Cell Plasma Membranes

These flashcards cover key concepts related to cell plasma membranes, including structure, functions, proteins, and processes involved in cellular transport and communication.

Phospholipid Bilayer

A double layer of phospholipids that makes up the fundamental structure of cell membranes.

Fluid Mosaic Model

A model describing the structure of the plasma membrane as a mosaic of various proteins embedded in a fluid bilayer of phospholipids.

Integral Proteins

Proteins that penetrate the hydrophobic core of the lipid bilayer; many are transmembrane proteins that span the membrane.

Peripheral Proteins

Proteins that are loosely bound to the surface of the membrane or to integral proteins.

What are the functions of the membrane?

1) Compartmentalization

2) Scaffolding for biochemical activities

3) Providing a selectively permeable barrier

4) Transporting solutes

5) Responding to external stimuli

6) Intra- Inter-cellular interaction

7) Energy transduction

What are the main types of membrane lipids?

1) Phosphoglycerides

2) Sphingolipids

3) Cholesterol

Phosphoglyceride

Contains a glycerol with 2 hydrophobic fatty acid chains attached and a hydrophilic phosphate with another charged or polar head group attached at the glycerol’s third hydroxyl.

Sphingolipids

derivatives of sphingosine, an amino alcohol with a hydrocarbon chain attached.

What is a ceramide?

A type of sphingolipid formed by the attachment of a fatty acid to sphingosine.

What is a sphingomyelin?

A class of sphingolipids that contain a phosphocholine or phosphoethanolamine head group

What happens to a ceramide when a sugar is attached?

it becomes a cerebroside glycolipid

What happens to a ceramide when an oligosaccharide is attached?

it becomes a ganglioside glycolipid

Where are glycolipids important?

The nervous system, particularly the myelin sheath of nerve axons

What is the difference between animal glycolipids and plant/ bacterial glycolipids?

Animals are produced from Sphingosine-Based Ceramide, where plant/ bacteria Glycolipids have Sugars Attached to Phosphoglycerides Instead

Where are glycolipids found?

exclusively in the outer leaflet of the plasma membrane

What is cholesterol?

a series of 4 hydrophobic rings with the terminal -OH group at the membranes surface

What does cholesterol do?

Cholesterol stabilizes the plasma membrane's fluidity and structure, helping to maintain membrane integrity.

What way do carbohydrates face in the membrane and inside the cell?

Membrane carbohydrates face the outside world (on outside of the cell), and internal carbohydrates face away from the cytosol

What ways do oligosaccharides join glycoproteins?

either N-linked or O-linked

What role do carbohydrate additions play?

mediate the interactions of the cell with its environment and allow the sorting of proteins into different membrane compartments

What determines ABO blood types

oligosaccharides attached to red blood cell membrane lipids and/or proteins

What is membrane sidedness?

asymmetryof the lipid bilayer and distribution of proteins, where different components are localized to either the outer or inner leaflets.

Integral proteins

span the membrane and are then transmembrane proteins, they make up 30% of all the cells proteins

Peripheral proteins

proteins that are not embedded in the lipid bilayer but are loosely attached to the exterior or interior surfaces of the membrane.

Lipid-anchored proteins

proteins that are covalently attached to lipids within the membrane, allowing them to associate with the lipid bilayer.

How do integral proteins make a seal that maintains the membranes permeability barrier?

They are surrounded by membrane lipids that interact with the hydrophobic parts of the membrane

What are the two techniques used to study integral proteins?

Freeze-fracture to show the embedded proteins using an electron microscope and immuno-gold labeling can identify which side of the membrane a component lies

Directed mutagenesis and cross-linking studies can be used to?

determine which parts of a protein are next to which others,

What are peripheral proteins?

Peripheral proteins are proteins that are loosely associated with the membrane surface, either through interactions with integral proteins or through binding to the polar head groups of lipids.

What are the possible reasons for peripheral proteins?

May Be Cytoskeletal Elements which Support the Membrane and Act as Anchors for Integral Proteins, May Act as Enzymes or Specialized Coats (COP1), or may be factors that transmit transmembrane signals

Lipid-anchored proteins

are proteins covalently attached to lipids in the membrane, allowing them to associate with the membrane surface.

GPI-anchored proteins

are a type of lipid-anchored protein that are tethered to the membrane via a short oligosaccharide to phosphotidylinositol, different receptors, enzymes or cell adhesion proteins

The physical state of membrane lipids is known as

fluidity or viscosity, when warmer the bilayer becomes more fluid and less rigid, allowing for greater movement of proteins and lipids within the membrane. When cooler the lipids become a crystalline gel and solidify where little movement occurs among the proteins and lipids.

What does transition temperature of membrane lipids depend on?

The saturation state, more saturated lipids lead to higher transition temperatures compared to unsaturated lipids whose double bonds and kink the chain lower the transition temperature.

How does cholesterol affect fluidity?

prevents close packing of fatty acid chains but also interferes with their mobility

How is membrane fluidity important for the function of the membrane?

it allows clusters of enzymes to assemble at particular sites to form intracellular junctions and facilitates the movement of molecules and proteins across the membrane, enhancing cellular communication and transport.

How does fluidity play a role in membrane assembly?

cell growth, division, movement, junction formation, secretion, and endocytosis all depend on and will only occur if the membrane has some fluidity

What do acyl-transferase enzymes do?

reshuffle unsaturated fatty acids (w/double bonds) so that some phospholipids have 2 unsaturated chains which greatly lowers the transition temperature

Lipid rafts

microdomains in the plasma membrane enriched with cholesterol and sphingolipids, essential for signaling and protein sorting.

What are the special enzymes called that move lipids from one leaflet to another?

Flippases, floppases, and scramblases are special enzymes that facilitate the movement of lipids between the inner and outer leaflets of the membrane, maintaining lipid asymmetry.

Spectrin

consists of 2 subunits (α and β) wound around each other and is attached indirectly to the membrane through non-covalent bonds to another protein called ankyrin

What gives RBCs the flexibility they need?

spectrin-actin cytoskeleton network

What shape is PE (phosphatidylethanolamine) responsible for?

occupies a conical shape in the membrane which allows the cell to produce a curved membrane structure

What shape is PC (phosphatidylethanolamine) responsible for?

occupies a cylindrical shape in the membrane

Which lipid/lipids are associated with the outer layer of the membrane?

The less fluid SM (Sphingomyelin) and PC (phosphatidylethanolamine)

which lipids are associated with the inner layer of the membrane?

The more fluid PE (phosphatidylethanolamine), PS (phosphatidylserine), and PI (phosphatidylinositol)

The inner leaflet of lipid rafts is composed of?

cholesterol and saturated glycerophospholipids that form the lipid rafts with lipid-anchored proteins

The outer leaflet of lipid rafts is composed of?

cholesterol, sphingolipids, glycoproteins, and PC (phosphatidylethanolamine) that are present with GPI anchored proteins

The membrane is most permeable to?

gases

The membrane is least permeable to?

charged ions and large polar molecules.

diffusion

the process by which molecules move from an area of higher concentration to an area of lower concentration. (down concentration gradients)

Active transport

the process of moving molecules across a membrane against their concentration gradient, requiring energy. occurs until the concentration everywhere is about the same

What maintains the membrane potential of the cell?

The movement of K+ ions out of the cell, down their concentration gradient

What is the partition coefficient?

the extent to which a solute partitions between water and hydrophobic octanol

what is turgor pressure?

Turgor pressure is the pressure exerted by the fluid inside the central vacuole of plant cells against the cell wall, providing structural support and maintaining cell shape.

what is plasmolysis?

Plasmolysis is the process in which plant cells lose water in a hypertonic solution, leading to the detachment of the plasma membrane from the cell wall.

How are aquaporins able to let water flow without allowing protons through?

by temporarily breaking the hydogen bonds between water molecules with positively charged amino acid side chains which bind the oxygen atoms

What are the important molecules that are moved by facilitated diffusion?

sugars, polar solutes, amino acids

What does the sodium pump do?

Moves 3 Na+ ions out and 2 K+ ions in

What do V-type ATPase do?

They transport protons across membranes, creating acidic environments in organelles.

What does the Na+/Ca2+ exchanger do?

it moves 3Na+ ions into the cell in exchange for 1Ca2+ out,

What do dendrites do?

receive incoming information from external sources

What do axons do?

conduct outgoing information away from the cell towards target cells

What are terminal knobs?

where information is transmitted to the target cell

What is the only open channel in a resting nerve cell?

potassium leak channels

What is the membrane potential required to oppose potassium’s concentration gradient?

-91mV

What is depolarization?

The process in which a cell's membrane potential becomes more positive, typically turning from a negative resting potential toward zero or even positive values, often in response to stimulus.

Where are action potentials generated?

axon hillock at the beginning of the axon

What is the purpose of the myelin sheath?

it is comprised of layers of lipids to prevent movement of ions across the membrane

Where are most of the Na+ channels located?

They are primarily located at the nodes of Ranvier in myelinated axons, facilitating rapid action potential propagation.

What is saltatory conduction?

Saltatory conduction is the process by which action potentials jump from one node of Ranvier to another in myelinated axons, increasing the speed of neural signal transmission.

presynaptic cell

conducts impulses towards the synapse

postsynaptic cell

receives the impulses

What are the two effects of neurotransmitters?

1) cation selective channels allow mostly Na+ to enter the postsynaptic cell, leading to depolarization; increasing the likelihood of a further action potential being generated

2) anion selective channels allow mainly Cl- to enter, causing hyperpolarization. decreasing the likelihood of further action potential