The Cellular Level of Organization (Q)

List the general functions of the plasma membrane.

The general functions of the plasma membrane include

physical isolation, regulation of exchange with the environment,

sensitivity to the environment, and structural support.

Identify the components of the plasma membrane that

allow it to carry out its functions.

The components of the plasma membrane that allow it to perform its characteristic functions are membrane lipids, membrane proteins, and

membrane carbohydrates.

Which component of the plasma membrane is primarily

responsible for the membrane’s ability to form a

physical barrier between the cell’s internal and external

environments?

The phospholipid bilayer of the plasma membrane forms a physical barrier between the cell’s internal and external environments.

Which type of integral protein allows water, ions, and

small water-soluble solutes to pass through the plasma

membrane?

Channel proteins are integral proteins that allow water, ions, and small water-soluble solutes to pass through the plasma membrane.

Explain the difference between the cytoplasm and the

cytosol.

Cytoplasm is the material between the plasma membrane and the nuclear membrane. Cytosol is the fluid portion of the cytoplasm; cytosol is also

called intracellular fluid. It is a mixture of water and various dis-

solved and insoluble materials, in which the organelles and inclu-

sions are suspended.

What are the major differences between cytosol and

extracellular fluid?

Cytosol has a higher concentration of

potassium ions and suspended proteins, and a lower concentration

of sodium ions, than does extracellular fluid. Cytosol also includes

small quantities of carbohydrates and large reserves of amino acids

and lipids.

Identify the nonmembranous organelles, and cite a

function of each.

The nonmembranous organelles and their functions

include (1) centriole = essential for movement of chromosomes dur-

ing cell division; organization of microtubules in cytoskeleton; (2)

cilia = movement of materials over cell surface (motile cilia), envi-

ronmental sensor (primary cilium); flagella = propel sperm (3) cyto-

skeleton = strength and support; movement of cellular structures

and materials; cell movement; (4) microvilli = increase surface area

to facilitate absorption of extracellular materials; (5) proteasomes =

breakdown and recycling of intracellular proteins; (6) ribosomes =

protein synthesis.

Identify the membranous organelles, and cite their

functions.

The membranous organelles and

their functions include (1) endoplasmic reticulum = synthesis of secretory products; intracellular storage and transport; (2) smooth ER =

lipid and carbohydrate synthesis; (3) rough ER = modification and packaging of newly synthesized proteins; (4) Golgi apparatus = storage, modification, and packaging of secretory products and lysosomal enzymes; (5) lysosomes = intracellular removal of damaged organelles or pathogens; (6) peroxisomes = neutralization of toxic compounds; (7) mitochondria = production of 95 percent of the ATP required by the cell.

Explain why certain cells in the ovaries and testes contain

large amounts of smooth endoplasmic reticulum.

The SER synthesizes lipids, such as steroids. Ovaries and testes produce large amounts of steroid hormones, which are lipids, and thus need large amounts of SER.

What does the presence of many mitochondria imply

about a cell’s energy requirements?

Mitochondria produce energy, in the form of ATP molecules, for the cell.

A large number of mitochondria in a cell indicates a high demand

for energy.

Describe the contents and structure of the nucleus

The nucleus is a cellular organelle that contains DNA, RNA, enzymes, and proteins. The nuclear envelope is a double membrane that surrounds the nucleus; the perinuclear space is the region between this double membrane. Nuclear pores enable chemical communication between the nucleus and the cytosol.

What is a gene?

A gene is a portion of a DNA strand that functions as a he-

reditary unit. Each gene is located at a particular site on a specific

chromosome and codes for a specific protein.

Define gene activation.

Gene activation is the process of uncoiling the segment of DNA containing

the nucleotide sequence of that gene, and temporarily removing his-

tones, so that the gene can affect the cell.

Describe transcription and translation.

Transcription is the synthesis of RNA from a DNA template. Translation uses the information from an mRNA strand to form a linear chain of amino acids,

or protein.

What process would be affected by the lack of the

enzyme RNA polymerase?

A cell that lacked the enzyme RNA polymerase would not be able to transcribe RNA from DNA.

What is meant by the term selectively permeable when

referring to a plasma membrane?

A selectively permeable membrane allows the passage of some sub-

stances and restricts the passage of others. It lies between two extremes: impermeable, which allows no substances to pass, and freely

permeable, which permits the passage of any substance.

Define diffusion.

Diffusion is the passive movement of a substance from an area of higher

concentration to an area of lower concentration. Diffusion proceeds

until the concentration gradient is eliminated (equilibrium is

reached).

List five factors that influence the diffusion of substances

in the body.

Five factors that influence the diffusion of substances

in the body are (1) distance, (2) ion or molecule size, (3) tempera-

ture, (4) concentration gradient, and (5) electrical forces.

How would a decrease in the concentration of oxygen in

the lungs affect the diffusion of oxygen into the blood?

Diffusion is driven by a concentration gradient. The steeper the concentration

gradient, the faster the rate of diffusion, and the less steep the con-

centration gradient, the slower the rate of diffusion. If the concentra-

tion of oxygen in the lungs were to decrease, the concentration

gradient between oxygen in the lungs and oxygen in the blood

would decrease (as long as the oxygen level of the blood remained

constant). Thus, oxygen would diffuse more slowly into the

blood.

Define osmosis.

Osmosis is the diffusion of water across a selectively

permeable membrane from one solution to another solution that

contains a higher solute concentration.

Some pediatricians recommend using a 10 percent salt

solution as a nasal spray to relieve congestion in infants

with stuffy noses. What effect would such a solution have

on the cells lining the nasal cavity, and why?

The 10 percent salt solution is hypertonic with respect to the cells lining the nasal cavity, because it contains a higher salt (solute) concentration than do the cells. The hypertonic solution would draw water out of the cells, caus-

ing the cells to shrink and adding water to the mucus, thereby dilut-

ing it and relieving congestion.

Describe the process of carrier-mediated transport.

In carrier-mediated transport, integral proteins bind specific ions or organic substrates and carry them across the plasma membrane. All forms of carrier-

mediated transport are specific, have saturation limits, and are regu-

lated (usually by hormones).

The concentration of hydrogen ions 1H in the stomach

contents during digestion rises to a higher level than in

the cells lining the stomach. Which transport process

must be operating?

An active transport process must be involved because it takes an expenditure of energy to move the hydrogen ions against their concentration gradient—that is, from a region where they are less concentrated (the cells lining the stomach) to a region where they are more concentrated (the interior of the

stomach).

Describe endocytosis.

Endocytosis is the movement of relatively large vol-

umes of extracellular material into the cytoplasm by the formation

of a membranous vesicle at the cell surface. Types of endocytosis are receptor-mediated endocytosis, the clathrin-coated vesicle-mediated

movement into the cytoplasm of specific target molecules (ligands);

pinocytosis, the vesicle-mediated movement into the cytoplasm of

extracellular fluid and its contents; and phagocytosis, the vesicle-me-

diated movement into the cytoplasm of extracellular solids, espe-

cially bacteria and debris.

Describe exocytosis.

Exocytosis is the ejection of cytoplasmic materials by the fusion of a membranous vesicle with

the plasma membrane.

What is the process called whereby certain types of white

blood cells engulf bacteria?

The process by which certain white

blood cells engulf bacteria is called phagocytosis.

What is the membrane potential of a cell, and in what

units is it expressed?

The membrane potential of a cell is the difference in electrical

potential that results from the uneven distribution of positive and

negative charges across the plasma membrane. It is expressed in millivolts (mV).

If the plasma membrane of a cell were freely permeable

to sodium ions (Na+), how would the membrane potential be affected?

If the plasma membrane of a cell were freely per-

meable to sodium ions, more of these positively charged ions would

move into the cell, and the membrane potential would become less

negative (move closer to zero).

Give the biological terms for (a) cellular reproduction

and (b) cellular death.

The biological term for cellular reproduction is cell division, and the term for

cell death is apoptosis.

What enzymes must be present for DNA replication to

proceed normally?

Important enzymes for DNA replication include helicases, DNA polymerase, and ligases.

Describe interphase, and identify its stages.

Interphase is a stage in a cell’s life when it performs all its normal

functions and, if necessary, prepares for cell division. The stages of

interphase include G1, S, G2, and G0. A cell in G0 is not preparing for cell division.

A cell is actively manufacturing enough organelles to

serve two functional cells. This cell is probably in what

phase of its life cycle?

This cell is likely in the G1 phase of its life cycle.

Define mitosis, and list its four stages.

Mitosis is the essential step in cell division in which a single cell nucleus divides to produce two identical daughter cell nuclei. The four stages of mitosis are prophase, metaphase, anaphase, and telophase.

What would happen if spindle fibers failed to form in a

cell during mitosis?

If spindle fibers failed to form during mitosis,

the cell would not be able to separate the chromosomes into two

sets. If cytokinesis occurred, the result would be one cell with two

sets of chromosomes and one cell with none.

Define growth factor, and identify several growth factors

that affect cell division.

A growth factor is a natural substance, such as a peptide or hormone,

that can stimulate the division of specific cell types. Representative

growth factors include M-phase promoting factor (maturation-pro-

moting factor), growth hormone, prolactin, fibroblast growth factor

(FGF), nerve growth factor (NGF), epidermal growth factor (EGF),

erythropoietin, thymosins and related compounds, and chalones.

An illness characterized by mutations that disrupt

normal control mechanisms and produce potentially

malignant cells is termed ___.

An illness characterized by mutations that disrupt

normal controls and produce potentially malignant cells is termed

cancer.

Define metastasis.

Metastasis is the spread of cancer cells to distant tissues

and organs, leading to the establishment of secondary tumors.

Define cellular differentiation.

Cellular differentiation is the development of specific cellular

characteristics and functions that are different from those of the

original cell. It results from genes in the cell being switched off,

restricting the cell’s potential functions.