5 Plasma membranes

Question 1

What are membranes formed from?

Answer 1

Membranes are formed from a phospholipid bilayer.

Question 2

Who proposed the fluid-mosaic model and when?

Answer 2

American scientists Singer and Nicolson

In 1972

Question 3

Why is it called the fluid mosaic model?

Answer 3

The bilayer is ‘fluid’ because the phospholipids are constantly moving and protein molecules are scattered through the bilayer, like tiles in a mosaic.

Question 4

What are the different components of a cell membrane?

Answer 4

Phospholipids, cholesterol, proteins, glycolipids and glycoproteins.

Question 5

What is an intrinsic protein?

Answer 5

• A protein molecule spanning the phospholipid layer.
• It has amino acids with hydrophobic R-groups on its external surface, which interacts with the hydrophobic core of the membrane, keeping it in place.

Question 6

What are some examples of intrinsic proteins?

Answer 6

• Channel proteins & carrier proteins.

- They are both involved in transport across the membrane.

Question 7

What do channel proteins do?

Answer 7

• Channel proteins provide a hydrophilic channel that allows the passive movement. of polar molecules and ions down a concentration gradient through membranes.
• They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.

Question 8

What do carrier proteins do?

Answer 8

• Carrier proteins have an important role in both passive transport (down a concentration gradient) and active transport (against a concentration gradient) into cells.
• This often involves the shape of the protein changing.

Question 9

What is a glycoprotein?

Answer 9

• A glycoprotein is an intrinsic proteins.
• recognition of cells as self and foreign for non-self
• cell signalling ; communication between cells
• play a role in cell adhesion
• acts as antigen
• receptor/ binding site for hormone

Question 10

What are glycolipids?

Answer 10

• Lipids with attached carbohydrate chains.

- It acts as a recognition site e.g for cholera toxins

Question 11

What is an extrinsic protein?

Answer 11

• A protein molecule lying on the surface.
• It is present in one side of the bilayer.
• It normally has hydrophilic R-groups on its outer surface and interacts with the polar heads of the phospholipids or with intrinsic proteins.

Question 12

What is cholesterol?

Answer 12

• Cholesterol is a lipid with a hydrophilic end and a hydrophobic end.
• It regulates the fluidity of the membrane.
• Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together.
• Cholesterol adds stability to the membrane.

Question 13

How does temperature affect membrane structure?

Answer 13

• When temperature is increased, the phospholipids will have more kinetic energy and will move more.
• This makes a membrane more fluid and it begins to lose its structure.
• If temperature continues to increase, the cells will eventually begin to break down.
• This loss of structure increases the permeability of the membrane, making it easier for particles to cross it.

Question 14

How do solvents affect membrane temperature?

Answer 14

• Organic solvents such as alcohol are less polar than water.
• Organic solvents will dissolve membranes, disrupting cells.
• When the membrane is disrupted it becomes more fluid and more permeable.

Question 15

How do cells communicate with each other?

Answer 15

• Cells communicate with each other using messenger molecules.
  1) One cell releases a messenger molecule (e.g a hormone)
  2) This molecule travels (e.g in the blood) to another cell.
  3) The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane.

Question 16

What are membrane-bound receptors?

Answer 16

• Proteins in the cell membrane which act as receptors for messenger molecules.
• Receptor proteins have specific shapes- only messenger molecules with a complementary shape can bind to.

Question 17

What are target cells?

Answer 17

• A cell that responds to a particular messenger molecule.

Question 18

Give an example of a messenger molecule binding to a receptor on a target cell.

Answer 18

Glucagon.

• Glucagon is a hormone that’s released when there isn’t enough glucose in the blood.
• It binds to receptors on liver cells, causing liver cells to break down stores of glycogen to glucose.

Question 19

What can also bind to cell membrane receptors?

Answer 19

Drugs.

• 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 20

What is an example of a drug?

Answer 20

Antihistamines.

• Cell damage causes the release of histamine.
• Histamine binds to receptors on the surface of other cells and causes inflammation.
• Antihistamines work by blocking histamine receptors on cell surfaces.
• This prevents histamine from binding to the cell and stops inflammation.

Question 21

How can you investigate the permeability of the cell membrane?

Answer 21

• You could investigate how temperature affects beetroot membrane permeability.
• The higher the permeability of the membrane, the more pigment leaks out of the cell.
  1) Cut five equal sizes pieces of beetroot and rinse them with distilled water to remove any pigment released during the cutting.
  2) Place the five pieces in five different test tubes, each with 5cm^3 of water.
  3) Place each test tube in a water bath at a different temperature e.g 10, 20, 30, 40, 50 degrees for the same length of time. (15 mins)
  4) Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
  5) Use a colorimeter and measure the absorbance. The higher the permeability of the membrane, the more pigment in released, so the higher the absorbance of the liquid.

Question 22

What is diffusion?

Answer 22

• Diffusion is the net movement of particles from a region of higher concentration to a region of a lower concentration.
• It is a passive process- no energy is required for it to happen.

Question 23

What diffuses through the cell membranes?

Answer 23

Small, non-polar molecules.
- Small, non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids.

Question 24

What factors affect the rate of diffusion?

Answer 24

• The concentration gradient: The higher the concentration, the faster the diffusion.
• The thickness of the exchange surface: The thinner the exchange surface (the shorter the distance the particles have to travel), the faster the rate of diffusion.
• The surface area: The larger the surface area, the faster the rate of diffusion.
• The temperature: The higher the temperature, the faster the rate of diffusion because the particles have more kinetic energy so they move faster.

Question 25

How can you investigate diffusion in model cells?

Answer 25

- You can investigate factors that affect the rate of diffusion. 1) First, make up some agar jelly with phenolphthalein and dilute sodium hydroxide. 2) Then fill a beaker with some dilute hydrochloric acid. Using a scalpel, cut out a few cubes from the jelly and put them in a beaker of acid. 3) If you leave the cubes for a while they'll eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide,

Question 26

How can you investigate surface area as a factor which affects the rate of diffusion?

Answer 26

- Cut the agar jelly into different sized cubes e.g 0.5*0.5*0.5, 1*1*1, 2*2*2 and work out their surface area to volume ratio. - Take out the cubes at the same, and measure the distance the acid travels in each cube. - You would expect the cubes with the largest surface area to volume ratio to have the largest distance travelled by acid.

Question 27

How can you investigate concentration gradient as a factor affecting the rate of diffusion?

Answer 27

- Prepare test tubes containing different concentrations of hydrochloric acid. - Put an equal sized cube of the agar jelly in each test tube and time how long it takes each one to turn colourless. - You would expect the cubes in the highest concentration of hydrochloric acid to go colourless fast.

Question 28

How can you investigate temperature as a factor affecting the rate of diffusion?

Answer 28

- Prepare several boiling tubes containing the same concentration of hydrochloric acid and put the tubes into water baths of varying temperatures. - Put an equal-sized cube of the agar jelly into each boiling tube and time how long it takes each cube to go colourless. - You would expect the cubes in the highest temperature to go colourless fastest.

Question 29

What is facilitated diffusion?

Answer 29

- Facilated diffusion is diffusion across a membrane through protein channels. - Larger molecules (e.g amino acids, glucose), ions and polar molecules diffuse via facilitated diffusion. - It is a passive process.

Question 30

What are the roles of carrier proteins in diffusion?

Answer 30

- Carrier proteins move large molecules into or out of cell, down their concentration gradient. - Different carrier proteins facilitate the diffusion of different proteins. 1) First, a large molecule attaches to a carrier protein in the membrane. 2) Then, the protein changes shape. 3) This releases the molecule on the opposite side of the membrane.

Question 31

What are the roles of channel proteins in diffusion?

Answer 31

- Channel proteins form pores in the membrane for charged particles to diffuse through. - Different channel proteins facilitate the diffusion of different charged particles.

Question 32

What is simple diffusion?

Answer 32

- Diffusion in the absence of a barrier or membrane.

Question 33

What is active transport?

Answer 33

- Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration. - This process requires energy as particles are being moved up a concentration gradient, in the opposite direction to diffusion. - Carrier proteins are also needed.

Question 34

What is the general process of active transport?

Answer 34

1) The molecule or ion to be transported binds to receptors in the channel of the carrier protein on the outside of the cell. 2) On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate. 3) Binding of the phosphate molecule to the carrier protein causes the protein to change shape- opening up to the inside of the cell. 4) The molecule or ion is released to the inside of the cell. 5) The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP. 6) The carrier protein returns to its original shape. - It is a selective process.

Question 35

What is bulk transport?

Answer 35

- A form of active transport where large molecules or whole bacterial cells are moved into or out of a cell by endocytosis or exocytosis.

Question 36

What is endocytosis?

Answer 36

- The bulk transport of material (e.g proteins, lipids and some carbohydrates) into cells.

Question 37

What are the two types of endocytosis?

Answer 37

- Phagocytosis (for solids) | - Pinocytosis (for liquids)

Question 38

What is the process of endocytosis?

Answer 38

1) The cell surface membrane first bends inwards when it comes into contact with the material to be transported. 2) The membrane enfolds the material until eventually the membrane fuses, forming a vesicle. 3) The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell. E.g vesicles containing bacteria are moved towards lysosomes, where the bacteria are digested by enzymes.

Question 39

What is exocytosis?

Answer 39

- The bulk transport of materials out of the cell. | - Substances produced by the cell (e.g digestive enzymes, hormones, lipids) need to be released from the cell.

Question 40

What is the process of exocytosis?

Answer 40

- Vesicles containing these substances pinch off from the sacs of the Golgi apparatus and move towards the plasma membrane. - The vesicles fuse with the cell surface membrane and the contents of the vesicle are then released outside of the cell.

Question 41

Is ATP required for bulk transport?

Answer 41

Yes, energy in the form of ATP is required for the movement of vesicles along the cytoskeleton, changing the shape of cells to engulf materials, and the fusion of the cell membranes as vesicles form or as they meet the cell-surface membrane.

Question 42

What is osmosis?

Answer 42

Osmosis is the net movement of water molecules from a high water potential to a low water potential.

Question 43

What is water potential ?

Answer 43

It is the measure of the quantity of water compared to solutes.

Question 44

How is water potential measured?

Answer 44

It is measured as the pressure created by the water molecules in kilopascals (kPa).

Question 45

What is hydrostatic pressure?

Answer 45

The pressure created by water in a closed system.

Question 46

What happens when an animal cell is placed in a solution with a higher water potential than that of the cytoplasm?

Answer 46

- Water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell. - The cell-surface membrane cannot stretch much and cannot withstand the increased pressure. - It will break and the cell will burst, an event called cytolysis.

Question 47

What happens when an animal cell is placed in a solution that has a lower water potential than the cytoplasm?

Answer 47

- It will lose water to the solution by osmosis down the water potential gradient. - This will cause a reduction in the volume of the cell and the cell crenates.

Question 48

What happens when water enters a plant cell by osmosis?

Answer 48

- The increased hydrostatic pressure pushes the cell surface membrane against the rigid cell walls. - This pressure against the cell wall is called turgor.

Question 49

What happens when water leaves a plant cell by osmosis?

Answer 49

- There is a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall. - The cell is said to be plasmolysed.

Question 50

what is cell signalling ?

Answer 50

communication between cells to trigger a response inside the cell

Question 51

functions of phospholipid bilayer :

Answer 51

- cell compartmentalisation | - to act as a barrier, selects what leaves and enters cell