Unit 1: Lipids, Membranes and Membrane Transport

Question 1

phospholipid (3)

Answer 1

• a type of lipid
• major component of the cell membrane
• made up of a glycerol backbone attached to a phosphate group and 2 fatty acids

Question 2

hydrophilic

Answer 2

• “water loving”

- describes a class of molecules with which water can undergo hydrogen bonding

Question 3

hydrophobic

Answer 3

• “water fearing”

- describes a class of molecules poorly able to undergo hydrogen bonding with water

Question 4

amphipathic

Answer 4

• having both hydrophilic and hydrophobic regions

Question 5

liposome

Answer 5

• an enclosed bilayer structure spontaneously formed by phospholipids in environments with neutral pH, like water

Question 6

bilayer

Answer 6

• a two layered structure of the cell membrane with hydrophilic “heads” pointing outward toward the aqueous environment and hydrophobic “tails” oriented inward away from water

Question 7

micelles

Answer 7

• spherical structure in which lipids with bulky heads and a single hydrophobic tail are packed

Question 8

How do amphipathic molecules react in a aqueous environment?

Answer 8

• they spontaneously arrange themselves into various structures where the polar head groups are on the outside interacting with water and the nonpolar tail groups come together on the inside away from water
• results from tendency of polar molecules to exclude nonpolar molecules

Question 9

How are lipids so dynamic?

Answer 9

• freely associate through extensive ID-ID interactions between fatty acid tails: weak interactions are easily broken and re-formed so lipid molecules can move within the plane
• lipids can rapidly rotate around vertical axis, and individual fatty acid chains are able to flex or bend

Question 10

Why is the membrane said to be fluid? (2)

Answer 10

• membrane lipids can move in the plane of the membrane
• degree of fluidity depends on which type of lipid make up the membrane: longer, saturated fatty acids are straight, tightly-packed, and have many interactions so the mobility of the cell is reduced, while unsaturated and shorter fatty acids introduce kinks reducing tightness and enhancing lipid mobility

Question 11

What is another type of lipid present in the cell membrane?

Answer 11

• cholesterol is a major component of of animal cell membranes
• cholesterol is amphipathic and structure allows it to insert itself into the lipid bilayer so that its head group interacts with the hydrophilic head, while the ring structure participated in interactions with the fatty acid chain

Question 12

How does cholesterol affect membrane fluidity?

Answer 12

• increases and decreases membrane fluidity depending on temperature to prevent dramatic transitions from fluid to solid state:
  1) at temperatures typically found in a cell, cholesterol decreases membrane fluidity because interaction of the rigid ring structure with the fatty tails reduces mobility of the phospholipids
  2) at low temperatures, cholesterol increases membrane fluidity because it prevents phospholipids from packing tightly with other phospholipids

Question 13

What are different functions of membrane proteins? (4)

Answer 13

• transporters: moving ions or other molecules across the membrane
• receptors: allow the cell to receive signals from the environment
• enzymes: catalyze chemical reactions
• anchors: attach to other proteins and help maintain cell structure and shape

Question 14

integral membrane proteins

Answer 14

• proteins permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself

Question 15

peripheral membrane proteins

Answer 15

• a protein temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions
• may be associated with either the internal or external side of the membrane

Question 16

transmembrane proteins (3)

Answer 16

• proteins that span the entire lipid bilayer
• most integral membrane proteins are transmembrane proteins
• composed of 3 regions: 2 hydrophilic regions, one protruding from each face of the membrane, and a connecting hydrophobic region that spans the membrane
• structure allows for separate functions and capabilities of each end of the protein (eg. hydrophilic region on external side can interact with signalling molecules and the hydrophilic region inside the cell often interacts with other proteins in the cytoplasm if the cell to pass along a message)

Question 17

fluid mosaic model

Answer 17

• a model that proposes that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of two types of molecules, lipids and proteins

Question 18

selectively permeable

Answer 18

• describes properties of the plasma membrane, which lets some molecules in and out freely, let others in and out only under certain conditions, and prevents other molecules from passing through at all to maintain homeostasis within the cell

Question 19

Describe membranes ability to act as a selective barrier

Answer 19

• hydrophobic interior prevents ions and charged polar molecules from moving across it
• macromolecules such as proteins and polysaccharides are too large to cross membrane on their own
• in contrast, gases and nonpolar molecules can move across lipid bilayer
• small uncharged polar molecules, such as water, are able to move through the lipid bilayer to a very limited extent

Question 20

How do macromolecules and charged molecules get across the cell membrane?

Answer 20

• protein channels and transporters in the membrane can greatly facilitate the movement of molecules, including ions, water, and nutrients that can’t on their own

Question 21

When does passive transport work to the cell’s advantage?

Answer 21

• if the concentration gradient supports nutrients needed tp take in and waste needed to be expelled

Question 22

active transport

Answer 22

• the “uphill” movement of substances against a concentration gradient requiring an inout of energy

Question 23

primary active transport

Answer 23

• active transport that uses the energy of ATP directly

Question 24

What are common names for pumps?

Answer 24

• proteins that pump molecules in different directions are called anitporters
• transporters that move 2 molecules in the same direction are called symporters or cotransporters

Question 25

electrochemical graident

Answer 25

• a gradient that combines the charge gradient and the chemical gradient of protons and other ions

Question 26

secondary active transport

Answer 26

• active transport that uses the potential energy of an electrochemical gradient to drive the movement of molecules

Question 27

facilitated diffusion

Answer 27

• diffusion through a membrane protein, bypassing the lipid bilayer

Question 28

diffusion

Answer 28

• random motion of molecules, with net movement occurring from areas of higher to lower concentration of the molecules

Question 29

osmosis

Answer 29

• net movement of a solvent, such as water, across a selectively permeably membrane toward side of higher solute concentration

Question 30

hypertonic (2)

Answer 30

• higher solute concentration outside the cell than inside the cell
• water leaves the cell by osmosis and the cell shrinks

Question 31

hypotonic (2)

Answer 31

• lower solute concentration outside the cell than inside the cell
• water moves into the cell by osmosis and the cell cell swells and may lyse

Question 32

isotonic

Answer 32

• same solute concentration inside and outside of cell

Question 33

contractile vacuoles (2)

Answer 33

• a type of cellular compartment that takes up excess water and waste products from inside the cell and expels them into the external environment
• useful for single-celled organisms

Question 34

turgor pressure

Answer 34

• pressure within a cell resulting from the movement of water into the cell by osmosis and the tendency of the cell wall to resist deformation

Question 35

What role does the cell wall play in plants, fungi, and bacteria

Answer 35

• helps maintain cell size and shape and provides structural support and protection for the cell
• because wall is rigid and resists expansion, it allows pressure to build up when water enters the cell

Question 36

What happens when a plant cell is placed in a hypertonic solution?

Answer 36

• water enters the cell by osmosis until the turgor pressure created by the cell wall resists being stretched and pushes back on the interior of the cell

Question 37

vacuole

Answer 37

• membrane-bound organelle present in some cells, including plants and fungal cells
• in some cases, it absorbs water and contributes to turgor pressure
• explain why plants cells are typically larger than animal cells and can store water, nutrients. ions and wastes

Question 38

What do plants wilt when dehydrated?

Answer 38

• loss of water from vacuoles reduces turgor pressure and cells can no longer maintain shape within the cell wall

Question 39

hydrophobic effect (3)

Answer 39

• water pushing the non-polar molecules together in order to maximize its own entropy (more stable)
• changes in enthalpy are negligible although the hydrophobic effect allows more water molecules to make stronger interactions (more stable)
• critical for formation of cell membranes, proteins, and DNA

Question 40

Why do phospholipids in water spontaneously form a bilayer?

Answer 40

• the initial state of phospholipids in water is less stable then the final bilayer state: change in free energy is negative because the entropy of water increases as the bilayer is formed

Question 41

What are some lipid formations?

Answer 41

• bilayer
• single sheet
• liposome
• micelle

Question 42

concentration ratio

Answer 42

• molecules inside/molecules outside

Question 43

aquaporin

Answer 43

• protein channel that allows water to cross the plasma membrane more readily than by diffusing through the lipid bilayer