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The organelles called mitochondria are often called the power plants of the cell. They are composed of inner and outer membranous sacs and are involved with energy-releasing chemical reactions (also called cellular respiration).

Each mitochondrion contains its own DNA molecule, which has the information for building and running the mitochondrion.

Lysosomes are membrane-enclosed vesicles containing digestive enzymes. This organelle has a protective function; they so-to-speak “eat” and digest microbes.

Lysosomes were formerly thought to be responsible for apoptosis (or programmed cell death), but now scientists know that is not true.

Peroxisomes are vesicles that also contain enzymes. This organelle, found in some cells, is important for metabolizing lipids for energy and detoxifying toxins.

Proteasomes are hollow cylinders of protein that break apart irregular proteins into amino acids that are recycled by the cell.

The centrosome is a microtubule-organizing region of cytoplasm near the nucleus of each cell.

Centrioles are paired organelles that lie at right angles to each other within the centrosome and function in moving chromosomes during cell reproduction.

The three major types of cell extensions are microvilli, cilia, and flagella.

Microvilli are short extensions of the plasma membrane that increase surface area and produce slight movements that enhance absorption by the cell.

Cilia are hairlike extensions with inner microtubules found on free or exposed surfaces of all cells; cilia serve sensory functions, but some are also capable of moving together in a wavelike fashion to propel mucus across a surface.

Flagella are single projections (much longer than cilia) that act as “tails” of sperm cells.

The nucleus controls the cell because it contains most of the genetic code (the genome), instructions for making proteins, which in turn determine cell structure and function.

Component structures of the nucleus include the nuclear envelope, nucleoplasm, nucleolus, and chromatin. Chromatin in the nucleus are made of proteins around which are wound segments of the long, threadlike molecules called deoxyribonucleic acid, abbreviated DNA. DNA molecules become tightly coiled chromosomes during cell division. 46 nuclear chromosomes contain DNA, which contains the genetic code.

Note that there is a relationship between cell structure and function.

Every human cell has a designated function: some help maintain the cell, and others regulate life processes. Specialized functions of a cell differ depending on the number and type of organelles.

It is important that you review the movements of substances through cell membranes. Transport processes move substances into and out of cells.

There are two types of transport: passive transport, which does not require the cell to expend energy; and active transport, which does require the cell to expend energy (from ATP).

Passive transport processes do not require added energy and result in movement “down a concentration gradient.” In a passive process, it is unnecessary to add energy to the system. The primary passive transport processes that move substances through membranes include diffusion, osmosis, dialysis, and filtration.

Diffusion, a type of passive transport, occurs when substances scatter themselves evenly throughout an available space, the particles moving from high to low concentration. Solute particles may thus move through channels or carriers in a membrane to reach an equilibrium (an equal concentration) of solution on both sides of the membrane.

Osmosis is the passive movement of water molecules when some solutes cannot cross the membrane. Similar to diffusion, in osmosis, water moves in a direction that produces an equilibrium. Because water moves, but not all the solutes, osmotic pressure may change across the membrane.

In dialysis, some solutes move across a selectively permeable membrane by diffusion and other solutes do not, thus resulting in uneven distribution of solute types.

Filtration is the movement of water and solutes caused by hydrostatic pressure on one side of membrane.

Next, we’ll review the active transport processes of ion pumps, phagocytosis, and pinocytosis.

Active transport processes occur only in living cells; movement of substances is “up the concentration gradient”; so it requires energy from ATP.

An ion pump is a protein complex in the cell membrane that uses energy from ATP to move substances across cell membranes against their concentration gradients. Examples of ion pumps include the sodium-potassium pump and the calcium pump. Some ion pumps work with other carriers so that glucose or amino acids are transported along with ions.

In phagocytosis (or “cell eating”), large particles are engulfed in a vesicle as a protective mechanism often used to destroy bacteria or debris from tissue damage.

In pinocytosis (or “cell drinking”), fluids or dissolved substances are engulfed into cells.

Both phagocytosis and pinocytosis are active transport mechanisms because they require cell energy from ATP to move the cytoskeleton in a way that engulfs material and pulls it into the cell.

Now we’ll review cell growth and reproduction.

Cell growth is managed by proteins that determine the structure and function of cells.

Protein synthesis is directed by two nucleic acids: DNA and ribonucleic acid (abbreviated RNA).

DNA makes up 46 chromosomes contained in the cell nucleus.

DNA is a large molecule shaped like a spiral staircase; sugar (deoxyribose) and phosphate units compose the sides of the molecule; base pairs (adenine-thymine or guanine-cytosine) compose the so-called “steps” of the staircase.

Base pairings are always the same, but the sequence of base pairs differs in different DNA molecules. Base pairings are referred to as complementary base pairing.

A gene is a specific sequence of base pairs within a DNA molecule. Genes dictate formation of enzymes and other proteins by ribosomes, thereby indirectly determining a cell’s structure and functions.

RNA molecules are made from genes that do not code directly for proteins. RNA molecules regulate cell processes, such as protein synthesis. RNA subunits are made up of nucleotides, but have ribose as their sugar and have the base uracil instead of thymine.

Protein synthesis occurs in cytoplasm; thus genetic information must pass from the nucleus to the cytoplasm.

In transcription, double-stranded DNA separates to form messenger RNA.

Each strand of messenger RNA is a copy or transcript of a particular gene base-pair sequence from a segment of DNA.

Messenger RNA molecules pass from the nucleus to the cytoplasm where they direct protein synthesis in ribosomes and endoplasmic reticulum.

Translation is the process of “translating” the genetic code in the messenger RNA transcript to synthesize proteins in cytoplasm in ribosomes.

A codon is a series of three nucleotide bases in messenger RNA that acts as a code for a specific amino acid. Transfer RNA carries a specific amino acid and has an anticodon, which is a 3-base sequence that complements the messenger RNA codon that signifies that amino acid.

Transfer RNA brings amino acids into place along the messenger RNA strand where it is held by a ribosome, thus forming a strand of amino acids.

To review cell reproduction, first recall this information about the cell life cycle.

The life cycle of a cell includes reproduction of the cell involving division of the nucleus (called mitosis) and the cytoplasm.

Two offspring cells result from the division.

Interphase is the period of a cell’s life cycle when the cell is not actively dividing.

DNA replication is the process by which each half of a DNA molecule becomes a whole molecule identical to the original DNA molecule; replication precedes mitosis.

Mitosis is the process in cell division that distributes identical nuclear chromosomes (DNA molecules) to each new cell formed when the original cell divides.

Be sure you understand what happens at each stage of mitosis.

In prophase, the first stage, chromatin become organized, chromosomes (which are pairs of linked chromatids) appear, and centrioles move away from nucleus. The nuclear envelope disappears, freeing genetic material, and spindle fibers appear.

In metaphase, the second stage, chromosomes align across the center of the cell, and spindle fibers attach themselves to each chromatid.

Anaphase is the third stage. In anaphase, centromeres break apart. The separated chromatids are now called chromosomes. Chromosomes are pulled to opposite ends of the cell, and a cleavage furrow develops at the end of anaphase.

In telophase, the fourth stage, cell division is completed, and nuclei appear in offspring cells. The nuclear envelope and nucleoli appear, and the cytoplasm is divided (called cytokinesis).

Offspring cells then become fully functional, thus ending mitosis and entering interphase.

Two identical cells result from cell division, growing tissues or replacing old or damaged cells.

Differentiation is the process by which offspring cells can specialize and form different kinds of tissues. Abnormalities of mitotic division can produce benign or malignant neoplasms, the medical term for tumors.

This concludes the audio review of Chapter 3.