This document summarizes different types of membrane transport proteins and how they transport molecules across cellular membranes. It discusses:
- Passive transport mechanisms like diffusion and facilitated diffusion.
- Active transport mechanisms like ATP-powered pumps that use ATP to transport molecules against their gradients. Examples given include the Na+/K+ ATPase pump and V-ATPase pump.
- Different classes of membrane transport proteins including channels, carriers, uniporters, symporters, antiporters and ABC transporters. Their structures and functions are described.
A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle.
More than half of all proteins interact with membranes.
Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter.
Structure and functions of endoplasmic reticulumICHHA PURAK
The presentation consists of 57 slides,describes following heads
• DISCOVERY
• INTRODUCTION
• BIOGENESIS OF ER
• ISOLATION OF MICROSOMES FROM E R
• STRUCTURE
• COMPONENTS OF ER
CISTERNAE
VESICLES
TUBULES
• MAIN FUNCTION OF ER
• TYPES OF ENDOPLASMIC RETICULUM
• SMOOTH ENDOPLASMIC RETICULUM (SER)
• FUNCTIONS OF SER
• ROUGH ENDOPLASMIC RETICULUM (RER)
• FUNCTIONS OF RER
• SUMMARY
• REFERENCES
• QUESTIONS
A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle.
More than half of all proteins interact with membranes.
Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter.
Structure and functions of endoplasmic reticulumICHHA PURAK
The presentation consists of 57 slides,describes following heads
• DISCOVERY
• INTRODUCTION
• BIOGENESIS OF ER
• ISOLATION OF MICROSOMES FROM E R
• STRUCTURE
• COMPONENTS OF ER
CISTERNAE
VESICLES
TUBULES
• MAIN FUNCTION OF ER
• TYPES OF ENDOPLASMIC RETICULUM
• SMOOTH ENDOPLASMIC RETICULUM (SER)
• FUNCTIONS OF SER
• ROUGH ENDOPLASMIC RETICULUM (RER)
• FUNCTIONS OF RER
• SUMMARY
• REFERENCES
• QUESTIONS
Cytoskeleton - microtubules ,microfilaments and intermediate filamentsBIOTECH SIMPLIFIED
The cytoskeleton is a structure that helps cells maintain their shape and internal organization, and it also provides mechanical support that enables cells to carry out essential functions like division and movement. There is no single cytoskeletal component. Rather, several different components work together to form the cytoskeleton.
Details of cytoskeleton element-microtubule. The Microtubule associated protein-type and function, Treadmilling and dynamic instability, Structure of cilia and flagella
Thermodynamic laws describe the flows and interchanges of heat, energy and matter.
Almost all chemical and biochemical processes are as a result of transformation of energy.
Laws can provide important insights into metabolism and bioenergetics.
The energy exchanges between the system and the surroundings balance each other.
There is a hierarchy of energetics among organisms
Cytoskeleton - microtubules ,microfilaments and intermediate filamentsBIOTECH SIMPLIFIED
The cytoskeleton is a structure that helps cells maintain their shape and internal organization, and it also provides mechanical support that enables cells to carry out essential functions like division and movement. There is no single cytoskeletal component. Rather, several different components work together to form the cytoskeleton.
Details of cytoskeleton element-microtubule. The Microtubule associated protein-type and function, Treadmilling and dynamic instability, Structure of cilia and flagella
Thermodynamic laws describe the flows and interchanges of heat, energy and matter.
Almost all chemical and biochemical processes are as a result of transformation of energy.
Laws can provide important insights into metabolism and bioenergetics.
The energy exchanges between the system and the surroundings balance each other.
There is a hierarchy of energetics among organisms
This presentation contains the introduction to the structure of plasma membrane. This gives an insight into the biochemistry of the plasma membrane and the singer and nicholsan model.
The plasma membrane, which is also called the cell membrane, has many functions, but the most basic one is to define the borders of the cell and keep the cell functional.
Characterization and the Kinetics of drying at the drying oven and with micro...Open Access Research Paper
The objective of this work is to contribute to valorization de Nephelium lappaceum by the characterization of kinetics of drying of seeds of Nephelium lappaceum. The seeds were dehydrated until a constant mass respectively in a drying oven and a microwawe oven. The temperatures and the powers of drying are respectively: 50, 60 and 70°C and 140, 280 and 420 W. The results show that the curves of drying of seeds of Nephelium lappaceum do not present a phase of constant kinetics. The coefficients of diffusion vary between 2.09.10-8 to 2.98. 10-8m-2/s in the interval of 50°C at 70°C and between 4.83×10-07 at 9.04×10-07 m-8/s for the powers going of 140 W with 420 W the relation between Arrhenius and a value of energy of activation of 16.49 kJ. mol-1 expressed the effect of the temperature on effective diffusivity.
Artificial Reefs by Kuddle Life Foundation - May 2024punit537210
Situated in Pondicherry, India, Kuddle Life Foundation is a charitable, non-profit and non-governmental organization (NGO) dedicated to improving the living standards of coastal communities and simultaneously placing a strong emphasis on the protection of marine ecosystems.
One of the key areas we work in is Artificial Reefs. This presentation captures our journey so far and our learnings. We hope you get as excited about marine conservation and artificial reefs as we are.
Please visit our website: https://kuddlelife.org
Our Instagram channel:
@kuddlelifefoundation
Our Linkedin Page:
https://www.linkedin.com/company/kuddlelifefoundation/
and write to us if you have any questions:
info@kuddlelife.org
Willie Nelson Net Worth: A Journey Through Music, Movies, and Business Venturesgreendigital
Willie Nelson is a name that resonates within the world of music and entertainment. Known for his unique voice, and masterful guitar skills. and an extraordinary career spanning several decades. Nelson has become a legend in the country music scene. But, his influence extends far beyond the realm of music. with ventures in acting, writing, activism, and business. This comprehensive article delves into Willie Nelson net worth. exploring the various facets of his career that have contributed to his large fortune.
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Introduction
Willie Nelson net worth is a testament to his enduring influence and success in many fields. Born on April 29, 1933, in Abbott, Texas. Nelson's journey from a humble beginning to becoming one of the most iconic figures in American music is nothing short of inspirational. His net worth, which estimated to be around $25 million as of 2024. reflects a career that is as diverse as it is prolific.
Early Life and Musical Beginnings
Humble Origins
Willie Hugh Nelson was born during the Great Depression. a time of significant economic hardship in the United States. Raised by his grandparents. Nelson found solace and inspiration in music from an early age. His grandmother taught him to play the guitar. setting the stage for what would become an illustrious career.
First Steps in Music
Nelson's initial foray into the music industry was fraught with challenges. He moved to Nashville, Tennessee, to pursue his dreams, but success did not come . Working as a songwriter, Nelson penned hits for other artists. which helped him gain a foothold in the competitive music scene. His songwriting skills contributed to his early earnings. laying the foundation for his net worth.
Rise to Stardom
Breakthrough Albums
The 1970s marked a turning point in Willie Nelson's career. His albums "Shotgun Willie" (1973), "Red Headed Stranger" (1975). and "Stardust" (1978) received critical acclaim and commercial success. These albums not only solidified his position in the country music genre. but also introduced his music to a broader audience. The success of these albums played a crucial role in boosting Willie Nelson net worth.
Iconic Songs
Willie Nelson net worth is also attributed to his extensive catalog of hit songs. Tracks like "Blue Eyes Crying in the Rain," "On the Road Again," and "Always on My Mind" have become timeless classics. These songs have not only earned Nelson large royalties but have also ensured his continued relevance in the music industry.
Acting and Film Career
Hollywood Ventures
In addition to his music career, Willie Nelson has also made a mark in Hollywood. His distinctive personality and on-screen presence have landed him roles in several films and television shows. Notable appearances include roles in "The Electric Horseman" (1979), "Honeysuckle Rose" (1980), and "Barbarosa" (1982). These acting gigs have added a significant amount to Willie Nelson net worth.
Television Appearances
Nelson's char
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Prevalence of Toxoplasma gondii infection in domestic animals in District Ban...Open Access Research Paper
Toxoplasma gondii is an intracellular zoonotic protozoan parasite, infect both humans and animals population worldwide. It can also cause abortion and inborn disease in humans and livestock population. In the present study total of 313 domestic animals were screened for Toxoplasma gondii infection. Of which 45 cows, 55 buffalos, 68 goats, 60 sheep and 85 shaver chicken were tested. Among these 40 (88.88%) cows were negative and 05 (11.12%) were positive. Similarly 55 (92.72%) buffalos were negative and 04 (07.28%) were positive. In goats 68 (98.52%) were negative and 01 (01.48%) was recorded positive. In sheep and shaver chicken the infection were not recorded.
WRI’s brand new “Food Service Playbook for Promoting Sustainable Food Choices” gives food service operators the very latest strategies for creating dining environments that empower consumers to choose sustainable, plant-rich dishes. This research builds off our first guide for food service, now with industry experience and insights from nearly 350 academic trials.
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The carbon cycle is a critical component of Earth's environmental system, governing the movement and transformation of carbon through various reservoirs, including the atmosphere, oceans, soil, and living organisms. This complex cycle involves several key processes such as photosynthesis, respiration, decomposition, and carbon sequestration, each contributing to the regulation of carbon levels on the planet.
Human activities, particularly fossil fuel combustion and deforestation, have significantly altered the natural carbon cycle, leading to increased atmospheric carbon dioxide concentrations and driving climate change. Understanding the intricacies of the carbon cycle is essential for assessing the impacts of these changes and developing effective mitigation strategies.
By studying the carbon cycle, scientists can identify carbon sources and sinks, measure carbon fluxes, and predict future trends. This knowledge is crucial for crafting policies aimed at reducing carbon emissions, enhancing carbon storage, and promoting sustainable practices. The carbon cycle's interplay with climate systems, ecosystems, and human activities underscores its importance in maintaining a stable and healthy planet.
In-depth exploration of the carbon cycle reveals the delicate balance required to sustain life and the urgent need to address anthropogenic influences. Through research, education, and policy, we can work towards restoring equilibrium in the carbon cycle and ensuring a sustainable future for generations to come.
1. Transportation Of Molecule In Different
membranes
Submitted to
Dr. J.J.Dhruve
Asst Research scientists,
BACA,AAU,
Anand. Submitted by
Joshi Prathmesh Govind
M.Sc (Agri) Biochemistry,
BACA,AAU, Anand.
Submission date:-
3. Membranes:-Definition
Membranes separate the cell from the outside world and separate organelles
inside the cell to compartmentalize important processes and activities.
Introduction:-
Cellular membranes have diverse functions depending on their location
within the organelles of a cell.
However, at the electron microscopic level, membranes share a common
structure following routine preparative steps.
The figure below shows a typical "Unit" membrane which resembles a
railroad track with two dense lines separated by a clear space.
4. This figure was obtained by cell fixation/sectioning and staining with osmium
tetroxide (an electron opaque agent that binds to a variety of organic compounds).
This figure actually shows two adjacent plasma membranes, both of which have
the "unit membrane" structure.
5. Membrane Transport: Definition
Membrane transport is defined as the movement of molecules across
cell membranes.
The bilayer is permeable to:
Small hydrophobic molecules
Small uncharged polar molecules
The bilayer is impermeable to:
Ions
Large polar molecules
Therefore, need membrane proteins to transport most molecules and all ions across
biomembranes
6. Gases diffuse freely, no
proteins required.
Water diffuses fast enough that
proteins aren’t required for
transport.
Water diffuses fast enough that
proteins aren’t required for
transport.
Sugars diffuse very slowly so
proteins are involved in
transport.
Charged molecules are virtually
impermeable.
Charged molecules are virtually
impermeable.
Fig:
The phospholipid bilayer is a
barrier that controls the
transport of molecules in and
out of the cell.
7. Selective transport across the lipid membrane requires transport proteins.
Transport proteins are integral membrane proteins that move molecules
and
ions.
There are two classes of transport proteins: transporters (pumps) and
channels.
8. Three main class of membrane protein
1. ATP- power pump( carrier, permease) couple with
energy source for active transport binding of specific solute to transporter
which undergo conformation change.
2. Channel protein (ion channel) formation of hydrophilic pore allow
passive movement of small inorganic molecule.
3. Transporters
uniport
symport
antiport
9. Differences
1. Transporters:
uniporters transport a single molecule down its gradient (passive).
co-transporters couple movement of a molecule down its gradient
with
moving a molecule up its gradient (active).
2. Pumps
hydrolyze ATP to move small molecules/ions up a concentration
gradient or electric potential (active).
3. Channels
transport water/ions/small molecules down their concentration
gradients or electric potentials (passive)
10. Two types of transport.
1. Passive transport: no metabolic energy is needed because the
solute is moving down its concentration gradient.
• In the case of an uncharged solute, the concentration of the solute on
each side of the membrane dictates the direction of passive
transport.
2. Active transport: metabolic energy is used to transport a solute
from the side of low concentration to the side of high concentration.
11.
12. Types of Diffusion
Free Diffusion
A. Non-channel mediated
-lipids, gasses (O2, CO2), water
B. Channel mediated
-ions, charged molecules
Facilitated diffusion
molecule moves down its electrochemical gradient.
-glucose, amino acids
13. ATP powered pump
1. P- class
2α, 2β subunit; can phosphorylation
i.e. Na+-K+ ATP ase, Ca+ATP ase, H+pump
2. F-class
locate on bacterial membrane , chloroplast and mitochondria
pump proton from exoplasmic space to cytosolic for ATP synthesis
3. V-class
maintain low pH in plant vacuole
similar to F-class
4. ABC (ATP-binding cassete) superfamily
several hundred different transport protein
14. Transport process requires ATP
hydrolysis in which the free energy is
liberated by breakdown of ATP into ADP
and phosphate.
Must phosphorylation
15.
16. The Fo integral membrane protein complex subscript o denoting its
inhibition by the drug oligomycin provides a transmembrane pore for
protons, and the peripheral protein F1 is a molecular machine that uses
the energy of ATP to drive protons uphill.
The reaction catalyzed by F-type ATPases is reversible, so a proton
gradient can supply the energy to drive the reverse reaction, ATP
synthesis.
17. V-class H+ ATP ase pump protons
across lysosomal and vacuolar
membrane
18. This class of proton-transporting ATPases structurally related to the F-type
ATPases, are responsible for acidifying intracellular compartments in many
organisms (thus V for vacuolar).
Proton pumps of this type maintain the vacuoles of fungi and higher plants
at a pH between 3 and 6, well below that of the surrounding cytosol (pH 7.5).
Vtype ATPases have a similar complex structure, with an integral
(transmembrane) domain (Vo) that serves as a proton channel and a
peripheral domain (V1) that contains the ATP-binding site and the ATPase
activity.
19. ABC Transporters
Largest family of membrane transport proteins 78 genes (5% of genome)
encode ABC transporters in E coli
Many more in animal cells Known as the ABC transporter superfamily
They use the energy derived from ATP hydrolysis to transport a variety
of
small molecules including:
Amino acids, sugars, inorganic ions, peptides.
ABC transporters also catalyze the flipping of lipids between monolayers
in membranes.
All ABC transporters each contain 2 highly conserved ATP- binding
domains.
20. Structure of ABC Transporter
2 T ( transmembrane ) domain,
each has 6 α- helix form pathways
for transported substance
2A ( ATP- binding domain) 30-
40% homology for membranes
21. The channels
The channels form membrane-spanning pores that allow molecules to diffuse down
the electrochemical gradient into or out of the cell.
Some channels are gated.
They are opened or closed by binding of a ligand or by altered membrane potential.
22. Example:-
The Acetylcholine Receptor Is a Ligand-Gated
Ion Channel
Another very well-studied ion channel is the nicotinic acetylcholine receptor,
essential in the passage of an electrical signal from a motor neuron to a muscle fiber
at the neuromuscular junction (signaling the muscle to contract).
Nicotinic receptors were originally distinguished from muscarinic receptors by the
sensitivity of the former to nicotine, the latter to the mushroom alkaloid muscarine.
Acetylcholine released by the motor neuron diffuses a few micrometers to the
plasma membrane of a myocyte, where it binds to the acetylcholine receptor.
This forces a conformational change in the receptor, causing its ion channel to open.
The resulting inward movement of positive charges depolarizes the plasma
membrane, triggering contraction.
The acetylcholine receptor allows Na+
, Ca2+
, and K+
to pass through with equal ease,
but other cations and all anions are unable to pass.
23. Movement of Na through an acetylcholine receptor ion channel is unsaturable (its rate
is linear with respect to extracellular [Na]) and very fast about 2 *107
ions/s under
physiological conditions.
This receptor channel is typical of many other ion channels that produce or
respond to electrical signals: it has a “gate” that opens in response to stimulation by
a signal molecule (in this case acetylcholine) and an intrinsic timing mechanism that
closes the gate after a split second.
Thus the acetylcholine signal is transient an essential feature of electrical signal
conduction.
24. Na+/K+ ATPase maintain the intracellular Na+ and
K+ concentration in animal cell
ATP-powered ion pumps generate and maintain ionic gradients
across cellular membranes.
Na+ transport out
K+ transport in
By Na+/K+ ATPase
Na+/K+ ATPase:-
Four major domains:
M - Membrane- bound domain, which is composed of 10
transmembrane segments
25. N- Nucleotide-Nucleotide binding domain, where adenine moiety of ATP
and ADP binds
P – Phosphatase domain, which contains invariant Asp residue, which
became phosphorylated during the ATP hydrolysis
A domain – essential for conformational transitions between E1 and E2
states
Na+K+ ATPase
26. In virtually every animal cell type, the concentration of Na is lower in the cell than in
the surrounding medium, and the concentration of K is higher.
This imbalance is maintained by a primary active transport system in the
plasma membrane.
The enzyme Na+K+ ATPase, discovered by Jens Skou in 1957, couples breakdown of
ATP to the simultaneous movement of both Na and K against their electrochemical
gradients.
For each molecule of ATP converted to ADP and Pi, the transporter moves two K ions
inward and three Na ions outward across the plasma membrane.
The Na+K +ATPase is an integral protein with two subunits (Mr ~50,000 and ~110,000),
both of which span the membrane.
27. Ion Channel (non-gate)
Nongated ion channels and the resting membrane potential
Gated need ligand to activation. Non-gated do not need ligand.
Generation of electrochemical gradient across plasma membrane
i.e. Ca+ gradient.
regulation of signal transduction , muscle contraction and triggers
secretion of digestive enzyme in to exocrine pancreastic cells.
i.e. Na+ gradient
uptake of a.a , symport, antiport; formed membrane potential
i.e. K+ gradient formed membrane potential.
Voltage-gated proton channels open with depolarization, but in a strongly pH-
sensitive . The result is that these channels open only when the
electrochemical gradient is outward, such that their opening will only allow
protons to leave cells.
i.e. Voltage -gated proton channels
28. Ion channels are selective pores in the membrane
Ion channels have ion selectivity - they only allow passage of specific
molecules
Ion channels are not open continuously, conformational changes open
and close
29. Ion-Selective Channels Allow Rapid
Movement of Ions across Membranes
Ion-selective channels—first recognized in neurons and now known to be present in
the plasma membranes of all cells, as well as in the intracellular membranes of
eukaryotes—provide another mechanism for moving inorganic ions across
membranes.
Ion channels, together with ion pumps such as the Na+K+ ATPase, determine a plasma
membrane’s permeability to specific ions and regulate the cytosolic concentration of
ions and the membrane potential.
In neurons, very rapid changes in the activity of ion channels cause the changes in
membrane potential (the action potentials) that carry signals
from one end of a neuron to the other.
In myocytes, rapid opening of Ca2+
channels in the sarcoplasmic reticulum releases the
Ca2+
that triggers muscle contraction.
31. The “activity” of an ion channel is estimated by measuring the flow of ions
through it, using the patch-clamp technique.
A finely drawn-out pipette (micropipette) is pressed against the cell surface, and
negative pressure in the pipette forms a pressure seal between pipette and
membrane.
As the pipette is pulled away from the cell, it pulls off a tiny patch of membrane (which
may contain one or a few ion channels).
After placing the pipette and attached patch in an aqueous solution, the researcher can
measure channel activity as the electric current that flows between the contents of the
pipette and the aqueous solution.
In practice, a circuit is set up that “clamps” the transmembrane potential at a given
value and measures the current that must flow to maintain this voltage.
With highly sensitive current detectors, researchers can measure the current flowing
through a single ion channel, typically a few picoamperes.
32. The trace showing the current as a function of time (in milliseconds) reveals how fast
the channel opens and closes, how frequently it opens, and how long it stays open.
Clamping the Vm at different values permits determination of the effect
of membrane potential on these parameters of channel function.
34. In voltage gated ion channels, a change in transmembrane electrical
potential (Vm) causes a charged protein domain to move relative to the
membrane, opening or closing the ion channel.
Both types of gating can be very fast.
A channel typically opens in a fraction of a millisecond and may remain
open for only milliseconds, making these molecular devices effective
for very fast signal transmission in the nervous system.
35. Transporters
Cotransporter:-
A cotransporter is an integral membrane protein that is involved in secondary active
transport.
It works by binding to two molecules or ions at a time and using the gradient of
one solute's concentration to force the other molecule or ion against its gradient.
It is sometimes equated with symporter, but the term "cotransporter" refers both to
symporters and antiporters .
The word "symporter" is a conjunction of the Greek syn- or sym- for "together, with"
and -porter.
In order for any protein to do work, it must harness energy from some source. In
particular, symporters do not require the splitting of ATP because they derive the
necessary energy for the movement of one molecule from the movement of the
another.
Overall, the movement of the two molecules still acts to increase entropy.
Proton-sucrose cotransporters are common in plant cell membranes.
36. Uniporter
A uniporter is an integral membrane protein that is involved in facilitated diffusion.
They can be either a channel or a carrier protein.
Uniporter carrier proteins work by binding to one molecule of solute at a time and
transporting it with the solute gradient.
Uniporter channels open in response to a stimulus and allow the free flow of specific
molecules.
Uniporters may not utilize energy other than the solute gradient.
Thus they may only transport molecules with the solute gradient, and not against it.
There are several ways in which the opening of uniporter channels may be regulated:
1.Voltage - Regulated by the difference in voltage across the membrane
2.Stress - Regulated by physical pressure on the transporter.
3.Ligand - Regulated by the binding of a ligand to either the intracellular or extracellular
side of the cell
37. Uniporters are involved in many biological processes, including impulse
transmission in neurons.
Voltage-gated sodium channels are involved in the propagation of a nerve
impulse across the neuron.
During transmission of the signal from one neuron to the next, calcium is transported
into the presynaptic neuron by voltage-gated calcium channels.
Calcium released from the presynaptic neuron binds to a ligand-gated calcium channel
in the postsynaptic neuron to stimulate an impulse in that neuron.
Potassium leak channels, also regulated by voltage, then help to restore the
resting membrane potential after impulse transmission.
In the ear, sound waves cause the stress-regulated channels in the ear to open,
sending
an impulse to the vestibulocochlear nerve,
38. Antiporter
An antiporter is an integral membrane protein involved in secondary active
transport of two or more different molecules or ions i.e., solutes across a phospholipid
membrane such as the plasma membrane in opposite directions.
In secondary active transport, one species of solute moves along its electrochemical
gradient, allowing a different species to move against its own electrochemical
gradient.
This movement is in contrast to primary active transport, in which all solutes are
moved against their concentration gradients, fueled by ATP.
Transport may involve one or more of each type of solute.
Example, the Na+
/Ca2+
exchanger, used by many cells to remove cytoplasmic calcium,
exchanges one calcium ion for three sodium ions.
39. Symporter
A symporter is an integral membrane protein that is involved in movement of two or
more different molecules or ions across a phospholipid membrane such as the plasma
membrane in the same direction, and is therefore a type of cotransporter.
Typically, the ion(s) will move down the electrochemical gradient, allowing the other
molecule(s) to move against the concentration gradient.
The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled
with the active transport of the molecule(s).
Although two or more types of molecule are transported, there may be several molecules
transported of each type.
Examples:- In the roots of plants, after pumping out H+, they use H+/K+ symporters to
create a chemiosmotic potential inside the cell. This allows the root hairs to take up
water, which moves by osmosis into the xylem so that way the root hair may stay in
a hypotonic environment.
41. Aquaporins
A family of integral proteins discovered by Peter Agre, the aquaporins
(AQPs), provide channels for rapid movement of water molecules
across all plasma membranes Ten aquaporins are known in humans,
each with its specialized role.
Erythrocytes, which swell or shrink rapidly in response to abrupt changes in
extracellular osmolarity as blood travels through the renal medulla, have a
high
density of aquaporin in their plasma membranes (2 ⨉ 105
copies of AQP-1 per
cell).
In the nephron (the functional unit of the kidney), the plasma
membranes of proximal renal tubule cells have five different aquaporin
types.
42. Need of Membrane Transporters
The lipid bilayer is an effective barrier to the movement of small hydrophilic
molecules.
Two factors govern the rate at which molecules can diffuse across the lipid
bilayer. These are:
(1)the membrane solubility of the specific molecules in question and the size of
the molecule that diffuses across the cell membrane.
(2) These cells reabsorb water during urine formation, a process for which
water movement across membranes is essential.
The plant Arabidopsis thaliana has 38 genes that encode various types of
aquaporins, reflecting the critical roles of water movement in plant
physiology.
Changes in turgor pressure, for example, require rapid movement of water
across a membrane.