Chapter 3: The Cellular Level of Organization
Chapter Objectives
THE PLASMA MEMBRANE
1. Describe the “fluid mosaic model” concept
2. Describe the components of the lipid bilayer.
3. Distinguish between integral and peripheral proteins in cell membranes.
TRANSPORT ACROSS THE PLASMA MEMBRANE
4. Distinguish between passive and active processes, including direction of particle movement and energy requirements.
5. Explain the concept of simple diffusion and list substances that pass through the bilayer by simple diffusion.
6. Explain the process of osmosis as the passive movement of water through a selectively permeable membrane
7. Explain the differences between simple and facilitated diffusion.
8. Describe the process of channel-mediated facilitated diffusion and identify substances transported by the process.
9. Describe the process of carrier-mediated facilitated diffusion and identify substances transported by the process.
10. Define active process and describe the energy source used to drive the process.
11. Describe the process of primary active transport, relating the process to the sodium/potassium pump.
12. Describe the process of secondary active transport and the role of symporters and antiporters.
13. Describe the process of exocytosis and explain its importance.
14. Describe the process of phagocytosis and explain its importance.
15. Describe the process of pinocytosis and explain its importance.
16. Describe the process of receptor-mediated endocytosis and explain its importance.
17. Show the relation of osmotic pressure to tonicity by the effect on red blood cells of different concentrations of solute in a surrounding solution.
ORGANELLES
18. Describe the basic structural features and functions of organelles.
19. Discuss the function of the nucleus, nuclear envelope, nucleoli, and chromatin.
20. Indicate the origin and components of ribosomes that allow them to produce proteins, and their association with other organelles in this process.
21. Compare and contrast the two forms of Endoplasmic reticulum and describe their functions.
22. Describe the structure and function of the Golgi apparatus.
23. Describe the structure and function of the lysosomes.
24. Describe the role of mitochondria in producing the energy-storage molecule ATP.
25. Distinguish the characteristics and functions of microfilaments, microtubules, and intermediate filaments.
26. Show the relationship of cytoskeletal elements to centrioles and the centrosome and note their purpose in dividing and nondividing cells
27. Describe the arrangement of microtubules in flagella and cilia that allow these projections to perform the different types of transport movements.
PROTEIN SYNTHESIS
28. List the steps of protein synthesis and describe what occurs in each.
Chapter Lecture Notes
Plasma Membrane
Plasma membrane – flexible yet sturdy barrier that surrounds and contains the cytoplasm of a cell (Fig 3.2)
Fluid mosaic model
Lipid bilayer
Phospholipids
Cholesterol
Glycolipids
Membrane proteins (Fig 3.3)
Integral proteins – extend into or through the lipid bilayer
Peripheral proteins – associated with the polar heads or integral proteins
Glycoproteins – integral or peripheral proteins that have carbohydrate molecules attached
Transport Across Membranes (Table 3.1)
Process / Energy Source / Description / ExamplesPassive processes
Simple Diffusion (Fig 3.4 & 3.5) / Kinetic energy / Net movement of particles from an area of their higher concentration to an area of their lower concentration, that is along their concentration gradient / Movement of fats, oxygen, carbon dioxide through the lipid portion of the membrane
Osmosis (Fig 3.8) / Kinetic energy / Simple diffusion of water through a selectively permeable membrane / Movement of water into and out of cells through the lipid bilayer and/or membrane pores (aquaporins)
Channel-Mediated Facilitated Diffusion (Figs 3.5 & 3.6) / Kinetic energy / Same as simple diffusion, but the diffusing substance goes through a membrane protein channel / Movement of ions into and out of cells
Carrier-Mediated Facilitated Diffusion (Figs 3.5 & 3.7) / Kinetic energy / The diffusing substance must attach to its membrane protein carrier / Movement of glucose into most body cells
Filtration / Hydrostatic pressure / 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
Active processes
Primary Active Transport (solute pumping) (Fig 3.10) / ATP (cellular energy) / Movement of a substance through a membrane against a concentration (or electrochemical gradient; requires a membrane carrier protein) / Movement of Na+ and K+ across the membrane against their concentration gradient
Secondary Active Transport (Fig 3.11) / ATP (cellular energy) / Energy stored in a Na+ or H+ concentration gradient is used to drive other substances across the membrane against their concentration gradients / Movement of calcium ions out of the cell while Na+ is moving in (antiporters); dietary glucose and amino acids moving into cells at the same time Na+ is also moving in (symporters)
Bulk transport
Exocytosis / ATP / 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 / Secretion of neurotransmitters, hormones, mucus, etc.; ejection of cell wastes
Phagocytosis (endocytosis) (Fig 3.13) / 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) (Fig 3.14) / 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
Receptor-mediated endocytosis (Fig 3.12) / 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 (Fig 3.9)
Isotonic – concentrations of solutes are the same on both sides of the membrane
0.9% NaCl 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 (Fig 3.1 & 3.24)
contains chromosomes - heredity material - called chromatin when cell is not dividing (Fig 3.25)
before cell division, chromosomes are copied by a process called replication (Fig 3.31)
Nucleolus - assembly plant for ribosomes
surrounded by nuclear membrane which has pores in it through which substances enter and exit
Ribosomes
Ribosomes - contain both rRNA and ribosomal proteins (Fig 3.18)
named ribosomes because of the high content of RNA (literally RNA bodies)
one ribosome consists of one larger and one smaller subunit
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
some ribosomes are attached to 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 (Fig 3.19)
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
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 Ca2+ in muscle (but called sarcoplasmic reticulum (SR) in muscle)
Golgi Complex
Golgi complex - 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) (Fig 3.20)
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
Protein Trip: ribosomes (site of protein synthesis) → rough ER → transport vesicle → Golgi → secretory vesicle → release to exterior by exocytosis (Fig 3.21)
proteins and lipids used in cell membrane or inside cell follow a similar route
Lysosomes (Fig 3.22)
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
programmed destruction of cells:
programmed destruction of tissue between fingers and toes during development so webbed hands and feet are absent
metamorphosis of tadpole to frog (lysosomes destroy cells of tail)
release of lysosomal enzymes outside of cell:
bone remodeling by osteoclasts (knee remodeled every 3-4 months)
rheumatoid arthritis due to leaking of lysosomal enzymes into joint cavity and destruction of joint cavity - cortisone and aspirin both help to maintain integrity of lysosomal membrane
lysosome works best in an acid pH (pH = 5) even though cytoplasm is pH 7:
contains proton (H+) pumps that gather H+ from cytoplasm and pump into lysosomes
lysosomes retain the dangerous hydrolytic enzymes but permit final products of digestion to escape
Mitochondria
Mitochondria - powerhouse of cell - double membraned - found in both animal and plant cells, but not in bacteria (Fig 3.23)
center of cellular respiration (Krebs cycle and electron transport chain) - 95 % of oxygen brought into body is used in mitochondria as the final hydrogen acceptor to make metabolic water (See Chapter 25)
Krebs cycle occurs in matrix - series of enzymes remove H from molecules such as glucose and fatty acids and are accepted by NAD to make NADH2
Electron transport chain (ETC) - occurs along cristae - accepts the NADH2 from Krebs cycle - ETC has a series of enzymes that split the hydrogen into electrons and protons - electrons are transported from high energy levels to low energy levels and ATP is then produced - at the end of ETC the proton and electron come back together to form hydrogen which is then accepted by O2 to make H2O
mitochondria contains its own DNA - can reproduce independently
Cytoskeleton
Cytoskeleton - elaborate network of protein structures = "bones and muscles" (Fig 3.15)
microfilaments - thin strands of actin - aid in cell movement (ex. amoeboid movement as in WBC), aid in cytokinesis
most highly developed in muscles
intermediate filaments - tough, insoluble protein fibers that are intermediate in size between microfilaments and microtubules - protein varies with cell types (ex. keratin filaments in epidermal cells)
microtubules - hollow tubes formed of globular proteins called tubulins
provide monorail system to move organelles/vesicles
also in centrioles, cilia, spindle fibers
Centrioles
Centrioles - paired cylindrical bodies each composed of 9 triplets of microtubules (protein straws) (Fig 3.16)
Functions:
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)
form the bases of cilia and flagella (having centrioles at each pole in mitosis provides a vehicle for transmission of centrioles to all cells)
Cilia/Flagella
Cilia/Flagella - Membrane bound sets of microtubules that move by means of ATP (Fig 3.17)
cannot produce cilia/flagella without centrioles
centrioles = 9 triplets of microtubules
cilia/flagella = 9 sets of doublets with central pair
Protein Synthesis
DNA in nucleus is used as instruction manual to produce proteins
Protein synthesis is a two step process: transcription and translation (Fig 3.26)
Transcription
Transcription - the synthesis of RNA under the direction of DNA, occurs in the nucleus (Fig 3.27)
Messenger RNA (mRNA) - RNA that carries a genetic message from the DNA to the protein-synthesizing machinery
Translation
Translation - the synthesis of a polypeptide, which occurs under the direction of mRNA (Fig 3.29)
Triplet code - instruction for a polypeptide chain written in three-nucleotide word, codons
Translation is done by the ribosomes in cytoplasm (free) and at endoplasmic reticulum (bound)