Chapter 7

Membrane Structure and Function

AP Biology

Overview: Life at the Edge

• The plasma membrane separates the living cell from its

surroundings

– It controls traffic into and out of the cell

– It also exhibits SELECTIVE PERMEABILITY

(allows some substance to cross it more easily than

others)

• In this chapter, we will learn how cell membranes control

the passage of substances

Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins

• Lipids and proteins are the main ingredients of membranes, though carbohydrates

are also important

– Of all the lipids that make up membranes, phospholipids are most abundant

• Phospholipids are AMPHIPATHIC molecules – have BOTH hydrophilic

and hydrophobic region

– How are phospholipids and proteins arranged in membranes

of cells?

• The currently accepted model is called the

FLUID MOSAIC MODEL

– This model states that the membrane is a FLUID

structure with a “MOSAIC” (many different)

of proteins stuck in or

attached to the bilayer

of phospholipids

The Sandwich Model

• Since 1925, we have reasoned that cell membranes must be phospholipid bilayers made of

protein and lipids (via chemical analysis)

– This is the only molecular arrangement that shields the hydrophobic tails of

phospholipids from water, while exposing the hydrophilic heads to water

– At this time, however, scientists were unsure of where proteins were located

• Scientists had observed that a pure phospholipid bilayer is less attracted to water

than the surface of a cell membrane

• Taking this observation in to account, the first model of the cell membrane was

proposed in 1935 by Hugh Davson and James Danielli

– This model, known as the SANDWICH MODEL, proposed that the

membrane was coated on both sides with hydrophilic proteins

Problems With the Sandwich Model

• Later studies found problems with the sandwich model, particularly:

– 1) All membranes of cells are not identical

• The plasma membrane is 7-8 nm thick but the mitochondria’s inner membrane is

only 6 nm thick

• Membranes look different under a microscope

– Plasma membrane appears as a 3-layered structure in electron micrographs

– Mitochondrial membranes look like a row of beads

• Membranes do not have identical composition

– Mitochondrial membranes have a higher percentage of proteins and different

kinds of phospholipids and other lipids

– 2) Membrane proteins are not very soluble in water because they are amphipathic (have

hydrophobic regions, along with hydrophilic regions)

• So if these proteins were layered on surface of membrane, even their hydrophobic

parts would be exposed to water

The Fluid Mosaic Model

• In 1972, S.J. Singer and G. Nicolson proposed that membrane proteins are dispersed and

individually inserted into the phospholipid bilayer with their hydrophilic regions protruding

– His molecular arrangement maximizes contact of hydrophilic regions of proteins and

phospholipids with water in the cytosol and extracellular fluid

• It also provides the hydrophobic portions of these proteins and phospholipids with

a nonaqueous environment

– Thus, in this fluid mosaic model, as it was eventually named , the membrane is a

mosaic of protein molecules

bobbing in a fluid bilayer of

phospholipids

Proof for the Fluid Mosaic Model: Freeze-Fracture

• A method of preparing cells for electron microscopy called freeze-fracture has

provided proof for the fluid mosaic model

– This technique involves freezing a cell and then fracturing it with a knife

• The fracture plane often follows the hydrophobic interior of a membrane,

splitting the phospholipid bilayer into 2 separated layers

– The membrane proteins remain whole, staying with one of the 2 layers

(like pulling apart a chunky peanut butter sandwich)

– Under the electron microscope, these membrane layers appear cobblestoned,

with protein particles interspersed in a smooth matrix

• Some proteins travel with one layer or the other (like peanut chunks in the

sandwich)

The Fluidity of Membranes

• Membranes are FLUID structures, since they are held together primarily by weak

hydrophobic interactions

– Most of the lipids and some proteins in a membrane can drift laterally (side-to-side) in

the plane of the membrane

• This is similar to people elbowing their way through a crowded room

– Lipids and proteins don’t usually flip flop transversely across the membrane (from one

layer to the other), however

• To do this, the hydrophilic portion of the molecule would have to cross the

hydrophobic core of the

membrane

Lateral Movement of Phospholipids and Proteins

• The lateral movement of phospholipids within the membrane is rapid

– Adjacent phospholipids switch positions ~107 times per second

• This means that a phospholipid can move ~2 µm in 1 second (the length of a

bacterial cell)

• Proteins are much larger than lipids so they move more slowly than phospholipids

– Some membrane proteins seem to move in a highly directed manner

• Scientists hypothesize that these membrane proteins may be driven along

cytoskeletal fibers by motor proteins connected to the membrane proteins’

cytoplasmic regions

– Other membrane proteins

seem to be held firmly in

place by their attachment

to the cytoskeleton

Experiment: Do Membrane Proteins Move?

• A classic experiment by David Frye and Michael Edidin proved that proteins do , in fact, drift

laterally along membranes

– Experiment: Plasma membrane proteins of a mouse cell and a human cell were labeled

with 2 different markers (red vs. blue) and then fused together

• Markers on the hybrid cells were then observed with a microscope

– Conclusion: There was some mixing of mouse and human membrane proteins

• This indicates that at least some membrane proteins move sideways within the

plane of the plasma membrane

Membrane Fluidity and Temperature

• Membranes switch from a fluid to solid state as temperature decreases (like bacon grease

forms lard when cooled)

– The composition of a membrane (what kinds of lipids it contains) determine at what

temperature it solidifies

• If phospholipids have unsaturated hydrocarbon tails (kinks due to double bonds),

they remain fluid to a lower temperature

• Recall: unsaturated fats are liquid at room temperature

– These kinks prevent close packing of phospholipid molecules, making the

membrane more

fluid

Cholesterol and Membrane Fluidity

• Other components of the membrane can also affect its fluidity:

– The steroid cholesterol (another type of lipid) is wedged between the phospholipid

molecules in animal cell plasma membranes

• Cholesterol has different effects of membrane fluidity at different temperatures:

– At higher temperatures (37o C – human body temperature), cholesterol makes

the membrane less fluid by restraining phospholipid movement

– At cooler temperatures, it maintains fluidity by preventing tight packing of

phospholipids

• This lowers the temperature required for the membrane to solidify

• We can thus think of cholesterol as a “temperature buffer”

– It resists changes in

membrane fluidity at

different temperatures,

ensuring the membrane

does not become too

fluid or too solid

Membrane Fluidity and Function

• Membranes must be fluid to work properly (about as fluid as salad oil)

– When a membrane solidifies, its permeability to different molecules can

change

– It can also inactivate certain membrane enzymes

• Ex) Some enzymes need to be able to move laterally within the membrane

to function

– To remain fluid, the lipid composition of cell membranes can change as an

adjustment to changing temperature

• Ex) In many cold-tolerant plants, the percentage of unsaturated

phospholipids increases in autumn to keep the membranes from

solidifying during winter

Membrane Proteins and Their Functions

• Now we will discuss the MOSAIC part of the fluid mosaic model

– Recall: membranes have many different proteins embedded in their fluid matrix

• Ex) More than 50 different kinds of proteins have been found in RBC plasma

membranes

– Though phospholipids make up the

main fabric of the membrane,

proteins determine most of the

membrane’s functions

• Thus, different cells have

different membrane proteins

• Different membranes in the same cell also have different proteins

Integral Proteins

• There are 2 major types of membrane proteins:

– 1) Integral proteins – penetrate the hydrophobic core

of the membrane

• Some integral proteins go all the way through the

membrane and are thus called

TRANSMEMBRANE PROTEINS

– Others only extend part of the way through

the membrane

• The hydrophobic regions of

integral proteins are made up

of nonpolar amino acids

stretches that often coil into

alpha helices

– Hydrophilic parts of

integral proteins are left

exposed on either surface

of the membrane

Peripheral Proteins

• There are 2 major types of membrane proteins:

– 2) Peripheral proteins – not embedded in the membrane but are loosely bound to the

surface of the membrane

• These proteins often bind to the exposed parts of integral proteins

• Many membrane proteins are also held in place by the cytoskeleton (on the

cytoplasmic side) or attached

to fibers of the extracellular

matrix (ECM) on the other side

Functions of Membrane Proteins

• There are six major functions of membrane proteins:

– Transport

– Enzymatic activity

– Signal transduction

– Cell-cell recognition

– Intercellular joining

– Attachment to the cytoskeleton and extracellular matrix

(ECM)

Functions of Membrane Proteins: Transport

• Some transmembrane proteins have hydrophilic channels in them

– These channels are usually selective for a particular solute (see

left)

– Other transport proteins move substances

from one side to the other by changing

shape (see right)

• Some must use ATP to actively

pump substances across the membrane

Functions of Membrane Proteins: Enzymatic Activity

• Some enzymes are built directly into the membrane

• The active sites of these enzymes are exposed to aqueous

solution

– Teams of enzymes can also work together to carry out a sequence

of steps in a metabolic pathway

(see illustration)

Functions of Membrane Proteins: Signal Transduction

• Some membrane proteins act as receptors

– These proteins have binding site with a specific shape that fits the shape of a

chemical messenger, called a signaling molecule

• Many signaling molecules are hormones

– Ex) Insulin, external signaling molecule,

causes the cell to take up glucose from

blood

Functions of Membrane Proteins: Cell-Cell Recognition

• Some glycoproteins act like ID tags

– These glycoproteins allow membrane proteins on other cells to

recognize each other

– Immune system cells can also pick

out foreign cells in this way

Functions of Membrane Proteins: Intercellular Joining

• Membrane proteins of cells next to one another can join these cells

using junctions

– Ex) Gap junctions allow

communication between cells

of a tissue (heart muscle)

Functions of Membrane Proteins: Attachment to Cytoskeleton & ECM

• Microfilaments can attach to membrane proteins, which helps to:

– Maintain cell shape

– Keep membrane proteins from moving

• Proteins can also bind to ECM molecules and coordinate

changes inside the cell

– This also allows communication between cells

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells need to be able to recognize each other

• Ex) Cell recognition allows cells to be sorted into tissues and organs during

embryonic development

– Cells recognize each other by binding to surface molecules like carbohydrates on each

others’ plasma membranes

• Some membrane carbohydrates have lipids bound to them

– These molecules are called glycolipids

• Most membrane carbohydrates are bound to proteins

– These molecules are called glycoproteins

– Cells recognize each other because these carbohydrates are all different

• They vary from species to species, members of same species, and from one cell

type to another

– Ex) A, B, AB, and O blood types reflect differences in carbohydrates found

on the surface of RBCs

Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces

– Their 2 lipid layers may differ in specific lipid composition

– Each protein has directional orientation in the membrane

• When a vesicle fuses with the plasma membrane, the outside of the vesicle

becomes continuous with the cytoplasmic (inner) layer of the plasma

membrane

– Thus, molecules that start out on the inside face of the ER end up on

the outside face of the plasma membrane

Synthesis and Sidedness of Membranes

• 1) The synthesis of membranes begins as membrane proteins and lipids are made in ER

– Carbohydrates (green) are added to proteins (purple), making glycoproteins

– The carbohydrate portions may then be modified

• 2) Glycoproteins are transferred to the GA for further

modification of carbohydrates

– Lipids from the ER acquire carbohydrates,

making glycolipids

• 3) Transmembrane proteins (purple dumbells),

glycoproteins, and secretory proteins (purple circles)

are transported from the GA to the plasma membrane

in vesicles

• 4) Vesicles fuse with the plasma membrane and

release their secretory proteins from the cell

– Fusion of vesicles positions carbohydrates of

glycoproteins and glycolipids on the outside

of the plasma membrane

Concept Check 7.1

• 1) The carbohydrates attached to some proteins and lipids

of the plasma membrane are added as the membrane is

made and refined in the ER and Golgi apparatus; the new

membrane then forms transport vesicles that travel to the

cell surface. On which side of the vesicle membrane are

the carbohydrates?

• 2) How would you expect the saturation levels of

membrane phospholipid fatty acids to differ in plants

adapted to cold environments and plants adapted to hot

environments?

Concept 7.2: Membrane structure results in

selective permeability

• It is essential that cells be able to regulate transport across

their boundaries

• This process is controlled by the plasma membrane

– Although a steady traffic of small ions and molecules

move across the plasma membrane in both directions,

these substances do not cross the barrier

indiscriminately

• The plasma membrane is selectively permeable

The Permeability of the Lipid Bilayer

• Because nonpolar molecules are hydrophobic, they can

easily cross through the lipid bilayer without the aid of

membrane proteins

– Hydrophilic ions and polar molecules (glucose, water) ,

however, can’t pass through as easily

• This is due to the hydrophobic core of the plasma

membrane

Transport Proteins: Channel Proteins

• Hydrophilic ions and polar molecules cross the lipid bilayer by passing through

transport proteins that span the membrane (like a bridge)

– Transport proteins called channel proteins have a hydrophilic channel that

polar or charged molecules can pass through

• Ex) Channel proteins called aquaporins help water molecules pass

through the plasma membrane

– 3 billion water molecules can pass through each aquaporin per second

– Water molecules can only pass through in a single file

– The channel fits 10 water molecules at a time

Transport Proteins: Carrier Proteins

• Another type of transport protein is called a carrier protein

– These transport proteins hold on to their passengers and change shape

• This change in shape allows molecule to be shuttled across the plasma

membrane

– Ex) Glucose carried by blood enters RBCs through carrier proteins,

allowing it to pass through 50,000 times faster than by simple

diffusion

– Carrier proteins are very specific for the substances they move

• Ex) Carrier proteins for glucose are so selective that they even reject

fructose (a structural isomer of glucose)

Concept Check 7.2

• 1) Two molecules that can cross a lipid bilayer without help from

membrane proteins are O2 and CO2. What properties allow this to

occur?

• 2) Why would water molecules need a transport protein to move

rapidly and in large quantities across a membrane?

• 3) Aquaporins exclude passage of hydronium ions (H3O+). But recent

research has revealed a role for some aquaporins in fat metabolism,

in which they allow the passage of glycerol, a 3-carbon alcohol (see

Figure 5.11), as well as H2O. Since H3O+ is much closer in size to

water than is glycerol, what do you suppose is the basis of this

selectivity?

Concept 7.3: Passive transport is diffusion of a

substance across a membrane with no energy investment

• Diffusion: the movement of molecules of any substance so that they spread out evenly into

an available space

– Though each molecule moves randomly, the movement of a population of molecules

may be directional (net movement)

– Once there are equal amounts of molecules on either side of a membrane, a DYNAMIC

equilibrium is reached

• Molecules still continue to

move but at equal rates in both

directions (no net movement)

Animation: Membrane Selectivity

Animation: Diffusion

Diffusion

Passive Transport

• Simple rule of diffusion:

– A substance will diffuse from where it is more concentrated to where it is less

concentrated

• Thus, the substance moves down its CONCENTRATION GRADIENT

• Diffusion is a spontaneous process

– No work must be done (no energy

needed)

• Ex) Dissolved oxygen (in

aqueous solution inside the body)

diffuses into cells across their

plasma membranes as cellular

respiration consumes it

– Diffusion of a substance across a

biological membrane is called

PASSIVE TRANSPORT

• Passive transport does not

require energy

Effects of Osmosis on Water Balance

• The diffusion of water across a selectively permeable membrane is called

OSMOSIS

– Water diffuses across a membrane from an area of lower SOLUTE

concentration to an area of higher

solute concentration

• At equilibrium, the solute

concentrations on both sides of

the membrane will be the same

Tonicity

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

– It depends (in part) on the concentration of solutes that cannot

cross the membrane (nonpenetrating solutes) relative to that

inside the cell

• Higher concentrations of nonpenetrating solutes in the

surrounding solution will cause water to leave the cell

• Lower concentrations of nonpentrating solutes in the

surrounding solution will cause water to enter the cell

Isotonic Solution

• For cells in an isotonic solution (iso = same):

– The solute concentration

in the surrounding solution

is the same as that inside

the cell

• Thus, there is no net

movement of water

across the plasma

membrane

– Water flows across the membrane at the same rate in both directions, creating

equal concentrations of solutes on both sides of the membrane

Hypertonic Solution

• For cells in hypertonic solution (more nonpentrating solutes):

– The solute concentration in the surrounding solution is greater than that inside cell

• This causes the cell to lose water and shrivel

Hypotonic Solution

• For cells in hypotonic solution (less nonpenetrating solutes):

– The solute concentration in the surrounding solution is lower than that inside the cell

• Water will therefore enter the cell faster than it leaves

– This will cause

the cell to

swell/burst

• Animal cells fare best in an

isotonic environment

– Plant cells do best in a

hypertonic environment

Water Uptake and Cells Without Walls • Cells without walls can’t tolerate excessive uptake or loss of water

– The resulting problem of water balance is automatically solved if the cell lives in

isotonic surroundings

• Ex) Sea water is isotonic to many marine invertebrates

• Ex) Our extracellular fluid is isotonic to the fluid inside cells

– Organisms without cell walls living in

hypertonic or hypotonic environments need to

have special adaptations for

OSMOREGULATION (control of water balance)

• Ex) The protist Paramecium is

hypertonic to its pond water environment

– It has a contractile vacuole that

functions as a pump to force water

out of cell as fast as it enters by

osmosis

Video: Chlamydomonas

Video: Paramecium Vacuole

Water Balance of Cells with Walls

• The cells of plants, prokaryotes, fungi, and some protists have walls

– In hypotonic solutions, this wall helps maintain water balance

• The cell wall is relatively inelastic and will expand only so much before it exerts

back enough pressure to oppose further water uptake

• At this point, the cell is TURGID (very firm)

– This is the healthy state

for most plant cells

– Plants (especially those

that are not woody)

depend on the mechanical

support of these turgid

cells

– If plant cells are in isotonic

solution, the cell become flaccid

(limp)

• This condition may cause the plant to wilt

Video: Plasmolysis

Video: Turgid Elodea

Animation: Osmosis

• In a hypertonic environment, the plant cell will also lose water like an animal cell

– As the plant cell shrivels, the plasma membrane pulls away from cell wall

• This process is known as plasmolysis

– Plasmolysis causes the plant to wilt , which can lead to plant death

Facilitated Diffusion: Passive Transport Aided by Proteins

• Polar molecules and ions cannot diffuse through membrane without the help of

transmembrane proteins

• Diffusion that requires the help of these transport proteins is called

FACILITATED DIFFUSION

– Proteins involved in facilitated diffusion are also referred to as channel proteins

because they provide corridors that allow specific molecules or ions to pass (like a

hallway)

• Their passages are hydrophilic, which allows polar or charged particles to cross

the hydrophobic core

of the membrane

Types of Channel Proteins

• There are 2 types of channel proteins:

– 1) Aquaporins: these water channel proteins help water diffuse at a much quicker rate

than it would otherwise

• Though water is small enough to diffuse without channel proteins, it is impeded

by its polarity

– Kidney cells, in particular, have large numbers of aquaporins, allowing them

to reclaim water from urine before excretion

• If water was completely excreted in the urine, a person would otherwise

have to drink 50 gallons of water per day

– 2) Ion (gated) channels: channel proteins that open or close in response to an electrical

or chemical stimulus

• If chemical, the stimulus is a substance other than the one to be transported

– Ex) In the sodium potassium pump, neurotransmitter molecules stimulate

gated channel to open in nerve cells, allowing sodium ions to move into cell

• Later, an electrical stimulus activates the ion channel protein so

potassium ions can rush out

Carrier Proteins

• Another type of protein involved in facilitated diffusion is the carrier protein

– Carrier proteins undergo a subtle change in shape that allows a specific solute

to move across the membrane during the shape change

• These changes in shape are usually triggered by the binding and release

of the transported molecule

Cystinuria

• In some inherited diseases, these specific transport systems are either

defective or missing

– Ex) In the disease cystinuria, an individual lacks a carrier protein to

transport cysteine and other amino acids across the membranes of their

kidney cells

• Though these amino acids are still absorbed from urine, the return

of these amino acids into the blood is impeded

• This causes the development of painful stones from the

accumulation and crystallization of amino acids in the kidneys

Concept Check 7.3

• 1) How do you think a cell performing cellular

respiration rids itself of the resulting carbon dioxide?

• 2) In the supermarket, produce is often sprayed with

water. Explain why this makes the vegetables look

crisp.

• 3) If a Paramecium swims from a hypotonic

environment to an isotonic one, will its contractile

vacuole become more active or less? Why?

Concept 7.4: Active transport uses energy to move

solutes against their gradients

Active Transport

• Even through facilitated diffusion requires the help of specific transport proteins, it

is still considered diffusion

• This is because the solute moves down its concentration gradient (It does

NOT alter the direction of transport)

– Some transport proteins, however, can move solutes against their

concentration gradient, from less concentrated to more concentrated

– This process, called ACTIVE TRANSPORT, requires the cell to use

energy (usually in the form of ATP)

• All of the transport proteins involved in active transport are carrier

proteins (no channel proteins)

– They must pick up the solutes and transport them rather than simply

allowing them to flow through

Animation: Active Transport

An Example of Active Transport: The Sodium-Potassium Pump

• Active transport lets cells have different concentrations of solutes inside and outside

the cell

– Ex) Animal cells have higher concentrations of K+ ions and lower

concentrations of Na+ ions due to the action of the sodium-potassium pump

Passive vs. Active Transport: A Review

• Passive transport: spontaneous diffusion down a concentration gradient

• No expenditure of energy by the cell

• The rate of diffusion can be greatly

increased by membrane transport

proteins

– A) Diffusion: transports hydrophobic

molecules

• Can also transport small uncharged

polar molecules at a very slow rate

– B) Facilitated diffusion: allows transport

of hydrophilic substances

• Assisted by transport proteins (channel or carrier proteins)

• Active transport: transport proteins acts as pumps

– Move substances against their concentration gradients

– Energy for work is usually supplied by ATP

How Ion Pumps Maintain Membrane Potential

• Cells have VOLTAGES across their plasma membranes (separation of opposite

charges)

– Their cytoplasm is NEGATIVELY charged relative to the extracellular fluid

• This is due to unequal distribution of anions and cations on opposite sides

of the membrane

– Voltage across a membrane is called MEMBRANE POTENTIAL

• Membrane potential ranges from –50 to –200 millivolts (mV)

– The minus sign indicates that the inside of the cell is negative relative

to the outside

The Electrochemical Gradient

• Because of the membrane potential (negatively charged cytoplasm relative to extracellular

fluid), the passive transport of cations into the cell and anions out of the cell is favored

– Therefore, 2 forces drive the diffusion of ions across the membrane:

• 1) A chemical force, due to the ion’s concentration gradient

• 2) An electrical force , due to the effect of the membrane potential on the ion’s

movement

– This combination of forces is called the ELECTROCHEMICAL GRADIENT

• Ex) The concentration of Na+ inside a resting nerve cell is much lower than

outside the cell

– Thus, the concentration gradient favors the movement of Na+ INTO the cell

– The membrane potential also favors movement of Na+ INTO the cell

(attracted to negative interior of cell)

• Electrical forces can also OPPOSE simple diffusion

– Active transport may be necessary in these cases

Electrogenic Pumps

• Membrane proteins involved in active transport of ions can contribute to membrane potential

– Ex) The sodium-potassium pump moves 3 Na+ ions out of the cell for every 2 K+ ions

brought in

• Each pump therefore provides a net transfer of one positive charge from the

cytoplasm to the extracellular fluid

– Transport proteins that generate voltage in this way across the membrane are called

ELECTROGENIC PUMPS

• The sodium-potassium pump is the main electrogenic pump in animal cells

• The main electrogenic pump of

plants, fungi, and bacteria is the

PROTON PUMP

The Proton Pump

• The proton pump actively transport H+ ions OUT of the cell

– This creates a negative charge INSIDE the cell relative to the more positively charged

outside

• This stores energy that

can be used for cellular

work

– Ex) ATP synthesis

during cellular

respiration

– Ex) A type of

membrane traffic called COTRANSPORT

Cotransport: Coupled Transport by a Membrane Protein

• In COTRANSPORT, the active transport of one solute indirectly drives the transport of

another solute

– A substance that is actively transported against its concentration builds up potential

energy

• This potential energy can then be used to do work as the substance diffuses back

down its concentration gradient

– In this case, the energy can be used

to drive the active transport of

another solute

• Ex) Plant cells use potential

energy built up by the

concentration of H+ outside the

cell to drive the active

transport of amino acids,

sugars, and other nutrients

Concept Check 7.4

• 1) When nerve cells establish a voltage across their

membrane with a sodium-potassium pump, does this

pump use ATP or does it produce ATP? Why?

• 2) Explain why the sodium-potassium pump in Figure

7.16 (pp. 136) would not be considered a

cotransporter.

• 3) What would happen if cells had a channel protein

allowing unregulated passage of hydrogen ions?

Concept 7.5: Bulk transport across the plasma

membrane occurs by exocytosis and endocytosis

• Large molecules (proteins, polysaccharides) can’t cross the plasma

membrane by diffusion or through transport proteins

– Generally, these large polymers cross the membrane in bulk,

packaged within vesicles

• This bulk transport requires energy

– Types of bulk transport include:

• Exocytosis

• Endocytosis

Bulk Transport of Large Molecules

Exocytosis

• In the process of exocytosis, transport vesicles move biological molecules OUT of the cell by

fusing with the plasma membrane

– Vesicles bud first from the ER and then the Golgi apparatus

• When the plasma membrane and the vesicle come into contact, the lipids of the 2

bilayers rearrange and their membranes FUSE

– The contents of the vesicle then spill outside of the cell

– The vesicle membrane then becomes part of the plasma membrane

– Many secretory cells use exocytosis to export products out of the cell

• Ex) Cells in the pancreas make insulin and excrete it into the extracellular fluid by

exocytosis

Animation: Exocytosis

Endocytosis

• In the process of endocytosis, the cell takes in material by forming new vesicles from the

plasma membrane

– Events look like the reverse of exocytosis

• A small area of plasma membrane sinks inward to form a pocket

• As the pocket deepens, it pinches in, forming a vesicle containing material from

outside the cell

– There are 3 types of endocytosis:

• 1) Phagocytosis – cellular eating

• 2) Pinocytosis – cellular drinking

• 3) Receptor-mediated endocytosis

Animation: Exocytosis and Endocytosis Introduction

Phagocytosis

• Phagocytosis can be considered “cellular eating”

– A cell engulfs a particle by wrapping pseudopodia around it

• Pseudopodia package the particle within a membrane-enclosed sac large enough to

be called a vacuole

– The particle is digested after the vacuole fuses with a lysosome containing hydrolytic

enzymes

Animation: Phagocytosis

Pinocytosis

• Pinocytosis can be considered “cellular drinking”

– The cell “gulps” droplets of extracellular fluid into tiny vesicles

• The fluid itself is not needed but rather the molecules dissolved in droplets

are required

– Pinocytosis is a nonspecific form of transport:

• Any and all solutes are taken into the cell

Animation: Pinocytosis

Receptor-Mediated Endocytosis

• Receptor-mediated endocytosis allows cells to acquire bulk quantities of specific substances,

even if these substances are not very concentrated in the extracellular fluid

– Proteins in the plasma membrane have specific receptor sites exposed to the

extracellular fluid

• These receptor proteins are usually clustered in regions of the membrane called

COATED PITS (called this because they are lined on their cytoplasmic side by a

fuzzy layer of coat proteins)

– The specific substances that bind to the

receptors are called LIGANDS

• When binding occurs, the coated pit forms

a vesicle containing the ligand molecules

• After the ingested material is freed into

the cell from the vesicle, receptors are

recycled to the plasma membrane by

the same vesicle

Animation: Receptor-Mediated Endocytosis

Concept Check 7.5

• 1) As a cell grows, its plasma membrane expands.

Does this involve endocytosis or exocytosis? Explain.

• 2) To send a signal, a neuron may carry out

exocytosis of signaling molecules that are recognized

by a second neuron. In some cases, the first neuron

ends the signal by taking up the molecules by

endocytosis. Would you expect this to occur by

pinocytosis or by receptor-mediated endocytosis?

Explain.

You should now be able to:

1. Define the following terms: amphipathic

molecules, aquaporins, diffusion

2. Explain how membrane fluidity is influenced

by temperature and membrane composition

3. Distinguish between the following pairs or

sets of terms: peripheral and integral

membrane proteins; channel and carrier

proteins; osmosis, facilitated diffusion, and

active transport; hypertonic, hypotonic, and

isotonic solutions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Explain how transport proteins facilitate

diffusion

5. Explain how an electrogenic pump creates

voltage across a membrane, and name two

electrogenic pumps

6. Explain how large molecules are transported

across a cell membrane

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings