proteins involves 7 hydrophobic
domain, because it is made of amino acids having domains inserted into the plasma membrane separated by
hydrophobic side chains, exists comfortably in the hydrophilic regions that are looped out alternatively into
hydrophobic inner layers of the plasma membrane. Because either the cytoplasm or the extracellular space.
these transmembrane domains anchor many proteins in the
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A3. MEMBRANE TRANSPORT MECHANISM & ION PUMPS
All cells acquire the molecules and ions they need from their
surrounding extracellular fluid (ECF). There is an unceasing The difference in the concentration of molecules across a
traffic of molecules and ions in and out of the cell through its space is called a concentration gradient. If these molecules
plasma membrane. Examples: glucose Na+, Ca2+. In diffusing across the membrane from an area of high
eukaryotic cells, there is also transport in and out of concentration to an area of low concentration were water
membrane‐bounded intracellular compartments such as the molecules the process would be called osmosis. Water also
nucleus, endoplasmic reticulum and mitochondria. moves from a low solute concentration to a high solute
Examples: proteins, mRNA, Ca2+, ATP concentration.
If uncharged solutes are small enough and lipid liking they
can move down their concentration gradients directly across The Two types of Passive Transport are
the lipid bilayer itself by simple diffusion. Examples of such
solutes are ethanol, carbon dioxide, and oxygen. Transport by simple diffusion and Osmosis
Facilitated diffusion: carrier proteins and ion
The major difficulty which the membranes pose for some channels
molecule is that the Lipid bilayers are not permeable to: ions
such as 1. Diffusion and Osmosis
K+, Na+, Ca2+ (called cations because when
subjected to an electric field they migrate toward 1. In diffusion, molecules move from higher to lower
the cathode [the negatively‐charged electrode]) concentration (i.e., down their concentration gradient).
Cl‐, HCO3‐ (called anions because they migrate A solution contains a solute, usually a solid, and a
toward the anode [the positively‐charged solvent, usually a liquid.
electrode]) In the case of a dye diffusing in water, dye is a
small hydrophilic molecules like glucose and solute and water is the solvent.
macromolecules like proteins and RNA 2. Membrane chemical and physical properties allow only a
few types of molecules to across by diffusion.
But we all know that that cell is intelligent enough to solve Lipid‐soluble molecules (e.g., alcohols) diffuse;
the above challenge posed by the membrane. This is how it lipids are membrane’s main structural
solves the problem components.
Gases readily diffuse through lipid bilayer.
Passive and active mechanisms move molecules across Movement of oxygen from air sacs (alveoli) to
membrane. blood in lung capillaries depends on
Passive transport moves molecules across concentration of oxygen in alveoli.
membrane without expenditure of energy by cell; 3. Osmosis is the diffusion of water across a differentially
includes diffusion and facilitated transport. permeable membrane from its higher concentration to
Active transport uses energy (ATP) to move lower concentration. Osmotic pressure is hydrostatic
molecules across a plasma membrane; include pressure, on side of membrane with higher solute
active transport, exocytosis, endocytosis, and concentration, produced by water diffusing to that side of
pinocytosis. membrane
4. Tonicity is strength of a solution in relationship to
I. Passive Transport osmosis; determines movement of water into or out of cells.
Isotonic is where the relative solute concentration
Cell membranes help organisms maintain homeostasis by of two solutions are equal.
controlling what substances may enter or leave the cells. Hypotonic is where a relative solute
Some substances such as water, oxygen, and carbon dioxide, concentration of one solution is less than another
can cross the cell membrane without any input of energy by solution.
the cell. The movement of such substances across the Hypertonic is where relative solute concentration
membrane is know as passive transport. The cell membrane of one solution is greater than another solution.
is said to be selectively permeable.
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CSIR Unit 2- Full Notes
CSIR Preparation (Amrita Vishwa Vidyapeetham)
Scan to open on Studocu
Studocu is not sponsored or endorsed by any college or university
Downloaded by Khushbu Shukla ()
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A1. MEMBRANES-STUCTURE
Cell membranes are crucial to the life of the cell. The plasma MEMBRANE ULTRASTRUCTURE AND FLUID MOSAIC
membrane encloses the cell, defines its boundaries, and MODEL
maintains the essential differences between the cytosol and
the extracellular environment. Inside the cell the This model, most forcefully proposed by S.J. Singer (A.R.
membranes of the endoplasmic reticulum, Golgi apparatus, Biochem. 1974, 43: 805‐826), is the currently most favored
mitochondria, and other membrane‐bounded organelles in model. Some consider it the 6 most current modification of
eucaryotic cells maintain the characteristic differences the original Danielli‐Davson model. It proposes:
between the contents of each organelle and the cytosol. Ion 1. A phospholipid bilayer with the hydrophobic (nonpolar)
gradients across membranes, established by the activities of fatty acid chains directed inwards and the polar hydrophilic
specialized membrane proteins, can be used to synthesize heads lining the surfaces.
ATP, to drive the transmembrane movement of selected
solutes, or, in nerve and muscle cells, to produce and 2. Proteins, glycoproteins, and lipoproteins, associate with
transmit electrical signals. In all cells the plasma membrane the inner and outer surfaces of the membranes. The polar
also contains proteins that act as sensors of external signals, portions of the proteins associate with the polar surface and
allowing the cell to change its behavior in response to may protrude from it while portions of the proteins may be
environmental cues; these protein sensors, or receptors, embedded in the bilayer of lipid. It is proposed that the
transfer information rather than ions or molecules across nonpolar portions of these proteins associate with the
the membrane. nonpolar fatty acid chains. Proteins having both polar and
nonpolar areas (residues) are termed amphipathic.
Despite their differing functions, all biological membranes 3. Under physiological conditions the membrane
have a common general structure: each is a very thin film of components are not rigidly fixed, but exist in a fluid and
lipid and protein molecules, held together mainly by non‐ dynamic state. Lipids of the membrane are fluid at
covalent interactions. Cell membranes are dynamic, fluid physiological temperatures, thus there may be fluid
structures, and most of their molecules are able to move movement of the lipid molecules and lateral movement
about in the plane of the membrane. The lipid molecules are (diffusion) of the proteins that are embedded in the
arranged as a continuous double layer about 5 nm thick. membrane. "Protein icebergs in a sea of lipid." (Singer) This
This lipid bilayer provides the basic structure of the forms a heterogeneous mosaic of proteins which may be
membrane and serves as a relatively impermeable barrier to continuously rearranged (but often with specificity.)
the passage of most water‐soluble molecules. Protein
molecules "dissolved" in the lipid bilayer mediate most of 4. Not only is there lateral mobility of lipids but there is also
the other functions of the membrane, transporting specific exchange from one monolayer of the bi‐leaflet to the other
molecules across it, for example, or catalyzing membrane‐ monolayer. This exchange (flip‐flop) is much slower than the
associated reactions, such as ATP synthesis. In the plasma lateral exchange, detected by nuclear resonance studies.
membrane some proteins serve as structural links that This process is facilitated by specific proteins (enzymes)
connect the membrane to the cytoskeleton and/or to either called flippases.
the extracellular matrix or an adjacent cell, while others
serve as receptors to detect and transduce chemical signals 5. Proteins may be integral or peripheral, probably most
in the cell's environment. As would be expected, cell being integral and displaying various different secondary
membranes are asymmetrical structures: the lipid and and tertiary forms. The proteins, being capable of secondary,
protein compositions of the outside and inside faces differ tertiary, and quaternary structure changes in response to
from one another in ways that reflect the different functions stimuli, may reorient in relation to the phospholipids
performed at the two surfaces of the membrane.
Downloaded bilayer.
by Khushbu Shukla ()
INSTITUTE FOR ADVANCED STUDIES, JODHPUR 1
, lOMoARcPSD|56746167
6. In a phospholipid bilayer, the long fatty acyl side chains in 10. As a phospholipid bilayer is heated, it undergoes a phase
each leaflet are oriented toward one another, forming a transition from a gel‐like to a more fluid state over a short
hydrophobic core; the polar head groups line both surfaces. temperature range.
7. The phospholipid bilayer forms the basic structure of all 11. Cholesterol is a major determinant of bilayer fluidity,
biomembranes, which also contain proteins, glycoproteins, although its effect depends on the composition of a
cholesterol and other steroids, and glycolipids. The presence membrane. Natural biomembranes generally have a fluid‐
of specific sets of membrane proteins permits each type of like consistency, and cells adjust their phospholipid
membrane to carry out distinctive functions. composition to maintain bilayer fluidity.
8. All cellular membranes line closed compartments and
have a cytosolic and an exoplasmic face. The asymmetry of 12. In all cells, proteins in the plasma membrane selectively
biological membranes is reflected in the specific orientation absorb nutrients, expel wastes, and maintain the proper
of each type of integral and peripheral membrane protein intracellular ionic composition. Proteins in the plasma
with respect to the cytosolic and exoplasmic faces. The membrane anchor the membrane to intracellular
presence of glycolipids exclusively in the exoplasmic leaflet cytoskeletal fibers and the extracellular matrix or cell wall.
also contributes to membrane asymmetry. In multicellular organisms, plasma membrane proteins also
act in the interactions and communication between cells,
9. Most integral proteins and lipids are laterally mobile in which are critical for proper functioning of multicellular
biomembranes. According to the fluid mosaic model, the tissues.
membrane is viewed as a two‐dimensional mosaic of
phospholipid and protein molecules. 13. In plants, the cell wall, which is built mainly of cellulose,
is the major determinant of cell shape and imparts rigidity to
cells. Animal cells, which lack a wall, are surrounded by an
extracellular matrix consisting of collagen, glycoproteins
A2. MEMBRANE PROTEINS
1. Transmembrane Proteins: lipid bilayer,these proteins are not free‐floating and cannot
be isolated and purified biochemically without first
Membrane proteins are either extrinsic or intrinsic. dissolving away the lipid bilayer with detergents. (Indeed,
Extrinsic membrane proteins are entirely outside of the much of the washing we do in our lives is necessitated by the
membrane, but are bound to it by weak molecular need to solubilize proteins that are embedded in lipid
attractions (ionic, hydrogen, and/or Van der Waals bonds). membranes using detergents!)
Intrinsic membrane proteins, the class we are mainly
interested in, are embedded in the membrane. Many of them 3. Glycoproteins:
extend from one side of the membrane to the other and are
referred to as transmembrane proteins. Cells are constantly For reasons that are not well understood, many
pumping ions in and out through their plasma membranes. transmembrane proteins are glycoproteins in the sense that
In fact, more than half the energy that are bodies consume is sugar side chains are covalently attached to their
used by cells to drive the protein pumps in the brain that do hydrophilic domains that protrude into the extracellular
nothing else but transport ions across plasma membranes of membrane. A typical mammalian cell may have several
nerve cells. How can ions be transported across membranes hundred distinct types of glycoprotein studding its plasma
that are effectively impermeable to them? membrane. Each of these glycoproteins will have its
extracellular domain glycosylated with a complex branching
Cells contain proteins that are embedded in the lipid bilayer bush of sugar residues covalently linked to the asparagine
of their plasma membranes and extend from one side of the side chains. Some glycoproteins may have 2 or 3 asparagine‐
membrane through to the other. Such transmembrane linked sugar side chains, others may have dozens.
proteins can function to effect ion transport in several ways.
But how can they cope with the energetically highly 4. Multimembranespanning proteins:
unfavorable situation in which an ion must pass through the
hydrophobic inner layers of the plasma membrane? An elaboration of this scheme depicting membrane proteins
having single transmembrane domains involves certain
2. Domains: membrane proteins that have multiple transmembrane
domains. As one scans along the amino acid sequence of
If we examine the detailed structures of many these proteins, it becomes apparent that hydrophilic
transmembrane proteins, we see that they often have three domains (i.e. having hydrophilic amino acids) alternate with
different domains, two hydrophilic and one hydrophobic. A hydrophobic domains. The protein chain as a whole when
hydrophilic domain (consisting of hydrophilic amino acids) embedded in the plasma membrane actually weaves back
at the N‐terminus is poking out in the extracellular medium, and forth between opposite sides of the plasma membrane.
a hydrophobic domain in the middle of the amino acid chain, Some think such proteins have the configuration of snakes
often only 20‐30 amino acids long, is threaded through the and hence term them serpentine membrane proteins. A
plasma membrane, and a hydrophilic domain at the C‐ commonly used type of structure seen in many hundreds of
terminus protrudes into the cytoplasm. The transmembrane serpentine transmembrane proteins involves 7 hydrophobic
domain, because it is made of amino acids having domains inserted into the plasma membrane separated by
hydrophobic side chains, exists comfortably in the hydrophilic regions that are looped out alternatively into
hydrophobic inner layers of the plasma membrane. Because either the cytoplasm or the extracellular space.
these transmembrane domains anchor many proteins in the
Downloaded by Khushbu Shukla ()
INSTITUTE FOR ADVANCED STUDIES, JODHPUR 2
, lOMoARcPSD|56746167
A3. MEMBRANE TRANSPORT MECHANISM & ION PUMPS
All cells acquire the molecules and ions they need from their
surrounding extracellular fluid (ECF). There is an unceasing The difference in the concentration of molecules across a
traffic of molecules and ions in and out of the cell through its space is called a concentration gradient. If these molecules
plasma membrane. Examples: glucose Na+, Ca2+. In diffusing across the membrane from an area of high
eukaryotic cells, there is also transport in and out of concentration to an area of low concentration were water
membrane‐bounded intracellular compartments such as the molecules the process would be called osmosis. Water also
nucleus, endoplasmic reticulum and mitochondria. moves from a low solute concentration to a high solute
Examples: proteins, mRNA, Ca2+, ATP concentration.
If uncharged solutes are small enough and lipid liking they
can move down their concentration gradients directly across The Two types of Passive Transport are
the lipid bilayer itself by simple diffusion. Examples of such
solutes are ethanol, carbon dioxide, and oxygen. Transport by simple diffusion and Osmosis
Facilitated diffusion: carrier proteins and ion
The major difficulty which the membranes pose for some channels
molecule is that the Lipid bilayers are not permeable to: ions
such as 1. Diffusion and Osmosis
K+, Na+, Ca2+ (called cations because when
subjected to an electric field they migrate toward 1. In diffusion, molecules move from higher to lower
the cathode [the negatively‐charged electrode]) concentration (i.e., down their concentration gradient).
Cl‐, HCO3‐ (called anions because they migrate A solution contains a solute, usually a solid, and a
toward the anode [the positively‐charged solvent, usually a liquid.
electrode]) In the case of a dye diffusing in water, dye is a
small hydrophilic molecules like glucose and solute and water is the solvent.
macromolecules like proteins and RNA 2. Membrane chemical and physical properties allow only a
few types of molecules to across by diffusion.
But we all know that that cell is intelligent enough to solve Lipid‐soluble molecules (e.g., alcohols) diffuse;
the above challenge posed by the membrane. This is how it lipids are membrane’s main structural
solves the problem components.
Gases readily diffuse through lipid bilayer.
Passive and active mechanisms move molecules across Movement of oxygen from air sacs (alveoli) to
membrane. blood in lung capillaries depends on
Passive transport moves molecules across concentration of oxygen in alveoli.
membrane without expenditure of energy by cell; 3. Osmosis is the diffusion of water across a differentially
includes diffusion and facilitated transport. permeable membrane from its higher concentration to
Active transport uses energy (ATP) to move lower concentration. Osmotic pressure is hydrostatic
molecules across a plasma membrane; include pressure, on side of membrane with higher solute
active transport, exocytosis, endocytosis, and concentration, produced by water diffusing to that side of
pinocytosis. membrane
4. Tonicity is strength of a solution in relationship to
I. Passive Transport osmosis; determines movement of water into or out of cells.
Isotonic is where the relative solute concentration
Cell membranes help organisms maintain homeostasis by of two solutions are equal.
controlling what substances may enter or leave the cells. Hypotonic is where a relative solute
Some substances such as water, oxygen, and carbon dioxide, concentration of one solution is less than another
can cross the cell membrane without any input of energy by solution.
the cell. The movement of such substances across the Hypertonic is where relative solute concentration
membrane is know as passive transport. The cell membrane of one solution is greater than another solution.
is said to be selectively permeable.
Downloaded by Khushbu Shukla ()
INSTITUTE FOR ADVANCED STUDIES, JODHPUR 3
