2.5 BIOLOGICAL MEMBRANES

Question 1

what are cell membranes?

Answer 1

partially permeable barriers meaning some small molecules can diffuse through the cell membrane between structural molecules, some can dissolve in the lipid layer and others can pass through protein channels of by carrier proteins

Question 2

what is the role of membranes at the surface of cells?

Answer 2

• Separates the cell components from the external environment
• regulates the transport of materials into and out of the cell
• may contain enzymes involved in specific metabolic pathways
• Has antigens so the immune system doesn’t attack itself
• May release chemicals that signal to other cells
• Contains receptors for the chemical signals
• may be the site of chemical reactions

Question 3

what is the role of membranes within cells?

Answer 3

• Barrier between organelle and cytoplasm
• can form vesicles
• partially permeable
• can be the site of chemical reactions
• provide large surface areas

Question 4

what is the fluid mosaic structure?

Answer 4

describes the arrangement of molecules in the membrane (1972)

Question 5

What comprises the fluid mosaic structure?

Answer 5

Phospholipids, cholesterol, protein, glycoproteins, glyoclipids

Question 6

what is the role of phospholipids in the cell membrane?

Answer 6

they form a barrier to dissolved substances, they have a water loving head (hydrophilic) and a water hating tail (hydrophobic), they arrange themselves into a layer and the heads face outwards

Question 7

what is the role of cholesterol in the cell membrane?

Answer 7

provides membrane stability, it fits between the phospholipids and causes them to pack closer together (making the membrane less fluid and more rigid)

Question 8

what is the role of proteins in the cell membrane?

Answer 8

they control what enters and leaves the cell, some form channel proteins and allow charged particles to enter the cell, carrier proteins also allow active transport and facilitated diffusion, can act as receptors for cell signalling

Question 9

what is the role of glycolipids and glycoproteins in the cell membrane?

Answer 9

• they act as receptors for messenger molecules
• Stabilise the membrane by forming hydrogen bonds with water molecules
• Sites where drugs, hormones and antibodies bind

Question 10

what is cell signalling and why does it occur?

Answer 10

it is how cells communicate with each other and cells receptors for messenger molecules
Stabilise the membrane by forming hydrogen bonds with water molecules
Sites where drugs, hormones and antibodies bind

Question 11

how do cells communicate?

Answer 11

They communicate with each other using messenger molecules
One cell releases a messenger molecule (hormone)
This molecule travels to another cell
The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Question 12

how do receptors play a role in cell signalling?

Answer 12

Proteins in the cell membrane act as receptors for messenger molecules (membrane bound receptors)

Question 13

what is a target cell?

Answer 13

A cell that responds to a particular messenger molecule

Question 14

how do drugs work?

Answer 14

Drugs also bind to cell membrane receptors. Many drugs work by binding to receptors in cell membranes. They either trigger a response in the cell, or block the receptor and prevent it from working.

Question 15

what is diffusion?

Answer 15

movement of molecules from an area of high concentration to an area of low concentration, it can be across a membrane and does not require energy

Question 16

how does simple diffusion work?

Answer 16

if there is an area of high concentration of a certain type of molecule then the molecules will randomly bump and spread away from each other until they are in lower concentration until they are evenly dispersed
When the molecules have moved down the concentration gradient they still move but remain evenly dispersed so there is no net diffusion

Question 17

how do the following substances pass through the cell membrane?

• oxygen and carbon dioxide
• fat-soluble molecules
• water molecules

Answer 17

Oxygen and carbon dioxide can pass through the membrane using simple diffusion because they are small molecules
Fat-soluble molecules can dissolve into the lipid bilayer and pass through also
Water molecules are polar and insoluble in the lipid layer so they are specific water channel proteins (aquaporins) which allow the water to travel across

Question 18

how is the concentration gradient maintained?

Answer 18

Many molecules entering the cells then pass into organelles and are used for metabolic reactions, for example, oxygen diffuses into the cytoplasm of respiring cells then diffuses into mitochondria and is used for aerobic respiration
carbon dioxide diffusing into the palisade mesophyll cells of a plant leaf will then diffuse into chloroplasts and be used for photosynthesis

Question 19

what are factors which affect the rate of diffusion?

Answer 19

• temperature
  as the temperature increases, molecules have more kinetic energy so rate of diffusion increases
• diffusion distance
  the thicker the membrane across which molecules have to diffuse = slower diffusion rate
• surface area
  more surface area = more diffusion
• size of diffusing molecule
  smaller ions or molecules diffuse quicker
• concentration gradient
  the steeper the gradient the faster the diffusion

Question 20

what is facilitated diffusion?

Answer 20

movement of molecules from an area of high concentration to an area of low concentration across a partially permeable membrane via protein channels or carriers, passive process

Question 21

why is facilitated diffusion needed?

Answer 21

Small molecules which have polarity are insoluble in lipid because they can’t interact with the hydrophobic tails of the lipid bilayer meaning they have to diffuse through water-filled protein channels embedded in the membrane

Question 22

what is osmosis?

Answer 22

passage of water molecules down their water potential gradient across a partially permeable membrane

Question 23

what is water measured in?

Answer 23

kilopascals (kPa)

Question 24

what water potential does pure water have?

Answer 24

highest water potential possible, 0kPa

Question 25

what happens to the water potential when solute molecules are added to pure water?

Answer 25

The water potential of the solution decreases it becomes more negative (numerical) more negative = the lower the water potential

Question 26

what happens when cells are placed in a solution of high water potential?

Answer 26

the water molecules move down the concentration through osmosis across the plasma membrane and into the cell

Question 27

what happens in an animal cell if too much water enters?

Answer 27

cytolysis

Question 28

what happens to a plant cell if it doesn’t get enough water?

Answer 28

plasmolysis

Question 29

what is active transport?

Answer 29

the movement of substances against their concentration gradient (from low to high concentration) across a cell membrane using ATP and carrier protein

Question 30

what are carrier proteins and how do they act as enzymes?

Answer 30

membrane proteins which have specific regions, or sites, that combine reversibly with only certain solute molecules or ions
They have a region that binds to and allows the hydrolysis of a molecule of ATP, to release energy, and in this way they act as enzymes
The energy helps the carrier proteins change its conformation and in doing so it carries the ion from one side of the membrane to the other

Question 31

what is bulk transport?

Answer 31

Some cells need to transport large molecules and particles that are too large to pass through a plasma membrane, in or out. They do this by bulk transport, a process that requires energy from ATP

Question 32

what is endocytosis?

Answer 32

• The bulk transport of molecules, too large to pass through a cell membrane even via channel or carrier proteins, into a cell
• Phagocytosis - eating by cells
• Pinocytosis - drinking of cells
• ATP is needed to provide energy to form vesicles and move them using motor proteins along cytoskeleton threads into the cell interior

Question 33

what is exocytosis?

Answer 33

• The bulk transport of molecules, too large to pass through a cell membrane even via a channel or carrier proteins, out of a cell
• an example is at synapses where chemicals in vesicles are moved to the presynaptic membrane
• ATP is needed to fuse the membranes together and for moving the vesicles

Question 34

what is the process of exocytosis?

Answer 34

1. A membrane-bound vesicle, containing the substance to be secreted is moved towards the cell surface membrane
2. The cell surface membrane and the membrane of the vesicle fuse together
3. The fused site opens, releasing the contents of the secretory vesicle

Question 35

how does increasing temperature affect membrane permeability?

Answer 35

• As the temperature increases the phospholipids move more (more energy) and this increases the permeability of the membrane.
• some proteins move around which could affect the rate of reactions they catalyse
• Could affect the infolding during phagocytosis
• Could impact exocytosis and cell signalling
• Cholesterol molecules buffers (to some extent) the effects of increasing heat as it reduces the increase in membrane fluidity

Question 36

how does decreasing temperature affect membrane permeability?

Answer 36

• The phospholipids don’t have much energy, so they can’t move very much.
• They’re rigid because they are packed closely together.
• Channel proteins and carrier proteins deform, increasing the permeability of the membrane.
• Ice crystals may form and pierce the membrane making it highly permeable when it thaws.
• saturated fatty acids become compressed
• Unsaturated fatty acids become compressed causing their tails to kink pushing the phospholipids away and maintaining membrane fluidity
• Cholesterol in the membrane buffers the effect of lower temperature to prevent a reduction in fluidity, it prevents the phospholipids from packing too closely together

Question 37

what happens to proteins when temperatures increase?

Answer 37

cause the atoms within their large molecules to vibrate and this breaks the hydrogen bonds and ionic bonds that hold their structure together and they begin to unfold
Their tertiary structure changes and cannot change (irreversible) back when they cool, they are denatured
If the membrane-embedded proteins and the cytoskeleton denature then the plasma membrane will begin to fall apart and become more permeable
Membrane-embedded enzymes will cease to function if they become denatured. If the shape of their active sites changes slightly or the enzymes move within the enzyme then the rate of reactions will be impacted

Question 38

how does changing the solvent affect membrane permeability ?

Answer 38

Surrounding cells in a solvent increases the permeability of cell membranes because it dissolves the lipids so the membrane structure is lost, some solvents increase permeability more than others (ethanol more than methanol)