1. The Plasma Membrane

Question 1

Why is it called the fluid mosaic model?

Answer 1

• Known as fluid because the fatty chains of phospholipids are like a thick oily liquid
• Referred to as mosaic because the external surface has a mosaic appearance due to embedded proteins set in a uniform background

Question 2

What is the structure of the phospholipid bilayer?

Answer 2

• Double layer of phospholipids
• Each phospholipid molecule consists of two fatty acid chains (hydrophobic tall) joined to a phosphate containing group (hydrophilic head)
• Phosphate head is exposed to external and internal cell environment and fatty acid tail is repelled by water and faces inwards away from the watery environment

Question 3

What is the difference between integral and peripheral proteins?

Answer 3

• integral proteins are fundamental components of the plasma membrane, embedded in the phospholipid bilayer. Span the width of the plasma membrane with exposed parts of the protein on both sides.
• Peripheral proteins are either anchored to the exterior of the plasma membrane through bonding with lipids or indirectly associated with the plasma me,brand through interactions with integral proteins

Question 4

Define plasma membrane

Answer 4

Selectively permeable boundary of a cell separating it from its physical surroundings, boundary entry to and exit of substances from a cell

Question 5

How does the plasma membrane form the active boundary of cells?

Answer 5

• Separates cell from external environment and other cells
• Selectively permeable
• Internal environment can be held within a narrow range of conditions different to external environment. Forms active boundary of cell organelles, membrane bound organelles can maintain internal environment different to cytosol
• Can perform different functions

Question 6

How does plasma membrane contribute to cell identity?

Answer 6

• Glycoproteins on the outer plasma membrane function as cell surface markers, enabling immune system to identify these cells as self, not foreign
• GLYCOLIPIDS on the plasma membrane play a part in tissue recognition

Question 7

How does the plasma membrane help in receiving external signals?

Answer 7

• Cells receive signals from the external environment through chemical compounds (hormones)
• Trans-membrane proteins on the outer surface of the plasma membrane are receptors. The signal binds to the receptor protein altering its shape starting a specific response in the cell

Question 8

How does the plasma membrane help with transport?

Answer 8

Transport proteins in the plasma membrane enable movement of substances that cannot cross the lipid bilayer of the membrane channel proteins carrier proteins proteins pumps

Question 9

Define simple diffusion

Answer 9

Simple diffusion is the net movement of a substance (molecule or ions) across the phospholipid bilayer from a region of high concentration to a region of low concentration of that substance

Question 10

What substances can cross through simple diffusion?

Answer 10

Gases such as oxygen and carbon dioxide, small uncharged molecules, small lipophilic molecules that can dissolve in the bilayer

Question 11

Define osmosis

Answer 11

Osmosis is the net movement of water from a low solute concentration (Hypotonic) to a high solute concentration (hypertonic) across a selectively permeable membrane

Question 12

What happens in hypotonic plant cells?

Answer 12

Plant cells contain a sap rich vacuole, a region of high concentration

• If the external concentration of solutes is hypertonic to the vacuole, water moves in via osmosis from adjacent cells
• Vacuole swells and pushes plasma membrane against the cell wall, making it turgid due to osmotic pressure

Question 13

What happens in external solute concentration in plant cells is hypertonic?

Answer 13

Water will move out of the vacuole via osmosis.

-Vacuole shrinks, plasma membrane moves away from cell wall, flaccid (plasmolysis)

Question 14

Osmosis in animal cells

Answer 14

• when animal cells are placed in a hypotonic solution, haemolysis occurs where the cells burst
• red blood cells will shrink and crinkle in a hypertonic solution (crenation) both cases result in cell death

Question 15

What is facilitated diffusion?

Answer 15

-Enables molecules that cannot diffuse across the phospholipid bilayer to move across the plasma membrane through the agency of transporter proteins (channel protein or carrier)

Question 16

What are channel proteins?

Answer 16

• have a water filled pore through which dissolved substances can pass down their concentration gradient
• Providing hydrophilic passage for water soluble molecules and charged particles (sodium and potassium ions)

Question 17

What are carrier proteins?

Answer 17

• Specific (each type of carrier to one kind of molecule)
• after binding to its specific cargo molecule, the carrier protein undergoes a change in shape as it delivers its cargo to the other side of the plasma membrane
• Allows passage of hydrophilic uncharged substances (glucose and amino acids)
• Open and close in response to certain signals

Question 18

What is transported in active transport?

Answer 18

Small polar molecules and ions
-Essential for key function of cells including pH balance, regulation of cell volume and uptake of needed nutrients even when concentrations outside the cell are very low

Question 19

Define exocytosis

Answer 19

ATP requiring progress in which substances are articles transported out of cell in vesicles containing secretions that are not permeable in the membrane
-Vesicles fuse with the plasma membrane before the contents are released out of the cell

Question 20

What is endocytosis?

Answer 20

Process of bulk transport of material into a cell where part of the plasma membrane encloses material and pinches off to form membranous vesicle that moves into the cytosol.

Question 21

How does molecular size affect diffusion?

Answer 21

At a given temperature smaller particles move fasten than larger ones. The smaller the particle the faster it will diffuse.

Question 22

How does temperature affect diffusion?

Answer 22

As temperature increases amount of energy available for diffusion increases and particles move faster, increasing the rate of diffusion.

Question 23

How does concentration difference affect diffusion?

Answer 23

When a substance is diffusing between two compartments the greater the concentration gradient difference the faster the substance will diffuse.

Question 24

How does diffusion distance affect rate of diffusion?

Answer 24

For a particle at a given temperature (moving at constant speeds) it takes longer for the particle to diffuse a greater distance. The more distance across which a particle must diffuse the longer it will take.

Question 25

How does pH affect the rate of diffusion?

Answer 25

Extremes can change the shape of proteins denaturing them so the membrane is freely permeable

Question 26

What is the protein pathway?

Answer 26

Packages of protein are transferred from the rough ER to the Golgi complete where they are modified and packaged. Secretary vesicles with protein cargo where they break free from the Golgi apparatus and move to and merge with the plasma membrane of the cell and discharge their contents.