Cell Structure and Function

Transcription

1 Cell Theory Cell Structure and Function Chapter 6 Pg What is a cell? The basic functional unit of all living things. The Cell Theory states All organisms are made of one or more cells. Cells are the basic units of structure and function in living things. Cells arise by division of pre-existing cells. Basically, cells come from other cells. Study of cells How do we do it? MICROSCOPES Light microscopes Electron microscopes Transmission electron microscope (TEM) Scanning electron microscope (SEM) Cytology Light Microscope Comparison of Scopes Visible light passes through specimen and refracts light so specimen is magnified Magnifies up to 1000x Specimen can be alive/moving Color Can t see organelles other than nucleus Electron Microscope Focuses a beam of electrons through/onto specimen Magnifies up to 1,000,000x Specimen is non-living and in vacuum Black and white; colored by computer Can see organelles Electron Microscope Comparison How do we know? Transmission (TEM) 2D Internal cell structures Flat image with extreme detail Enhance contrast by staining atoms with heavy metal dyes Scanning (SEM) 3D Detailed surface structures Great field of depth Sample covered with thin film of gold, beam excites electrons on surface Methods of studying cell structure and function: Cell fractionation: take apart cells and separate major organelles Ultracentrifuge: applies 1,000,000x force of gravity to separate cell organelles Most dense are at the bottom 1

2 Importance of Cell Size As a cell grows, its volume increases faster than its surface area. This makes the surface area-to-volume (SA/V) ratio DECREASE. So a larger cell has a smaller SA/V ratio. This can make chemical exchange difficult. Cells must remain small to maintain a large SA/V ratio This allows exchange between the cell and the environment to occur the most efficiently Cell Types Prokaryotes (prokaryotic cells) Domain Bacteria and Archaea Typically these cells only consist of a plasma membrane, DNA, ribosomes, cytoplasm, and often a cell wall Eukaryotes (eukaryotic cells) Domain Eukarya Remaining organisms plants, animals, protists, fungi, etc. Prokaryotes No nucleus DNA stored in nucleoid No organelles other than ribosomes Small size Primitive Comparing Cell Types Include bacteria and Archaea Eukaryotes Nucleus and nuclear envelope DNA stored in nucleus Many specialized membranebound organelles Large size Complex Include plants, animals, fungi, and protists Both have cytosol/cytoplasm, plasma membrane, ribosomes 2

3 Organelles Cytoplasmic structures that separate metabolic reactions that occur within eukaryotic cells Chemical reactions are isolated and without interference or competition Many provide large surface area to maximize space for chemical reactions There are different concentrations of organelles in different cells, depending on their job Major Parts of the Cell Cell membrane Nucleus Ribosomes Endoplasmic reticulum Golgi apparatus Lysosomes Peroxisomes Mitochondria Chloroplasts Microtubules, intermediate filaments, and microfilaments Flagella and cilia Centrioles and basal bodies Vacuoles and vesicles Extracellular structures Cell Membrane A.K.A. plasma membrane Separates internal metabolic events from the external environment Controls movement of materials into and out of the cell Consists of a phospholipid bilayer, or a double phospholipid membrane Nucleus Enclosed by a nuclear envelope two phospholipid bilayers Nuclear pores allow proteins and RNA molecules to exit the nucleus Nucleus contains DNA (genetic material) and acts as the control center of the cell DNA is typically in loose chromatin form, but condenses into rod-like chromosomes during cell division Also contains nucleolus (plural: nucleoli), which synthesizes ribosomes Nuclear envelope attaches to rough endoplasmic reticulum membrane 3

4 Ribosomes Endoplasmic Reticulum (ER) Makes proteins Large and small subunits Composed of rrna and proteins Types: Free ribosomes float in the cytosol and produce proteins used within the cell Bound ribosomes are attached to the rough ER and make proteins for export from the cell Extend from the outer layer of the nuclear envelope Network of membranes and sacs Types Rough ER: ribosomes are present; creates glycoproteins, packages proteins for secretion, sends transport vesicles to Golgi, makes replacement membranes Smooth ER: no ribosomes; synthesis of lipids and steroid hormones, metabolizes carbs, stores calcium and helps regulate muscle contraction, breakdown of toxins, drugs, and toxic by-products from cellular reactions Golgi Apparatus (a.k.a. Golgi body, Golgi complex) Group of cisternae (flattened sacs) arranged like a stack of bowls Collect and modify proteins and lipids made in other areas of the cell and package them into vesicles As modified proteins exit, they are packaged into a secretory vesicle that migrates to and merges with the plasma membrane, releasing its contents to the outside of the cell Other packaged substances may be retained within the cell for other purposes Lysosomes Vesicles from the Golgi apparatus that contain hydrolytic enzymes (enzymes that break down molecules by hydrolysis) Breaks down food, cellular debris, and foreign invaders like bacteria Contribute to recycling cellular nutrients A low ph is favorable to the enzymes and is maintained inside the lysosome Any enzyme that might escape from the lysosome remains inactive in the neutral ph of the cytosol (so the cell will not be damaged by rupturing lysosomes) Only found in animal cells In the Real World Lysosomal diseases are often fatal Digestive enzyme does not work in the lysosome, so biomolecules cannot be digested. Lysosome ends up with undigested material, growing larger and larger until it disrupts cell and organ function. More than 40 lysosomal storage diseases are known. Example: Tay-Sachs disease, a buildup of undigested fat in brain cells Lysosomes and Apoptosis Lysosomes can be used to kill cells when they are supposed to be destroyed, as in during development. Apoptosis: programmed cell destruction where lysosomes break open and kill the cell Example: tadpole tail, webbed fingers/toes Cancer cells override this function to enable tumor growth. Typically apoptosis would be triggered in cells growing uncontrollably. 4

5 Peroxisomes Mitochondria Break down various substances, including hydrogen peroxide (H 2 O 2 ), fatty acids, and amino acids Common in liver and kidney cells, where they break down toxic substances In plant cells, peroxisomes modify byproducts of photorespiration One large to thousands of smaller ones may be in a single cell The more activity a cell performs, the more mitochondria it will have Carry out aerobic cellular respiration Generates ATP in the presence of O 2 Catabolic reaction Have 2 bilayer membranes Separates metabolic processes Inner membrane is folded to increase surface area, enhancing productivity The inner compartment, the mitochondrial matrix, is fluidfilled Chloroplasts Carry out photosynthesis Plant process of incorporating energy from sunlight into carbohydrates Contains chlorophyll (green pigment) for capturing solar energy Also have two membranes like the mitochondria Only found in plants Cytoskeleton Network of protein fibers extending from nucleus to plasma membrane in eukaryotic cells Several functions Structural support: maintains cell shape, anchors organelles Motility: cell motion Regulation: organize structures and activities of cells Made up of microtubules, intermediate filaments, and microfilaments Microtubules, intermediate filaments, and microfilaments Establish the shape of or coordinate movements of the cytoskeleton; made and broken down as needed Microtubules: tubulin; provides support and motility for cellular activities; found in spindle fibers, flagella, and cilia Intermediate filaments: keratin; provide support for maintaining cell shape Microfilaments (actin filaments): actin; involved in cell motility; found in muscle cells and amoeba; in plants, they promote cytoplasmic streaming Centrioles Also called the microtubule organizing center Located within the centrosome, the region from which microtubules grow Make up and organize the spindle apparatus during cell division separates chromosomes Only found in animal cells 5

6 Cilia and Flagella Vacuoles and vesicles Extensions of the cytoskeleton; allow cell to move Flagellum (plural: flagella): whip-like movement; cell moves in same direction as flagellum; few and long Cilium (plural: cilia): oar-like movement; cell moves perpendicular to cilium; many and short Fluid-filled, membrane-bound bodies Primary function is storage Transport vesicles: move materials throughout cell Food vacuoles: temporary receptacles of nutrients, often merge with lysosomes for digestion Contractile vacuoles: collect and pump excess water out of the cell Central vacuoles: large bodies occupying most of the interior of many plant cells Functions include: maintain rigidity of cell wall, store nutrients/waste/pigments, digestion, allows cell to grow, creates large cytoplasmic surface area Extracellular Structures Cell walls: found in plants, fungi, and many protists; provide support for the cell on the OUTSIDE of the plasma membrane Plants: mostly made of cellulose Fungi: mostly made of chitin (modified polysaccharide; also found in insect exoskeletons) Protects cell, maintains shape, prevents excessive water uptake Extracellular matrix: found in animals between adjacent cells; occupied by fibrous structural proteins, adhesion proteins, and glycoproteins secreted by cells Provides mechanical support; helps bind adjacent cells together takes the place of a cell wall Most common substance in this region is collagen Function: strengthens tissues, helps with cell signaling, support, adhesion, movement, and regulation Cell Junctions Anchors cells to one another or provide a passageway for cellular exchange Includes Anchoring junctions: ANIMALS Desmosomes: binds adjacent cells for mechanical stability of tissues Tight junctions: ANIMALS Tightly stitched seams, typical in the digestive tract where materials pass through layers of cells to enter blood stream Communicating junctions: transfer chemical or electrical signals as a means of communication Gap junctions: ANIMALS Plasmodesmata: PLANTS Plant or Animal? Plant cells can generally be distinguished from animal cells by the following: The presence of cell walls, chloroplasts, and central vacuoles in plant cells and their absence in animal cells. The presence of centrioles, lysosomes, and cholesterol in animal cells and their absence in plant cells. 6