Transport Across Membranes

Question 1

Phospholipid Structure:
Glycerol combines with 2 fatty acid tails and 1 phosphate head

Question 2

The hydrophobic fatty acid tails are orientated inwards towards the middle, and point away from water and the hydrophilic heads are orientated towards the outside and point towards the water. The hydrophilic heads make contact with watery cytoplasm and extracellular fluid. This causes the phospholipids to form a phospholipid bilayer.

Answer 2

This phospholipid bilayer forms the basis of all cell membranes (cell surface plasma membrane and all membrane bound organelles)

Question 3

The entry and exit of molecules in cells is controlled by the cell-surface membrane, which surrounds the cytoplasm of a cell. The membrane is described as selectively permeable.

Question 4

The fluid-mosaic model

Phospholipid molecules form a BILAYER (double layer) that are constantly moving around relative to one another, giving the membrane a fluid structure.

The different protein molecules are unevenly distributed throughout the membrane, forming a mosaic.

The selective permeability of the cell-surface membrane is related to the type and distribution of specific proteins and phospholipid molecules present in the membrane.

Question 5

Explain why a cell
membrane may be
described as a fluid-mosaic?

Answer 5

1) The position of the molecules within the membrane is fluid – they are able to move around within the membrane

2) Membrane is made up from a variety of different molecules arranged into a mosaic

Question 6

Explain the arrangement of
phospholipids in a cell-
surface membrane.

Answer 6

1) Bilayer OR Water is present inside and outside a cell; ·

2) Hydrophobic (fatty acid) tails point away/are repelled from water OR Hydrophilic (phosphate) heads point to/are in/are attracted to water

Question 7

The cell membrane consists of: proteins, glycoproteins, phospholipids, cholesterol and carbohydrate

Question 8

Phospholipids

1) Hydrophobic fatty acid tails of phospholipid molecules are attracted towards each other, and the hydrophilic phosphate heads are orientated either inwards towards the cytoplasm or outwards towards the watery extra-cellular fluid, forming the phospholipid bilayer

2) Most abundant molecule found in ALL membranes

3) Phospholipid bilayer allows lipid-soluble (non-polar) molecules to pass through by simple diffusion but prevents the passage of small polar/charged molecules (like ions) and larger molecule (e.g. Glucose)

Question 9

Cholesterol:

1) Decreases permeability and increases stability of the membrane

2) MORE cholesterol = LESS fluidity of the membrane

Question 10

Channel Proteins

1) Only allow specific charged ions or small molecules to move across the membrane by facilitated diffusion (facilitate means to help)

2) They can be open or closed and are intrinsic (allow ions to pass straight though)

3) Na+ ions can only pass through Sodium ion channel proteins embedded within and through the membrane

4) Proteins have specific tertiary structures, so they are specific and can only transport molecules that are complementary to the shape of the channel protein

Question 11

Carrier Proteins

1) Aids the transport of ions/polar molecules and large molecules e.g. glucose and amino acids by facilitated diffusion and active transport.

Question 12

Receptor Proteins:

1) Act as specific receptors for complementary molecules

2) Only specific molecules can bind to specific receptor protein

3) Specific cells have specific receptor

Question 13

Enzymes:

1) Found embedded in the cell membrane

2) Shape of the enzymes active site is specific and complementary to its substrate, allowing them to bind and form enzyme-substrate complexes