Topic 1: Plasma Membranes

Question 1

What does the fluid mosaic model mean?

Answer 1

The term fluid comes from the fact that the fatty chains of the phospholipids are like a thick oily liquid
The term mosaic comes from the fact that the external surface has the appearance of a mosaic because of the various embedded proteins set in a uniform background

Question 2

What is the function of the plasma membrane?

Answer 2

• selectively controls molecules entering and leaving the cell
• establishes compartments within the cell, separating cell components
• restricts movement of substance between one part of the cell and another therefore regulating chemical reactions and enzyme action
• has receptors involved in intercellular communication by hormones and nerves
• involved in cell recognition and adhesion so cells adhere to form tissues and can recognise and respond to self and non self molecules and cells
• produce electric activity in excitable cells (neurons)

Question 3

What is the structure of phospholipids?

Answer 3

• Membranes in a cell are made up of a double layer of phospholipid molecules forming the phospholipid bilayer.
• A phospholipid molecule consists of a hydrophilic phosphate head and two hydrophobic fatty acid tails

Question 4

Why is the positioning of the phospholipid molecule important?

Answer 4

The phosphate head is attracted to water so it’s found facing the outside and the inside of the cell whereas the fatty acid tails are repelled by water and face inwards away from the watery environment.
The lipid structure of the membrane gives it the unique property of being flexible soft and able to repair itself.

Question 5

What are the transport proteins involved in passive transport?

Answer 5

Channel proteins and carrier proteins allow some substances such as ions to move through the membrane by facilitated diffusion.

Question 6

What is a gated protein channel?

Answer 6

A transport protein that opens to transport a specific molecule in response to contact with another molecule, such as a hormone

Question 7

What are the transport proteins involved in active transport?

Answer 7

A protein pump or Carrier proteins is a transport protein involved in active transport

Question 8

What are catalytic proteins?

Answer 8

Enzymes are catalytic proteins that speed up the rate of chemical reactions.

Question 9

What are receptor proteins?

Answer 9

Receptor proteins have hormones and other substances bind to them affecting cell activities (glycoproteins)

Question 10

What are recognition proteins?

Answer 10

Recognition proteins act as markers called antigens which enable immune system to recognise self cells from foreign cells

Question 11

What are adhesion proteins?

Answer 11

Adhesion proteins join cells together in a multicellular organism

Question 12

What are cytoskeleton proteins?

Answer 12

They anchor the cytoskeleton to the membrane

Question 13

What is cholesterol?

Answer 13

Steroid lipid that is amphipathetic and is embedded within the phospholipid bilayer
Regulates membrane consistency and maintains firmness and stability of the membrane by preventing it from becoming overly fluid
Maintains fluidity by separating fatty acid tails so they don’t crystallise.
Reduces permeability to some solutes

Question 14

What are GLYCOLIPIDS?

Answer 14

Found on the plasma membrane and play a role in tissue recognition
Glyco refers to a carbohydrate molecule attacked to the phospholipid molecule

Question 15

What are glycoproteins?

Answer 15

Found on the outer plasma membrane and function as cell surface markers or antigens.
In mammals these markers enable the immune system to identity these cells as self not foreign cells.

Question 16

What is the surface area to volume ratio?

Answer 16

The surface area to volume ratio of a cell imposes upper limits on size, as the volume increases much faster than does the surface area, thus limiting the rate at which substances dissolve from the interior across the cell membrane to interstitial spaces or other cells.
Changes in the Sa:V ratio have important implications for limits or constraints on organism growth and help explain some of the modifications seen in some cells.

Question 17

What is simple diffusion?

Answer 17

Net movement of a substance across the phospholipid bilayer from a region of high concentration to a region of low concentration of that substance

Question 18

Does simple diffusion require energy?

Answer 18

It is a passive process and does not require energy.
It occurs along the concentration gradient. The end point is reached when equal concentrations of the substance are reached on both sides of the plasma membrane

Question 19

What can cross the plasma membrane through simple diffusion?

Answer 19

Gases such as o(2) and co(2) and small uncharted molecules can cross the plasma membrane via simple diffusion as well as small hydrophobic molecules that can dissolve in the lipid bilayer.

Question 20

What is osmosis?

Answer 20

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 21

Does osmosis require energy?

Answer 21

It occurs down the concentration gradient and does or require energy. Movement occurs more slowly compared to movement through specific protein channels

Question 22

Hypotonic

Answer 22

Hypotonic refers to a solution with a lower concentration of solutes compared to the solute concentration inside a cell. There will be a net movement of water into the cell from surrounding cells

Question 23

Hypertonic

Answer 23

Hypertonic refers to a solution with a higher concentration of solutes compared to the solute concentration inside a cell. There will be a net movement of water out of the cell into surrounding solution

Question 24

Isotonic

Answer 24

When the surrounding fluid is of equal concentration to the solution inside the cell it is said to be isotonic. There will be no net movements of water (water will diffuse equally in both directions).

Question 25

What happens to plant cells in a hypotonic solution?

Answer 25

Plant cells contain a freely permeable cell wall and a large vacuole which contain sap rich in solutes a solution of high concentration.
If the external concentration of solute molecules is hypotonic than inside the vacuole water will move freely into the vacuole via osmosis from adjacent cells. The vacuole swells and pushes the cell membrane against the cell wall making the cell turgid through osmotic pressure.

Question 26

What happens to plant cells in a hypertonic solution?

Answer 26

The the concentration of solutes in the external environment is greater than the vacuole water will move out via osmosis.
This reduces the size of the vacuole the plasma membrane moves away from the cell wall and the cell becomes flaccid through plasmolysis.

Question 27

What happens to animal cells place in hypotonic solution?

Answer 27

Animal cells such as red blood cells placed in a hypotonic solution will burst. Called haemolysis.

Question 28

What happens to animal cells placed in a hypertonic solution?

Answer 28

In a hypertonic solution, red blood cells will shrink and crinkle. This is called crenation.

Question 29

What is facilitated diffusion?

Answer 29

Facilitated diffusion enables molecules that cannot diffuse across the phospholipid bilayer to move across the plasma membrane through the agency of transporter proteins.

Question 30

What can cross through facilitated diffusion?

Answer 30

Transport proteins in the plasma membrane assists the diffusion of charged ion and larger molecules such as Glucose to diffuse across the bilayer.
These proteins form water filled connection across the lipid bilayer which allow water soluble (hydrophilic) substances to cross.

Question 31

What are channel proteins and what do they allow to cross?

Answer 31

Channel proteins are channels which open and close to allow specific water soluble molecules and ions through.

Question 32

What are carrier proteins?

Answer 32

Carrier proteins combine with the molecule or ion and carry it across the membrane. Once the molecule has bound to the protein the protein changes shape and releases its content to the other side. Carrier proteins allow the diffusion of hydrophilic uncharged substances such as glucose and amino acids.

Question 33

What is active transport?

Answer 33

Active transport is the movement of molecules or ions from a region of low concentration of a region of high concentration.

Question 34

Does active transport require energy?

Answer 34

Occurs against the concentration gradient and requires energy. Movement involves carrier proteins for each substance being transported.

Question 35

Why is active transport important?

Answer 35

It allows cells to take up nutrients even when concentration outside the cell are very low and to excrete waste, maintaining homeostasis.

Question 36

Exocytosis

Answer 36

Substances are actively transported out of the cell by moving them in small vacuoles called vesicles. The membrane of vesicles are filled up with secretions that are not permeable in the membrane (digestive enzymes, hormones)
The vesicle membrane fuses with the plasma membrane release the content to the outside of the cell.
Requires ATP

Question 37

Endocytosis

Answer 37

Substances are actively transported into the cell, requires ATP. Part of the membrane engulfs the solid to form a vesicle which moves into the cytosol. The vesicle then fuses with another vesicle containing digestive enzymes that digest the contents

Question 38

Pinocytosis

Answer 38

Bulk transport of material that is in solution being transported into cells

Question 39

Phagocytosis

Answer 39

Bulk movement of solids material into cells

Question 40

What is receptor mediated endocytosis

Answer 40

Process by which cells internalise molecules or viruses. Depends on the interaction of that molecule with a specified binding protein in the cell membrane called a receptor

Question 41

How does size of particle affect diffusion?

Answer 41

At a given temperature a smaller particle moves faster than a lager one, the rate of diffusion is inversely proportional to the size of molecules, a smaller molecule will diffuse faster.

Question 42

How does temperature affect rate of diffusion?

Answer 42

As temperature increases amount of energy available for diffusion increases. A given particle will move faster at t a higher temperature. Rates of diffusion will be faster as temperature increases.

Question 43

How does concentration difference affect rate of diffusion?

Answer 43

When a substance is diffusion between two compartments the greater the concentration gradient between them the faster the rate of diffusion.

Question 44

How does diffusion distance affect rate of diffusion?

Answer 44

For a particle at a given temperature, it takes longer for the particle to diffuse a farther distance. The farther a particle will diffuse the longer it Will take and the slower the rate of diffusion.

Question 45

How does surface area affect rate of diffusion?

Answer 45

When a substance is diffusing between two compartments the greater the surface area of the membrane the greater the probability that a particle will pass through it. Larger SA equals greater rate of diffusion

Question 46

What factors affect active transport?

Answer 46

Number and type of carrier proteins and protein pumps per unit area
Surface area
ATP availability 
Temperature
Ph

Question 47

How does pH affect membranes?

Answer 47

Different acid and alkaline levels can also change the shape of proteins causing denaturation so that the membrane becomes freely permeable

Question 48

What is selectivity of the cell membrane determined by?

Answer 48

The phospholipid bil hare and the number of and types of proteins channels, carrier proteins and protein pumps.

Question 49

What are the steps for protein synthesis?

Answer 49

• rRNA is stored in the nucleoli and passes out into the cytoplasm where it becomes part of the ribosomes.
• mRNA is produced in the nucleus and passes out into the cytoplasm where it binds to a ribosome.
• protein synthesis is initiated
• proteins are synthesised by ribosomes with tRNA carrying specific amino acids into position. The proteins pass into the cavity of the ER then to the Golgi apparatus
• protein is further modified and packaged for export in vesicle