The Fluidity of Membranes

• Phospholipids in the plasma membrane can move within the bilayer

• Most of the lipids, and some proteins, drift laterally

• Rarely does a molecule flip-flop transversely across the membrane

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Movement of phospholipids

• As temperatures cool, membranes switch from a fluid state to a solid state

• The temperature at which a membrane solidifies depends on the types of lipids

• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids

• Membranes must be fluid to work properly; they are usually about as fluid as salad oil

The Fluidity of Membranes

• Some proteins in the plasma membrane can drift within the bilayer

• Proteins are much larger than lipids and move more slowly

• To investigate whether membrane proteins move, researchers fused a mouse cell and a human cell

The Fluidity of Membranes

Membrane Proteins and Their Functions

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

• Proteins determine most of the membrane’s specific functions

• Peripheral proteins are not embedded

• Integral proteins penetrate the hydrophobic core and often span the membrane

• Integral proteins that span the membrane are called transmembrane proteins

• The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

Membrane Proteins and Their Functions

EXTRACELLULARSIDEN-terminus

C-terminusCYTOPLASMICSIDE

Helix

• Six major functions of membrane proteins:– Transport– Enzymatic activity– Signal transduction– Cell-cell recognition– Intercellular joining– Attachment to the cytoskeleton and extracellular

matrix (ECM)

Membrane Proteins and Their Functions

EnzymesSignal

ReceptorATP

Transport Enzymatic activity Signal transduction

LE 7-9b

Glyco-protein

Cell-cell recognition Intercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell

Recognition• Cells recognize each other by binding to

surface molecules, often carbohydrates, on 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

Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces

• 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

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

Transport Proteins

• 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

Transport Proteins

Passive transport is diffusion of a substance across a membrane with

no energy investment

• Diffusion is the tendency for molecules to spread out evenly into the available space

• Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction

• At dynamic equilibrium, as many molecules cross one way as cross in the other direction

• Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another

• 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 it requires no energy from the cell to make it happen

Passive transport is diffusion of a substance across a membrane

with no energy investment

Effects of Osmosis on Water Balance

• Osmosis is the diffusion of water across a selectively permeable membrane

• The direction of osmosis is determined only by a difference in total solute concentration

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

Lowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can

H2O

Osmosis

Water Balance of Cells Without Walls

• Tonicity is the ability of a solution to cause a cell to gain or lose water

• Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

• Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water

• Hypotonic solution: solute concentration is less than that inside the cell; cell gains water

• Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment

• To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance

• The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Water Balance of Cells Without Walls

Water Balance of Cells with Walls

• Cell walls help maintain water balance• A plant cell in a hypotonic solution swells until the

wall opposes uptake; the cell is now turgid (firm)• If a plant cell and its surroundings are isotonic,

there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

• In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Facilitated Diffusion: Passive Transport Aided by Proteins

• In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane

• Channel proteins provide corridors 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

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

The Need for Energy in Active Transport

• 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

• The sodium-potassium pump is one type of active transport system

Maintenance of Membrane Potential by Ion Pumps

• Membrane potential is the voltage difference across a membrane

• 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 the voltage across a membrane

• The main electrogenic pump of plants, fungi, and bacteria is a proton pump

Maintenance of Membrane Potential by Ion Pumps

H+

ATP

CYTOPLASM

EXTRACELLULARFLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

Cotransport: Coupled Transport by a Membrane Protein

• Cotransport occurs when active transport of a solute indirectly drives 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

H+

ATP

Proton pump

Sucrose-H+

cotransporter

Diffusionof H+

Sucrose

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+

–

–

–

–

–

–

Bulk transport across the plasma membrane occurs by exocytosis

and endocytosis

• Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

• Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

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

The Energy of Life

• The living cell is a miniature chemical factory where thousands of reactions occur

• The cell extracts energy and applies energy to perform work

• Some organisms even convert energy to light, as in bioluminescence

An organism’s metabolism transforms matter and energy,

subject to the laws of thermodynamics

• Metabolism is the totality of an organism’s chemical reactions

• Metabolism is an emergent property of life that arises from interactions between molecules within the cell

Organization of the Chemistry of Life into Metabolic Pathways

• A metabolic pathway begins with a specific molecule and ends with a product

• Each step is catalyzed by a specific enzyme

Enzyme 1

A B

Reaction 1

Enzyme 2

C

Reaction 2

Enzyme 3

D

Reaction 3

ProductStarting

molecule

• Catabolic pathways release energy by breaking down complex molecules into simpler compounds

• Anabolic pathways consume energy to build complex molecules from simpler ones

• Bioenergetics is the study of how organisms manage their energy resources

Metabolic Pathways

Forms of Energy

• Energy is the capacity to cause change

• Energy exists in various forms, some of which can perform work

• Kinetic energy is energy associated with motion– Heat (thermal energy) is kinetic energy associated

with random movement of atoms or molecules

• Potential energy is energy that matter possesses because of its location or structure– Chemical energy is potential energy available for

release in a chemical reaction

• Energy can be converted from one form to another

Forms of Energy

On the platform,the diver hasmore potentialenergy.

Diving convertspotentialenergy to kinetic energy.

Climbing up convertskinetic energy ofmuscle movement topotential energy.

In the water, the diver has lesspotential energy.

The Laws of Energy Transformation

• Thermodynamics is the study of energy transformations

• A closed system, such as that approximated by liquid in a thermos, is isolated from its surroundings

• In an open system, energy and matter can be transferred between the system and its surroundings

• Organisms are open systems

The First Law of Thermodynamics

• According to the first law of thermodynamics, the energy of the universe is constant

– Energy can be transferred and transformed– Energy cannot be created or destroyed

• The first law is also called the principle of conservation of energy

The Second Law of Thermodynamics

• During every energy transfer or transformation, some energy is unusable, often lost as heat

• According to the second law of thermodynamics, every energy transfer or transformation increases the entropy (disorder) of the universe

Chemical energy

Heat CO2

First law of thermodynamics Second law of thermodynamics

H2O

• Living cells unavoidably convert organized forms of energy to heat

• Spontaneous processes occur without energy input; they can happen quickly or slowly

• For a process to occur without energy input, it must increase the entropy of the universe

Biological Order and Disorder

• Cells create ordered structures from less ordered materials

• Organisms also replace ordered forms of matter and energy with less ordered forms

• The evolution of more complex organisms does not violate the second law of thermodynamics

• Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases

The free-energy change of a reaction tells us whether the

reaction occurs spontaneously

• Biologists want to know which reactions occur spontaneously and which require input of energy

• To do so, they need to determine energy changes that occur in chemical reactions

Free-Energy Change, G

• A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell

• The change in free energy (∆G) during a process is related to the change in enthalpy, or change in total energy (∆H), and change in entropy (T∆S):

∆G = ∆H - T∆S• Only processes with a negative ∆G are

spontaneous• Spontaneous processes can be harnessed to

perform work

Free Energy, Stability, and Equilibrium

• Free energy is a measure of a system’s instability, its tendency to change to a more stable state

• During a spontaneous change, free energy decreases and the stability of a system increases

• Equilibrium is a state of maximum stability• A process is spontaneous and can perform work

only when it is moving toward equilibrium

Gravitational motion Diffusion Chemical reaction

Free Energy and Metabolism

• The concept of free energy can be applied to the chemistry of life’s processes

Exergonic and Endergonic Reactions in Metabolism

• An exergonic reaction proceeds with a net release of free energy and is spontaneous

• An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous

Reactants

EnergyProducts

Progress of the reaction

Amount ofenergy

released(G < 0)

Fre

e en

erg

y

Exergonic reaction: energy released

ReactantsEnergy

Products

Progress of the reaction

Amount ofenergy

required(G > 0)

Fre

e en

erg

y

Endergonic reaction: energy required

Equilibrium and Metabolism

• Reactions in a closed system eventually reach equilibrium and then do no work

• Cells are not in equilibrium; they are open systems experiencing a constant flow of materials

• A catabolic pathway in a cell releases free energy in a series of reactions

• Closed and open hydroelectric systems can serve as analogies

G = 0

A closed hydroelectric system

G < 0

An open hydroelectric system

G < 0

A multistep open hydroelectric system

G < 0G < 0

G < 0