What is Cells & transport on the Living Environment Regents?

Cells & transport is a Cell Biology topic on the Living Environment Regents. RegentsLab has 22 practice questions on it, each with a full worked solution.

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Cells & transport: Living Environment Regents Practice

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Question 1 · 2 pts

The cell membrane of a living cell helps maintain homeostasis primarily by

(A) releasing energy from nutrient molecules
(B) regulating the passage of materials into and out of the cell ✓
(C) assembling proteins from amino acids
(D) directing all of the cell's genetic activities

Worked solution

$\text{Homeostasis} = \text{maintaining a stable internal environment}$ Definition the question targets.

$\text{Cell membrane is selectively permeable} \Rightarrow \text{controls what enters and exits}$

Answer: regulating the passage of materials into and out of the cell

The selectively permeable cell membrane controls which substances enter and leave the cell, which is essential for maintaining homeostasis.

Question 2 · 2 pts

Oxygen molecules move from a region where they are highly concentrated to a region where they are less concentrated, without the cell using energy. This process is known as

(A) synthesis
(B) active transport
(C) cellular respiration
(D) diffusion ✓

Worked solution

$\text{Net movement: high concentration} \rightarrow \text{low concentration}$

$\text{No energy required (passive)} \Rightarrow \text{diffusion}$

Answer: diffusion

Diffusion is the net movement of molecules from an area of higher concentration to an area of lower concentration and does not require energy.

Question 10 · 2 pts

Embedded in the cell membrane are receptor molecules. Which statement best describes the role of these receptor molecules?

(A) They produce the energy (ATP) needed for active transport across the membrane.
(B) They build the proteins that the cell needs for growth and repair.
(C) They store the genetic information that controls the cell's activities.
(D) They allow the cell to recognize and respond to specific molecules, such as hormones or pathogens. ✓

Worked solution

$\text{Receptor molecules are proteins located on the cell membrane.}$ Identify what and where they are.

$\text{Each receptor has a specific shape that binds a specific molecule.}$ Shape determines what it recognizes.

$\text{binding} \Rightarrow \text{the cell recognizes a signal and responds}$ This underlies hormone signaling and immune recognition.

Answer: Receptor molecules let the cell recognize and respond to specific molecules in its environment.

Membrane receptors have specific shapes that bind specific signal molecules, allowing cells to communicate, respond to hormones, and recognize pathogens.

Question 14 · 2 pts

A paramecium is a single-celled organism that lives in fresh water. Water continually moves into the cell by osmosis. To survive, the paramecium uses a contractile vacuole to pump the excess water back out of the cell. This activity best illustrates

(A) the active transport of glucose into the cell.
(B) the diffusion of water from a region of low water concentration to a region of high water concentration.
(C) the synthesis of new water molecules inside the cell.
(D) maintenance of homeostasis by removing the water that enters the cell by osmosis. ✓

Worked solution

$\text{fresh water: water concentration outside} > \text{water concentration inside}$ The environment is hypotonic to the cell.

$\text{water enters by osmosis (high} \rightarrow \text{low water concentration)}$ Net water flows into the cell.

$\text{contractile vacuole pumps excess water out} \Rightarrow \text{stable internal conditions}$ Keeping conditions stable is homeostasis.

Answer: Pumping out excess water that enters by osmosis maintains homeostasis.

Water enters a freshwater cell by osmosis; the contractile vacuole removes the excess to keep the cell's internal environment stable, which is homeostasis.

Question 4 · 2 pts

Which cell structure is responsible for packaging and modifying proteins before they are transported out of the cell?

(A) mitochondrion
(B) nucleus
(C) Golgi apparatus ✓
(D) ribosome

Worked solution

$Ribosomes synthesize proteins from mRNA.$ not the answer

$The Golgi apparatus receives proteins, modifies them, packages them into vesicles, and directs them for secretion or use elsewhere in the cell.$

Answer: Golgi apparatus

The Golgi apparatus modifies, sorts, and packages proteins made by ribosomes on the rough endoplasmic reticulum, preparing them for transport out of or within the cell.

Question 13 · 2 pts

A cell is placed in a solution that has a much lower concentration of solutes than the inside of the cell. Water will most likely move

(A) out of the cell, causing it to shrink
(B) into the cell at the same rate it moves out, causing no change
(C) into the cell, causing it to swell ✓
(D) out of the cell using active transport that requires ATP

Worked solution

$The solution outside the cell has fewer solutes than the cytoplasm, so it is hypotonic to the cell.$ Compare solute concentrations

$Water always diffuses (osmosis) from a region of lower solute concentration to a region of higher solute concentration.$ Definition of osmosis

$Since the outside has less solute, water moves from outside into the cell.$ Direction of net water movement

$Net water gain causes the cell to swell.$ Result

Answer: into the cell, causing it to swell

Osmosis is the diffusion of water across a membrane from an area of lower solute concentration to an area of higher solute concentration; since the surrounding solution is hypotonic, water enters the cell and it swells.

Question 12 · 1 pt

Which cell structure is the site where most of a cell's energy (ATP) is released from nutrients?

(A) vacuole
(B) ribosome
(C) cell wall
(D) mitochondria ✓

Worked solution

$\text{respiration} \Rightarrow \text{mitochondria}$ Cellular respiration releases ATP here.

Answer: \text{mitochondria}

Mitochondria carry out cellular respiration, releasing energy from nutrients and storing it as ATP, the cell's powerhouse.

Question 3 · 2 pts

A red blood cell is placed in a beaker of distilled (pure) water. The fluid inside the cell has a much higher concentration of dissolved solutes than the water around it. What is most likely to happen to the cell?

(A) Solute will move out of the cell into the water.
(B) Water will move out of the cell, causing it to shrink.
(C) Water will move into the cell, causing it to swell. ✓
(D) No net movement of water will occur.

Worked solution

$\text{Osmosis: water moves toward the region of higher solute concentration}$

$\text{Inside solute} > \text{outside solute} \Rightarrow \text{water enters the cell}$ The surrounding solution is hypotonic to the cell.

Answer: Water will move into the cell, causing it to swell.

In osmosis, water crosses a selectively permeable membrane from lower solute concentration to higher solute concentration; here water enters the cell and it swells.

Question 4 · 2 pts

A certain ion is already more concentrated inside a cell than outside, yet the cell continues to move more of that ion inward. To accomplish this, the cell must use

(A) no energy, because the ion is simply diffusing through the membrane
(B) energy, because the ion is being moved against its concentration gradient ✓
(C) no energy, because osmosis is moving the ion
(D) no energy, because the membrane is selectively permeable

Worked solution

$\text{Against gradient: low concentration} \rightarrow \text{high concentration}$

$\text{Movement against the gradient} = \text{active transport} \Rightarrow \text{requires energy (ATP)}$

Answer: energy, because the ion is being moved against its concentration gradient

Active transport moves materials against their concentration gradient and therefore requires the cell to expend energy in the form of ATP.

Question 6 · 2 pts

Dye molecules diffuse through a beaker of water until the concentration is the same everywhere. After this point is reached, the dye molecules

(A) are broken down once they are evenly spread out
(B) stop moving completely because equilibrium has been reached
(C) move back together and become concentrated in one region again
(D) continue to move randomly, but there is no net change in concentration ✓

Worked solution

$\text{Equilibrium} \Rightarrow \text{concentration is uniform everywhere}$

$\text{Random motion continues, but movement balances} \Rightarrow \text{no net change}$ This is called dynamic equilibrium.

Answer: continue to move randomly, but there is no net change in concentration

At dynamic equilibrium, molecules keep moving randomly, but movement in all directions is balanced so the overall concentration no longer changes.

Question 8 · 2 pts

Specific molecules on the surface of the cell membrane allow a cell to recognize and respond to chemical signals such as hormones. These surface molecules are best described as

(A) waste products stored within the membrane
(B) genes that code for proteins
(C) enzymes that digest the cell membrane
(D) receptor molecules ✓

Worked solution

$\text{Cell membrane carries specific receptor molecules}$

$\text{Receptor binds matching signal (e.g., hormone)} \Rightarrow \text{cell responds}$ Basis of cell communication and recognition.

Answer: receptor molecules

Receptor molecules on the cell membrane have specific shapes that bind particular signal molecules, allowing a cell to recognize and respond to its environment.

Question 9 · 2 pts

Most living cells remain microscopic in size. Which statement best explains why cells cannot grow indefinitely large and instead divide or stay small?

(A) As a cell increases in size, it contains more DNA than it is able to copy, so growth stops.
(B) The cell membrane stops growing as soon as the nucleus is fully formed.
(C) Smaller cells contain more mitochondria per unit of volume, which forces them to stay small.
(D) As a cell increases in size, its volume increases faster than its surface area, so the membrane cannot exchange materials with the environment fast enough. ✓

Worked solution

$\text{Model the cell as a cube of side } s:\quad SA = 6s^{2},\ \ V = s^{3}$ Approximate the cell with a simple shape.

$\frac{SA}{V} = \frac{6s^{2}}{s^{3}} = \frac{6}{s}$ The ratio of membrane to interior depends on size.

$s \uparrow \;\Rightarrow\; \frac{SA}{V} \downarrow$ A bigger cell has less membrane per unit of volume.

$\text{less membrane per volume} \Rightarrow \text{slower exchange of food, gases, wastes}$ So cells stay small or divide to keep transport efficient.

Answer: As a cell grows, its volume increases faster than its surface area, so the membrane cannot move materials in and out fast enough.

The cell membrane (surface area) limits how fast materials cross; because volume grows faster than surface area, large cells cannot supply their interior, so cells stay small or divide.

Question 11 · 2 pts

A drop of dye is added to the water around a cell. The dye diffuses into the cell until its concentration inside the cell equals its concentration outside. Once this point is reached, which statement best describes what happens to the dye molecules?

(A) The cell uses energy to keep the inside and outside concentrations equal.
(B) All movement of molecules across the membrane stops completely.
(C) Molecules keep moving across the membrane in both directions at equal rates, so there is no net change in concentration. ✓
(D) Molecules move only out of the cell until the inside is empty of dye.

Worked solution

$\text{At equilibrium: concentration inside} = \text{concentration outside}$ Concentrations are equal, not zero.

$\text{Molecules continue to move randomly across the membrane both ways.}$ Random motion never stops.

$\text{rate in} = \text{rate out} \;\Rightarrow\; \text{net movement} = 0$ Movement continues, but there is no NET change.

Answer: Molecules continue moving across the membrane in both directions at equal rates, so there is no net change.

Dynamic equilibrium means molecules keep moving in both directions at equal rates; concentrations stay balanced even though motion never stops.

Question 12 · 2 pts

A model cell made of dialysis (selectively permeable) tubing is filled with starch solution and placed in a beaker of water containing iodine. Iodine is an indicator that turns blue-black when starch is present. After 20 minutes, the liquid inside the tubing turns blue-black, but the water in the beaker stays clear. Which statement best explains these results?

(A) Starch diffused out of the tubing and reacted with the iodine in the beaker.
(B) The membrane used energy to actively pump iodine into the tubing.
(C) Iodine molecules are small enough to diffuse through the membrane into the tubing, but starch molecules are too large to pass out. ✓
(D) Water carried the starch across the membrane and out into the beaker.

Worked solution

$\text{starch} = \text{large molecule},\quad \text{iodine} = \text{small molecule}$ Compare molecule sizes.

$\text{selectively permeable: small molecules pass, large molecules do not}$ Key property of the membrane.

$\text{iodine diffuses IN} \rightarrow \text{meets starch} \rightarrow \text{blue-black inside}$ Color forms where starch is.

$\text{starch cannot exit} \Rightarrow \text{beaker stays clear}$ Matches the observed results.

Answer: Small iodine molecules diffuse into the tubing while large starch molecules cannot diffuse out.

A selectively permeable membrane lets small molecules (iodine) diffuse through but blocks large molecules (starch), so the indicator only changes color inside the tubing.

Question 5 · 2 pts

A red blood cell is placed in a solution that has a much lower concentration of solutes than the inside of the cell. Water will move by osmosis

(A) out of the cell, causing it to shrink
(B) into the cell, causing it to shrink
(C) equally in both directions, so no net change occurs
(D) into the cell, causing it to swell ✓

Worked solution

$Osmosis is the diffusion of water across a membrane from an area of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration).$ definition

$The surrounding solution has fewer solutes than the cell's interior, so it has a higher concentration of free water molecules than the cell.$

$Water moves from the solution into the cell, causing the red blood cell to swell.$

Answer: into the cell, causing it to swell

When a cell is placed in a solution with lower solute concentration than its cytoplasm (a hypotonic solution), water moves into the cell by osmosis, causing it to swell.

Question 21 · 2 pts

An enzyme's ability to speed up a chemical reaction depends on its three-dimensional shape, which allows it to bind to a specific substrate. If the temperature of the environment becomes very high, the enzyme often stops working because

(A) high temperature increases the enzyme's specificity for a completely different substrate
(B) the enzyme is used up completely after one reaction regardless of temperature
(C) the high temperature denatures the enzyme, changing its shape so it can no longer bind its substrate ✓
(D) the high temperature converts the enzyme into a new type of substrate

Worked solution

$Enzymes are proteins whose function depends on maintaining a specific three-dimensional shape.$ Structure-function relationship

$Excessive heat breaks the bonds that hold the protein's shape, a process called denaturation.$ Denaturation

$Once denatured, the enzyme's active site no longer matches the shape of its substrate.$ Loss of active site fit

$Without proper binding, the enzyme can no longer catalyze the reaction efficiently.$ Loss of function

Answer: the high temperature denatures the enzyme, changing its shape so it can no longer bind its substrate

Enzymes are proteins that depend on a precise shape to bind substrates at their active site; high temperatures denature this shape, so the enzyme loses its ability to catalyze reactions, which is why enzyme activity drops sharply above an organism's optimal temperature.

Question 52 · 4 pts · constructed response

A red blood cell is placed into a solution that has a much higher concentration of dissolved solutes than the inside of the cell. Describe what will happen to the cell and explain the process by which water moves into or out of the cell to cause this change.

Model answer: The red blood cell will shrink (crenate) because water will move out of the cell, from an area of higher water concentration (inside the cell) to an area of lower water concentration (the surrounding hypertonic solution), through the process of osmosis.

Worked solution

$\text{Solution is hypertonic (more solute, less water) compared to the cell}$ Compare solute concentrations

$\text{Water moves down its concentration gradient}$ Osmosis is diffusion of water across a selectively permeable membrane

$\text{Water exits the cell, moving toward the higher-solute solution}$ Net water movement direction

$\text{Cell loses water and shrinks (crenation)}$ Resulting effect on the cell

Answer: The red blood cell will shrink (crenate) because water will move out of the cell, from an area of higher water concentration (inside the cell) to an area of lower water concentration (the surrounding hypertonic solution), through the process of osmosis.

Osmosis causes water to move from a region of lower solute concentration to a region of higher solute concentration across the cell membrane, so in a hypertonic solution the cell loses water and shrinks.

Question 5 · 2 pts

As a cell grows larger, its volume increases faster than its surface area, so its surface-area-to-volume ratio $\frac{SA}{V}$ decreases. This change makes it increasingly difficult for the cell to

(A) hold all of its organelles inside the cell
(B) replicate its DNA before it divides
(C) transport materials across its membrane fast enough to supply its entire volume ✓
(D) maintain a cell wall around its outer surface

Worked solution

$\frac{SA}{V} = \frac{\text{surface area}}{\text{volume}}$

$\text{size} \uparrow \Rightarrow \text{volume grows faster than surface area} \Rightarrow \frac{SA}{V} \downarrow$ Less membrane is available per unit of volume.

$\text{smaller } \frac{SA}{V} \Rightarrow \text{membrane cannot exchange materials fast enough}$

Answer: transport materials across its membrane fast enough to supply its entire volume

A falling surface-area-to-volume ratio limits how quickly a large cell can exchange materials through its membrane, which restricts the maximum size a cell can reach.

Question 7 · 2 pts

A plant cell is placed in a concentrated salt solution whose solute concentration is higher than that of the cell's cytoplasm. Which change is most likely to be observed in the cell?

(A) No water moves because the cell wall blocks osmosis.
(B) The cell gains water and bursts open.
(C) The cell wall dissolves and the cell is destroyed.
(D) The cell loses water, and its membrane pulls away from the cell wall. ✓

Worked solution

$\text{outside solute} > \text{inside solute} \Rightarrow \text{hypertonic solution}$

$\text{Water moves out of the cell (toward higher solute)}$

$\text{Membrane shrinks away from the rigid wall} = \text{plasmolysis}$ The cell wall itself stays in place.

Answer: The cell loses water, and its membrane pulls away from the cell wall.

In a hypertonic solution, water leaves the cell by osmosis, causing the plant cell's membrane to pull away from the rigid cell wall.

Question 13 · 2 pts

In a plant root cell, the concentration of potassium ions inside the cell is $80$ units, while in the surrounding soil water it is only $10$ units. Even so, the root cell continues to absorb potassium ions from the soil. Which statement best explains how this is possible?

(A) The ions enter freely because the membrane is fully permeable to all substances.
(B) The ions are moving from low to high concentration, against the gradient, so the cell must use energy (ATP) for active transport. ✓
(C) The ions move from high to low concentration by simple diffusion.
(D) Osmosis carries the potassium ions from the soil into the cell.

Worked solution

$\text{inside} = 80\ \text{units},\quad \text{outside} = 10\ \text{units}$ Concentration is higher inside the cell.

$\text{cell still absorbs K}^{+}: \ 10 \rightarrow 80 \ \text{(low to high)}$ Movement is against the concentration gradient.

$\text{against gradient} \Rightarrow \text{requires ATP} \Rightarrow \text{active transport}$ Diffusion alone cannot do this.

Answer: The cell uses energy (ATP) for active transport to move ions against the concentration gradient.

Moving a substance from a region of lower concentration to higher concentration is active transport, which requires the cell to spend energy supplied as ATP.

Question 15 · 2 pts

Potato cores of equal starting mass were placed in three sucrose solutions for 30 minutes and then reweighed. Results: in $0.0\,\text{M}$ sucrose the core gained mass, in $0.2\,\text{M}$ sucrose the core showed no change, and in $0.6\,\text{M}$ sucrose the core lost mass. Which conclusion is best supported by these data?

(A) In the $0.6\,\text{M}$ solution, sucrose moved into the cells by active transport, causing the mass to increase.
(B) In the $0.6\,\text{M}$ solution, water moved out of the cells because that solution had a lower water concentration than the cells. ✓
(C) In the $0.0\,\text{M}$ solution, water moved out of the cells, causing them to gain mass.
(D) In the $0.2\,\text{M}$ solution, water moved rapidly into the cells.

Worked solution

$0.6\,\text{M solution: core LOST mass} \Rightarrow \text{water left the cells}$ Mass change tracks net water movement.

$\text{water moves toward lower water concentration (higher solute)}$ Osmosis direction rule.

$\text{so the } 0.6\,\text{M solution had lower water concentration than the cells}$ The cells were hypotonic to that solution.

$0.0\,\text{M gain},\ 0.2\,\text{M no change} \Rightarrow \text{equilibrium near } 0.2\,\text{M}$ Confirms the trend across solutions.

Answer: In the 0.6 M solution the cells lost water because that solution had a lower water concentration than the cells.

Mass loss means water left the cells by osmosis, which happens when the surrounding solution has a higher solute (lower water) concentration than the cells.

Question 16 · 2 pts

Active transport requires energy in the form of ATP, which is supplied mainly by mitochondria. Based on this relationship, which cell would be expected to contain the greatest number of mitochondria?

(A) A mature red blood cell that relies mostly on diffusion of gases.
(B) A dead skin cell at the surface of the body.
(C) A kidney cell that actively pumps large amounts of ions across its membrane. ✓
(D) A storage cell whose main job is holding water in a large vacuole.

Worked solution

$\text{active transport} \Rightarrow \text{requires ATP}$ Energy is needed to move against gradients.

$\text{ATP is produced by mitochondria}$ Link energy demand to the organelle.

$\text{more active transport} \Rightarrow \text{more ATP needed} \Rightarrow \text{more mitochondria}$ High energy demand means many mitochondria.

$\text{kidney cell pumping ions} = \text{highest active-transport demand}$ Best match among the choices.

Answer: A kidney cell that actively pumps ions does the most active transport, so it needs the most mitochondria.

Cells doing large amounts of active transport need lots of ATP, and since mitochondria make ATP, those cells contain many mitochondria.

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