2.5 Biological membranes

Question 1

Simple diffusion

Answer 1

Movement of molecules from high to low concentration
Passive so doesn’t require ATP
may/may not be across a membrane

Question 2

Facilitated diffusion

Answer 2

Diffusion across a membrane through protein channels from high to low concentration
Passive so doesn’t require ATP
Helps ions pass through

Question 3

Osmosis

Answer 3

Movement of water over a partially permeable membrane/from a dilute solution to a more concentrated one
From high to low concentration
Passive so doesn’t require ATP

Question 4

Active transport

Answer 4

Movement of molecules from low to high concentration
Active so requires ATP
E.g. movement of glucose up a xylem vessel
Uses carrier proteins

Question 5

Bulk transport

Answer 5

Mass movement of molecules in/out of a cell from low to high concentration
Active so requires ATP
Examples of endocytosis/exocytosis

Question 6

Roles of the membrane

Answer 6

Acts as a partially permeable barrier between the cell and its environment, between organelles and cytoplasm, within organelles
Site of chemical reactions
Site of cell communication (cell signalling)

Question 7

What is compartmentalisation?

Answer 7

Formation of separate membrane-bound areas in a cell- containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions

Question 8

What are the parts of a plasma membrane?

Answer 8

Phospholipids, glycoproteins, glycolipids, carrier/channel proteins, cholesterol

Question 9

Phospholipids info

Answer 9

Comprised of glycerol, 2 hydrophobic fatty acid tails and a hydrophilic phosphate group, in bilayer heads point outwards and tails point toward themselves

Question 10

Cholesterol

Answer 10

In between phospholipids, helps with fluidity/flexibility, maintains fluidity (from being too stiff/fluid), has a philic/phobic ends that interact with corresponding ends on phospholipids to pull membrane together

Question 11

Glycoproteins and Glycolipids

Answer 11

Branching carb attached to outside of a protein/phosphate head, acts as recognition sites- membrane-bound receptor molecules to detect presence of hormones/drugs. Cell signalling occurs- when chemical binds to receptor it creates a response from the cell or sets off a cascade of events)

Question 12

Channel proteins

Answer 12

Provide a hydrophilic channel that allows diffusion of polar molecules from high to low concentration through the membrane

Question 13

Carrier proteins

Answer 13

Play a role in active and passive transport- shape of protein changes when a specific molecule binds to fit molecule

Question 14

Intrinsic proteins

Answer 14

Transmembrane proteins (embedded through both layers)- have amino acids with hydrophobic R-groups on their surfaces that interact with the membrane’s hydrophobic core to keep them in place. Examples are channel and carrier proteins

Question 15

Extrinsic proteins

Answer 15

Present in one side of the bilayer- normally have hydrophilic R-groups on their outer surfaces and interact with polar phospholipid heads or with intrinsic proteins. Examples are glycoproteins

Question 16

How does temperature affect diffusion rate?

Answer 16

The higher the temperature, the higher the rate of diffusion as molecules have increased KE so move at higher speeds- increases permeability if molecules have increased KE

Question 17

How does concentration affect diffusion rate?

Answer 17

The greater the difference in concentration, the steeper the gradient between 2 regions, the faster the rate of diffusion as there are more molecules able to diffuse

Question 18

How does diffusion distance affect diffusion rate?

Answer 18

The thicker the membrane across which molecules have to diffuse, the slower rate of diffusions as molecules have to diffuse over a larger distance

Question 19

How does surface area affect diffusion rate?

Answer 19

The larger the area of an exchange surface, the more space their is for molecules to be taken in, the higher the diffusion

Question 20

How does concentration gradient affect diffusion rate?

Answer 20

The steeper the gradient, the faster the diffusion of molecules from the side with more molecules to the side with fewer, down the concentration gradient

Question 21

How do molecules in simple diffusion move?

Answer 21

From their own KE, which is affected by the surface area of a cell, concentration, temperature and PH
Diffusion stops when equilibrium is reached (no longer high to low as the high concentration is no longer high)

Question 22

Fick’s law with simple diffusion is the rate of movement of a substance across an exchange surface. It’s worked out by:

Answer 22

Surface area x concentration difference all ÷ by diffusion path

Question 23

How do molecules in facilitated diffusion move?

Answer 23

Large, polar molecules like glucose sometimes unable to pass through membrane as phospholipid tails are hydrophobic, so they use channel/carrier proteins to transport them- carrier proteins change shape for the molecule
Rate of diffusion affected by temperature, concentration gradient, surface area, diffusion distance and amount of proteins present

Question 24

How do molecules in Bulk transport move?

Answer 24

Membrane is fluid so movement of large quantities of materials move into cells (endo) and out of cells (exo)
Active process so requires ATP/energy to form vesicles and move them

Question 25

How do molecules in active transport move?

Answer 25

Molecule binds to receptors in the channel of the carrier protein and ATP binds to the protein where its hydrolysed into ADP and a phosphate, phosphate molecule binds and causes shape to change opening a pathway to the cell, the molecule goes through into the cell and the phosphate is released to recombine with ADP forming ATP again, then the protein returns to its original shape

Question 26

Endocytosis involves: (Plus its process)

Answer 26

Phagocytosis- movement of solids Pinocytosis- movement of liquids Membrane bends when in contact with transported material and enfolds the material until eventually the membrane fuses forming a vesicle, vesicle separates and moves into the cytoplasm to transfer the materials for further processing like how bacteria are moved towards lysosomes to be digested

Question 27

Phagocytosis

Answer 27

Bulk transport of solids, takes place in specialised cells called phagocytes White blood cells ingest/engulf bacteria as part of our body's defence system Amoebas do this as a food source

Question 28

Pinocytosis

Answer 28

Bulk transport of liquids, takes place in human egg cell to take up nutrients for possible zygote

Question 29

Exocytosis involves: (Plus its process)

Answer 29

Reversal of endocytosis where materials are removed from the cell Vesicle carrying large molecules moves to and fuses with the membrane, the fused site opens and releases contents of the secretory vesicle Happens in phagocytes and plant cells during cell wall binding

Question 30

How does temperature affect membrane structure and permeability?

Answer 30

Increased temperature gives KE to phospholipids so they move more and membrane becomes more fluid (losing its structure), this loss increases permeability making it easier for molecules to diffuse across, channel/carrier proteins become denatured and if temp continues to increase then the cells break down completely

Question 31

How do solvents affect membrane structure and permeability?

Answer 31

Water (a polar solvent) essential in bilayer formation as tails orientated away from water to from bilayer with phobic core ad heads interact with water keeping bilayer intact. Organic solvents are less polar than water e.g. alcohols(benzene is a non-polar example)- organic solvents dissolve membranes, disrupting cells (like why alcohol used in antiseptic wipes as the alcohol dissolves the bacteria membranes in a wound, killing them and reducing infection risk) Membrane becomes more fluid/permeable when disrupted

Question 32

Actual correct terms used for osmosis

Answer 32

Water molecules move from a region of high water potential to a region of low water potential (down a water potential gradient), across a partially permeable membrane and equilibrium eventually reached

Question 33

What is water potental?

Answer 33

Measure of tendency (how likely) water is to move from one place to another The higher the KE, the higher the water potential Always from higher to lower

Question 34

Unit and symbol for water potential

Answer 34

Measured in kilopascals (kPa) Pure water has a water potential of 0 kPa (the highest possible water potential) Symbol looks like a trident (Ψ)

Question 35

What does more of a solute do to water potential?

Answer 35

This lowers the water potential, it therefore becomes negative- the more concentrated a solution is, the more negative water potential is

Question 36

Solute potential

Answer 36

The amount by which solute molecules lower the water potential

Question 37

Isotonic

Answer 37

Solutions with the same solute potential so no net movement of water

Question 38

Hypotonic

Answer 38

Solutions with low concentrations of solutes so high Ψ, water moves from hypotonic solutions to other solutions

Question 39

Hypertonic

Answer 39

Solutions with high concentration of solutes so low Ψ, water molecules will move from other solutions to hypertonic solutions

Question 40

The closer to 0 kPa, the...

Answer 40

Higher the water potential

Question 41

Hydrostatic pressure

Answer 41

When water diffuses into a closed system like a cell it increases the hydrostatic pressure (diffusion of water into a solution increases the solution's volume)

Question 42

Water potential and animal cells with cytolysis and crenation

Answer 42

If an animal cell is placed in a solution with a higher Ψ than that of its cytoplasm, water will move into the cell by osmosis, increasing its hydrostatic pressure- the membrane cannot stretch and withstand the pressure so will burst- this is called cytolysis If solution has lower Ψ than the cell's cytoplasm then it will lose water by osmosis and "pucker"/shrivel and shrink, called crenation

Question 43

Define turgid and flaccid in terms of plant cells

Answer 43

Turgid- plant cell that's fully inflated with water Flaccid- plant cell that's limp through pressure reduction inside cell

Question 44

Haemolysis

Answer 44

Rupturing of red blood cell- if placed in concentrated solution, cell absorbs water and possibly bursts/ruptures

Question 45

Plasmolysis

Answer 45

When plant cells are put in solution with a lower Ψ than theirs, water is lost from the cell by osmosis and this leads to a reduction in the cytoplasm's volume- this pulls the membrane away from the cell wall- plasmolysis occurs