Chapter Three (PART TWO)

The Cell Nucleus

• The nucleus contains genetic material, DNA, which controls cell activities.

• The fairly large nucleus is surrounded by a double-layered nuclear envelope, containing relatively large nuclear pores that allow the passage of certain substances.

• Contains a fluid called nucleoplasm.

• The nucleolus is a small, dense body in the nucleus, composed of RNA and protein; it is the site of ribosome production.

• Chromatin consists of loosely coiled fibers of protein and DNA found in the nucleus.

• Condensed DNA is referred to as chromosomes; this form of DNA is present during cell division.

Structures and Functions of Cell Parts (Table 3.1)

• Cell Part: Cell membrane

  • Structure: Membrane composed of protein and lipid molecules

  • Function: Maintains integrity of cell and controls passage of materials into and out of the cell

• Cell Part: Ribosomes

  • Structure: Particles composed of protein and RNA molecules

  • Function: Synthesize proteins

• Cell Part: Endoplasmic reticulum

  • Structure: Complex of interconnected membrane-bounded sacs and canals

  • Function: Transports materials within the cell, provides attachment for ribosomes, and synthesizes lipids

• Cell Part: Vesicles

  • Structure: Membranous sacs

  • Function: Contain and transport various substances

• Cell Part: Golgi apparatus

  • Structure: Stack of flattened, membranous sacs

  • Function: Packages protein molecules for transport and secretion

• Cell Part: Mitochondria

  • Structure: Membranous sacs with inner partitions

  • Function: Release energy from nutrient molecules and change energy into a usable form

• Cell Part: Lysosomes

  • Structure: Membranous sacs

  • Function: House enzymes that digest worn cellular parts or substances that enter cells

• Cell Part: Peroxisomes

  • Structure: Membranous sacs

  • Function: House enzymes that catalyze diverse reactions, including breakdown of hydrogen peroxide and fatty acids, and alcohol detoxification

• Cell Part: Microfilaments and microtubules

  • Structure: Thin rods and tubules

  • Function: Support the cytoplasm and help move substances and organelles within the cytoplasm

• Cell Part: Centrosome

  • Structure: Nonmembranous structure composed of two rodlike centrioles

  • Function: Helps distribute chromosomes to new cells during cell division

• Cell Part: Cilia and flagella

  • Structure: Motile projections attached beneath the cell membrane

  • Function: Cilia propel fluid over cellular surfaces, and a flagellum enables a sperm cell to move

• Cell Part: Nuclear envelope

  • Structure: Double membrane that separates the nuclear contents from the cytoplasm

  • Function: Maintains integrity of nucleus and controls passage of materials between nucleus and cytoplasm

• Cell Part: Nucleolus

  • Structure: Dense, nonmembranous body composed of protein and RNA

  • Function: Site of ribosome synthesis

• Cell Part: Chromatin

  • Structure: Fibers composed of protein and DNA

  • Function: Contains information for synthesizing proteins

Movements Into and Out of the Cell

• The cell membrane is a selective barrier that controls which substances pass through it.

• Mechanisms of movement across the membrane may be passive, requiring no energy from the cell, or active, requiring cellular energy.

• Examples of passive transport mechanisms: diffusion, facilitated diffusion, osmosis, and filtration.

• Examples of active transport mechanisms: active transport, endocytosis, and exocytosis.

Passive Mechanisms: Diffusion

• Diffusion (Simple diffusion): Movement of molecules or ions in a liquid or gas from a region of higher concentration to one of lower concentration.

• This is referred to as moving down the concentration gradient.

• Caused by the random motion and collisions of particles.

• At diffusional equilibrium, particles have become uniformly distributed; they continue to move, but there is no net change in concentration.

• Diffusion enables oxygen and carbon dioxide molecules to be exchanged between the air and the blood in the lungs, and between blood and cells.

• Diffusion across cell membranes occurs only if cell membrane is permeable to the substance and there is a concentration gradient.

• A sugar cube placed into a glass of water will disperse over time, by the process of diffusion.

Figure 3.11: Example of Diffusion: Dissolving a Sugar Cube in a Glass of Water

• A sugar cube placed into a glass of water will disperse over time, by the process of diffusion.

Figure 3.12: Example of Diffusion Across a Permeable Membrane

• Membrane is permeable to both water and the solute.

• Equilibrium is reached when the concentrations of water and the solute are equal in both compartments.

Diffusion Video

Figure 3.13: Example of Diffusion: Exchange of O2 and CO2 Between the Capillaries & Body Cells

Passive Mechanisms: Facilitated Diffusion

• Facilitated diffusion is a method of transport of substances across the cell membrane, using membrane proteins to carry the substances across the phospholipid bilayer.

• One type of facilitated diffusion involves the use of ion channels to transport ions across the cell membrane.

• Substances transported by facilitated diffusion, such as glucose and amino acids, cannot pass through the phospholipid bilayer, since they are water-soluble or too large; they require the help of a specific carrier molecule.

• The number of carrier molecules in the cell membrane limits the rate of this process.

Figure 3.14: Facilitated Diffusion

• Diagram shows transported substance, protein carrier molecule, region of lower concentration, ion channel, region of higher concentration, and cell membrane.

Passive Mechanisms: Osmosis

• Osmosis: Movement of water across a selectively permeable membrane, into an area that contains an impermeant solute.

• Osmosis is a special case of diffusion, in which water moves across a selectively permeable membrane, from an area of greater water concentration (where there is lower osmotic pressure and a lower concentration of solutes) to an area of lower water concentration (where there is greater osmotic pressure and a higher concentration of solutes).

• During osmosis, water moves from a region of lower solute concentration to a region of higher solute concentration.

• The cell membrane is permeable to water but impermeable to water-soluble solutes.

• Osmotic pressure is the pressure needed to lift a volume of water; the greater the concentration of impermeable solutes in a solution, the greater its osmotic pressure.

Figure 3.15: Osmosis

• Osmotic pressure is the pressure needed to lift a volume of water; more impermeable solutes creates more pressure.

3.4: Tonicity

• Tonicity refers to composition of dissolved solutes in a solution surrounding a cell.

• Tonicity affects the size and shape of a cell.

• The greater the impermeable solute concentration in a solution, the greater its osmotic pressure.

• Water moves toward solutions with greater osmotic pressure.

• Cell membranes are usually permeable to water, but impermeable to many solutes, so water equilibrates by osmosis, and equilibrates the osmotic pressure in the intracellular and extracellular fluids.

• Isotonic; a solution with the same osmotic pressure as body fluids is called isotonic; 0.9\% \mathrm{NaCl} solution is isotonic to human cells.

• Hypertonic: higher osmotic pressure; cells placed in a hypertonic solution will lose water and shrink.

• Hypotonic: lower osmotic pressure; cells placed in a hypotonic solution will gain water and swell.