04-27-16: Lecture 9 Membrane Structure and Function

All Membranes are semipermeable

Proteins can move about the membrane – not a static structure

Fluid Mosiac Model

http://www.youtube.com/watch?v=Qqsf_UJcfBc 04-27-16: Lecture 9 Membrane Structure and Function

All Membrane are semipermeable Fundamental unit is phospholipid It allows some substances to cross it more easily than others

Proteins can move about the membrane – not a static structure

Fluid Mosiac Model Proteins move back and forth

Proteins inside the lipid bilayer should have non-polar side chains (R groups)

Interactions with hydrophobic interior

Importance of Fluidity Adjust the fluidity by changing the degree of saturation

Cholesterol is also helpful in maintaining fluidity 04-27-16: Lecture 9 Membrane Structure and Function Fission of Membranes Fusion of Membranes

Examples: Examples: Vesicle leaving ER or leaving Golgi Vesicle from ER fusing to Golgi Division (cytokinesis) fertilization

Endocytosis Virus infection Exocytosis Membrane Structure and Function04-27-16: Lecture 9

Membrane Interaction – an Up-close view

Apposition Hemifusion Full fusion (content mixing) Membrane Structure and Function04-27-16: Lecture 9

Membrane Interaction – an Up-close view

Invagination Hemifusion Separation 04-27-16: Lecture 9 Membrane Structure and Function Endocytosis Brings nutrients into the cell Brings signals into the cell Invagination of the plasma membrane - Vesicle fission

Phagocytosis Brings foods into cell  vesicle  (digested)

Pinocytosis Brings soluble components into cell  water  04-27-16: Lecture 9 Membrane Structure and Function

Receptor mediated Endocytosis Involves proteins in plasma membrane – called a receptor transmembrane

Binds a molecule (ligand) on the outside of the cell (e.g. sugar)

Clusters on cell surface – a pit is formed

Then coated pits formed – by attachment of clathrin (a protein) inside cell

Invagination  form vesicle  coat falls off  vesicle can go to its destination 04-27-16: Lecture 9 Membrane Structure and Function Endocytosis Receptor mediated Endocytosis

•Ligand • •Receptor

 RECEPTOR-MEDIATED ENDOCYTOSIS •Receptors cluster Coat protein  Receptor Coated vesicle •Invagination 

•Recruit clathrin (coat protein) Coated pit  Ligand •Pinch off vesicle (fission) Coat  protein Transmission •Clathrin falls off EM of a coated pit

Plasma membrane 0.25 µm 04-27-16: Lecture 9 Membrane Structure and Function Permeability of Membranes Most permeable:

CO2, CH4, O2

Least permeable: Sugars, amino acids, proteins 04-27-16: Lecture 9

Diffusion Rate of diffusion is proportional to concentration of a solute

Osmosis is the diffusion of water across a selectively permeable membrane Isotonic solution: equal concentrations

Hypertonic solution: cell in a solution w/ higher solute concentration

Hypotonic solution: cell has greater solute conc than solution. 04-27-16: Lecture 9 Diffuson

One solute

Net diffusion Net diffusion Equilibrium

Two solutes

Net diffusion Net diffusion Equilibrium

Net diffusion Net diffusion Equilibrium 04-27-16: Lecture 9 Membrane Structure and Function Types of proteins that reside at membranes

Integral membrane Proteins – example of structure

Extracellular side N-terminus

Note: N-term (amino) and C-term (carboxyl) can be on either side and same side depending how the protein is inserted into membrane

C-terminus CYTOPLASMIC a Helix Figure 7.8 SIDE 04-27-16: Lecture 9 Membrane Structure and Function Types of proteins that reside at membranes

Integral membrane Proteins

•Penetrate the hydrophobic core

•Transmembrane – span the entire lipid bilayer - ~10-20 amino acids in length

Peripheral Proteins ~20 a.a •Loosely bind to plasma membrane

•Typically by interacting with integral membrane proteins

•Some peripheral proteins can partially embed in plasma membrane 04-27-16: Lecture 9 Membrane Structure and Function Integral membrane Proteins Six major functions

1. Transport

ATP

Enzymes 2. Enzymatic Activity

Signal

3. Signal Transduction 04-27-16: Lecture 9 Membrane Structure and Function Integral membrane Proteins Six major functions

4. Cell Recognition Glyco- protein

5. Intercellular joining

6. Attachment of the to the extracellular matrix 04-27-16: Lecture 9 Internal Cytoskeleton

•Microtubules (MT) Polymers of tubulin

• (MF) Polymers of •Non-covalent

•Intermediate filaments Polymers of keratin

4˚ structure : protein-protein interactions which make long chains

•Cell shape

•Rigidity/flexibility

•Transport “roadway”

•Movement 04-27-16: Lecture 9 Internal Cytoskeleton

•Microtubules •Microfilaments •Intermediate filaments

•Cell Shape (compression- •Cell Shape (tension- •Cell Shape (tension- resisting) bearing) bearing)

•Cell •Cell shape changes! • anchorage

•Chromosome movement in •Cell motility cell division •Muscle contraction •Organelle movement (vesicle movement thru •Cell Division ) 04-27-16: Lecture 9 Cytoskeleton 04-27-16: Lecture 9 Internal Cytoskeleton

Microtubules: cell transport and motility

•Central assembly point for MT in the cell is called the (MT organizing center)

•Motor proteins move along MT

•Kinesin move things away from the nucleus

•Dynein move things towards the nucleus

•Cell motility – Flagella, or cilia – use Dynein motor

Show Kinesin Animation Show Kinesin Animation

See Video links on website : migration 04-27-16: Lecture 9 Internal Cytoskeleton

Microfilaments: cell shape and motility

•Changes in cell shape is related to motility

•Three types of cell shape

•Microvilli – projections on surface – increase surface area

•Lamellipodia – membrane ruffles help sense environment – and direct movement

•Filopodia – like microvilli but less stable – also sense environment (can turn into lamellipodia) 04-27-16: Lecture 9 Actin-based motility – Filopodia and Lammellipodia of a neuron

5 uM Betz et al., 2009 See Video links on website : neuron migration 04-27-16: Lecture 9 Actin-based motility Actin polymerization at the leading edge (lamellipodia) 04-27-16: Lecture 9 Actin-based motility – neuron migrating past another neuron

See Video links on website : neuron migration 04-27-16: Lecture 9 Linking the Extracellular Matrix (ECM) to the Cytoskeleton – SUPPORT! ECM made of: •Glycoprotein (proteins modified with sugars) •collagen (most predominant)

•Proteoglycans Plus Integrin (integral membrane protein) •fibronectin

EXTRACELLULAR FLUID Polysaccharide Collagen molecule A proteoglycan complex Carbo- hydrates

Core protein Fibronectin

Proteoglycan Plasma molecule membrane Integrins

CYTOPLASM Integrin Micro- filaments Figure 6.29 04-27-16: Lecture 9 Linking the Cells together by Cytoskeleton – Why?

Fluid Mosaic Model •proteins are not stationary - THEY MOVE in the MEMBRANE

Tight Junction: Physical Barrier to proteins •Link neighboring cells together so components Can’t get thru.

•Anchored by cytoskeleton

•Barrier to movement in membrane

•Have to send proteins to the right membrane surface Tight Junction: Physical Barrier to proteins 04-27-16: Lecture 9 •Link neighboring cells together so components Can’t get thru. 04-27-16: Lecture 9 Moving molecules across a membrane: Transport

Diffusion: Entropy at work - molecules move from high to low concentration

•Small and nonpolar molecules can freely diffuse across membrane (CO2, O2, etc)

•Protein dependent diffusion

•Facilitated Diffusion channel protein through which small molecules (such as water) can pass.

Carrier protein - integral membrane protein switches between 2 conformation states – moving molecules across as the shape of protein changes

•Movement in Both directions – WHY? •Depends on the concentration gradient!! 04-27-16: Lecture 9 Moving molecules across a membrane: Transport

Facilitated Diffusion -

EXTRACELLULAR FLUID •Example of a channel protein

Channel protein Solute

Figure 7.15

•Example of a carrier protein

Carrier protein Solute

Figure 7.15 04-27-16: Lecture 9 Moving molecules across a membrane: Transport

Facilitated Diffusion Rate of transport of Rate 04-27-16: Lecture 9 Moving molecules across a membrane: Transport

Active Transport :transport of a molecule against its concentration gradient

•A way to concentrate molecules in the cell – against conc. Gradient

•Involves a transporter or pump

•Uses Energy

•Primary active transport

•Uses ATP as energy (ATP  ADP + Pi) •Conformational change in pump due transfer of phosphate to protein (phosphorylation)

•Secondary Active transport •Energy is provided by a concentration gradient Previously made using ATP! •Co-transporter protein •Example: Sugar-H+ pump which moves high of H+ and sugar together

Don’t forget Allosteric regulation! 04-27-16: Lecture 9 Cell Signaling

Phosphorylation: Why is it important in signaling?

from ATP Serine O Threonine OH + HO P O- Tyrosine O-

Enzyme (Kinase) H20

O O P O- O- 04-27-16: Lecture 9 Moving molecules across a membrane: Transport

Active Transport :transport of a molecule against its concentration gradient

•Primary active transport •Example – Sodium-Potassium Pump!!

•Sodium-Potassium Pump exchanges sodium (Na+) for potassium (K+) across the plasma membrane

•Membrane Potential •Voltage across their plasma membranes •Voltage – is electrical potential energy – separation of opposite charges •Cytoplasm of cells is negative compared to extracellular fluid •Unequal distribution of cations and anions

•Membrane potential is like a battery that effects the traffic of all charged molecules.

•Cell is negative so this membrane potential helps to drive transport of cations into cell

•ELECTROCHEMICAL GRADIENT – two forces: chemical and electrical 04-27-16: Lecture 9 Sodium-Potasium Pump: Maintenance of membrane potential

1 Cytoplasmic Na+ binds to [Na+] high 2 Na+ binding stimulates the sodium-potassium pump. [K+] low phosphorylation by ATP. Na+ Na+ + Na+ Na Na+

+ ATP + [Na ] low P Na + CYTOPLASM [K ] high ADP

Na+ Na+

Na+

3 K+ is released and Na+ K+ 4 Phosphorylation causes the sites are receptive again; protein to change its conformation, the cycle repeats. K+ P expelling Na+ to the outside.

K+

K+ K+ K+ 5 Loss of the phosphate 6 Extracellular K+ binds to the restores the protein’s protein, triggering release of the original conformation. Phosphate group. 04-27-16: Lecture 9 Mechanism of a Action Potential

Resting Membrane Potential 04-27-16: Lecture 9

Depolarization – initial phase of an action potential Na+ rushes in to make inside of cell more positive 04-27-16: Lecture 9 Repolarization – later phase of the action potential K+ (potassium rushes out) Restore resting membrane potential

Na+ and K+ channels are voltage gated – meaning they open and close at specific membrane potentials!! 04-27-16: Lecture 9 Propagation of the Action Potential

Along an

Afterwhich the sodium-potasium pump restores high K+/low Na+ inside and high Na+/low K+ outside 04-27-16: Lecture 9 Moving molecules across a membrane: Transport

Active Transport :transport of a molecule against its concentration gradient

•Secondary Active transport

– + ATP H+ H+ – +

+ Proton pump H H+

– + + – H + + H Diffusion + Sucrose-H+ of H cotransporter H+

– +

– + Sucrose

Figure 7.19 04-27-16: Lecture 9 Moving molecules across a membrane: Transport

Passive Transport Active Transport

ATP

Diffusion Facilitated Diffusion