1. Chapter 6
The Cell

2. Overview: The Fundamental Units of Life
All organisms are made of cells
The cell is the simplest collection of matter that can be alive
Cell structure is correlated to cellular function
All cells are related by their descent from earlier cells

4. Concept 6.2 Prokaryotic and Eukaryotic Cells
The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic
Basic features of all cells:
Plasma membrane
Semifluid substance called cytosol
Chromosomes (carry genes)
Ribosomes (make proteins)

5. A major difference between prokaryotic and eukaryotic cells is the location of chromosomes.
In an eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus.
In a prokaryotic cell, the DNA is concentrated in the nuclear area or nucleoid without a membrane separating it from the rest of the cell.

7. Eukaryotic cells are generally much bigger than prokaryotic cells.
Most bacteria are 1-10 microns in diameter.
Eukaryotic cells are typically microns in diameter.

8. The plasma membrane functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the cell.

9. A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments.
These membranes also participate in metabolism as many enzymes are built into membranes.
The barriers created by membranes provide different local environments that facilitate specific metabolic functions.
We can think of a cell as a factory with different division producing different products.

12. Concept 6.3 Nucleus and Ribosomes
The nucleus contains the genes in a eukaryotic cell.
The nucleus is separated from the cytoplasm by a double membrane.
Where the double membranes are fused, a pore allows large macromolecules and particles to pass through.

14. In a normal cell they appear as diffuse mass.
Within the nucleus, the DNA and associated proteins are organized into fibrous material, chromatin.
In a normal cell they appear as diffuse mass.
However when the cell prepares to divide, the chromatin fibers coil up to be seen as separate structures, chromosomes.
Each eukaryotic species has a characteristic number of chromosomes.
A typical human cell has 46 chromosomes, but sex cells (eggs and sperm) have only 23 chromosomes.
The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis

15. Ribosomes Ribosomes contain rRNA and protein.
A ribosome is composed of two subunits that combine to carry out protein synthesis.

16. Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol.
Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic reticulum.
Bound ribosomes synthesize proteins that are included into membranes or exported from the cell.

17. Concept 6.4 Membrane Bound Organelles
Components of the endomembrane system:
Nuclear envelope
Endoplasmic reticulum
Golgi apparatus
Lysosomes
Vacuoles
Plasma membrane
These components are either continuous or connected via transfer by vesicles

18. Endoplasmic Reticulum
The endoplasmic reticulum (ER) accounts for half the membranes in a eukaryotic cell.
The ER includes membranous tubules and internal, fluid-filled spaces, the cisternae.
The ER membrane is contiguous with the nuclear envelope.

19. There are two regions of ER that differ in structure and function.
Smooth ER looks smooth because it lacks ribosomes.
Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside, including the outside of the nuclear envelope.

20. Functions of Smooth ER The smooth ER Synthesizes lipids
Metabolizes carbohydrates
Detoxifies drugs and poisons
Stores calcium ions

21. Functions of Rough ER The rough ER
Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates)
Distributes transport vesicles, proteins surrounded by membranes
Is a membrane factory for the cell

22. Golgi apparatus
Many transport vesicles from the ER travel to the Golgi apparatus for modification of their contents.
The Golgi is a center of manufacturing, warehousing, sorting, and shipping.
The Golgi apparatus is especially extensive in cells specialized for secretion.
Functions of the Golgi apparatus
Modifies products of the ER
Manufactures certain macromolecules
Sorts and packages materials into transport vesicles

23. The Golgi apparatus consists of flattened membranous sacs - cisternae - looking like a sac of pita bread.
The membrane of each cisternae separates its internal space from the cytosol
One side of the Golgi, the cis side, receives material by fusing with vesicles, while the other side, the trans side, buds off vesicles that travel to other sites.

25. Lysosomes
The lysosome is a membrane-bounded sac of hydrolytic enzymes that digests macromolecules like proteins, fats, polysaccharides, and nucleic acids .

26. The lysosomal enzymes and membrane are synthesized by rough ER and then transferred to the Golgi.
At least some lysosomes bud from the trans face of the Golgi.

27. Vacuole
Vesicles and vacuoles (larger versions) are membrane-bound sacs with varied functions.
Food vacuoles, from phagocytosis, fuse with lysosomes.
Contractile vacuoles, found in freshwater protists, pump excess water out of the cell.
Central vacuoles are found in many mature plant cells.

29. The Endomembrane System: A Review
The endomembrane system is a complex and dynamic player in the cell’s compartmental organization

30. Nucleus Rough ER Smooth ER Plasma membrane Figure 6.15-1
Figure 6.15 Review: relationships among organelles of the endomembrane system.
Plasma membrane

31. Nucleus Rough ER Smooth ER cis Golgi Plasma membrane trans Golgi
Figure
Nucleus
Rough ER
Smooth ER
cis Golgi
Figure 6.15 Review: relationships among organelles of the endomembrane system.
Plasma membrane
trans Golgi

32. Nucleus Rough ER Smooth ER cis Golgi Plasma membrane trans Golgi
Figure
Nucleus
Rough ER
Smooth ER
cis Golgi
Figure 6.15 Review: relationships among organelles of the endomembrane system.
Plasma membrane
trans Golgi

33. Concept 6.5 Mitochondria and Chloroplast
Mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work.
Mitochondria are the sites of cellular respiration, generating ATP from the catabolism of sugars, fats, and other fuels in the presence of oxygen.

34. Almost all eukaryotic cells have mitochondria.
There may be one very large mitochondrion or hundreds to thousands in individual mitochondria.
The number of mitochondria is correlated with aerobic metabolic activity.

35. The Endosymbiont theory
An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an endosymbiont relationship with its host
The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion
At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts

36. Endoplasmic reticulum Nucleus
Figure 6.16
Endoplasmic reticulum
Nucleus
Engulfing of oxygen- using nonphotosynthetic prokaryote, which becomes a mitochondrion
Nuclear envelope
Ancestor of eukaryotic cells (host cell)
Mitochondrion
Engulfing of photosynthetic prokaryote
At least one cell
Figure 6.16 The endosymbiont theory of the origin of mitochondria and chloroplasts in eukaryotic cells.
Chloroplast
Nonphotosynthetic eukaryote
Mitochondrion
Photosynthetic eukaryote

37. Mitochondria have a smooth outer membrane and a highly folded inner membrane, the cristae.
This creates a fluid-filled space between them.
The cristae provides ample surface area for the enzymes that synthesize ATP.
The inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA, ribosomes, and enzymes.

39. Chloroplasts: Capture of Light Energy
Chloroplasts, found in plants and eukaryotic algae, are the site of photosynthesis.
The chloroplast is one of a group of plant organelles, called plastids
They convert solar energy to chemical energy and synthesize new organic compounds from CO2 and H2O.
The chloroplast produces sugar via photosynthesis.
Chloroplasts gain their color from high levels of the green pigments of chlorophyll.

40. The processes in the chloroplast are separated from the cytosol by two membranes.
Inside the innermost membrane is a fluid-filled space, the stroma, in which float membranous sacs, the thylakoids.
The stroma contains DNA, ribosomes, and enzymes for part of photosynthesis.
The thylakoids, flattened sacs, are stacked into grana and are critical for converting light to chemical energy.

42. Concept 6.6: The cytoskeleton is a network of fibers that organizes structures and activities in the cell
The cytoskeleton is a network of fibers extending throughout the cytoplasm
It organizes the cell’s structures and activities, anchoring many organelles
It is composed of three types of molecular structures
Microtubules
Microfilaments
Intermediate filaments
For the Cell Biology Video The Cytoskeleton in a Neuron Growth Cone, go to Animation and Video Files
For the Cell Biology Video Cytoskeletal Protein Dynamics, go to Animation and Video Files.

43. Figure 6.20
Figure 6.20 The cytoskeleton.
10 m

44. Roles of the Cytoskeleton: Support and Motility
The cytoskeleton helps to support the cell and maintain its shape
It interacts with motor proteins to produce motility
Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton
Recent evidence suggests that the cytoskeleton may help regulate biochemical activities

45. Column of tubulin dimers Keratin proteins Actin subunit
Fibrous subunit (keratins coiled together)
25 nm
7 nm
8–12 nm  
Tubulin dimer

46. Concept 6.7: Extracellular components and connections between cells help coordinate cellular activities
Most cells synthesize and secrete materials that are external to the plasma membrane
These extracellular structures include:
Cell walls of plants
The extracellular matrix (ECM) of animal cells
Intercellular junctions

47. Functions of the Extra Cellular Matrix (ECM)
Support
Adhesion
Movement
Regulation

49. Intercellular Junctions
Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact
Intercellular junctions facilitate this contact
There are several types of intercellular junctions
- Plasmodesmata are channels that perforate plant cell walls
- At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid
- Desmosomes (anchoring junctions) fasten cells together into strong sheets
- Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

51. Understanding the Complexity
While the cell has many structures that have specific functions, they must work together.
Food vacuoles are digested by lysosomes, a product of the endomembrane system of ER and Golgi.
The enzymes of the lysosomes are synthesized at the ribosomes.
The information for these proteins comes from genetic messages sent by DNA in the nucleus.
All of these processes require energy in the form of ATP, most of which is supplied by the mitochondria.
A cell is a living unit greater than the sum of its parts

53. Figure 6.UN01 Summary table, Concepts 6.3–6.5
Nucleus
(ER)
(Nuclear envelope)
Figure 6.UN01 Summary table, Concepts 6.3–6.5

54. Figure 6.UN01a Summary table, Concept 6.3
Nucleus
(ER)
Figure 6.UN01a Summary table, Concept 6.3

55. Figure 6.UN01b Summary table, Concept 6.4
(Nuclear envelope)
Figure 6.UN01b Summary table, Concept 6.4

56. Figure 6.UN01c
Figure 6.UN01c Summary table, Concept 6.5