Unit 3 Plasma Membrane: Structure, Composition, and Transport Mechanisms

Plasma membrane

1. Structure of the membrane 2. Composition of the membrane 3. Membrane transport 4. Cell communication 5. Specializations of the membrane: Cell junctions

Fibers of extra- celludat matriz (ECM)

1

Carbohydrate Glyco- protein

EXTRACELLULAR SIDE CE MEMBRANE

Cholesterol

Maoflament of cytoskeleton

Parpheril proteins

İndegral protein

A Figure 7.3 Updated model of an animal cell's plasma membrane (cutaway view).

CYTOPLASMIC SIDE OF MEMBRANE

QUESTION EVERYTHING

ul Universidad Europeaue

Membrana Plasmática: Comunicación

Cell communication

The signaling cell produces a signaling molecule that is detected by the target cell.

SENDING CELL

TARGET CELL

NON-TARGET CELL

RECEPTOR

NO RECEPTOR FOR LIGAND

LIGAND

RESPONSE

CHEMICAL MESSENGERS INSIDE THE CELL RELAY - THE SIGNAL

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In order to detect a signal (that is, to be a target cell), a cell must have the right receptor for that signal.

Signaling molecules are the ligands of those receptors.

• Most animal cells send and receive signals, acting as both signaling cells and target cellsue

Membrana Plasmática: Comunicación

Cell communication: What needs to happen for cells to communicate?

1. Stimulus reception: A message molecule is recognized by a specific receptor on the membrane or inside the cell.
2. Transduction of signal: The signal is transmitted across the membrane to effector molecules on the inner surface or within the cytoplasm.
3. Cellular response: Changes in gene expression, enzymatic activity, or ionic concentration.
4. End of response by destruction or inactivation of the signal molecule.

Signal Molecule

Receptor

1

Reception

Cytoplasm

Extracellular Fluid

Plasma Membrane

Relay Molecule

2

Transduction

Signal-Transduction Pathway

3

Response

Activation of Cellular Responses

Membrana Plasmática: Comunicación

Importance of cell-to-cell communication

"To regulate their development and tissue organization " To control their growth and division " To coordinate their functions

They communicate in three ways

1. They secrete chemical compounds that carry signals to other distant cells
2. Through direct contacts between membrane-bound proteins
3. GAP junctions: directly communicate the two cytoplasms

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Moreculos de comunicación a Satinca

Señalización por contacto

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Membrana Plasmática: Comunicación

Comunicación Celular

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Through: Signal molecules

Through: GAP junctions

Through: Direct contact

Unión tipo gap

Antígeno

Célula presentadora

Conexón

Moléculas de comunicación intercelular a distancia

Célula T helper

Señalización por contacto

Membrana Plasmática: Comunicación

Comunicación Celular

ue

Through: Signal molecules

Through: GAP junctions

Through: Direct contact

Unión tipo gap

Antígeno

Célula presentadora

Conexón

Moléculas de comunicación intercelular a distancia

Célula T helper

Señalización por contacto

Inside Chemical Signaling

Signaling mechanisms

Autocrine/Paracrine

Receptor

Cells secrete local chemical signals, which are so quickly captured, destroyed, or immobilized that they only act on nearby cells (or the sending cell itself, autocrine)

(Short distance)

Synaptic or neuronal

Neurotransmitter

Neurotransmitter

Neuron

Neuron

Electrical signal

Electrical signal

Target cell

Neuron

Electrical signal

It is a special kind of paracrine signalling but only occurs in the nervous system.

Cells secrete neurotransmitters into the synaptic cleft as a response to an electrical signal.

Then neurotransmitters bind to receptors in the postsynaptic cell.

Endocrine

Endocrine cell

Cell without receptor

Cell with receptor

-Target cell

No response

Response

Specialized endocrine cells secrete hormones, which travel through the bloodstream and act on target cells that are widely distributed throughout the body.

(Long distance)

Neuroendocrine

Neurohormone

Neuron

Cell without receptor

Cell with receptor

No response

Response

Same mechanism as in the endocrine system, but hormones are secreted by neurons.

Hormone

Membrana Plasmática: Comunicación

EXTRACELLULAR SIGNAL MOLECULE

1

2

O

0

C

O

O

0

-

O

O

0

Active cell-surface receptor

INTRACELLULAR SIGNALING

Transduction

TARGET PROTEINS

metabolic enzyme

gene regulatory protein

cytoskeletal protein

CELLULAR RESPONSE

altered metabolism

altered gene expression

altered cell shape or movement

76

Nucleus

Modification of gene expression, development

• The same signal can induce different responses in different target cells.

5

Responding cell

8

Signal transduction proteins and second messengers

6

Modification of cellular metabolism, function, movement

7a

Effector protein

What happens once the signal molecule reaches its target cell?

RECEPTOR PROTEIN

Signaling cell

3

9

INTRACELLULAR SIGNALING PROTEINS

Inactive cell-surface receptor

Membrana Plasmática: Comunicación

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According to the chemical nature of the signal molecules

• Hydrophilic
• Hydrophobic

According to the type of signalling molecule

• Neurotransmitters
• Hormones
• Growth factors
• Pheromones

Receptor in cytosol

Lipophilic signal molecules

-

Receptor in nucleus

Lipophobic or lipophilic signal molecule

Receptor on surface of cell membrane

Membrana Plasmática: Comunicación

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Chemical nature of signal molecules

Depending on the chemical nature of the signal molecule, the mechanism of action is different

Hydrophilic/polar

Hydrophobic/nonpolar

External environment

Cytoplasm

Signal transduction pathway

Cellular response

Membrane receptor

Hydrophilic ligand

Signal transduction pathway

Cellular response

Intracellular receptor

Hydrophobic ligand

Plasma membrane

Membrana Plasmática: Comunicación

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Chemical nature of signal molecules: Hydrophobic/nonpolar

• They are transported associated with proteins in plasma
• They have cytoplasmic or nuclear receptors on the target cell

(B)

INTRACELLULAR RECEPTORS

small hydrophobic signal molecule

target cell

carrier protein

nucleus

intracellular receptor protein

Mechanism of action of hydrophobic molecules

1. Binding of the signal molecule to its receptor in the cytoplasm or nucleus. 2. Receptor-ligand enters the nucleus of the target cell and interacts with the DNA 3. Induces the expression of specific genes 4. Synthesis of specific proteins

i.e. Steroid hormones

Adosterone

CHOH

Cortinal

CHOH

0

HSC

HO

HC

0

Estradiol

Pregnencione

DO

HIG

HC

HO

Testosterone

OH

HC

HC

Lipid-soluble hormone

Plasma membrane

1

Nuclear membrane

Ribosome

2

Nuclear receptor

3

Hormone-receptor complex

mRNA

5

Hormone- response element

mRNA synthesis

6

4

Proteins produced

mRNA

Nuclear pore

OH

Progesterone

CH

Membrana Plasmática: Comunicación

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Chemical nature of signal molecules: Hydrophilic/polar

• They travel in the bloodstream dissolved in plasma
• Their receptors are in the plasma mebrane of the target cells.

(A)

CELL-SURFACE RECEPTORS

plasma membrane

cell-surface receptor protein

hydrophilic signal molecule

target cell

Membrana Plasmática: Comunicación

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Mechanism of action of hydrophilic molecules

1. The signal molecule arrives and binds to the specific receptor 2. A signaling cascade occurs that amplifies the signal through second messengers 3. Cellular response occurs

2203

Intracellular signaling cascade through second messengers.

-One signal molecule

G-protein coupled receptor

Adenylyl cyclase

ATP

G protein

- CAMP

Protein kinase A

Phosphorylated- protein

Cell response

Extracellular fluid

Receptor ligand complex

L

Membrane

R

Intracellular fluid

One ligand is amplified into many intracellular molecules

Membrana Plasmática: Comunicación

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Mechanism of action of hydrophilic molecules: Ligand-gated channels

The arrival of the signal molecule opens the channel, which allows the passage of specific ions (Ca2+, Cl- , Na+)

1 Acetylcholine binds to two of the five AChR subunits, causing the channel to change shape and open.

Outside of cell

Na+

Acetylcholine (ACh)

Plasma membrane

000

2 The channel is lined with negatively charged amino acids, allowing Na+ to flow into the cell.

9

Acetylcholine receptor (AChR)

Inside of cell

3 Na+ buildup in cells leads to muscle contraction.

LIFE 8e, Figure 15.5

LIFE THE SCIENCE OF BIOLOGY, Eighth Edition @ 2007 Sinaver Associates, Inc. and W. H. Freeman & Co.

Mechanism of action of hydrophilic molecules

Types of receptors (summary)

señal

Receptor

Exterior celular

Membrana plasmática

Citoplasma

Ligand-gated ion channels

The arrival of the signal molecule opens the channel, which allows the flow of ions (Ca2+, Cl-, Na+)

G-protein-coupled receptors

The arrival of the ligand causes the receptor to interact with an intracellular protein called protein G, which activates a signaling cascade.

Receptors with enzymatic activity

The binding of the ligand to the receptor modifies the enzymatic activity (tyrosine kinase, tyrosine phosphatase ... ) of another area of the protein on the cytoplasmic or inner side of the cell, causing an intracellular signaling cascade

Membrana Plasmática: Comunicación

ue

Mechanism of action of hydrophilic molecules: G protein-coupled receptors

(A)

(B)

(C)

1 Hormone binding to the receptor activates the G protein. GTP replaces GDP.

2 Part of the activated G protein activates an effector protein that causes changes in cell function.

Outside of cell

Signal (hormone)

Activated effector protein

/ GDP

GTP

GDP

G protein- linked receptor

nactive G protein

Inactive effector protein

Activated G protein

The GTP on the G protein is hydrolyzed to GDP.

Cellular responses

Inside of cell

When a G-protein-coupled receptor binds its extracellular signal molecule, the activated receptor signals to a G protein on the opposite side of the plasma membrane, which then turns on (or off) an enzyme (or an ion channel; not shown) in the same membrane.

G protein coupled receptors

Mechanism of action of hydrophilic molecules

G protein-coupled receptors

Ligand Receptor G protein Enzyme

(1st messenger)

2nd messenger

Ligand" (1st messenger) binds to the receptor. The receptor changes shape and activates.

The activated receptor binds to a G protein and activates it.

The G protein changes shape (turns "on"), causing it to release GDP and bind GTP (an energy source).

Activated G protein activates (or inactivates) an effector protein by causing its shape to change.

Extracellular fluid

Effector protein (e.g., an enzyme

Ligand

Receptor

GTP

C

Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell.

Common 2nd messengers include cyclic AMP and Ca2+.)

GTP

G protein

GTP

GDP

Inactive 2nd- messenger

Active 2nd messenger

Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes.

Activated Kinase enzymes

388

• Ligands include hormones and neurotransmitters.

Cascade of cellular Responses

(The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.)

Kinase enzymes activate other enzymes. Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various metabolic and structural changes in the cell.

Intracellular fluid

Second messengers:

ue

They are intracellular molecules that participate in the transmission of the signal to the target molecule.

Signal amplification occurs

Signaling cascade

L

Cellular response

Ca2+

Common 2nd messengers:

AMPc

GMPc

IP 3