1. Chapter 3: Cells

2. Composite Cell All cells vary in size, shape, content, and function
Composite cell includes many of the known cell structures
Cells have most but not all of these structures
Cells consist of three main parts:
1. Cell membrane
2. Cytoplasm
3. Nucleus

3. Cell Membrane (plasma membrane)
Actively functioning part of the cell
Regulates movement of substances in and out of the cell
Certain molecules receive stimulation from outside the cell
Transmits it inside the cell  signal transduction
Helps cells adhere to certain other cells
Important in forming tissues

4. Cell Membrane General Characteristics:
Extremely thin (can only be seen with an electron microscope)
Flexible & somewhat elastic
Controls which substances exit & enter the cell  selectively permeable

5. Cell Membrane Structure:
Mainly composed of lipids, proteins, and some carbohydrates
Has a double layer  phospholipid bilayer
Oxygen and carbon dioxide can pass through the bilayer easily because they are soluble in lipids
Impermeable to water-soluble molecules (amino acids, sugars, proteins, nucleic acids, & various ions)
Embedded cholesterol molecules aid in making the membrane less permeable to water-soluble substances

7. Cell Membrane Structure
Has a few lipid molecules & many kinds of proteins
Membrane proteins are classified according to their positions
Membrane-spanning (trans-membrane) proteins extend through the lipid bilayer
Peripheral membrane proteins are only on one side of the bilayer
Vary in shape  blobby or globular & elongated (rod-like)

8. Cell Membrane structure
Membrane protein functions:
Some form receptors on the cell surface  bind incoming hormones or growth factors
Starts signal transduction
Transport ions or molecules across the cell membrane
Others form selective channels that allow certain ions to enter or leave the cell

9. Composite Cell Assignment
Provide information on the organelles in a cell
Smooth ER, Rough ER, Ribosomes, Golgi apparatus, Mitochondria, Lysosomes, Peroxisomes, Centrosome, Microfilaments & microtubules, Cilia & flagella, vesicles, and Nucleus
Function in the cell
What is the structure/composition
Where it is located in the body

10. Membrane protein functions
Proteins that protrude from the inside of the cell:
Anchor the cell to rods and tubules that support the cell
Proteins that protrude from the outside of the cell:
Mark the cell as part of a particular tissue or organ
Cellular adhesion molecule (CAM)
Determines a cell’s interactions with other cells
Ex. A series of CAMs helps a white blood cell move to the site of an injury

11. Cytoplasm
Appears as a clear jelly through a light microscope with specks scattered throughout
An electron microscope will show you that the cytoplasm contains networks or membranes and organelles
These are suspended in a clear liquid called cytosol
Also has protein rods and tubules that form a cytoskeleton

12. Cytoplasm
Cell activities occur mainly here where nutrients are received, processed, and used
Contains the following organelles:
Endoplasmic reticulum (rough and smooth), ribosomes, golgi apparatus, mitochondria, lysosomes, peroxisomes, microfilaments and microtubules, centrosome, cilia & flagella, & vesicles

13. Endoplasmic Reticulum
Composed of membrane-bound, flattened sacs, elongated canals, and fluid-filled bubble sacs called vesicles
These are interconnected & communicate w/ the cell membrane, nuclear envelope & other organelles
Transports molecules from one cell part to another
Smooth ER
Contains enzymes responsible for lipid synthesis
Carbohydrate metabolism & Detoxification
Abundant in liver and gonad cells
Rough ER
Responsible for protein synthesis
Ribosomes cover it
Proteins then travel to Golgi apparatus through tubules for further processing
Found in hepatocytes

14. Ribosomes
Many are attached to the rough ER, others scattered throughout the cytoplasm
Composed of protein and RNA molecules
Provide enzymes & structural support for RNA molecules that come together during protein synthesis

15. Golgi Apparatus
Looks like a stack of pancakes

16. Mitochondria

17. Lysosomes

18. Peroxisomes

19. Microfilaments & Microtubules

20. Centrosome

21. Cilia & Flagella

22. Vesicles

23. Cell Nucleus Houses DNA Directs all cell activities
Large spherical structure that is covered by a double-layered nuclear envelope
Consists of inner and outer lipid bilayer membranes
Protein lined channels (nuclear pores)  allow certain molecules to exit the nucleus
Contains a fluid that suspends the necleolus and chromatin
Nucleolus (“little nucleus”)
Composed largely of RNA and protein
Ribosomes form here and migrate through nuclear pores to the cytoplasm
Chromatin
Consists of loosely coiled fibers of protein and DNA
DNA contains the information for protein synthesis
When the cell divides, fibers of chromatin coil tightly into chromosomes

24. Movements through Cell Membranes
Passive Mechanisms
Diffusion (simple diffusion)  process where molecules or ions move to areas from regions where there is a high concentration to areas where there is a lower concentration
They spread out evenly
Molecules diffuse along or down their concentration gradient

25. Diffusion
When diffusion happens the molecules eventually become uniformly distributed in a mixture
This is known as equilibrium; molecules continue to move after equilibrium but the concentrations no longer change
The plasma membrane is a physical barrier to free diffusion  because of its hydrophobic core
A molecule will diffuse through the membrane if the molecule is: 1) lipid soluble, 2) small enough to pass through membrane channels, or 3) assisted by a carrier molecule

26. Facilitated Diffusion
Only moves molecules from higher concentrations to lower concentrations
Certain molecules (glucose & other sugars, amino acids, and ions) are transported passively even though they are unable to get through the lipid bilayer.
They are transported by either a protein carrier in the membrane that ferries the molecule across or through water-filled protein channels

27. Facilitated Diffusion
Protein Carriers
Channels
Small polar or charged particles diffuse through membrane channels constructed by channel proteins.
Large, lipid-insoluble molecules (glucose) are moved across the membrane via a transport protein.

28. Osmosis
Occurs whenever water molecules diffuse from a higher water concentration area to a lower water concentration area across a selectively permeable membrane (cell membrane)
If a cell has too much glucose in it, water will diffuse across the cell membrane to put it back in equilibrium
Isotonic – when a solution has the same osmotic pressure as body fluids
Hypertonic – solutions with higher osmotic pressure than body fluids
Hypotonic – solutions that have a lower osmotic pressure than body fluids

29. Filtration
Process that forces water and solutes through a membrane or capillary wall by hydrostatic pressure
Commonly used to separate solids from water
Example: hydrostatic pressure exerted by blood forces fluid out of the capillaries
This fluid contains solutes that are vital to the tissues

30. Active Mechanisms Active Transport
Moves particles through membranes from a region of lower concentration to a region of higher concentration
Needs carrier proteins to move the molecules across the cell membrane
Example: moving sodium ions to the outside of the cell, the sodium-potassium pump

31. Sodium-potassium pump
Carrier enzyme is called Na+-K+ ATPase
Concentration of K+ inside the cell is some 10 times higher that outside the cell, the reverse is true of Na+
These ionic concentration differences are essential for excitable cells like muscle and nerve cells to function normally and for all body cells to maintain their normal fluid volume
Since Na+ and K+ leak continuously through the plasma membrane along their concentration gradient, the Na+-K+ pump operates to drive Na+ out of the cell and pump K+ back in against a steep concentration gradient

32. Endocytosis & Exocytosis
Uses cellular energy to move substances
Molecules or other particles that are too large to enter a cell by diffusion or active transport must enter the cell through vesicles
3 forms: pinocytosis, phagocytosis, & receptor-mediated endocytosis
Exocytosis  the reverse process, secreting a molecule or substance by a vesicle from the cell
Example: nerve cells releasing neurotransmitter chemicals that signal other nerve cells, muscle cells, or glands

33. Pinocytosis “cell drinking”
Cells take in tiny droplets of liquid from their surroundings by an infolding of the plasma membrane
The droplet enters the cell and fuses with an endosome
This is a routine activity of most cells

34. Phagocytosis “cell eating”
Similar to pinocytosis but the cell takes in solids rather than liquids
Certain kinds of white blood cells are called phagocytes
Can take in solid particles like bacteria
Particle attaches to the phagocyte’s cell membrane, this stimulates a portion of the membrane to protrude outward, surround the particle, and slowly draw it inside the cell
Part of the membrane that surrounds the particle detaches from the cell’s surface, this forms a vesicle containing the particle (phagosome)

35. Receptor-mediated endocytosis
Moves very specific kinds of particles into the cell by binding them first
Protein molecules extend through a portion of the cell membrane to the outer surface
They serve as receptors to which only specific substances from outside the cell can bind
Cholesterol molecules enter cells by this mechanism

36. The Cell Cycle
A series of changes a cell goes through from the time it forms to the time is divides
Split into different stages: Interphase, mitosis, cytokinesis, & differentiation
Interphase:
Split up into different phases or checkpoints: G1, S, & G2
G1 – determines the fate of the cell; whether it will continue and divide, stay in the G1 phase as a specialized cell, or die
S – DNA is replicated
G2 – growth happens and final preparation for cell division

37. Mitosis
Division of the nucleus must be precise because it contains the DNA
Each cell resulting from mitosis must have a complete and accurate copy of this information to survive
DNA is replicated in Interphase, but it’s equally distributed into two new cells in mitosis
Consists of four different stages
Prophase, Metaphase, Anaphase, Telophase

38. Prophase
First indication of cell division is the appearance of chromosomes in the nucleus
Each prophase chromosome is composed of two identical portions (chromatids)  they are temporarily a region on each called the centromere
Centrioles of the centrosome replicate before mitosis begins, so during prophase these new centriole pairs move to opposite ends of the cell
The nuclear envelope and nucleolus break up, disperse, and are no longer visible
Microtubules are assembled from tubulin proteins in cytoplasm & associate w/ the centrioles & chromosomes
Spindle-shaped array of microtubules (spindle fibers) forms between the centrioles as thy move apart

39. Metaphase Chromosomes line up about midway between the centrioles
Spindle fibers attach to the centromeres of each chromosome
A fiber from one pair of centrioles contacts one centromere & a fiber from the other pair of centrioles attaches to another centromere

40. Anaphase Centromeres begin to be pulled apart
As chromatids separate they begin to be individual chromosomes
Separated chromosomes move in opposite directions guided by microtubule activity
Spindle fibers shorten & pull their attached chromosomes toward the centrioles at opposite ends of the cell

41. Telophase
Begins when chromosomes complete their migration toward the centrioles
As chromosomes approach the centrioles they start to get longer and unwind from rod-like into thread-like chromatin
A nuclear envelope forms around each chromosome set
Nucleoli appear within the newly formed nuclei
Finally, the microtubules disassemble into free tubulin molecules

42. Cytokinesis
Begins during anaphase when the cell membrane starts to constrict down the middle of the cell
This constriction lasts through telophase
A ring of microfilaments begins to contract  this assembles in the cytoplasm and attach to the inner surface of the cell membrane
These rings pinch inwards and separates the two newly formed nuclei
About half of the organelles are distributed into each
New cells may differ in size & number of organelles but they have the same genetic information

43. Cell Differentiation
All body cells form by mitosis & contain the same DNA information, however they do not look or function the same
Sometime during development the cells begin to specialize  develop special structures or begin to function in different ways
Differentiation – process by which cells develop different characteristics in structure and function

44. Cell Death
A cell that doesn’t divide or specialize has one other option  death
Apoptosis – a form of cell death that is actually a normal part of development
Examples: carving away of webbing between fingers & toes, removes extra brain cells, sunburn – peels away cells so damaged they might turn cancerous