Membrane proteins and transport mechanisms

What are membranes made of in general

Lipids (phospholipids, glycolipids, sterols) + proteins + carbohydrates (small amount)

What makes up the backbone of membranes? describe its structure

Phospholipids

glycerol (3C) backbone + 2 fatty acids + negatively charged phosphate group attached to a highly polar organic alcohol

Describe implications of amphipathic properties of lipids

The hydrophilic and hydrophobic regions allow phospholipids to spontaneously align in a bilayer (tails are shielded, heads pointed outwards)

Why is a membrane fluid and flexible on its own (no cholesterol)

Since fatty acids tails are not strongly attracted to another

Can large uncharged molecules or ions pass through bilayer easily? why or why not?

No

In case of large molecules —> phospholipid molecules are tightly packed together

In case of charged molecules —> hydrophobic interior prevents passing

Can small uncharged polar molecules + nonpolar molecules pass through bilayer easily? Why or why not

Yes

Give examples of large uncharged molecules and charged ions

Glucose, sucrose

Na+, K+

Give examples of nonpolar molecules + small uncharged polar molecules

Steroids, CO2, O2

H2O

What is the easiest mechanism of transport for molecules across bilayer? Define

Simple diffusion

Definition: when particles move from areas of high solute concentration to low solute concentration (movement of molecules down a concentration gradient)

It is spontaneous and passive

Results in equilibrium

Give an example of simple diffusion

1. Movement of small, nonpolar molecules like O2 and CO2 in cellular respiration

O2 diffuses inside the cell (lower concentration O2 inside cell)

CO2 (made by mito respiration) difusses outside

2. gas exchange

O2 diffuses down concentration gradient from air in the alveoli into the capillaries which carry it into tissues, where O2 diffuses from erythrocytes into cells

CO2 diffuses from cells into the blood, carried to lungs, etc

What are 2 types of membrane proteins

Integral

• amphipathic (hydrophobic part embedded in membrane, hydrophilic part interact with phospholipids head or aq. environment)

• difficult to isolate (need to disrupt lipid bilayer)

• most are transmembrane, but can be found on one side of bilayer

Peripheral

• hydrophilic (interacts only with hydrophilic parts of integral membrane or with phospholipid heads)

• found on surface of membrane

• easier to remove from membrane

List 5 functions of membrane proteins

1. recognition

• most proteins have carbs attached as “tags”

• this allows distinguishing of self vs non self cells

2. reception

• membrane proteins have binding sites for hormones/neurotransmiter

• helps receive chemical signals

• binding triggers intercellular reactions

3. enzymes

• can show enzymatic activity

• example: glucose-6-phosphatase in ER allowing production of glucose for blood glucose levels

• found on interior and exterior

4. transport

• using either channel (transmembrane, has pores) or carrier (goes through conformational change) proteins

5. cell adhesion

• CAMs

• some protein types allow temporary or permanent junctions between cells (gap or tight)

What are the different types of membrane proteins (based on function) (list multiple)

1. hormone binding (has shapes that fit size of certain hormones, in which attachment between them changes shape of protein resulting in a message)

2. enzymatic

3. cell adhesion

4. cell to cell communication

5. channel

6. pumps for active transport

What are the 2 types of membrane transport.

1. active

• requires energy (released by an exergonic reaction like breakdown of ATP)

• substance moves against [ ] gradient

• molecules move from low [ ] to high [ ]

• equilibrium not reached

2. passive

• no energy

• substance moves from high [ ] to low [ ] 

• movement occurs along or down [ ] gradient

• energy for movement comes from Ek of particles

• if undisturbed, continues until equilibrium reached

What is osmosis

• type of passive transport

• movement of H2O (diffusion) across a selectively permeable membrane from low solute [ ] (HIGH water [ ]) to high solute concentration (LOW water [ ] )

  • water moves from hypotonic solution —> hypertonic solution (has higher [ ] solutes)

• the [ ] gradient allowing this to occur is a result of differing solute [ ] on either sides of membrane

*There is no net movement of H2O if a isotonic solution (has same [ ] solutes as another solution) is used

• continues until equilibrium

What allows the fast rapid movement of water across a selectively permeable membrane?

Aquaporins

• allow H2O to transport at much higher rates than simple diffusion

• they are:

  • integral proteins

  • tetrameric structure + 4 monomeric subunits

  • each subunit has a water channel (lined with specific hydrophilic side chains of AA residues specific to water and not ions)

  • H2O goes through subunits in single file

• bidirectional

• amount of AQP depends on volume of water needed to transport (Ex. more of them are in kidneys)

What is facilitated diffusion?

• movement of solutes down [ ] gradient using 2 types of integral proteins

  • channel

    • has pores allow specific sizes and charges of molecules to pass

    • has gates that open and close to stimuli

    • only carries hydrophilic molecules

  • Carrier

    • changes shape to carry a certain substance (usually ions) across a membrane —> has sites specific 

    • can carry along [ ] gradient (facilitated diffusion) or against (active transport)

    • carries hydrophilic + hydrophobic molecules

Give example of channel proteins

• Ion channels (K+, Na+)

• voltage-gated

• ligand-gated

What determines the selectivity of channel proteins

The binding sites of the hydrophilic AA side chains lining the channel + stimuli used

Give an example of carrier proteins

The GLUT transporter (helps transport glucose in RBC down [ ] gradient)

What makes cell membranes selectively permeable?

Use of channel and carrier proteins allowing specific ions to pass at specific times

What determines the rate of facilitated diffusion?

The number of carrier/channel proteins and the [ ] difference

What type of transport protein is mainly used in active transport?

Pump

What are functions of active transport, give example

• helps take up essential nutrients

  • example: uptake of glucose from intestine to epithelial cells in small intestine

• removes secretory materials from cell

• maintain correct [ ] ions in cell

Name example of active tranposrt

Na+/K+ pump

What are the 2 types of active tranposrt

1. direct

• energy released from exergonic (breakdown of ATP) reaction directly transports molecules across membrane

• transport proteins used are ATPase pumps

2. indirect

• movement of one solute down [ ] gradient drives the movement another another against [ ] gradient

Movement of molecules by simple diffusion depends on…

size + charge

Diffusion of simple small molecules is selective or not? Implications

No. Harmful or useful molecules can pass

Why do we call the membrane selectively permeable?

Due to facilitated diffusion + active transport

What is fluid mosaic model?  Who proposed it

Singer + nicolson

states that

• lipid bilayer is fluid (fatty acids + cholesterol)

• proteins are embedded in bilayer resulting in a mosaic

Describe glycolipids and glycoporteins and functions

Glycolipids

• amphipathic

• carb group (polar) extends to ECM

• nonpolar lipid embedded in bilayer

• 2 types: glycosphingolipid (derivatives of sphingosphine) + glycerol based lipid

• for membrane stability (forms H bonds with water)

Glycoproteins

• carb groups stick out to ECM

functions

• cell recognition

• cell adhesion (CAM)

• cell signalling

• for ABO blood groups

• carb groups form glycocalyx

Define glycocalyx

• sticky layer

• common in animal cells

• for cell adhesion, recognition, reception

• also found on bacterial and fungal cells (for protection and adhesion)

• in plant cells, only used to anchor cell membrane to cell wall

Membrane fluidity depends on what 2 things

• fatty acid composition

• cholesterol

Describe the implications of the fatty acid composition in membrane

• unsaturated —> ensures fluidity due to lower melting point, meaning it survives cooler temperatures

• saturated —> for stability, higher melting point, effective on higher temp

• at lower temp —> phospholipids move closer together making membrane gel like (thus, the kink is important by preventing too close packing)

Why is the lipid bilayer stable on its own

• phospholipids form H bonds with H2O, but can freely move

Give examples of the importance of fatty acid composition for fluidity

• cold blooded organisms or hibernating animals have more unsaturated fatty acids

Where is cholesterol found

• hydrophobic region in lipid bilayer

• not in plants

Why is cholesterol important, what is it

• for fluidity

• allows membranes to function at wider range of temps by interacting with fatty acid tails

• an amphipathic steroid

  • hydrophilic part: polar OH group

  • hydrophobic part: 4 steroid rings + HC side chain

• at higher temp —> stabilizes membrane and reduces permeability

• at lower temp —> prevents tight packing of FA tails preventing freezing

Where in the cell would you find more cholesterol

• in plasma membrane versus ER membrane (subject to more temp differences)

what transport mechanisms help move large molecules and large amounts of it across membrane

• endocytosis

  • pinching off of plasma membrane to enclose particulates in vesicle

  • vesicle enters cytoplasm

  • changes membrane shape

  • fluidity of membrane is important (Regions ends)

• exocytosis

  • bulk transport of material secreted outside of cell

  • vesicles moves to plasma membrane and fuses

Endocytosis and exocytosis are passive or active

Active

Give example of exocytosis

• proteins made in ribosomes in rough ER is packed into vesicles in the lumen

• vesicles fuses w/cis side of golgi

• protein is modified inside golgi and leaves on trans side in another vesicle

• vesicle fuses w/cell membrane

Give examples of endocytosis

• phagocytosis (intake of large solid particles)

  • in WBC (membrane projections, pseudopodia, surround the foreign particles to form a phagosome, food vacuole, before fusing w/lysosome to digest), amoeba

  • pinocytosis (intake of liquids)

  • receptor-mediated endocytosis

Why is membrane fluidity important in endo/exocytosis

• helps w/structural stability of vesicles in fusion/formation

Describe gated-ion channels and List examples

• allows ions to pass quickly

• gates open/close due to stimuli (voltage, ligand, mechanically)

• movement of ions through channels controls electrical potential across membranes

Describe voltage-gated ion channels w/example

example: voltage gated ion channels for nerve signalling

• at rest, neuron cell membrane has a potential differnece (inside is more negative)

• electrochemical gradient also exists across the membrane, driving force for ion movement

• opened by changes in membrane polarity

• example: Na+ and K+ channels

  • electrical stimulus opens and closes it

  • Na+ open first (diffuse from outside to inside) to depolarize membrane —> creates action potential that travels down nerve fiber

  • K+ open next (inside diffuses to outside) repolarization

  • Resting membrane potential is reestablished using Na+/K+ pump

  • defective repolarizqation can lead to death, etc

• voltage must hit threshold (min value) to open or close gates

• channels can undergo channel inactivation (particle blocks pore)

Describe neurotransmitter/chemically gated ion channel (ligand gated) w/example and application

• example: nicotinic acetylcholine receptor

• found at skeletal neuromuscular junctions

  • can bind to ACh, which results in a conformational change that opens the channel

• Na+ diffuse down [ ] grad, making interior of cell more positive (depolarization) —> this genereates impulse (membrane pot. changes)

• in a milisecond, cholinesterase breaks down ACh to close channel

• after depolarization, K+ channels open for repolarization to restore resting potential

application: this is used so impulses can be carried along connnected neurons for responses

• for muscle movement —> neurotransmitter is released between junction from nerve and muscle

• Myasthenia gravis —> produces antibodies binding to ACh reducing response to muscle movement

Give and describe an example of direct active tranposrt

Na+/K+ pump

• in cell mmebranes of all animal cells

• Na+/K+ ATPase is an enzyme that hydrolyzes ATP to generate action for this movement of ions against [ ] gradient

• animal cells have higher [ ] K+ inside vs. outside, and opposite for NA+

  • due to this difference, there is a membrane potential (difference in charge) and electrochemical gradient

What is the main purpose of Na+/K+ pump

• to maintain membrane potential (electrical charge difference between inside and outside of cell) —> after passage of nerve impulse

• does not directly create impulse

what is structure of Na+/K+ pump

• transmembrane protein

• has 3 binding sites for Na+, 2 for K+

Describe the 5 steps of Na+/K+ pump

1. the pump protein (w/ATP attached) binds to 3 intracellular Na+

2. binding of Na+ causes pump to split ATP (causing phosphorylation as Pi is attached to the pump)

3. this phosphorylation creates a conformational change releasing the Na+ —> this shape change results in a higher affinity for K+

4. 2 K+ extracellular bind to binding sites to detach Pi

5. release of Pi restores protein’s OG shape, releasing the K+

Give an example of indirect active transport

• transport of glucose into intestinal animal cells

  • There is higher concentration glucose outside cell vs. inside

  • ATP is used to move glucose against [ ] gradient, allowing Na+ and K+ to be transported by the same carrier protein

• 1. there is more Na+ outside than inside intestiinal cell

• Na+ and glucose bind to a transport protein

• Na+ passes through carrier down [ ] gradient, carrier captures this energy

• captured energy is used to transport glucose

*the carrier used is SGLT

*requires 2 protein pumps: ATP made by Na+/K+ pumps is needed for this

What are CAMs

• glycoproteins that help bind cells with other adjacent cells

• example: cadherins, integrins, selectins, immunoglobulin family

• cell connections help coordinated behaviour and with structure

  • desmosomes: form sturdy and flexible sheets of cells in organs like the heart (allowing stretch)

  • plasmodesmata: tubes connecting cytoplasm of adjacent plant cells for exchanging materials

• for cell junctions

What are cell junctions

• connect cells to each other for intracellular transport and communication

• help cell migration, cell proliferation, prevent unregulated movement of materials

• made using CAMs

• types

  • adhesive/anchoring junctions(desmosomes) = in epithelial and cardiac cells for cell-cell adhesion for structural stability and mechanical stress

  • tight junctions = epithelial cells, forming tight seal between 2 cells (prevent unregulated movement of molecuels)

  • gap = found in several cell types, the channels physically connect cells for movement of molecules (also called communicating junctions)