Membrane Structure and Function

Membrane Function

• Membranes organize the chemical activities of cells.

• The outer plasma membrane – forms a boundary between a living cell and its

surroundings– Exhibits selective permeability

• Controls traffic of molecules in and out

Membrane Function

• Internal membranes provide structural order for metabolism

• Form the cell's organelles• Compartmentalize chemical reactions

Membrane Structure

Phospholipids are the major structural component of membranes.

Phospholipid

Membrane Structure

All membranes are phospholipid bilayers with embedded proteins.

Label the:

Hydrophilic heads

Hydrophobic tails

Phospholipid Bilayer

Membrane Structure

• Embedded in the bilayer are proteins– Most of the membrane’s functions are

accomplished by the embedded proteins. • Integral proteins span the membrane• Peripheral proteins are on one side or the other of the

membrane

Plasma Membrane

• Glycoproteins and glycolipids are proteins/lipids with short chain carbohydrates attached on the extracellular side of the membrane.

Cholesterol

Glycoprotein

Glycolipid

Carbohydrate ofglycoprotein

Phospholipid

Microfilamentsof cytoskeleton

Integrin

Membrane Proteins

Types of Membrane Proteins1. Recognition proteins2. Integrins3. Cell junction proteins4. Enzymes 5. Receptor proteins 6. Transport proteins

– Passive and active

• Recognition Proteins - identify type of cell and identify a cell as “self” versus foreign– Most are glycoproteins

• Carbohydrate chains vary between species, individuals, and even between cell types in a given individual.

• Glycolipids also play a role in cell recognition

Integrins

• Integrins are a type of integral protein– The cytoskeleton attaches to integrins on the

cytoplasmic side of the membrane– Integrins strengthen the membrane

• Cell Junction proteins - help like cells stick together to form tissues

• Many membrane proteins are enzymes– This is especially important – on the membranes of organelles.

Receptor Proteins

• Receptor proteins bind hormones and other substances on the outside of the cell.– Binding triggers a change inside the cell.

• Called signal transduction• Example: The binding of insulin to insulin receptors

causes the cell to put glucose transport proteins into the membrane.

Messenger molecule

Activatedmolecule

Receptor

Transport Proteins

• Passive Transport Proteins – allow water soluble substances (small polar

molecules and ions) to pass through the membrane without any energy cost

• Active Transport Proteins– The cell expends energy to transport water

soluble substances against their concentration gradient

Transport of Substances Across the Plasma Membrane (PM)

1. Passive Transport – (Simple) Diffusion– Facilitated diffusion – Osmosis

2. Active Transport3. Bulk Flow - endocytosis and exocytosis

Passive Transport

• In passive transport substances cross the membrane by diffusion– Diffusion - net movement of substances from an

area of high concentration to low concentration• no energy required

Factors Affecting Diffusion Rate

• Steepness of concentration gradient– Steeper gradient, faster diffusion

• Molecular size– Smaller molecules, faster diffusion

• Temperature– Higher temperature, faster diffusion

Simple Diffusion

• Nonpolar, hydrophobic molecules diffuse directly through the lipid bilayer– Simple diffusion does not require the use of

transport proteins.– Examples: O2, CO2, steroids

• Polar, hydrophilic substances cannot pass directly through the lipid bilayer – Examples: water, ions, carbohydrates

Simple Diffusion

small, nonpolar molecules(ex. O2, CO2)

Polar molecules(ex. Glucose, water)

ions(ex. H+, Na+, K+)

LIPID-SOLUBLELIPID-SOLUBLE WATER-SOLUBLEWATER-SOLUBLE

LIPID-SOLUBLELIPID-SOLUBLE

Facilitated Diffusion

• In facilitated diffusion small polar molecules and ions diffuse through passive transport proteins.– No energy needed

• Most passive transport proteins are solute specific

• Example: glucose enter/leaves cells through facilitated diffusion

Facilitated Diffusion

Passive transport protein

Lower concentration

Higher concentration of

Osmosis

• Osmosis – diffusion of water across a selectively permeable membrane

• Water moves from an area of _______ water concentration to an area of _____ water conc.– Is energy required ?

Osmosis TermsConsider two solutions separated

by a plasma membrane.• Hypertonic

– solution with a relatively high concentration of solute• Hypotonic

– solution with a relatively low concentration of solute• Isotonic

– solutions with the same solute concentration

Watermolecule

Selectivelypermeablemembrane

Solutemolecule

H2O

Lowerconcentration

of solute

Higherconcentration

of solute

Equalconcentration

of solute

Solute molecule withcluster of water molecules

Net flow of water

Osmosis and Animal Cells

Osmosis and Plant Cells

Osmosis

• When a Cell is Placed in a Hypotonic Solution– Water concentration is _________ the cell.– Water flows ___________ the cell.

Osmosis

• When a Cell is Placed in a Hypertonic Solution– Water concentration is _________ the cell.– Water flows ___________ the cell.

Isotonic solution Hypotonic solution Hypertonic solution

H2O H2O

(1) Normal (2) Lysed

H2O

H2O H2O H2O

Animalcell

Plantcell

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

(3) Shriveled

Plasmamembrane

H2O

H2O

See page 83

Osmosis Summary

•When a cell is placed in a Hypotonic solution:– Cell gains water through osmosis– Animal cell lyses; plant cell becomes turgid (firm)

•When a cell is placed a Hypertonic solution:– Cell loses water through osmosis– Animal cell shrivels; plant cell plasmolyzes

Active Transport

• Active transport proteins move substances across the PM against their concentration gradient.– Requires energy (ATP)– Active transport proteins are highly selective– Active transport is needed for proper functioning

of nerves and muscles

Active Transport of “X”– Active transport proteins span the plasma

membrane– They have openings for “X” on only one

side of the membrane– “X” enters the channel and binds to

functional groups inside the transport protein.

– Cytoplasmic ATP binds to the transport protein

Active Transport of “X”– A phosphate group is transferred from ATP

to the transport protein – protein is energized by the added –P.

– The energized transport protein changes shape and releases “X” on the other side of the cell.

– The phosphate group is released from the transport protein and it resumes its original shape.

– Process repeats.

Transportprotein

Solute

Solute binding1

Transportprotein

Solute

Solute binding1 Phosphorylation2

Transportprotein

Solute

Solute binding1 Phosphorylation2 Transport3

Proteinchanges shape

Transportprotein

Solute

Solute binding1 Phosphorylation2 Transport3

Proteinchanges shape

Protein reversion4

Phosphatedetaches

Active Transporttell the story…

ATPATP P

ADP

Bulk Flow

• Vesicles are used to transport large particles across the PM.– Requires energy

• Types:– Exocytosis– Endocytosis

• Phagocytosis, pinocytosis, receptor-mediated

Vesicle

Fluid outside cell

Protein

Cytoplasm

ExocytosisExocytosis

Bulk Flow

• Exocytosis– Cytoplasmic vesicle merges with the PM

and releases its contents– Example:

• Golgi body vesicles merge with the PM an release their contents

• How nerve cells release neurotransmittors

Vesicle forming

EndocytosisEndocytosis

Endocytosis can occur in three waysEndocytosis can occur in three ways• Phagocytosis ("cell eating")Phagocytosis ("cell eating")• Pinocytosis ("cell drinking")Pinocytosis ("cell drinking")• Receptor-mediated endocytosisReceptor-mediated endocytosis

Endocytosis

• Endocytosis– PM sinks inward, pinches off and forms a vesicle– Vesicle often merges with Golgi for processing and

sorting of its contents

Endocytosis - terms

• Phagocytosis – cell eating– Membrane sinks in and captures solid particles for

transport into the cell – Examples:

• Solid particles often include: bacteria, cell debris, or food

• Pinocytosis – cell drinking– Cell brings in a liquid

Endocytosis - comments

• Phagocytosis and pinocytosis are not selective– Membrane sinks inward and captures whatever

particles/fluid present.– Vesicle forms and merges with the Golgi body…

Receptor Mediated Endocytosis

• Receptor Mediated Endocytosis is a highly specific form of endocytosis.

– Receptor proteins on the outside of the cell bind specific substances and bring them into the cell by endocytosis

Receptor Mediated Endocytosis

1. Receptor proteins on PM bind specific substances (vitamins, hormones..)

2. Membrane sinks in and forms a pit– Called a coated pit

3. Pit pinches closed to form a vesicle around bound substances• Cytoskeleton aids in pulling in the membrane and

vesicle formation

Coatedvesicle

Coatedpit

Specificmolecule

Receptor-mediated endocytosis

Coat proteinReceptor

Coatedpit

Material boundto receptor proteins

Plasma membrane

Phagocytosis

EXTRACELLULARFLUID

Pseudopodium

CYTOPLASM

Foodvacuole

“Food” orother particle

Pinocytosis

Plasmamembrane

Vesicle

Coatedvesicle

Coatedpit

Specificmolecule

Receptor-mediated endocytosis

Coat proteinReceptor

Coatedpit

Material boundto receptor proteins

Plasma membrane

Foodbeingingested

Plasma Membrane Notes Continued…Plasma Membrane Notes Continued…What Are Three Forms of Transport Across the MembraneWhat Are Three Forms of Transport Across the Membrane ?

57

Objective:

1. Compare active transport with passive transport.

2. Distinquish between endocytosis and exocytosis

Review…Review…Passive TransportPassive Transport

is a type of Simple Diffusion which…is a type of Simple Diffusion which…

1.1. Doesn’tDoesn’t require energyrequire energy

2.2. Moves solids, liquids and Moves solids, liquids and gases from gases from high to lowhigh to low concentrationconcentration

Biological connectionBiological connection OxygenOxygen oror waterwater diffusing into lung cells and diffusing into lung cells and

carbon dioxidecarbon dioxide diffusing out diffusing out.

Passive Transport can Passive Transport can

also occur as …also occur as … Facilitated diffusion which…

Doesn’t require energy

Uses transport proteins to move high to low concentrationBiological connection:Biological connection:Glucose Glucose or Protein in the form of or Protein in the form of amino acidsamino acids move from blood move from blood into a cell.into a cell.

In cells proteins (amino acids) are Critical In cells proteins (amino acids) are Critical to Membrane Function because of 4 main to Membrane Function because of 4 main

reasons which are…reasons which are…

22 Types of Transport Proteins are…Types of Transport Proteins are…• Channel proteins are embedded in the cell

membrane & have a pore for materials to cross

• Carrier proteins can change shape to move material from one side of the membrane to the other

Facilitated DiffusionFacilitated Diffusion

Molecules will randomly move Molecules will randomly move through the through the porespores in in Channel Channel ProteinsProteins..

Facilitated DiffusionFacilitated Diffusion

• SomeSome Carrier proteinsCarrier proteins do not extend through do not extend through the membrane.the membrane.

• TheyThey bond and drag bond and drag moleculesmolecules through the through the lipid bilayer and lipid bilayer and release them on the release them on the opposite side. opposite side.

Carrier ProteinsCarrier Proteins• Other carrier Other carrier

proteinsproteins change change shapeshape to move to move materials across materials across the cell the cell membranemembrane

Active TransportActive Transport

Requires energy or ATP

Moves materials from LOW to HIGH concentration

AGAINST concentration gradient

What is the name of this organelle?

• What is the mitochondria function?• How does it transport the materials it needs to function?

• Why is this important?

Moving the “Big Stuff”Moving the “Big Stuff”

Molecules are Molecules are moved out moved out of the cell by of the cell by vesiclesvesicles that that fusefuse with the with the plasma membrane.plasma membrane.

ExocytosisExocytosis-

moving things

out.

This is how many This is how many hormoneshormones are secreted and how are secreted and how nerve cells nerve cells communicate with one anothercommunicate with one another.

ExocytosisExocytosisExocytic vesicle Exocytic vesicle immediately after immediately after fusion with plasma fusion with plasma membrane.membrane.

Moving the “Big Stuff”Moving the “Big Stuff”Large molecules move materials into the cell by Large molecules move materials into the cell by

one of one of three forms of endocytosisthree forms of endocytosis.

PinocytosisPinocytosis

Most Most commoncommon form of endocytosis form of endocytosis.

Takes in Takes in dissolved dissolved molecules as a vesiclemolecules as a vesicle.

PinocytosisPinocytosis

• Cell forms an Cell forms an invaginationinvagination

• MaterialsMaterials dissolve dissolve in waterin water to be to be brought into cellbrought into cell

• CalledCalled “Cell “Cell Drinking”Drinking”

Example of Example of PinocytosisPinocytosispinocytic vesicles forming mature transport vesicle

Transport across a capillary cell (blue).

Receptor-Mediated EndocytosisReceptor-Mediated Endocytosis

Some Some integral proteins integral proteins have have receptorsreceptors on their on their surface to recognize & take in surface to recognize & take in hormones,hormones, cholesterolcholesterol, etc., etc.

Receptor-Mediated EndocytosisReceptor-Mediated Endocytosis

Endocytosis – Phagocytosis Endocytosis – Phagocytosis

Used to Used to engulf large particles engulf large particles such as such as food, food, bacteriabacteria, etc. into vesicles, etc. into vesicles

Called Called “Cell Eating”“Cell Eating”

Phagocytosis About to OccurPhagocytosis About to Occur

This is a picture of a White Blood Cell…

about to flow around and eat a Prokaryotic Bacteria

PhagocytosisPhagocytosis - Capture of a Yeast Cell (round yellow object ) bythe White Blood Cells (BLUE)extending found in the Immune System Cell

ExocytosisExocytosis The opposite of endocytosis is exocytosis. The opposite of endocytosis is exocytosis. Large molecules Large molecules that are that are

manufactured in the cell are manufactured in the cell are releasedreleased through the cell membrane through the cell membrane..

From inside the Cell out to the … cells environment which will then

distribute the molecule to the proper system to get rid of the material