Transport Proteins

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Chapter 07
Membrane Structure and
Function
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
세포막: 8nm 두께의 울타리, 그 위대한 업적
• Overview: Life at the Edge (생명유지 및 정체성
제공을 위한 울타리)
• The plasma membrane is the boundary that separates the
living cell from its surroundings
• The plasma membrane exhibits selective permeability,
allowing some substances to cross it more easily than
others
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 7.1: Cellular membranes are fluid
mosaics of lipids and proteins
• Phospholipids are the most abundant lipid in the plasma
membrane
• Phospholipids are amphipathic molecules, containing
hydrophobic and hydrophilic regions
• The fluid mosaic model (유동성 모자이크 모델) states
that a membrane is a fluid structure with a “mosaic” of
various proteins embedded in it
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Membrane Models: Scientific Inquiry
•
Membranes have been chemically analyzed and found to be made of
proteins and lipids
•
Scientists studying the plasma membrane reasoned that it must be a
phospholipid bilayer
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
In 1935, Hugh Davson and
James Danielli proposed a
sandwich model in which
the phospholipid bilayer lies
between two layers of
globular proteins
•
Later studies found
problems with this model,
particularly the placement of
membrane proteins, which
have hydrophilic and
hydrophobic regions
•
In 1972, J. Singer and G.
Nicolson proposed that the
membrane is a mosaic of
proteins dispersed within
the bilayer, with only the
hydrophilic regions exposed
to water
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: 유동성 모자이크 모델는 실험적으로 어떻게 증명할 수 있나?
Freeze-fracture (동결 파단):
a cell membrane can be split into its two layers, revealing the
ultrastructure of the membrane’s interior
TECHNIQUE
RESULTS: SEMs
Extracellular
layer
Proteins
Knife
Plasma membrane
Inside of extracellular layer
Cytoplasmic layer
Inside of cytoplasmic layer
Fig. 7-4:
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Figure 7.5
최신 유동성 모자이크 모델
(Updated Fluid mosaic model)
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: 막 유동성 (membrane fluidity)의 증거는?
•
Phospholipids in the plasma membrane can move within the bilayer
•
Most of the lipids, and some proteins, drift laterally (지질분자, 단백질의
측면운동)
•
Rarely does a molecule flip-flop transversely across the membrane
(플립플롭 이동=역공중제비)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: 막단백질의 운동은 어떻게 증명할 수 있나?
• Proteins in the plasma membrane can drift (떠다님) within
the bilayer
EXPERIMENT
RESULTS
Researchers (David Frye and Michael Edidin at Johns’ Hopkins Univ.) labeled
the plasma membrane proteins of a mouse cell and a human cell with two
different markers and fused the cells. Using a microscope, they observed the
markers on the hybrid cell.
Membrane proteins
+
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
CONCLUSION The mixing of the mouse and human membrane proteins indicates that at
Figure 7.7
least some membrane proteins move sideways within the plane of the plasma
membrane.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: 막 유동성에 영향을 주는 요소는?
(1) The type of hydrocarbon tails in phospholipids affects the membrane
fluidity 온도가 낮아지면 막은 유체 상태에서 고체 상태로 전환된다. 이때
막의 고체화 정도는 지질분자의 종류와 연관됨
(2) The steroid cholesterol: 온도
완충제 역할
–
따뜻한 온도 (37C): 인지질
이동 억제로 막유동성 감소
–
낮은 온도: 인지질의
규칙적인 배열을 억제로
막의 고체화 방지
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Figure 7.5
막단백질의 종류와 기능
(Membrane proteins and functions)
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
•
Integral proteins
N-terminus
–
Penetrate the hydrophobic
core of the lipid bilayer
–
Are often transmembrane
proteins, completely
spanning the membrane
(막관통 단백질)
C-terminus
Figure 7.9
a Helix
CYTOPLASMIC
SIDE
EXTRACELLULAR
SIDE
•
Peripheral proteins
–
Are appendages loosely bound to the surface of the membrane
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Figure 7.10a
막단백질의 주요 기능
Figure 7.10b
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The Role of Membrane Carbohydrates in Cell-Cell
Recognition
•
Cells recognize each other by
binding to surface molecules,
often containing carbohydrates,
on the extracellular surface of
the plasma membrane
•
Membrane carbohydrates may
be covalently bonded to lipids
(forming glycolipids) or more
commonly to proteins (forming
glycoproteins)
•
Carbohydrates on the
external side of the plasma
membrane vary among
species, individuals, and even
cell types in an individual
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: 세포막의 안팎은 어떻게 차이가 생기나?
•
Membranes have distinct
inside and outside faces
(막은 안팎 구분이 있다)
•
The asymmetrical
distribution of proteins,
lipids, and associated
carbohydrates in the plasma
membrane is determined
when the membrane is built
by the ER and Golgi
apparatus
막의 안팎에 존재하는
단백질, 지질분자, 첨가된
탄수화물의 비대칭적인
분포는 막이
소포체/골지체에서 합성될
때 결정된다
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 7.2: Membrane structure results in
selective permeability (선택적 투과성)
•
A cell must exchange materials with its surroundings, a process
controlled by the plasma membrane (세포와 주변간의 물질교환은
세포막에 의해 조절됨)
•
Plasma membranes are selectively permeable, regulating the cell’s
molecular traffic
The Permeability of the Lipid Bilayer
•
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can
dissolve in the lipid bilayer and pass through the membrane rapidly
•
Polar molecules, such as sugars, do not cross the membrane easily
Q: 그렇다면 극성분자를 포함하는 다양한 물질의 선택적
막투과는 어떻게 가능한가?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transport Proteins (수송단백질)
• Transport proteins allow passage of hydrophilic
substances across the membrane
• Some transport proteins, called channel proteins,
have a hydrophilic channel that certain molecules or
ions can use as a tunnel
• Channel proteins called aquaporins facilitate the
passage of water
• Other transport proteins, called carrier proteins, bind
to molecules and change shape to shuttle them
across the membrane
• A transport protein is specific for the substance it
moves
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 7.3: Passive transport is diffusion of a
substance across a membrane with no energy
investment
수동수송은 에너지 소모 없이 확산 의존성임
에너지 소모 없이 자발적으로 농도구배를 따라 물질 이동이
일어남
수동수송은 막단백질의 도움이 있으면 확산속도는 급격히
증가됨
• Substances diffuse down their concentration gradient (농도구배), the
region along which the density of a chemical substance increases or
decreases
•
No work must be done to move substances down the concentration
gradient
•
The diffusion of a substance across a biological membrane is passive
transport because no energy is expended by the cell to make it happen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Effects of Osmosis on Water Balance
•
Osmosis (삼투현상)
–
Is the movement of
water across a
semipermeable
membrane (반투과성
막을 따라 일어나는
물이 이동하고자 하는
힘)
–
무질서도가 높아지는
방향 (용매와
용질분자가 잘 혼합되는
방향 또는 농도 평형을
이루고자 하는
방향)으로 용매인
물분자가 이동하는 현상
–
결국 물은 용질의
농도가 낮은 곳에서
높은 곳으로 막을 통해
확산된다
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Water Balance of Cells Without Walls
•
Tonicity
–
Is the ability of a solution to cause a cell to gain or lose water (세포로
하여금 물의 획득 또는 손실을 유도하는 용액의 능력)
–
Has a great impact on cells without walls (세포벽이 없는 세포에 큰 영향을
미침)
Q: 세포막 안팎의 용질의 농도차이가 세포의
tonicity에 미치는 영향은 무엇인가?
세포밖의 용액의 종류와 세포막을 통한 물의
이동?
•
If a solution is isotonic (등장성)
–
The concentration of solutes is the same as it is inside the cell
(용질의 농도가 세포 안과 동일한 상태)
–
There will be no net movement of water
(물의 안팎의 이동속도가 동일하여 순이동은 없는 것으로 인식됨)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
•
If a solution is hypertonic (고장성)
–
The concentration of solutes is greater than it is inside the cell
(용질의 농도가 세포 안보다 높음)
–
삼투현상으로 세포 안의 물분자가 세포밖으로 이동하게 되며,
따라서 세포는 물을 잃게 되고 세포는 마르게 됨
If a solution is hypotonic (저장성)
–
The concentration of solutes is less than it is inside the cell
(용질농도가 세포 안보다 낮음)
–
The cell will gain water : 삼투현상으로 세포밖의 물분자가
세포안으로 들어오게 되고, 세포는 풍선처럼 터짐 (용혈현상:
염색체 이상을 검사하는 karyotyping할 때 세포를 터뜨릴 때 이
현상을 이용함 )
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
동물세포처럼 세포벽이 없는 경우
• Water balance in cells without walls
Hypotonic solution
(a) Animal cell. An
animal cell fares best
in an isotonic environment unless it has
special adaptations to
offset the osmotic
uptake or loss of
water.
Figure 7.15
H2O
Isotonic solution
H2O
H2O
Lysed
Normal
용혈
정상
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Hypertonic solution
H2O
Shriveled
터진풍선처럼
찌그러진
삼투조절의 필요성?
•
Hypertonic or hypotonic environments create osmotic problems
for organisms
•
Osmoregulation, the control of water balance, is a necessary
adaptation for life in such environments
Q: 짚신벌레처럼 세포안이 주변환경인 연못보다 고장성일 경우 이
생물체에는 무슨 일이 벌어지게 될까? 문제발생시 해결방법은?
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그렇다면 식물과 같이 세포벽이 있는 세포에서의
물균형은?
• Water balance in cells with walls
(b) Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
H2O
H2O
H2O
Turgid (normal)
팽만
Figure 7.15
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Flaccid
축 늘어진
H2O
Plasmolyzed
원형질 분리된
지금까지는 단순확산에 대해 공부하고 있다.
만약 세포가 막을 통한 물질 수송이
단순확산에만 의존한다면 어떻게 될까?
단순확산 이외의 다른 방법은 없을까?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Facilitated Diffusion: Passive Transport Aided by Proteins
• In facilitated diffusion (촉진확산 : 단백질이 관여하는
수동수송)
– Transport proteins speed the movement of
molecules across the plasma membrane
 Channel protein (채널단백질)
 Aquaporins, for facilitated diffusion of water
 Ion channels that open or close in response to a stimulus
(gated channels)
 Carrier protein (운반체 단백질)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
Channel proteins
–
•
Provide corridors (통로)
that allow a specific
molecule or ion to cross
the membrane
Carrier proteins
–
Undergo a subtle change
in shape that translocates
the solute-binding site
across the membrane
(용질결합부위를 위치이동
시키도록 단백질
구조변형을 동반함)
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Q: 채널단백질이나 운반체단백질의 기능에
문제가 생기면 개체에는 어떤 일이
벌어질까?
(1) 낭포성 섬유증 (cystic fibrosis): CFTR 단백질
결핍으로 염소이온의 수송에 문제발생으로 땀, 소화액
등 점액생산에 문제생김
(2) 신장결석 (cystinuria): 신장세포막에 존재하는
시스테인 등의 수송을 담당하는 막단백질의 결핍으로
발생
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Active transport (능동수송)란?
• Concept 7.4: Active transport uses energy
to move solutes against their gradients
•
Facilitated diffusion is still passive because the solute moves
down its concentration gradient
•
Some transport proteins, however, can move solutes against their
concentration gradients
능동수송의 특징은?
•
Active transport moves substances against their concentration gradient
•
Active transport requires energy, usually in the form of ATP
•
Active transport is performed by specific proteins embedded in the
membranes
•
Active transport allows cells to maintain concentration gradients that differ
from their surroundings
•
The sodium-potassium pump is one type of active transport system
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The sodium-potassium pump (Na+-K+ 펌프)
EXTRACELLULAR
[Na+] high
FLUID
(1) Cytoplasmic Na+ binds to
the sodium-potassium pump.
Na+
[K+] low
Na+
Na+ [Na+] low
CYTOPLASM [K+] high
(6) K+ is released and Na+
sites are receptive again;
the cycle repeats
(3 Na+- out and 2 K+ in).
(2) Na+ binding stimulates
phosphorylation by ATP.
P
ADP
Na+
ATP
(3) Phosphorylation
causes the
protein to change its
conformation,
expelling Na+ to the
outside.
Na+
Na+
K+
K+
(5) Loss of the phosphate
restores the protein’s
original conformation.
Na+
Na+
Na+
P
K+
K+
K+
Figure 7.18
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
K+
P
Pi
(4) Extracellular K+
binds to the
protein, triggering
release of the
Phosphate group.
수동수송과 능동수송의 비교?
• Review: Passive and active transport compared
Passive transport. Substances diffuse spontaneously
 에너지 소모 없이
down their concentration gradients, crossing a
자발적으로 농도구배를 membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
따라 물질 이동이
proteins in the membrane.
일어남
 수동수송은
막단백질의 도움이
있으면 확산속도는
급격히 증가됨
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse
through the lipid bilayer.
Facilitated diffusion. Many
hydrophilic substances diffuse
through membranes with the
assistance of transport proteins,
either channel or carrier proteins.
Figure 7.19
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How ion pumps maintain membrane potential
•
Membrane potential is the voltage difference across a membrane
•
Voltage is created by differences in the distribution of positive and
negative ions
•
Two combined forces, collectively called the electrochemical gradient, drive
the diffusion of ions across a membrane:
–
A chemical force (the ion’s concentration gradient)
–
An electrical force (the effect of the membrane potential on the ion’s
movement)
•
An electrogenic pump is a transport protein that generates voltage across a
membrane
•
The sodium-potassium pump is the major electrogenic pump of animal
cells
•
The main electrogenic pump of plants, fungi, and bacteria is a proton pump
•
Electrogenic pumps help store energy that can be used for cellular work
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
An electrogenic
pump (전기발생 펌프)
–
•
Cotransport (공동수송)
–
•
Is a transport
protein that
generates the
voltage across a
membrane
Occurs when active transport of a
specific solute indirectly drives the
active transport of another solute
Plants commonly use the gradient
of hydrogen ions generated by
proton pumps to drive active
transport of nutrients into the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 7.5: Bulk transport across the
plasma membrane occurs by exocytosis
and endocytosis
Q: 작은 분자나 물분자는 수송단백질의 도움으로 세포막
안팎으로 이동이 가능함을 알게 되었다. 그렇다면
앞서 배운 거대분자들은 어떻게 세포막 안으로 들어
올 수 있나?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Exocytosis
•
In exocytosis, transport vesicles migrate to the membrane, fuse with it,
and release their contents
•
Many secretory cells use exocytosis to export their products
Endocytosis
•
In endocytosis, the cell takes in macromolecules by forming vesicles from the
plasma membrane
•
Endocytosis is a reversal of exocytosis, involving different proteins
•
There are three types of endocytosis:
–
Phagocytosis (“cellular eating”)
–
Pinocytosis (“cellular drinking”)
–
Receptor-mediated endocytosis
•
In phagocytosis a cell engulfs a particle in a vacuole
•
The vacuole fuses with a lysosome to digest the particle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 7.22a
Phagocytosis
EXTRACELLULAR
FLUID
Solutes
Bacterium
Food vacuole
Pseudopodium
1 m
Pseudopodium
of amoeba
“Food”
or other
particle
An amoeba engulfing a bacterium
via phagocytosis (TEM).
식세포작용
Food
vacuole
CYTOPLASM
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Figure 7.22b
0.5 m
Pinocytosis
Plasma
membrane
Pinocytosis vesicles forming
in a cell lining a small blood
vessel (TEM).
음세포작용
Vesicle
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Figure 7.22c
수용체 매개 endocytosis
Receptor-Mediated Endocytosis
Plasma
membrane
Receptor
Coat
proteins
Ligand
0.25 m
Coat proteins
Top: A coated pit. Bottom: A
coated vesicle forming during
receptor-mediated endocytosis
(TEMs).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Coated
pit
Coated
vesicle
Q: If a marine algal cell is suddenly transferred
from seawater to freshwater, the algal cell will
initially
(a) lose water and decrease in volume.
(b) stay the same: neither absorb nor lose water.
(c) absorb water and increase in volume.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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