Structure and function of Plasma Membranes: Chapter 5

What is the quote of the author from our book that describes the plasma membrane?

“A cells plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment”

What are the foundation of the cell membrane?

Lipids and proteins

Most lipids in the membrane are what?

Phospholipids

The plasma membrane is _ permeable. What does that mean?

Selectively; It allows some things in and out the cell but not all things.

What is the fluid mosaic model of cell membranes?

That the cell membrane is a flexible sea of phospholipids and all the other molecules floating in it

Membranes are dynamic. What does that mean?

They move a lot

They often move laterally, or rotate, but they rarely move transversely (flip from one phase of a membrane to another

Do proteins move in the plane of the membrane?

Yes

What famous experiment helped show that proteins actually do move in the plane of the membrane?

Frye and Edidin

They fused a mouse and human cell and made a hybrid cell.

They saw that the proteins on the cell membrane of the two cells spreaded out and got mixed. Showing that proteins do actually move

What is another experiment that helps show that proteins do actually move?

There were stained proteins with green dye on a cell membrane and they bleached a square on the cell membrane making all proteins white

The white square started to turn a little bit green after a while meaning that the stained proteins with green dye moved it the bleached square

Membrane fluidity is affected by what?

Temperature (The hotter it is the more fluidity) (The colder it is the less fluidity)

Lipid Content

Unsaturated fatty acids make membrane more fluid

While the saturated fatty acids makes the membrane less fluid (viscous)

Cholesterol: Makes membranes more fluid at low temperatures and less fluid at high temperatures; “fluidity buffer” (not found in bacterial membranes). Homeostasis

The fluidity of the membrane affects what?

The permeability the more fluid the more permeability

And the functions of membrane proteins

Some organisms can change the lipid component in membranes in response to temperature

When it gets cold, they add more what?

If they get hot, they add more?

Unsaturated fatty acids; Saturated fatty acids

There are differences in lipid concentration of membranes. What are they?

Between different sides (faces of the same membrane

Between different membranes in the same cell

Between different cells in the same organism

Between different organisms

There are membrane protein locations. Name them and describe them.

Integral proteins: penetrate hydrophobic core of lipid bilayer

Transmembrane proteins: completely span the membrane (is a integral protein)

Peripheral proteins: loosely bound to surface

What are the different functions of membrane proteins?

Intercellular joining

Enzymatic activity

Transport (active / passive)

Cell-cell recognition

Anchorage / attachment

Signal transduction

What would you predict about CCR5 that would allow HIV to bind to it? And how could a drug interfere with this binding?

They have to be complementary in shape

A drug can change the shape of the receptor so HIV cannot bind anymore removing the CCR5

Some proteins cannot move freely within the membrane. Give a few examples.

In prokaryotes when the proteins are attached to one another if one protein wants to move than all of them would have to move

In Eukaryotes

What it is attached to the extracellular matrix, cytoskeleton or attached to proteins on another cell

Proteins can also not move through tight junctions

Membrane carbohydrates

They are usually attached to something else (like they are going to be a glycolipids [attached to lipids] or glycoproteins [attached to proteins])

They are also involved in cell-cell recognition

• Sorting of cells in embryos

• Immune recognitiion

The plasma membrane is _ permeable.

Selectively

What is the real barrier of the cell membrane during diffusion?

The hydrophobic tails (or hydrophobic core)

What can quickly pass through the lipid bylayer?

Hydrophobic/ nonpolar molecules

What can pass slowly through the lipid bilayer?

Hydrophillic/ Polar

What can pass very slowly through the liquid bilayer?

Ions

What is diffusion?

The tendency of molecules to spread out evenly over available space; molecules move down their own concentration gradient

NOTE: molecules keep moving, even after equilibrium is reached ( molecules are in constant, random motion

Also remember that each substance is diffusing down its own concentration gradient

What is passive transport?

Diffusion across cell membrane; cell does not expend energy

Simple diffusion across a membrane

Facilitated diffusion

Passive transport through protein channels or carrier proteins

Allows water and other hydrophilic substances to move across membrane

These are specific they only allow one thing to cross

What are the proteins that are involved in facilitated diffusion?

Chanel proteins: provide corridors for specific molecule/ion

Aquaporins: channel proteins for water molecules

Ion channels: Channel proteins for specific ions

Gated channels: chemical or electrical signal causes ion channels to open or close

Carrier proteins: change shape to translocate substances across membrane

What are some factors that can affect diffusion?

Concentration gradient

Mass of molecules diffusing

Temperature

Solvent density

Solubility (hydrophobic or hydrophilic)

Surface area and thickness of membrane

Distance travelled 

Osmosis

Diffusion of water across a membrane

What are the 3 types of tonicity?

Isotonic solution

Hypertonic solution

Hypotonic solution

Isotonic solution

Same solute concentration as inside cell

Hypertonic solution

Higher solute concentration than inside cell

Hypotonic solution

Lower solute conentration than inside cell

Label the tonicities

Label the toxicities

What is osmoregulation?

Contractile vesicles that controls aquaporins for allowing water in or out

Active transport

Substances pumped across membrane against concentration gradient

Requires cellular energy

Carrier proteins

Specific

Allows cells to maintain/establish concentrations of substanses

Usually against the concentration gradient

Electrochemical gradient

Combined gradient of concentration and electrical charge (ions)

What is primary active transport and secondary active transport?

Primary active transport - Moves ions across a membrane and creates a difference in charge cross the membrane (electrochemical pump)

Secondary active transport - moves material by an electrochemical gradient established by primary active transport

What are the three types of transporters? Describe them.

Uniporter - only goes in one direction and one compound

Symporter - only goes in one direction and two compounds

Antiporter - goes in opposite directions and two compounds

What are two types of bulk transport?

Endocytosis and exocytosis

Endocytosis

Transport of large molecule into cell

Plasma membrane invaginates to form vesicles

What are the three types of endocytosis?

Phagocytosis (cell eating) Not specific

Examples consist of lysosomes and immune cells

Pinocytosis (cellular drinking) Not specific

Bring in fluid inside the cell

Receptor-mediated endocytosis: Specific

Receptor ligand interactions

Exocytosis

Transport of large substances out of cell

Vesicles fuses with plasma membrane