Cell Structure and Function (Pt. 2) Plasma Membrane & related Functions/Mechanisms: 2.3-2.9

What's the chemical structure of a phospholipid?

• A polar/hydrophilic head: Glycerol (3-carbon alcohol) & phosphate (OPO3 2-)

• Nonpolar/hydrophobic tails: hydrocarbons (fatty acids)

How/why does the structure of the phospholipid lead to the formation of a bilayer?

- The tails of the phospholipid go on the inside while the heads are on the outside ( O= =O)

-The polar/hydrophilic heads being on the outside allow the bilayer to interact with water (polar attracted to polar) while the nonpolar/hydrophobic tails on the inside are able to interact with each other and avoid interacting with water (nonpolar not attracted polar)

What does the fluid mosaic model describe about a plasma membrane?

-Describes the membrane's flexibility, changing, and ability to move with its surrounding & the variety of proteins/molecules that are apart of the membrane

How does TEMPERATURE affect the fluidity of the plasma membrane?

- High temp. --> molecules move faster --> very fluid
-Low temp. --> molecule movement low --> less fluid

How does FATTY ACID SHAPE affect the fluidity of the plasma membrane?

-saturated fatty acids (no double bonds) --> straight --> less space --> less fluid

-unsaturated fatty acids (1+ more double bonds) --> bent --> more space --> more fluid

How does CHOLESTEROL affect the fluidity of the plasma membrane?

• wedges between phospholipids

• at body temp. --> restricts lipid movement --> less fluid

• at lower temp. --> hinders close-packing of lipids, so even lower temperatures are required to fully solidify (more flexible than at body temp)

What types of membrane proteins are found in a phospholipid bilayer? (Describe)

• Integral Proteins: penetrate through the hydrophobic interior

• Transmembrane Proteins: (Subtype of integral) span entire membrane EX. Channel, Carrier

• Peripheral Protein: loosely bound to surface of membrane

What does it mean for the a membrane to be "selectively permeable"?

- means that only certain molecules can pass through while other cannot

Explain why smaller molecules generally have an easier time crossing the phospholipid bilayer.

- The size allows for the molecule to easily "slip through" the polar head and past the nonpolar tails without getting stuck

Explain why charged molecules generally have a harder time crossing a phospholipid bilayer.

- While charged molecules can interact with the polar heads of the phospholipids, the nonpolar tails are not attracted to the charge of the molecules and deflect rather than passing through successfully

What's the difference between channel and carrier proteins?

• Channel proteins do not require energy and are typically just open from one end to the other (w/ exception of some ion channels that have a "lid" and use a molecule, pressure, or charge to open them); typically facilitated diffusion

• Carrier proteins sometimes requires energy and typically changes in shape to account for the substances being moved from one side to another; typically active transport

Explain the factors influencing permeability of the cell wall.

• Semi-permeable

• Size NOT polarity

• Fully permeable to smaller proteins and other molecules

Summarize the difference between passive and active transport

- Passive: doesn't require energy (ATP), flows WITH the gradient [High] to [Low],

-Active: requires energy (ATP) & carrier proteins, typically flows AGAINST the gradient [Low] to [High]

How can dynamic equilibrium be achieved through diffusion?

- Substance will move naturally with their gradients through the plasma membrane until the concentrations inside of a cell and outside of a cell are roughly equal, but movement of substances inside and out of a cell is still continuous

How are diffusion and osmosis similar and different?

• Similar: Passive transport (with the gradient)

• Different: Diffusion is the movement of molecules and particles, Osmosis is the diffusion of just water

(i.e Osmosis is a form of diffusion)

What are aquaporins, and what are they used for?

- Aquaporins are channel proteins that allow for the movement of large quantities of water in the process of osmosis (faster than through the membrane)

How are diffusion and facilitated diffusion similar and different?

• Similar: Passive Transport (with the gradient)

• Different: Diffusion is directly through the phospholipid bilayer while facilitated diffusion uses transport proteins (channel or carrier) to pass solutes through the membrane

What type(s) of substances are usually moved across a membrane using active transport?

• Solutes (usually ions)

• Large molecules

• Any molecule moving against the gradient (depending on Transport protein)

How does HEAT ENERGY influence the movement of materials across a phospholipid bilayer?

- Higher Temp. --> Faster molecules move --> more likely to pass through membrane

How does DIFFERENCE IN CONCENTRATION influence movement across a phospholipid bilayer?

- Molecules move naturally from [High] to [Low] so if a molecule is in an area with a high concentration of "itself" the molecule will move to a lower concentration

How does SIZE OF MOLECULE influence movement across a phospholipid bilayer?

- Smaller molecules can typically pass through the phospholipid bilayer w/o the usage of transport proteins (diffusion)
-Larger molecules need a transport protein to pass through the phospholipid bilayer (facilitated diffusion)

How is bulk transport used by cells?

• Used to move very large or a lot of molecules at once

• The cell creates vesicles using the phospholipid bilayer to let contents into (endocytosis) or out of (exocytosis) the cell (requires energy)

How are exocytosis and endocytosis similar and different?

- Similar: usage of vesicles, transport of materials, involved with the phospholipid bilayer, Active/Bulk transport

-Different:
Exocytosis --> movement of materials out of the cell, protein secretion, merges with the phospholipid bilayer

Endocytosis --> movement of materials into the cell, buds off the phospholipid bilayer

Differ between the 3 specific types of endocytosis.

Phagocytosis: Cell eating; engulfing a food particle

Pinocytosis: Cell drinking; bringing in extracellular fluid and dissolving solutes

Receptor-mediated Endocytosis: movement of a specific substance into a cell after it binds to a receptor protein on cell's membrane

Differ between the various types of ion channels used in facilitated diffusion

Ligand-gated channel: Only opens when another molecule binds to the protein

Mechanically-gated channel: Only opens when physical pressure is applied to the cell membrane

Voltage-gated channel: Only opens when the overall charge on one side of the membrane is at a certain point

Explain how ATP and electrochemical gradients are used in active transport

• ATP will bind to a carrier protein, hydrolyze into ADP and inorganic phosphate, the inorganic phosphate will remain bound to the carrier protein changing the shape of the protein, then when removed the shape changes back

• Electrochemical gradients are a difference in charge and concentration across the membrane, causing ions to move down the gradient; due to direct or indirect use of ATP

Differ between uniporter, symporter, and antiporter. What is the Na+/K+ pump an example of?

Uniporter: Relies on ATP, transports ONE type of molecule/ion in ONE direction

Symporter: Relies on an electrochemical gradient, transports TWO types of molecules/ions in the SAME direction

Antiporter: Relies on ATP or electrochemical gradient, transports TWO types of molecules/ions in OPPOSITE directions

Na+/K+ pump is an example of an antiporter, movement of Na+ in one direction and K+ in another direction using ATP

Differ between primary and secondary active transport. How are they related?

Primary: Directly uses ATP as an energy source

Secondary: Relies on electrochemical gradient

Secondary Active Transport is set up by Primary Active Transport

Differ between isotonic, hypotonic, and hypertonic solutions

Isotonic: roughly equal concentrations on the inside and outside (No net movement of water/Homeostasis)

Hypotonic: [Water]: Outside > Inside, [Solute]: Outside < Inside, (Net movement of water into cell); water diffuses into cell at greater rate

Hypertonic: [Water]: Outside < Inside, [Solute]: Outside > Inside, (Net movement of water out of cell); water diffuses out of a cell at a greater rate

What is the effect on an animal cell AND plant cell when surrounding solution is Isotonic, Hypotonic, or Hypertonic?

Isotonic

• Animal: normal

• Plant: limp, plants wilt b/c central vacuole is smaller

Hypotonic

• Animal: swells in size, may burst (lyse)

• Plant: healthy --> turgid (very firm), due to turgor pressure that cell wall exerts back on expanding cell

Hypertonic

• Animal: shrivels and dies (crenated)

• Plant: shrivels and cytoplasm pulls away from cell wall (plasmolysis)

Water Potential Interpretation (Tips/Reminder)

• Water moves from high water potential to low water potential (reminder: the larger the negative, the smaller the value)

• Open container of aqueous solution has a Wp of 0

• Salts (i = 2)

• Sugars (i = 1)

• T should be in K not Celsius (K = 273 + C)

-Practice problems in the study guide on canvas

• Water moves to hypertonic areas (so if water has a net flow in a certain direction, where it came from is hypotonic) (hypotonic and hypertonic go hand in hand description wise)

How does the surface area to volume ratio differ between small and large cells?

• Small cells --> larger SA:V ratio --> more efficient at diffusion

• Larger cells --> smaller SA:V ratio --> less efficient at diffusion

*Think of the ratio as a fraction and the larger the denominator (aka Volume) the smaller the fraction (the amount of SA for a given volume)

What adaptations do prokaryotic cells and eukaryotic cells have to increase their efficiency?

Prokaryotic Cells

• Cell size, tend to be smaller since less distance to travel into the cell & creates a larger SA:V ratio which is more effective in diffusion

Eukaryotic Cells

• Small size

• Folded internal membranes (ex. inner mitochondrial membrane (cristae), ER, and Goldi) --> larger surface area in tissues and organs due to folds