Cells Chapter 3 Bio 160 Plasma Membrane Plasma

Cells Chapter 3 Bio 160

Plasma Membrane • Plasma membrane – flexible yet sturdy barrier that surrounds and contains the cytoplasm of a cell. Fluid mosaic model Lipid bilayer Membrane proteins

Plasma Membrane The plasma membrane is selectively permeable, which means that only certain substances can enter or leave the cell • Cytoplasm – Everything inside the cell including the fluid (cytosol) and organelles.

Transport Across Membranes Process Energy Source Description Examples Passive processes Simple diffusion Kinetic energy Net movement of Movement of fats, particles (ions, oxygen, carbon molecules, etc. ) dioxide through from an area of their the lipid portion higher concentration of the to an area of their membrane, and lower concentration, ions through that is along their protein channels concentration under certain gradient conditions Osmosis Kinetic energy Simple diffusion of water through a selectively permeable membrane Movement of water into and out of cells via membrane pores

Transport Across Membranes Facilitated diffusion Kinetic energy Same as simple diffusion, but the diffusing substance is attached to a lipidsoluble membrane carrier protein Movement of glucose into cells Filtration Movement of water and solutes through a semipermeable membrane from a region of higher hydrostatic pressure to a region of lower hydrostatic pressure, that is along a pressure gradient Movement of water, nutrients, and gasses through a capillary wall; formation of kidney filtrate Hydrostatic pressure

Transport Across Membranes Active processes Active transport (solute pumping) ATP (cellular Movement of a substance energy) through a membrane against a concentration (or electrochemical gradient; requires a membrane carrier protein) Movement of amino acids and most ions across the membrane ATP Secretion of neurotransmitters, hormones, mucus, etc. ; ejection of cell wastes Bulk transport Exocytosis Secretion or ejection of substances from a cell; the substance is enclosed in a membranous vesicle, which fuses with the plasma membrane and ruptures, releasing the substance to the exterior

Transport Across Membranes Phagocytosis (endocytosis) ATP “Cell eating”; A large external particle (proteins, bacteria, dead cell debris) is surrounded by a “seizing foot” and becomes enclosed in a plasma membrane In the human body, occurs primarily in protective phagocytes (some white blood cells, macrophages) Pinocytosis (endocytosis) ATP “Cell drinking”; Plasma membrane sinks beneath an external fluid droplet containing small solutes; membrane edges fuse, forming a fluid-filled vesicle Occurs in most cells; important for taking in solutes by absorptive cells of the kidney and intestine

Transport Across Membranes Receptor-mediated endocytosis ATP Selective endocytosis process; external substance binds to membrane receptors, and coated pits are formed Means of intake of some hormones, cholesterol, iron, and other molecules

Tonicity • Isotonic – concentrations of solutes are the same on both sides of the membrane 0. 9% Na. Cl solution (normal physiological saline) is isotonic for red blood cells (RBC) • Hypotonic – a solution that has lower concentration of solutes than the cytosol Lysis or hemolysis in RBC • Hypertonic – a solution that has greater concentration of solutes than the cytosol Crenation – cell shrinkage

Nucleus • Nucleus – control center contains chromosomes - heredity materialcalled chromatin when cell is not dividing nucleolus - assembly plant for ribosomes surrounded by nuclear membrane which has pores in it through which substances enter and exit

Ribosomes • Ribosomes - contain both r. RNA and ribosomal proteins functions as the workbench for protein synthesis some ribosomes are free ribosomes - no attachment to organelles - concerned primarily with synthesizing proteins for use inside cell

Ribosomes some ribosomes are attached to the endoplasmic reticulum (ER), hence rough ER, involved in the synthesis of proteins for insertion in the cell membrane or for export from the cell

Endoplasmic Reticulum • Endoplasmic reticulum (ER) - system of membrane-enclosed channels continuous with nuclear membrane and Golgi complex rough ER - has attached ribosomes - proteins synthesized are stored by the ER and sugar groups may be added to form glycoproteins then transported from ER to Golgi

Endoplasmic Reticulum (2) smooth ER - no ribosomes attached - provides a surface area for chemical reactions – site of steroid, fatty acid, phospholipid synthesis (ex: in testis provides surface for enzymes involved in testosterone synthesis) – site of carbohydrate synthesis, detoxification of alcohol, pesticides, carcinogens (ex: liver synthesis of glycogen) – stores Ca++ in muscle (but called sarcoplasmic reticulum (SR) in muscle)

Golgi Apparatus • Golgi apparatus - consists of stacks of flattened sacs (like pancakes) that can form vesicles for exocytosis, lysosomes, or for storage (vesicles are membrane bound sacs that are smaller than vacuoles) Golgi receives proteins, carbohydrates, lipids from vesicles made from ER and collects, sorts, packages as new vesicles, and delivers vesicles for storage, membrane use, or exocytosis, lysosomes

Lysosomes • Lysosome - formed by Golgi and contain powerful digestive (hydrolytic) enzymes that: recycle monomers in a cell from polymers (proteins, carbohydrates, lipids, nucleic acids) destroy bacteria engulfed by white blood cells (WBC) when the phagocytic vesicle fuses with lysosome

Mitochondria • Mitochondria - powerhouse of cell - double membraned - found in both animal and plant cells, but not in bacteria Major site of chemical reactions that release energy from certain nutrient molecules and transfer it to ATP, a chemical form the cell can use.

Cytoskeleton • Cytoskeleton - elaborate network of protein structures = "bones and muscles" microfilaments - thin strands of actin - aid in cell movement (ex. amoeboid movement as in WBC), aid in cytokinesis – most highly developed in muscles

Cytoskeleton microtubules - hollow tubes formed of globular proteins called tubulins – provide monorail system to move organelles/vesicles – also in centrioles, cilia, spindle fibers

Centrosomes and Centrioles • Centrosomes – a structure near the Golgi and nucleus that contains two centrioles. • Centrioles - paired cylindrical bodies, each composed of microtubules Organize spindle fibers and asters during mitosis in animal cells (may not be necessary for this purpose because plants produce spindle fibers during mitosis but they lack centrioles)

Centrosomes and Centrioles – Having centrioles at each pole in mitosis provides a vehicle for transmission of centrioles to all cells Form the bases of cilia and flagella

Cilia/Flagella • Cilia/Flagella - Membrane bound sets of microtubules that move by means of ATP cannot produce cilia/flagella without centrioles – Cilia – tiny, hairlike structures that fringe the surface of some cells – Flagellum – long, usually single extension, ex. tail of a sperm