Transcript 29 - Alamo Colleges

Chapter 3
Cells: The Living Units
Cell Theory
1.
2.
3.
4.
The cell is the basic structural and functional
unit of life
Organismal activity depends on individual and
collective activity of cells
Biochemical activities of cells are dictated by
subcellular structure
Continuity of life has a cellular basis
Chromatin
Nuclear envelope
Nucleus
Nucleolus
Plasma
membrane
Smooth endoplasmic
reticulum
Cytosol
Lysosome
Mitochondrion
Centrioles
Rough
endoplasmic
reticulum
Ribosomes
Centrosome
matrix
Golgi apparatus
Microvilli
Secretion being released
from cell by exocytosis
Microfilament
Microtubule
Intermediate
filaments
Peroxisome
Figure 3.2
Plasma Membrane



Separates intracellular fluids from extracellular
fluids
Plays a dynamic role in cellular activity
Glycocalyx – “fingerprint” of the cell
Fluid Mosaic Model


Double bilayer of lipids with imbedded proteins
Bilayer consists of phospholipids, cholesterol,
and glycolipids
Glycolipids are lipids with bound carbohydrate
 Phospholipids have hydrophobic and hydrophilic poles

Fluid Mosaic Model
Figure 3.3
Functions of Membrane Proteins

Transport

Enzymatic activity

Receptors for signal
transduction
Figure 3.4.1
Functions of Membrane Proteins

Intercellular adhesion

Cell-cell recognition

Attachment to
cytoskeleton and
extracellular matrix
Figure 3.4.2
Membrane Junctions

Tight junction – impermeable junction that joins
cells

Desmosome – anchoring junction scattered along
the sides of cells

Gap junction – allows chemical substances to
pass between cells for communication
Membrane Junctions: Tight
Junction
Figure 3.5a
Membrane Junctions:
Desmosome
Figure 3.5b
Membrane Junctions: Gap
Junction
Figure 3.5c
Passive Membrane Transport:
Diffusion


Diffusion is movement of substances from high
to low concentrations
Diffusion occurs along a concentration gradient
Passive Membrane Transport:
Simple Diffusion

Simple diffusion – nonpolar and lipid-soluble
substances
Diffuse directly through the lipid bilayer
 Diffuse through channel proteins
 Nonpolar, lipid-soluble, and hydrophobic are
synonyms

Passive Membrane Transport:
Facilitated Diffusion

Facilitated diffusion
Transport of glucose, amino acids, and ions
 Transported substances bind carrier proteins or pass
through protein channels
 Carrier proteins are transmembrane and are specific
for certain polar molecules
 Polar, lipid-insoluble, and hydrophilic are synonyms

Diffusion Through the Plasma
Membrane
Extracellular fluid
Lipidsoluble
solutes
Lipid-insoluble
solutes
Small lipidinsoluble
solutes
Water
molecules
Lipid
bilayer
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis, diffusion
through a specific
channel protein
(aquaporin) or
through the lipid
bilayer
Figure 3.7
Passive Membrane Transport:
Osmosis



Diffusion of water across a semipermeable
membrane due to differences in concentration
of a solvent across the membrane
Osmolarity – total concentration of solute
particles in a solution
Tonicity – how a solution affects cell volume
Effect of Membrane Permeability
on Diffusion and Osmosis
Figure 3.8a
Effect of Membrane Permeability
on Diffusion and Osmosis
Figure 3.8b
Effects of Solutions of Varying
Tonicity



Isotonic – solutions with the same solute
concentration as that of the cytosol
Hypertonic – solutions having greater solute
concentration than that of the cytosol
Hypotonic – solutions having lesser solute
concentration than that of the cytosol
Passive Membrane Transport:
Filtration


The passage of water and solutes through a
membrane by hydrostatic pressure
Pressure gradient pushes solute-containing fluid
from high to low pressure
Active Transport


Uses ATP to move solutes across a membrane
Requires carrier proteins
Extracellular fluid
K+ is released and
Na+ sites are ready to
bind Na+ again; the
cycle repeats.
Binding of cytoplasmic
Na+ to the pump protein
stimulates phosphorylation
by ATP.
Na+
Na+
Na+
Na+
Na+
Cytoplasm
Na+
K+
ATP
P
K+
ADP
Cell
K+
K+
Na+
Na+
Phosphorylation
causes the
protein to
change its shape.
Na+
Concentration gradients
of K+ and Na+
Na+
Na+
K+
K+
K+
P
K+
Loss of phosphate
restores the original
conformation of the
pump protein.
P
Pi
The shape change
expels Na+ to the
outside, and
extracellular K+ binds.
K+ binding triggers
release of the
phosphate group.
Figure 3.10
Types of Active Transport


Symport system – two substances are moved
across a membrane in the same direction
Antiport system – two substances are moved
across a membrane in opposite directions
Types of Active Transport


Primary active transport – hydrolysis of ATP
phosphorylates the transport protein causing
movement of solutes
Secondary active transport – indirect use of a
primary pump to drive the transport of other
solutes through a second channel
Types of Active Transport
Figure 3.11
Active Transport:
Vesicular Transport



Transcytosis – moving substances into, across,
and then out of a cell
Vesicular trafficking – moving substances from
one area in the cell to another
Phagocytosis – pseudopods engulf foreign objects
and bring them into the cell
Active Transport:
Vesicular Transport




Fluid-phase endocytosis – the plasma membrane
brings extracellular fluid and solutes into the cell
Receptor-mediated endocytosis – a ligand binds
to a receptor on the cell membrane, and the
complex is brought into the cell
Exocytosis – release of a substance from a cell
through a vesicle
Transcytosis, receptor-mediated endocytosis,
and phagocytosis all use clathrin-coated pits
Exocytosis
Figure 3.12a
Clathrin-Mediated Endocytosis
Extracellular
fluid
Cytoplasm
Clathrincoated
pit
Recycling of
membrane and
receptors (if present)
to plasma membrane
1
Ingested
substance
Exocytosis
of vesicle
contents
Clathrin
protein
Uncoated
vesicle
Detachment
of clathrincoated
vesicle
Plasma
membrane
Extracellular
fluid
Plasma
membrane
Clathrincoated
vesicle
Endosome
3
Transcytosis
Uncoating
Uncoated
vesicle
fusing with
endosome
2
To lysosome
for digestion
and release
of contents
(a) Clathrin-mediated endocytosis
Figure 3.13a
Phagocytosis
Figure 3.13b
Receptor Mediated Endocytosis
Figure 3.13c
Passive Membrane Transport –
Review
Process
Energy Source
Example
Simple diffusion
Kinetic energy
Movement of O2 through membrane
Facilitated diffusion
Kinetic energy
Movement of glucose into cells
Osmosis
Kinetic energy
Movement of H2O in & out of cells
Filtration
Hydrostatic pressure
Formation of kidney filtrate
Active Membrane Transport –
Review
Process
Energy Source
Example
Active transport of solutes
ATP
Movement of ions across
membranes
Exocytosis
ATP
Neurotransmitter secretion
Endocytosis
ATP
White blood cell phagocytosis
Fluid-phase endocytosis
ATP
Absorption by intestinal cells
Receptor-mediated endocytosis
ATP
Hormone and cholesterol uptake
Membrane Potential


Voltage across a membrane
Resting membrane potential
Ranges from –20 to –200 mV
 Results from Na+ and K+ concentration gradients
across the membrane


Steady state – potential maintained by active
transport of ions (sodium-potassium pump)
Generation and Maintenance of Resting
Membrane Potential



Potassium leak channels allow K+ to exit the cell
Negatively charged proteins are trapped within
the cell
The sodium-potassium pump ejects 3 positive
charges for every 2 positive charges that it brings
into the cell
Roles of Membrane Receptors




Contact signaling – important in normal
development and immunity
Electrical signaling – voltage-regulated “ion
gates” in nerve and muscle tissue
Chemical signaling – neurotransmitters bind to
chemically gated receptors in nerve and muscle
tissue
G protein-linked receptors – ligands bind to a
receptor which activates a G protein, causing the
release of a second messenger
Operation of a G Protein


An extracellular ligand (first messenger), binds to a
specific receptor
The receptor activates a G protein that relays the
message to an effector protein
Operation of a G Protein




The effector is an enzyme that produces a second
messenger inside the cell
The second messenger activates a kinase
The activated kinase can trigger a variety of
cellular responses
This pathway is important for amplification of a
signal
Operation of a G Protein
Extracellular fluid
First messenger
(ligand)
Effector
(e.g., enzyme)
1
4
3
2
Membrane
receptor
G protein
Active
second
messenger
(e.g., cyclic
AMP)
5
Inactive
second
messenger
Activated
(phosphorylated)
kinases
6
Cascade of cellular responses
(metabolic and structural changes)
Cytoplasm
Figure 3.16
Cytoplasm

Cytoplasm – material between plasma
membrane and the nucleus
Cytosol – largely water with dissolved protein, salts,
sugars, and other solutes
 Cytoplasmic organelles – metabolic machinery of the
cell

Cytoplasmic Organelles


Specialized cellular compartments
Membranous


Mitochondria, peroxisomes, lysosomes, endoplasmic
reticulum, and Golgi apparatus
Nonmembranous

Cytoskeleton, centrioles, and ribosomes
Mitochondria



Double membrane structure with shelf-like
cristae
Provide most of the cell’s ATP via aerobic
cellular respiration
Contain their own DNA and RNA
Mitochondria
Figure 3.17a, b
Ribosomes




Granules containing protein and rRNA
Site of protein synthesis
Free ribosomes synthesize soluble proteins
Membrane-bound ribosomes synthesize proteins
to be incorporated into membranes
Endoplasmic Reticulum (ER)



Interconnected tubes and parallel membranes
enclosing cisternae
Continuous with the nuclear membrane
Two varieties – rough ER and smooth ER
Endoplasmic Reticulum (ER)
Figure 3.18a, c
Rough ER



External surface studded with ribosomes
Manufactures all secreted proteins
Responsible for the synthesis of integral
membrane proteins and phospholipids for cell
membranes
Signal Mechanism of Protein
Synthesis



mRNA – ribosome complex attaches to rough
ER at a signal-recognition particle (SRP)
SRP is released and polypeptide grows into
cisternae
The protein is released into the cisternae and
sugar groups are added
Signal Mechanism of Protein
Synthesis


The protein folds into a three-dimensional
conformation
The protein is transported to the Golgi
apparatus
Signal Mechanism of Protein
Synthesis
Cytosol
Transport
vesicle
budding off
Coatomercoated
transport
vesicle
5
Ribosomes
mRNA
3
4
Sugar
group
2
1
Released
glycoprotein
Signal
Receptor
sequence site
Signalrecognition
particle
(SRP)
Growing
polypeptide
Signal
sequence
removed
ER
cisterna
ER
membrane
Figure 3.19
Smooth ER


Tubules arranged in a looping network
Catalyzes the following reactions
In the liver – lipid and cholesterol metabolism,
breakdown of glycogen, and detoxification of drugs
 In the testes – synthesis of steroid-based hormones
 In the intestinal cells – absorption, synthesis, and
transport of fats
 In skeletal and cardiac muscle – storage and release
of calcium
