The document provides an overview of the cellular level of organization, including key concepts such as: - Cells are the basic unit of structure and function in living things, and all cells contain a plasma membrane, cytoplasm, and nucleus. - The plasma membrane is selectively permeable and regulates what enters and exits the cell. Transport across the membrane includes passive diffusion and active transport processes. - The cytoplasm contains cytosol and various organelles that carry out specialized functions. Organelles include the endoplasmic reticulum, Golgi complex, lysosomes, mitochondria and more. - The nucleus houses the cell's DNA within chromosomes and controls gene expression through transcription and translation. Cell division occurs through mitosis and

1. Chapter 3The Cellular Level of
Organization
Lectures by Dr. Yukti Sharma

2. Learning Objectives
 Define a cell, and discuss modern cell theory.
 Discuss structure and function of plasma membrane.
 Discuss transport of substances across plasma membrane.
 Describe cytoplasm and its components.
 Cytosol and organelles: cytoskeleton, centrosome, cilia and flagella, ribosomes,
endoplasmic reticulum, golgi complex, lysosomes, peroxisomes, proteasomes,
mitochondria.
 Define a nucleus and discuss its parts: nuclear membrane, nucleoplasm, nucleolus,
chromosomes.
 Discuss structure of chromosomes, and processes of transcription and translation.
 Describe cell division
 Discuss cellular diversity.
 Discuss medical terminology related to changes in normal structure, shape and function
of the cells

3.  A cell is the basic unit of all living things.
 Prokaryotic cells
 Eukaryotic cells
The Cell

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

5. 1. The plasma membrane forms the cell’s
outer boundary and separates the cell’s
internal environment from the outside
environment.
A Generalized Cell

6. 2. The cytoplasm contains all the cellular
contents between the plasma membrane
and the nucleus.
A Generalized Cell

7. 3. 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.
A Generalized Cell

8. A Generalized Cell
 Figure shows a generalized body cell labeled with the
plasma membrane, cytoplasm (and organelles) and nucleus.

9.  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
The Plasma Membrane

10. 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.

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
• Peripheral proteins

13. The Plasma Membrane
 Glycoproteins are membrane proteins with
a carbohydrate group attached that protrude
into the extracellular fluid.
 The Glycocalyx

14. The Plasma Membrane
 The Functions of the membrane proteins
 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. Functions of Membrane Proteins
1. Transport
2. Receptors for signal transduction
3. Attachment to cytoskeleton and extracellular matrix
(1) Transport (2) Receptors (3) Attachment

16. Functions of Membrane
Proteins
4. Enzymatic activity
5. Intercellular joining
6. Cell-cell recognition
CAMs
Enzymes
Glycoprotein
Enzymatic activity Intercellular joining Cell-cell recognition

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

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

19.  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
The Plasma Membrane
courtesy of Dr. Jim Hutchins

20. 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.

21. 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].

22. Transport Processes
 Passive 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.

23. Concentration gradients
 Concentrations of some key ions are very
different on the inside versus the outside
of cells creating a gradient
IN:
[Na+] = low
[K+] = high
[Ca2+] = very
low
[Cl-] = low
OUT:
[Na+] = high
[K+] = low
[Ca2+] = low
[Cl-] = high
(blood,
interstitial fluid)

24. Passive Transport Processes
 Diffusion is the passive spread of particles
through random motion, from areas of high
concentration to areas of low concentration.
 Amount of substance and the steepness of the
concentration gradient.
 Temperature
 Surface area
 Diffusion distance

25. What can/can’t diffuse through the cell
membrane?

26. Passive Transport Processes
 Channel-Mediated Facilitated Diffusion
 Carrier-Mediated Facilitated Diffusion

27. 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.

28. Diffusion Through the Plasma Membrane
Figure 3.7
Extracellular fluid
Cytoplasm
Lipid-
soluble
solutes
Lipid
bilayer
Lipid-insoluble
solutes
Water
molecules
Small lipid-
insoluble
solutes
(a) Simple diffusion
directly through the
phospholipid bilayer
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
(d) Osmosis, diffusion
through a specific
channel protein
(aquaporin) or
through the lipid
bilayer

29. 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)

30. Effect of Membrane Permeability
on Diffusion and Osmosis
Figure 3.8a

31. Effect of Membrane Permeability on
Diffusion and Osmosis
Figure 3.8b

32. Osmosis in Cells

33. KEY CONCEPT
 Concentration gradients tend to even out due
to random motion of particles
 In the absence of a membrane, diffusion
eliminates concentration gradients
 When different solute concentrations exist on
either side of a selectively permeable
membrane, either:
1. diffusion of permeable molecules equalizes
concentrations OR
2. osmosis moves water through the
membrane to equalize the concentration
gradients

34. Active Transport Processes
Cytoplasm
Extracellular fluidK+ is released and
Na+ sites are ready to
bind Na+ again; the
cycle repeats.
Cell
ADP
Phosphorylation
causes the
protein to
change its shape.
Concentration gradients
of K+ and Na+
The shape change
expels Na+ to the
outside, and
extracellular K+ binds.
Loss of phosphate
restores the original
conformation of the
pump protein.
K+ binding triggers
release of the
Binding of cytoplasmic
Na+ to the pump protein
stimulates phosphorylation
by ATP.Na+
Na+
Na+
Na+Na+
K+K+
K+
K+
Na+
Na+
Na+
ATP
P
P
Na+
Na+
Na+
K+
K+
P
Pi
K+
K+

35. Secondary Active Transport Mechanisms

36. 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

37. Receptor-Mediated Endocytosis

38. Phagocytosis

39. Bulk-phase Endocytosis

40. 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

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

42.  Types
 Microfilaments
 Intermediate
filaments
 Microtubules
The Cytoskeleton

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

44. 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

45. Organelles

46. Organelles
 Ribosomes - sites of protein synthesis

47. Organelles
 Endoplasmic
reticulum - network of
membranes in the
shape of flattened sacs
or tubules
- Rough ER - connected
to the nuclear envelope,
a series of flattened
sacs, surface is studded
with ribosomes,
produces various
proteins

48. Organelles
 Golgi complex - consists of 3–20 flattened,
membranous sacs called cisternae.

49. Processing and Packaging

50. Organelles
 Lysosomes - vesicles that form from the Golgi
complex and contain powerful digestive enzymes

51. Organelles
 Peroxisomes
 Smaller than lysosomes
 Detoxify several toxic substances such as alcohol
 Abundant in the liver
 Proteasomes
 Continuously destroy unneeded, damaged, or
faulty proteins
 Found in the cytosol and the nucleus

52. Organelles
Mitochondria - the “powerhouses” of the cell
 Generate ATP
 Have inner and outer mitochondrial membranes similar in
structure to the plasma membrane
 Cristae - the series of folds of the inner membrane
 Matrix - the large central fluid-filled cavity
 Self-replicate during times of increased cellular demand or
before cell division
 Contain own DNA
• Inherited only from your mother

53. Mitochondria

54. Nucleus

55. Packing of DNA into a
Chromosome of a Dividing Cell

56. Overview of Gene Expression
The Central Dogma
DNA > RNA > Protein.

57. Translation

58. Translation

59. Translation

60. Translation

61. Translation

62. Translation

63. Translation

64. Transcription

65. Somatic Cell Division - Mitosis
 The cell cycle is a sequence of events in which a body
cell duplicates its contents and divides in two
 Human somatic cells contain 23 pairs of chromosomes
(total = 46)
 The two chromosomes that make up each pair are called
homologous chromosomes (homologs)
 Somatic cells contain two sets of chromosomes and are
called diploid cells

66. Cell Division
 Interphase - the cell is not dividing
- The cell replicates its DNA
- Consists of three phases, G1, S, and G2,
replication of DNA occurs in the S phase
Mitotic phase - consists of a nuclear
division (mitosis) and a cytoplasmic
division (cytokinesis) to form two identical
cells

67. The Cell Cycle

68. DNA Replication

69. Nuclear Division: Mitosis
 Prophase - the chromatin fibers change into chromosomes.
 Metaphase - microtubules align the centromeres of the chromatid
pairs at the metaphase plate.
 Anaphase - the chromatid pairs split at the centromere and move
to opposite poles of the cell; the chromatids are now called
chromosomes.
 Telophase - two identical nuclei are formed around the identical
sets of chromosomes now in their chromatin form.

70. Cytoplasmic Division: Cytokinesis
 Division of a cell’s cytoplasm to form two
identical cells
 Usually begins in late anaphase
 The plasma membrane constricts at its
middle, forming a cleavage furrow
 The cell eventually splits into two daughter
cells.

71. Mitosis

72. Mitosis

73. Mitosis

74. Mitosis

75. Mitosis

76. Mitosis

77. 1
Pericentriolar material
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
(a) INTERPHASE
Centrioles
Centrosome:
all at 700xLM
1
LateEarly
Pericentriolar material
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
Chromosome
(two chromatids
joined at
centromere
(a) INTERPHASE
(b) PROPHASE
Centrioles
Centrosome:
Fragments of
nuclear envelope
Mitotic spindle
(microtubules)
Kinetochore
2
all at 700xLM
Centromere
1
Pericentriolar material
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
Metaphase plate
(a) INTERPHASE
Centrioles
Centrosome:
(c) METAPHASE
2
3
LateEarly (b) PROPHASE
Fragments of
nuclear envelope
Mitotic spindle
(microtubules)
Kinetochore
all at 700xLM
Chromosome
(two chromatids
joined at
centromere
Centromere
1
Early
Late
(d) ANAPHASE
Pericentriolar material
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
Chromosome
(a) INTERPHASE
Centrioles
Centrosome:
(c) METAPHASE
2
3
4
Cleavage
furrow
LateEarly (b) PROPHASE
Fragments of
nuclear envelope
Mitotic spindle
(microtubules)
Kinetochore
Metaphase plate
all at 700xLM
Chromosome
(two chromatids
joined at
centromere
Centromere
1
Early
Late
(d) ANAPHASE
Pericentriolar material
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
(a) INTERPHASE
Centrioles
Centrosome:
Cleavage furrow
(e) TELOPHASE
(c) METAPHASE
2
3
4
5
Cleavage
furrow
LateEarly (b) PROPHASE
Fragments of
nuclear envelope
Mitotic spindle
(microtubules)
Kinetochore
Metaphase plate
Chromosome
all at 700xLM
Chromosome
(two chromatids
joined at
centromere
Centromere
1
Early
Late
(d) ANAPHASE
Pericentriolar material
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
(a) INTERPHASE
Centrioles
Centrosome:
(f) IDENTICAL CELLS IN INTERPHASE
Cleavage furrow
(e) TELOPHASE
(c) METAPHASE
Cleavage
furrow
2
3
4
5
6
LateEarly (b) PROPHASE
Fragments of
nuclear envelope
Mitotic spindle
(microtubules)
Kinetochore
Metaphase plate
Chromosome
all at 700xLM
Centromere
Chromosome
(two chromatids
joined at
centromere
Mitosis

78. Reproductive Cell Division
 During sexual reproduction, each new
organism is the result of the union of two
gametes (fertilization), one from each
parent.
 Meiosis - reproductive cell division that
occurs in the gonads (ovaries and testes) that
produces gametes with half the number of
chromosomes.
 Haploid cells - gametes contain a single set
of 23 chromosomes.
 Fertilization restores the diploid number of
chromosomes (46).

79. Reproductive Cell Division
 Meiosis occurs in two successive stages: meiosis I and
meiosis II .
 Each of these two stages has 4 phases: prophase,
metaphase, anaphase, and telophase.
 Summary - Meiosis I begins with a diploid cell and ends
with two cells having the haploid number of
chromosomes; in Meiosis II, each of the two haploid cells
divides, and the net result is four haploid gametes that are
genetically different from the original diploid starting cell.

80. Meiosis and
Cytokinesis

81. Comparison
of Mitosis
and Meiosis

82. Cellular Diversity
 The average adult has
nearly 100 trillion cells.
 There are about 200
different types of cells.
 Cells come in a variety of
shapes and sizes.
 Cellular diversity permits
organization of cells into
more complex tissues and
organs.