1. Chapter 3 Cell Structure and Function 1

2. Basic Structure of the Cell Plasma or cell membrane Cytoplasm containing organelles Nucleus 2

3. Functions of the Cell Basic unit of life Protection and support through production and secretion of various kinds of molecules. Movement - Various kinds occur because of specialized proteins produced in the cell. Communication - Cells produce and receive electrical and chemical signals. Cell metabolism and energy release Inheritance - Each cell contains DNA. Some cells are specialized cells called gametes for exchange during reproduction. 3

4. Plasma Membrane Intracellular = inside cell Extracellular = outside cell Intercellular = between cells 4

5. Plasma Membrane Fluid mosaic model Plasma membrane is neither rigid nor static in structure. Membrane is highly flexible and can change shape/composition. Phospholipid bilayer with “floating” cholesterol and proteins. Membrane is selectively permeable. 5

6. Membrane Lipids Phospholipid bilayer – Nonpolar tails are directed toward center = hydrophobic. – Polar heads are directed toward surfaces = hydrophilic. Cholesterol = important in determining fluid nature of membrane. 6

7. Membrane Proteins Integral or intrinsic – Extend deeply into membrane, often extending from one surface to the other – Can form channels through the membrane Peripheral or extrinsic – Attached to integral proteins at either the inner or outer surfaces of the lipid bilayer Functioning depends on 3-D shape and chemical characteristics. Markers, attachment sites, channels, receptors, enzymes, or carriers. 7

8. Marker Molecules: Glycoproteins and Glycolipids Glyco = sugar Allow cells to identify one another or other molecules. – Immunity – Recognition of oocyte by sperm cell – Intercellular communication 8

9. Attachment Sites (Integrins) Membrane-bound proteins Attachment sites to other cells or to extra/ intracellular molecules. 9

10. Channel Proteins (Integral) Allow polar ions to move through membrane. Nongated ion channels: – always open – Responsible for the permeability of the plasma membrane to ions when the plasma membrane is at rest. Gated ion channels: – can be open or closed – Ligand gated ion channel: open in response to small molecules that bind to a receptor site. – Voltage-gated ion channel: open when there is a change in charge across the plasma membrane. 10

11. Receptor Molecules Proteins in membranes with an exposed receptor site. Can attach to specific ligand molecules and act as an intercellular communication system. Ligand can attach only to cells with that specific receptor. 11

12. Receptors Linked to Channel Proteins Attachment of receptor- specific ligands to receptors causes change in shape of channel protein. Channel opens or closes. Changes permeability of cell to some substances. – Cystic fibrosis: defect in genes causes defect(s) in channel proteins. 12

13. A Receptor Linked to a Mediator A ligand on the outside of a cell affects the cell without entering it. Alter activity on inner surface of plasma membrane. Activates a mediator (intracellular chemical signals) that affect cell function. Some hormones function in this way. 13

14. Enzymes and Carrier Protein Enzymes: some act to catalyze reactions at outer/inner surface of plasma membrane. Example: Surface cells of small intestine produce enzymes that digest proteins. Carrier proteins: integral proteins move ions from one side of membrane to the other. – Have specific binding sites. – Protein change shape to transport ions or molecules. 14

15. Movement through the Plasma Membrane Diffusion Osmosis Filtration Mediated transport mechanisms – Facilitated diffusion – Active transport – Secondary active transport 15

16. Mediated Transport Mechanisms Involve carrier proteins or channels in the cell membrane. Characteristics – Specificity for a single type of molecule. – Competition among molecules of similar shape. – Saturation: rate of transport limited to number of available carrier proteins. 16

17. Saturation of a Carrier Protein 17 As the concentration of solutes increases, so does the rate of transport until a maximum rate is reached.

18. Mediated Transport Mechanisms Move large, water soluble molecules or electrically charged molecules across the plasma membrane. For example, amino acids and glucose in, manufactured proteins out. – Facilitated diffusion: carrier- or channel-mediated. Passive. – Active transport – Secondary active transport 18

19. Active Transport Substances are pumped by a carrier protein from ↓ to ↑ concentration. Substances move against concentration gradient. Requires ATP. The use of energy allows the cell to accumulate substances. Example: Na/K exchange pump 19

20. Secondary Active Transport Is the movement of glucose an example of symport or antiport? One ion pumped against the concentration gradient across cell membrane so when it moves back down the concentration gradient through a carrier protein, it brings another substance with it. symport - Ions or molecules move in same direction. antiport - Ions or molecules move in different direction. 20

21. Endocytosis Bringing substances into the cell by formation of a vesicle. Active transport = requires energy in form of ATP. – Phagocytosis = solid particles or entire cells are ingested by cell. – Pinocytosis = molecules dissolved in liquid ingested by cell. 21

22. Exocytosis Active transport = requires energy in form of ATP. Vesicle contents expelled from cell by merging with plasma membrane. Examples – Secretion of digestive enzymes by pancreas. – Secretion of mucous by salivary glands. – Secretion of milk by mammary glands. 22

23. Cytoplasm Cellular material outside nucleus but inside plasma membrane. Composed of Cytosol, Cytoskeleton, and Organelles Cytosol: fluid portion. Dissolved molecules (ions in water) and suspended molecules (proteins in water). 23

24. Cytoskeleton Supports the cell but has to allow for movements like changes in cell shape and movements of cilia. – Microtubules: hollow, made of tubulin. Internal scaffold, transport, cell division – Intermediate filaments: mechanical strength – Microfilaments: made of actin. Structure, support for microvilli, contractility, movement 24

25. Organelles 25 Small specialized structures with particular functions. Most have membranes that separate interior of organelles from cytoplasm. Related to specific structure and function of the cell.

26. Nucleus Large double-membrane bound structure near center of cell. The DNA in a cell is located here. Nucleoplasm – Liquid part of nucleus Nucleolus – Site of DNA activity Nuclear envelope – Double membrane with pores 26

27. Chromosome Structure Chromatin: DNA complexed with proteins. During cell division, chromatin condenses into pairs of chromatids called chromosomes. Each pair of chromatids is joined at centromere. 27

28. Centrioles and Spindle Fibers Located in centrosome: specialized zone near nucleus. Center of microtubule formation. Before cell division, centrioles divide, move to ends of cell and organize spindle fibers. 28

29. Cilia Structures projecting from cell surfaces. Capable of movement. Moves materials over the cell surface. 29

30. Flagella Similar to cilia but longer. Usually only one per cell. Move the cell itself in wave-like fashion. Example: sperm cell 30

31. Microvilli Extension of plasma membrane. Not on all cells. Increase the cell surface area for absorption. Normally many per cell. One tenth to one twentieth size of cilia. Do not move. 31

32. Ribosomes Sites of protein synthesis. Made of RNA. Composed of a large and a small subunit. Types – Free (in cytoplasm) – Attached (to endoplasmic reticulum) 32

33. Endoplasmic Reticulum – Rough Has attached ribosomes. Proteins produced and modified here. 33 Smooth No attached ribosomes. Manufactures lipids. Stores calcium.

34. Golgi Apparatus Flattened membrane sacs stacked on each other. Compartments called cisternae. Modification, packaging, distribution of proteins and lipids for secretion or internal use. Packaged into vesicles. 34

35. Function of Golgi Apparatus 35

36. Lysosomes Digest foreign particles taken into cell or structures of the cell no longer functional (autophagia). Membrane-bound sacs containing hydrolytic enzymes that pinch off from Golgi apparatus. Especially present in white blood cells that digest bacteria. 36

37. Mitochondria Major site of ATP synthesis 2 Membranes Cristae: Infoldings of inner membrane Matrix: Substance located in space formed by inner membrane Mitochondria increase in number when cell energy requirements increase. Mitochondria contain DNA that codes for some of the proteins needed for mitochondria production. 37

38. DNA  Protein Nucleic acids like DNA and RNA are molecules made from 4 nucleotides (A,C,G,U). Proteins are also polymers, but made from 20 different amino acids. How does DNA code for a protein? By translating from DNA language to protein language.

39. Transcription The 1 st step in making a protein. The DNA double strand is pulled apart. The template strand is used to make an RNA copy of the DNA. This process is catalyzed by the enzyme RNA polymerase.

40. Transcription Transcription starts at the promoter DNA. The RNA polymerase makes an RNA copy of the DNA template strand one base at a time. Transcription stops at the terminator DNA. The mRNA copies moves out of the nucleus into the Rough Endoplasmic Reticulum or the Cytoplasm.

41. Translation In translation, the instructions in mRNA gets translated (interpreted) into protein. How does DNA code for a protein? The instructions for a protein are written in the DNA (and copied to mRNA) as a series of three-nucleotide words. The DNA and mRNA base triplets are called codons.

42. The DNA code 64 possible “words” can be made from a triplet code. Since there are 64 possible code words, and only 20 amino acids, some amino acids are encoded by more than one code word. There are also start and stop codons.

43. Translation The interpreter between codon and amino acid is another type of RNA. molecule called transfer RNA (tRNA). Transcribed from DNA Each amino acid binds to a different tRNA. Each type of tRNA associates a particular mRNA codon with a particular amino acid.

44. Translation Translation does not start with 1 st mRNA base. There are ‘handles’. Begins with Start codon = AUG First amino acid is always Methionine (Met). 2 tRNA are held at a time. The ribosome adds each amino acid brought to it by tRNA to the growing end of a polypeptide chain. Translation ends at a Stop codon.

45. Transcription and Translation

47. Regulation of Protein Synthesis All nucleated cells except germ cells have the full complement of DNA. During development, differentiation occurs and some segments of DNA are turned off in some cells while those segments remain “on” in other cells. During the lifetime of a cell, the rate of protein synthesis varies depending upon chemical signals that reach the cell. – Example: thyroxine from the thyroid causes cells to increase their metabolic rate. More thyroxine, higher metabolic rate; less thyroxine, lower metabolic rate. 47