Exercise 5 The Cell: Transport Mechanism and Permeability

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Molecular Motion

- reflects the kinetic energy of molecules

- is random and erratic


Velocity of molecular movement

- is lower in larger molecules

- increases with increasing temperature


Sac 1: 40% glucose suspended in distilled water

Glucose diffused from the sac into the water; using the Benedict’s test indicated the presence of the glucose that passed through the membrane. Water moved into the sac by osmosis; sac gained weight


Sac 2: 40% glucose suspended in 40% glucose

There was no net diffusion of glucose or osmosis because the water concentration on both sides of the membrane was the same. Net movement occurs only when there is a concentration gradient.


Sac 3: 10% NaCl in distilled water

NaCl diffused from the sac into the water; silver nitrate added to the water showed the presence of Cl–. Osmosis caused water to enter the sac because the solution in the sac was hypertonic to the distilled water in the beaker.


Sac 4: 40% sucrose and Congo red dye in distilled water

The Congo red dye did not diffuse from the sac into the water; the water in the beaker did not turn red. The sucrose did not diffuse from the sac; upon boiling, some of the sucrose bonds are hydrolyzed, releasing glucose and fructose. Using Benedict’s test then indicates the presence of glucose if sucrose passed through the membrane; the Benedict’s test was negative. Water moved into the sac by osmosis; the sac gained weight.


What single characteristic of the differentially permeable membranes used in the laboratory determines the substances that can pass through them? In addition to this characteristic, what other factors influence the passage of substances through living membranes?

Size of pores. Solubility in the lipid portion of the membrane and/or presence of membrane “carriers” for the substance(s).


A semipermeable sac containing 4% NaCl, 9% glucose, and 10% albumin is suspended in a solution with the following com- position: 10% NaCl, 10% glucose, and 40% albumin. Assume that the sac is permeable to all substances except albumin. State whether each of the following will (a) move into the sac, (b) move out of the sac, or (c) not move.

glucose: a, moves into sac
albumin:c, does not move
water: b, moves out of sac
NaCl: a, moves into sac


Diffusion through living membranes—the egg

Egg 1 in distilled water:
Water moves by osmosis from an area of higher water concentration into an area of lower water concentration.
Egg 2 in 30% sucrose:
solution. Water moves by osmosis from an area of higher water concentration into an area of lower water concentration.

card image

Microscope fields containing RBCs. Arrows show the direction of net osmosis.
Which field contains a hypertonic solution? _ The cells in this field are said to be _________. Which field contains an isotonic bathing solution? _ Which field contains a hypotonic solution? _ What is happening to the cells in this field? _________

- C. Hypertonic

- Crenated

- B. Isotonic

- A. Hypotonic

- Cells take on water by osmosis until they become bloated and burst in a hypotonic solution. Hemolysis, bursting as excessive water entry occurs


The faster-diffusing gas is _________________.

NH4OH ammonium hydroxide


The precipitate forms closer to the _________________ end.

HCl hydrochloric acid


What determines whether a transport process is active or passive?

Whether or not the cell must provide ATP for the process; if so, the process is active.


Passive process

- account for the movement of fats and respiratory gases through the plasma membrane

- include osmosis, simple diffusion, and filtration

- use hydrostatic pressure or molecular energy as the driving force


Active process

- explain solute pumping, phagocytosis, and pinocytosis

- may occur against concentration and/or electrical gradients

- move ions, amino acids, and some sugars across the plasma membrane


For the osmometer demonstration, explain why the level of the water column rose during the laboratory session.

The thistle tube was immersed in a dialysis sac which, in turn, was immersed in water. Since water will move down its concentration gradient if it is able, water diffused from the beaker into the sac, where its concentration was much lower.
As a result, the fluid column (molasses and entering water) rose in the thistle tube.


Selective permeability

Some molecules can cross. but others cannot



Movement of molecules from a region of their higher concentration to an area where they are in lower concentration


Simple diffusion

Movement of molecules from a region of their higher concentration to a lower region of their concentration. Its driving force is kinetic energy of the molecules themselves.


Facilitated diffusion

Movement of molecules across cell membranes in response to a concentration gradient with the aid of a membrane protein



Diffusion of water through a semipermeable or differentially permeable membrane. Water moves from an area of higher water concentration to an area of lower water concentration, from hypotonic to hypertonic solution



Passage of substances across a membrane from an area of higher hydrostatic pressure to an area of lower hydrostatic pressure


Vesicular transport

The movement of vesicles within the cell with the aid of the cytoplasm



Large molecules or particles move into cells



Intracellular vesicles more to the cell membrane and then release their contents to the extracellular fluid

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