Chapter 3: Cells

Composite Cell All cells vary in size, shape, content, and function Composite cell includes many of the known cell structures Cells have most but not all of these structures Cells consist of three main parts: 1. Cell membrane 2. Cytoplasm 3. Nucleus

Cell Membrane (plasma membrane) Actively functioning part of the cell Regulates movement of substances in and out of the cell Certain molecules receive stimulation from outside the cell Transmits it inside the cell  signal transduction Helps cells adhere to certain other cells Important in forming tissues

Cell Membrane General Characteristics: Extremely thin (can only be seen with an electron microscope) Flexible & somewhat elastic Controls which substances exit & enter the cell  selectively permeable

Cell Membrane Structure: Mainly composed of lipids, proteins, and some carbohydrates Has a double layer  phospholipid bilayer Oxygen and carbon dioxide can pass through the bilayer easily because they are soluble in lipids Impermeable to water-soluble molecules (amino acids, sugars, proteins, nucleic acids, & various ions) Embedded cholesterol molecules aid in making the membrane less permeable to water-soluble substances

Cell Membrane Structure Has a few lipid molecules & many kinds of proteins Membrane proteins are classified according to their positions Membrane-spanning (trans-membrane) proteins extend through the lipid bilayer Peripheral membrane proteins are only on one side of the bilayer Vary in shape  blobby or globular & elongated (rod-like)

Cell Membrane structure Membrane protein functions: Some form receptors on the cell surface  bind incoming hormones or growth factors Starts signal transduction Transport ions or molecules across the cell membrane Others form selective channels that allow certain ions to enter or leave the cell

Composite Cell Assignment Provide information on the organelles in a cell Smooth ER, Rough ER, Ribosomes, Golgi apparatus, Mitochondria, Lysosomes, Peroxisomes, Centrosome, Microfilaments & microtubules, Cilia & flagella, vesicles, and Nucleus Function in the cell What is the structure/composition Where it is located in the body

Membrane protein functions Proteins that protrude from the inside of the cell: Anchor the cell to rods and tubules that support the cell Proteins that protrude from the outside of the cell: Mark the cell as part of a particular tissue or organ Cellular adhesion molecule (CAM) Determines a cell’s interactions with other cells Ex. A series of CAMs helps a white blood cell move to the site of an injury

Cytoplasm Appears as a clear jelly through a light microscope with specks scattered throughout An electron microscope will show you that the cytoplasm contains networks or membranes and organelles These are suspended in a clear liquid called cytosol Also has protein rods and tubules that form a cytoskeleton

Cytoplasm Cell activities occur mainly here where nutrients are received, processed, and used Contains the following organelles: Endoplasmic reticulum (rough and smooth), ribosomes, golgi apparatus, mitochondria, lysosomes, peroxisomes, microfilaments and microtubules, centrosome, cilia & flagella, & vesicles

Endoplasmic Reticulum Composed of membrane-bound, flattened sacs, elongated canals, and fluid-filled bubble sacs called vesicles These are interconnected & communicate w/ the cell membrane, nuclear envelope & other organelles Transports molecules from one cell part to another Smooth ER Contains enzymes responsible for lipid synthesis Carbohydrate metabolism & Detoxification Abundant in liver and gonad cells Rough ER Responsible for protein synthesis Ribosomes cover it Proteins then travel to Golgi apparatus through tubules for further processing Found in hepatocytes

Ribosomes Many are attached to the rough ER, others scattered throughout the cytoplasm Composed of protein and RNA molecules Provide enzymes & structural support for RNA molecules that come together during protein synthesis

Golgi Apparatus Looks like a stack of pancakes

Mitochondria

Lysosomes

Peroxisomes

Microfilaments & Microtubules

Centrosome

Cilia & Flagella

Vesicles

Cell Nucleus Houses DNA Directs all cell activities Large spherical structure that is covered by a double-layered nuclear envelope Consists of inner and outer lipid bilayer membranes Protein lined channels (nuclear pores)  allow certain molecules to exit the nucleus Contains a fluid that suspends the necleolus and chromatin Nucleolus (“little nucleus”) Composed largely of RNA and protein Ribosomes form here and migrate through nuclear pores to the cytoplasm Chromatin Consists of loosely coiled fibers of protein and DNA DNA contains the information for protein synthesis When the cell divides, fibers of chromatin coil tightly into chromosomes

Movements through Cell Membranes Passive Mechanisms Diffusion (simple diffusion)  process where molecules or ions move to areas from regions where there is a high concentration to areas where there is a lower concentration They spread out evenly Molecules diffuse along or down their concentration gradient

Diffusion When diffusion happens the molecules eventually become uniformly distributed in a mixture This is known as equilibrium; molecules continue to move after equilibrium but the concentrations no longer change The plasma membrane is a physical barrier to free diffusion  because of its hydrophobic core A molecule will diffuse through the membrane if the molecule is: 1) lipid soluble, 2) small enough to pass through membrane channels, or 3) assisted by a carrier molecule

Facilitated Diffusion Only moves molecules from higher concentrations to lower concentrations Certain molecules (glucose & other sugars, amino acids, and ions) are transported passively even though they are unable to get through the lipid bilayer. They are transported by either a protein carrier in the membrane that ferries the molecule across or through water-filled protein channels

Facilitated Diffusion Protein Carriers Channels Small polar or charged particles diffuse through membrane channels constructed by channel proteins. Large, lipid-insoluble molecules (glucose) are moved across the membrane via a transport protein.

Osmosis Occurs whenever water molecules diffuse from a higher water concentration area to a lower water concentration area across a selectively permeable membrane (cell membrane) If a cell has too much glucose in it, water will diffuse across the cell membrane to put it back in equilibrium Isotonic – when a solution has the same osmotic pressure as body fluids Hypertonic – solutions with higher osmotic pressure than body fluids Hypotonic – solutions that have a lower osmotic pressure than body fluids

Filtration Process that forces water and solutes through a membrane or capillary wall by hydrostatic pressure Commonly used to separate solids from water Example: hydrostatic pressure exerted by blood forces fluid out of the capillaries This fluid contains solutes that are vital to the tissues

Active Mechanisms Active Transport Moves particles through membranes from a region of lower concentration to a region of higher concentration Needs carrier proteins to move the molecules across the cell membrane Example: moving sodium ions to the outside of the cell, the sodium-potassium pump

Sodium-potassium pump Carrier enzyme is called Na+-K+ ATPase Concentration of K+ inside the cell is some 10 times higher that outside the cell, the reverse is true of Na+ These ionic concentration differences are essential for excitable cells like muscle and nerve cells to function normally and for all body cells to maintain their normal fluid volume Since Na+ and K+ leak continuously through the plasma membrane along their concentration gradient, the Na+-K+ pump operates to drive Na+ out of the cell and pump K+ back in against a steep concentration gradient

Endocytosis & Exocytosis Uses cellular energy to move substances Molecules or other particles that are too large to enter a cell by diffusion or active transport must enter the cell through vesicles 3 forms: pinocytosis, phagocytosis, & receptor-mediated endocytosis Exocytosis  the reverse process, secreting a molecule or substance by a vesicle from the cell Example: nerve cells releasing neurotransmitter chemicals that signal other nerve cells, muscle cells, or glands

Pinocytosis “cell drinking” Cells take in tiny droplets of liquid from their surroundings by an infolding of the plasma membrane The droplet enters the cell and fuses with an endosome This is a routine activity of most cells

Phagocytosis “cell eating” Similar to pinocytosis but the cell takes in solids rather than liquids Certain kinds of white blood cells are called phagocytes Can take in solid particles like bacteria Particle attaches to the phagocyte’s cell membrane, this stimulates a portion of the membrane to protrude outward, surround the particle, and slowly draw it inside the cell Part of the membrane that surrounds the particle detaches from the cell’s surface, this forms a vesicle containing the particle (phagosome)

Receptor-mediated endocytosis Moves very specific kinds of particles into the cell by binding them first Protein molecules extend through a portion of the cell membrane to the outer surface They serve as receptors to which only specific substances from outside the cell can bind Cholesterol molecules enter cells by this mechanism

The Cell Cycle A series of changes a cell goes through from the time it forms to the time is divides Split into different stages: Interphase, mitosis, cytokinesis, & differentiation Interphase: Split up into different phases or checkpoints: G1, S, & G2 G1 – determines the fate of the cell; whether it will continue and divide, stay in the G1 phase as a specialized cell, or die S – DNA is replicated G2 – growth happens and final preparation for cell division

Mitosis Division of the nucleus must be precise because it contains the DNA Each cell resulting from mitosis must have a complete and accurate copy of this information to survive DNA is replicated in Interphase, but it’s equally distributed into two new cells in mitosis Consists of four different stages Prophase, Metaphase, Anaphase, Telophase

Prophase First indication of cell division is the appearance of chromosomes in the nucleus Each prophase chromosome is composed of two identical portions (chromatids)  they are temporarily attached @ a region on each called the centromere Centrioles of the centrosome replicate before mitosis begins, so during prophase these new centriole pairs move to opposite ends of the cell The nuclear envelope and nucleolus break up, disperse, and are no longer visible Microtubules are assembled from tubulin proteins in cytoplasm & associate w/ the centrioles & chromosomes Spindle-shaped array of microtubules (spindle fibers) forms between the centrioles as thy move apart

Metaphase Chromosomes line up about midway between the centrioles Spindle fibers attach to the centromeres of each chromosome A fiber from one pair of centrioles contacts one centromere & a fiber from the other pair of centrioles attaches to another centromere

Anaphase Centromeres begin to be pulled apart As chromatids separate they begin to be individual chromosomes Separated chromosomes move in opposite directions guided by microtubule activity Spindle fibers shorten & pull their attached chromosomes toward the centrioles at opposite ends of the cell

Telophase Begins when chromosomes complete their migration toward the centrioles As chromosomes approach the centrioles they start to get longer and unwind from rod-like into thread-like chromatin A nuclear envelope forms around each chromosome set Nucleoli appear within the newly formed nuclei Finally, the microtubules disassemble into free tubulin molecules

Cytokinesis Begins during anaphase when the cell membrane starts to constrict down the middle of the cell This constriction lasts through telophase A ring of microfilaments begins to contract  this assembles in the cytoplasm and attach to the inner surface of the cell membrane These rings pinch inwards and separates the two newly formed nuclei About half of the organelles are distributed into each New cells may differ in size & number of organelles but they have the same genetic information

Cell Differentiation All body cells form by mitosis & contain the same DNA information, however they do not look or function the same Sometime during development the cells begin to specialize  develop special structures or begin to function in different ways Differentiation – process by which cells develop different characteristics in structure and function

Cell Death A cell that doesn’t divide or specialize has one other option  death Apoptosis – a form of cell death that is actually a normal part of development Examples: carving away of webbing between fingers & toes, removes extra brain cells, sunburn – peels away cells so damaged they might turn cancerous