Chapter 07

1. Chapter 07 Membrane Structure and Function

2. 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

3. Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins • Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathicmolecules, containing hydrophobic and hydrophilic regions • The fluid mosaic model (유동성모자이크 모델)states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

4. 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

5. In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipidbilayer 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. Nicolsonproposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

6. Q: 유동성모자이크 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 Inside of extracellular layer Knife Cytoplasmic layer Plasma membrane Inside of cytoplasmic layer Fig. 7-4:

7. Figure 7.5 최신 유동성모자이크 모델 (UpdatedFluid mosaic model) Fibers of extra-cellular matrix (ECM) Glyco-protein Carbohydrate Glycolipid EXTRACELLULARSIDE OFMEMBRANE Cholesterol Microfilamentsof cytoskeleton Peripheralproteins Integralprotein CYTOPLASMIC SIDEOF MEMBRANE

8. 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)

9. Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell + Figure 7.7 Q: 막단백질의 운동은 어떻게 증명할 수 있나? • Proteins in the plasma membrane candrift (떠다님) within the bilayer Researchers (David Frye and Michael Edidin at Johns’ Hopkins Univ.) labeled the plasma membrane proteins of a mousecell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. EXPERIMENT RESULTS The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the planeof the plasma membrane. CONCLUSION

10. Q: 막 유동성에영향을 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

11. Figure 7.5 막단백질의 종류와 기능 (Membrane proteins and functions) Fibers of extra-cellular matrix (ECM) Glyco-protein Carbohydrate Glycolipid EXTRACELLULARSIDE OFMEMBRANE Cholesterol Microfilamentsof cytoskeleton Peripheralproteins Integralprotein CYTOPLASMIC SIDEOF MEMBRANE

12. N-terminus C-terminus CYTOPLASMIC SIDE a Helix Figure 7.9 • Integral proteins • Penetrate the hydrophobic core of the lipid bilayer • Are often transmembrane proteins, completely spanning the membrane (막관통 단백질) EXTRACELLULAR SIDE • Peripheral proteins • Are appendages loosely bound to the surface of the membrane

13. 막단백질의주요기능 Figure 7.10a Figure 7.10b

14. 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

15. 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

16. 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?

17. Transport Proteins (수송단백질) • Transport proteins allow passage of hydrophilic substances across the membrane • Some transport proteins, calledchannel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel • Channel proteins called aquaporinsfacilitate 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

18. 수동수송은 에너지 소모 없이 확산 의존성임 • Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment 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

19. 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

20. 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 tonicity에미치는 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)

21. 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 )

22. Hypotonic solution Hypertonic solution Isotonic solution (a) Animal cell. An animal cell fares best in an isotonic environ- ment unless it has special adaptations to offset the osmotic uptake or loss of water. H2O H2O H2O H2O Normal Shriveled Lysed Figure 7.15 동물세포처럼 세포벽이 없는 경우 • Water balance in cells without walls 터진풍선처럼 찌그러진 용혈 정상

23. 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?

24. (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environ- ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H2O H2O H2O H2O Turgid (normal) Flaccid Plasmolyzed Figure 7.15 그렇다면 식물과 같이 세포벽이 있는 세포에서의 물균형은? • Water balance in cells with walls 원형질 분리된 팽만 축 늘어진

25. 지금까지는 단순확산에 대해 공부하고 있다. 만약 세포가 막을 통한 물질 수송이 단순확산에만 의존한다면 어떻게 될까? 단순확산 이외의 다른 방법은 없을까?

26. 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)

27. 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)

28. Q: 채널단백질이나 운반체단백질의 기능에 문제가 생기면 개체에는 어떤 일이 벌어질까? (1) 낭포성섬유증 (cystic fibrosis): CFTR 단백질 결핍으로 염소이온의 수송에 문제발생으로 땀, 소화액 등 점액생산에 문제생김 (2) 신장결석 (cystinuria): 신장세포막에존재하는 시스테인 등의 수송을 담당하는 막단백질의 결핍으로 발생

29. 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

30. Na+ Na+ Na+ K+ P K+ EXTRACELLULAR FLUID The sodium-potassium pump (Na+-K+펌프) [Na+] high [K+] low (2) Na+ binding stimulates phosphorylation by ATP. Na+ Na+ (1) Cytoplasmic Na+ binds to the sodium-potassium pump. Na+ Na+ Na+ Na+ P [Na+] low [K+] high ADP ATP CYTOPLASM (3) Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. (6) K+ is released and Na+ sites are receptive again; the cycle repeats (3 Na+- out and 2 K+ in). (4) Extracellular K+ binds to the protein, triggering release of the Phosphate group. (5) Loss of the phosphate restores the protein’s original conformation. K+ K+ K+ K+ P Figure 7.18 P i

31. Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. 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. 수동수송과 능동수송의 비교? • Review: Passive and active transport compared  에너지 소모 없이 자발적으로 농도구배를 따라 물질 이동이 일어남  수동수송은 막단백질의 도움이 있으면 확산속도는 급격히 증가됨 Figure 7.19

32. How ion pumps maintain membrane potential • 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) • Membrane potential is the voltage difference across a membrane • Voltage is created by differences in the distribution of positive and negative ions • 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

33. An electrogenic pump (전기발생 펌프) • Is a transport protein that generates the voltage across a membrane • Cotransport (공동수송) • 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

34. 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?

35. 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

36. Figure 7.22a Phagocytosis EXTRACELLULARFLUID Solutes Pseudopodiumof amoeba Pseudopodium Bacterium 1 m Food vacuole “Food”or otherparticle An amoeba engulfing a bacteriumvia phagocytosis (TEM). 식세포작용 Foodvacuole CYTOPLASM

37. Figure 7.22b Pinocytosis Plasma membrane 0.5 m Pinocytosis vesicles formingin a cell lining a small bloodvessel (TEM). 음세포작용 Vesicle

38. Figure 7.22c 수용체 매개 endocytosis Receptor-Mediated Endocytosis Receptor Plasma membrane Coatproteins Ligand Coat proteins Coatedpit 0.25 m Coatedvesicle Top: A coated pit. Bottom: Acoated vesicle forming duringreceptor-mediated endocytosis(TEMs).

39. 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.