Lecture Notes - Chapter 5
Homework - Review questions 1, 4, 8, 10
I.
An Overview of Cell Structure
All cells share a similar overall organization.
A. The Plasma Membrane Surrounds the Cell
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cell is enclosed by a phospholipid bilayer.
proteins embedded in the layer are responsible for cell’s ability to interact with
the environment.
B. The Central Portion of the Cell Contains the Genetic Material
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Nucleoid –
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Nucleus –
C. The Cytoplasm Comprises the Rest of the Cell’s Interior -
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semifluid matrix which completely fills the space between the nucleus and the
plasma membrane
contains the components, molecules, proteins needed to sustain life processes.
II. Most Cells Are Very Small
A. The Cell Theory
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describes the importance of cells to life, and includes 3 principles:
1) All organisms are composed of one or more cells.
2) Cells are the smallest living things, the basic units of organization of all
organisms.
3) Cells arise only by division of a previously existing cell.
B. Why Aren’t Cells Larger? –
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Cell size influences the speed at which materials are able to diffuse between the
plasma membrane and the center of the cell.
Smaller cells have a greater surface-to-volume ratio, allowing for better control
of cell function relative to the interaction with the environment.
III. The Structure of Simple Cells: Bacteria
Membrane-bound cells which are surrounded by a rigid cell wall and contain no distinct
interior compartments.
A. Strong Cell Walls
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carbohydrate matrix cross-linked by short polypeptides
Gram-positive vs. gram-negative
B. Simple Interior Organization
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few or no membrane-bound internal compartments or organelles
whole bacterium operates as a single unit.
C. Rotating Flagella
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locomotion and feeding
IV. The Structure of Eukaryotic Cells: An Overview
All have membrane-bound organelles to compartmentalize processes occurring in the cell.
Most organelles are found in both plant and animal eukaryotic cells.
V. The Nucleus: Information Center for the Cell
Largest “organelle”, usually located near the center.
Contains all the genetic information responsible for the activities carried out by the cell.
A. Getting In and Out: The Nuclear Envelope
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composed of two phospholipid bilayer membranes.
Outer membrane is continuous with the e.r.
membranes are fused together in several areas to form nuclear pores.
B. The Chromosomes of Eukaryotes are Complex
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chromosomes – provide compact packaging of DNA for cell division
chromatin – loosely packed DNA
VI. Proteins Are Synthesized on Ribosomes -
Ribosomes read/translate mRNA (copy of a gene from the DNA) to synthesize proteins.
Made of rRNA and a complex of many proteins.
Ribosomal assembly area is easily visible within the nucleus, in dark-staining regions
called nucleoli.
The nucleolus is not membrane-bound.
VII. The Endoplasmic Reticulum: Compartmentalization of the Cell
Composed of very thin, interconnected membranes (lipid bilayers)
Functions to divide the cell into compartments, assist the transport of molecules through
the interior of the cell, and provide surfaces on which the enzymes may act.
A. Rough E.R.: Manufacturer of Proteins for Export
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Studded with ribosomes
Most of the proteins synthesized by the rough e.r. are eventually destined for
export from the cell. Pass through the lumen of the e.r. for processing.
Proteins synthesized by free ribosomes may also need to be processed and
packaged.
The signal hypothesis - as a new protein is being made by a free ribosome, a
short amino acid sequence (signal sequence) at the tip of the protein recognizes a
“docking site” on the surface of the e.r. As the protein continues to be
synthesized, it passes into the interior of the e.r. When synthesis is complete, the
protein can pass through the e.r. to the Golgi complex, where it can be processed
and packaged for use or export from the cell.
B. Smooth E.R. - Organizer of Internal Activities
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contains few to no bound ribosomes.
contains many enzymes which are involved in the synthesis of carbohydrates and
lipids, and in certain organs (liver in particular) are capable of drug
detoxification.
Acts as a passageway for some processed proteins originating in the rough e.r.
VIII. The Golgi Complex: The Delivery System of the Cell
Golgi bodies - flattened stacks of membranes. Collectively - Golgi complex.
Functions to package and distribute the proteins and lipids synthesized on the e.r.
Modifications which may occur include the addition of short sugar chains to form
glycoproteins and glycolipids.
The molecules collect in areas of the Golgi body know as the cisternae. The membranes
pinch off, surrounding the glycoproteins or glycolipids and forming transport vesicles
known as liposomes which deliver the molecules to their appropriate destinations, such
as the plasma membrane.
IX. Lysosomes: Producers of Digestive Enzymes For the Cell
Small membrane-bound sacs that contain the digestive enzymes of the cell, and assist in
the breakdown of proteins, nucleic acids, lipids, and carbohydrates.
Also break down particles and foreign materials brought into the cell by phagocytosis, as
well as old organelles, and recycling the components to create new organelles for the
cell.
Lysosomes that are not functioning actively are called primary lysosomes. When the
enzymes of a primary lysosome are activated (by fusion with a vesicle, etc.), it becomes a
secondary lysosome.
X. Peroxisomes: Detoxifiers of Hydrogen Peroxide
Small membrane-bound sacs that contain the enzymes responsible for many of the
oxidative reactions within the cell.
Hydrogen peroxide is prevalent in eukaryotic cells as a by-product of the oxidative
activities of the peroxisomes; although it is dangerous to cells, the peroxisomes are able
to enzymatically break it down to water and oxygen.
XI. Mitochondria: The Cell’s Chemical Furnaces
Bounded by two membranes (each consisting of a phospholipid bilayer).
The outer mitochondrial membrane is smooth, the inner mitochondrial membrane is
organized into numerous folds called cristae.
Space between the two membranes is called the intermembranous space.
Area inside the inner membrane is known as the matrix.
Proteins embedded on and within the inner membrane are responsible for oxidative
metabolism, the process by which the cell produces energy in the form of ATP.
Mitochondria contain their own DNA and are therefore capable of self-replication,
though this process is tied to the presence of proteins encoded for by the DNA of the
nucleus and translated by cytoplasmic ribosomes.
XII. Centrioles: Microtubule Assembly Plants
Occur in pairs located at right angles to each other near the nuclear membranes, and
together form a centrosome.
Involved in the assembly of microtubules, as well as moving the chromosomes during
cell division, influencing cell shape, and moving flagella and cilia (via actions of
microtubules).
XII. The Cytoskeleton: Interior Framework of the Cell
Network of protein fibers within cells.
The fibers are constantly formed and disassembled by polymerization, where identical
protein subunits spontaneously assemble into long chains, or are removed subunit by
subunit. There are 3 types of cytoskeletal fibers present in plants and animals:
A. Actin filaments –
- long fibers about 7 nm in diameter composed of 2 protein strands loosely twisted
around one another.
- strands are made of many globular proteins called actin.
- function in cellular movements such as contraction, crawling, “pinching” during
division, and formation of cellular extensions.
B. Microtubules –
- hollow tubes about 25 nm in diameter, composed of a ring of 13 protofilaments,
which are each composed of heterodimers of alpha and beta tubulin subunits.
- form from the centrosome near the center of the cell and radiate out toward the
periphery.
- almost constantly polymerized and depolymerized.
- allow for cell movement (cilia, flagella), and are responsible for movements of
materials within the cell itself (chromosomes, organelles).
- Two specific proteins which assist in this movement are kinesin (moves
organelles toward cell periphery) and dynein (moves organelles toward center of
cell).
C. Intermediate filaments –
- tough, fibrous protein fibers, 8-10 nm in diameter
- generally stable and are not broken down frequently.
- several types of intermediate filaments, including vimentin (important in
structural support), keratin (present in epithelial cells, as well as hair and nails),
and neurofilaments (intermediate filaments of the nervous system).
- Along with actin filaments, they provide mechanical support to the membrane
and prevent stretching of the cell.
XIII. Flagella and Cilia: Motility for the Cell
A. Flagella
- consist of a circle of nine microtubule pairs surrounding two central
microtubules - referred to as the 9 + 2 structure.
- pairs of microtubules move past one another which causes the entire flagellum to
undulate rather than rotate.
- flagella are outward projections of the cell - they contain cytoplasm and are
enclosed by the plasma membrane.
- the basal body is located just below the point at which the flagellum protrudes
from the surface of the cell, and it is from this structure that the flagellum is
derived.
B. Cilia
- also composed of the 9 + 2 arrangements of microtubules
- often arranged in rows along the cell surface, and are much shorter and more
numerous than flagella.
- responsible for the movement of water or materials along the cell surface. They
also function in sensory perception (hearing).
XIII. Plant Cell Specializations
A. Chloroplasts: Where Photosynthesis Takes Place
These are found primarily in the photosynthetic cells of plants and algae, and give
these organisms the unique ability to manufacture their own food. Similar to the
mitochondria, chloroplasts have 2 membranes, as well as a closed compartment of
stacked membranes called grana that lie inside the internal membrane. The grana
contain thylakoids, which contain the photosynthetic pigments capable of capturing
light. Chloroplasts contain DNA, but like mitochondria, are unable to replicate
without several components located in the nucleus. Chloroplasts that are deprived of
light for long periods of time are called leucoplasts, and many function primarily as
sites of starch storage (amyloplasts).
B. Central Vacuole
A central, membrane-bound sac that is the water-storage area for the plant cell. Also
contains sugars, ions, pigments.
C. Cell Wall
A protective structure comprised of cellulose (a fibrous, rigid polysaccharide).