Pre-mitosis: Stem cells, transport, cell membrane structure (1)

Question 1

Describe hypotonic solutions, hypertonic solutions, and isotonic solutions, and how the former two affect cells (Hint: try to include “solute concentration” and “osmosis”)

Answer 1

Hypotonic solution: a solution in which the solute concentration outside the cell is lower than the solute concentration inside the cell. Water will enter the cell through osmosis.

Hypertonic solution: a solution in which the solute concentration inside the cell is lower than the solute concentration outside the cell. Water will exit the cell through osmosis.

Isotonic solution: a solution in which the solute concentration outside the cell is the same as the solute concentration inside the cell.

Question 2

Describe the process of cell specialization.

Answer 2

1. A zygote goes through rapid cleavage to form a blastula.
2. The blastula forms a blastocyst, its cells arranging itself into an inner cell mass and an outer layer.
3. The cells in the blastula rearrange to form three layers (gastrulation)

(Extra notes: three layers are endoderm, mesoderm, and ectoderm. Ectoderm forms skin and nervous system, endoderm forms intestinal organs, mesoderm forms rest of organs)

Question 3

Describe gene activation in a cell and what it enables.

Answer 3

Every cell in a multicellular organism contains all its genes, but not all genes are activated in each cell. Activated genes encode for proteins, which decide the structure/function of the cell. This selective activation allows cells to differentiate and form specialized tissues.

(Extra note: This is generally regulated during transcription)

Question 4

Why are stem cells valuable? Name the five types of stem cells in order of decreasing potency (Toilet Paper In My Underwear)

Answer 4

Self-renewal: they can continuously undergo mitosis without differentiating
Potency: they are able to differentiate into several different cell types.

1. Totipotent: Can form any cell type, as well as extra-embryonic tissue (e.g. zygote)
2. Pluripotent: Can form any cell type (e.g. embryonic stem cell)
3. Induced Pluripotent: Pluripotent stem cell that comes from an adult stem cell
4. Multipotent: Can differentiate into closely related cell types (e.g. cells in bone marrow)
5. Unipotent: Cannot differentiate, but can self-renew (e.g. muscle stem cells)

Question 5

Explain either Stargardt’s or Parkinson’s Disease and how stem cells can assist with it.

Answer 5

Stargardt’s: An inherited form of juvenile macular degeneration that causes progressive vision loss. Retinal photoreceptors degenerate due to impairing gene mutation. Functional retina cells from stem cells can replace dead ones.

OR

Parkinson’s: Brain disorder that causes uncontrollable movement that can impair walking and talking. Caused by a loss of nerve cells in the substantia nigra (part of brain), which produces dopamine to control movement. Dopamine producing neurons from stem cells can replenish loss.

Question 6

Define osmosis and how the involved substance travels based on concentration levels.

Answer 6

Osmosis is the diffusion of water across a semi-permeable membrane.

(Extra note: The semi permeable membrane acts like a filter that lets only the water through)

Water always goes from the area of higher water concentration to the area of lower water concentration.

(A good way to remember is think of the particles other than water as salt! Salt Sucks!)

Question 7

Define diffusion.

Answer 7

Diffusion is the movement of particles from areas of higher concentration to areas of lower concentration. It is a natural, random process that it does not require extra energy input.

Question 8

What are two differences between diffusion and osmosis?

Answer 8

• Osmosis can only function in a liquid medium, but diffusion can occur in all three mediums.
• Osmosis requires a semi-permeable membrane, while diffusion does not.

Question 9

What is the plasma membrane and what is it composed of?

Answer 9

The plasma membrane is the boundary that separates the living cell from its surroundings with selective permeability. It is composed of phospholipids and protein.

Question 10

Describe the two possible membrane models.

Answer 10

Davson-Danielli Model (Sandwich Model): Model where there are protein layers surrounding the phospholipids. Explained how the thin membranes could be an effective barrier.

Singer and Nicholson Model (Fluid Mosaic Model): Proteins are found at various positions in the membrane. They are either peripheral (attached to a surface), or integral (embedded and run through the membrane). Proteins can move with phospholipids.

Question 11

Which is correct, and what is one way that it was proved?

Answer 11

The fluid mosaic model is correct!

1. Freeze-Etched Electron micrographs: The cell is rapidly frozen in liquid nitrogen, then cleaved along the fracture plane that splits the lipid bilayer. Embedded proteins were observed as bumps.

OR

2. Structure of Membrane Proteins: Membranous proteins extracted and analyzed were found to be different sizes and globular in structure, unlike those that would form continuous layers on the periphery.

OR

3. Fluorescent Antibody Tagging: Red and green markers were attached to the antibodies that bind to membrane proteins. One had red, the other had green; they were then fused together. The colours mixed together throughout the membranes, proving that proteins could flow throughout the membrane.

Question 12

What is the cellular membrane composed of?

Answer 12

Cellular membranes consist mainly of two macro molecules:
- Lipids (mainly phospholipids and some cholesterol)
- Proteins (various types)

Question 13

Wild carrrrrd! Go look at a labelled diagram of the fluid mosaic model and try to remember where these parts are:

• Fibers of extracellular membrane
• Glycoprotein
• Microfilaments of cytoskeleton
• Cholesterol
• Integral proteins
• Peripheral proteins
• Glycolipid

Question 14

Wild carrrrrd! Look at Figure 3 on this website: https://www.visionlearning.com/es/library/Biologia/2/Lipids/207

Try to remember the glycerol, carboxyl group, fatty acid, and double bond to label!

Question 15

Are the glycerol head and fatty acid tails of a phospholipid hydrophilic or hydrophobic, respectively?

Answer 15

The glycerol head is hydrophilic; the fatty acid tails are hydrophobic.

Question 16

What is the point of hydrophobia and hydrophilia, especially in phospholipids?

Answer 16

Hydrophobic (non-polar) and Hydrophilic (polar) layers prevent substances from randomly moving into/out of the cell.

Phospholipids are amphipathic, containing regions of both.

Hydrophobic regions are pushed towards the middle of the bilayer (away from water).
Hydrophilic regions face towards the cytoplasm and extracellular fluid (towards water)

This allows the lipid membrane to to spontaneously form a bilayer and maintain its structure.

Question 17

How does cholesterol help regulate membrane fluidity?

Answer 17

• At warm temperatures, cholesterol restrains movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing tight packing

Question 18

No need to answer here! Take a look at some structural properties of the phospholipid bilayer! Definitely try to remember a couple though :)

Answer 18

• Phospholipids are held together in a bilayer by hydrophobic interactions.
• Phospholipids with short or unsaturated fatty acids are more fluid.
• Phospholipids can move horizontally or occasionally laterally to increase fluidity
• Fluidity allows for the breaking / remaking of membranes (exocytosis / endocytosis)

Question 19

What is the difference between passive and active transport?

Answer 19

Passive transport requires no cellular energy, as substances are moved along their concentration gradient.
Active transport requires the expenditure (use) of cellular energy, moving substances against their concentration gradient.

(Extra Note: Passive transport is primarily driven by diffusion, and the energy for active transport is primarily found in ATP)

Question 20

What are the 3 factors affecting diffusion rate? (That’s So Cool!)

Answer 20

• Temperature
• Size of particles
• Concentration gradient

Question 21

Why is there no net diffusion at dynamic equilibrium?

Answer 21

A concentration gradient causes net movement from high to low concentration. But at dynamic equilibrium, many molecules cross in both directions, so there is no net movement

Question 22

What is tonicity?

Answer 22

The ability of a solution to cause a cell to gain or lose water.

Question 23

How do animal and plant cells react to different tonicities?

Answer 23

Animal cells: Isotonic solutions keep the cell in a normal state–in hypotonic solutions the cell becomes lysed and bursts, and in hypertonic solutions it shrivels.

Plant cells: Hypotonic solutions keep the cell in a normal (turgid) state–in isotonic solutions the cell becomes flaccid, and in hypertonic solutions it becomes plasmolyzed.

Question 24

Explain water intoxication.

Answer 24

1. Too much water enters the body’s cells, and tissues swell with the excess fluid.
2. Too much water outside the cells due to an imbalance of electrolytes produces a hypotonic solution.
3. Permeable electrolytes and water move across the cell membrane to balance the concentration.
4. The excess water dilutes blood sodium levels and causes fluids to move inside cells, which then swell.

(

• Symptoms include headache, nausea, and vomiting.
• More severe symptoms include increased blood pressure, confusion, double vision, drowsiness, trouble breathing, and muscle cramping.
• At its worst, water intoxication can cause brain damage, coma, and even death

)

Question 25

What types of molecules can cross the synthetic bilayer unobstructed and obstructed, respectively?

Answer 25

Unobstructed:

Small hydrophobic molecules: e.g. O2, N2, CO2, benzene
Small uncharged hydrophilic molecules: e.g. H2O, glycerol, ethanol

Obstructed:

Larger uncharged hydrophilic molecules: e.g. amino acids, glucose, nucleotides
Ions: e.g. H+, Na+, HCO3-, K+ Ca(2+), Cl-, Mg(2+).

(Maybe try and remember one of each?)

Question 26

What enables the passage of hydrophilic substances across the membrane?

Answer 26

Transport proteins:
- Channel Proteins: have a hydrophilic channel that certain molecules or ions can use as a tunnel
- Carrier Proteins: bind to molecules and change shape to shuttle them across the membrane

Question 27

How does the concentration gradient affect the rate of diffusion?

Answer 27

The rate of diffusion decreases as the concentration gradient decreases.

Question 28

How do you access the energy for active transport?

Answer 28

The energy can be accessed via:
- Direct hydrolysis of ATP (primary active transport)
- Indirectly coupling transport with another molecule moving along its gradient (secondary active transport)

Question 29

List the steps in the process of a sodium-potassium pump. (Hint: 5 steps!)

Answer 29

1. 3 sodium ions bind to intracellular sites on sodium-potassium pump
2. Phosphate group is transferred to the pump via the hydrolysis of ATP
3. Pump undergoes a conformational change, translocating sodium across the membrane
4. Conformational change exposes two potassium binding sites on pump’s extracellular surface
5. Phosphate group is released, causing pump to return to its original conformation
6. Potassium is translocated across the membrane, completing the ion exchange

(A ridiculous and overcomplicated rhyme that I came up with to help; feel free to use:

3 sodiums bind to intra-pump sites
Phosphate transfers to pump via ATP hyd-s
Con-change makes sodium translocate
Pot binding sites on surface in an exposed state
Phosphate’s let go, pump’s con goes og
Then the pot translocates and we’re finally free!)

Question 30

Define cotransport.

Answer 30

When the active transport of a solute indirectly drives transport of another solute

(Extra Note: They are transported through symports and antiports. One example is cotransport is glucose transport in the intestines.)

Question 31

Describe bulk transport and its two methods.

Answer 31

Large molecules (e.g. proteins, polysaccharides) have to cross the membrane in bulk via vesicles, through the processes of:
- Endocytosis (pinocytosis, phagocytosis, receptor mediated)
- Exocytosis

(Extra notes: Bulk transport requires energy. Microtubules are used to move vesicles / change membrane position)

Question 32

Define exocytosis.

Answer 32

Transport vesicles migrate to the membrane, fuse with it, and release their contents.

(Extra note: Many secretory cells use exocytosis to export their products)