2.1.5 Biological Membranes

Question 1

what are the membranes like between the cell and its environment?

Answer 1

• partially permeable
• plasma membrane allows transfer of substances in and out of the cell
• e.g water, ions

Question 2

what is the role of the membrane between organelles and cytoplasm?

Answer 2

• compartmentalisation
• in a lysozome, the hydrolytic enzymes need to be kept seperate from other organelles
• controls what enters and leaves organelle

Question 3

what is the role of membranes within organelles?

Answer 3

• allows for concentration gradients to be created
• site of attatchment of enzymes into the membrane

Question 4

what are the other roles of membranes inside a cell?

Answer 4

• sites of chemical reactions
• sites of cell communication

Question 5

what happens in cell communication?

Answer 5

• communication between cells
• cell identification
• cells working together
• to trigger a response

Question 6

why do phospholipids arrange themselves in a bilayer at the cell surface?

Answer 6

• hydrophilic heads orientate towards water and form hydrogen bonds
• hydrophobic tails orientate away from water
• water is present both outside and inside of the cell

Question 7

what makes up a phospholipid bilayer in a fluid mosaic model?

Answer 7

• glycoproteins and glycolipids
• proteins
• cholesterol
• phospholipids

Question 8

what is the function of glycolipids/glycoproteins?

Answer 8

• cell recognition
• cell signalling
• acts as antigens
• acts as receptors
• involved in cell adhesion (cells bind together)

Question 9

what do carrier proteins do?

Answer 9

transports large and polar substances

Question 10

what do channel proteins do?

Answer 10

transports polar substances

Question 11

where is cholesterol found in the model and what does it do?

Answer 11

• found between the tails of the phospholipid
• it regulates fluidity of the membrane
• more cholesterol = less fluid

Question 12

what factors affect membrane permeability?

Answer 12

• temperature
• solvent

Question 13

what happens to permeability when you increase temperature?

Answer 13

• phospholipids gain kinetic energy
• causes gaps to appear between phospholipids
• membrane becomes more permeable
• at high temperatures, the proteins will denature

Question 14

what happens to the permeability when temp decreases and water freezes?

Answer 14

• water expands and forms ice crystals
• break and pierce cell membrane making it more permeable

Question 15

what is diffusion?

Answer 15

the net movement of molecules down a concentration gradient until equilibrium is reached

Question 16

what factors affect the rate of diffusion?

Answer 16

1. temperature
   —> higher temperature = more KE so particles move faster
2. thick membrane
   —> thicker membrane = more distance so slower rate of diffusion
3. surface area
   —> larger SA = faster rate of diffusion
4. conc gradient
   —> higher conc gradient = faster rate of diffusion

Question 17

what is facilitated diffusion?

Answer 17

using carrier or channel proteins for diffusion

Question 18

how do carrier proteins work in diffusion?

Answer 18

1. large molecule attaches to carrier proteins
2. protein changes shape
3. releases molecule on the other side of the

Question 19

how do channel proteins help in facilitated diffusion?

Answer 19

form pores for charged particles to diffuse through

Question 20

why is it called fluid mosaic model?

Answer 20

fluid = phospholipids can move around
mosaic = phospholipids resemble a mosaic image

Question 21

what’s an advantage of the structure of a membrane being fluid?

Answer 21

• membrane proteins can diffuse to areas where they are needed
• endocytosis

Question 22

what are some functions of proteins that occupy the plasma membrane?

Answer 22

• act as enzymes
• act as receptors
• act as antigens
• carrier proteins
• channel proteins

Question 23

how does endocytosis occur?

Answer 23

• happens when some molecules are too large to be taken into a cell by carrier proteins
• a cell surrounds the molecule with a section of plasma membrane
• membrane pinches off to form a vesicle inside the cell containing the molecule

Question 24

how does exocytosis occur?

Answer 24

1. vesicles containing substance pinches off from Golgi and moves towards plasma membrane
2. vesicle fuses with plasma membrane
3. contents of vesicle released outside cell

Question 25

how does temperature affect membrane permeability?

Answer 25

below 0°C: • phospholipids packed **closely together** • **proteins deform**, increasing permeability • **ice crystal forms** and **breaks membrane** making it **very permeable** when it freezes between 0 and 45°C: • as temp increases, **phospholipids gain energy** and move faster • increases permeability so now **partially permeable** above 45°C: • phospholipid **bilayer melts** • **channel and carrier proteins deform** increasing permeability by a lot

Question 26

what are the effects of solvents on membrane permeability?

Answer 26

- putting cells into a solvent **increases permeability** of cell surface membranes - **lipids** in cell membrane **dissolve into the solvent** so **membrane loses its structure** - **some** solvents increase permeability **more than others** - **increasing solvent concentration** **increases permeability** of the membrane

Question 27

why is osmosis important in plants?

Answer 27

- **water** enters **vacuole** - **pressure** against **cell walls** - cells are **turgid**

Question 28

what’s required for endocytosis?

Answer 28

atp

Question 29

what happens when there’s less proteins in the membrane?

Answer 29

- **less receptors** - **endocytosis reduced** - **fewer** proteins for **cell recognition**

Question 30

what factors affect membrane **fluidity**?

Answer 30

1. temperature —> **high temp = more fluid** 2. cholesterol content —> **more = decreases fluidity** 3. length of fatty acid tails —> **longer = less fluid** —> **more intermolecular interactions** between fatty acid tails

Question 31

what’s the role of hydrophobic tails

Answer 31

**barrier** to **polar molecules**

Question 32

what are the differences between channel and carrier proteins?

Answer 32

- **carrier proteins change shape**, channel doesn’t - **channel is always passive** while carrier can be active or passive - active transport needs **carrier** proteins as it has **site of ATP hydrolysis**, channel doesn’t

Question 33

why is there lower water potential in aqeous solutions?

Answer 33

- **pure water** has **no solute molecules** - so **all water molecules** free to **diffuse across membrane**