Movement of substances

Question 1

what is a cell surface membrane made out of?

Answer 1

protein and lipid, together with a small amount of carbohydrate.

Question 2

what is the lipid of membranes?

Answer 2

phospholipids and cholesterol

Question 3

what is the “head” of a phospholipid molecule composed of?

Answer 3

a glycerol which is attached to an ionised phosphate group

Question 4

what property does the phosphate group have?

Answer 4

the phosphate group has hydrophilic properties. (water-loving), making the head of the phospholipid molecule hydrophilic as well –> a glycerol-phosphate hydrophilic head

Question 5

what property does the tail of the phospholipid molecule have?

Answer 5

the tails have hydrophobic properties. (water-hating)

Question 6

what is a phospholipid molecule’s response to water?

Answer 6

the molecules float with their hydrophilic heads in contact with the water molecules, and their hydrocarbon tails exposed and away from the water.

Question 7

what is the phospholipid molecules’ arrangement in a cell surface membrane?

Answer 7

they arrange themselves as a bilayer, with hydrocarbon tails facing together

Question 8

what are the proteins and carbohydrates in a cell surface membrane? + specific places of where they are
(fluid mosaic model)

Answer 8

integral proteins - partially or fully buried in the lipid bilayer
peripheral proteins - superficially attached on either surface of the lipid bilayer
glycoproteins - polysaccharides (they look like cactus) attached to proteins
glycolipids - polysaccharides attached to lipids

Question 9

role of glycoproteins in cell surface membrane

Answer 9

glycoproteins act as receptor sites for chemical signals. They are also important for cell-cell recognition and assist in the binding together of cells to form tissue

Question 10

role of glycolipids in cell surface membrane

Answer 10

glycolipids help to maintain the stability of the cell surface membrane as well as facilitate cell-cell recognition

Question 11

functions of membrane proteins

Answer 11

channel protein - a passage through the membrane, each channel allows one specific substance to pass

pump protein - active transport across the membrane. ATP is required to move substances across

binding protein - for attachment of a specific hormone

Question 12

why is the membrane described as fluid?

Answer 12

this is because the components (lipids and proteins) move around within the layer.

Question 13

what are the movements of the components like?

Answer 13

the movement of lipid molecules are rapid.

the mobile proteins move more slowly.

Question 14

why is the membrane described as mosaic?

Answer 14

it describes the scattered pattern of the proteins, when viewed from above.

Question 15

how do phospholipids keep the membrane fluid?

Answer 15

the phospholipids have a mixture of saturated and unsaturated fatty acid tails.
an excess of unsaturated fatty acid tails makes the membrane more fluid. This is because the kinks in the tails prevent close-packing of the lipids.

Question 16

what makes a membrane less fluid?

Answer 16

the presence of cholesterol reduces fluidity by preventing or reducing the movements of the lipid molecules.

if the temperature of a membrane falls, it also becomes less fluid

Question 17

why must the membrane be fluid?

Answer 17

so that the proteins in the membrane can move about and function correctly. If the membrane isn’t fluid, it risks solidifying.

Question 18

definition of diffusion?

Answer 18

diffusion is the net movement of molecules from a region of high concentration to low concentration, down a concentration gradient, using kinetic energy from the continuous random movement of molecules.

Question 19

what is the link between diffusion and its role in nutrient uptake and gaseous exchange in plants and humans?

Answer 19

The smaller the cell is, the more quickly and easily can materials be exchanged between its cytoplasm and environment by diffusion.

When the cell increases in size, the volume increases faster than the surface area, making the surface area to volume ratio fall. Thus, less of the cytoplasm has access to the cell surface for exchange of gases and supply of nutrients.

Question 20

definition of osmosis?

Answer 20

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

Question 21

how does a plant cell become turgid through osmosis?

Answer 21

when the external solution is diluted and has a higher water potential than the cell, there is a net flow of water into the cell. The cell solution becomes more diluted. The volume of the cell increases (due to the water intake). Thus, the cytoplasm may come to press hard against the cell wall. If this happens, the cell is described as turgid. Further net uptake of water stops.

the cellulose cell wall protected the cell contents from damage from osmosis.

Question 22

how does a plant cell become plasmolysed and flaccid through osmosis?

Answer 22

when the external solution is concentrated and has a lower water potential, there is a net flow of water out of the cell. The cell solution becomes more concentrated. The volume of the cell solution decreases and the cytoplasm starts to pull away from the parts of the cell wall. (however, contact is still maintained at the points where there are cytoplasmic connection between cells). When this happens, the cells are described as plasmolysed and are flaccid.

Question 23

effects on plants due to osmosis

Answer 23

usually the cytoplasm of plant cells is undamaged by a period of plasmolysis.
but if the plant is short of water for a long period of time, it will wilt and die.

Question 24

effects on animal cells due to osmosis

Answer 24

animal cells do not have cell walls to protect them.

when the external solution has a lower water potential than the cell, the cell will crenate (shrink and become a weird shape).

When the external solution has a higher water potential than the cell, the cell will become swollen/expand and may lyse (burst)

Question 25

definition of active transport

Answer 25

active transport involved molecules from a region of lower concentration to a region of higher concentration, against a concentration gradient, using metabolic energy in the form of adenosine triphosphate (ATP) produced by the cell.

Question 26

how is active transport able to occur against the concentration gradient?

Answer 26

the cell membrane retains useful molecules and ions valuable in metabolism. When more useful molecules or ions become available, they are actively absorbed into the cells.

Question 27

what is the protein needed for active transport?

Answer 27

pump molecules

Question 28

properties of pump molecules?

Answer 28

they require ATP to move
most are specific to particular molecules (selective)
unidirectional/bi-directional (it can transport substances both ways)