1. CELL MEMBRANE, CYTOSKELETON & CELL-CELL INTERACTIONS
Cells are the most basic units of life because they exhibit all of the characteristics of life.
This is a picture of a human white blood cells (T lymphocyte). It’s inner skeleton and surface features enable it to move in the body and to recognize “foreign” cell surfaces.
Chapter 4

2. A. Cell Membrane Structure
Cell membrane is a phospholipid bilayer embedded with mobile proteins.
Phosphate head of phospholipid is hydrophilic.
Fatty acid tails are hydrophobic.
Proteins
Hydrophilic - “water loving”
Hydrophobic - “water fearing”

3. Types of membrane proteins:
Transport proteins - move substances across membrane
Cell surface proteins - establish “self”
Cellular adhesion molecules (CAMs) - enable cells to stick to each other
Receptor proteins - receive & transmit messages into a cell

4. Additional molecules may be associated with proteins & phospholipids:
cholesterol
sugar molecules
glycoproteins
glycolipids
Glycolipids & glycoproteins function in cell recognition.
Note: plant cells do not have carbohydrates extending from the protein-phospholipid bilayer.

5. B. Movement Across Membranes
Cell membranes are selectively permeable.
1. Simple diffusion (passive)
Substance moves across phospholipids from an area of high to an area of low concentration, without using energy.
Substance moves down its concentration gradient
Ex. O2, CO2
Selectively permeable - allow some substances to pass through membrane, while preventing others.

6. Simple Diffusion Cell membrane Transport protein
Notice that transport proteins are not involved in simple diffusion.
Dynamic equilibrium - point of equal movement back & forth across membrane; no NET movement of molecules.
Diffusion continues until dynamic equilibrium is reached.

7. Movement of water across membranes by simple diffusion is called osmosis.
Water is driven to move because the membrane is impermeable to solute(s).

8. Isotonic - both solutions have the same solute concentrations.
Tonicity
Refers to differences in solute concentration on either side of a semipermeable membrane.
Isotonic - both solutions have the same solute concentrations.
Hypotonic - solution with the lower solute concentration.
Hypertonic - solution with the higher solute concentration.
Solutions are composed of solutes & solvents.
solute - substance dissolved in a solution.
solvent - the dissolving agent. Most versatile dissolving agent is water.

9. What is effect of immersing an animal cell in a hypertonic or hypotonic solution?
Plasma is normally isotonic to cytoplasm of RBC. Cell is in dynamic equilibrium with plasma & maintains its shape.
If RBC is placed in a hypertonic solution, solute concentration is greater in solution than inside the cell. Since RBC membrane is impermeable to solutes, water is driven to move. Thus, water tends to leave the cell to dilute the outside solute. The cell shrinks.
If RBC is placed in a hypotonic solution, solute concentration is greater in cytoplasm of RBC. Here again, water is driven to move because the RBC membrane is impermeable to solutes. Thus, water tends to enter the cell. The cell swells & may even burst.
Note: Some single-celled protists (paramecium) live in fresh water. They use structures called contractile vacuoles to rid themselves of excess water that is continuously diffusing inward.

10. What is effect of immersing a plant cell in a hypertonic or hypotonic solution?
If plant cell is placed in a hypertonic solution, solute concentration is greater in solution than inside the cell. Since plant cell membrane is impermeable to solutes, water is driven to move. Thus, water tends to leave the cell to dilute the outside solute. The cell shrinks & pulls away from the cell wall. Plant wilts.
If plant cell is placed in a hypotonic solution (left), solute concentration is greater in cytoplasm of plant cell. Here again, water is driven to move because the plant cell membrane is impermeable to solutes. Thus, water tends to enter the cell. The cell swells, but will not rupture because of the surrounding cell wall. Plant stands erect.
Cell immersed in hypertonic solution
Cell immersed in hypotonic solution

11. 2. Facilitated Diffusion (passive)
Substance moves through a transport protein from an area of high to an area of low concentration, without using energy.
Substance moves down its concentration gradient
Ex. glucose

12. Facilitated Diffusion
Cell membrane
Transport protein

13. In which direction will sucrose, glucose, fructose & water move?
Cell: Environment:
1% sucrose % sucrose
1% glucose % glucose
1% fructose % fructose
97% water % water
Assume cell membrane is permeable to water, glucose & fructose, but impermeable to sucrose.
In which direction will sucrose, glucose, fructose & water move?
Key to figuring this problem out is to remember that in diffusion, substances always move from a higher concentration to a lower concentration.
Sucrose would diffuse into the cell if it could; however, the membrane is impermeable to sucrose.
Glucose diffuses into the cell until dynamic equilibrium is reached.
Fructose is at dynamic equilibrium, so would experience no NET diffusion.
Water diffuses out of the cell until dynamic equilibrium is reached. [Water diffuses outward because the solute concentration is greater outside the cell than in the surrounding environment].

14. 3. Active Transport (active)
Substance moves through a transport protein from an area of low to an area of high concentration; requires energy.
Substance moves against its concentration gradient
Ex. ions (Na+, K+, Cl-)

15. Active transport
Cell membrane
Transport protein
ATP

16. Active transport of Na+ & K+ through the sodium-potassium pump (transport protein).
Cells must contain high concentrations of potassium ions & low concentrations of sodium ions to function. The only way cells can maintain these concentrations is by activity of sodium-potassium pumps in the cell membrane.
Note: 3 sodium ions are pumped outward for every 2 potassium ions pumped inward.

17. 4. Cotransport
The active transport of one substance is coupled to the passive transport of another.
Ex. sucrose (plant cells)
Example of cotransport: sucrose loading
Energy is used to actively transport protons to the outside of the cell, creating a concentration gradient. Protons passively flow back through the cell membrane through a symporter, which couples the movement of sucrose with the movement of protons.

18. 5. Exocytosis, Endocytosis & Transcytosis
Movement of large particles across membranes with the help of vesicles.
Exocytosis - vesicles move particles out of the cell.
Exocytosis - vesicle fuses with cell membrane, expelling contents to the outside of the cell.
Acrosomal enzymes are found in the head of a sperm. They are released by exocytosis when the sperm encounters an egg.
Nerve cells release neurotransmitters by exocytosis.
Ex. release of enzymes from head of sperm; neurotransmitter release

19. Endocytosis - vesicles move particles into the cell.
Three types of endocytosis:
Receptor-mediated endocytosis
Pinocytosis
Phagocytosis
Endocytosis - part of cell membrane surrounds substance & pinches off forming a vesicle.
Pinocytosis - “cell drinking”; vesicle brings water containing substances into the cell.
Phagocytosis - “cell eating”; vesicle brings large clumps of nutrients into the cell. [WBCs phagocitize bacteria]
Receptor-mediated endocytosis - substance must bind to a receptor protein on cell membrane before it can be brought into the cell. [liver cells envelop cholesterol by receptor-mediated endocytosis]

20. Transcytosis - combines endocytosis & exocytosis.
Vesicles rapidly transport particles through cells.
Ex. transport of nutrient monomers through cells lining digestive tract & into the bloodstream

21. 1. Microtubules - hollow, thick elements made of the protein tubulin.
C. Cytoskeleton
The structural framework of a cell.
1. Microtubules - hollow, thick elements made of the protein tubulin.
Functions:
move chromosomes apart during cell division
form cilia & flagella
Cilia & flagella have a arrangement of microtubules.
Cilia are short, numerous structures that function to move cells (paramecium) or move materials past cells (ciliated cells line the upper respiratory tract).
Flagella are long, whip-like structures that function to propel cells. Human sperm have only a single flagellum.

22. 2. Microfilaments - long, thin elements made of the protein actin.
Functions:
connect cells to each other
move vesicles & organelles within cytoplasm
help cells move

23. 3. Intermediate filaments - elements with diameters in between that of microtubules & microfilaments.
Made of various proteins (ie. keratin)
Functions:
maintain cell shape
connect cells to each other & to underlying tissue (skin cells)
Abundant in skin & nerve cells.

24. D. Intercellular Junctions
Structures that connect cells of multicellular organisms to form tissues.
1. Animal cell Connections
Tight Junctions - cell membranes of adjacent cells are fused, creating a tight seal.
Ex. cells lining small intestine; cells lining capillaries in brain

25. Desmosomes - intermediate filaments weld cell membranes of adjacent cells together in isolated spots.
Ex. skin cells
Gap Junctions - channels that link the cytoplasm of adjacent cells, allowing exchange of materials.
Ex. heart muscle cells

26. 2. Plant Cell Connections
Plasmodesmata - channels that link the cytoplasm of adjacent plant cells, allowing the exchange of cytoplasm & organelles.
Ex. cells conducting water & nutrients

27. E. Cell-Cell Interactions 1. Cell Adhesion
Process that uses membrane proteins called cellular adhesion molecules (CAMs) to direct the migration of cells.
Various CAMs function in sequence to:
guide WBCs to injury sites
guide embryonic cells to help form placenta
establish nerve connections involved in learning & memory

28. CAMs directing WBCs to injury sites.
CAMs direct WBCs to injury sites.
CAMs directing WBCs to injury sites.

29. 2. Signal Transduction
Process by which cells receive, amplify, & respond to outside stimuli.
Outside stimulus (first messenger) is received by a receptor protein in the cell’s membrane. This triggers a series of chemical reactions on the cell’s surface. Eventually, a second messenger is activated which triggers the cell’s response.