1. Department of Kinesiology and Applied Physiology
Chapter 3: Cells
Overview
Plasma membrane: structure
Plasma membrane: transport
Resting membrane potential
Cell-environment interactions
Cytoplasm
Nucleus
Cell growth & reproduction
Extracellular materials
Developmental aspects
Department of Kinesiology and Applied Physiology

2. Do exercise scientists need to think about cells? Exercise in a Pill
“AMPK and PPARδ Agonists Are Exercise Mimetics”
AICAR activates intracellular pathways that are also activated by exercise. Mice taking AICAR look like mice on exercise. Mice on AICAR plus exercise are supermice.
Department of Kinesiology and Applied Physiology

3. (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

4. 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

5. 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

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

7. 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

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

9. 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
PLAY
Animation: Structural Proteins
PLAY
Animation: Receptor Proteins

10. 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

11. (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

12. (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

13. (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

14. (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

15. (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

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

17. (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

18. (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

19. (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

20. Membrane Transport: How things get in and out of cells
Plasma membranes are selectively permeable: some molecules easily pass through the membrane; others do not

21. 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

22. Passive Processes
What determines whether or not a substance can passively permeate a membrane?
Lipid solubility of substance
Channels of appropriate size
Carrier proteins
PLAY
Animation: Membrane Permeability

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

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

25. (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

26. 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

27. (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

28. (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

29. 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

30. (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

31. When osmosis occurs, water enters or leaves a cell
Importance of Osmosis
When osmosis occurs, water enters or leaves a cell
Change in cell volume disrupts cell function
PLAY
Animation: Osmosis

32. Tonicity
Tonicity: How much dissolved material there is in a solution. Tonicity determines whether a solution will make cells shrink or swell.
Isotonic: A solution with the same solute concentration as the inside of a normal cell
Hypertonic: A solution with a greater solute concentration than than a normal cell
Hypotonic: A solution with a lesser solute concentration than a normal cell

33. 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

34. Membrane Transport: Active Processes
Two types of active processes:
Active transport
Vesicular transport
Both use ATP to move solutes across a living plasma membrane

35. Requires carrier proteins (solute pumps)
Active Transport
Requires carrier proteins (solute pumps)
Moves solutes against a concentration gradient
Types of active transport:
Primary active transport
Secondary active transport

36. Primary Active Transport
Energy from breakdown of ATP causes shape change in transport protein to “pump” molecules across the membrane
Example: Sodium-potassium pump (Na+-K+ ATPase)
Located in all plasma membranes
Involved in primary and secondary active transport of nutrients and ions
Maintains “electrochemical gradients” essential for functions of muscle and nerve tissues

37. Figure 3.10 Extracellular fluid Na+ Na+-K+ pump ATP-binding site K+
Na+ bound
Cytoplasm 1
Cytoplasmic Na+ binds to pump protein. P
ATP
K+ released
ADP 6
K+ is released from the pump protein and Na+ sites are ready to bind Na+ again. The cycle repeats. 2
Binding of Na+ promotes phosphorylation of the protein by ATP.
Na+ released
K+ bound P Pi K+ 5
K+ binding triggers release of the phosphate. Pump protein returns to its original conformation. 3
Phosphorylation causes the protein to change shape, expelling Na+ to the outside. P 4
Extracellular K+ binds to pump protein.
Figure 3.10

38. Secondary Active Transport
Energy stored in ionic gradients is used indirectly to drive transport of other solutes
Always involves cotransport – transport of more than one substance at a time
Two substances transported in same direction (Na+, glucose)
Two substances transported in opposite directions (Na+, H+)
Mod WCR

39. The ATP-driven Na+-K+ pump stores energy by creating a
Extracellular fluid
Glucose
Na+-glucose
symport
transporter
loading
glucose from
ECF
Na+-glucose
symport transporter
releasing glucose
into the cytoplasm
Na+-K+
pump
Cytoplasm 1
The ATP-driven Na+-K+ pump
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell. 2
As Na+ diffuses back across the
membrane through a membrane
cotransporter protein, it drives glucose
against its concentration gradient
into the cell. (ECF = extracellular fluid)
Figure 3.11

40. Vesicular Transport
Transport of large particles, macromolecules, and fluids across plasma membranes
Requires cellular energy (e.g., ATP)
Functions:
Exocytosis—transport out of cell
Endocytosis—transport into cell (receptor mediated; phago-; pino-)
Transcytosis—transport into, across, and then out of cell
Vesicular transport within a cell (see the video)
Mod WCR

41. The cell engulfs a large particle by forming pro-
Endocytosis:
Phagocytosis
The cell engulfs a large
particle by forming pro-
jecting pseudopods (“false
feet”) around it and en-
closing it within a membrane
sac called a phagosome.
The phagosome is
combined with a lysosome.
Undigested contents remain
in the vesicle (now called a
residual body) or are ejected
by exocytosis. Vesicle may
or may not be protein-
coated but has receptors
capable of binding to
microorganisms or solid
particles.
Phagosome
Figure 3.13a

42. The cell “gulps” drops of extracellular fluid containing
(b)
Endocytosis:
Pinocytosis
The cell “gulps” drops of
extracellular fluid containing
solutes into tiny vesicles. No
receptors are used, so the
process is nonspecific. Most
vesicles are protein-coated.
Vesicle
Figure 3.13b

43. Extracellular substances bind to specific receptor
Receptor-mediated
endocytosis
Extracellular substances
bind to specific receptor
proteins in regions of coated
pits, enabling the cell to
ingest and concentrate
specific substances
(ligands) in protein-coated
vesicles. Ligands may
simply be released inside
the cell, or combined with a
lysosome to digest contents.
Receptors are recycled to
the plasma membrane in
vesicles.
Vesicle
Receptor recycled
to plasma membrane
Figure 3.13c

44. Exocytosis Membrane-bound vesicle migrates to
Plasma membrane
SNARE (t-SNARE)
Extracellular
fluid
Fusion pore formed
Secretory
vesicle
Membrane-bound vesicle migrates to
plasma membrane. 1
Vesicle
SNARE
(v-SNARE)
Vesicle and plasma membrane fuse and pore opens up. 3
Molecule to
be secreted
Cytoplasm
Proteins at vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). 2
Vesicle contents
released to cell exterior. 4
Fused
v- and
t-SNAREs
Figure 3.14a

45. Summary of Active Processes
Energy Source
Example
Primary active transport
ATP
Pumping of ions across membranes
Secondary active transport
Ion gradient
Movement of polar or charged solutes across membranes
Exocytosis
Secretion of hormones and neurotransmitters
Phagocytosis
White blood cell phagocytosis
Pinocytosis
Absorption by intestinal cells
Receptor-mediated endocytosis
Hormone and cholesterol uptake