Ch.3 - Cell Structure and Function

3.1 What is a Cell?
Organisms are composed of cells which are quite
small and can be seen only under a microscope.
The Cell Theory
According to the cell theory, the cell is the basic unit
of life, all living things are composed of cells, and
new cells arise only from preexisting ones.
Cell Size
Cells are small to maintain an optimal surface areato-volume ratio.
Micrographs are photographs of objects obtained by
using a microscope. The compound light microscope
passes light rays through specimens and uses glass
lenses to view objects. The transmission electron
microscope passes electrons through a specimen and
projects images onto a fluorescent screen or
photographic film. The scanning electron microscope
provides a three-dimensional view of the surface of
an object.
3.2 How Cells are Organized
Biologists classify cells into two broad categories-the
prokaryotes and the eukaryotes. Both types of cells
have a plasma membrane, which is selectively
permeable, and cytoplasm.
Internal Structure of Eukaryotic Cells
The most prominent organelle within the eukaryotic
cell is a nucleus. Prokaryotic cells lack a nucleus.
Evolutionary History of the Animal Cell
The first cells to arise were prokaryotic cells, lacking
a membrane-enclosed nucleus. Prokaryotic cells
today are represented by the bacteria and archaea.
The eukaryotic cell, which does have a nucleus, is
believed to have evolved from the archaea. During
evolution, the plasma membrane may have
invaginated to form a nucleus. Other organelles in
eukaryotic cells may have arisen by engulfing
prokaryotic cells.
3.3 The Plasma Membrane and How Substances
Cross It
The plasma membrane is a bilayer of phospholipids
with embedded or attached proteins. The fluidmosaic model is a working description of membrane
structure. Glycoproteins and glycolipids serve as
identification markers. Embedded proteins may
serve as hormone receptors, transport channels, and
enzymes in metabolic reactions.
Plasma Membrane Functions
The plasma membrane keeps a cell intact. It is
selectively permeable, and therefore regulates the
entrance and exit of molecules.
Diffusion is the random movement of molecules from
an area of higher concentration to one of lower
Osmosis is the diffusion of water across a plasma
membrane. Solutions that cause cells to swell due to
the intake of water are considered hypotonic.
Solutions that cause cells to shrink are considered
hypertonic. In an isotonic solution, there is the same
concentration of solutes and water on both sides of
the membrane and the cells maintain their size and
Facilitated Transport
A protein carrier helps a molecule across the
membrane with the concentration gradient without
using energy.
Active Transport
A protein carrier transports a molecule across the
concentration gradient and requires energy.
Endocytosis and Exocytosis
Endocytosis occurs as the plasma membrane forms a
vesicle around a particle as it enters the cell.
Phagocytosis and Pinocytosis are forms of
endocytosis. Exocytosis is the reverse process.
3.4 The Nucleus and Endomembrane System
The Nucleus
The nucleus stores genetic information in the form of
chromatin, a combination of DNA molecules and
proteins. The nucleus also contains nucleoli where
ribosomal RNA is produced. The nucleus is separated
from the cytoplasm by a double membrane known as
the nuclear envelope which is continuous with the
endoplasmic reticulum.
Ribosomes are a mix of rRNA and proteins. They are
the sites of protein synthesis in the cytoplasm.
The Endomembrane System
The endomembrane system consists of the nuclear
envelope, the endoplasmic reticulum, the Golgi
apparatus, lysosomes, and vesicles.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a series of
membranes that form tubular channels within the
cytoplasm. Rough ER (with ribosomes) specializes in
protein synthesis. Smooth ER produces different
compounds in different cells.
The Golgi Apparatus
The Golgi apparatus, a stack of flattened vacuoles
with vesicles near the edges, packages, stores, and
distributes the proteins the ER produces.
The Golgi apparatus produces lysosomes, which
contain hydrolytic enzymes that digest unwanted
materials inside the cell, including worn-out cell
3.5 The Cytoskeleton, Cell Movement, and Cell
Microtubules, actin filaments, and intermediate
filaments form a cytoskeleton that serves as a
framework for the cell's interior.
Cilia and Flagella
Cilia and flagella are hair-like projections of cells
responsible for locomotion. Each cilium and flagellum
shares the same organization of microtubules within
a plasma membrane. Motor molecules help them
Junctions Between Cells
Adhesion junctions serve to mechanically attach
adjacent cells. In tight junctions, connections
between the plasma membrane proteins of
neighboring cells produce a zipperlike barrier. Gap
junctions serve as communication portals between
3.6 Mitochondria and Cellular Metabolism
Mitochondria are organelles bounded by a double
membrane. The inner membrane is folded into
cristae. The gel-like material between the cristae is
matrix. Mitochondria convert the energy stored in
glucose into ATP molecules in a process called
cellular respiration.
Cellular Respiration and Metabolism
Cellular respiration is a very important component of
metabolism which includes all the chemical reactions
that occur in a cell. Chemical reactions in a cell are
organized into metabolic pathways where every
reaction has its own enzyme.
Enzymes are named for their substrates and have an
active site specific for the substrate.
Many enzymes have nonprotein molecules, called
coenzymes, to assist them. Some coenzymes, like
NAD+, remove hydrogen from substrates.
Cellular Respiration
The breakdown of glucose into carbon dioxide and
water requires three subpathways: glycolysis, the
citric acid cycle, and the electron transport system.
Glycolysis needs no oxygen and takes place in the
cytoplasm of the cell. During glycolysis, glucose
breaks down into two molecules of pyruvate. If
oxygen is present, pyruvate can enter the
mitochondrion and the citric acid cycle. If no oxygen
is present, fermentation occurs.
The citric acid cycle is a cyclical series of enzymatic
reactions that occurs in the matrix of the
mitochondria and completes the breakdown of
glucose. Carbon dioxide is released and hydrogen
atoms are carried away by NADH. NADH molecules
from glycolysis and the citric acid cycle deliver
electrons to the electron transport chain, a group of
carrier proteins embedded in the cristae of a
mitochondrion. Oxygen serves as the final acceptor
of the electrons at the end of the chain.
When no oxygen is present for pyruvate to enter the
citric acid cycle, pyruvate is converted to lactic acid
through fermentation, producing few ATP.
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