Cells

Cell Variations Function dictates form Nerve cell Red blood cells White blood cells Muscle cell Gland cell

Cells All human cells are microscopic. Most range in size from 10 to 15 µm. In all cases, a cell’s function dictates its form. long extensions on nerve cells allow quick transmission of impulses elongated muscle cells can shorten for movement concave shape of red blood cells allows them to squeeze into vessels sacs of gland cells release substances

Cell Structure

The cell’s basic structures include the following: Plasma membrane: boundary of the cell Nucleus: center of the cell Cytoplasm: gel-like substance surrounding the nucleus and packed with various organelles and molecules, each of which serves a specific function

Plasma Membrane Phospholipids Cholesterol Protein Regulates the passage of substances into and out of the cell Consists of phospholipids, cholesterol, and protein.

Plasma Membrane Phospholipids form the bulk of the membrane. They have a head and twin tails: the heads are “water loving” (hydrophilic) and the tails are “water fearing” (hydrophobic). They are positioned in a double layer (bilayer): some heads point toward the fluid-filled cell interior and others point toward the fluid surrounding the cell’s exterior; the tails point toward each other, forming a “hydrophobic” core. They are not stationary: they move slowly, which keeps membrane fluid.

Plasma Membrane Cholesterol molecules are scattered within the phospholipid molecules; they stiffen and strengthen the plasma membrane. The membrane is selectively permeable: Some substances pass through easily, whereas others do not.

Plasma Membrane Proteins are embedded in various spots in the membrane. They fulfill various functions: Some pass through the membrane and act as channels. Some attach to the surface and serve as receptors for specific chemicals or hormones. Others have carbohydrates attached to their outer surface (forming glycoproteins, which act as markers to help the body distinguish its own cells from foreign invaders).

Nucleus The most important part of the cell Nuclear envelope Nuclear pores Chromatin Nucleolus

Nucleus The nucleus contains all of a cell’s genetic information. Most cells have only one nucleus, although a few (like liver and skeletal muscle cells) contain more than one; red blood cells have no nucleus. The nuclear envelope surrounds the nucleus. Nuclear pores allow molecules (like materials needed for construction of RNA and DNA) to pass into and out of nucleus.

Nucleus Extending throughout the nucleoplasm are strands composed of DNA and protein called chromatin. The center of the nucleus is the nucleolus; it manufactures ribosomes (the cell’s protein- producing structures).

Question A cell’s genetic information is contained in the: A. cytoplasm. B. nucleus. C. proteins within the plasma membrane. D. plasma membrane.

Organelles Smooth endoplasmic reticulum Rough endoplasmic reticulum Ribosomes Organelles fill the cytoplasm and perform specific tasks in metabolism.

Organelles Organelles include the endoplasmic reticulum (ER), ribosomes, Golgi apparatus, lysosomes, centrioles, and mitochondria. ER is a network of membranous canals and curving sacs throughout the cytoplasm. Ribosomes, which manufacture proteins, dot the surface of some of the ER (called rough ER). Smooth ER has no ribosomes; smooth ER synthesizes certain lipids and carbohydrates.

Golgi Apparatus

The Golgi apparatus receives proteins from the endoplasmic reticulum (ER), and prepares and packages them for export to other parts of the body using the following sequence:

Golgi Apparatus 1. The ER delivers a protein molecule to the Golgi apparatus. 2. The protein passes through each of the sacs of the Golgi apparatus, undergoing modifications along the way. 3. The Golgi apparatus envelopes the protein and then pinches off the portion of itself containing the protein, creating a vesicle. 4. The vesicle migrates to the surface of the cell and pops open to secrete the protein inside. Some of the protein becomes part of the plasma membrane; some become secretory vesicles that store substances like breast milk or digestive enzymes; others become lysosomes.

Mitochondria

Mitochondria function as the cell’s “powerhouses.” They have two membranes: an outer membrane and an inner membrane. The inner membrane folds back and forth across its interior; these folds are called cristae. Spaces between the cristae contain enzymes that the organelle uses to convert organic compounds into ATP, which cells use for energy. Cells that do a lot of work (such as liver cells) contain more mitochondria than cells that do less work (such as sperm cells).

Cytoskeleton Framework of the cell Determines cell shape Strengthens cell Allows cell to move May have extensions: microvilli, cilia, or flagella

Cytoskeleton Microvilli are folds of the cell membrane that greatly increase the surface area of the cell; they are found in cells charged with absorbing nutrients (intestines). Cilia are hair-like processes on the cell surface; they beat in waves to propel a substance forward. Flagella have a whip-like motion to move a cell; in the human body, flagella occur only in sperm.

Question Cells depend on which organelle for energy? A. Golgi apparatus B. Cytoskeleton C. Mitochondria D. ATP

Movement Through the Cell Membrane Passive transport Passive transport Requires no energy Includes diffusion, osmosis, filtration, and facilitated diffusion Active transport Active transport Requires energy Includes transport by pumps and vesicles

Diffusion View animation on “Diffusion”

Diffusion Diffusion involves the movement of particles from an area of higher to lower concentration; it may occur in air or water. Diffusion continues until the particles are evenly distributed. A difference in concentration from one point to another is called a concentration gradient. When particles move from an area of greater to lesser concentration, they are said to move down the concentration gradient.

Osmosis View animation on “Osmosis”

Osmosis Osmosis involves the diffusion of water down the concentration gradient through a selectively permeable membrane.

Tonicity The ability of a solution to affect the fluid volume and pressure in a cell through osmosis is called tonicity. Isotonic:same Isotonic: A solution with the same concentration of solutes as that inside the cell. Hypertonic:higher Hypertonic: A solution with a higher concentration of solutes as that inside the cell. Hypotonic:lower Hypotonic: A solution with a lower concentration of solutes as that inside the cell. View animation on “Osmolarity and Tonicity”

Tonicity The concentration of solutes in the fluid inside cells determines whether, and how much, fluid moves into or out of a cell. For example, when a red blood cell is placed in an isotonic solution, water moves into and out of the cell at an equal rate, and the cells remain normal in size and water content. If a red blood cell is immersed in a hypertonic solution, water will diffuse out of the cell, causing it to shrivel. If a red blood cell is placed in a hypotonic solution, water will move by osmosis into the cell, causing it to swell and burst.

Filtration View animation on “Filtration” In filtration, water and dissolved particles are forced across a membrane from an area of higher to lower pressure.

Filtration In the body, the pressure of blood inside capillaries forces water and dissolved materials (such as nutrients) into the surrounding tissue fluid. Filtration is the method by which the body’s cells receive nutrients and the kidneys remove waste products from the blood.

Question Osmotic pressure is: A. the force driving osmosis. B. the force driving solutes through capillary walls. C. the pressure that aids venous return. D. the water pressure that develops from osmosis.

Facilitated Diffusion Solute Channel protein View animation on “Facilitated diffusion” In facilitated diffusion, molecules move down the concentration gradient—from an area of greater to lesser concentration.

Sodium-Potassium Pump View animation on “Sodium-potassium pump” By actively pumping, cells can move ions and other particles to specific areas. The most important active transport in the body is the sodium-potassium pump. The sodium-potassium pump regulates the volume of fluid within cells, provides the electrical potential necessary for nervous system activity, and helps in heat production.

Sodium-Potassium Pump The sequence of events in the sodium-potassium pump are as follows: 1. Three sodium ions (Na + ) from inside the cell funnel into receptor sites on a channel protein. 2. Fueled by ATP, the channel protein releases the sodium ions into the extracellular fluid, causing them to move from an area of lower to higher concentration. 3. Meanwhile, two potassium ions (K + ) from outside the cell enter the same channel protein. 4. The potassium ions are then released inside the cell. This keeps the concentration of potassium higher, and the concentration of sodium lower, within the cell.

Transport by Vesicles Endocytosis:into Endocytosis: Brings substances into the cell – Phagocytosis: “Cell eating” – Pinocytosis: “Cell drinking” Exocytosis: outside Exocytosis: Uses vesicles to release substances outside the cell Vesicles allow cells to move large particles or numerous molecules at once through the plasma membrane.

Question Which of the following is a key function of the sodium-potassium pump? A. Movement of large particles into the cell across the plasma membrane B. Provision of electrical potential necessary for nervous system activity C. Release of substances outside the cell D. Provision of nutrients to the body’s cells

Deoxyribonucleic Acid (DNA) DNA is one of the largest and most complex of all molecules. It stores all of a cell’s genetic information. The structure of DNA resembles a twisted ladder, called a double helix.

Deoxyribonucleic Acid (DNA) The building blocks of DNA are millions of pairs of nucleotides. Each nucleotide consists of one sugar, one phosphate group, and one of four possible types of nitrogenous bases. The four types of bases are: adenine (A), thymine (T), guanine (G), and cytosine (C).

Ribonucleic Acid (RNA) One of DNA’s main functions is to provide information for building proteins. Because DNA is too large to leave the nucleus—and because protein synthesis takes place in the cytoplasm— DNA needs help from another nucleic acid called ribonucleic acid (RNA).

Ribonucleic Acid (RNA) RNA is a long chain of nucleotide units consisting of a sugar, a phosphate group, and a nitrogenous base. The structure of RNA differs from that of DNA in three key ways: RNA is a single strand. RNA contains the sugar ribose (instead of deoxyribose). RNA contains the base uracil (U) instead of thymine (T). RNA exists in three forms. Two of these forms— messenger RNA (mRNA) and transfer RNA (tRNA)— are crucial to protein synthesis.

Protein Synthesis Transcription Triplet (codon) Manufacture of proteins occurs in two main phases: transcription and translation. The length of mRNA consists of a series of three bases (triplets). Each triplet, called a codon, is the code for one amino acid.

Translation tRNA molecule Amino acid chain View animation on “Protein synthesis” Protein Synthesis

Question Which statement about DNA is correct? A. It stores all of a cell’s genetic information. B. It contains the sugar ribose. C. It contains the four bases adenine, cytosine, guanine, and uracil. D. It exists as a single strand.

Cell Cycle For life to continue, cells must periodically divide. The cell life cycle encompasses the period from the beginning of one division until the beginning of the next.

Cell Cycle It includes the following sequence of events: First gap phase (G 1 ) Synthesis phase (S) Second gap phase (G 2 ) Mitotic phase (M) The time between mitotic phases (which includes phases G 1, S, and G 2 ) is called interphase. After mitosis, most cells repeat this cycle and divide again. Some cells leave the cycle and enter a period of rest in which they do not divide; this is called G 0 (G-zero) phase and can last for days, years, or even decades.

Mitosis Prophase Metaphase Anaphase Telophase View animation on “Mitosis” Mitosis occurs when a cell splits into two identical daughter cells.

Mitosis Mitosis includes the following phases: Prophase Metaphase Anaphase Telophase