Cellular Membrane (1.3)

Question 1

What are the primary molecules of the cell membrane?

Answer 1

Phospholipids

Question 2

How do Phospholipids help play a key role in homeostasis?

Answer 2

The membrane maintains the internal environment of the cell from the outside and any changes in the membrane structure are likely to affect the exchange of substances.

Question 3

What organelle synthesize the phospholipids?

Answer 3

Smooth Endoplasmic Reticulum

Question 4

What causes the membrane to be considered flexible, permeable, or “fluid?”

Answer 4

The movement of the phospholipids

Question 5

What composes a phospholipid?

Answer 5

Phosphate-glycerol head (HYDROPHILIC) and two fatty-acid tails, chains of Hydrogen and Carbon atoms, termed “hydrocarbon chain” (HYDROPHOBIC).

Question 6

What makes a substance Hydrophobic or Hydrophilic?

Answer 6

• Hydrophilic: Polar or ionic molecules that are capable of forming bonds with water molecules; attract water molecules.
• Hydrophobic: Nonpolar molecules that are incapable of forming bonds with water molecules; repel the water molecules.

Question 7

Define amphipathic and outline the amphipathic properties of phospholipids.

Answer 7

• Phospholipids are AMPHIPATHIC
  
  • Definition: a single molecule that contains both hydrophilic and hydrophobic parts.
  • Hydrophilic (“water-loving”) head portions are exposed to water.
  • Hydrophobic (“water fearing”) tail portions of are oriented inside the bilayer.

Question 8

Explain why phospholipids form bilayers in water, with reference to hydrophilic phosphate heads and two hydrophobic hydrocarbon tails.

Answer 8

There is water on either side of the cell membrane.
- Inside = cytoplasm
- Outside = extracellular fluid
Phospholipids will arrange themselves as a bilayer so that the hydrophilic head associates with water inside and outside of the membrane and the hydrophobic tails face each other, within the membrane, away from the water.

Question 9

State the primary function of the cell membrane.

Answer 9

• Composed of a phospholipid bilayer with embedded proteins, the cell membrane regulates the movement of substances in and out of cells.
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis, and active transport*

Question 10

Membrane proteins can be classified into two broad categories— integral and peripheral, contrast them.

Answer 10

Integral membrane proteins are embedded in the phospholipid bilayer. Most integral proteins span the entire phospholipid bilayer. Peripheral membrane proteins are indirectly or loosely attached to the surface of the cell membrane but may dip slightly into the lipid bilayer. Integral proteins are amphipathic. Peripheral proteins are polar (hydrophilic) and are attached to the outside of the plasma membrane.

Question 11

What are the types of proteins within a cell membrane?

Answer 11

Channel, Carrier, Receptor, Recognition, Enzyme.

Question 12

Define channel protein and cite function.

Answer 12

Channel proteins form pores that extend across the lipid bilayer; when these pores are open, they allow ions/substances to cross.

Question 13

Define carrier protein and cite function.

Answer 13

Carrier proteins bind the specific solute to be transported and undergo a series of shape changes to transfer the bound solute across the membrane.

Question 14

Define recognition protein and cite function

Answer 14

Recognition proteins are proteins that are embedded in the cellular membrane that allow cells to identify each other and interact.
Recognition proteins are often “glycoproteins,” proteins with small chains of carbohydrates (oligosaccharides) attached.

Question 15

Define receptor protein and cite function

Answer 15

Receptor proteins are proteins that are embedded in the cellular membrane that bind to specific chemical signals from outside the cell. When the chemical signal binds, the membrane protein triggers a response by the cell.

Question 16

Define enzymes and site function

Answer 16

A large number of cellular enzymes are located in membranes. Enzymes are proteins that catalyze (speed up) chemical reactions

Question 17

What are glycolipids?

Answer 17

Glycolipids are a phospholipid and a carbohydrate attached together. They are important in maintaining the structure of the cell membrane and in cells differentiating between self and non-self cells.

Question 18

What is cholesterol?

Answer 18

Cholesterol is a LIPID and acts to modulate membrane fluidity and permeability to some solutes.
Cholesterol is a steroid made up of a non-polar part comprising four ring structures, a hydrocarbon tail, and a polar hydroxyl group (hydrophilic).

Question 19

Describe the structural placement of cholesterol within the cell membrane.​

Answer 19

Cholesterol inserts into the bilayer of phospholipids:
The polar hydroxyl group forms hydrogen bonds with the phospholipid head group.
Nonpolar hydrocarbon rings and tails positioned near the hydrocarbon chains of phospholipid tails

Question 20

Describe the function of cholesterol molecules in the cell membrane and how temperature affects cell membrane fluidity.

Answer 20

restricts the movement of phospholipids and other molecules, thus reducing membrane fluidity.

However, at low temperatures, it disrupts the regular packing of the hydrocarbon tails of phospholipid molecules which prevents the solidification of the membrane.

This enables the membrane to stay more fluid at lower temperatures, allowing the membrane to function properly.

Question 21

Draw a cellular membrane

Answer 21

Question 22

Who came up with our current cellular membrane representation?

Answer 22

Singer and Nicolson in 1972

Question 23

What was the Davson-Danielli model?

Answer 23

the model suggested that the cell membrane comprises a lipid bilayer where two layers of polar lipid molecules are arranged with their hydrophilic heads outward. The lipid bilayer is itself sandwiched between two protein layers on either side of the membrane

Question 24

How was the Davson-Danielli model proven wrong?

Answer 24

Evidence from freeze-fracture techniques also confirmed that proteins are embedded in the membrane. Using these techniques, biologists delaminated (separated) membranes along the middle of the bilayer. When viewed with an electron microscope, the fracture revealed an irregular rough surface inside the phospholipid bilayer. The globular structures appearing on the fractured surface were interpreted as trans-membrane proteins (that span the whole membrane) and it was deduced that proteins penetrate into the hydrophobic interior of the membrane.

Question 25

What made Davson and Danielli think they were right?

Answer 25

This model was put forward following evidence obtained from electron microscopy which showed the membrane as having three layers (see Figure 2). In high-magnification electron micrographs, membranes appeared as two dark parallel lines with a lighter-colored region in between.

Since proteins normally appear dark in electron micrographs and phospholipids appear lighter, it was deduced that membranes comprised two protein layers, one on either side of a phospholipid core.