cell membrane and models discussing about the membranes bound organelles.ppt

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extracellular space).[1][2] The cell membrane is a lipid bilayer, usually consisting of phospholipids and glycolipids; eukaryotes and some prokaryotes typically have sterols (such as cholesterol in animals) interspersed between them as well, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins, including integral proteins that span the membrane and serve as membrane transporters, and peripheral proteins that attach to the surface of the cell membrane, acting as enzymes to facilitate interaction with the cell's environment.[3] Glycolipids embedded in the outer lipid layer serve a similar purpose.

The cell membrane controls the movement of substances in and out of a cell, being selectively permeable to ions and organic molecules.[4] In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity, and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall and the carbohydrate layer called the glycocalyx, as well as the intracellular network of protein fibers called the cytoskeleton. In the field of synthetic biology, cell membranes can be artificially reassembled.[5][6][7][8]

History
Main article: History of cell membrane theory
Robert Hooke's discovery of cells in 1665 led to the proposal of the cell theory. Initially it was believed that all cells contained a hard cell wall since only plant cells could be observed at the time.[9] Microscopists focused on the cell wall for well over 150 years until advances in microscopy were made. In the early 19th century, cells were recognized as being separate entities, unconnected, and bound by individual cell walls after it was found that plant cells could be separated. This theory extended to include animal cells to suggest a universal mechanism for cell protection and development.

By the second half of the 19th century, microscopy was still not advanced enough to make a distinction between cell membranes and cell walls. However, some microscopists correctly identified at this time that while invisible, it could be inferred that cell membranes existed in animal cells due to intracellular movement of components internally but not externally and that membranes were not the equivalent of a plant cell wall. It was also inferred that cell membranes were not vital components to all cells. Many refuted the existence of a cell membrane still towards the end of the 19th century. In 1890, a revision to the cell theory stated that cell membranes existed, but were merely secondary structures. It was not until later studies with osmosis and permeability that cell membranes gained more recognition.

1. The Plasma Membrane – Structure and Function
–Is the boundary that separates the living cell from its nonliving
surroundings
–The basic structure is a phospholipid bilayer.

2. –Are the most abundant lipid in the plasma membrane
–Are amphipathic, containing both hydrophobic and hydrophilic
regions
Phospholipids

9. Fluid-Mosaic Model
•The model states that: The plasma membrane is
a phospholipid bilayer with many things floating in
it (also amphipathic molecules).
Figure 7.1

10. Fluid-Mosaic Model
•Most of the mosaic molecules are proteins,
including many for transport.
1. Channels- gates that can open/close, mainly for ion transport
2. Carriers - pick up a molecule and rotate into the cell.
3. Receptors- mainly for hormones. When the hormone binds to
the receptor, the receptor protein changes shape and stimulates a
secondary messenger within the cell.
4. Cell-cell recognition- mainly for self-identification. e.g. H-Y
antigen present on all cells of males, makes organ donation not
work from male to female. e.g. Blood types (ABO) and the Rh
factor.
  Recognition proteins are apparently not functional in fetuses
or newborns.
 

11. Fluid-Mosaic Model
•Carbohydrates can also be part of self-
recognition, or for membrane-membrane
interactions (e.g. tissue glue that holds cells
together).
 
•Be sure to review the structure of the
membrane and predict what types of
molecules can and can't pass through the
phospholipid bilayer.

12. Figure 7.2
Hydrophilic
head
Hydrophobic
tail
WATER
WATER
•Membranes have been chemically
analyzed, and they have in fact
been found to be composed of
proteins and lipids.
•Scientists studying the plasma
membrane reasoned that it must be
a phospholipid bilayer.
Membrane Models

13. Davson-Danielli sandwich model of membrane structure
–Stated (1935) that the membrane was made
up of a phospholipid bilayer sandwiched
between two protein layers
–Was supported by electron microscope
pictures of membranes.
–But predicted that all membranes were of the
same makeup, which they are not.
–And does not fit with the fact that many
membrane proteins are amphipathic and have
hydrophobic regions (are not water-soluble).

14. In 1972, Singer and Nicolson
–Proposed that membrane proteins are
dispersed and individually inserted into the
phospholipid bilayer
Figure 7.3
Phospholipid
bilayer
Hydrophobic region
of protein
Hydrophobic region of protein

15. Freeze-fracture studies of the plasma membrane
–Supported the fluid mosaic model of
membrane structure
Figure 7.4
A cell is frozen and fractured with a knife. The fracture plane often follows
the hydrophobic interior of a membrane, splitting the phospholipid bilayer
into two separated layers. The membrane proteins go wholly with one of
the layers.
Extracellular layer Cytoplasmic layer
APPLICATION
A cell membrane can be split into its two layers, revealing the
ultrastructure of the membrane’s interior.
TECHNIQUE
Extracellular
layer
Proteins
Cytoplasmic
layer
Knife
Plasma
membrane
These SEMs show membrane proteins (the “bumps”) in the two layers,
demonstrating that proteins are embedded in the phospholipid bilayer.
RESULTS

16. The Fluidity of Membranes
•Phospholipids in the plasma membrane
–Can move within the bilayer
•Fluidity affects transport and the functioning of proteins.
Figure 7.5 A
Lateral movement
(~10
7
times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids

17. Proteins in the plasma membrane
–Can drift within the bilayer
EXPERIMENT
Researchers labeled the plasma membrane proteins of a mouse
cell and a human cell with two different markers and fused the cells. Using a microscope,
they observed the markers on the hybrid cell.
Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
RESULTS
CONCLUSION
The mixing of the mouse and human membrane proteins
indicates that at least some membrane proteins move sideways within the plane
of the plasma membrane.Figure 7.6
+

18. The type of hydrocarbon tails in phospholipids
–Affects the fluidity of the plasma membrane
Figure 7.5 B
Fluid Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails
(b) Membrane fluidity

19. The steroid cholesterol
–Has different effects on membrane fluidity at
different temperatures
•Reduces phospholipid movement (fluidity)
•But, hinders solidification at low temperatures
Figure 7.5 (c) Cholesterol within the animal cell membrane
Cholesterol

20. Figure 7.7
Glycoprotein
Carbohydrate
Microfilaments
of cytoskeletonCholesterolPeripheral
protein
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
EXTRACELLULAR
SIDE OF
MEMBRANE
Glycolipid
Membrane Proteins and Their Functions
•A membrane
–Is a collage of different proteins embedded in
the fluid matrix of the lipid bilayer
Fibers of
extracellular
matrix (ECM)

21. Integral proteins
–Penetrate the hydrophobic core of the lipid
bilayer
–Are often transmembrane proteins, completely
spanning the membrane
EXTRACELLULAR
SIDE
Figure 7.8
N-terminus
C-terminus
 Helix
CYTOPLASMIC
SIDE

22. Peripheral proteins
–Are appendages loosely bound to the surface
of the membrane
Figure 7.7
Peripheral
protein
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
EXTRACELLULAR
SIDE OF
MEMBRANE

23. Six major functions of membrane proteins
Figure 7.9
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor

24. Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions (see Figure 6.31).
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes (see Figure 6.29).
(d)
(e)
(f)
Glyco-
protein
Figure 7.9

25. The Role of Membrane Carbohydrates in Cell-Cell Recognition
•Cell-cell recognition
–Is a cell’s ability to distinguish one type of
neighboring cell from another
•Membrane carbohydrates
–Interact with the surface molecules of other
cells, facilitating cell-cell recognition

26. Synthesis and Sidedness of Membranes
•Membranes have distinct inside and outside
faces
•This affects the movement of proteins
synthesized in the endomembrane system

27. Membrane proteins and lipids
–Are synthesized in the ER and Golgi apparatus
ER
Figure 7.10
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus
Vesicle
Transmembrane
glycoprotein
Membrane glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Secreted
protein
4
1
2
3

28. Selective Permeability
•Membrane structure results in selective
permeability
•A cell must exchange materials with its
surroundings, a process controlled by the
plasma membrane

29. A Selectively Permeable Barrier
•The plasma membrane exhibits selective permeability. It
controls what substances enter and leave the cell.
–It allows some substances to cross more easily than others

30. The Permeability of the Lipid Bilayer
•Hydrophobic molecules
–Are lipid soluble and can pass through the
membrane rapidly
•Hydrophilic substances
–Do not cross the membrane rapidly
–Includes polar molecules and ions

31. A Selectively Permeable Barrier
•Things that pass easily through the bilayer:
1. small non-polar molecules (hydrocarbons, O
2
, N
2
)
2. small polar uncharged molecules (H
2O, CO
2,
glycerol, urea)
 

•Things that don’t pass easily through the
bilayer: (Require transport)
1. large polar molecules (glucose)
2. ions (H
+
, Na
+
, Cl
-
, Mg
++
, PO
4
2-
)

32. Transport Proteins
•Transport proteins
–Allow passage of hydrophilic substances
across the membrane
•Transport may be passive by diffusion, which
follows the concentration gradient of the
molecules and requires no expenditure of
energy by the cell.
•Transport may be active, against the
concentration gradient, requiring an energy
source, generally ATP.