1. Cells: The Living Units: Part A3
Cells: The Living Units: Part A

2. Cell Theory
The cell is the smallest structural and functional living unit
Organismal functions depend on individual and collective cell functions
Biochemical activities of cells are dictated by their specific subcellular structures
Continuity of life has a cellular basis

3. Cell Diversity
Over 200 different types of human cells
Types differ in size, shape, subcellular components, and functions

4. (a) Cells that connect body parts, form linings, or transport gases
Erythrocytes
Fibroblasts
Epithelial cells
(a) Cells that connect body parts,
form linings, or transport gases
Nerve cell
Skeletal
Muscle
cell
Smooth
muscle cells
(e) Cell that gathers information
and control body functions
(b) Cells that move organs and
body parts
Sperm
Macrophage
(f) Cell of reproduction
Fat cell
(c) Cell that stores nutrients
(d) Cell that
fights disease
Figure 3.1

5. All cells have some common structures and functions
Generalized Cell
All cells have some common structures and functions
Human cells have three basic parts:
Plasma membrane—flexible outer boundary
Cytoplasm—intracellular fluid containing organelles
Nucleus—control center

6. Chromatin Nuclear envelope Nucleolus Nucleus Smooth endoplasmic
reticulum
Plasma
membrane
Mitochondrion
Cytosol
Lysosome
Centrioles
Centrosome
matrix
Rough
endoplasmic
reticulum
Ribosomes
Golgi apparatus
Secretion being
released from cell
by exocytosis
Cytoskeletal
elements
• Microtubule
• Intermediate
filaments
Peroxisome
Figure 3.2

7. Plays a dynamic role in cellular activity
Plasma Membrane
Bimolecular layer of lipids and proteins in a constantly changing fluid mosaic
Plays a dynamic role in cellular activity
Separates intracellular fluid (ICF) from extracellular fluid (ECF)
Interstitial fluid (IF) = ECF that surrounds cells

8. Extracellular fluid (watery environment) Cholesterol Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outward-
facing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Inward-facing
layer of
phospholipids
Bimolecular
lipid layer
containing
proteins
Peripheral
proteins
Nonpolar
tail of
phospholipid
molecule
Cytoplasm
(watery environment)
Figure 3.3

9. Membrane Lipids 75% phospholipids (lipid bilayer) 5% glycolipids
Phosphate heads: polar and hydrophilic
Fatty acid tails: nonpolar and hydrophobic (Review Fig. 2.16b)
5% glycolipids
Lipids with polar sugar groups on outer membrane surface
20% cholesterol
Increases membrane stability and fluidity

10. Lipid Rafts
~ 20% of the outer membrane surface
Contain phospholipids, sphingolipids, and cholesterol
May function as stable platforms for cell-signaling molecules

11. Membrane Proteins Integral proteins
Firmly inserted into the membrane (most are transmembrane)
Functions:
Transport proteins (channels and carriers), enzymes, or receptors

12. Membrane Proteins Peripheral proteins
Loosely attached to integral proteins
Include filaments on intracellular surface and glycoproteins on extracellular surface
Functions:
Enzymes, motor proteins, cell-to-cell links, provide support on intracellular surface, and form part of glycocalyx

13. Extracellular fluid (watery environment) Cholesterol Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outward-
facing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Inward-facing
layer of
phospholipids
Bimolecular
lipid layer
containing
proteins
Peripheral
proteins
Nonpolar
tail of
phospholipid
molecule
Cytoplasm
(watery environment)
Figure 3.3

14. Functions of Membrane Proteins
Transport
Receptors for signal transduction
Attachment to cytoskeleton and extracellular matrix

15. A protein (left) that spans the membrane
(a) Transport
A protein (left) that spans the membrane
may provide a hydrophilic channel across
the membrane that is selective for a
particular solute. Some transport proteins
(right) hydrolyze ATP as an energy source
to actively pump substances across the
membrane.
Figure 3.4a

16. (b) Receptors for signal transduction Signal
A membrane protein exposed to the
outside of the cell may have a binding
site with a specific shape that fits the
shape of a chemical messenger, such
as a hormone. The external signal may
cause a change in shape in the protein
that initiates a chain of chemical
reactions in the cell.
Receptor
Figure 3.4b

17. (c) Attachment to the cytoskeleton and extracellular matrix (ECM)
Elements of the cytoskeleton (cell’s
internal supports) and the extracellular
matrix (fibers and other substances
outside the cell) may be anchored to
membrane proteins, which help maintain
cell shape and fix the location of certain
membrane proteins. Others play a role in
cell movement or bind adjacent cells
together.
Figure 3.4c

18. Functions of Membrane Proteins
Enzymatic activity
Intercellular joining
Cell-cell recognition

19. (d) Enzymatic activity
Enzymes
A protein built into the membrane may
be an enzyme with its active site
exposed to substances in the adjacent
solution. In some cases, several
enzymes in a membrane act as a team
that catalyzes sequential steps of a
metabolic pathway as indicated (left to
right) here.
Figure 3.4d

20. (e) Intercellular joining
Membrane proteins of adjacent cells
may be hooked together in various
kinds of intercellular junctions. Some
membrane proteins (CAMs) of this
group provide temporary binding sites
that guide cell migration and other
cell-to-cell interactions.
CAMs
Figure 3.4e

21. (f) Cell-cell recognition
Some glycoproteins (proteins bonded
to short chains of sugars) serve as
identification tags that are specifically
recognized by other cells.
Glycoprotein
Figure 3.4f

22. Membrane Junctions
Three types:
Tight junction
Desmosome
Gap junction

23. Membrane Junctions: Tight Junctions
Prevent fluids and most molecules from moving between cells
Where might these be useful in the body?

24. (a) Tight junctions: Impermeable junctions prevent molecules
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Interlocking
junctional proteins
Intercellular
space
(a) Tight junctions: Impermeable junctions prevent molecules
from passing through the intercellular space.
Figure 3.5a

25. Membrane Junctions: Desmosomes
“Rivets” or “spot-welds” that anchor cells together
Where might these be useful in the body?

26. (b) Desmosomes: Anchoring junctions bind adjacent cells together
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular space
Plaque
Intermediate
filament (keratin)
Linker glycoproteins
(cadherins)
(b) Desmosomes: Anchoring junctions bind adjacent cells together
and help form an internal tension-reducing network of fibers.
Figure 3.5b

27. Membrane Junctions: Gap Junctions
Transmembrane proteins form pores that allow small molecules to pass from cell to cell
For spread of ions between cardiac or smooth muscle cells

28. (c) Gap junctions: Communicating junctions allow ions and small mole-
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular
space
Channel
between cells
(connexon)
(c) Gap junctions: Communicating junctions allow ions and small mole-
cules to pass from one cell to the next for intercellular communication.
Figure 3.5c

29. Membrane Transport
Plasma membranes are selectively permeable
Some molecules easily pass through the membrane; others do not

30. Types of Membrane Transport
Passive processes
No cellular energy (ATP) required
Substance moves down its concentration gradient
Active processes
Energy (ATP) required
Occurs only in living cell membranes

31. Passive Processes
What determines whether or not a substance can passively permeate a membrane?
Lipid solubility of substance
Channels of appropriate size
Carrier proteins

32. Passive Processes
Simple diffusion
Carrier-mediated facilitated diffusion
Channel-mediated facilitated diffusion
Osmosis

33. Passive Processes: Simple Diffusion
Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer

34. (a) Simple diffusion of fat-soluble molecules
Extracellular fluid
Lipid-
soluble
solutes
Cytoplasm
(a) Simple diffusion of fat-soluble molecules
directly through the phospholipid bilayer
Figure 3.7a

35. Passive Processes: Facilitated Diffusion
Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:
Exhibit specificity (selectivity)
Are saturable; rate is determined by number of carriers or channels
Can be regulated in terms of activity and quantity

36. Facilitated Diffusion Using Carrier Proteins
Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids)
Binding of substrate causes shape change in carrier

37. (b) Carrier-mediated facilitated diffusion via a protein
Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
Figure 3.7b

38. Facilitated Diffusion Using Channel Proteins
Aqueous channels formed by transmembrane proteins selectively transport ions or water
Two types:
Leakage channels
Always open
Gated channels
Controlled by chemical or electrical signals

39. (c) Channel-mediated facilitated diffusion
Small lipid-
insoluble
solutes
(c) Channel-mediated facilitated diffusion
through a channel protein; mostly ions
selected on basis of size and charge
Figure 3.7c

40. Passive Processes: Osmosis
Movement of solvent (water) across a selectively permeable membrane
Water diffuses through plasma membranes:
Through the lipid bilayer
Through water channels called aquaporins (AQPs)

41. (d) Osmosis, diffusion of a solvent such as
Water
molecules
Lipid
billayer
Aquaporin
(d) Osmosis, diffusion of a solvent such as
water through a specific channel protein
(aquaporin) or through the lipid bilayer
Figure 3.7d

42. Passive Processes: Osmosis
Water concentration is determined by solute concentration because solute particles displace water molecules
Osmolarity: The measure of total concentration of solute particles
When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

43. Solute and water molecules move down their concentration gradients
(a) Membrane permeable to both solutes and water
Solute and water molecules move down their concentration gradients
in opposite directions. Fluid volume remains the same in both compartments.
Left
compartment:
Solution with
lower osmolarity
Right
compartment:
Solution with
greater osmolarity
Both solutions have the
same osmolarity: volume
unchanged
H2O
Solute
Solute
molecules
(sugar)
Membrane
Figure 3.8a

44. (b) Membrane permeable to water, impermeable to solutes
Solute molecules are prevented from moving but water moves by osmosis.
Volume increases in the compartment with the higher osmolarity.
Both solutions have identical
osmolarity, but volume of the
solution on the right is greater
because only water is
free to move
Left
compartment
Right
compartment
H2O
Solute
molecules
(sugar)
Membrane
Figure 3.8b

45. Importance of Osmosis
When osmosis occurs, water enters or leaves a cell
Change in cell volume disrupts cell function

46. Tonicity
Tonicity: The ability of a solution to cause a cell to shrink or swell
Isotonic: A solution with the same solute concentration as that of the cytosol
Hypertonic: A solution having greater solute concentration than that of the cytosol
Hypotonic: A solution having lesser solute concentration than that of the cytosol

47. Figure 3.9 (a) Isotonic solutions (b) Hypertonic solutions
(c) Hypotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
Figure 3.9

48. Summary of Passive Processes
Energy Source
Example
Simple diffusion
Kinetic energy
Movement of O2 through phospholipid bilayer
Facilitated diffusion
Movement of glucose into cells
Osmosis
Movement of H2O through phospholipid bilayer or AQPs
Also see Table 3.1