The Structure and Function of Cells

The Structure and Function of Cells The Cell is the basic functional unit of ALL living things. There are 2 basic types of cells: 1. Eukaryotic Cells = contain plasma membrane (cell membrane), organelles, nucleus, cytoplasm Most cells are of this type (body cells, plant cells, Protists, etc.) Evolutionarily this is a recent advancement 2. Prokaryotic Cells = contain plasma membrane (cell membrane) and cytoplasm but NO TRUE ORGANELLES, NO NUCLEUS Bacteria cells are of this type Ribosomes are the only other real internal structure The oldest of cell types are prokaryotic IMPORTANT PARTS OF THE CELL 1. Plasma Membrane: separates the internal metabolic events from the external environment surrounding the cell Function = control the movement of materials into and out of the cell Structure = a double phospholipid membrane (Lipid Bilayer), Polar hydrophilic water loving heads pointing toward outside and inside of the cell; Nonpolar hydrophobic water fearing tails point toward the inside of the membrane. Fluid Mosaic Model = describes the structure of the cell membrane. PROTEINS are scattered throughout the flexible phospholipid bilayer. These proteins can be of several different types: 1. Peripheral Proteins: attached loosely to the inner or outer surface of the membrane 2. Integral Proteins: extend into the membrane 3. Transmembrane Proteins: an integral protein that spans across the membrane and appears at both surfaces The CELL MEMBRANE is Semi-Permeable = only small, uncharged, polar, hydrophobic molecules pass freely across the membrane Permeable (Pass freely) = H 2 O, CO 2, O 2, lipid soluble hydrocarbons NON-Permeable = large molecules, glucose, all ions 2. Proteins in the plasma membrane provide a wide range of functions including: 1. Channel Proteins: provide passageways through the membrane for certain water soluble substances (i.e. polar & charged molecules) 2. Transport Protein: use energy to transport materials across the membrane against a concentration gradient 3. Recognition Proteins (Glycoproteins): used to distinguish the identity of neighboring cells 1

4. Adhesion Proteins: attach cells to neighboring cells, provide stability 5. Receptor Proteins: provide a binding site for hormones or other chemical triggers 6. Electron Transfer Proteins: transfer electrons from one molecule to another during chemical reactions 3. Cholesterol = provide some rigidity to the fluid nature of the membrane in animal cells. Act as a Cellular Antifreeze to keep a cell from freezing in cold temps. or melting in warm temps. 4. Glycocalyx = carbohydrate coat covering the outer surface of the cell membrane. Used for cell to cell recognition. ORGANELLES OF THE CELL Organelle = a structure in the cytoplasm of the cell that serves a specific function related to the metabolism of the cell. They can be thought of as tiny organs that carry out the life functions of a cell 1. Nucleus: contains DNA, the hereditary information of the cell. Surrounded by a nuclear envelope allows for the movement of proteins in, ribosome subunits out, mrna out of the nucleus Normal DNA = Chromatin: spread out in a threadlike matrix Chromosomes: visible when the cell begins to divide 2. Nucleolus: concentrations of DNA involved in the process of manufacturing (synthesizing) and storage of the components of Ribosomes 3. Ribosomes: assemble amino acids into proteins in the cytoplasm structure is comprised of a small and large subunit (one on top of the other) 4. Endoplasmic Reticulum (ER): stacks of flattened sacs involved in the production of various products, looks like a maze when viewed in crosssection, often connected to the nucleus Rough ER: has ribosomes on it, specializes in protein synthesis Smooth ER: NO ribosomes, specializes in the production of lipids, detoxification, storage etc. (depends on the cell type) 5. Golgi Apparatus (Complex or Body): a group of flattened sacs arranged like a stack of bowls Function to modify and package proteins and lipids Puts the products into Vesicles Contents of vesicles get released outside the cell 6. Lysosomes: vesicles that contain digestive enzymes. NOT FOUND IN PLANT CELLS 2

7. Peroxisomes: breakdown various substances O 2 + H 2 H 2 O 2 Common in liver and kidney cells 8. Mitochondria: carry out aerobic respiration, create energy, the POWERHOUSE of the cell kidney bean shaped structure double membrane stack with internal foldings called Cristae Energy in the form of ATP is obtained from carbohydrates 9. Chloroplasts: carry out photosynthesis in plants; this is where plants incorporate energy from the sun to make carbohydrates STRUCTURE: a double membrane sac containing stacks of membranes called thylakoids (grana) 10. Cytoplasm: The Semifluid medium found inside the cell Suspends all internal organelles keeps them in place provides a medium through which substances can travel and move in the cell 11. Cytoskeleton: internal structure of the cytoplasm Microtubules = provide support and motility, made of protein tubulin (spindle fibers during cell division; cilia & flagella) Intermediate Filaments = support, maintain cell shape Microfilaments = made of protein actin, involved in cell motility 12. Flagella and Cilia: protrude from the cell membrane, used for movement Flagella = long and few Cilia = short and many Both arranged in 9+2 arrangement 13. Centrioles and Basal Bodies: Microtubule organizing centers 14. Microtubules: thin cylinders containing 13 rows of the globular protein tubulin. Found in various organelles such as Cilia and Flagella in animal cells,and the centriole which is used for attachment to spindle fibers during cell division 15. Cell Wall: found in plants; outside of plasma membrane, provides support, made of Cellulose 16. Vacuoles and Vesicles: fluid filled, membrane bound bodies Transport Vesicles = move materials between organelles Food Vacuoles = temporary food storage Storage Vacuoles = store starch, pigments, and toxins 3

Central Vacuole = fill up most of the interior in plant cells only, maintain rigidity of the cell, how plant cells can grow larger in size take in more water to fill central vacuole Contractile Vacuoles = collect and pump excess water out of a cell 17. Cell Junctions: serve to anchor cells to one another Desmosomes = act as spot welds to hold adjacent animal cells together in high stress tissues (skin, heart muscle) Tight Junctions = stitched seams between animal cells, prevent movement of material between the cells (cells lining the digestive tract). Gap Junctions = narrow tunnels between animal cells, allow passage of small ions and molecules, communication through electrical impulses Plasmodesmata = narrow channels between plant cell Plant vs. Animal Cells 1. Plants = cell walls, large vacuole, chloroplasts 2. Animal = lysosomes, centrioles Prokaryotes vs. Eukaryotes Prokaryotes No nucleus No Organelles Single stranded naked DNA Only Bacteria Eukaryotes Have a nucleus Lots of Organelles DNA in chromosomes All other cells The Movement of Substances The cell membrane and the membrane of other organelles are Semipermeable = only certain substances can cross the membrane Selective Permeability = allowing only specific substances to pass Water = solvent; dissolved particles = solute Movement of substances may be from higher to lower concentrations (down or with the concentration gradient) or the reverse (up or against the gradient) The movement of substances may be active or passive Solute concentrations may be compared: Hypertonic = higher concentration of solutes Hypotonic = lower concentration of solutes Isotonic = equal concentration of solutes relative to another region. Bulk Flow = collective movement of substances in the same direction in response to some force or pressure (blood moving through a vessel) 4

Passive vs. Active Transport Passive Transport = Movement of molecules down a concentration gradient (high low) Does not require the cell to expend any energy! 1. Diffusion: due to the random nature of molecules in motion (dye in a beaker of water) Rates of diffusion are determined by: a. Concentration gradient b. Shape and size of the molecules c. Temperature (higher temp. = faster diffusion time) d. Liquid vs. gas (slower in liquids) 2. Osmosis: the diffusion of water molecules across a semi-permeable membrane Osmotic Pressure = force water exerts against the inside of a cell Turgor Pressure = pressure water exerts against the inside of a plant cell, used for structural support 3. Dialysis: diffusion of solutes across a semi-permeable membrane 4. Plasmolysis: movement of water out of a cell, results in the collapse of the cell 5. Facilitated Diffusion: use channel proteins embedded in the membrane to transport solutes Active Transport = requires the cell to expend energy to move substances against the concentration gradient (low high) Usually utilizes transport proteins to move: small ions (Na +, K +, Cl -, H + ), amino acids, simple sugars 1. Exocytosis: Large particles are released from the cell by vesicles forming and fusing with the cell membrane 2. Endocytosis: Large particles enter the cell by vesicles forming and fusing with the cell membrane Phagocytosis = cell eating (white blood cells attacking bacteria) Pinocytosis = cell drinking Receptor Mediated Endocytosis = special receptors attract certain molecules (cholesterol transport in the blood, hormones) 5