fluid mosaic model

Question 1

state the 3 functions of cell surface membrane / phospholipid bilayer

from tutorial 2C

Answer 1

1. barrier between cytoplasm and aqueous surroundings
2. barrier to water soluble / hydrophilic molecules / ions
3. partially permeable membrane allowing small and lipid soluble/non-polar molecules to cross it

Question 2

describe the cell surface membrane

Answer 2

consists of bilayer of phospholipids with different proteins attached or embedded with the bilayer. carbohydrate chains attached to phospholipids or proteins are found on the side of membrane in contact with external environment of cell

Question 3

contrast peripheral and integral proteins

Answer 3

peripheral: attached to membrane surface
integral: extend completely through lipid bilayer

Question 4

describe the structure of a phospholipid and explain why it is amphipathic

Answer 4

a molecule of phospholipid consists of 2 fatty acid chains linked to 2 of the 3 carbons of glycerol molecule, and a phosphate group attached to the last carbon of glycerol molecule.

phosphate group is electrically charged and hence hydrophilic. fatty acid chains are hydrophobic.
therefore amphipathic due to distinctive hydrophobic regions.

Question 5

describe and explain why phospholipid molecules arrange themselves

Answer 5

hydrophilic heads are in contact with aqueous environment, hydrophobic tails are shielded from aqueous environment.
this is because cytosol and external environment are aqueous

Question 6

what does “fluid” and “mosaic” refer to?

Answer 6

fluid: phospholipid and proteins moving freely and laterally within phospholipid bilayer

mosaic: protein and cholesterol (only in animal cells) molecules that are randomly embedded and scattered among the phospholipid molecules

Question 7

name and state the functions of the 5 different proteins found in cell surface membrane

Answer 7

1. channel proteins and carrier proteins: allow passage of specific ions and molecules across the membrane
2. enzymes that catalyse reactions
3. receptors that have specific binding sites, allowing chemical messenger to bind to the protein, resulting in relay of message to the inside of the cell
4. glycoproteins: play a role in cell-cell recognition and adhesion. they consist of carbohydrate chains covalently bonded to proteins and are found on the side of membrane facing the exterior of cell

Question 8

name and state the functions of 2 non-protein components in cell surface membrane

Answer 8

1. glycolipids: consist of carbohydrate molecules covalently bonded to phospholipid molecules. the carbohydrate molecules are attached to side of membrane facing exterior of cell. they are involved in cell recognition
2. cholesterol: regulates the fluidity of cell surface membrane / increases membrane fluidity at low temperatures and decreases membrane fluidity at high temperatures

Question 9

how do fatty acids affect membrane fluidity

Answer 9

• unsaturated fatty acids: increases membrane fluidity
• saturated fatty acids: decreases membrane fluidity

Question 10

define simple diffusion

Answer 10

movement of molecules from a region of higher concentration to a region of lower concentration across the partially permeable cell surface membrane without the help of transport proteins

Question 11

define facilitated diffusion

Answer 11

movement of molecules from a region of higher concentration to a region of lower concentration across the partially permeable cell surface membrane with the help of transport proteins

Question 12

define active transport

Answer 12

movement of molecules from a region of lower concentration to a region of higher concentration across the cell surface membrane. carrier proteins and energy from the hydrolysis of ATP are required for process

if it’s a carrier protein: energy is required for carrier protein to change its shape to transport the molecule across the cell surface membrane

Question 13

what can the cross the cell surface membrane by simple diffusion?

Answer 13

oxygen, carbon dioxide, nitrogen, and relatively small polar molecules like water and glycerol

Question 14

state examples of polar molecules

Answer 14

glucose, amino acids with polar R groups, ions

Question 15

why can’t polar molecules cross the cell surface membrane (without transport proteins)?

Answer 15

they are hence hydrophilic and unable to cross the hydrophobic core of phospholipid bilayer

Question 16

how do water molecules enter the cell? describe the process

Answer 16

water molecules pass through channel of aquaporins by osmosis. there is a net movement of water molecules from a region of higher water potential to a region of lower water potential through the partially permeable membrane.

Question 17

why are aquaporins necessary?

Answer 17

they allow water molecules to move more quickly across the hydrophobic core of cell membrane by providing a channel that is hydrophilic.

Question 18

why do cells that carries out a lot of active transport have more mitochondria?

Answer 18

it requires a lot of energy to constantly transport molecules by active transport

Question 19

define osmosis

Answer 19

net movement of water molecules from a region of higher water potential to a region of lower water potential through the partially permeable cell surface membrane