Anatomy and Physiology Cells
Introduction • There are 75 trillion cells that make up the human body. • Cells of different tissues have much in common but vary in size and shape. • Typically, their shapes make possible their functions. Ex: nerve cells have long, threadlike extensions that transmit bioelectrical impulses.
Composite Cell • A “typical” cell consists of three major parts--the nucleus, the cytoplasm, and the cell membrane. • The nucleus is usually in the center of the cell and is surrounded by a thin nuclear envelope. • Cytoplasm surrounds the nucleus and is encircled by the thinner cell membrane (plasma membrane).
Organelles • Within the cytoplasm are specialized structures called cytoplasmic organelles, which are suspended in a liquid called cytosol. • Organelles (little organs) perform specific functions which are directed by the nucleus. • The cell membrane determines which substances enter or leave the cell.
Cell Membrane • The cell membrane regulates movement of substances in and out of the cell and is the site of much biological activity. • Molecules that are part of the cell membrane receive stimulation from outside the cell and transmit it into the cell, a process called signal transduction. • The cell membrane also holds cells together.
Cell Membrane Characteristics • The cell membrane is extremely thin but flexible and elastic. • The cell membrane is selectively permeable (semipermeable) in that it controls which substances enter and exit.
Cell Membrane Structure • A cell membrane is composed mainly of lipids, proteins, and some carbohydrates. • It is a bilayer of phospholipid molecules. • Each phospholipid molecule includes a phosphate group and two fatty acids bound to a glycerol molecule. • The lipid molecules can move around forming a soft and flexible fluid film.
Membrane Solubility • Because the membrane’s interior consists of fatty acids, it is oily. Only substances soluble in lipids can pass through this layer. • O2 and CO2 can pass through easily but water-soluble molecules, such as amino acids, sugars, proteins, nucleic acids, and various ions, cannot pass through. • Cholesterol molecules help to stabilize the membrane.
Membrane Proteins • Membrane proteins are classified according to their positions within the membrane. • Membrane-spanning (trans-membrane) proteins extend through the lipid bilayer and may protrude from 1 or both faces. • Peripheral membrane proteins are associated with one side of the bilayer.Membrane proteins also vary in shape--globular or elongated.
Membrane Protein Functions • Some proteins form receptors on the cell surface that bind incoming hormones or growth factors, starting signal transduction. • Others transport ions or molecules across the cell membrane. • Some membrane proteins form selective channels that allow only particular ions to enter or leave.
More Protein Functions • Proteins that protrude from the inner face of the cell anchor it to the cytoskeleton (rods and tubules that support the inner cell). • Proteins that extend from the outer surface mark the cell as part of a particular tissue or organ--important identification for the immune system. • CAM (cellular adhesion molecule) determines a cell’s interactions with other cells.
Cytoplasm • Cytoplasm contains networks of membranes and organelles suspended in a clear liquid called cytosol. • Cytoplasm also includes many protein rods and tubules that form a framework called a cytoskeleton. • Cell activities occur mainly in the cytoplasm, where nutrients are received, processed, and used.
Endoplasmic Reticulum Ribosomes Golgi Apparatus Mitochondria Lysosomes Peroxisomes Microfilaments and microtubules Centrosome Cilia and Flagella Vesicles Cell Nucleus nucleolus chromatin Cytoplasmic Organelles
Endoplasmic Reticulum • Endoplasmic Reticulum (ER) is composed of membrane-bound, flattened sacs, elongated canals, and fluid-filled bubble-like sacs called vesicles. • ER provides a vast tubular network that transports molecules from one cell part to another.
Functions of ER • ER participates in synthesis of protein and lipid molecules. • These molecules may leave the cell as secretions, or be used within the cell to produce new ER or cell membranes as the cell grows.
Two Types of ER • Rough ER contains ribosomes on its surface; smooth ER is ribosome-free. • The ribosomes of rough ER are sites or protein synthesis. The proteins may then move through ER tubules to the Golgi apparatus for further processing. • Smooth ER contains enzymes important in lipid synthesis.
Ribosomes • All ribosomes are composed of protein and RNA molecules. • Many ribosomes are attached to ER and others are scattered throughout the cytoplasm. • Ribosomes provide enzymes as well as a structural support for the RNA molecules that come together as the cell synthesizes proteins from amino acids.
Golgi Apparatus • The Golgi apparatus is composed of a stack of about 6 flattened, membranous sacs. • This organelle refines, packages, and delivers proteins synthesized on ribosomes associated with the ER. • Proteins arrive at the Golgi apparatus enclosed in vesicles composed of the ER membrane.
Golgi Function • Vesicles arriving at the Golgi apparatus fuse at the innermost end, which is specialized to receive glycoproteins. • As the glycoproteins pass from layer to layer of the Golgi, they are modified chemically. • When they reach the outermost layer, they are packaged in bits of Golgi membrane, which bud off and form transport vesicles. • Vesicles may move to the cell membrane and release its contents to the outside or be used within the cell--vesicle trafficking.
Mitochondria • Mitochondria are elongated, fluid-filled sacs that have an outer and inner layer. • The inner layer folds extensively to form partitions called cristae. • Cristae contain enzymes that control chemical reactions that release energy from glucose. • Mitochondria are the major sites for cellular respiration--the power house of the cell.
More Mitochondria • Mitochondria contain their own DNA--much like that of bacteria. • According to the endosymbiont theory, mitochondria are the remnants of once free-living bacteria-like cells that were swallowed by more complex primitive cells. • You can only inherit mitochondria from your mother.
Lysosomes • Lysosomes, the ‘garbage disposals of the cell’, are tiny membranous sacs containing powerful enzymes that break down nutrient molecules or foreign particles. • Certain white blood cells can engulf bacteria which are then digested by the lysosomal enzymes. • Lysosomes also destroy worn cellular parts.
Peroxisomes • Peroxisomes, membranous sacs abundant in liver and kidney cells, house enzymes that catalyze a variety of biochemical reactions, including synthesis of bile acids; detoxification of hydrogen peroxide; breakdown of certain lipids and rare biochemicals; and detoxification of alcohol.
Microfilaments and Microtubules • Microfilaments and microtubules are 2 types of thin, threadlike strands within the cytoplasm that form the cytoskeleton.
Microfilaments are tiny rods of actin protein that form meshworks or bundles. They provide cell motility. In muscle cells, microfilaments form myofibrils to help muscle cells contract. Microtubules are long, slender tubes composed of globular tubulin proteins . Usually 2-3 times larger than microfilaments, microtubules are arrayed in a pattern called 9 + 2. Cytoskeleton
Centrosome • The centrosome is a nonmembranous structure near the nucleus of animal cells. • It consists of 2 hollow cylinders called centrioles, made of microtubules. • The centrioles lie at right angles to each other. During mitosis, they help distribute chromosomes to newly forming cells.
Cilia and Flagella • Cilia and flagella are motile extensions from the surfaces of certain cells, made of microtubules in a 9 + 2 arrangement. • Their main difference is in length. • Cilia is a tiny, hairlike structure attached beneath the cell membrane that moves in a ‘to-and-fro’ pattern. • Flagella are longer than cilia. Usually a cell may have only 1 cilia which moves in a whip-like motion.
Vesicles • Vesicles are membranous sacs formed by part of the cell membrane folding inward and pinching off. • Vesicles that hold food or water are called vacuoles. • The Golgi apparatus and ER also form vesicles that play a role in secretion.
Cell Nucleus • The nucleus houses the genetic material (DNA), which directs all cell activity. • It is a large spherical structure enclosed in a double-layered lipid nuclear envelope. • The nuclear envelope has protein channels called nuclear pores that allow certain molecules to exit the nucleus. • A nuclear pore is not just a hole, but a complex opening formed from 100+ proteins.
Within the Nucleus • Nucleolus=“little nucleus”-a small, dense body composed largely of RNA and protein. It has no membrane. Ribosomes form in the nucleolus, then migrate through nuclear pores to the cytoplasm. • Chromatin=loosely coiled fibers of protein and DNA. When the cell begins to divide, chromatin fibers coil tightly into rodlike chromosmes.
Movement Through Cell Membranes • The cell membrane is a selective barrier that controls which substances enter and leave the cell. • Passive mechanisms do not require energy: diffusion, facilitated diffusion, osmosis, and filtration. • Active mechanisms use cellular energy: active transport, endocytosis, and exocytosis.
Passive Mechanisms: Diffusion • Diffusion is the process by which molecules or ions scatter or spread spontaneously from regions of higher concentrations to regions of lower concentrations. • Molecules move at random and mix molecules together. • Equilibrium occurs when the molecules are equally mixed. Movement still occurs but there is no change in concentration.
Examples of Diffusion at Work • Oxygen molecules diffuse through cell membranes and enter cells if these molecules are more highly concentrated on the outside than on the inside. • Carbon dioxide molecules also diffuse through cell membranes in the same way. • Diffusion enables oxygen and carbon dioxide molecules to be exchanged between the air and blood in the lungs and between the blood and cells.