THE CELL
Cells and extracellular material together comprise all the tissues that make up the organs of
multicellular animals. In all tissues, cells themselves are the basic structural and functional
units, the smallest living parts of the body. Animal cells are eukaryotic, with distinct membrane
limited nuclei surrounded by cytoplasm which contains various membrane-limited organelles
and the cytoskeleton. In contrast, the smaller prokaryotic cells of bacteria typically have a cell
wall around the plasmalemma and lack nuclei and membranous cytoplasmic structures.
Рlasma membrane
The plasma membrane (cell membrane or plasmalemma) that envelops every eukaryotic cell
consists of phospholipids, cholesterol, and proteins, with oligosaccharide chains covalently
linked to many of the phospholipid and protein molecules.
The plasma membrane is approximately 7.5 nm thick and consists of two leaflets, known as the
lipid bilayer that houses associated integral and peripheral proteins.
1. The inner leaflet of the plasma membrane faces the cytoplasm.
2. The outer leaflet faces the extracellular environment.
Functions:
1. The plasma membrane envelops the cell and maintains its structural and functional integrity.
2. It acts as a semipermeable membrane between the cytoplasm and the external environment.
3. It permits the cell to recognize macromolecules and other cells as well as to be recognized by
other cells.
4. It participates in the transduction of extracellular signals into intracellular events.
5. It assists in controlling interaction between cells.
6. It maintains an electrical potential difference between the cytoplasmic and extracellular
sides.
,The lipid bilayer is freely permeable to small, lipid-soluble, nonpolar molecules but is
impermeable to charged ions.
The lipid bilayer is composed of:
1. Phospholipids.
2. Glycolipids.
3. Cholesterol, of which, in most cells, phospholipids constitute the highest percentage.
Phospholipids are amphipathic molecules, consisting of one polar (hydrophilic) head and two
nonpolar (hydrophobic) fatty acyl tails, one of which is usually unsaturated.
The two leaflets are not identical; instead the distribution of the various types of phospholipids
is asymmetrical. The polar head of each molecule faces the membrane surface, whereas the tails
project into the interior of the membrane, facing each other. The tails of the two leaflets are
mostly 16–18 carbon chain fatty acids, and they form weak noncovalent bonds that attach the
two leaflets to each other.
Glycolipids are restricted to the extracellular aspect of the outer leaflet. Polar carbohydrate
residues of glycolipids extend from the outer leaflet into the extracellular space and form part of
the glycocalyx. Some of the outer layer’s lipids, known as glycolipids, include oligosaccharide
chains that extend outward from the cell surface and contribute to a delicate cell surface coating
called the glycocalyx.
Cholesterol, constituting 2% of plasmalemma lipids, is present in both leaflets, and helps
maintain the structural integrity of the membrane. Cholesterol and phospholipids can form
microdomains, known as lipid rafts, that can affect the movement of integral proteins of the
plasmalemma.
,Membrane proteins include:
1. Integral proteins.
2. Peripheral proteins and, in most cells, constitute approximately 50% of the plasma membrane
composition.
Integral proteins are dissolved in the lipid bilayer. Transmembrane proteins span the entire
thickness of the plasma membrane and may function as membrane receptors, enzymes, cell
adhesion molecules, cell recognition proteins, molecules that function in message transduction,
and transport proteins.
Most transmembrane proteins are glycoproteins. Transmembrane proteins are amphipathic and
contain hydrophilic and hydrophobic amino acids, some of which interact with the hydrocarbon
tails of the membrane phospholipids. Most transmembrane proteins are folded so that they pass
back and forth across the plasmalemma; therefore, they are also known as multipass proteins.
Integral proteins may also be anchored to the inner (or occasionally outer) leaflet via fatty acyl
or prenyl groups.
Peripheral proteins do not extend into the lipid bilayer. These proteins are located on the
cytoplasmic aspect of the inner leaflet. The outer leaflets of some cells possess covalently
linked glycolipids to which peripheral proteins are anchored; these peripheral proteins thus
project into the extracellular space. Peripheral proteins bind to the phospholipid polar groups or
integral proteins of the membrane via noncovalent interactions.
They usually function as electron carriers (e.g., cytochrome c) part of the cytoskeleton or as
part of an intracellular second messenger system. They include a group of anionic, calcium-
dependent, lipid-binding proteins known as annexins, which act to modify the relationships of
other peripheral proteins with the lipid bilayer and also to function in membrane trafficking and
the formation of ion channels; synapsin I, which binds synaptic vesicles to the cytoskeleton;
and spectrin, which stabilizes cell membranes of erythrocytes.
Functional characteristics of membrane proteins:
1. The lipid-to-protein ratio (by weight) in plasma membranes ranges from 1:1 in most cells to
as much as 4:1 in myelin.
2. Some membrane proteins diffuse laterally in the lipid bilayer; others are immobile and are
held in place by cytoskeletal components.
Glycocalyx (cell coat), located on the outer surface of the outer leaflet of the plasmalemma,
varies in appearance (fuzziness) and thickness (up to 50 nm). The glycocalyx consists of polar
oligosaccharide side chains linked covalently to most proteins and some lipids (glycolipids) of
the plasmalemma. It also contains proteoglycans (glycosaminoglycans bound to integral
proteins).
Functions:
1. The glycocalyx aids in attachment of some cells (e.g., fibroblasts but not epithelial cells) to
extracellular matrix components.
2. It binds antigens and enzymes to the cell surface.
3. It facilitates cell-cell recognition and interaction.
4. It protects cells from injury by preventing contact with inappropriate substances.
5. It assists T cells and antigen-presenting cells in aligning with each other in the proper fashion
and aids in preventing inappropriate enzymatic cleavage of receptors and ligands.
, 5. In blood vessels, it lines the endothelial surface to decrease frictional forces as the blood
rushes by and it also diminishes loss of fluid from the vessel.
The plasma membrane is the site where materials are exchanged between the cell and its
environment. Most small molecules cross the membrane by the general mechanisms explained
as follows:
1. Diffusion transports small, nonpolar molecules directly through the lipid bilayer. Lipophilic
(fat-soluble) molecules diffuse through membranes readily, water very slowly.
2. Channels are multipass proteins forming transmembrane pores through which ions or small
molecules pass selectively. Cells open and close specific channels for Na+, K+, Ca2+ and other
ions in response to various physiological stimuli. Water molecules usually cross the plasma
membrane through channel proteins called aquaporins.
3. Carriers are transmembrane proteins that bind small molecules and translocate them across
the membrane via conformational changes. Diffusion, channels, and carrier proteins operate
passively, allowing movement of substances across membranes down a concentration gradient
due to its kinetic energy. In contrast, membrane pumps are enzymes engaged in active
transport, utilizing energy from the hydrolysis of adenosine triphosphate (ATP) to move ions
and other solutes across membranes, against often steep concentration gradients. Because they
consume ATP pumps they are often referred to as ATPases.
Macromolecules normally enter cells by being enclosed within folds of plasma membrane
(often after binding specific membrane receptors) which fuse and pinch off internally as
cytoplasmic vesicles (or vacuoles) in a general process known as endocytosis.
Three major types of endocytosis are recognized:
1. Phagocytosis (“cell eating”) is the ingestion of particles such as bacteria or dead cell
remnants. Certain blood derived cells, such as macrophages and neutrophils, are specialized for
this activity. When a bacterium becomes bound to the surface of a neutrophil, it becomes
surrounded by extensions of plasmalemma and cytoplasm which project from the cell in a
process dependent on cytoskeletal changes. Fusion of the membranous folds encloses the
bacterium in an intracellular vacuole called a phagosome, which then merges with a lysosome
for degradation.
Cells and extracellular material together comprise all the tissues that make up the organs of
multicellular animals. In all tissues, cells themselves are the basic structural and functional
units, the smallest living parts of the body. Animal cells are eukaryotic, with distinct membrane
limited nuclei surrounded by cytoplasm which contains various membrane-limited organelles
and the cytoskeleton. In contrast, the smaller prokaryotic cells of bacteria typically have a cell
wall around the plasmalemma and lack nuclei and membranous cytoplasmic structures.
Рlasma membrane
The plasma membrane (cell membrane or plasmalemma) that envelops every eukaryotic cell
consists of phospholipids, cholesterol, and proteins, with oligosaccharide chains covalently
linked to many of the phospholipid and protein molecules.
The plasma membrane is approximately 7.5 nm thick and consists of two leaflets, known as the
lipid bilayer that houses associated integral and peripheral proteins.
1. The inner leaflet of the plasma membrane faces the cytoplasm.
2. The outer leaflet faces the extracellular environment.
Functions:
1. The plasma membrane envelops the cell and maintains its structural and functional integrity.
2. It acts as a semipermeable membrane between the cytoplasm and the external environment.
3. It permits the cell to recognize macromolecules and other cells as well as to be recognized by
other cells.
4. It participates in the transduction of extracellular signals into intracellular events.
5. It assists in controlling interaction between cells.
6. It maintains an electrical potential difference between the cytoplasmic and extracellular
sides.
,The lipid bilayer is freely permeable to small, lipid-soluble, nonpolar molecules but is
impermeable to charged ions.
The lipid bilayer is composed of:
1. Phospholipids.
2. Glycolipids.
3. Cholesterol, of which, in most cells, phospholipids constitute the highest percentage.
Phospholipids are amphipathic molecules, consisting of one polar (hydrophilic) head and two
nonpolar (hydrophobic) fatty acyl tails, one of which is usually unsaturated.
The two leaflets are not identical; instead the distribution of the various types of phospholipids
is asymmetrical. The polar head of each molecule faces the membrane surface, whereas the tails
project into the interior of the membrane, facing each other. The tails of the two leaflets are
mostly 16–18 carbon chain fatty acids, and they form weak noncovalent bonds that attach the
two leaflets to each other.
Glycolipids are restricted to the extracellular aspect of the outer leaflet. Polar carbohydrate
residues of glycolipids extend from the outer leaflet into the extracellular space and form part of
the glycocalyx. Some of the outer layer’s lipids, known as glycolipids, include oligosaccharide
chains that extend outward from the cell surface and contribute to a delicate cell surface coating
called the glycocalyx.
Cholesterol, constituting 2% of plasmalemma lipids, is present in both leaflets, and helps
maintain the structural integrity of the membrane. Cholesterol and phospholipids can form
microdomains, known as lipid rafts, that can affect the movement of integral proteins of the
plasmalemma.
,Membrane proteins include:
1. Integral proteins.
2. Peripheral proteins and, in most cells, constitute approximately 50% of the plasma membrane
composition.
Integral proteins are dissolved in the lipid bilayer. Transmembrane proteins span the entire
thickness of the plasma membrane and may function as membrane receptors, enzymes, cell
adhesion molecules, cell recognition proteins, molecules that function in message transduction,
and transport proteins.
Most transmembrane proteins are glycoproteins. Transmembrane proteins are amphipathic and
contain hydrophilic and hydrophobic amino acids, some of which interact with the hydrocarbon
tails of the membrane phospholipids. Most transmembrane proteins are folded so that they pass
back and forth across the plasmalemma; therefore, they are also known as multipass proteins.
Integral proteins may also be anchored to the inner (or occasionally outer) leaflet via fatty acyl
or prenyl groups.
Peripheral proteins do not extend into the lipid bilayer. These proteins are located on the
cytoplasmic aspect of the inner leaflet. The outer leaflets of some cells possess covalently
linked glycolipids to which peripheral proteins are anchored; these peripheral proteins thus
project into the extracellular space. Peripheral proteins bind to the phospholipid polar groups or
integral proteins of the membrane via noncovalent interactions.
They usually function as electron carriers (e.g., cytochrome c) part of the cytoskeleton or as
part of an intracellular second messenger system. They include a group of anionic, calcium-
dependent, lipid-binding proteins known as annexins, which act to modify the relationships of
other peripheral proteins with the lipid bilayer and also to function in membrane trafficking and
the formation of ion channels; synapsin I, which binds synaptic vesicles to the cytoskeleton;
and spectrin, which stabilizes cell membranes of erythrocytes.
Functional characteristics of membrane proteins:
1. The lipid-to-protein ratio (by weight) in plasma membranes ranges from 1:1 in most cells to
as much as 4:1 in myelin.
2. Some membrane proteins diffuse laterally in the lipid bilayer; others are immobile and are
held in place by cytoskeletal components.
Glycocalyx (cell coat), located on the outer surface of the outer leaflet of the plasmalemma,
varies in appearance (fuzziness) and thickness (up to 50 nm). The glycocalyx consists of polar
oligosaccharide side chains linked covalently to most proteins and some lipids (glycolipids) of
the plasmalemma. It also contains proteoglycans (glycosaminoglycans bound to integral
proteins).
Functions:
1. The glycocalyx aids in attachment of some cells (e.g., fibroblasts but not epithelial cells) to
extracellular matrix components.
2. It binds antigens and enzymes to the cell surface.
3. It facilitates cell-cell recognition and interaction.
4. It protects cells from injury by preventing contact with inappropriate substances.
5. It assists T cells and antigen-presenting cells in aligning with each other in the proper fashion
and aids in preventing inappropriate enzymatic cleavage of receptors and ligands.
, 5. In blood vessels, it lines the endothelial surface to decrease frictional forces as the blood
rushes by and it also diminishes loss of fluid from the vessel.
The plasma membrane is the site where materials are exchanged between the cell and its
environment. Most small molecules cross the membrane by the general mechanisms explained
as follows:
1. Diffusion transports small, nonpolar molecules directly through the lipid bilayer. Lipophilic
(fat-soluble) molecules diffuse through membranes readily, water very slowly.
2. Channels are multipass proteins forming transmembrane pores through which ions or small
molecules pass selectively. Cells open and close specific channels for Na+, K+, Ca2+ and other
ions in response to various physiological stimuli. Water molecules usually cross the plasma
membrane through channel proteins called aquaporins.
3. Carriers are transmembrane proteins that bind small molecules and translocate them across
the membrane via conformational changes. Diffusion, channels, and carrier proteins operate
passively, allowing movement of substances across membranes down a concentration gradient
due to its kinetic energy. In contrast, membrane pumps are enzymes engaged in active
transport, utilizing energy from the hydrolysis of adenosine triphosphate (ATP) to move ions
and other solutes across membranes, against often steep concentration gradients. Because they
consume ATP pumps they are often referred to as ATPases.
Macromolecules normally enter cells by being enclosed within folds of plasma membrane
(often after binding specific membrane receptors) which fuse and pinch off internally as
cytoplasmic vesicles (or vacuoles) in a general process known as endocytosis.
Three major types of endocytosis are recognized:
1. Phagocytosis (“cell eating”) is the ingestion of particles such as bacteria or dead cell
remnants. Certain blood derived cells, such as macrophages and neutrophils, are specialized for
this activity. When a bacterium becomes bound to the surface of a neutrophil, it becomes
surrounded by extensions of plasmalemma and cytoplasm which project from the cell in a
process dependent on cytoskeletal changes. Fusion of the membranous folds encloses the
bacterium in an intracellular vacuole called a phagosome, which then merges with a lysosome
for degradation.
