CAMPBELL

BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson

© 2014 Pearson Education, Inc.

TENTH EDITION

CAMPBELL

BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson

© 2014 Pearson Education, Inc.

TENTH EDITION

7 Membrane Structure and Function

Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick

Life at the Edge • The plasma membrane is a

boundary separates the living cell from its surroundings.

• It controls traffic into and out of the cell.

• The cell must be able to be selective in its chemical exchanges with its environment.

• Like all biological membranes, the plasma membrane is selectively permeable- allowing some substances to cross more easily than others.

© 2014 Pearson Education, Inc.

Figure 7.1

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

© 2014 Pearson Education, Inc.

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 (fatty acid tails) and hydrophilic (polar head) regions.

• A phospholipid bilayer can exist as a stable boundary between two aqueous compartments.

Fig. 7.2

© 2014 Pearson Education, Inc.

• 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

© 2014 Pearson Education, Inc.

Fluid Mosaic Model • The fluid mosaic model

states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it.

• Proteins are not randomly distributed in the membrane.

• The hydrophilic regions of proteins and phospholipids are in maximum contact with aqueous environments, and the hydrophobic regions are in a non-aqueous environment within the membrane.

Fig. 7.3

© 2014 Pearson Education, Inc.

Figure 7.3

Glyco- protein Carbohydrate Glycolipid

EXTRACELLULAR SIDE OF MEMBRANE

Microfilaments of cytoskeleton

Fibers of extra- cellular matrix (ECM)

Cholesterol

Peripheral proteins Integral

protein CYTOPLASMIC SIDE OF MEMBRANE

The Fluidity of Membranes

• Phospholipids in the plasma membrane can move within the bilayers.

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

• Rarely does a molecule flip-flop transversely across the membrane – The lateral movements of phospholipids are rapid. – Adjacent phospholipids switch positions about 107

times per second. – Some large membrane proteins drift within the

phospholipids bilayers, although they move more slowly than the phospholipids.

© 2014 Pearson Education, Inc.

Fig. 7-4

RESULTS

Membrane proteins

Mouse cell Human cell Hybrid cell

Mixed proteins after 1 hour

Protein of two organism of Class- Mammals are very similar in structure both are mixed after few hours .

© 2014 Pearson Education, Inc.

Fig. 7.5

(a) Movement of phospholipids

Lateral movement (∼107 times per second)

Flip-flop (∼ once per month)

© 2011 Pearson Education, Inc.

The Fluidity of Membranes • Membrane fluidity is

influenced by temperature. To work properly with active enzymes and appropriate permeability, membranes must be about as fluid as salad oil.

• As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids pack more closely.

• Membrane fluidity is also influenced by the components of the membrane.

• Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids because kinks in the unsaturated fatty acid tails at the locations of the double bonds prevent tight packing.

Fig. 7.5 a

© 2014 Pearson Education, Inc.

Unsaturated tails prevent packing

Saturated tails pack together.

The Fluidity of Membranes

• The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells.

• At moderate temperatures-(warm temperatures such as 37°C),Cholesterol reduces membrane fluidity but at low –cool temperatures cholesterol hinders solidification and maintains fluidity by preventing tight packing.

• Thus, cholesterol acts as a “temperature buffer” for the membrane, resisting changes in membrane fluidity as temperature changes.

• .

Fig. 7.5b

© 2014 Pearson Education, Inc.

Evolution of Differences in Membrane Lipid Composition • Variations in lipid composition of cell

membranes of many species appear to be adaptations to specific environmental conditions

• Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary

© 2014 Pearson Education, Inc.

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

• - Integral Proteins the membrane penetrate the hydrophobic core • - Peripheral Proteins are bound to the

surface of the membrane

© 2014 Pearson Education, Inc.

Integral Proteins

• Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane as transmembrane proteins.

• Other integral proteins extend partway into the hydrophobic core. • The hydrophobic regions embedded in the membrane’s core consist of

stretches of nonpolar amino acids, usually coiled into alpha helices. • The hydrophilic regions of integral proteins are in contact with the

aqueous environment. • Some integral proteins have a hydrophilic channel through their center

that allows passage of hydrophilic substances.

Fig. 7.3

© 2014 Pearson Education, Inc.

Transmembrane Proteins

• Integral proteins that span the entire lipid bilayer are transmembrane proteins. – N-terminus outside cell, C-terminus inside cell – Hydrophobic areas orient themselves towards lipid

core (inside) of the membrane and hydrophilic regions of protein orient themselves towards outside/inside of bilayer, or inside of channel/pore.

© 2014 Pearson Education, Inc.

Fig. 7.6

Peripheral and Intracellular/Extracellular Proteins

• Peripheral proteins are not embedded in the lipid bilayer at all. – Instead, peripheral proteins are loosely bound to the surface of

the membrane, often to integral proteins. • On the cytoplasmic side of the membrane, some membrane proteins

are attached to the cytoskeleton. • On the exterior side of the membrane, some membrane proteins

attach to the fibers of the extracellular matrix. – These attachments combine to give animal cells a stronger

framework than the plasma membrane itself could provide.

Fig. 7.3

© 2014 Pearson Education, Inc.

Six functions of membrane proteins

a. Transport of specific solutes into or out of cells b. Enzymatic activity- sometimes catalyzing one

of a number of steps of a metabolic pathway c. Signal transduction-relaying hormonal

messages to the cell d. Cell-cell recognition- allowing other proteins

to attach two adjacent cells together e. Intercellular joining of adjacent cells with gap

or tight junctions f. Attachment to the cytoskeleton and

extracellular matrix- maintaining cell shape and stabilizing the location of certain membrane proteins

Fig. 7.7

(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transduction

Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco- protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Let us check details of each in next slides

Transport and Enzymatic Activity

a. Transport: Channels that are selective

for a particular solute Some transport proteins

require ATP to actively shuttle substances across the membrane

b. Enzymatic Activity: Enzymes may be

sequestered in the membrane with their active sites exposed in the cytoplasm. Several enzymes may be located in close proximity to each other and function sequentially

© 2014 Pearson Education, Inc.

Fig. 7.7a

Signal Transduction and Cell:Cell Recognition

c. Signal Transduction: A chemical messenger binds to

a membrane receptor resulting in a shape change of the receptor that then triggers a series of events within the cell

d. Cell:Cell Recognition: Membrane glycoproteins serve

as ID tags that are recognized be other cells

© 2014 Pearson Education, Inc. Fig. 7.7b

Membrane carbohydrates are important for cell-cell recognition

• Cell-cell recognition- the ability of a cell to distinguish one type of neighboring cell from another.

– Cell-cell recognition is the basis for the rejection of foreign cells by the immune system.

• Cells recognize other cells by binding to surface molecules, often carbohydrates, on the plasma membrane.

– Membrane carbohydrates are usually branched oligosaccharides(sugar groups).

• Membrane carbohydrates may be covalently bonded to lipids, forming glycolipids, or more commonly to proteins, forming glycoproteins.

• The sugar groups on the extracellular side of the plasma membrane vary from species to species, from individual to individual, and even from cell type to cell type within an individual.

– This variation distinguishes each cell type. – The four human blood groups (A, B, AB, and O) differ in the external

carbohydrates on red blood cells.

Fig. 7.3

Figure 7.8

Receptor (CD4)

Co-receptor (CCR5)

HIV

Receptor (CD4) but no CCR5 Plasma

membrane

HIV must bind to the immune cell surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell. HIV can infect a cell that has CCR5 on its surface, as in most people.

HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals.

Intercellular Joining and ECM Attachment

e. Intercellular Joining: Membrane proteins on

adjacent cells may attach to form gap or tight junction

f. Attachment to ECM or Cytoskeleton:

Membrane proteins can form non-covalent bonds with microfilaments to maintain cell shape and stabilize proteins in a certain location

© 2014 Pearson Education, Inc. Fig. 7.7c

The inside and outside faces of membranes may differ in lipid composition

• Each protein in the membrane has a directional orientation in the membrane so they have distinct inside and outside faces.

• The asymmetrical arrangement of proteins, lipids, and their associated carbohydrates in the plasma membrane (PM) is determined as the membrane is built by the endoplasmic reticulum (ER) and Golgi apparatus. Membrane lipids and proteins are synthesized in the ER. Carbohydrates are added to proteins in the ER, and the resulting glycoproteins are further modified in the Golgi apparatus. Glycolipids are also produced in the Golgi apparatus.

Figure 7.9

Transmembrane glycoproteins

ER ER lumen

Glycolipid

Plasma membrane: Cytoplasmic face Extracellular face

Secretory protein

Golgi apparatus

Vesicle

Transmembrane glycoprotein Secreted

protein Membrane glycolipid

Synthesis and Sidedness of Membranes 1. Membrane proteins and lipids are

synthesized in the ER and carbohydrates are added to the proteins.

2. Glycoproteins undergo carbohydrate modifications in the Golgi, and lipids acquire carbohydrate chains.

3. Transmembrane proteins, membrane glycolipids, and secretory proteins leave the Golgi in vesicles and travel to the PM.

4. At the PM the vesicles fuse with the PM and secretory proteins are released from the cell. Vesicles fuse to the PM and the carbohydrate chains are positioned so that they are on the outside of the PM. The outside layer of the vesicle becomes continuous with the cytoplasmic (inner) layer of the plasma membrane.

• Molecules that originate on the inside face of the ER end up on the outside face of the plasma membrane.

• Carbohydrate

• Proteins

Membrane structure results in selective permeability

• A cell must exchange materials with its surroundings, a process controlled by the plasma membrane – sugars, amino acids, and other nutrients enter cells and

metabolic waste products leave. – Inorganic ions such as Na+, K+, Ca2+, and Cl− are

shuttled in and out of cells. • Plasma membranes are selectively permeable,

regulating the cell’s molecular traffic – 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-they require transport proteins to enter the cell.

Transport Proteins and Selective Permeability

• Transport proteins allow passage of hydrophilic substances and specific ions across the membrane

• Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

© 2014 Pearson Education, Inc.

Aquaporin is a Channel Protein • The passage of water

through the membrane is facilitated by channel proteins known as aquaporins.

• Each aquaporin allows entry of as many as 3 billion (109) water molecules per second, passing single file through its central channel, which fits 10 at a time.

• Without aquaporins, only a

tiny fraction of these water molecules would diffuse through the same area of the cell membrane in a second, so the channel protein greatly increases the rate of water movement.

Carrier Proteins

• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.

• Each transport protein is specific for the substance that it translocates, carries across the membrane.

• For example, the glucose transport protein in the liver carries glucose into the cell but does not transport fructose, its structural isomer.

• The glucose transporter causes glucose to pass through the membrane 50,000 times as fast as it would diffuse through on its own.

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 – Diffusion is driven by the intrinsic kinetic energy

(thermal motion or heat) of molecules. – The movements of individual molecules are random.

However, the movement of a population of molecules may be directional.

• At dynamic equilibrium, as many molecules cross

one way as cross in the other direction

© 2014 Pearson Education, Inc.

Diffusion of One Solute

• Example: A permeable membrane with microscopic pores separating a solution with dye molecules from pure water.

• Each dye molecule wanders randomly, but there is a net movement of the dye molecules across the membrane to the side that began as pure water.

• The net movement of dye molecules across the membrane continues until both sides have equal concentrations of the dye.

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

• In the absence of other forces, a substance diffuses from where it is more concentrated to where it is less concentrated, down its concentration gradient.

• No work must be done to move substances down the concentration gradient; diffusion is a spontaneous process, needing no input of energy.

Fig.7.10

© 2014 Pearson Education, Inc.

Diffusion of Two Solutes

• Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances. – Example: There is a net diffusion of the purple dye to the left,

even though the total solute concentration (orange + purple) is greater on the left side.

Fig.7.10a

© 2014 Pearson Education, Inc.

The diffusion of a substance across a biological membrane is passive transport because it requires no

energy from the cell to make it happen.

The diffusion of water across a selectively permeable membrane is called osmosis.

• Imagine that two sugar solutions differing in

concentration are separated by a membrane that allows water through, but not sugar. How does this affect the water concentration?

• Water diffuses across the membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentrations on both sides of the membrane are equal.

• The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms.

• Both solute concentration and membrane permeability affect tonicity, the ability of a solution to cause a cell to gain or lose water.

Fig.7.11 © 2014 Pearson Education, Inc.

Water Balance of Cells Without Walls

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

• Hypotonic solution: Solute concentration is less outside than that inside the cell; cell gains water and lyses

• Isotonic solution: Solute concentration is the same both outside and inside the cell; no net water movement across the plasma membrane

• Hypertonic solution: Solute concentration is greater outside than that inside the cell; cell loses water and becomes shriveled

© 2014 Pearson Education, Inc.

Figure 7.12

Hypotonic Isotonic Hypertonic

H2O

Lysed Normal Shriveled

Plasma membrane

Cell wall

Turgid (normal) Flaccid Plasmolyzed

(a) A

nim

al c

ell

(b) P

lant

cel

l

Plasma membrane

H2O H2O

H2O H2O H2O H2O

H2O

Water Balance in Cells with Walls • The cells of plants, prokaryotes, fungi, and some protist

have walls. • A plant cell in a solution hypotonic to the cell contents

swells due to osmosis until the elastic cell wall exerts a back-pressure on the cell that opposes further uptake. – At this point the cell is turgid (very firm), a healthy

state for most plant cells. Turgid cells contribute to the mechanical support of the plant.

• If a plant cell and its surroundings are isotonic, there is no movement of water into the cell. The cell becomes flaccid (limp), and the plant may wilt.

• The cell wall provides no advantages when a plant cell is immersed in a hypertonic solution. – As the plant cell loses water, its volume shrinks.

Eventually, the plasma membrane pulls away from the wall. This plasmolysis is usually lethal.

– The walled cells of bacteria and fungi also plasmolyzed in hypertonic environments.

Osmoregulation-The control of Water Balance

• Organisms without rigid cell walls have osmotic problems in either a hypertonic or a hypotonic environment.

• Water balance is not a problem if such a cell lives in isotonic surroundings, however. – Seawater is isotonic to many marine invertebrates. – The cells of most terrestrial animals are bathed in

extracellular fluid that is isotonic to the cells. • Animals and other organisms without rigid cell

walls living in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance.

Osmoregulation • The protist Paramecium

is hypertonic to the pond water in which it lives.

• In spite of a cell membrane that is less permeable to water than other cells, water continually enters the Paramecium cell.

• To solve this problem, Paramecium cells have a specialized organelle, the contractile vacuole, that functions as a bilge pump to force water out of the cell.

Fig.7.13

© 2014 Pearson Education, Inc.

Proteins facilitate the passive transport of water and selected solutes

• Many polar molecules and ions that can’t easily pass through the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane.

• The passive movement of molecules down their concentration gradient via transport proteins is called facilitated diffusion.

• Most transport proteins are very specific: They transport only particular substances but not others.

• Two types of transport proteins facilitate the movement of molecules or ions across membranes:

channel proteins and carrier proteins.

Figure 7.14

(a) A channel protein

(b) A carrier protein

Carrier protein

Channel protein Solute

Solute

EXTRACELLULAR FLUID

CYTOPLASM

Facilitated Diffusion: Passive Transport Aided by Proteins

• Channel proteins provide hydrophillic corridors that allow a specific molecule or ion to cross the membrane

• Channel proteins include – Aquaporins, for

facilitated diffusion of water

– Many Ion channels are Gated they open or close in response to a stimulus

© 2014 Pearson Education, Inc.

Carrier Proteins and Passive Transport

• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.

• Each transport protein is specific for the substance that it translocates.

• For example, the glucose transport protein in the liver carries glucose into the cell but does not transport fructose, its structural isomer.

• The glucose transporter causes glucose to pass through the membrane 50,000 times as fast as it would diffuse through on its own.

• Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease-cystinuria.

Active transport uses energy to move solutes against their gradients

• Active transport moves substances against their concentration gradient which requires energy ATP.

• ATP supplies the energy for most active transport by transferring its terminal phosphate group directly to the transport protein.

• Active transport is performed by specific carrier proteins embedded in the membranes

• Active transport usually induces a conformational change in the transport protein, translocating the bound solute across the membrane.

© 2014 Pearson Education, Inc.

Active Transport: The Sodium-Potassium Pump

• The sodium-potassium pump exchanges sodium ions (Na+) for potassium ions (K+) across the plasma membrane of animal cells.

• The plasma membrane helps maintain these steep gradients by pumping 3 sodium ions out of the cell and 2 potassium ions into the cell.

© 2014 Pearson Education, Inc.

Figure 7.15a

CYTOPLASM

2

[Na+] low [K+] high

[Na+] high [K+] low

Na+

Na+

Na+

Na+

Na+

Na+

ATP

ADP

P

EXTRACELLULAR FLUID

1 Cytoplasmic Na+ binds to the sodium- potassium pump. The affinity for Na+ is high when the protein has this shape.

Na+ binding stimulates phosphorylation by ATP.

Figure 7.15b

4 3 Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside.

The new shape has a high affinity for K+, which binds on the extracellular side and triggers release of the phosphate group.

Na+

Na+

P

Na+ K+

K+

P P i

Figure 7.15c

6 K+ is released; affinity for Na+ is high again, and the cycle repeats.

Loss of the phosphate group restores the protein’s original shape, which has a lower affinity for K+.

5

Figure 7.16

ATP

Passive transport Active transport

Diffusion Facilitated diffusion

How Ion Pumps Maintain Membrane Potential

• Membrane potential is the voltage difference across a membrane and ranges from −50 to −200 millivolts (mV).

• Voltage is created by differences in the distribution of positive and negative ions The inside of the cell is negative compared to the outside. – The cytoplasm of a cell is negative in charge relative to

the extracellular fluid because of an unequal distribution of cations and anions on opposite sides of the membrane.

– Because the inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell.

© 2014 Pearson Education, Inc.

• 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 ion does not simply diffuse down its concentration gradient but diffuses down its electrochemical gradient. – For example, there is a higher concentration of Na+

outside a resting nerve cell than inside. – When the neuron is stimulated, gated channels open

and Na+ diffuses into the cell down the electrochemical gradient.

– The diffusion of Na+ is driven by the concentration gradient and by the attraction of cations to the negative side of the membrane.

© 2014 Pearson Education, Inc.

1. An electrogenic pump is a transport protein that generates voltage across a membrane. Electrogenic pumps help store energy that can be used for cellular work.

2. The sodium-potassium pump is the major electrogenic pump of animal cells.

3. The proton pump main electrogenic pump of plants, fungi, and bacteria is actively transports H+ out of the cell and transferring positive charge from the cytoplasm to the extracellular solution.

Electrogenic pumps help store energy that can be used for cellular work.

Active Transport by three different types of Electrogenic Pump

© 2014 Pearson Education, Inc.

Proton Pump-Plants and Bacteria

• The proton pump uses ATP for energy to translocate H+ (positive charge) out of the cell. This creates membrane voltage, or stored energy. The voltage and H+ concentration gradient are dual energy sources that can be utilized to drive the uptake of nutrients.

Fig.7.17

© 2014 Pearson Education, Inc.

Cotransport: Coupled Transport by a Membrane Protein

• Cotransport occurs when active transport of a solute indirectly drives transport of another solute. As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration gradient.

© 2014 Pearson Education, Inc.

Cotransport is active transport driven by a concentration gradient

Plants use the gradient of hydrogen ions generated by proton pumps to drive the active transport of amino acids, sugars, and other nutrients into the cell. A carrier protein such as the sucrose-H+ cotransporter uses the diffusion of H+ down its electrochemical gradient to drive sucrose uptake into the cell. The H+ gradient is maintained by an ATP-driven proton pump that expels H+ from the cell, thus storing potential energy that can be used for the active transport of sucrose.

Fig.7.18

© 2014 Pearson Education, Inc.

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 through bulk transport via vesicles

• Bulk transport requires energy

© 2014 Pearson Education, Inc.

Exocytosis

• In exocytosis, transport vesicles that have budded off the Golgi migrate to the membrane, fuse with it, and release their contents

• Many secretory cells use exocytosis to export their products – Pancreatic cells release insulin – Neuronal cells release norepinephrine

and acetylcholine

© 2014 Pearson Education, Inc.

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-allows the bulk

transport of molecules that might not be abundant in the environment

© 2014 Pearson Education, Inc.

Figure 7.19

Phagocytosis Pinocytosis Receptor-Mediated

Endocytosis

Solutes

Pseudopodium

“Food” or other particle

Food vacuole

CYTOPLASM

EXTRACELLULAR FLUID

Plasma membrane

Coated pit

Coated vesicle

Coat protein

Receptor

• In phagocytosis a cell engulfs a particle in a vacuole

• The vacuole fuses with a lysosome to digest the particle

• For example- Digestion of food of Amoeba and

• WBC engulf bacteria.

Phagocytosis

© 2014 Pearson Education, Inc.

Pinocytosis

• In pinocytosis the cell gulps droplets of

extracellular fluid into tiny vesicles. The cell utilizes the

molecules dissolved in the extracellular fluid, not the fluid itself.

© 2014 Pearson Education, Inc.

Receptor-Mediated Endocytosis • Example: Human cells use a

RME process to take in cholesterol for use in the synthesis of membranes and as a precursor for the synthesis of steroids.

• Cholesterol travels in the blood in low-density lipoproteins (LDL), complexes of protein and lipid.

• These lipoproteins act as ligands by binding to LDL receptors on membranes and entering the cell by endocytosis.

• In an inherited disease called familial hypercholesterolemia, the LDL receptors are defective, leading to an accumulation of LDL and cholesterol in the blood.

• This condition contributes to early atherosclerosis.

© 2014 Pearson Education, Inc.