Cell Membranes, Channels, and Transporters

What do higher ratio mean for membrane surface area to cell volume ratio

Higher means more efficient movement of resources, heat, and waste

What are the roles of the plasma membrane

protection, shape, holding cell contents, cell recognition, cell identity, attachment,cell communication, selective barrier, cell movement

How much of the cell volume is organelles

50%

What do membranes allow organelles to do

Carry out unique functions

Role of membranes in reactions

They are sites of reaction like ETC in mitochondria or phospholipid generation in the ER

Which ER has ribosomes

rough ER

Where are membrane lipids synthesized

smooth ER

where are proteins synthesized

rough ER

What do ER-derived vesicles fuse with

Golgi apparatus or plasma membrane

What do golgi apparatus and vesicles do

redistributes lipids before they reach the plasma membrane

How thick is plasma membrane

5-10 nm

What is plasma membrane composed of

It’s a tripartite structure made up of 2 hydrophilic layers (leaflets) sandwiching a hydrophobic core. Also contains transmembrane proteins and other proteins/glycolipids

Is membrane lipid to protein ratio the same for all membranes

No, 80% lipid for myelin membrane versus 25% lipid for IMM

What’s fluid mosaic model

fluid indivates that molecules diffuse laterally, and mosaic refers to the many types of molecules residing in the plasma membrane

What happens to phospholipid molecules in water

It forms bilayers because hydrophilic head is in contact with water and hydrophobic tails of the 2 layers are in contact → the plasma membrane separates extracellular from cytosol

What does it mean for something to be amphipathic

contains both hydrophobic and hydrophilic parts

How many carbon atoms does each tail of a phospholipid contain

14-24 carbon atoms

Unsaturated hydrocarbon tail

has double bond → more space between lipid molecules

What’s phosphatidylcholine

Most common phospholipid (has choline attached to the phosphate group)

What do cis (on the same side) arrangement of hydrogens cause

forms double bond → kink in one chain → adds to fluidity of membrane

In which ways can and cant phospholipids move

can flex, rotate, and diffuse (can’t flip flop)

What occurs to membrane at higher temperatures

Faster diffusion which means the membrane is more fluid

What do cells insert into the membrane in response to higher temperatures

They may insert phospholipid molecules with longer tails so they move more slowly or they insert phospholipid molecules with fewer double bonds so they pack more tightly together, decreasing fluidity

When is cholesterol increased in membrane and what does increasing cholestrol do

It is increased in response to higher temperatures and it fills spaces between tails which stiffens the bilayer and makes it less fluid, also widening the bilayer

Where are phospholipids assembled

The cytosolic side of the smooth ER

How are phospholipids distributed in the ER membrane

Randomly distributed

How are phospholipids able to be assembled int he smooth ER

There are enzymes embedded in ER membrane

Process by which phospholipids are assembled in the ER

Phospholipid synthesis adds to the cytosolic half of the bilayer of the ER, and then scramblase catalyzes the transfer of RANDOM phospholipids from one monolayer to another equally, which leads to symmetric growth of both halves of the bilayer

What happens to phospholipids after assembly in the ER

Shipped to Golgi

What happens to delivery of new membrane from the ER in the Golgi

The new membrane from the ER enters lipid bilayer of Golgi apparatus and then flippase catalyzes the trasnfer of SPECIFIC phospholipids to the cytosolic monolayer

Is phospholipid distribution in golgi and other cell membranes symmetric and how does that affect the membrane

No they are asymmetrical between the two sides which curves the membrane

What occurs to the phospholipid after Golgi

Golgi-derived vesicles fuse with plasma membrane, and the phospholipids in the golgi lumen becomes part of the outer leaflet of the plasma membrane.

What is glycosylation and where does it occur

Addition of sugars to lipids and proteins, occurs in the golgi lumen

What happens to orientation of Golgi membrane and associated protiens at the plasma membrane

The orientation is maintained

Whats the distribution at the plasma membrane

asymmetrical, phospholipid heads differ between extracellular and cytosolic sides

Where are glycolipids found and how are they made

outer leaflet only → they are added to lipids in the golgi lumen (glycosylation)

What’s a glycolipid composed of

lipid + oligosaccharide (<15 sugar units)

What’s a glycoprotein composed of

protein + oligosaccharide (<15 sugar units)

what’s a proteoglycan composed of

protein + longer polysaccharide

What’s glycocalyx composed of

glycolipid+glycoprotein+proteoglycan

What does glycocalyx do

protexts surface and serves in cell-cell recognition

Where does glycocalyx occur

only on external surface of plasma membrane

hat are the two types of membrane proteins

Integral membrane proteins and peripheral membrane proteins

What are integral membrane proteins

attach cells to each other and to the extracellular matrix, and form channels, transporters and receptors (ex, transmembrane, monolayer-associated, lipid-linked)

What are peripheral membrane proteins

attached by weak bonds and can be removed by gentle procedures, leaving bilayer intact (ex: protein-attached)

Which molecules can readily diffuse across artificial lipid bilayers (aka no proteins)

small molecules and nonpolar molecules (like O2 and CO2)

How does glucose move across artificial lipid bilayers (aka no proteins)

Diffuses slowly (in membranes, most glucose transport is via transporters)

Similarities between channels and transporters

both are integral membrane proteins, both are selective for certain soluted

Difference between channel and transporter/pump/permease/carroier

Channels: hydrophilic pore which allow for faster transfer of soluted between the in and out of the cell

Transporters: morph (undergo conformational changes) upon binding to solute to trasnfer them across the membrane

What is simple diffusion

just crossing through the cell membrane

Do both channels and transporters allow passive transport/facilitated diffusion

Yes

Is passive transport agaisnt the gradient

No it follows the gradient

Do both transporters and channels allow active transport

No, only transporters do

What does active transport need

Requires energy like ATP to pump molecules because it is going agaisnt the concentration gradient

Which way does passive transport move

Area of high concentration to area of low concentration (down the concentration gradient) → no energy needed

Which way does active transport move

molecules move from area of low concentration to area of high concentration (up the concentration gradient) → requires energy

What are are the two conformational states in a transporter that mediates passive transport

(1) binding sites for the solute are exposed on the outside of the membrane

(2) the same sites are exposed on the other side of the membrane

What happens if concentration gradient and voltage work in the same direction

membrane potential increases passage across membrane for pos ions which enter the cell readily

the electrochemical gradient is high

What happens if concentration gradient and voltage work in the opposite directions

positive ions cross membrane due to the concentration gradient but are slowed by the membrane potential

electrochemical gradient is small

Whats electrochemical gradient

concentration gradient + membrane potential

Basis of membrane potentials

When the charges are balanced on each side, membrane potential is 0, but if pos ions cross membrane it sets up a nonzero membrane potential

What is cell membrane potential normally between in mV

-80 to -40 mV

What’s membrane potential

voltage difference across a membrane due to a slight excess of pos ions on one side and neg ions on the other side

3 types of transporters/pumps

uniport, symport, antiport

uniport

transports only one solute

symport

transports multiple solutes in same direction

antiport

transports multiple solutes in opposite directions

Similiarity between symport and antiport

Both coupled transport by gradient-driven pumps

Example of trasnporters changin shape in passive transport

glucose transporter

Whats the glucose transporters

transmembrane protein that switches between different conformations

What is flow of glucose in or out of cell depend on

concentration gradient

What happens when theres high concentration of blood glucose

Glucose binds to exterior site and is released into the cell

What happens when theres low concentration of blood glucose

glucose binds to interior site and leaves cell

What does it mean if glucose is electrically uncharged

The concentration gradient alone drives the passive transport

the 3 ways transporters perform active transport

Gradient-driven pump, ATP-driven pump, light-driven pump

What’s Gradient-driven pump

the movement of one moledule down its concentration gradient is coupled to movement of another molecule up its concentration gradient

What’s ATP-driven pump

movement of one molecule up its concentration gradient is coupled to hydrolysis of ATP to ADP (ex; proton pump on membrane of lysosome pumps protons agaisnt electrochemical gradient)

What’s light-driven pump

Movement of one molecule up its concentration gradient is coupled to light energy (ex; bacteriorhodopsin pumps protons agaisnt the electrochemical gradient)

Whats the glucose-Na+ symport protein

pump that can oscillate randomly between different configurations (occluded-empty, outward-open, occluded-occupied, inward-open)

What happens to glucose-Na+ pump when both glucose and sodium enter

Changes to occluded-occupied state

What happens when glucose-Na+ symport protein changes to inward-open

Glucose and sodium enter the cytosol

Is glucose-Na+ symport protein a gradient-driven pump and why

yes because import of Na+ (down its cocnetration gradient) drives glucose import (agaisnt its concentration gradient)

What’s the Na+/K+ pump

An antiport that uses a lot of ATP to maintian homeostasis (lwo intracellular sodium and high intracellular potassium)

What does hydrolysis of ATP to ADP do to NA+/K+ ATPase

changes its conformation which then pumps out Na+ and pumps in K+

Electrogenic pump meaning

Active transport. membrane protein that consumes ATP to move ions across cell membranes resulting in net transfer of charge and the generation of an electrical gradient (membrane potential)

How many sodium and potatssium ions go in and out of Na+/K+ pump

3 sodium and 2 potassium

Relation between Na+/K+ pump and Glucose/Na+ pump

TWhen sodium leaves the Na+/K+ pump, it then re-enters the cell and as it does that, it also helps import glucose into the cell

Why does sodium re-enter the cell after leaving

Becuase of. concentration gradient and voltage gradient

Why does cell need to control concentration of Na+ and K+ acorss its membrane

It’s improtant for cell to maintain high concentration of Na+ on extracellular side because movement of Na+ into the cell is coupled to the movement of otehr important molecules into the cell

It’s important for the cell to maintain high conentration of K+ on the cytoplasmic side of the membrane for proper functioning of the nervous system

How long does. na+/K+ pump process take

around 10 millisecs

What types of transporters do gut epithelial cells use

All 3

How do gut epithelial cells use symport transporters

glucose and Na+ are both needed in oral rehydration therapy to replace loss of salts

How do gut epithelial cells use antiport transporters

sodium and potassium

How do gut epithelial cells use uniport transporters

allows glucose to leave cell passively due to concentration gradient

How are gut epitherlial cell transporters maintained

in separate domains (apical surface versus basolateral surface) by tight junctions

What’s osmosis

the diffusion of water through the lipid bilayer and membrane channels

Why does osmosis occur

keep equilibrium

Whats the primary conduit for water moving across membranes

Aquaporin channels

How much water does kidney reabsorb because of aquaporin channels

99%

Which is faster, pumps or gated/ion-selective channels

channels, a single one can transfer >100 million ions/sec which is 10^5 faster than pumps