membrane functions 2015 ss

© 2016 Pearson Education, Inc.

PowerPoint Lecture prepared by Dr. Judi Roux,

University of Minnesota Duluth

Chapter 3

Membrane Transport

Passive Transport

Active Transport

MEMBRANE STRUCTURE AND FUNCTION

Membranes organize and compartmentalize the chemical reactions in cells

Membranes

• Provide structural order for metabolism

• Provide an efficient way for making ATP using energy from electrons in the chemical bonds of your food

• Use enzyme-like proteins and other organic chemicals for transporting substances across the membrane


VISUALIZING THE CONCEPT: Membranes are fluid mosaics of lipids and proteins with many functions

Biologists use the fluid mosaic model to describe a membrane’s structure, a patchwork of diverse protein molecules embedded in a phospholipid bilayer.

• The plasma membrane exhibits selective permeability.

• The proteins embedded in a membrane’s phospholipid bilayer perform various functions.

Phospholipid

Cholesterol

Membraneproteins

Microfilamentsof cytoskeleton

Extracellular sideof membrane

Fibers ofextracellularmatrices (ECM)

Cytoplasmic sideof membrane

O2 CO2

Junctionprotein

Junctionprotein

Junction Proteins• Form intercellular junctions that

attach adjacent cells

Attachedsugars

Glycoprotein

Protein thatrecognizesneighboringcell

Glycoproteins• Serve as ID tags• May be recognized by membrane

proteins of other cells

Figure 5.1-5

Extracellularmatrix

Attachmentprotein


Attachment Proteins• Attach to the extracellular matrix and cytoskeleton• Help support the membrane• Can coordinate external and internal changes

Enzymes

Initialreactant

Product ofreaction

Enzymes• Some membrane proteins are enzymes• Enzymes may be grouped to carry out

sequential reactions

Signalingmolecule

Receptorprotein

Receptor Proteins• Signaling molecules bind to receptor proteins• Receptor proteins relay the message by

activating other molecules inside the cell

Solute molecules

Channelprotein

Activetransportprotein

Transport Proteins

ATP

• Allow specific ions or moleculesto enter or exit the cell

The membrane is a “fluid mosaic”

of phospholipids and proteins• proteins and other molecules embedded in a liquid

phospholipid bilayer

Fibers of the extracellular matrix

Carbohydrate(of glycoprotein)

Glycoprotein


Phospholipid

CholesterolProteins

Plasmamembrane

Glycolipid

Cytoplasm

Phospholipid

Cholesterol

Membraneproteins


Extracellular sideof membrane

Fibers ofextracellularmatrices (ECM)

Cytoplasmic sideof membrane

O2 CO2

Substances and Signalscan cross the membrane

“Selectively Permeable”= only certain types of things can cross

The structure and components of the membrane determine which types of substances can enter/leave the cell, and how much, and under what types of circumstances.

Membrane phospholipids form a bilayerPhospholipids

• Have a hydrophilic head and two hydrophobic tails

• Are the main structural components of membranes

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH

CH

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2

CH2

CH3

CH3

CH3N+

O

O O–P

OCH2CHCH2

C O C O

O O

Phosphategroup

Symbol

Hydrophilic head

Hydrophobic tails

Phospholipids in water form a two-layer sheetCalled a phospholipid bilayer, with the heads facing outward and the tails facing inward

Water

Water

Hydrophilicheads

Hydrophobictails


3.3 Transport Across Membranes

Membrane transport

Plasma membrane: phospholipid bilayer is differentially permeable

Hydrophobic substancespass more easily through the hydrophobic interior

Water can cross, too albeit slowly - one water molecule in about 1 million phosholipids

Small nonpolar molecules such as O2 and CO2

and even ethanol and some organic solvents –

• Diffuse easily across the phospholipid bilayer of a membrane

• Water can’t “grab onto” small molecules that don’t have a charge, so they can move into the fatty acid chains in the middle of the membrane

• These molecules always cross cell membranes

Diffusion of smallnonpolar molecules

O2CO2


Transport proteins can facilitate diffusion across membranes

• Because water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow.

• The very rapid diffusion of water into and out of certain cells, such as in the kidneys, is made possible by a protein channel called an aquaporin.

Water molecules (red/gray) can freely cross mem branes

Through “aquaporins” (blue) - protein channels.

Phospholipids in green (hydrophobic tails) & yellow (charged heads)

Passive transport is diffusion across a membrane with no energy investment

Diffusion across a cell membrane does not require energy, so it is called passive transport.

Diffusion down concentration gradients is the sole means by which oxygen enters your cells and carbon dioxide passes out of cells.


3.3 Transport Across MembranesPassive transport: movement of molecules without energy

• Diffusion

• Facilitated diffusion

• Osmosis


3.3 Transport Across Membranes

Equilibrium

• Concentrations of substances are equal on both sides of the membrane

• Substances move back & forth across membranes until equilibrium condition is reached, which is the lowest energy state for that system

Passive transport is diffusion across a membrane with no energy investment

Diffusion is the tendency of particles to spread out evenly in an available space.

• Particles move from an area of more concentrated particles to an area where they are less concentrated.

• This means that particles diffuse down their concentration gradient.

• Eventually, the particles reach dynamic equilibrium, where particles still move across the membrane, but there is no net change in concentration on either side of the membrane.

Passive transport is diffusion across a membraneIn passive transport, substances diffuse through

membranes without work by the cell

• Spreading from areas of high concentration to areas of low concentration

EquilibriumMembraneMolecules of dye

Equilibrium


3.3 Transport Across MembranesDiffusion: passive transport from area of high concentration to low concentration

• Very small, hydrophobic molecules



• Hydrophobic substances easily diffuse across a cell membrane.

• However, polar or charged substances do not easily cross cell membranes and, instead, move across membranes with the help of specific transport proteins in a process called facilitated diffusion, which

• does not require energy and• relies on the concentration gradient.

Transport proteins may facilitate diffusion across membranes

• Many kinds of molecules do not diffuse freely across membranes

• For these molecules, transport proteins provide passage across membranes through a process called facilitated diffusion

Solutemolecule

Transportprotein


3.3 Transport Across MembranesFacilitated diffusion: transport proteins help move hydrophilic and charged molecules across the membrane

• From high to low concentration

• Without using energy



• Some proteins function by becoming a hydrophilic tunnel for passage of ions or other molecules.

• Other proteins bind their passenger, change shape, and release their passenger on the other side.

• In both cases, the transport protein helps a specific substance diffuse across the membrane down its concentration gradient and thus requires no input of energy.

Transport proteins can facilitate diffusion across membranes• Because water is polar, its diffusion through a

membrane’s hydrophobic interior is relatively slow.

• The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin.

• This is a type of facilitated diffusion.

Water molecules (red/gray) can freely cross mem branes

Through “aquaporins” (blue) - protein channels.

Phospholipids in green (hydrophobic tails) & yellow (charged heads)


What process allows charged amino acids to enter the cell?

A. passive diffusion

B. osmosis

C. facilitated diffusion

D. exocytosis


Osmosis is the diffusion of water across a membrane

• One of the most important substances that crosses membranes by passive transport is water.

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


3.3 Transport Across MembranesOsmosis: diffusion of water across a membrane from high to low concentration


• If a membrane, permeable to water but not to a solute, separates two solutions with different concentrations of solute, water will cross the membrane, moving down its own concentration gradient, until the solute concentration on both sides is equal.


• In osmosis, water travels from a solution of lower solute concentration to one of higher solute concentration

Lowerconcentration

of solute

Higherconcentration

of solute

Equalconcentration

of solute

H2OSolutemolecule

Selectivelypermeablemembrane

Watermolecule

Solute molecule withcluster of water molecules

Net flow of water

The control of water balance in organisms• Is called osmoregulation

Some definitions:solute = chemical dissolved in a solventsolvent = chemical that dissolves a solutesolution = a solute dissolved in a solventhypotonic = less concentrated solutionhypertonic = more concentrated solutionisotonic = two solutions of equal concentrations

Water balance between cells and their surroundings is crucial to organisms

• Tonicity is a term that describes the ability of a surrounding solution to cause a cell to gain or lose water.

• The tonicity of a solution mainly depends on its concentration of solutes relative to the concentration of solutes inside the cell.



How will animal cells be affected when placed into solutions of various tonicities?

• In an isotonic solution, the concentration of solute is the same on both sides of a membrane, and the cell volume will not change.

• In a hypotonic solution, the solute concentration is lower outside the cell, water molecules move into the cell, and the cell will expand and may burst.

• In a hypertonic solution, the solute concentration is higher outside the cell, water molecules move out of the cell, and the cell will shrink.


• Osmosis causes cells to shrink in hypertonic solutions and swell in hypotonic solutions

• In isotonic solutions, animal cells are normal, but plant cells are limp

Plantcell

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2OPlasma

membrane

(1) Normal (2) Lysed (3) Shriveled

(4) Flaccid (5) Turgid(6) Shriveled (plasmolyzed)

Isotonic solution Hypotonic solution Hypertonic solution

Animalcell

Animalcell

Plantcell

Hypotonic solution(lower solute levels)

Isotonic solution(equal solute levels)

Hypertonic solution(higher solute levels)

H2O

Lysed Normal Shriveled

Turgid (normal) Flaccid

Plasmamembrane

Shriveled (plasmolyzed)

H2O H2O H2O

H2OH2OH2O


For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation, the control of water balance.


The cell walls of plant cells, prokaryotes, and fungi make water balance issues somewhat different.

• The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much water and bursting when placed in a hypotonic environment.

• But in a hypertonic environment, plant and animal cells both shrivel.

What results if the solute concentration is higher outside the cell?

A. Osmosis will cause water to move into the cell.

B. Osmosis will cause water to move out of the cell.

C. Osmosis will not cause any water to move.

D. Osmosis will cause the cell to burst.

Solute molecules

Channelprotein

Activetransportprotein

Transport Proteins

ATP

• Allow specific ions or moleculesto enter or exit the cell


3.3 Transport Across MembranesActive transport

• Uses proteins to move molecules from low to high concentration

• Powered by energy from ATP

Membrane proteins also function in transport• Moving substances across the membrane

ATP

Passive TransportRequires no energy inputRelies on diffusion

“Facilitated”Passive TransportUses proteins

ActiveTransportRequires energy

Concentrates a chemical

Runs diffusion in reverse


Cells expend energy in the active transport of a solute

• In active transport, a cell must expend energy to move a solute against its concentration gradient.

• The energy molecule ATP supplies the energy for most active transport.

PP PProtein

changes shapePhosphatedetaches

ATPADPSolute

Transportprotein

Solute binding1 Phosphorylation2 Transport3 Protein reversion4

Cells expend energy for active transport• Transport proteins can move solutes against a

concentration gradient that needs ATP

“Pumps” = pump a chemical across a membrane

Exocytosis and endocytosis transport large molecules across membranes

A cell uses two mechanisms to move large molecules across membranes.

1. Exocytosis is used to export bulky molecules, such as proteins or polysaccharides.

2. Endocytosis is used to take in large molecules.

• In both cases, material to be transported is packaged within a vesicle that fuses with the membrane.

Fluid outside cell

Cytoplasm

Protein

Vesicle

Active Transport - “Bulk” transport

Exocytosis and endocytosis transport large molecules and/or large amounts of molecules

• To move large molecules or particles through a membrane, a vesicle may fuse with the membrane and expel its contents (exocytosis)


3.3 Transport Across MembranesExocytosis: a membrane-bound vesicle fuses with the membrane and expels the large molecule outside the cell

Membranes may fold inward, enclosing material from the outside (endocytosis)

Vesicle forming


3.3 Transport Across MembranesEndocytosis: a vesicle pinches the plasma membrane inward and brings a large molecule into the cell

Exocytosis and endocytosis transport large molecules across membranes

• There are two kinds of endocytosis.1. Phagocytosis is the engulfment of a particle by

the cell wrapping cell membrane around it, forming a vacuole.

Pinocytosis = engulf only water2. Receptor-mediated endocytosis uses

membrane receptors for specific solutes. The region of the membrane with receptors pinches inward to form a vesicle.• Receptor-mediated endocytosis is used to take in

cholesterol from the blood.

Endocytosis can occur in three ways• Phagocytosis• Pinocytosis• Receptor-mediated endocytosis

Pseudopodium of amoeba Food being ingested

Phagocytosis Pinocytosis Receptor-mediated endocytosis

Material bound to receptor proteins

PIT

Cytoplasm

Plasma membrane

TEM

54,

000

TEM

96,

500

LM 2

30

Phagocytosis

Pseudopodium

EXTRACELLULARFLUID

CYTOPLASM

“Food” orother particle

Foodvacuole

Receptor-mediated endocytosisCoat protein

Receptor

Specificmolecule

Coatedpit

Coatedvesicle

CONNECTION

Faulty membranes can overload the blood with cholesterol

• Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors

LDL particle

Protein

Phospholipid outer layer

CytoplasmReceptorprotein

Plasmamembrane

Vesicle

Cholesterol


Which describes active transport?

A. K+ will move from high concentration to low concentration; ATP is used.

B. K+ will move from low concentration to high concentration; ATP is used.

C. K+ will move from high concentration to low concentration; ATP is not used.

D. K+ will move from low concentration to high concentration; ATP is not used.

PowerPoint Lecture prepared by Dr. Judi Roux,

University of Minnesota Duluth

Chapter 4

How Cells Harvest Chemical

Energy

Cellular Respiration

The membrane of a mitochondrion:

Proteins make the membrane a mosaic of function

• Many membrane proteins function as enzymes

• Membrane proteins also function in transporting substances across the membrane

ATP

Membranes are Selectively Permeable

The membranes that form the

plasma (cell) membrane

And

organelles like ER, nucleus, Golgi body, lysosome

Are BARRIERS to certain chemicals and molecules

- They won’t let solutions on opposite sides ofthe membrane mix together – So -

So, why you need to know about membranes:

The most efficient mechanism to make ATP is

Chemiosmosis, or the

Electron transport pathway, or

Oxidative phosphorylation

------ Terms for the same process -----

- Takes place on the inner mitochondrial membrane & depends on the

diffusion of H+ across that membrane

H2O

NAD+

NADH

ATP

H+

H+

Controlled release of energy for

synthesis of ATP

Electron transport chain

2 O2

2e

2e+

1

2

Electron Transport Pathway

In oxidative phosphorylation, the mitochondrion –- transfers energy in electrons removed from the chemical bonds in your food, and

- uses that energy to concentrate H+ on one side of a membrane.

- Then the H+ diffuses across the membrane through a protein channel next to

- an enzyme that makes high energy bonds of ATP.

Then, diffusion & phosphorylation of ADP

Intermembrane space

Inner mitochondrial membrane

Mitochondrial matrix

Protein complex

Electron flow

Electron carrier

NADH NAD+

FADH2 FAD

H2OATPADP

ATP synthase

H+ H+ H+

H+

H+ H+

H+

H+

H+

H+

H+

H+

H+

H+

+ P

O2

Electron Transport Chain Chemiosmosis

.

OXIDATIVE PHOSPHORYLATION

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