2.1.5 Biological membranes

Question 1

cell surface(plasma) membrane

Answer 1

-partially permeable barrier
-allow cell recognition(distinguishing cells/foreign material).
-allow cell communication

Question 2

membranes that bind to organelles

Answer 2

-divide the cell into different compartments, barrier between organelle and cytoplasm
-partially permeable to let in molecules using receptors.
-isolate organelles for/site of chemical reactions,internal barriers,form vesicles for transporting.

Question 3

Fluid mosaic model structure

Answer 3

-hydrophilic, outward facing head(phosphate group) and hydrophobic, inward facing tails(fatty acids) forming a bilayer
phospholipids and some proteins are free to move in their layer of the membrane
proteins scattered throughout, between phospholipids

Question 4

phospholipids

Answer 4

-main part of cell membrane(flexible shape) that controls the movement of substances across the membrane
-head(phosphate group) is HYDROPHILIC (attracted to and soluble in water) and tail(fatty acids/lipids) is HYDROPHOBIC.(repelled by and insoluble in water)
-bilayer because heads are polar and face water and tails go in center and are hydrophobic and act as barrier from dissolved substances and only allows only non polar, hydrophobic, uncharged molecules to dissolve and pass through i.e lipid soluble substances

Question 5

cholesterol

Answer 5

-type of lipids that bind to the fatty acid tails of the phosolipids for stability
-regulate membrane fluidity
-at high temps, they bind hydrophobic tails together –>less fluid and more rigid(prevents water/ions from leaking out of the cell)
-at lower temps, it prevents tightness by increasing packing of the membrane and therefore increases membrane fluidity
-has h-phobic regions so act as barrier to polar substances.

Question 6

proteins

Answer 6

-CHANNEL PROTEINS= in both layers(intrinsic) , let through small, charged particles.
-CARRIER PROTEINS= both layers, transport larger molecules and charged particles across membrane(changes shape)
-surface proteins provide mechanical support
-protein receptors= act as receptors in cell signaling, when molecule binds to it, it triggers reaction.

Question 7

Extrinsic protein

Answer 7

binding sites and is on the surface of the membrane i.e glycoprotein and protein receptors
-can provide mechanical support or connect to proteins/lipids

Question 8

Intrinsic protein

Answer 8

-embedded in the membrane, electron carriers i.e channel and carrier proteins
-involved in transporting moleucles

Question 9

glycoproteins/lipids

Answer 9

-attached to a carbohydrate
-stabilise the membrane with H-bonds with surround water molecules.
-receptors for messengers in cell signaling.
-antigens, involved in self-recognition or recognition of specific chemicals and immune response—> allows cells to attach to one another and form tissues

Question 10

FACTORS AFFECTING MEMBRANE PERMEABILITY

Question 11

solvents

Answer 11

-membranes are surrounded by water which is polar and solvents with lower polarity than water can disrupt cells
-some solvents dissolve lipids in membrane so it loses its structure
-some increase permeability more than others i.e ethanol more than methanol
-increasing conc of solvent would
increase i.e higher alc conc, higher permeability
-too non polar then membrane breaks down completely

Question 12

temperature

Answer 12

-affects how much phospholipids in the bilayer can move, affecting permeability and structure

Question 13

below 0 degrees

Answer 13

-phospholipids don’t have much kinetic energy so don’t move much
-packed close together and rigid membrane
-channel and carrier proteins in membrane denature, increasing permeability of membrane
-ice crystals may pierce membrane, making it highly permeable when thawed

Question 14

between 0 and 45 degrees

Answer 14

-phospholipids can move around and aren’t as tightly packed=partially permeable
-as temp increases, phospholipids move more as they gain more energy= increase permeability

Question 15

above 45 degrees

Answer 15

-phospholipid bilayer starts to melt + lost its structure and increases permeability.
-water inside cell expands, putting pressure on membrane
-channel and carrier proteins in membrane denature so can’t control what enters/leaves= increases permeability.
-temp gets too high then membrane breaks down completely

Question 16

Diffusion

Answer 16

net movement of particles/molecules from an area of higher concentration to lower concentration down a conc gradient
-sometimes across a partially permeable membrane
-continues until even distribution of particles

Question 17

what kind of process is diffusion?

Answer 17

-passive process, no ATP required

Question 18

Simple diffusion

Answer 18

-occurs with lipid-based molecules e.g CO2, 02
-they are non polar so can easily diffuse through spaces between phospholipids.

Question 19

Factors affecting rate of diffusion

Answer 19

-temp: more KE means they move faster + vibrate more, higher rate
-Diffusion distance: thicker the membrane or exchange surface, the slower the rate (particles travel longer) i.e alveoli is one cell thick, short
-conc gradient difference: steeper=faster
-surface area: more diffusion can take place, higher rate
-size of molecule: smaller molecules/ions diffuse faster

Question 20

why does facilitated diffusion happen?

Answer 20

-to help larger molecules(amino acids,glucose) and charged particles(ions and polar molecules) diffuse quicker using plasma membrane protein channels
-selective permeability= cell can choose what substances move into it

Question 21

carrier protein role

Answer 21

move large molecules down the conc gradient:
-molecule attaches to binding site of carrier
-protein changes shape
-allows molecule to pass through opposite side of membrane

Question 22

channel protein role

Answer 22

-form pores in the membrane for smaller ions and polar molecules to diffuse through, down conc gradient e.g calcium ions

Question 23

What is Active transport?

Answer 23

-movement of particles from an area of low conc to high conc against a conc gradient with the use of ATP.
-uses carrier proteins to transport molecules
i.e mineral ions move into root hair cells

Question 24

what kind of process is active transport?

Answer 24

-Active process, requires ATP
-ATP is hydrolysed into ADP + inorganic phosphate for conformational change in shape for protein carriers(selective)

Question 25

what is exocytosis?

Answer 25

movement of molecules out of a cell
-bulk transport(active transport of larger molecules)
i.e fusion of vesicles into plasma membranes

Question 26

process of exocytosis

Answer 26

-vesicles buds off from golgi apparatus and moves to plasma membrane
-fuses w membrane and releases substances out of cell OR inserted into membrane
-uses ATP as energy source

Question 27

What is Endocytosis?

Answer 27

-movement of large molecules or objects into a cell
-uses ATP for energy
-bulk transport(active transport of larger molecules)
i.e phagocytosis, pinocytosis(englufing liquids)

Question 28

process of endocytosis

Answer 28

-cell surrounds the substance with a section of its plasma membrane
-membrane pinches off to form a vesicle inside the cell containing substance
EXAMPLE: white blood cells use endocytosis to take in invading cells

Question 29

What is Osmosis?

Answer 29

movement of water molecules from a high water potential to a low water potential across a partially permeable membrane

Question 30

water potential

Answer 30

likelihood of water molecules to diffuse out of/into a solution
(is always negative—>more negative, stronger conc of solutes so lower potential)

Question 31

pure water

Answer 31

-has a high water potential(zero)
-water molecules are free to move

Question 32

what are aquaporins?

Answer 32

little gaps in the membrane that help water move faster.

Question 33

hypertonic solution

Answer 33

-external solution that has a high solute conc and lower water potential than inside the cell

Question 34

what happens in a hypertonic solution?

Answer 34

-animal cell will shrink as water leaves the cell
-plant cell will become flaccid
-PLASMOLYSIS occurs in plant cells=cytoplasm and membrane will pull away from cell wall

Question 35

hypotonic solution

Answer 35

-external solution with a low solute conc and high water potential than inside the cell

Question 36

what happens in a hypotonic solution?

Answer 36

-animal cell would swell and burst as water moves into the cell
-plant cell would become turgid(contents of vacuole and cytoplasm swell and push upon the cell wall)

Question 37

isotonic solution

Answer 37

external solution has the same water potential and solute conc to the cell

Question 38

Investigating cell membrane permeability

Answer 38

-cut five equal pieces of beetroot using a scalpel and add to test tubes with 5 cm3 of water.
-each test tube into a water bath of diff temps for the same length of time
-use colorimeter on blue filter to read absorbance of each solution(higher absorbance more pigment released)

Question 39

What is membrane fluidity

Answer 39

-allows proteins to diffuse laterally

Question 40

process of active transport

Answer 40

1. particle with ATP binds to to transport protein
2. ATP hydrolysed to ADP and inorganic phosphate
3. energy released triggers protein to change shape so particle is transported across membrane
   (for co-transporting, second particle binds to second binding site, inorganic phosphate released, triggers protein to change shape so second particle is transported across the membrane)