The working cell • Energy in biology

• The nature of energy • The molecule ATP and cellular work

• Enzymes – biological catalysis • The nature of catalysis • How enzymes work • The regulation of enzymes

• Functions of biomembranes • Passive transport • Osmosis and water balance • Active transport • Exocytosis and endocytosis • Signaling

Several movies today

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Membrane Function • Working cells must control the flow of materials to and from the environment.

– Membrane proteins are essential for this task. • Membrane proteins perform a variety of functions.

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Passive Transport: Diffusion Across Membranes • Molecules contain heat energy • They vibrate and wander randomly (“diffusion”) • This eliminates concentration gradients

Passive transport is one example of diffusion across membranes

• Dynamic equilibrium: no net movement

• No energy is needed

• But selective permeability allows regulation of cell composition

• Examples: CO2 and O2 transport, H2O transport

Facilitated diffusion

Simple diffusion

• Most substances that need to be transported into cells are hydrophilic (polar), but the inner parts of membranes are hydrophobic (non-polar) membranes are poorly permeable, simple diffusion across membranes is inefficient

• Facilitated diffusion is the passive transport of hydrophilic substances by specific transport proteins that act as “open doors” in a selective manner

Movie facilitated diffusion – 27 sec http://www.youtube.com/watch?v=s0p1ztrbXPY&feature=related

Movie Channels and carriers – 29 sec http://www.youtube.com/watch?v=pQ1pmBUViIs&feature=related

•Facilitated diffusion can occur through channels or carriers

•Typically ions •Are not saturable

•Typically small organic molecules like glucose • Are saturable

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Osmosis and Water Balance in Cells • Osmosis: the passive transport (diffusion) of water across a selectively permeable

membrane.

Osmotic pressure

Movie osmosis – 48 sec http://www.youtube.com/watch?v=sdiJtDRJQEc&feature=related

Hypertonic solution •Has a higher concentration of solute (= low concentration of water)

Hypotonic solution (저장액) •Has a lower concentration of solute (= high concentration of water)

Isotonic solution (등장액) • equal concentrations of solute

3:30 min http://www.youtube.com/watch?v=6MWl3DCa2uM

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Survival of cells in different osmotic situations

Red blood cell volume remains normal

Cell gains water, swells, and may burst

(lyse) if there is no wall

Cell shrinks and may die from water loss

Osmoregulation in animals •Terrestrial vertebrates use kidneys to regulate osmotic pressure

Osmoregulation in plants • Turgor (= back pressure from cell wall) is necessary for plant to retain upright posture

In a hypotonic solution, a plant cell is turgid (firm)

In an isotonic solution, a plant cell is flaccid (floppy), and a plant wilts

A paradox: It was long assumed that water simply diffuses through the membrane, even though it was known that the membrane is hydrophobic We know now that water cannot easily diffuse through the membrane

The passive transport of water

Specialized water channel proteins (aquaporins) in the plasma membrane allow the water to pass the membrane

Aquaporin Video – 39 sec http://www.youtube.com/watch?v=1Uw6u0fzNsE

Note the turn of the water molecule in yellow

Peter Agre, Noble Prize 2003, for the discovery of aquaporins

The level of one of the aquaporins was reduced by a genetic procedure Now the plant formed a larger root system to be able to take up sufficient water

Normal plant Plant with reduced aquaporin level

Aquaporins are being used to develop a new water purification technology

Traditional filtration technology •Need high pressure • High energy costs

Aquaporin-based technology under development

• Pure water with low pressure • Application: Desalination of sea water

http://www.techamender.com/aquaporin-natures-clean-water-filter/

Active Transport: The Pumping of Individual Molecules Across Membranes • Molecules move across the membrane against a concentration gradient. • This requires energy

Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Movie active transport – 21 sec http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related

• Our cells are batteries The enzyme Na+ K+ ATPase uses ATP to maintain a gradient of Na+ and K+ ions and an electrical membrane potential

This enzyme consumes ~ 25% of all ATP in the resting body

• Note that [K+] plus [Na+] inside the cell (152 mM) is larger than [K+] plus [Na+] outside the cell (149 mM) paradox?

• Solution: The Na+ K+ ATPase creates the primary electrical potential. The actual resting potential of the cell arises also from secondary movements of other ions

• Primary (direct) active transport (for example Na/K ATPase) - solute accumulation is coupled directly to ATP hydrolysis (ATP ADP + Pi) • Secondary (indirect) active transport - uphill transport of solute is coupled to downhill flow of a different solute

Overview of types of membrane transport of individual molecules = solutes

Exocytosis and Endocytosis: The mode of bulk transport through membranes (especially proteins and particles)

• Exocytosis – Secretes substances outside of the cell.

Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

• Endocytosis – Takes material into the cell.

Movie exocytosis and endocytosis – 36 sec http://www.youtube.com/watch?v=4gLtk8Yc1Zc&feature=related

Types of endocytosis

• In phagocytosis (“cellular eating”), a cell engulfs a particle and packages it within a food vacuole.

Movie (30 sec): White blood cell chases a bacterium and eats it: http://www.youtube.com/watch?v=fpOxgAU5fFQ&feature=related

Movie (1 min 3 sec): Amoeba eats bacterium: http://www.youtube.com/watch?v=W6rnhiMxtKU&feature=related

• In pinocytosis (“cellular drinking”), a cell “gulps” droplets of fluid by tiny vesicles.

images.yahoo.com

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

•Receptor-mediated endocytosis –Is triggered by the binding of external molecules to membrane proteins. –LDL (low density lipoprotein: cholesterol) receptor

The logic of transport through membranes

• Individual molecules or ions are usually sufficiently abundant to justify the “construction” (by evolution) of specialized transporter molecules (passive or active). Further, it is hard to imagine a selective bulk transport mechanism for small ions.

•Proteins are too diverse to justify the construction of a new transporter for each type of protein are transported in bulk, in “buckets” (membrane vesicles)

•Particles (especially bacteria) also require a membrane vesicle to be engulfed

•A specific situation arises for lipids. Lipids are insoluble transport in microdroplets (lipid particles). A receptor mechanism organizes the distribution of the lipid droplets to the different target cell types.

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Role of Membranes in Cell Signaling •Cellular communication –Begins with the reception of an extracellular signal. •The signal transduction pathway –Consists of proteins and other molecules that relay the signal.

http://www.youtube.com/watch?v=GigxU1UXZXo

Nice animation (8 minutes) showing various aspects of membrane function with the example of a white blood cell (leucocyte) invading a site of inflammation (Note: A cytokine is a hormone-like signaling substance that attracts the leukocyte)

Summary – Functions of biomembranes

•Biomembranes define, separate, and connect compartments (the cell itself and its organelles). •The lipid bilayer, which is made of molecules with polar heads and hydrophobic tails, provides the ground structure of all biomembranes.

•The lipid bilayer is impermeable for hydrophilic substances (= water and water-soluble substances). •Proteins that are embedded in the lipid bilayer specify most membrane functions such as transport, cell-cell attachment, cell-matrix attachment, cell-cell recognition, signal perception, catalysis.

•Passive transport (based on diffusion, no energy expenditure). •Channels (continuous, non-saturable passage of solutes, mainly ions and water itself) and carriers (valve-like, saturable passage, mainly of small organic molecules).

•Special case of water: Aquaporins (water channels) and osmosis (water diffusing through otherwise impermeable membrane separating two compartments with different concentrations of solutes).

•Active transport (against chemical or electrical gradients, energy is needed). •Primary active transport (typically ion pumps) uses energy (e.g., ATP ) directly. Na+/K+ ATPase and others. Pumps maintain a membrane potential (cells are batteries). •Secondary active transport (diffusion of an ion along gradient drives other substance to move against gradient) .

•Exocytosis and Endocytosis. •Bulk transport of fluid, proteins, or particles in “buckets” (= membrane vesicles). •Special case: receptor-mediated endocytosis (e.g., directing uptake of lipids in form of lipoproteins). •Phagocytosis refers to endocytosis of particles such as bacteria; may or may not involve receptors.

•The logic of transport through membranes. •Transport proteins handle molecules individually. Transporters are constructed only for highly abundant or unusually important molecules. Proteins in general are too diverse to merit individual transporters.

요약- 생체막의 역할

• 생체막은 부위를 정의하고, 나누고, 그리고 연결한다 (세포와 세포 소기관들).

• 소수성 꼬리와 극성의 머리로 구성된 분자로 이루어진, 지질 이중막은 모든 생체막의 기초 구조이다. • 지질 이중막은 친수성 물질에 비투과적이다 (물과 물에 녹는 물질들). • 지질 이중막에 박혀있는 단백질은, 수송, 세포와 세포간의 부착, 세포와 세포간충물질(matrix) 간의 부착, 세포와 세포간의 인식, 신호인식, 촉매 등, 대부분의 막 기능을 한다.

• 수동수송 (확산을 기반으로 함, 에너지 소모 없음).

• 통로 (Channel, 지속적, 용질의 비포화적 투과, 주로 물과 이온)와 운반체 (Carrier, 밸브와 같음, 포화적 투과, 주로 작은 유기분자들)가 있다. • 물의 특이한 예: Aquaporin (물 통로) 와 삼투현상 (농도가 낮은 곳에서 높은 곳으로 선택적 투과성 막을 통한 물의 확산)이 있다.

• 능동수송 (화학적 또는 전기적 농도와 반대로 이동, 에너지가 필요).

• 일차 능동수송 (일반적으로 이온펌프)은 직접 에너지를 사용한다 (예, ATP). Na+/K+ 펌프 등 (Na+/K+ ATPase). 펌프는 막전위를 유지한다 (세포는 건전지와 같다). • 이차 능동수송 (농도기울기에 의한 이온의 확산을 이용하여, 농도와 반대로 다른 물질을 이동시킴).

• 세포외배출 (exocytosis)와 세포내유입 (endocytosis).

• "용기 (=소낭, membrane vesicle)“ 내 유체, 단백질, 미립자의 대규모 수송. • 특이한 예: 수용체 매개 세포내유입 작용 (예, 지질단백질에서 지질의 직접적인 흡수). • 식균작용 (phagocytosis)는 세균과 같은 미립자의 세포내유입 작용을 뜻한다; 수용체와 관련이 있기도 없기도 한다.

• 막을 통한 수송이론.

• 수송 단백질은 분자를 각각 다룬다. 수송 단백질은, 오직 양이 많거나 중요하고 특이한 분자들을 위해서만 만들어진다. 일반적으로 단백질은 그 종류가 다양하여, 각각의 수송 단백질을 가지기 어렵다.