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Table 5-1
Membrane Proteins
• Constitute 1-10% of total molecules but 50% of the weight because of their larger size
• Types of proteins– Integral or intrinsic proteins
• Pass through the mem- brane
• Can form channels through the membrane
– Peripheral or extrinsic• Attached to integral
proteins or lipids at either the inner or outer surfaces of the lipid bilayer
Functions of Membrane Proteins
1. Transport - by channel or carrier proteins
2. Enzymatic activity
3. Receptors
4. Cell-cell recognition
5. Cell-adhesion functions
Extracellular signalingmolecules released by cells occurs over distancesfrom a few microns - autocrine (c)and paracrine (b) signaling toseveral meters in endocrine (a)signaling. In some instances,receptor proteins attached to themembrane of one cell interactdirectly with receptors on anadjacent cell (d).
© 2000 by W. H. Freeman and Company. All rights reserved.
General Schemes of Intercellular Signalling
• Cells may communicate by direct contact.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 11.4
Immune connection: Macrophage will use direct contact to alert T cells that invaders are present
Epithelial Cell Junction Types
Action Potential
Figure 11.15
• easily travels through the blood - hydrophilic• but cannot diffuse through plasma membrane!• therefore absolutely requires the expression of receptors
on the cell surface – integral membrane proteins that act as first messenger
• the receptor protein activates a series of signaling events within the cells– e.g. epinephrine binds to receptor and activates an
adjacent G-protein in membrane– G-protein activates adenylate cyclase to convert ATP
to cyclic AMP (cAMP) in the cytosol– cAMP acts as a 2nd messenger– cAMP activates a series of proteins in the cytosol
called kinases– kinases act to phosphorylate their targets – either
activating them or inhibiting them– this speeds up/slows down physiological responses
within the cell– phosphodiesterase inactivates cAMP quickly
• many second messengers are made in cells in response to specific hormones
– e.g. calcium, IP3, DAG• Cell response is turned off unless new hormone
molecules arrive• this mechanism allows for amplification – one H-R
combination can activate two G proteins which activates 4 kinases which activate 16 more kinases etc…….
Action of Water-Soluble Hormones: Endogenous signaling
Action of Lipid-Soluble Hormones: Endogenous signaling
• Hormone must be carried by a transport protein that allows it to dissolve within the aqueous (watery) environment of the blood plasma
• Hormone diffuses through phospholipid bilayer & into cell
• the receptor is located within the cell (cytoplasm or the nucleus)
• binding of H to R results in its translocation into the nucleus
• the H then binds directly to specific sequences within the DNA = response elements
• this binding turns on/off specific genes – activates or inhibits gene transcription
• if turned on - new mRNA is formed & directs synthesis of new proteins
• new protein alters cell’s activity• if turned off – no new protein results
and the cell’s activity is altered
HeLa cell dying, coloured scanning electron micrograph (SEM). This cell appears spherical because it is undergoing apoptosis, or programmed cell death. Apoptosis occurs when a cell becomes old or damaged. Blebs (vesicles) called apoptotic bodies form on its surface, which prevent toxic or immunogenic substances from leaking when it is phagocytosed (engulfed and digested) by specialist cells. HeLa cells are a continuously cultured cell line of human cancer cells, which are immortal and so thrive in the laboratory.
Necrosis vs. Apoptosis
• Cellular condensation• Membranes remain intact• Requires ATP• Cell is phagocytosed, no
tissue reaction • Ladder-like DNA
fragmentation• In vivo, individual cells
appear affected
• Cellular swelling• Membranes are broken• ATP is depleted• Cell lyses, eliciting an
inflammatory reaction • DNA fragmentation is
random, or smeared• In vivo, whole areas of
the tissue are affected
Necrosis Apoptosis
mitochondria
Meiosis KM 23
Differentiation: Embryonic Stem cells
• the ES cells are said to be totipotent – have the ability to specialize or differentiate into ALL cells of the embryo
• the blastocyst then begins a process of differentiation and these ES cells form populations of stem cells with more restricted potentials
• the ES cells first differentiate into two layers called the embryonic disc – divides the blastocyst cavity into an amniotic cavity and a yolk sac (primitive hematopoietic organ)
• these two layers then continue to differentiate into the three germ layers of the embyro– ectoderm, mesoderm and endoderm
• the formation of these germ layers marks the gastrula embryonic stage
• the blastocyst is a hollow ball of cells containing an outer rings of progenitor cells = trophoblast and an inner mass of cells at one end of the embryo = inner cell mass
• it is these ICM cells that are the source for the derivation of embryonic stem (ES) cells
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