1.4: Membrane Transport

Question 1

Passive transport

Answer 1

involves the movement of substances along a concentration gradient (high=low)

does not require the expenditure of energy

3 main types:
-simple diffusion
-osmosis
-facilitated diffusion

Question 2

Simple diffusion

Answer 2

the net movement of molecules from an area of high concentration to a low concentration

movement of small/lipophilic molecules (O2, CO2)

rate of diffusion is affected by temperature
molecular size
steepness of gradient

Question 3

Osmosis

Answer 3

the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration

essentially the diffusion of free water molecules since it occurs from regions of low solute concentrations

Question 4

Facilitated diffusion

Answer 4

the passive movement of molecules across the cell membrane with the aid of membrane proteins

e.g
channel proteins
carrier proteins
-potassium channels

Question 5

Active transport

Answer 5

involves the movement of molecules against a concentration gradient (low=high)

requires the expenditure of energy

2 main types:
-primary(direct)
-secondary(indirect)

Question 6

Osmolarity

Answer 6

measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)

Solutions may be loosely categorised as hypertonic, hypotonic or isotonic according to their relative osmolarity

Solutions with a relatively higher osmolarity are categorised as hypertonic (high solute concentration ⇒ gains water)

Solutions with a relatively lower osmolarity are categorised as hypotonic (low solute concentration ⇒ loses water)

Solutions that have the same osmolarity are categorised as isotonic (same solute concentration ⇒ no net water flow)

Question 7

Estimating osmolarity

Answer 7

The osmolarity of a tissue may be interpolated by bathing the sample in solutions with known osmolarities

The tissue will lose water when placed in hypertonic solutions and gain water when placed in hypotonic solutions

Water loss or gain may be determined by weighing the sample before and after bathing in solution

Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)

Question 8

Vesicular Transport

Answer 8

Materials destined for secretion are transported around the cell in membranous containers called vesicles

These vesicles are:
Endoplasmic reticulum
Golgi apparatus
Plasma membrane

Question 9

Endoplasmic reticulum

Answer 9

Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use

Smooth ER is involved in lipid synthesis and also plays a role in carbohydrate metabolism

Question 10

Golgi Apparatus

Answer 10

The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex

Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face
While within the Golgi apparatus, materials may be structurally modified

Question 11

Plasma membrane

Answer 11

Vesicles containing materials destined for extracellular use will be transported to the plasma membrane

The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid

Materials sorted by the Golgi apparatus may be either:

Released immediately into the extracellular fluid (constitutive secretion)
Stored within an intracellular vesicle for a delayed release in response to a cellular signal (regulatory secretion)

Question 12

Bulk Transport

Answer 12

The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis

Question 13

Endocytosis

Answer 13

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

because:
An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
The invagination is then sealed off to form an intracellular vesicle containing the material

2 main types:
Phagocytosis – The process by which solid substances are ingested (usually to be transported to the lysosome)
Pinocytosis – The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

Question 14

Exocytosis

Answer 14

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane

Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment
The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis