1. Chapter 8 –
Cells and Their Environment

2. Cell Membrane Structure and Function
Separates the internal environment of the cell from its surroundings.
Regulates what materials enter and leave a cell
Made of 2 layers of phospholipids (phospholipid bilayer) with proteins embedded throughout.
Fluid consistency (like a light oil) and a mosaic pattern of proteins.

4. http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane fluidity.swf

5. Because of consistency and pattern of components, referred to as fluid-mosaic model of membrane structure

6. Cells live in fluid environments, with water inside and outside the cell.
Components of plasma membrane:
2 layers of phosphlipids
Polar head and nonpolar tail
Proteins
Cholesterol
Carbohydrates

7. Phospholipid bilayer:
Polar heads (hydrophilic) (water-loving)
Face outside and inside of cell.
Nonpolar tails (hydrophobic) (water-fearing)
Extend to the interior of the plasma membrane.

8. Proteins:
Some pass through entire membrane, others go part-way through
Help large molecules pass through cell membrane

9. Cholesterol:
Strengthens the plasma membrane.
Carbohydrates:
Cell surface markers – helps to identify each type of cell
Help cells to work together

10. The Permeability of the Plasma Membrane
Selectively permeable – meaning only certain materials can cross the membrane.
Two mechanisms of transport:
Active – requires energy
Passive – does not require energy

11. How do materials get across the plasma membrane?
Small, uncharged molecules pass through the membrane, following their concentration gradient -
Gradual change in chemical concentration from one area to another
Molecules tend to move from area of high to low concentration
Larger macromolecules or tiny charged molecules rely on proteins to help them to get across.

12. Diffusion and Osmosis
Diffusion is the movement of molecules from a higher to a lower concentration until equilibrium is reached.
Equilibrium –
State in which all materials are evenly concentrated
Movement of molecules still occurs, but there is no NET movement of molecules
Gases move through plasma membrane by diffusion.

14. Simple diffusion:
Small, nonpolar molecules can pass directly across the membrane
Ex: oxygen, carbon dioxide, natural steroid hormones

15. Facilitated diffusion:
Substances pass through a carrier protein following their concentration gradients (high  low concentration).
Does not require energy.
Brings in materials such as glucose & amino acids; proteins are specific to molecules taken in.
Some molecules can be taken in faster than others – differential permeability.

16. Transport by Carrier & Channel Proteins
Some materials cannot enter or leave cell due to their size and/or nature.
Some of those molecules use the channel proteins and carrier proteins that span the membrane in order to enter or leave the cell.
Carrier proteins – transport specific substances that fit within a binding site
Channel proteins – transport ions, sugars, and amino acids
Animation of carrier protein

17. Osmosis
Defined as the diffusion of water across a selectively permeable membrane due to concentration differences.
Molecules always move from higher to lower concentration.

18. Osmosis in cells
A solution contains a solute (solid) and a solvent (liquid).
Cells are normally isotonic (iso = same) to their surroundings, and the solute concentration is the same inside and out of the cell.
Cell is in equilibrium
There is no net movement of water across the cell membrane.

19. Fresh water = Isotonic solution
X = Water

20. Cells in isotonic solutions

21. Hypotonic (“hypo” means less than)
Lower concentration of solute, higher concentration of water outside than inside cell.
Net movement of water into the cell.
Animal cells - may burst
Plant cells –
Increased turgor pressure (makes plant cell rigid)
Plant cells do not burst because they have a cell wall.

22. Distilled water = Hypotonic solution
X = Water

23. Cells in a hypotonic solution

24. Hypertonic (“hyper” means more than)
Higher concentration of solute, lower concentration of water outside than inside cell.
Net movement of water out of the cell.
Animal cells - shrivel
Plant cells - shrink
Turgor pressure is lost as plant cells shrink – plant will wilt or feel soft and bendable.

25. Salt water = Hypertonic solution
X = Water

26. Cells in a hypertonic solution

27. Active transport
Ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration.
Energy in the form of ATP is required for the carrier protein to combine with the transported molecule.
Occurs in cells such as kidney cells (taking sodium from urine), thyroid gland cells (taking in iodine), and cells in digestive tract (absorbing nutrients).

28. Active transport

29. Exocytosis and Endocytosis
Exocytosis and Endocytosis
For molecules that are too large to be transported with carrier proteins – requires energy.
Use vesicles – small storage units
Exocytosis - vesicles fuse with the plasma membrane for secretion.
Causes cell membrane to enlarge – process occurs during growth.

30. Exocytosis

31. When vesicle fuses with lysosome, digestion occurs.
Endocytosis - cells take in substances when a portion of the plasma membrane folds in, and forms a vesicle around the substance.
When vesicle fuses with lysosome, digestion occurs.
Endocytosis occurs as:
Phagocytosis – large particles (food or other cells)
Pinocytosis – small particles or liquids
Receptor-mediated endocytosis – form of pinocytosis for specific particles such as vitamins, hormones, or lipoproteins

32. Phagocytosis Seen in unicellular organisms like amoebas.
Seen in unicellular organisms like amoebas.
White blood cells can use this process to take in bacteria & worn out red blood cells

33. Pinocytosis Used by root cells of plants.
Used by root cells of plants.
Seen in blood cells, cells that line kidneys and intestines.

34. Receptor-mediated endocytosis
Seen during exchange of maternal & fetal blood in placenta; & intake of cholesterol in body cells (failure to do so results in high blood pressure, blocked arteries, and heart attacks).