BSC 215 Lecture Notes - Lecture 5: Intermediate Filament, Messenger Rna, Centrosome
8 Jun 2018
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The Cell and Its Membrane
The cell is the basic functional unit of all living things. The plasma membrane (cell
membrane) bounds the cell and encloses the nucleus (discussed presently)
and cytoplasm. The cytoplasm consists of specialized bodies called organelles
suspended in a fluid matrix, the cytosol, which consists of water and dissolved
substances such as proteins and nutrients.
The plasma membrane
The plasma membrane separates internal metabolic events from the external
environment and controls the movement of materials into and out of the
cell. The plasma membrane is a double phospholipid membrane (lipid
bilayer), with the nonpolar hydrophobic tails pointing toward the inside of
the membrane and the polar hydrophilic heads forming the inner and outer
faces of the membrane (Figure 1).
Proteins and cholesterol molecules are scattered throughout the flexible
phospholipid membrane. Proteins may attach loosely to the inner or outer
surface of the plasma membrane (peripheral proteins), or they may lie
across the membrane, extending from inside to outside (integral proteins).
The mosaic nature of scattered proteins within a flexible matrix of
phospholipid molecules describes the fluid mosaic model of the cell
membrane. Additional features of the plasma membrane follow:
• The phospholipid bilayer is semi‐permeable. Only small, uncharged,
polar molecules, such as H 2O and CO 2, and hydrophobic molecules—
nonpolar molecules such as O 2 and lipid soluble molecules such as
hydrocarbons—can freely cross the membrane.
• Channel proteins provide passageways through the membrane for
certain hydrophilic (water‐soluble) substances such as polar and
charged molecules.
• Transport proteins spend energy (ATP) to transfer materials across the
membrane. When energy is used to provide a passageway for
materials, the process is called active transport.
• Recognition proteins (glycoproteins) distinguish the identity of
neighboring cells. These proteins have oligosaccharide (short
polysaccharide) chains extending from their cell surface.
• Adhesion proteins attach cells to neighboring cells or provide anchors for
the internal filaments and tubules that give stability to the cell.
• Receptor proteins initiate specific cell responses once hormones or other
trigger molecules bind to them.
• Electron transfer proteins are involved in moving electrons from one
molecule to another during chemical reactions.
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Organelles are bodies within the cytoplasm that serve to physically separate
the various metabolic activities that occur within cells. They include the
following (Figure 2):
• The nucleus is bounded by the nuclear envelope, a phospholipid bilayer
similar to the plasma membrane. The nucleus contains DNA
(deoxyribonucleic acid), the hereditary information of the cell.
Normally, the DNA is spread out within the nucleus as a threadlike
matrix called chromatin. When the cell begins to divide, the chromatin
condenses into rod‐shaped bodies called chromosomes, each of which,
before dividing, is made up of two long DNA molecules and various
histone molecules. The histones serve to organize the lengthy DNA,
coiling it into bundles called nucleosomes. Also visible within the
nucleus are one or more nucleoli, each consisting of RNA that is
involved in the process of manufacturing the components of
ribosomes. The components of ribosomes move to the cytoplasm to
form a complete ribosome. The ribosome will eventually assemble
amino acids into proteins. The nucleus also serves as the site for the
separation of chromosomes during cell division.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
The cell is the basic functional unit of all living things. The plasma membrane (cell membrane) bounds the cell and encloses the nucleus (discussed presently) and cytoplasm. The cytoplasm consists of specialized bodies called organelles suspended in a fluid matrix, the cytosol, which consists of water and dissolved substances such as proteins and nutrients. The plasma membrane separates internal metabolic events from the external environment and controls the movement of materials into and out of the cell. The plasma membrane is a double phospholipid membrane (lipid bilayer), with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar hydrophilic heads forming the inner and outer faces of the membrane (figure 1). Proteins and cholesterol molecules are scattered throughout the flexible phospholipid membrane. Proteins may attach loosely to the inner or outer surface of the plasma membrane (peripheral proteins), or they may lie across the membrane, extending from inside to outside (integral proteins).
