biology 7.1: Cellular membranes are fluid mosaics of lipids and proteins

Question 1

most abundant lipids in most membranes are

Answer 1

phospholipids

Question 2

A phospholipid is an ____________________, meaning it has both a hydrophilic (“water-loving”) region and a hydrophobic (“water-fearing”) region

Answer 2

amphipathic molecule

Question 3

A phospholipid bilayer can exist as a stable boundary between__________________________ because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water

Answer 3

two aqueous compartments

Question 4

Like membrane lipids, most membrane proteins are

Answer 4

amphipathic

Question 5

the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

Answer 5

fluid mosaic model

Question 6

life depends on plasma membranes, these thin films envelop all cells, separating the cell’s.

Answer 6

interior, or cytoplasm, from the surrounding extracellular environment

Question 7

But this edge of life is not

Answer 7

solid or static

Question 8

it regulates a steady flow of material into and out of

Answer 8

a cell and performs many crucial functions

Question 9

the structure of the plasma membrane, whose key components are

Answer 9

phospholipids and proteins

Question 10

Phospholipids form a bilayer in which their

Answer 10

nonpolar hydrophobic tails are oriented toward the interior of the membrane, and their hydrophilic phosphate heads face the watery environment on either side of the membrane

Question 11

suspended in and attached to this bilayer of phospholipids are

Answer 11

various proteins.

Question 12

biologist use a __________________ to describe a membrane

Answer 12

fluid mosaic model

Question 13

the kinky tails of many phospholipids, along with cholesterol, found in animal cell membranes,

Answer 13

keep the molecules from packing tightly.

Question 14

The membrane remains _________, and its components can drift about like party-goers elbowing their way through a crowded room.

Answer 14

fluid

Question 15

Just as a mosaic is formed by differently colored and shaped tiles, a membrane is

Answer 15

mosaic of proteins with different shapes and functions.

Question 16

Groups of proteins are often associated in long-lasting,

Answer 16

specialized patches, where they carry out common functions

Question 17

Researchers have found specific lipids in these patches as well and have proposed naming them

Answer 17

lipid rafts

Question 18

there is ongoing controversy about whether such structures exist in

Answer 18

living cells or are an artifact of biochemical techniques

Question 19

A membrane is held together mainly by

Answer 19

hydrophobic interactions, which are much weaker than covalent bonds

Question 20

a lipid may flip-flop across the membrane, switching from one

Answer 20

phospholipid layer to the other.

Question 21

Adjacent phospholipids switch positions about 10(small 7 above 10) times per second, which means that a phospholipid can travel about

Answer 21

2 µm—the length of many bacterial cells—in 1 second.

Question 22

A membrane remains fluid as temperature ___________ until the phospholipids settle into a closely packed arrangement and the membrane solidifies, much as bacon grease forms lard when it cools.

Answer 22

decreases

Question 23

The steroid ________________, which is wedged between phospholipid molecules in the plasma membranes of animal cells, has different effects on membrane fluidity at different temperatures

Answer 23

cholesterol

Question 24

At relatively high temperatures—at 37°C, the body temperature of humans, for example—cholesterol makes the membrane less fluid by restraining

Answer 24

phospholipid movement.

Question 25

cholesterol can be thought of as a ___________________ for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature.

Answer 25

“fluidity buffer”

Question 26

Membranes must be _______to work properly; the fluidity of a membrane affects both its __________________________________ to where their function is needed.

Answer 26

fluid, permeability and the ability of membrane proteins to move

Question 27

Somewhat like a tile mosaic, a membrane is a collage of different proteins, often clustered together in groups, embedded in the fluid matrix of the

Answer 27

lipid bilayer

Question 28

Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane’s

Answer 28

functions

Question 29

penetrate the hydrophobic interior of the lipid bilayer.

Answer 29

Integral proteins

Question 30

majority are ______________________, which span the membrane; other integral proteins extend only partway into the hydrophobic interior.

Answer 30

transmembrane proteins

Question 31

The hydrophobic regions of an integral protein consist of __________________________, typically 20–30 amino acids in length, usually coiled into helices

Answer 31

one or more stretches of nonpolar amino acids

Question 32

Some proteins also have ___________________that allow passage through the membrane of hydrophilic substances

Answer 32

one or more hydrophilic channels

Question 33

are loosely bound to the surface of the membrane, often to exposed parts of integral protein

Answer 33

Peripheral proteins

Question 34

On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the

Answer 34

cytoskeleton.

Question 35

A single cell may have cell-surface membrane proteins that carry out several different functions, such as

Answer 35

transport through the cell membrane, enzymatic activity, or attaching a cell to either a neighboring cell or the extracellular matrix

Question 36

Left: A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Right: Other transport proteins shuttle a substance from one side to the other by changing shape (see Figure 7.14b). Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

Answer 36

Transport

Question 37

A protein built into the membrane may be an enzyme with its active site (where the reactant binds) exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

Answer 37

Enzymatic activity

Question 38

A membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein

Answer 38

Signal transduction.

Question 39

Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell-cell binding is usually short-lived

Answer 39

Cell-cell recognition.

Question 40

Q
Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions

Answer 40

Intercellular joining.

Question 41

Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes

Answer 41

Attachment to the cytoskeleton and extracellular matrix (ECM).

Question 42

transport proteins may be carrier proteins that shuttle a molecule, such as

Answer 42

glucose across the membrane by changing shape

Question 43

other transport proteins provide channels through which specific

Answer 43

molecules or ions, such as sodium, can diffuse

Question 44

some transport proteins act as _________, using the energy of _____ to actively move molecules against their concentration gradient.

Answer 44

pumps, ATP

Question 45

what other types of functions do membrane proteins perform?

Answer 45

some transfer information, signaling molecules outside a cell may bind to receptor proteins, which transmit the message into the cell.

Question 46

example: when insulin binds to receptor on a cell, it signals the cell to take up

Answer 46

glucose

Question 47

provide support by connecting to the cell’s cytoskeleton and to the extracellular matrix

Answer 47

attachment proteins

Question 48

these attachments can also relay information about

Answer 48

external and internal conditions

Question 49

enable cells to recognize each other

Answer 49

glycoproteins

Question 50

the prefix glycol refers to

Answer 50

short chains of sugar molecules extending from the protein

Question 51

other membrane proteins can recognize these

Answer 51

ID tags

Question 52

the sorting of cells within a developing embryo relies on such

Answer 52

cell-to-cell recognition

Question 53

form long-lasting connections between cells

Answer 53

junction proteins

Question 54

example: holds muscle cells together

Answer 54

anchoring junctions

Question 55

create a leak-proof barrier between the cells lining the intestine

Answer 55

tight junctions

Question 56

some membrane proteins are enzymes, which may be arranged in

Answer 56

clusters that may carry out sequential reactions

Question 57

in addition to its plasma membrane, a eukaryotic cell is packed with

Answer 57

membranes enclosing a variety of organelles

Question 58

what might you conclude about the importance of membranes in the life of a eukaryotic cell?

Answer 58

internal membranes share many of the functions of the plasma membrane

Question 59

helps the human immunodeficiency virus (HIV) infect these cells, leading to acquired immune deficiency syndrome (AIDS).

Answer 59

protein called CD4 on the surface of immune cells

Question 60

Comparing their genes with the genes of infected individuals, researchers learned that resistant people have an unusual form of a gene that codes for an immune cell-surface protein

Answer 60

called CCR5.

Question 61

Further work showed that although CD4 is the main HIV receptor, HIV must also bind to CCR5 as a

Answer 61

“co-receptor” to infect most cells

Question 62

Cells recognize other cells by binding to molecules, often containing

Answer 62

carbohydrates, on the extracellular surface of the plasma membrane

Question 63

a lipid with one or more covalently attached carbohydrates

Answer 63

glycolipids

Question 64

However, most are covalently bonded to proteins, which are thereby

Answer 64

glycoproteins