Cell Biology - 1.4 Membrane Transport

Question 1

Passive transport

Answer 1

• a way in which materials move across the cell membrane (by diffusion, facilitated diffusion, osmosis (and active transport)) - does NOT use energy as molecules move from a high to low concentration (ie. down the concentration gradient)

Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)

Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)

• simple diffusion
• osmosis
• facilitated diffusion

Question 2

Active transport

Answer 2

(ion pumps and phagocytosis) - USES ENERGY as molecules move from a LOW TO HIGH concentration AGAINST the concentration gradient

Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)

Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)

Can either be:

1. direct
2. indirect

Question 3

Semi-permeable

Answer 3

• the membrane is Semi-permeable ie. it allows some molecules through but not all molecules (controls what goes in and out) - some small molecules can easily diffuse across, but most need help

(only certain materials may freely cross – large and charged substances are typically blocked)

Question 4

Concentration gradient

Answer 4

• the changing in concentration of a substance as it travels over a distance
• where concentration is different across an area = there is a high and low concentration area
• the gradient of concentrations of different particles within an area (eg. the spray in a classroom)
• a concentration gradient exists until the diffused substance is evenly distributed
• if there is a large difference in concentration between the two areas, there is a steep concentration gradient so the diffusion is faster

Question 5

Simple Diffusion (Passive Transport)

Answer 5

it is the passive movement of particles (atoms, ions molecules) from an area of higher concentration to an ara of lower concentration down a concentration gradient.

• requires no energy
• diffusion occurs as a consequence of the random motion of particles

(eg. glucose diffuses into a cell and is used to make energy by respiration - movement of small or lipophilic molecules (e.g. O2, CO2, etc.))

Question 6

Factors of diffusion rates

Answer 6

1. molecule size - small = move faster
2. temperature - higher temperature = move faster
3. concentration - diffusion is faster down the steeper gradients

Question 7

Facilitated diffusion (Passive Transport)

Answer 7

(Simple diffusion but requires proteins to support it)

movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)

It is the passive transport of molecules or ions across a cell membrane via specific transmembrane integral proteins

THE EXCEPTION = POLAR molecules (excluding water) CANNOT diffuse freely across the plasma membrane - due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the lipid bilayer - all polar molecules that enter the cells are transported by proteins in the form of transmembrane channels

Question 8

Lipid bilayer

Answer 8

The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells.

it is a part of the membrane that is virtually impermeable - materials (eg. proteins, hormones, water, etc) are transported across it by being “facilitated” by integral proteins that span the membrane.

Question 9

Channel proteins (ion channels)

Answer 9

Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other

Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli

Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)

Channel proteins have a much faster rate of transport than carrier proteins

Question 10

Osmosis (passive transport)

Answer 10

Water always wants to go: 1. high water -> low water, 2. low salt -> high salt

“the movement of water molecules from an area of high concentration to an area low water concentration through a semi-permeable membrane”

is 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 (until equilibrium is reached)

• does not use energy to make it happen
• (dependent on solute concentrations)

Question 11

Osmolarity

Answer 11

“measure of the concentration of solutes in a solution” as defined by the number of osmoles of a solute per litre of solution (osmol/L)

can be categorised as hypertonic, hypotonic or isotonic

Question 12

Hypotonic

Answer 12

• high water levels
• hypotonic area
• low solute concentration

When a cell is placed in a hypotonic solution it will gain water and swell (become TURGID/lysis) - in a plant cell the cell membrane will be constrained by the cell wall - it could BURST

(Vacuole swells and pushes against the cell membrane === turgent)

water flows into cell

Question 13

Isotonic

Answer 13

• equal concentration (of solute in both areas - in and outside the shell)

no change = water outside and inside stays equal

Question 14

Hypertonic

Answer 14

• lower water level
• high solute
• hypertonic area

When a cell is placed in a hypertonic solution it will lose water and shrink (become FLACCID/crenation) - in a plant cell the cell membrane will pull away from the cell wall - in extreme cases the cell cannot recover and is said to be PLASMOLYSED = PLASMALISED

Water flows out of cell

Question 15

Tonicity

Answer 15

determining if a solution is hypotonic, isotonic or hypertonic

• > high water solution = water wants to go into the cell (opposite applies for cell in salty solution) —> If cells absorb too much water they will pop!!
• must compare it (the solution) to something eg A to B

Question 16

solution “equation”

Answer 16

solution = solvent (eg. H2O) + solute (dissolved)

Question 17

Isotonic saline solutions - application

Answer 17

they are used in medical procedures (eg. kidney transplants and intravenous therapy to rehydrate patients) - they must be bathed in the same osmolarity as human cytoplasm

Application - tissues or organs to be used in medical procedures must be bathed in a solution the same osmolarity as the cytoplasm to prevent osmosis

Question 18

osmoregulation

Answer 18

The process of controlling the amount of water in cells

Question 19

Why osmosis is important to plants

Answer 19

• plants absorb water through the roots and reabsorption of water by the proximal and distal convoluted tubules of the nephron

1. needed for photosynthesis
2. allows them to gain water and stay alive -> “support”

(Freshwater - cells gain water as the cytoplasm contains dissolved substances = incise cell has lower water concentration than outside = USEFULL AS: keeps water flowing into cells ensuring they are turgid

saltwater - cells tend to lose water = concentration of water is much higher in cells than sea)

Question 20

Practical: Osmosis in living potato cells

COMPULSORY PRACTICAL YOU MUST KNOW

Answer 20

Skill = Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions

Aim = determine the osmolarity of potato cells
IV = concentration of sucrose/sugar in H2O
DV = Osmosis in potato cells
CV = potato types, volume of sugar solutions, etc. 

Method:

1. use cork borer to cut 10 cylinders from potato (equal length)
2. weigh each potato
3. place into boiling tube x 2 (one with 0.7 mol/l sugar solution, “” 0.5, “” 0.3, “” 0.1, ““0)
4. leave overnight, next day re-weigh potatos
5. calculate averages and put onto graph

Reasons for results for hypertonic = when the sucrose concentration was greater the percentage decrease was great -> due to the H2O in the potato diffused out of the potato —- opposite applies for hypotonic solutions.

Question 21

Salt in the human body

Answer 21

Our bodies naturally require salt & because of this when put into a SLIGHTLY hypertonic solution the cells will withhold a “normal” mass - ie. cells were not killed.

Question 22

Potassium Ion Channels - example of facilitated diffusion

Answer 22

• the most common type of ion channels - they form potassium-selective pores that span cell membranes

Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
The channel is comprised of four

transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest
region that restricts passage of alternative ions

Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage

Structure and Function of Potassium Channels in Axons:
- Axons of neuron contain potassium channels that as used during an action potential (They are closed when the axon is polarised but open in response to depolarisation of the axon membrane allowing K+ ions to exit by facilitated diffusion which repolarizes the axon - they will only remain open for a very short time before a blobular sub-unit blocks the pore, the channel then returns to its original closed conformation)

Question 23

Potassium Ion Channels - Process

Answer 23

• they are used to move K+ ions across the membrane (In axons of neurons, the channels work with the sodium-potassium pump)
  => the axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane:
• at rest the the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within the cell
• When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels.

Question 24

Active Transport

Answer 24

• is the movement of molecules from an area of low concentration to an area of high concentration against the concentration gradient and REQUIRES energy (ATP)
• Integral protein pumps use the energy from the hydrolysis of ATP to move ions or large molecules across the cell membrane

eg.
1. uptake of glucose in the intestines in humans
2. the exchange of Na and K ions in nerve axons
3. uptake of mineral ions into root hair cells of plants

Question 25

Two types of active transport

Answer 25

1. protein pumps

2. use of vesicles

Question 26

Protein Pumps

Answer 26

They only transport specific substances and in a specific direction. The substance binds to the protein and energy (ATP) is used to more the substance through.

A specific solute will bind to the protein pump on one side of the membrane

The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump

The solute molecule is consequently translocated across the membrane (against the gradient) and released

Question 27

Uniport

Answer 27

transports one substance in one direction

Question 28

Symport

Answer 28

transports two different substances in the same directions

Question 29

Antiport

Answer 29

Transports two different substances in opposite directions

Question 30

Sodium-Potassium Pumps in Nerve Axons

Alternating facilitated diffusion and active transport

Answer 30

Application: Structure and function of sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons

Nerve cells conduct nerve umpulse via a “Mexican Wave” on Na+ and K+ ions in and out of the nerve - the axons contain a pump protein that move the Na and K ions across the membrane - the 2 ions are move in opposite directions (antiport). The pump has two states: 1. there is more binding site attraction for K+ ions outside the membrane so K moves in and Na+ moves out 2. the reverse of 1 occours (ATP causes the switch from one state to another

1 ATP molecule can move 3 Na+ and 2 K+ ions

this helps establish a NET CHARGE across the PLASMA MEMBRANE with the interior of the cell being NEGATIVELY charged with respect to the exterior (ie. prepares nerve/muscle cells for the propagation of nerve impulses)

Question 31

“Use of Vesicles”

Answer 31

Understanding: the fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

Exocytosis and Endocytosis can occur due to the fluidity of the cell membrane (its phospholipid bilayer)

Question 32

Exocytosis (secretion) - use of vesicles

Answer 32

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

Vesicles are usually small vacuoles contains proteins that bud off from the endoplasmic reticulum or the golgi bodies and travel to the cell membrane - vesicle fuses with the cell membrane (contents are expelled and the membrane flattens out again

eg. the secretion of substances (eg hormones or contractile vacuoles in paramecium)

Question 33

Endocytosis

Answer 33

Part of the cell membrane is pulled inwards - the material is engulfed and the vesicle is pinched off. - The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

Process: 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

1. Phagocytosis (is engulfing solid material) - The process by which solid substances are ingested (usually to be transported to the lysosome)
2. Pinocytosis (is engulfing fluid material) - The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

Question 34

Selective (membrane)

Answer 34

(membrane proteins may regulate the passage of material that cannot freely cross)

Question 35

Direct - active transport

Answer 35

Primary (direct) active transport – Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport

Question 36

Indirect - active transport

Answer 36

Secondary (indirect) active transport – Involves coupling the molecule with another moving along an electrochemical gradient

Question 37

Diffusion

Answer 37

Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration

• This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium)
• Small and non-polar (lipophilic) molecules will be able to freely diffuse across cell membranes (e.g. O2, CO2, glycerol)

Question 38

Aspects of osmosis

Answer 38

Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes)

Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions

At a higher solute concentration there are less free water molecules in solution as water is associated with the solute

Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration

Question 39

Carrier Proteins

Answer 39

Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane

Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction

Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)

Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)

Question 40

Energy for active transport - how to get it:

Answer 40

This energy may either be generated by:

The direct hydrolysis of ATP (primary active transport)

Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)

Question 41

Vesicular Transport

Answer 41

Endoplasmic Reticulum
Golgi Apparatus
Plasma Membrane

https://ib.bioninja.com.au/standard-level/topic-1-cell-biology/14-membrane-transport/vesicular-transport.html

Question 42

Cell Chemistry - what molecules are made up of:

Answer 42

carbohydrates (eg. glucose, starch, cellulose)
Lipids (eg. fats, waxes, oils)
Proteins (eg. enzymes, hormones, collagen, muscle)
Nucleic acids (eg. DNA)
Minerals (eg. calcium, iron, sodium)
water

Question 43

Carbon in organic compounds

understanding + skills

Answer 43

Organic compounds contain carbon and are found in living things
Each carbon atom forms 4 covalent bonds
- Covalent bonds are relatively strong so molecules are stable - weaker bonds can form between molecules

Understanding:
Carbon atoms can form four covalent bonds allowing a diversity of stable compounds to exist. Life is based on C compounds including carbohydrates, lipids, proteins and nucleic acids

Skills:
Identification of biochemicals such as sugars, lipids or amino acids from molecular diagrams. Drawing molecular diagrams of glucose, ribose, a saturated fatty acid and generalised amino acid.

Question 44

Theory of Vitalism

Answer 44

Original thought: you had to have a living thing to create other living things (ie. a VITAL FORCE)

Falsified by discoveries in biochem - Synthesis of Urea

Question 45

Synthesis of Urea

Answer 45

Urea was discovered in human urine in 18th century - in 1828 Fredrich Wohler synthesised urea artificially using silver isocyanate and ammonium chloride.

The chemical synthesis of organic compounds is a key aspect of Organic Chemistry and today most organic compounds that makeup living organisms can be synthesised.

Application:
urea as an example of a compound that is produced by living organisms but can also be artificially synthesized.

Question 46

Metabolism

Answer 46

• every single biochemical reaction within your body

Understanding:
Metabolism is the web of all the enzyme - catalysed reactions in a cell or organism

The reaction may be anabolic or catabolic - a metabolic pathway is a series of chemical reactions within a cells

Question 47

Anabolism

Answer 47

Understanding:
Anabolism is the synthesis of complex molecules from simpler molecules including the formation of macromolecules from monomers by condensation reactions (simple -> complex)

Anabolic pathways eg. the synthesis of sugar from CO2, photosynthesis, the synthesis of large proteins from amino acid

Question 48

Catabolism

Answer 48

Understanding:
It is the breakdown of complex molecules into a simpler molecules including the hydrolysis of macromolecules into monomers (complex -> simple)

Catabolic pathway eg. respiration where glucose is broken down into CO2 and H2O to produce energy in the form of ATP