chapter 05

Organisms are made of , which are mostly small and can only be visualized with microscopes.
- • Most cells are microscopic in size, though some vertebrate eggs are visible the unaided eye.
  Prokaryotic cells are generally 1-2 micrometers (µm) across. Eukaryotic cells are usually larger, about 10-100 µm in diameter.
- • Microscopes provide better resolution to help visualize small objects.
  Light microscopes:
  1. bright-field
  2. dark-field
  3. phase-contrast
  4. differential-interference-contrast
  5. fluorescence
  6. confocal
  Electron microscopes:
  1. transmission
  2. scanning
- • A bright-field microscope transmits light through a thin specimen. Staining the specimen can increase contrast but often kills the cell.
- • A dark-field microscope directs light at an angle toward the specimen; a condenser lens transmits only light reflected off the specimen. The specimen is light against a dark background.
- • A phase-contrast microscope uses out-of-phase light rays to enhance contrast by amplifying variations in density within the specimen.
- • A differential-interference-contrast microscope (Nomarski) separates polarized light into two beams taking different paths through the sample; the interfere enhances variation in optical density of the sample, especially along edges. Video:
- • A fluorescence microscope shines UV light on objects that have been tagged by fluorescent dyes or antibodies, which then emit visible light.
- • A confocal microscope uses lasers to focus on one plane of the specimen. Out-of-focus objects above and below the plane are excluded by a computer.
- • A transmission electron microscope passes a beam of electrons, which have shorter wavelengths than visible light, through a thin section of a specimen, and detects the scattered electrons.
- • A scanning electron microscope scans a beam of electrons across the surface of a specimen and can show a 3D image of the object.
The plasma that encloses all cells consists of a phospholipid in which proteins are embedded.
- • All cells possess a plasma membrane. The membrane is a double layer (bilayer) of phospholipids with a nonpolar (hydrophobic) interior made of fatty acid hydrocarbon "tails" attached to polar (hydrophilic) "heads" on the exterior.
- • The phosphate "head" of a phospholipid molecule is polar (water-soluble, or hydrophilic)
  The fatty acid hydrocarbon "tail" is nonpolar (water-insoluble, or hydrophobic).
- • Various membrane proteins serve as channels, receptors, and cell surface markers.
  Carbohydrate chains may bind to the proteins or to phospholipids and serve as identification tags, unique to each type of cell.
  Cholesterol molecules help keep animal cell membranes fluid.
Prokaryotic cells lack nuclei or other membrane-bound and are encased in a rigid cell .
- • Both prokaryotes and eukaryotes have a plasma membrane enclosing a semifluid cytoplasm within which are protein-synthesizing ribosomes.
  Prokaryotes lack a nucleus or other membrane-bound organelles.
  Prokaryotic DNA is located in a nucleoid region, and the cell is encased in a rigid cell wall.
  Labeling exercise:
Eukaryotic cells contain and can be categorized into two main types: cells and cells
- • A typical animal cell comprises numerous organelles, including a nucleus which contains most of the cell's DNA.
  The organelles are contained in a semifluid cytoplasm and enclosed in a plasma membrane.
- • Most plant cells contain several structures not found in animal cells.
  A central vacuole stores material and chloroplasts perform photosynthesis for the cell.
  Plant cells also have rigid cell walls outside the plasma membrane, and cytoplasmic connections through openings in the cell wall called plasmodesmata.
The contains the cell's genetic material and serves as the command and control center.
- • The nucleus is composed of a double membrane called a nuclear envelope which encloses the nucleoplasm containing DNA. The nucleolus is the site of synthesis of ribosomes. Nuclear pores extend through the envelope and allow for exchange of material between the nucleoplasm and the cytoplasm.
The nucleus is closely associated with 2 other membranous organelles: the reticulum and the complex.
- • The endoplasmic reticulum (ER) comprises folded membranes continuous with the nuclear envelope.
  Rough ER has embedded ribosomes and is involved in protein synthesis.
  Smooth ER has no ribosomes and is involved in lipid and carbohydrate synthesis.
- • The Golgi complex is a stack of membranes that processes and packages material made in the ER to other organelles via vesicles that fuse with its membranes. Vesicles can transport the material to other organelles, or fuse with the plasma membrane to secrete material outside the cell.
Two organelles provide energy for the cell.
- The mitochondrion is called the powerhouse of the cell because it is the site of aerobic , providing energy for the cell.
  - • Cells convert energy from food into the energy storage molecule ATP in a series of chemical reactions called aerobic respiration.
    These reactions take place in mitochondria.
    A mitochondrion has a double membrane where enzymes involved in aerobic respiration are embedded.
    Muscle cells that require a lot of energy are rich in this organelle.
- The chloroplast is the site of and are present in plants and algal cells.
- • The chloroplast has a double membrane. Within is a series of folded membranes called thylakoids, which form columns called grana. The interior is bathed in a semifluid stroma. Photosynthesis occurs on thylakoid membranes to produce food by converting light energy to chemical energy.
  Labeling exercise:
Central are storage centers in plant cells.
- • A central vacuole stores water and dissolved substances.
  It is usually the largest observable structure in a plant cell.
Plants have cell that function to support and protect the cell.
- • Plant cell walls are composed of the polysaccharide cellulose.
  Sometimes a thick secondary wall provides further support inside the primary wall.
  The middle lamella lies between the walls of adjacent cells and glues the cells together.
The interior of the cell contains a network of protein fibers, called the , that provides support and mobility for the cell.
- • The cytoskeleton is a network of 3 types of protein fibers.
  - Microfilaments are made of 2 strands of actin and function in cell movement, such as crawling of an amoeba.
    Video: amoeba crawling
  - Microtubules are made of tubulin subunits that provide transport.
    Video: organelle movement
  - Intermediate filaments are made of overlapping protein fibers that anchor organelles in place and hold cells together.
    Video: skin cells
- Cilia and are locomotor appendages that propel cells through the environment.
- • Eukaryotic flagella and cilia are composed of a ring of nine pairs of microtubules with two microtubules (9 + 2) in its core.
  Flagella are relatively longer and fewer than cilia.
  Videos:
  ciliated protists
  ciliated epithelium
  flagellated protists
  flagellated sperm
- • The surface of this paramecium is covered with a dense forest of cilia, which have the same 9 + 2 structure of microtubules as flagella.
  Video: flagellates and ciliates

Materials pass into and out of the cell passively through and .

• Diffusion is the mixing process that spreads molecules across a fluid medium; solute molecules move from areas of high concentration to areas of low concentration (a gradient) until movement in both directions are equal (equilibrium).
Review:

• Osmosis is the diffusion of water through a semi-permeable membrane.
The largely hydrophobic plasma membrane is impermeable to large molecules and to polar molecules except water.
Water will diffuse to the side of higher solute concentration.
A cell needs to adapt to different osmotic environments to maintain homeostasis.
Review:

•
- An isotonic solution contains equal concentration of solutes outside the cell as inside the cell.
- A hypertonic solution has higher concentration of solutes outside the cell than inside; water moves out of the cell.
- A hypotonic solution has lower concentration of solutes outside the cell than inside; water moves into the cell.

•


A red blood cell is normally in an isotonic environment, with equal concentration of solutes outside the cell as inside the cell. There is no net movement of water across the membrane.	If placed in pure water, the hypotonic environment causes water to move into the cell, causing the cell to swell.

Large substances move into and out of the cell through transport mechanisms of endocytosis and exocytosis without crossing the plasma membrane.
- • Endocytosis is the cellular intake of material by folding plasma membrane around it, forming a vesicle.
  When the material is an organism or other large particle, the process is called phagocytosis (cell eating).
  When the material is small or a liquid, the process is called pinocytosis (cell drinking).
- • Exocytosis is the discharge of material from vesicles at the cell surface. Molecules are secreted from cells in small packets of secretory vesicles whose membranes fuse with the plasma membrane, releasing their contents to the outside. Review:
Plasma membranes contain protein channels that allow permeability: the transport of specific materials across the membrane.
- Facilitated transports substances their concentration gradient through a protein.
  - • In facilitated diffusion, carrier proteins bind to specific molecules and allow them to diffuse down their concentration gradients. This does not require energy.
    Review:
- transport requires energy to move substances (against) concentration gradients through a protein; an example is the -potassium pump.
- • Active transport. The sodium-potassium pump uses the energy of one ATP molecule to move 3 Na⁺ outward and 2 K⁺ into the cell, against their concentration gradients, resulting in a negative voltage inside nerve cells.
  Review: