Which of the following results best refutes the alternative hypothesis that selective permeability is a consequence of the cell wall?

A certain type of specialized cell contains an unusually large amount of rough

Skill:

•  Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model 

    
The fluid-mosaic model was not the first scientifically accepted paradigm to describe membrane structure

The first model that attempted to describe the position of proteins within the bilayer was proposed by Hugh Davson and James Danielli in 1935

When viewed under a transmission electron microscope, membranes exhibit a characteristic 'trilaminar’ appearance

• Trilaminar = 3 layers [two dark outer layers and a lighter inner region]

Electron Micrograph of Plasma Membrane

Danielli and Davson proposed a model whereby two layers of protein flanked a central phospholipid bilayer

• The model was described as a 'lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers
• The dark segments seen under electron microscope were identified [wrongly] as representing the two protein layers

Skill:

•  Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model 

    
There were a number of problems with the lipo-protein sandwich model proposed by Davson and Danielli:

• It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio 
• It assumed all membranes would have symmetrical internal and external surfaces [i.e. not bifacial]
• It did not account for the permeability of certain substances [did not recognise the need for hydrophilic pores] 
• The temperatures at which membranes solidified did not correlate with those expected under the proposed model 

Falsification Evidence:

Membrane proteins were discovered to be insoluble in water [indicating hydrophobic surfaces] and varied in size

• Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane

Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place

• Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively
• When the two cells were fused, the markers became mixed throughout the membrane of the fused cell
• This demonstrated that the membrane proteins could move and did not form a static layer [as per Davson-Danielli]

Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane 

• These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure

New Model:

In light of these limitations, a new model was proposed by Seymour Singer and Garth Nicolson in 1972

According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers

This model, known as the fluid-mosaic model, remains the model preferred by scientists today [with refinements]

Models of Membrane Structure

CIncubate the cells in the presence of several different concentrations of the protein.DIncubate the cells in the presence of the protein for several different lengths of time.16.Lysosomes digest food particles brought into a cell by endocytosis. After a vesiclecontaining food particles fuses with a lysosome,ions are transported into thelysosome from the cytosol. This significantly lowers theof the lysosome relative to thecytosol and activates the enzymes that digest the particles.Which of the following best predicts what will happen to the lysosomal enzymes if theproteins that transportions from the cytosol into the lysosome are damaged?AThe lysosomal enzymes will not become active, since there will be no active transport ofions.BThe lysosomal enzymes will not become active, sinceions will diffuse out of thelysosome.CThe lysosomal enzymes will become active, since facilitated diffusion will moveionsinto the lysosome.DThe lysosomal enzymes will become active, since passive diffusion will moveions intothe lysosome.17.Which of the following transport mechanisms will be affected most directly by a temporaryshortage ofmolecules inside the cell?

AP BiologyScoring GuideUnit 2 Progress Check: MCQCopyright © 2017. The College Board. These materials are part of a College Board program. Use or distribution of these materials online orin print beyond your school’s participation in the program is prohibited.Page 13 of 26AThe movement of water molecules through aquaporinsBThe diffusion of oxygen molecules across the plasma membraneCThe transport of glucose molecules against a concentration gradientDThe facilitated diffusion ofions into the cell

AP BiologyScoring GuideUnit 2 Progress Check: MCQCopyright © 2017. The College Board. These materials are part of a College Board program. Use or distribution of these materials online orin print beyond your school’s participation in the program is prohibited.Page 14 of 2618.Some membrane proteins help maintain the concentrations of ions inside a cell bytransporting the ions across the cell’s plasma membrane. Other membrane proteins formpores in the plasma membrane through which the ions can diffuse. A model showing theinfluence of membrane proteins on the movement of sodium [] and potassium []ions across a plasma membrane is presented in Figure 1.Figure 1. Section of a cell’s plasma membrane, showing ion concentrations andmembrane proteinsBased on the model presented in Figure 1, which of the following changes will most likelyresult from a depletion of availablestores inside the cell?ATheconcentration outside the cell will increase.BTheconcentration inside the cell will increase.CTheconcentration inside the cell will increase.DTheconcentration outside the cell will decrease.

AP BiologyScoring GuideUnit 2 Progress Check: MCQCopyright © 2017. The College Board. These materials are part of a College Board program. Use or distribution of these materials online orin print beyond your school’s participation in the program is prohibited.Page 15 of 26

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Which of the following best predicts what will happen to the lysosomal enzymes if the proteins that transport H+ ions from the cytosol into the lysosome are damaged? The lysosomal enzymes will not become active, since there will be no active transport of H+ ions.

Which of the following claims about the origin of the euglenid chloroplast is best supported by the three-membrane structure of the envelope? It originated from the incorporation of a photosynthetic prokaryote into a eukaryotic cell by two endosymbiotic events.

Which of the following indicates where testosterone enters a cell and why it is able to cross at that point? because testosterone is nonpolar and can diffuse through the membrane. Reason: Steroids such as testosterone are hydrophobic lipids.

Which of the following cellular deficiencies would most likely be related to these MT-ND1 mutations? The cell is unable to synthesize most proteins required for normal cell functions.