Download Plasma membrane

Histology
Lecture 2 Cell Structure
CELL
CELLS ARE THE
STRUCTURAL AND
FUNCTIONAL UNITS OF
LIFE.
PROKARYOTES AND
EUKARYOTES:There are
two basic cell types. Prokaryotic
cells typically are small,singlecalled organisms(eg,bacteria)that
lack a nuclear envelope,histones
and membranous organelles.
EUKARYOTIC CELLS
are larger than prokaryotes and have a nuclear envelope,histones and
membrane-limited organelles
All cells require energy captured from their environment.
The typical molecule involved in storing this energy is ATP.
Energy from sun, chemicals, eating other cells.
Cells must make DNA, RNA and proteins
Cells use sugars, nucleotides, amino acids, lipids and ATP to make components
General features of metabolism similar in all cells, details can vary
Prokaryotes
Eukaryotes
Cell components
…The living substances of the cell is known as
Protoplasm
…Protoplasm has two compartments: Cytoplasm and
Karyoplasm
…Cytoplasm extending from the plasma membrane to
the nuclear envelope
…Karyoplasm forming the contents of the nucleus
Volumes of Intracellular Compartments
(liver cell)
Cytosol
Mitochondria
RER
Smooth ER & Golgi
Nucleus
Peroxisomes
Lysosomes
Endosomes
%age of Total Cell Volume
54
22
9
6
6
1
1
1
DIFFERENTIATION
The process that
The cells synthesize specific
proteins, change their shape,
And become very
Efficient in specialized
Functions.
Eukaryotic cell structure
Cell Membrane
The cell membrane
forms a selectively
permeable barrier
between the cytoplasm
and the external milieu.
Plasma membrane studied at the EM level
Functions
Selective Barrier
Facilitated transport
Exocytosis
Endocytosis
Phagocytosis
Pinocytosis
Attachment (via junctions
Cell Membrane
Molecular Composition
*phospholipids,
Cell membrane is based on a lipid bilayer contains
glycolipids and cholesterol
*It is composed of a phospholipid bilayer and associated
integral and peripheral proteins
*Membrane proteins carry out most of the specialized
functions of cell membrane by participating in membrane
transport mechanisms, by forming attachments to cytoskeletal
and extracellular proteins and by constituting the receptors of
the plasma membrane
*Glycoclayx composed usually of carbohydrate chains, coats
the cell surface which are not in contact with cytosol
Amounts of Membrane Types in 2 Cells
Plasma membrane
RER
SER
Golgi
Mitochondria
OM
IM
Nucleus IM
Lysosome, perox., endosome
%age of Total Membrane
Liver
Pancreas
2
5
35
60
16
<1
7
10
7
32
0.2
1.2
4
17
0.7
UNK
Table 10-1
% Lipid Composition of Different Cell Membranes
Chol
Liver PM
17
RBC PM
23
E. coli memb
0
PE
7
18
70
PS
4
7
trace
PC
24
17
0
SM
19
18
0
GL
7
3
0
Lipid rafts (~70 nm dia) are typically
rich in sphingolipids & cholesterol.
The thicker membrane in the raft
can accommodate some of the
larger transmembrane proteins, so
they tend to accumulate in LR.
Asymmetrical distribution
of PL in membrane of
rbc.
PC (r) & SM (b)
PS (-) & PE
Cholesterol is not shown, but
is ~equal in the layers.
Animal cells use PL asymmetry to distinguish between live and dead cells.
Before a cell undergoes apoptosis the PS is facing the cytoplasm. At death,
the PS translocates to the outer leaflet. This signal activates adjacent
macrophages that cell is dying.
PS translocation is by 2 mechanisms:
PL translocator that normally moves PS from the outer leaflet to the
inner leaflet is inactivated.
The “scramblase” that normally moves PL between leaflets
nonspecifically, is activated.
Membrane protein content can vary widely from just 25% in
myelin
membranes to 75% in mitochondria and chloroplast
membranes.
In most cells the membrane is about 50% protein.
Bcz proteins have much higher MW than PLs, the ratio is about
50 PL to 1 protein.
Like glycolipids, membrane proteins often have carbohydrate
chains on
their exposed regions contributing to the glycocalyx.
Single-pass and multi-pass transmembrane proteins
Membrane Proteins are
associated with the membrane
in various ways
Integral membrane proteins
Peripheral membrane proteins
Glycophorin
1 million per
cell
Function
unknown
Spectrin is a peripheral membrane
protein on the inner surface of the rbc
plasma membrane, MW 220-240 Kd
The majority of transmembrane proteins are glycosylated
Detergent
micelle in
water
Plasma membrane is less than 5%
of typical eukaryotic cell total
membrane.
Plasma membrane studied at the EM level
Functions
Selective Barrier
Facilitated transport
Exocytosis
Endocytosis
Phagocytosis
Pinocytosis
Attachment (via junctions
2 main classes of membrane transport proteins:
carriers and channels
Conformational change leading to passive transport
Comparison of Active and Passive Transport
Electrical gradient
Chemical gradient
3 Ways of Powering Active Transport
Amino acid sequence of carrier proteins mediating passive transport
are similar to those of carriers involved in active transport suggesting
an evolutionary relationship between these families of proteins.
Transport of material in or out of a cell
takes place by the processes of
ENDOCYTOSIS & EXOCYTOSIS
The invaginated cell membrane fuses
To form an ENDOCYTOTIC VESICLE or
ENDOSOME
Endocytosis divided into two categories:
Ù
Phagocytosis and Pinocytosis
•Pinocytosis is used when cell take up small molecules and fluid.
•the two main vesicles involved in transport of substances into cells
are derived from surface membrane invaginations called coated pits
and caveoli
phagocytosis
Phagocytosis is the
process of engulfing
large particulate
matter such as
microorganisms
PINOCYTOSIS
CAVEOLI are
invaginations
Braced by the
Protein CAVEOLI
Pinocytosis and receptor mediated endocytosis
Caveolae in plasma membrane of
fibroblast (no protein coat); caveolin is
integral membrane protein
Coated Pits and Vesicles
The coated pits and vesicles function in receptor mediated
endocytosis and in the pathway for cell secretion
There are at least two types of coated vesicles: clathrin
coated and nonclathrin coated
Clathrin coated vesicles are associated
with transport vesicles which function in:
1- The regulated secretory pathway;
2- In transport of lysosomal proteins
3- In receptor mediated endocytosis
×Polypeptides of clathrin form a basket-like structure
which coats the cytoplasmic surface of the membrane
×Nonclathrin coated vesicles
are either those which move
between the endoplasmic reticulum
and the Golgi complex, those which
are a part of the constitutive
secretory pathway, or those
which move between the Golgi
cisternae