2.1.5 Biological membranes - Factors affecting membrane structure

Question 1

What factors can affect the movement of molecules across membranes?

Answer 1

Factors that can affect the movement of molecules across membranes are:

• Temperature
• Concentration gradient
• Distance
• Surface area
• Mass of particles
• Solute viscosity
• Solvents

Question 2

How does temperature affect the movement of molecules across membranes?

Answer 2

Increased temperature means that particles have an increase in kinetic energy therefore increased rate of diffusion.

Question 3

How does concentration gradient affect the movement of molecules across membranes?

Answer 3

The steeper the concentration gradient, the greater the net movement if particles.

Question 4

How does distance affect the movement of molecules across membranes?

Answer 4

The shorter the distance particles have to travel the greater the rate of diffusion.

Question 5

How does surface area to volume ratio gradient affect the movement of molecules across membranes?

Answer 5

Increased area equals greater space for diffusion so more particles can move down the concentration gradient in any given time.

Question 6

How does the mass of particles affect the movement of molecules across membranes?

Answer 6

Heavier particles move more slowly, therefore they diffuse more slowly.

Question 7

How does solute viscosity affect the movement of molecules across membranes?

Answer 7

As the viscosity of a solute increases the rate of diffusion decreases. This is because molecules slow down as they have greater difficulty getting through the denser medium.

Question 8

How do solvents affect the movement of molecules across membranes?

Answer 8

Non-polar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing for a faster rate of diffusion.

Question 9

How does the thickness of the membrane affect the movement of molecules across membranes?

Answer 9

The thinner the exchange surface, the higher the rate of diffusion.

Question 10

Define Diffusion

Answer 10

Diffusion is the net, or overall movement of particles from a region of higher concentration to a region of lower concentration.
It is a passive process, and continues until there is a concentration equilibrium.

Question 11

Define Facilitated diffusion

Answer 11

Facilitated diffusion is diffusion across a membrane through protein channels.
Facilitated diffusion is the movement of molecules down a concentration gradient and does not require external energy unlike active transport.

Question 12

Why are membranes described as partially permeable?

Answer 12

Small polar molecules pass through more easily than larger ones. For example, non-polar molecules such as Oxygen (O2) diffuse through freely down a concentration gradient BUT polar molecules such as water (H2O) can diffuse through membranes but only at a very slow rate.

Question 13

Why are transport molecules required to move water molecules and other substances across a membrane?

Answer 13

Ions, sugars, amino acids and sometimes water cannot diffuse across the phospholipid bilayer at sufficient rates to meet the cell’s needs and must therefore be transported by a group of integral membrane proteins including channels, transporters, and ATP-powered ion pumps.

Question 14

What factors affect the rate of facilitated diffusion?

Answer 14

The rate of facilitated diffusion is dependent in the temperature, concentration gradient, membrane surface are and thickness, but is also affected by the number of channel proteins present. The more protein channels, the higher the rates of diffusion overall.

Question 15

Define Active Transport

Answer 15

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.
The process requires energy and carrier proteins.

Question 16

What is an example of a carrier protein?

Answer 16

Carrier proteins span the membrane and act as ‘pumps’. The sodium-potassium pump is the most important active transport channel, which expends metabolic energy to actively pump sodium ions (Na+) in one direction, out of cells, and potassium ions (k+) in one direction, into cells.
More than one-third of all the energy expended by your body’s cell is spent driving Na+K+ pump channels. This energy is derived from ATP.
The transportation of two different ions in opposite directions happens because energy causes a change in shape of the membrane protein carrier.

Question 17

Define endocytosis

Answer 17

Is the bulk transport of material into cells.

Question 18

How does endocytosis occur?

Answer 18

In endocytosis, an invagination of the membrane forms a flask-like depression which envelopes the material; the invagination is then sealed off forming a vesicle containing substances that had been outside the cell.

Question 19

Define Phagocytosis

Answer 19

The process by which solid substances (e.g. food particles, foreign pathogens) are ingested (usually to be transported to the lysosome for break down) [“cellular-eating”].

Question 20

Define Pinocytosis

Answer 20

The process by which liquids/solutions (e.g. dissolved substances) are ingested by the cell (allows quick entry for large amounts of substances) [“cellular-drinking”]

Question 21

Define Invaginates

Answer 21

bends inwards

Question 22

Describe how phagocytosis occurs.

Answer 22

In phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac that is large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

Question 23

Describe how Pinocytosis occurs.

Answer 23

In pinocytosis, the cell gulps droplets of extracellular fluid into tiny vesicles.

Question 24

Why is pinocytosis non-specific in the substances it transports?

Answer 24

Pinocytosis is non-specific in the substances it transports as fluid is taken up by the cell even though the molecules dissolved in the droplets are needed, not the actual fluid. As any and all included solutes are taken into the cell, pinocytosis is described as non-specific.

Question 25

Define Receptor-mediated endocytosis

Answer 25

Enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid.

Question 26

What are coated pits? And how does receptor-mediated endocytosis occur?

Answer 26

Embedded in the membrane are proteins with specific receptors sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins.

The specific substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. After this ingested material is liberated form the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

Question 27

Why is receptor-mediated endocytosis more efficient than pinocytosis?

Answer 27

It is more efficient in attaining the nutrients that the cell needs. Only takes up the specific substance rather than all of the solute/fluid.

Question 28

Define Exocytosis

Answer 28

Is the reverse of endocytosis

Question 29

Describe how exocytosis occurs.

Answer 29

Vesicles, usually formed by the Golgi body, move towards and fuse with the cell surface membrane. The contents of the vesicle are then release outside of the cell.

Question 30

Define Osmosis

Answer 30

Osmosis is the diffusion of water molecules across a partially permeable membrane. Water molecules diffuse from an area of high water potential to an area of low water potential, down a water potential gradient.

Question 31

Define Water potential

Answer 31

Water potential is the pressure exerted by water molecules as they collide with a membrane, measured in pascals (Pa)
Water potential refers to the concentration of free water molecules.
Increased solute concentration = decreased water potential

Question 32

What is the water potential of pure water?

Answer 32

Pure water has a water potential of 0kPa.

Water is always going to move to the more negative number (water potential)

Question 33

What is hydrostatic pressure?

Answer 33

Diffusion of water molecules in a closed system, such as a cell, results in an increase in pressure called hydrostatic pressure.

Question 34

Hypotonic -

Answer 34

higher water potential (less negative) than that of the cytoplasm.

Question 35

Isotonic -

Answer 35

water potential the same as that of the cytoplasm

Question 36

Hypertonic -

Answer 36

lower water potential (more negative) than that of the cytoplasm

Question 37

What is the effect of a hypotonic solution on animal cells?

Answer 37

Net movement of water into the cell.
(high water potential to low water potential).
This will cause the cell to swell and burst, cytolysis.

Question 38

What is the effect of an Isotonic solution on animal cells?

Answer 38

No net movement of water since no water potential gradient.

Question 39

What is the effect of a hypertonic solution on animal cells?

Answer 39

Net movement of water out of the cell.
(low water potential to high water potential)
This will cause the cell to shrink, and become crenated/crenation.

Question 40

Why can’t a plant cell burst?

Answer 40

Plant cells have a strong cellulose cell wall surrounding the cell membrane, which can withstand water pressure,

Question 41

What is turgor pressure?

Answer 41

Pressure against the cell wall from increased hydrostatic pressure is called turgor pressure.
As turgor pressure increases it resists the further movement of water into the cell, and the cell is said to be turgid.

Question 42

What is the effect of a hypotonic solution on plant cells?

Answer 42

Net movement of water into the cell.

This causes the cell to swell and become turgid.

Question 43

What is the effect of an isotonic solution on animal cells?

Answer 43

No net movement of water since there is no water potential gradient.

Question 44

What is the effect of a hypertonic solution on animal cells?

Answer 44

Net movement of water out of the cell.

This causes the cell to become plasmolysed and the contents of the cell to shrink.