Transcript
Chapter 3The Cellular Level of
Organization
Lectures by Dr. Yukti Sharma
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
A cell is the basic unit of all living things. Prokaryotic cells Eukaryotic cells
The Cell
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
1. The plasma membrane forms the cell’s outer boundary and separates the cell’s internal environment from the outside environment.
A Generalized Cell
2. The cytoplasm contains all the cellular contents between the plasma membrane and the nucleus.
A Generalized Cell
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
A Generalized Cell Figure shows a generalized body cell labeled with
the plasma membrane, cytoplasm (and organelles) and nucleus.
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
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 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
The Plasma Membrane Glycoproteins are membrane proteins with
a carbohydrate group attached that protrude
into the extracellular fluid.
The Glycocalyx
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.
Functions of Membrane Proteins1. Transport
2. Receptors for signal transduction
3. Attachment to cytoskeleton and extracellular matrix
(1) Transport (2) Receptors (3) Attachment
Functions of Membrane Proteins
4. Enzymatic activity
5. Intercellular joining
6. Cell-cell recognition
CAMs
Enzymes
GlycoproteinEnzymatic activity Intercellular joining Cell-cell recognition
The Plasma Membrane Examples of different membrane proteins
include
Ion channels
Carriers
Receptors
The Plasma Membrane
Examples of different
membrane proteins
include
Enzymes
Linkers
Cell identity markers
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
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.
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].
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.
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)
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
What can/can’t diffuse through the cell membrane?
Passive Transport Processes
Channel-Mediated Facilitated Diffusion Carrier-Mediated Facilitated Diffusion
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.
Diffusion Through the Plasma Membrane
Figure 3.7
Extracellular fluid
Cytoplasm
Lipid-solublesolutes
Lipidbilayer
Lipid-insolublesolutes
Watermolecules
Small lipid-insolublesolutes
(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
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)
Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8a
Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8b
Osmosis in Cells
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
Active Transport Processes
Cytoplasm
Extracellular fluidK+ is released andNa+ sites are ready tobind Na+ again; thecycle repeats.
Cell ADP
Phosphorylationcauses theprotein tochange its shape.
Concentration gradientsof K+ and Na+
The shape change expels Na+ to the outside, and extracellular K+ binds.
Loss of phosphaterestores the originalconformation of thepump protein.
K+ binding triggersrelease of thephosphate group.
Binding of cytoplasmic Na+ to the pump proteinstimulates phosphorylationby ATP.Na+
Na+
Na+
Na+Na+
K+K+
K+
K+
Na+
Na+
Na+
ATPP
P
Na+
Na+Na+
K+
K+
P
Pi
K+
K+
Secondary Active Transport Mechanisms
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
Receptor-Mediated Endocytosis
Phagocytosis
Bulk-phase Endocytosis
Cytoplasm - 2 Components1. 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
Network of protein filaments throughout the cytosol
Provides structural support for the cell
The Cytoskeleton
Types Microfilaments
Intermediate
filaments
Microtubules
The Cytoskeleton
Organelles Centrosome - located near the nucleus, consists of two
centrioles and pericentriolar material
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
Organelles
Organelles Ribosomes - sites of protein synthesis
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
-Smooth ER - a network of membrane tubules, does not have ribosomes, synthesizes fatty acids and steroids, detoxifies certain drugs
Organelles Golgi complex - consists of 3–20 flattened,
membranous sacs called cisternae.
Processing and Packaging
Organelles Lysosomes - vesicles that form from the Golgi
complex and contain powerful digestive enzymes
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
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
Mitochondria
Nucleus
Packing of DNA into a Chromosome of a Dividing Cell
Overview of Gene Expression
The Central DogmaDNA > RNA > Protein.
Translation
Translation
Translation
Translation
Translation
Translation
Translation
Transcription
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
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
The Cell Cycle
DNA Replication
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.
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.
Interphase begins when cytokinesis is complete .
Mitosis
Mitosis
Mitosis
Mitosis
Mitosis
Mitosis
1
Pericentriolar material
NucleolusNuclear envelopeChromatin
Plasma membraneCytosol
(a) INTERPHASE
CentriolesCentrosome:
all at 700xLM
1
LateEarly
Pericentriolar material
NucleolusNuclear envelopeChromatin
Plasma membraneCytosol
Chromosome(two chromatidsjoined atcentromere
(a) INTERPHASE
(b) PROPHASE
CentriolesCentrosome:
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
2
all at 700xLM
Centromere
1
Pericentriolar material
NucleolusNuclear envelopeChromatin
Plasma membraneCytosol
Metaphase plate
(a) INTERPHASE
CentriolesCentrosome:
(c) METAPHASE
2
3
LateEarly (b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
all at 700xLM
Chromosome(two chromatidsjoined atcentromere
Centromere
1
EarlyLate(d) ANAPHASE
Pericentriolar material
NucleolusNuclear envelopeChromatin
Plasma membraneCytosol
Chromosome
(a) INTERPHASE
CentriolesCentrosome:
(c) METAPHASE
2
3
4
Cleavage furrow
LateEarly (b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
Metaphase plate
all at 700xLM
Chromosome(two chromatidsjoined atcentromere
Centromere
1
EarlyLate(d) ANAPHASE
Pericentriolar material
NucleolusNuclear envelopeChromatin
Plasma membraneCytosol
(a) INTERPHASE
CentriolesCentrosome:
Cleavage furrow
(e) TELOPHASE
(c) METAPHASE
2
3
4
5
Cleavage furrow
LateEarly (b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
Metaphase plate
Chromosome
all at 700xLM
Chromosome(two chromatidsjoined atcentromere
Centromere
1
EarlyLate(d) ANAPHASE
Pericentriolar material
NucleolusNuclear envelopeChromatinPlasma membraneCytosol
(a) INTERPHASE
CentriolesCentrosome:
(f) IDENTICAL CELLS IN INTERPHASE
Cleavage furrow
(e) TELOPHASE
(c) METAPHASE
Cleavage furrow
2
3
4
5
6
LateEarly (b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
Metaphase plate
Chromosome
all at 700xLM
Centromere
Chromosome(two chromatidsjoined atcentromereMitosis
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).
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.
Meiosis and Cytokinesis
Comparison of Mitosisand Meiosis
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.
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