T2: Transport across Cell Membranes

Question 1

what are the functions of the cell surface membrane?

Answer 1

Forms the boundary between the cell and its environment
Controls the movement of substances in and out of the cell

Question 2

phospholipid structure

Answer 2

hydrophilic head - interacts with water
hydrophobic tail - orientates itself away from water

fatty acid molecules repel water (non-polar/hydrophobic)
phosphate molecules attract water (polar/hydrophilic) - because of the charge on the phosphate
one fatty acid tail is saturated and the other is unsaturated

Question 3

describe how phospholipids are arranged in a plasma membrane

Answer 3

in a phospholipid bilayer
hydrophobic fatty acid tails to inside
hydrophilic phosphate heads to outside

Question 4

what is the ‘fluid mosaic’ model of the cell membrane?

Answer 4

‘fluid’ - phospholipid and proteins can move around within each layer so the membrane is flexible

‘mosaic’ - studded with proteins, arrangement varies

Question 5

what are the 2 types of proteins found in cell membranes?

Answer 5

intrinsic - within the bilayer from one side to the other, have hydrophobic portions in contact with hydrophobic tails

extrinsic - found on the surface of one side, hydrophillic

Question 6

what is the function of membrane proteins?

Answer 6

provide structural support
act as receptors for molecules (ext)
enzymes (ext)
act as channel and carrier proteins (intrinstic)

Question 7

what is the function of cholesterol in the membrane?

Answer 7

ONLY found in animal cell membranes

prevents loss of water and dissolved ions from the cell, stops the cell from being too leaky
adds strength and rigidity to the membrane, pulls together fatty acid tails of phospholipid reducing movement

Question 8

structure and function of glycoproteins and glycolipids

Answer 8

carbohydrate chain attached to a protein/lipid

provide membrane stability
act as cell-surface receptors for binding of molecules or recognition sites (cell-recognition)
also help cells to attach to each other (to form tissues)
glycolipids on the cell surface of RBC’s determine ABO blood groups

Question 9

define simple diffusion

Answer 9

the net movement of particles from an area of higher concentration to a region of lower concentration gradient (down the conc gradient)

doesn’t require energy
partially permeable

Question 10

how does concentration gradient affect diffusion?

Answer 10

the greater the concentration gradient, the faster the rate of diffusion

Question 11

how does particle charge + size affect diffusion?

Answer 11

the cell membrane contains a hydrophobic core so charged particles CANNOT pass through (water is an exception, small)

generally the smaller the particle the greater the rate of diffusion

Question 12

how does temperature affect diffusion?

Answer 12

the higher the temperature the faster the rate of diffusion because particles have more KE

Question 13

how does surface area of the membrane affect diffusion?

Answer 13

diffusion takes place faster if the membrane has a larger SA

Question 14

how does distance affect diffusion?

Answer 14

the shorter the diffusion distance, the faster the rate of diffusion

Question 15

name all the factors affecting diffusion

Answer 15

temperature
concentration gradient
particle charge + size
SA of the membrane
diffusion distance

Question 16

define facilitated diffusion

Answer 16

hydrophilic substances diffuse across the cell membrane via protein molecules
allowing them to cross without interacting with the hydrophobic centre of the phospholipid bilayer

Question 17

define osmosis

Answer 17

the diffusion of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane

Question 18

how does water potential gradient affect osmosis?

Answer 18

the higher the water potential gradient, the faster the rate of osmosis

as osmosis occurs the water potential gradient decreases so the rate of osmosis levels off over time

Question 19

how does thickness of membrane affect osmosis?

Answer 19

the thinner the membrane, the faster the rate of osmosis

Question 20

how does SA of membrane affect osmosis?

Answer 20

the bigger the SA the faster the rate of osmosis

Question 21

define active transport

Answer 21

the movement of particles into/out of a cell from a region of lower concentration to a region of higher concentration (against the conc gradient) using ATP and protein pumps

requires ATP
involves water soluble molecule (ions and polar)
carrier proteins act as pumps

Question 22

what are all the factors affecting osmosis

Answer 22

water potential gradient
thickness of membrane
SA of membrane

Question 23

how is active transport similar to facilitated diffusion?

Answer 23

a molecule attaches to the carrier protein
the protein changes shape moving the molecule across the membrane
the molecule is released

Question 24

how is active transport different to facilitated diffusion?

Answer 24

active transport requires energy from the hydrolysis of ATP

active transport moves substances from low to high concentrations, facilitated diffusion does the opposite

both use carrier proteins, only f.diffusion uses channel proteins

Question 25

what are co transporter proteins?

Answer 25

a type of carrier protein
bind to 2 molecules at once, the conc gradient of one of the molecules is used to move the other against the concentration gradient

Question 26

describe the co transport system for glucose and amino acids

Answer 26

Sodium ions are actively pumped out by the Na+K+ATPase pump using ATP.
Sodium ions inside the cell are at a lower concentration than in the lumen.

Sodium moves into the cell from the lumen down its concentration gradient through a carrier protein.
Glucose moves in with it against its concentration gradient, creating a high concentration inside the cell.

Glucose moves into the blood by facilitated diffusion from a high concentration inside to a low concentration in the blood where it is quickly removed by the flow, maintaining a concentration gradient.

Question 27

what factors affect active transport?

Answer 27

speed of individual carrier proteins
number of carrier proteins
the rate of respiration in a cell (availability of ATP), if respiration is inhibited then active transport can’t take place