1. Chapter 3 The Cellular Level of Organization
Lecture slides prepared by Curtis DeFriez, Weber State University

2. The Cell A cell is the basic unit of all living things.
Prokaryotic cells are simple cells that are Pro (“before”) karyon (“nucleus”) – they have no nucleus.
Most are unicellular bacteria.
Eukaryotic cells are complex cells with a nucleus and subcellular structures (organelles).
All fungi, plants, and animals are eukaryotes.

3. A Generalized Cell
All eukaryotic cells are composed of three main parts:
Plasma membrane or “plasmalemma”
Cytoplasm - a gelatin-like substance, plus structural fibers and organelles (but
not the nucleus)
Nucleus - contains the
genetic library of the cell

4. A Generalized Cell
The plasma membrane forms the cell’s outer boundary and separates the cell’s internal environment from the outside environment.
It is a selectively permeable barrier,
allowing the passage of some
things and not others.
It plays a role in
cellular communication.

5. A Generalized Cell
The cytoplasm contains all the cellular contents between the plasma membrane and the nucleus.
The cytosol is the fluid portion (mostly water).
Organelles are subcellular
structures embedded
in the cytosol, having
characteristic shapes
and specific functions.

6. A Generalized Cell
The nucleus is a large organelle that contains DNA in molecules called chromosomes.
Each chromosome consists of
a single molecule of DNA and
associated packaging proteins.
A chromosome contains
thousands of hereditary
units called genes.

7. A Generalized Cell
Fig. 3.1 shows a generalized body cell labeled with the plasma membrane, cytoplasm (and organelles) and nucleus.

8. The Plasma Membrane
The plasma membrane is much more than just a “fence” – it is a flexible yet sturdy, “intelligent” semipermeable regulator that:
Covers and protects the cell
Controls what goes in and comes out
Links to other cells
Flies certain “flags” to tell
other cells “who” it is

9. The Plasma Membrane
The Fluid Mosaic Model describes the arrangement of molecules within the membrane: They resemble a sea of phospholipids with protein “icebergs” floating in it.
The lipids act as a barrier to certain polar substances.
The proteins act as
“gatekeepers”, allowing
passage of specific
molecules and ions.

10. The Plasma Membrane Interactions Animation
Membrane Functions
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11. The Plasma Membrane The structure of the membrane
Phospholipids form a lipid bilayer - cholesterol and glycolipids (sugar-lipids) also contribute.
Integral proteins - extend into or through the bilayer.
Transmembrane proteins (most integral proteins) span the entire lipid bilayer.
Peripheral proteins attach to the inner or outer surface but do not extend through the membrane.

12. The Plasma Membrane

13. The Plasma Membrane
Glycoproteins are membrane proteins with a carbohydrate group attached that protrude into
the extracellular fluid.
The Glycocalyx is the entire
“sugary coating” surrounding
the membrane (made up of
the carbohydrate portions
of the glycolipids and
glycoproteins).

14. The Plasma Membrane The Functions of the membrane
Some integral proteins are ion channels.
Transporters - selectively move substances through the membrane.
Receptors - for cellular recognition; a ligand is a molecule that binds with a receptor.
Enzymes - catalyze chemical reactions
Others act as cell-identity markers.

15. The Plasma Membrane Examples of different membrane proteins include
Ion channels
Carriers
Receptors

16. The Plasma Membrane Examples of different membrane proteins include
Enzymes
Linkers
Cell identity markers

17. The Plasma Membrane
Because of the distribution of lipids and the proteins embedded in it, the membrane allows some substances across but not others; this is called Selective permeability
Rule of thumb: small, neutrally-charged, lipid-soluble substances can freely pass. Water is a special case - it is highly polar,
yet still freely
permeable.
courtesy of Dr. Jim Hutchins

18. Membrane Permeability
For those substances that are needed by the cell but for which the membrane is impenetrable (impermeable), transmembrane proteins act as channels and transporters.
They assist the entrance of certain substances that either can’t pass at all (glucose) or for which the cell needs to hasten passage
(ions).

19. Transport Processes
Passive processes involve substances moving across the cell membranes without the input of any energy - they are said to move “with” or “down” their concentration gradient ([gradient] , where [ ] indicates “concentration”).
Active processes involve the use of energy, primarily from the breakdown of ATP, to move a substance against its [gradient].

20. Transport Processes Passive processes Active processes
Diffusion of solutes
Diffusion of water (called osmosis)
Facilitated diffusion (requires a specific channel or a carrier molecule, but no energy is used)
Active processes
Various types of transporters are used, and energy is required.

21. Passive Transport Processes
Diffusion is the passive spread of particles through random motion, from areas of high concentration to areas of low concentration.
It is affected by the amount of
substance and the steepness of
the concentration gradient.
Temperature
Surface area
Diffusion distance

22. Passive Transport Processes
Channel-Mediated Facilitated Diffusion
Carrier-Mediated Facilitated Diffusion

23. Passive Transport Processes
An example of Channel-Mediated Facilitated Diffusion is the passage of potassium ions through a gated K+ Channel
An example of Carrier-Mediated Facilitated Diffusion is the passage of glucose across the cell membrane.

24. Passive Transport Processes
Osmosis is the net movement of water through a selectively permeable membrane from an area of high water concentration to one of lower water concentration.
Water can pass through plasma membrane in 2 ways:
through lipid bilayer by simple diffusion
through aquaporins (integral membrane proteins)

25. Passive Transport Processes
In the third tube, the force generated by the movement of water from the left to the right side is called osmotic pressure.

26. Passive Transport Processes
In the body, tonicity refers to the concentration of salt solutions in the blood and elsewhere. Since semipermeable membranes separate these fluid compartments, osmosis of water is free to occur between
any fluid space and another.
The effect of changing tonicity
is demonstrated in this graphic,
as water moves in and out
of red blood cells.

27. Active Transport Processes
Solutes can also be actively transported across a plasma membrane against their concentration gradient ([low] to [high]) by using energy (usually in the form of ATP).
The sodium-potassium pump is found in all cells.

28. Secondary Active Transport Mechanisms
Antiporters carry two substances across the membrane in opposite directions.
Symporters carry two substances across the membrane in the same direction.

29. Transport in Vesicles
Vesicle - a small spherical sac formed by budding off from a membrane
Endocytosis - materials move into a cell in a vesicle formed from the plasma membrane
three types: receptor-mediated endocytosis
phagocytosis
bulk-phase endocytosis (pinocytosis)
Exocytosis - vesicles fuse with the plasma membrane, releasing their contents into the extracellular fluid
Transcytosis - a combination of endocytosis and exocytosis

30. Receptor-Mediated Endocytosis

31. Phagocytosis

32. Bulk-phase Endocytosis

33. Interactions Animation
Transport
Transport Across the Plasma Membrane
Interactions Animation
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34. Transport Interactions Animations
Transport Across the Plasma Membrane
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35. Cytoplasm - 2 Components
1. Cytosol - intracellular fluid, surrounding the organelles
- The site of many chemical reactions
- Energy is usually released by these reactions.
- Reactions provide the building blocks for cell maintenance, structure, function and growth.
2. Organelles
- Specialized structures within the cell

36. The Cytoskeleton Network of protein filaments throughout the cytosol
Provides structural support for the cell

37. The Cytoskeleton Types Microfilaments Intermediate filaments
Microtubules

38. Organelles
Centrosome - located near the nucleus, consists of two centrioles and pericentriolar material

39. Organelles
Cilia - short, hair-like projections from the cell surface, move fluids along a cell surface
Flagella - longer than cilia, move an entire cell; only example is the sperm cell’s tail

40. Organelles