Chapter 3: Cells: The Living Units

Objectives: 1. Define cell.

2. Discuss structure and function of cell organelles.

3. Describe the chemical composition of the plasma membrane and relate it to membrane function.

4. Compare the structure and function of tight junctions, desmosomes, and gap junctions.

5. Relate the plasma membrane structure to active and passive transport mechanisms.

6. List several roles of membrane receptors.

Cell Theory:

1. The cell is the basic unit of structure and function in living things.

2. Activity of the organism depends on activity of cells.

3. Cells come from preexisting cells.

Parts of Cell: Structure and Function

Structure / Function
Cytosol / Holds organelles, contains dissolved nutrients
Mitochondria / Site of aerobic respiration for production of ATP
Ribosomes / Site of protein synthesis w/ mRNA and tRNA
Rough endoplasmic reticulum / Attachment site for ribosomes, transport of proteins to Golgi
Smooth endoplasmic reticulum / Detoxification of cell, lipid synthesis, storage of calcium
Golgi apparatus / Packages and secretes proteins from cell
Lysosomes and peroxisomes / Contain digestive enzymes to break up things brought into cell
Cytoskeleton / Made of microtubules, intermediate and microfilaments to support cell
Centrioles / Used by animal cells to pull chromosomes apart during mitosis/meiosis
Cilia and flagella / Cell locomotion
Nuclear envelope and pores / Contains chromatin w/in nucleus, allows material to enter and leave nucleus
Nucleoli / Site of ribosome assembly in nucleus
Chromatin / Genetic information made of DNA
Chloroplasts / Site of photosynthesis in plant cells

Cell Membrane: fluid mosaic model of phospholipids, cholesterol (for fluidity), proteins, glycoproteins, and glycolipids. Also called a phospholipid bilayer.

I. Protein functions: found either all the way through the membrane or on only one side

A. Integral: found all the way through the membrane; act as transporters, enzymes, receptors, for intercellular joining, cell-cell recognition, attachment to cytoskeleton and extracellular matrix (ECM).

B. Peripheral (Glycocalyx): found on one side of membrane in gooey, carbohydrate-rich area at cell surface to provide highly specific biological markers for recognition.

Membrane Junctions

I. Tight junctions: proteins fuse together; ex. epithelial cells of intestines

II. Desmosomes: act like rivets to hold skin, heart, muscle, neck of uterus together

III. Gap junctions: allow chemicals to pass between adjacent cells; heart and smooth muscle synchronize contractions

Cell Transport

Interstitial Fluid: soup of nutrients that surrounds cells. Cells contain a selectively permeable membrane around themselves. Their ability to select is lost with damage to cell, as in burns.

Membrane Tranport

I.  Passive Processes: no energy (ATP) is required

A.  Concentration Gradient: a continuum of solute particles from an area of high amount of solute, gradually decreasing in amounts until reaching an area of low amount of solute.

B.  Diffusion: movement along concentration gradient (high to low). Size of molecule, temperature, lipid solubility and presence of carrier protein all determine rate of diffusion. Oxygen, carbon dioxide, vitamins A, D, E, alcohol, water and glucose with a carrier all enter cells by diffusion.

C.  Osmosis: diffusion of solvent from dilute solution to concentrated solution (along the concentration gradient of water). The following terms refer to the environment outside of the cell:

1. Isotonic: no net osmosis. Concentrations inside and outside of cell are equal.

2. Hypertonic: net water movement out of cell. Cells will shrivel or crenate.

3. Hypotonic: net water movement into cell. Cells swell and lyse.

D. Filtration: occurs along a pressure gradient (high pressure to low pressure). Not a selective process, common in kidney.

II. Active Transport: requires ATP for movement against a concentration gradient, from low to high.

A. Sodium/Potassium Pump: used to establish membrane potential. Energy (ATP) is used against a concentration gradient to put more sodium outside of the cell and more potassium inside (3:2 ratio)

B. Exocytosis: vacuoles and vesicles fuse with cell membrane, material is released.

C. Endocytosis: large material that is needed inside of the cell is engulfed.

1. Phagocytosis: engulf with pseudopodia

2. Receptor-mediated: engulf specific substances

Extracellular Matrix: materials outside of cell made of proteins and polysaccharides that can hold the cells together, particularly found in connective tissue.