Membrane Structure and Function (Ch. 7)

Question 1

About how thick is the plasma membrane?

Answer 1

About 8 nm

Question 2

What is selective permeability?

Answer 2

A property of biological membranes whereby it allows some substances to cross it more easily than others

Question 3

What are the most important ingredients of membranes?

Answer 3

Lipids and proteins mostly; some carbohydrates

Question 4

What are the most abundant lipids in the membrane?

Answer 4

Phospholipids

Question 5

Are phosopholipids hydrophilic, hydrophobic, or both?

Answer 5

Both – this is called being amphipathic. They have a hydrophilic region and a hydrophobic region

Question 6

How are phospholipids and proteins arranged in the membranes of cells?

Answer 6

• They are arranged in the fluid mosaic model.
• The membrane is a fluid structure with a mosaic of various proteins embedded in or attached to a phospholipid bilayer.

Question 7

Envision:

• Fibers of extra-cellular matrix
• Glycoproteins
• Carbohydrates
• Glycolipids
• Integral proteins
• Membrane proteins
• Cholesterol
• Microfilaments of cytoskeleton

Answer 7

Question 8

What primarily holds together membranes?

Answer 8

Mostly hydrophobic interactions. Much weaker than covalent bonds.

Question 9

How and how often do the proteins and lipids in the bilayer shift around?

Answer 9

Laterally – very often, 10^7 times per second

Vertical flip-flop - rarely, once per month

Question 10

What causes kinks in the tails of bilayer lipids, and what effect do those kinks have? Why?

Answer 10

Kinks are caused by double bonds that create unsaturated hydrocarbons. The kinks lower the solidification temperature of the membrane because the layers cannot pack together as closely as saturated tails.

Question 11

What does cholesterol do to the membrane?

Answer 11

Cholesterol is a “fluidity buffer” that makes it resist changes in membrane fluidity caused by temprature.

Question 12

Describe evolution and its impact on cell membrane lipid composition.

Answer 12

Adaptations to environments: Fish living in cold environments evolved bilayers with lots of unsaturated lipid tails; bacteria in super hot geysers developed bilayers with lipids that prevented excessive fluidity.

Question 13

What are the two major populations of membrane proteins?

Answer 13

1. Integral proteins
2. Peripheral proteins

Question 14

Describe integral proteins.

Answer 14

Integral proteins penetrate the hydrophobic interier.

Most are transmembrane proteins, spanning the whole membrane.

Others go only part way into the hydrophobic interior.

Question 15

Describe peripheral proteins.

Answer 15

Peripheral proteins are not embedded in the bilayer; they are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins.

Question 16

What are the six major functions performed by proteins of the plasma membrane?

Answer 16

1. Transport
2. Enzymatic activity
3. Signal Transduction
4. Cell-cell recognition
5. Intercellular joining
6. Attachment to the cytoskeleton and extracellular matrix

Question 17

What membrane macromolecule is most crucial to cell-cell recognition?

Answer 17

Membrane carbohydrates are the most important.

Question 18

Describe the structure of membrane carbohydrates. How long are they?

Answer 18

They are usually short, branched chains of fewer than 15 sugar units. They might be bonded covalently to lipids (–> glycolipids) or proteins (–> glycoproteins)

Question 19

Are membranes sided? If so, how does that sidedness arise?

Answer 19

The asymmetrical arrangement of proteins/lipids/carbs in the membrane is determined as the membrane is being built by the ER and Golgi apparatus

Question 20

Describe the steps of membrane synthesis

Answer 20

1. Membrane proteins and lipids are synthesized in the ER.
2. In Golgi, glycoproteins’ carbs are modified, and lipids get carbs, becoming glycolipids
3. Glycoproteins, glycolipids, and secretory proteins are transported in vesicles to the membrane
4. Vesicles fuse with the membrane and release the secretory proteins (exocytosis)

Question 21

What moves easily across the plasma membrane in both directions?

Answer 21

small molecules, ions, nonpolar molecules (because they are hydrophobic)

Polar molecules have trouble because they are hydrophilic, Charged molecules/atoms also have trouble because they’ll have a surrounding shell of water.

Question 22

How can some specific ions and a varity of polar molecules pass through the cell membrane? How does it work?

Answer 22

transport proteins (aka channel proteins) that span the membrane.

They act as a sort of tunnel through the membrane

Question 23

What transports water (polar…) across the membrane?

Answer 23

transport proteins called aquaporins

Question 24

What are carrier proteins?

Answer 24

Carrier proteins are a type of transport protein that hold onto their passengers and change shape enough to send them across.

Question 25

What is diffusion?

Answer 25

Diffusion is the movement of molecules to spread out evenly into an available space, caused by thermal energy of the molecules

Question 26

Define osmosis

Answer 26

the diffusion of free water across a selectively permeable membrane. aims to establish water balance.

Question 27

Define tonicity, hypertonic, hypotonic, isotonic

Answer 27

concentration and membrane permability more concentrated less concentrated equally concentrated

Question 28

Compare the effects of different tonicities in animal/plant cells without/with a cell wall

Answer 28

Question 29

Define facilitated diffusion

Answer 29

When a molecule (particularly a polar molecule or an ion) diffuses passively through the membrane with the help of transport transmembrane protein.

Question 30

What is the difference between channel proteins and carrier proteins?

Answer 30

Channel proteins are corridors; carrier proteins actually change shape to bring the molecule across the membrane.

Question 31

What are ion channels?

Answer 31

Ion channels are channel proteins that channel ions

Question 32

What are gated channels?

Answer 32

Gated channels are a type of ion channel (a type of channel protein) that open or close in response to a stimulus. Stimulus could be electrical or when a specific third-party substance binds to the channel

Question 33

Define active transport

Answer 33

using energy to pump a solute across a membrane against its gradient.

Question 34

Describe the nature of the K+ and Na+ concentrations in an animal cell

Answer 34

An animal cell needs way more K+ compared to Na+, so it needs sodium-potassium pumps to maintain this

Question 35

Describe the steps of a sodium-potassium pump

Answer 35

1. Cytoplasmic Na+ binds to the pump, because it is in its high-Na+-affinity shape 2. Na+ binding stimulates phosphorylation by ATP (now ATP is a P -- on the protein -- and ADP) 3. Phosphorylation makes the protein change shape, releasing Na+ outside 4. The new shape likes K+, which binds on the outside and triggers the release of the phophate inside 5. The loss of the phosphate group restores original shape 6. K+ is released; cycle repeats

Question 36

What is membrane potential?

Answer 36

The voltage across a membrane caused because the cytoplasmic side is negative in charge compared to the extracellular side.

Question 37

Which way does the membrane potential favor movement (re: voltage across the cell)

Answer 37

Because the inside of the cell is negative compared with the outside, the potential favors the passive transport of cations (+) into the cell and anions (-) out of the cell.

Question 38

What drives diffusion of an ion across a membrane? What is this called?

Answer 38

1. Chemical Force (ion's concentration gradient) 2. electrical force (effect of the membrane potential on the ion's movement) Together, called the elctrochemical gradient

Question 39

What is an electrogenic pump?

Answer 39

A transport protein that generates voltage across a membrane. Think: Na-K pump. Pumps out three Na+ for 2 K+ in; that's a net +1 getting pumped out.

Question 40

What is the main electrogenic pump in plants, fungi, and bacteria?

Answer 40

Proton pump. Actively transports protons out of the cell. Transfers positive charge from cytoplasm to extracellular solution.

Question 41

What is cotransport?

Answer 41

Cotransport is when a pump uses active transport and then the molecule that was pumped out diffuses down into the cell again, but this time via a cotransporter that brings another molecule along with it.

Question 42

What is exocytosis? What are some examples of its use?

Answer 42

How the cell secretes certain biomolecules by fusing vesicles with the plasma membrane Ex.: 1. Pancreas cells that make insulin secrete insulin 2. neurons release neurotransmitters 3. plant cells delivering proteins and carbs from Golgi vesicles outside the cell when they are making walls

Question 43

Define endocytosis

Answer 43

Takes in biomolecules by forming new vesicles from the plasma membrane.

Question 44

What are the three types of endocytosis?

Answer 44

1. phagocytosis ("cellular eating") 2. pinocytosis ("cellular drinking") 3. receptor-mediated endocytosis

Question 45

What is an example of receptor-mediated endocytosis?

Answer 45

Taking in cholesterol for membrane synthesis and the synthesis of other steroids