Biology of the Cell презентация

Understanding the Cell
All body processes dependent upon cells for their activities
Cells known as “the functional units of the body”
Knowledge of cell structure and function crucial for understanding anatomy and physiology

Introduction to Cells: How Cells Are Studied
Cells
Studied through the discipline of cytology
Discovered after the invention of microscopes
Measured in micrometers (1/10,000 cm)
Microscopy
The use of a microscope to view small-scale structures
Accomplished through staining techniques to provide contrast

Microscopy

TEM vs. SEM

Figure 4.1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
TEM 50,000x
(c) Scanning electron microscopy
(b) Transmission electron microscopy
(a) Light microscopy
SEM 3000x
LM 720x
a: © The McGraw-Hill Companies, Inc./Al Telser, photographer; b: © VVG/SPL/Photo Researchers, Inc.
c: © Eye of Science/Photo Researchers, Inc.
Cilia
Cilia
Cilia

Introduction to Cells: Cell Size and Shape
Cells vary greatly in size and shape
E.g., an erythrocyte between 7-8 nm
E.g., an oocyte of 120 nm
Most microscopic
Shapes spherical, cubelike, columnlike, cylindrical, disc-shaped, or irregular

Figure 4.2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Electron microscope
Unaided eye
Light microscope
Most bacteria
Large macromolecules (proteins)
Small molecules (amino acids)
Atom
0.1 nm
1 nm
10 nm
100 nm
1µm
10 µm
100 µm
Human
oocyte
1 mm
1 cm
0.1 m
1 m
10 m
Size
Human height
Some muscle and
nerve cells
Ostrich egg
Red blood cell
Mitochondrion
Viruses
Ribosomes
Most plant and animal cells
(average ~ 30 µm)

Figure 4.3
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Irregular: Nerve cells
Biconcave disc: Red blood cells
Cube-shaped: Kidney tubule cells
Column-shaped: Intestinal
lining cells
Spherical: Cartilage cells
Cylindrical: Skeletal muscle cells

Introduction to Cells: Common Features
and General Functions
Overview of Cellular Components
Plasma membrane
Forms the outer limiting barrier
Separates internal contents of cell from external environment
Cilia, flagellum, microvilli
modified extension of plasma membrane

Plasma Membrane

Introduction to Cells: Common Features
and General Functions
Overview of Cellular Components (continued)
Nucleus
Largest structure in the cell
Enclosed by a nuclear envelope
Contains the genetic material, DNA
Inner fluid called nucleoplasm
Cytoplasm
Cellular contents between plasma membrane and the nucleus
Includes cytosol, organelles, and inclusions

Nucleus

Cytoplasm
Cytoplasm
Nucleus
Mitochondria
Peroxisomes
Vesicles

Introduction to Cells: Common Features
and General Functions
Cytoplasmic Components
Cytosol (intracellular fluid)
Viscous fluid of the cytoplasm
High water content
Contains dissolved macromolecules and ions

Introduction to Cells: Common Features
and General Functions
Cytoplasmic Components (continued)
Organelles
Organized structures within cells
“Little organs”
Unique shape and function
Membrane-bound organelles
enclosed by a membrane
separates contents from the cytosol
e.g., endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria

Introduction to Cells: Common Features
and General Functions
Cytoplasmic Components
Organelles (continued)
Non-membrane-bound organelles
not enclosed within a membrane
generally composed of protein
e.g., ribosomes, cytoskeleton, centrosome, proteasomes
Inclusions
Large diverse group of molecules
not bound by membrane
not considered organelles
e.g., pigments, glycogen, triglycerides

Figure 4.4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Flagellum
Non-membrane-
bound organelles
Ribosomes
Free
ribosomes
Fixed
ribosomes
Centrosome
Proteasome
Cytoskeleton
Cytosol
(intracellular fluid)
Inclusions
Vesicle
Cilia
Microvilli
Modifications of
plasma membrane
Plasma membrane
Cytoplasm
Nucleus
Nuclear membrane
Nucleoplasm
Nucleolus
Membrane-bound organelles
Rough endoplasmic reticulum
Smooth endoplasmic reticulum
Mitochondrion
Golgi apparatus
Peroxisome
Lysosome

The Structure of a Cell

Introduction to Cells: Common Features
and General Functions
General Cell Functions
Performed by most cells
Maintain integrity and shape of cell
dependent on plasma membrane and internal contents
Obtain nutrients and form chemical building blocks
harvest energy for survival
Dispose of wastes
avoid accumulation disrupting cellular activities

Introduction to Cells: Common Features
and General Functions
General Cell Functions (continued)
Performed by some cells
Cell division
make more cells of the same type
help maintain the tissue by providing new cells

Plasma Membrane
Inner leaflet
Outer leaflet
Plasma membranes
Cytoplasm
Extracellular matrix

Membrane Lipids
Membrane Proteins
Membrane Carbohydrates
Components of Plasma Membrane

Plasma Membrane

Chemical Structure of the Plasma Membrane: Lipid Components
Phospholipids
Most membrane lipids of this type
Polar “head” and two hydrophobic “tails”
Form two parallel sheets of molecules
Lie tail to tail with tails forming internal area membrane
Head directed outward
Structure termed phospholipid bilayer
Ensures cytosol and fluid surrounding cells remain separate
surrounding fluid termed interstitial fluid

Phospholipid Bilayer

Phospholipid Molecules
Fatty Acid Tails
Polar Heads

Outer Leaflet
Inner Leaflet

Chemical Structure of the Plasma Membrane: Lipid Components
Cholesterol
Scattered within phospholipid bilayer
Strengthens the membrane
Stabilizes the membrane against temperature extremes
Glycolipids
Lipids with attached carbohydrate groups
Located on outer phospholipid region only
Helps to form the glycocalyx
the “coating of sugar” on cell’s surface

Figure 4.5
b: © Don W. Fawcett/Photo Researchers, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a) Plasma membrane
(b) Phospholipid bilayer
Cytosol
Polar head of
phospholipid
molecule
Phospholipid
Glycolipid
Carbohydrate
Interstitial fluid
Cholesterol
Integral protein
Peripheral protein
Filaments of
cytoskeleton
1. Physical barrier: Establishes a flexible boundary, protects cellular contents, and supports cell structure. Phospholipid bilayer separates substances inside and outside the cell
2. Selective permeability: Regulates entry and exit of ions, nutrients, and waste molecules
through the membrane
3. Electrochemical gradients: Establishes and maintains an electrical charge difference
across the plasma membrane
4. Communication: Contains receptors that recognize and respond to molecular signals
Functions of Plasma Membrane
Cytosol
Phospholipid
bilayer
Phospholipid
bilayer
Cytosol
Protein
Glycoprotein
Nonpolar tails
of phospholipid
molecule
Phospholipid bilayer

Membrane Lipid
Cholesterol

Membrane Lipid
Glycolipid

Membrane Carbohydrates
Glycocalyx

Chemical Structure of the Plasma Membrane: Membrane Proteins
Membrane proteins
Compose half of plasma membrane by weight
Can “float” and move about fluid bilayer
Most of a membrane’s functions determined by resident proteins
Classified as integral or peripheral proteins

Membrane Protein

Transmembrane Proteins

Chemical Structure of the Plasma Membrane: Membrane Proteins
Integral proteins
Embedded within and extend across lipid bilayer
Hydrophobic regions interacting with hydrophobic interior
Hydrophilic regions interacting with hydrophilic regions
Often glycoproteins with carbohydrate portion
Peripheral proteins
Not embedded in lipid bilayer
Attach loosely to surfaces of the membrane

Channel Pore

Peripheral Protein

Glycoprotein

Chemical Structure of the Plasma Membrane: Membrane Proteins
Often categorized functionally
Transport proteins
regulate movement of substances across membrane
e.g., channels, carriers, and pumps
Cell surface receptors
bind ligand molecules released from a specific cell
bind receptors on another cell
e.g., neurotransmitters and hormones

Chemical Structure of the Plasma Membrane: Membrane Proteins
Often categorized functionally (continued)
Identity markers
communicate to other cells
e.g., immune system cells distinguishing healthy cells from foreign cells
Enzymes
catalyze chemical reactions

Chemical Structure of the Plasma Membrane: Membrane Proteins
Often categorized functionally (continued)
Anchoring sites
Secure cytoskeleton to plasma membrane
Cell-adhesion proteins
Perform cell to cell attachments

Figure 4.6
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Interstitial
fluid
Cytosol
Transport protein
Receptor
Ligand
Identity marker
Enzyme
Product
Substrate
Anchoring site
Cytoskeleton protein
Cell-adhesion protein
Interstitial
fluid

Membrane Transport
One important function of plasma membrane
Regulating movement of materials into and out of a cell
requires substances from interstitial fluid
requires waste elimination into interstitial fluid
occurs through processes of membrane transport
can be categorized as passive or active transport

Membrane Transport
Passive processes of membrane transport
Do not require energy
Depend on substances moving down concentration gradient
move from where there is more of a substance to where there is less
Two types:
diffusion
osmosis

Membrane Transport
Active processes of membrane transport
Require energy
E.g., movement of a substance up its concentration gradient
termed active transport
E.g., release of a membrane-bound vesicle
termed vesicular transport

Membrane Transport—
Passive Processes: Diffusion
Environmental conditions affecting rate of diffusion
“Steepness” of concentration gradient
measure of the difference in concentration between two areas
steeper gradient with a faster rate of diffusion
Temperature
reflects kinetic energy and random movement
higher movement with higher temperature
results in faster rate of diffusion

Figure 4.7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Membrane Transport—
Passive Processes: Diffusion
Cellular Diffusion
Simple diffusion
Molecules passing between phospholipid molecules
Solutes small and nonpolar
Include respiratory gases (O2 and CO2), some fatty acids, ethanol, urea
Cannot be regulated by plasma membrane
Movement dependent on concentration gradient alone
Continue to move as long as gradient exists

Figure 4.8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Small nonpolar solutes move down
their concentration gradients.
Interstitial
fluid
Cytosol
Carbon dioxide
Oxygen

Membrane Transport—
Passive Processes: Diffusion
Cellular Diffusion (continued)
Facilitated diffusion
Transport process for small charged or polar solutes
Require assistance from plasma membrane proteins
Two types of facilitated diffusion
channel-mediated diffusion
carrier-mediated diffusion
Maximum rate of transport determined by number of channels and carriers
higher rate with greater number of transport proteins

Membrane Transport—
Passive Processes: Diffusion
Cellular Diffusion (continued)
Facilitated diffusion
Transport process for small charged or polar solutes
Require assistance from plasma membrane proteins
Two types of facilitated diffusion
channel-mediated diffusion
carrier-mediated diffusion
Maximum rate of transport determined by number of channels and carriers
higher rate with greater number of transport proteins

Membrane Transport—
Passive Processes: Diffusion
Cellular Diffusion (continued)
Channel-mediated diffusion
Movement of small ions through water-filled protein channels
Channels specific for one ion type
Leak channels
continuously open
Gated channel
usually closed
open in response to stimulus

Figure 4.9a
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ions move down their concentration
gradient through water-filled channels.
Na+
K+ leak
channel
Interstitial
fluid
Cytosol
Na+ leak channel
K+
(a) Channel-mediated diffusion

Membrane Transport—
Passive Processes: Diffusion
Cellular Diffusion (continued)
Na+ channels
Na+ leak channels
allow Na+ to pass through continuously
Chemically gated Na+ channels
allow Na+ to move through in response to a particular chemical

Membrane Transport—
Passive Processes: Diffusion
Cellular Diffusion (continued)
Carrier-mediated diffusion
Small, polar molecules assisted across membrane by carrier protein
Transport substances such as glucose
Binding of substance causing change in carrier protein shape
Releases substances on other side of membrane
Move substances down their gradient
Carrier transporting only one substance termed a uniporter

Figure 4.9b
Interstitial
fluid
Cytosol
(b) Carrier-mediated diffusion
Glucose carrier protein
Carrier proteins change shape to transport
molecules across the plasmamembrane.
Glucose
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Membrane Transport—
Passive Processes: Osmosis
Osmosis
Passive movement of water through selectively permeable membrane
membrane allowing passage of water
membrane preventing passage of most solutes
Occurs in response to differences in water concentration
different concentrations on either side of a membrane

Membrane Transport—
Passive Processes: Osmosis
Plasma Membrane: A Selectively Permeable Membrane
Two ways water crosses membrane
“Slip between” molecules of phospholipid bilayer
Moves through integral protein water channels
termed aquaporins

Membrane Transport—
Passive Processes: Osmosis
Plasma Membrane: A Selectively Permeable Membrane (continued)
Two types of solutes
Permeable solutes
pass through bilayer
small and nonpolar solutes
e.g., oxygen, carbon dioxide
Nonpermeable solutes
prevented from passing through bilayer
charged, polar, or large solutes
e.g., ions, glucose, proteins

Membrane Transport—
Passive Processes: Osmosis
Concentration Gradients Across the Plasma Membrane
Differences in solute concentration across membrane
May exist between cytosol and interstitial fluid
Also cause water concentrations to exist
Greater concentration of solutes with lower concentration of water

Membrane Transport—
Passive Processes: Osmosis
Movement of Water Into or Out of a Cell by Osmosis
Net movement of water by osmosis
Dependent on concentration gradient between cytosol and solution
Moves down its gradient
E.g., moves from solution of 1% solutes to solution containing 3% solutes
Moves until equilibrium is reached
Equal concentration of water inside and outside cell
Moves toward solution with lower water concentration

Plasma membrane
Cytosol
Protein
Water
molecule
Interstitial
fluid
Aquaporin
Ca2+
Cl-–
Impermeable
to most solutes
(charged, polar, large)
Lower water
concentration
(higher solute
concentration)
Concentration
gradient
Glucose
Higher water
concentration
(lower solute
concentration)
Permeable to water
Na+
K+
Figure 4.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Membrane Transport—
Passive Processes: Osmosis
Osmotic Pressure
Pressure exerted by movement of water across semipermeable membrane
Due to difference in solution concentration
Steeper gradient, more water moved by osmosis
Steeper gradient, greater osmotic pressure

Membrane Transport—
Passive Processes: Osmosis
Osmotic Pressure (continued)
Figure 4.11
Semipermeable membrane allowing for passage of water only
Side A with more solutes initially
Water moving from side B to side A by osmosis
Continues until fluids equal in concentration

Figure 4.11
Side A
Side B
Side A
Side B
Semipermeable
membrane
Final setup: Water moved by osmosis from side B down the water gradient to side A until the concentrations of side A and side B are equal.
Non-permeable solutes (glucose, Na+, protein)
Water molecules
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Semipermeable
membrane
Higher solute
concentration,
lower water
concentration
Lower solute
concentration,
higher water
concentration
Initial setup: Side A contains proportionately
more solute and less water.

Membrane Transport—
Passive Processes: Osmosis
Osmotic Pressure (continued)
Can be measured indirectly
Could put stopper on side A in figure 4.11b
Could exert force to return fluid to original level
Would create hydrostatic pressure within the tube
the pressure exerted by a fluid on wall of its container
Osmotic pressure equal to hydrostatic pressure applied
= total pressure needed to return fluid to original level

Membrane Transport—
Passive Processes: Osmosis
Osmosis and Tonicity
Cell gains or loses water with osmosis
Accompanying change in cell volume and osmotic pressure
Tonicity
ability of a solution to change the volume or pressure of the cell by osmosis

Membrane Transport—
Passive Processes: Osmosis
Osmosis and Tonicity (continued)
Isotonic solution
Both cytosol and solution with same relative concentration of solutes
E.g., physiological saline with a concentration of 0.9% NaCl
Isotonic to erythrocytes
No net movement of water

Membrane Transport—
Passive Processes: Osmosis
Osmosis and Tonicity (continued)
Hypotonic solution
Solution with a lower concentration of solutes than cytosol
E.g., erythrocytes in pure water
Water moving down concentration gradient
from outside the cell to inside
Increased volume and pressure of cell
May cause cell lysis (rupture)
hemolysis, term for ruptured red blood cells

Membrane Transport—
Passive Processes: Osmosis
Osmosis and Tonicity (continued)
Hypertonic solution
Solution with a higher concentration of solutes than cytosol
E.g., erythrocytes in 3% NaCl pure water
Water moves down concentration gradient
Moves from inside the cell to outside
Decreased volume and pressure of cell
May cause cell to shrink
termed crenation

Figure 4.12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
SEM 11,550x
Isotonic solution
Hypotonic solution
Interstitial fluid is the same
concentration as cytosol.
Interstitial fluid is less
concentrated than cytosol.
Hypertonic solution
Interstitial fluid is more
concentrated than cytosol.
Water
enters
cell.
No net
movement
of water.
Water
leaves
cell.
(c)
Erythrocytes undergoing crenation
(b)
Erythrocytes nearing hemolysis
SEM 9030x
SEM 6900x
(a)
Normal erythrocytes
a: © Dennis Kunkel Microscopy, Inc./Phototake; b: © Dennis Kunkel Microscopy, Inc./Phototake; c: © Dennis Kunkel Microscopy, Inc./Phototake
Erythrocyte
Erythrocyte
Erythrocyte

Membrane Transport: Active Processes
Active Transport
Opposes the movement of solutes by diffusion
Solutes moved against a concentration gradient
Maintains gradient between cell and interstitial fluid

Membrane Transport: Active Processes
Active Transport (continued)
Primary active transport
Uses energy directly from breakdown of ATP
Phosphate group added to transport protein
Results in a change in protein’s shape
Results in movement of solute across membrane
Addition of phosphate to protein termed phosphorylation

Membrane Transport: Active Processes
Ion pumps
Active transport proteins that move ions across membrane
Help cell maintain internal concentration of ions
E.g., Ca2+ pumps in plasma membrane of erythrocytes
prevent cell rigidity from accumulated calcium
Figure 4.13
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATP
ADP
+Pi
Ca2+
Interstitial
fluid
Erythrocyte
Cytosol
Ca2+
pump

Membrane Transport: Active Processes
Active Transport (continued)
Sodium-potassium pump
Special kind of ion pump, an exchange pump
Moves one ion into cell against gradient
Moves another ion out of cell against gradient
Three Na+ pumped out for two K+ pumped in
Maintains steep membrane gradient
Requires ATP

Membrane Transport: Active Processes
Active Transport
Sodium-potassium pump (continued)
Maintains an electrochemical gradient
electrical charge difference across plasma membrane
due to unequal distribution of positive and negative substances across membrane
voltage differences termed membrane potential
at rest, termed resting membrane potential

Figure 4.14
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Three sodium ions (Na+) and ATP bind to sites on the
cytoplasmic surface of the Na+/ K+ pump.
This transport protein reverts back to its original
shape, resulting in the release of the K+ ions into
the cytosol. The Na+/ K+ pump is now ready to
begin the process again.
ATP is split into ADP and Pi, resulting in both the
binding of the Pi to the pump and release of energy
that causes the Na+/ K+ pump to change
conformation (shape) and release the Na+ ions into
the interstitial fluid.
Two K+ ions from the interstitial fluid then bind to
sites on the outer cellular surface of the Na+/ K+
pump. At the same time, the Pi produced earlier
by ATP hydrolysis is released into the cytosol.
1
2
3
P
P
Phospholipid bilayer
Cytosol
Interstitial
fluid (IF)
IF
Cytosol
4
ATP
binding
site
ATP
Na+
Breakdown of ATP
(releases energy)
Transport protein changes
shape (requires energy
from ATP breakdown)
Transport protein
resumes original
shape
Na+
K+
Cytosol
IF
ADP
Cytosol
Na+
K+
K+
Transport
protein
K+
IF
Na+/K+
Pump

Membrane Transport: Active Processes
Active Transport (continued)
Secondary active transport
Moves substance against concentration gradient
Uses energy provided by movement of second substance down gradient
Kinetic energy providing “power” to pump other substance
Na+ moving down concentration gradient
Ultimately dependent on Na+/K+ pumps for energy

Membrane Transport: Active Processes
Active Transport
Secondary active transport (continued)
Two substances moved in same direction
proteins termed symporters
process symport secondary active transport
e.g., glucose transported up its gradient into cell
Na+ and glucose moved in same direction

Membrane Transport: Active Processes
Active Transport
Secondary active transport (continued)
Two substances moved in opposite directions
proteins termed antiporters
process termed antiport secondary active transport
e.g., H+ transported up its gradient out of cell
Na+ and H+ moved in opposite directions

Figure 4.15
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Na+ diffuses down
its gradient into cell.
(b) Antiporter: Substances
move in opposite directions.
(a) Symporter: Substances
move in the same direction.
H+ is transported up
its gradient out of cell.
Antiporter
Symporter
Cytosol
Interstitial fluid
Glucose is transported
up its gradient into cell.

Membrane Transport: Active Processes
Vesicular Transport
Requires vesicles
membrane-bounded sac filled with materials
Requires energy to transport vesicles
Exocytosis
vesicle fuses with membrane
releases substances outside the cell
Endocytosis
vesicle encloses material outside cell
fuses with membrane to release inside cell

Membrane Transport: Active Processes
Vesicular Transport (continued)
Exocytosis
How large substances are secreted from cell
Macromolecules too large to be moved across membrane
Material packed within intracellular transport vehicles
Vesicle and plasma membrane fusion
requires ATP
Contents released to outside of cell
E.g., release of neurotransmitters from nerve cells

Figure 4.16
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
2
3
4
Fusion of vesicle membrane with plasma membrane
Plasma membrane opens to outside of cell
Release of vesicle components into the interstitial
fluid and integration of vesicle membrane
components into the plasma membrane
Plasma
membrane
opens
Membrane
proteins
Plasma
membrane
Vesicle membrane
Secretory
vesicle
Interstitial fluid
Cytosol
Vesicle nears plasma membrane

Membrane Transport: Active Processes
Vesicular Transport (continued)
Endocytosis
Cellular uptake of large substances from external environment
Used for the uptake of materials for digestion
Used for retrieval of membrane from exocytosis
Used for regulating membrane protein composition
to alter cellular processes
Three types:
phagocytosis, pinocytosis, and receptor-mediated endocytosis

Membrane Transport: Active Processes
Vesicular Transport (continued)
Steps of endocytosis
Substances within interstitial fluid packaged into a vesicle
Vesicle formed at cell surface
Inward fold of membrane to form pocket
termed invagination
Deepens and pinches off when layer fuses
requires energy
Intracellular vesicle with material formerly outside cell

Membrane Transport: Active Processes
Vesicular Transport (continued)
Phagocytosis
Occurs when cell engulfs large particle external to cell
Forms large extensions termed pseudopodia
Surround particle, enclosing it in membrane sac
Fuses with lysosome
contents digested here
Only in a few cell types
E.g., white blood cells engulfing microbes

Figure 4.17a
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a) Phagocytosis
Cytosol
Pseudopodia
Particle
Invagination
Interstitial
fluid
Plasma
membrane
Newly
formed
vesicle

Membrane Transport: Active Processes
Vesicular Transport (continued)
Pinocytosis
Internalization of droplets of interstitial fluid
Multiple, small vesicles formed
All dissolved solutes taken into cell
Performed by most cells
E.g., cells of capillary wall

Figure 4.17b
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(b) Pinocytosis
Cytosol
Interstitial
fluid
Plasma
membrane
Vesicle

Membrane Transport: Active Processes
Vesicular Transport (continued)
Receptor-mediated endocytosis
Movement of specific molecules from interstitial environment into a cell
Requires binding to a receptor
Enables cell to obtain bulk quantities of substances
E.g., transport of cholesterol from blood to a cell
cholesterol in blood in structures termed low-density lipoproteins
LDLs internalized by this process

Membrane Transport: Active Processes
Vesicular Transport (continued)
Steps of receptor-mediated endocytosis
Molecule binding to protein receptors in membrane
Form ligand-receptor complex
Accumulate at special regions containing clathrin protein
Fold inward to form clathrin-coated pit
Form clathrin-coated vesicle
Moves into cytosol
Fusion of lipid bilayers requiring ATP

Membrane Transport: Active Processes
Vesicular Transport (continued)
Steps of receptor-mediated endocytosis
Molecule binding to protein receptors in membrane
Form ligand-receptor complex
Accumulate at special regions containing clathrin protein
Fold inward to form clathrin-coated pit
Form clathrin-coated vesicle
Moves into cytosol
Fusion of lipid bilayers requiring ATP

Figure 4.17c
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(c) Receptor-mediated endocytosis
Cytosol
Interstitial
fluid
Plasma
membrane
Clathrin-
coated pit
Clathrin-
coated
vesicle
Receptors

Figure 4.18a
DIFFUSION: Movement of a solute from an area of higher concentration to an area of lower concentration.
Simple Diffusion: Small and nonpolar substances move between phospholipid molecules
of the plasma membrane.
Facilitated Diffusion: Small, charged, or polar substances move assisted by a transport protein (channel or carrier).
Channel-Mediated: Ion (e.g., Na+)
movement is facilitated by channels
across the plasma membrane.
Carrier-Mediated: Small polar molecule
movement (e.g., glucose) is facilitated by
protein carriers across the plasma membrane.
(a) Passive Processes
Do not require expenditure of cellular energy; substance moves
into or out of a cell down its concentration gradient.
OSMOSIS: Movement of water across a selectively permeable membrane from an area of higher water concentration
to an area of lower water concentration.
Oxygen
Interstitial fluid
Cytosol
Carbon dioxide
Carrier
Channel
Interstitial fluid
Cytosol
Interstitial fluid
Water
Cytosol
Solute
Plasma membrane
Glucose
Na+
Aquaporin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 4.18b
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Require expenditure of cellular energy; substance moved up its
concentration gradient or involves a vesicle.
Secondary Active Transport: Pumps are powered by energy harnessed as a second substance
(usually Na+) moves through a channel down a concentration gradient.
Primary Active Transport: Pumps are powered directly by splitting an ATP molecule.
Antiport: Two
substances are moved
in opposite directions.
Phagocytosis:
Movement of
large substances
into a cell.
Pinocytosis: Movement
of fluid into a cell.
Glucose
H+
ACTIVE TRANSPORT: Movement of a substance up its concentration gradient via a protein pump.
(b) Active Processes
ADP + P
ATP
Cytosol
Na+
Interstitial fluid
Receptors
Vesicles
Vesicle
Cytosol
Interstitial
fluid
Pseudopodia
Particle
Receptor-Mediated Endocytosis:
Movement of a specific substance
into a cell following the binding of
the substance to a receptor.
Plasma
membrane
opens
Transport protein changes
shape (requires energy
from ATP breakdown)
Note: The two ion species
are not simultaneously
attached to the pump
Interstitial
fluid
Cytosol
Symport: Two
substances are moved
in the same direction
VESICULAR TRANSPORT: Movement of a substance across the plasma membrane via a vesicle.
Exocytosis: Movement of a
substance out of
a cell via a vesicle.
Endocytosis: Movement of a substance into a cell via a vesicle. The three types of endocytosis
include phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Na+
K+

Membrane Transport: Active Processes
Clinical View: Familial Hypercholesteremia
Inherited genetic disorder
Defects in LDL receptor or proteins of LDLs
Interfere with normal receptor-mediated endocytosis of cholesterol
Results in greatly elevated cholesterol
Causes atherosclerosis
Greatly increased risk of heart attack