12.7 Transport across membranes

Question 1

What are phospholipids similar in structure to?

Answer 1

Triglycerides

Question 2

What is the difference between phospholipids and triglycerides?

Answer 2

One of the fatty acid groups is replaced with a phosphate group
The glycerol combines with two fatty acids, instead of two

Question 3

Which way do the hydrophobic fatty acid tails point?

Answer 3

Towards the middle, away from the water

Question 4

Which way do the hydrophilic heads point?

Answer 4

Towards the outside of the cell, towards the water
They make contact with the cytoplasm and extracellular fluid

Question 5

How would you describe the cell surface membrane?

Answer 5

Semi permeable

Question 6

What does the cell surface contain?

Answer 6

Proteins, glycoproteins, phospholipids, cholesterol and carbohydrates

Question 7

Why is it called a fluid structure?

Answer 7

The phospholipid bilayer is constantly moving

Question 8

Why is it called a mosaic structure?

Answer 8

The protein molecules are unevenly distributed throughout the membrane

Question 9

Function of phospholipids

Answer 9

Bilayer allows lipid soluble molecules to pass through via simple diffusion but prevents small polar molecules (ions) and larger molecules (glucose)

Question 10

Function of cholesterol

Answer 10

Decreases permeability, increases stability of membrane
More cholesterol = less fluidity
Embedded in bilayer

Question 11

Function of channel proteins

Answer 11

Only allow specific charged ions across the membrane via facilitated diffusion
Proteins have specific tertiary structure and can only transport molecules with complimentary shape to channel protein
Eg. Na+ ions can only pass through sodium channel protein

Question 12

Function of carrier proteins

Answer 12

Aid the transport of ions, polar and large molecules by facilitated diffusion and active transport

Question 13

Function of receptor proteins

Answer 13

Act as receptors to complementary molecules
Eg hormones such as insulin bin do the insulin receptor protein which increases the cells permeability to glucose

Question 14

Function of enzymes

Answer 14

Shape of active site is complimentary to substrate, allowing them to form enzyme-substrate complexes
Embedded within cell membrane

Question 15

Function of glycoproteins

Answer 15

Composed of carbohydrates and protein
On the outer surface of cell membrane and important in cell recognition
Produced in the Golgi body

Question 16

Function of aquaporins

Answer 16

Special type of channel proteins specific to water
A cell with many aquaporins will be permeable to water and carry out osmosis easily

Question 17

Diffusion definition

Answer 17

The net movement of molecules from an area of higher concentration to an area of lower concentration across a partially permeable membrane

Question 18

What type of molecules move via simple diffusion?

Answer 18

Non polar
Small
Lipid soluble molecules
O2, CO2 and oestrogen

Question 19

What is Ficks law?

Answer 19

Rate of diffusion = surface area x concentration gradient / diffusion distance

Question 20

Factors that increase diffusion

Answer 20

Higher surface area
Higher concentration gradient
Higher diffusion pathway
Higher temperature

Question 21

what molecules can move via facilitated diffusion?

Answer 21

Specific molecules to the complementary binding site as facilitated diffusion is facilitated via channel and carrier proteins

Question 22

Is facilitated diffusion active or passive?

Answer 22

Passive
It does not require ATP

Question 23

What does the curve of facilitated diffusion look like?

Answer 23

It plateaus when all the carrier protein binding sites are saturated
The number of carrier proteins becomes the limiting factor

Question 24

Osmosis definition

Answer 24

The net movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane

Question 25

What liquid has the highest water potential

Answer 25

Pure water

Question 26

When does the water potential of a liquid change?

Answer 26

When more solute is added, the water potential becomes more negative

Question 27

What does more solute mean in terms of water potential?

Answer 27

More solute
More negative water potential
Less free water molecules to push on membrane so water moves in

Question 28

Active transport definition

Answer 28

Used to transport molecules across a membrane from a lower concentration to a higher concentration via carrier proteins and the use of ATP

Question 29

How does active transport work?

Answer 29

The molecule for transport binds to the carrier protein binding site
The hydrolysis of ATP from ADP + Pi provides a small amount of energy for the carrier protein
This causes a conformational change
This change in protein shape transports the molecule across the membrane into an area of higher concentration

Question 30

Two types of active transport

Answer 30

Endocytosis
Exocytosis

Question 31

Endocytosis definition

Answer 31

Molecules are bulk transported into the cell via the cell surface membrane

Question 32

Exocytosis defintion

Answer 32

Molecules are transported to the outside of the cell via the cell surface membrane

Question 33

Describe the process of exo/endocytosis

Answer 33

The cell surface membrane is pulled inside or pushed outside to create a vesicle
Any molecules next to that part off the cell membrane are pulled into the vesicle
This process requires ATP

Question 34

Describe how substances move across a cell surface membrane

Answer 34

1. Small/non polar/lipid soluble molecules pass via the phospholipid bilayer
2. via simple or facilitated diffusion from high to low concentration down the concentration gradient
3. Water moves via aquaporins by osmosis from higher to lower water potential
4. Active transport is movement from low to high concentration by facilitated diffusion by carrier proteins
5. Active transport requires ATP