membrane structure and function. membrane function membranes organize the chemical activities of...
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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
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.
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
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
• 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 Phosphorylation2 Transport3
Proteinchanges shape
Protein reversion4
Phosphatedetaches
Bulk Flow
• Vesicles are used to transport large particles across the PM.– Requires energy
• Types:– Exocytosis– Endocytosis
• Phagocytosis, pinocytosis, receptor-mediated
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.
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