Chapter 5 – Membranes: The Interface Between Cells and Their Environment

Bio 190A

The two primary structural components of membranes are ___ and ___; membranes perform ___ functions

phospholipids and proteins; many

Carbohydrates may be attached to membrane ___ and ___, forming ___ and ___

lipids and proteins; glycolipids and glycoproteins

Phospholipid bilyer

framework of the membrane

Phospholipids are ___ molecules

amphipathic (hydrophobic “tails” face in, hydrophilic “heads” face out)

In phospholipids, the two ___ (halves of bilayer) are ___

leaflets; asymmetrical

Fluid-mosaic model

describes the membrane as a mosaic of lipid, protein, and carbohydrate molecules, where the lipids and proteins can move relative to each other within the membrane

Proteins are categorized based on their

association with the membrane

The three protein categories are

transmembrane proteins, lipid-anchored proteins, peripheral membrane proteins

Transmembrane proteins

span both leaflets of the membrane

Lipid-anchored proteins

an amino acid of the protein is covalently attached to a lipid

Peripheral membrane proteins

noncovalently bound to regions of other proteins or to he polar portions of phospholipids

Both transmembrane and lipid-anchored proteins are

integral membrane proteins (have a portion that is integrated into the hydrophobic region of the membrane)

Membranes are ___, as lipids and proteins can move in ___ (within the plane of the membrane)

semifluid; 2D

Lipid rafts

anchor certain proteins when lipid strongly associate with each other

The ___ ___ of phospholipids affect fluidity

biochemical properties

Length of the nonpolar tails (14-24 carbons)

shoter tails are less likely to interact —> more fluid membrane

Presence of double bonds

puts a kink in the lipid tail, making it harder for neighboring tails to interact and making the bilayer more fluid

Presence of cholesterol (in animal cells)

tends to stabilize membranes; effects vary depending in temperature

An experiment conducted in 1970 verified

the lateral movement of transmembrane proteins

Mouse membrane protein H-2 fluorescently labeled

• Cells at 0°C —> label stays on mouse side

• Cells at 37°C —> label moves over entire fused cell

Depending on cell type, ___% of membrane proteins may be restricted in their movement

10-70%

Membrane trasport

key function of membranes

The plasma membrane is

selectively permeable (allows the passage of some ions and molecules but not others)

Substances can cross the membrane in three general ways:

simple diffusion, facilitated diffusion, or active transport

___ transport does not require and input of energy, where as ___ transport does require energy

passive; active

The phospholipid bilayer is a barrier to

hydrophilic molecules and ions due to its hydrophobic interior

The ability of solutes to cross the bilayer by simple diffusion depends on

size, polarity, charge

Size

small molecules diffuse faster than large

Polarity

nonpolar molecules diffuse faster than polar

Charge

noncharged molecules diffuse faster than charged

Living cells maintain a

relatively constant internal environment that is different from their external environment

Transmembrane/concentration gradient

present when the concentration of a solute is higher on one side of a membrane than the other

Electromechanical gradient

a dual gradient with both electrical and chemical components (ion gradients)

Solute gradient affect

the movement of water across membranes

There are three options for how solutions on opposite sides of a membrane relate to each other (typically, the solution outside the cell is compared to the solution inside the cell)

Isotonic (same solute concentration), hypertonic (more solute), hypotonic (less solute)

There is an inverse relationship between solute and water concentration

more solute —> less free water

less solute —> more free water

Osmosis

the diffusion of water across a membrane

Water moves, from high to low, down its gradient

area of more water —> area of less water

which corresponds to

hypotonic (less solute) —> hypertonic (more solute)

If the extracellular fluid is hypotonic,

a plant cell with take up a small amount of water, but the cell wall prevents osmotic lysis

Some freshwater microorganisms live in

extremely hypotonic environments

Water consistently moves

into the cell by osmosis

Transport proteins

transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes

Two classes based on transport protein structure:

channels, transporters

Channels

provide and open passageway that can facilitate the diffusion of hydrophilic molecules or ions

Gated

transition between open and closed states of channels is based on regulatory signals

In contrast to transporters, channels do not have

a specific binding site (pocket) for their solutes

Transporters (carriers)

bind their solute in a hydrophilic pocket and undergo a conformational change that switches the exposure of the pocket from one side of the membrane to the other

Transporters provide the principal pathway for

uptake of organic molecules, such as sugars, amino acids, and nucleotides; they are also involved in expelling various waste material out of cells

Transporters are named according to the number of solutes they bind and the direction they transport those solutes

Uniporter (singles solute moves in one direction), symporter (two solutes move in the same direction), and antiporter (two solutes move in opposite directions)

Active transport

the movement of a solute across a membrane against

Primary active transport

directly uses energy (typically released from ATP hydrolysis) to transport a solute against its gradient

Secondary active transport

involves the use of energy stores in a pre-existing gradient to drive the active transport of another solute

Na⁺/K⁺-ATPase

an antiporter that actively transports Na⁺ and K⁺ against their gradients using the energy from the ATP hydrolysis

Ion pumps

maintain ion gradients that drive many important cellular processes; cells have many different types in their membranes

Cells invest a tremendous amount of their energy (up to 70%) into

ion pumping

Intercellular channels

allow direct movement of substances between adjacent cells (in addition to channels and transporters)

Gap junctions

abundant in tissues where cells need to communicate with each other (e.g. cardiac muscle)

Six membrane proteins called ___ assemble to form a ___, which, when of adjacent cells, align to form a channel

connexins; connexon

Gap junctions allow the passage of

ions and small molecules (amino acids, sugars, and signaling molecules)

Plasmodesmata

channels connecting the cytoplasm of adjacent plant cells; similar in function to gap junctions but different in structure

The plasma membrane of one cell is coninuous with

the plasma membrane of an adjacent cell, forming a pore that permits diffusion of small molecules between cells

Desmotubule

connects ER membranes of adjacent cells

Endocytosis and exocytosis are

mechanisms of vesicular transport that move large material into or out of cells

Exocytosis

materials inside the cell are packages into vesicles and excreted to the extracellular environment

Endocytosis

the plasma membrane invaginates (folds inward) to form a vesicle that brings substances into the cell

Three types of endocytosis:

1. Receptor-mediated endocytosis

2. Pinocytosis

3. Phagocytosis

Receptor-mediated endocytosis

uses receptor proteins to bring in specific cargo

Pinocytosis

primarily brings in fluid, allowing cells to sample the extracellular environment

Phagocytosis

involves bringing in very large particles (e.g. a bacterial cell); only some cells are phagocytes

Animals are multicellular; to become multicellular, cells must be

linked together

Anchoring junctions

link cells to each other and to the ECM

Cell adhesion molecules (CAMs)

are integral membrane proteins that participate in forming these junctions

Two types of CAMs:

cadherins, integrins

Anchoring junctions are grouped into four main categories:

adherens junctions, desmosomes, hemidesmosomes, focal adhesions

Adherens junctions

connect cells to each other, use cadherins, and bind actin filaments

Desmosomes

connect cells to each other, use cadherins, and bind intermediate filaments

Hemidesmosomes

connect cells to ECM, use integrins, and bind intermediate filaments

Focal adhesions

connect cells to the ECM, use integrins, and bind actin

Tight junctions

form a tight seal between cells and prevent material from leaking between adjacent cells