BIOL 107 Part B - Cell Membranes and Transport (Questions))

Why are cells small?

1. Surface area to volume ratio must be small enough to import nutrients and export waste products

2. Cells specialize to perform certain functions

Humans have ___ types of cells.

200

What is the structure of the plasma membrane?

Phospholipid bilayer, with the hydrophobic tails of the phospholipids pushing together and the hydrophilic heads facing water outwards

Why is the phospholipid bilayer flexible?

The phospholipids are not actually attached to one another, only held together by hydrophobic interactions

Why do phospholipid bilayers form spontaneously?

It is energetically favourable for a sealed compartment to be formed in order to ensure no hydrophobic edges are exposed to water

Describe the fluidity of plasma membranes.

Remains moderately fluid until it reaches a temperature in which it solidifies

How does the saturation of the hydrocarbon tails of phospholipid bilayers affect their fluidity?

Phospholipids with unsaturated tails enhance membrane fluidity because their kinked tails prevent proper packing, while phospholipids with saturated tails pack together to decrease membrane fluidity

How does cholesterol moderate fluidity of the plasma membrane?

Prevents leaking at moderate temperatures by by reducing phospholipid movement (reducing fluidity); prevents solidification at low temperatures by disrupting the regular packing of phospholipids

Peripheral membrane proteins (are/are not) embedded into the phospholipid bilayer.

Are not

Integral membrane proteins have a hydrophobic region made of ___ coiled into ___ secondary structure.

Stretches of nonpolar amino acids, alpha helix

What are the functions of transmembrane proteins?

Transport, enzymatic activity, signal transduction, glycoproteins as identification tags for cell-cell recognition, intercellular joining, attachment to the cytoskeleton and extracellular matrix

What are the characteristics of animal cell plasma membranes?

Contain phospholipids, membrane proteins, cholesterols, membrane carbohydrates

H, A, and B antigens in human blood cells are ___.

Glycolipids

D antigens in human blood cells are ___.

Integral membrane proteins

Patients can receive blood transfusions from blood types that ___.

Do not contain antigens they don’t already have

What are the characteristics of plant cell plasma membranes?

Have a cell wall for protection, contain just phospholipids and membrane proteins

What are the characteristics of prokaryote cell plasma membranes?

Have a cell wall for protection, contain just phospholipids and membrane proteins

Cells want to increase membrane surface area because ___.

More membrane means more membrane proteins to be able to carry out essential functions

What are the ways in which cells increase membrane surface area?

Outfoldings, infoldings, internal membranes

Animal cells prefer ___ environments because ___.

Isotonic; no net movement of water in and out of the cell, allowing it to maintain a stable volume

Plant cells prefer ___ environments because ___.

Hypotonic; the turgid environment is maintained by the cell wall, providing pressure that gives plants structure

In an isotonic environment, plant cells become ___ due to ___.

Flaccid (wilted), lack of sufficient pressure

In a hypotonic environment, animal cells become ___ due to ___.

Lysed (burst), excess pressure on the membrane of the cell

In a hypertonic environment, animal cells become ___ and plant cells become ___ due to ___.

Shriveled, plasmolyzed, excess water loss

Which molecules can pass through the phospholipid bilayer by diffusion and why?

Small, nonpolar molecules (steroid hormones and gases) that are either small enough to diffuse through or are soluble across the hydrophobic interior of the bilayer

Describe how is oxygen transported into muscle cells.

Oxygen concentration inside muscles is low, causing oxygen in the bloodstream to diffuse into the muscle. Once oxygen is used by the muscle, concentration drops, allowing for more diffusion into the cells

How are steroid hormones able to pass through the phospholipid bilayer?

They are small and mostly nonpolar (hydrophobic) and thus can dissolve in the membrane to move inside

Which molecules can pass through the phospholipid bilayer by membrane transport proteins?

Small, polar molecules (ions, amino acids, monosaccharides)

Channel proteins are involved in (active/passive) transport, and (do/do not) change shape.

Passive, do not

Passive transporters are involved in (active/passive) transport, and (do/do not) change shape.

Passive, do

Do passive transporters require energy to change shape? Why or why not?

No, they are attached to the cell membrane which spontaneously changes shape on its own

What are the pros and cons of channel proteins?

Pros: quick molecule transport due to not needing to change shape, cons: unspecific

What are the pros and cons of passive transporters?

Pros: transports specific molecules across the membrane, cons: needs longer to change shape

What powers active transport?

Hydrolysis of ATP, which produces ADP, an inorganic phosphate, and energy

Describe the workings of a proton pump (concentrations and molecules in and out of cell).

Pumps H+ ions from low concentration inside the cell to high concentration outside the cell. Uses energy from ATP hydrolysis, resulting in ADP creation and an inorganic phosphate to transferred to the pump

Describe the workings of the sodium-potassium pump (concentrations and molecules in and out of cell).

3 Na+ inside the cell binds to the sodium-potassium pump in its first shape (affinity for sodium), stimulating phosphorylation by ATP (energy created powers the proton pump, ADP created and inorganic phosphate produced attaches to the pump). Phosphorylation of the pump leads the protein to change into its second shape (affinity for potassium), releasing the 3 Na+ outside the cell and binding to 2 K+ outside the cell. Binding of K+ causes the pump to release the phosphate group, restoring its original shape and causing it to release the K+ inside the cell.

Describe the workings of the sucrose-proton transporter (concentrations and molecules in and out of cell). How does it relate to the proton pump?

The sucrose-proton cotransporter carries both sucrose and H+ ions across the membrane. H+ diffuses into the cell with its concentration gradient, powering the active transport of sucrose either in or out against its concentration gradient. The proton pump is not directly involved in the cotransport process, but maintains the H+ gradient so that H+ can diffuse into the cell and power the cotransport.

ATP (directly/indirectly) powers proton cotransport. Why?

Indirectly, ATP powers the proton pump which provides the gradient necessary for cotransport

How is sucrose imported into root cells (processes) in Arabidopsis plants?

Leaf cells make sucrose via photosynthesis, which is actively imported into the root cells (high concentration) via SUC1 transporters

How is glucose uptaken by humans?

Small intestine cells contain a sodium-potassium pump, maintaining a sodium gradient. Glucose is actively imported into the cell from low to high concentration via SGLT and passively exported out of the cell from high concentration to low concentration via GLUT

The sodium-potassium pump in human intestine cells exists on the (extracellular/internal) side to power glucose transport because ___.

Extracellular; Na+ needs to be pumped out of the cell to maintain a sodium gradient that will bring glucose into the cell, Na+ is needed in the bloodstream

Explain how immature red blood cells uptake iron.

Iron proteins are embedded into iron transport proteins (transferrin), which binds to corresponding receptors on the RBC and allows for receptor-mediated endocytosis