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Chapter 7

BIOCHEM 2B03 Chapter Notes - Chapter 7: Cell Membrane, Nuclear Lamina, Digestive Enzyme


Department
Biochemistry
Course Code
BIOCHEM 2B03
Professor
Margaret Fahnestock
Chapter
7

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Bio 1A03
Unit Two: Cell Structure and Function
Chapter 7: Inside the Cell
7.1 What’s Inside the Cell?
The cell is the basic unit of structure and function in living organisms
There are two fundamental cell types (according to morphology): Prokaryotic Cells and Eukaryotic Cells
Prokaryotic Cells
Archae and Bacteria
Most prokaryotic cells are relatively smaller in size compared to eukaryotic cells
Are structurally simpler in design
Plasma membrane surrounds the cytoplasm, which collectively refers to the contents of the cell
Have few or no subdivisions delimited by internal membranes
Interestingly, recent research has identified membrane bound organelles or cytoskeletons of protein in a few
prokaryotic species
Do not contain a nucleus
Figure 7.1, page 126; Prokaryotic Cell
The cell/plasma membrane consists of a phospholipid bilayer and proteins that either span the bilayer or
attach to one side
Cytoplasm the contents of a cell collectively
o Contains high concentration of solutes, in most habitats it is hypertonic relative to the surrounding
environment
o When it is hypertonic water enters the cell via osmosis and makes the cell volume expand
Cell wall tough and fibrous layer that surrounds the plasma membrane; resists pressure from expanding
cytoplasm volume; protects the organism and gives it shape and rigidity
Glycolipids lipids that contain carbohydrate groups
Chromosome singular, circular, consists of a large DNA molecule associated with a small number of proteins
o DNA molecule contains information contains genes (a segment of DNA that contains the information
for building an RNA molecule or a polypeptide)
o Proteins provide structural support for the DNA
o Found in a localized area of the cell called the nucleoid
Nucleoid is usually found in the center of the cell and typically represents about 20 percent of
the cell’s total volume
o To fit into the cell, the DNA double helix coils on itself with the aid of enzymes to form the highly
compact, “supercoiled” structure – resemble a string that has been held at either end and then twisted
until coils back upon itself
Figure 7.3, page 127; Bacterial DNA Supercoiled
o Cells may also contain about a hundred, small, usually circular, supercoiled DNA molecules called
plasmids
Plasmids contain genes but are physically independent of the main, cellular chromosome
Genes carried by plasmids are not required under normal circumstances; instead they help cells
adapt to unusual circumstances (such as the sudden presence of a poison in an environment)
Plasmids can be considered auxiliary genetic elements
Ribosomes manufacture proteins
o Bacterial ribosomes are complex structures consisting of a total of three distinct RNA molecules and
over 50 different proteins
o These molecular components are organized into two major structural elements, called the large subunit
and small subunit
Figure 7.4, page 127; Bacterial Ribosome
Flagella (sing. flagellum) 0 rotate to power swimming in aquatic species
o Not all bacterial species have flagella
Photosynthesis the suite of chemical reaction responsible for converting the energy in sunlight into chemical
energy stored in sugars

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Bio 1A03
o The photosynthetic membranes observed in prokaryotes contain the enzymes and pigment molecules
required for these reactions to occur and develop as infoldings of the plasma membrane
o In other cases, vesicles pinch off as the plasma membrane folds in
o In some cases, flattened stacks of photosynthetic membrane form from the infolded sections of the
plasma membrane
Figure 7.4,page 128; Photosynthetic Membrane
Organelles membrane-bound compartment in the cytoplasm that contains enzymes or structures specialized
for a particular function
o One type of bacterial organelle is specialized for storing calcium ions
o Another type contains crystals of the mineral magnetite, which function like a compass needle to help
cells orient themselves in a magnetic field and swim in a directed way
Bacteria and Archae contain long, thin fibers that serve a structural role inside the cell
o Protein Fibres these filaments are essential foe cell division to take place, or they help maintain cell
shape; form the bases of the cytoskeleton
Eukaryotic Cells
Fungi, Protists, Plants and Animals
Are relatively larger in size compared to prokaryotic cells
Contain a nucleus
Figure 7.6, page 129; Animal and Plant Cells
The large size poses a problem large molecules cannot diffuse across a large volume quickly
o Eg/ ATP is used up on one side of the cell, ATP from the other side of the cell would have to diffuse to
that location solved by compartmentalization
Compartmentalization offers to key advantages
o Incompatible chemical reactions can be separated
Eg/ new fatty acids can be synthesized in one organelle while excess or damaged fatty acids are
degraded and recycled in different organelles
o The efficiency of chemical reactions is increased
First the substrates required for particular reactions can be localized and maintained at high
concentration within organelles
Second, if the substrates are used up in a particular part of the organelle, hey can be replaced by
substrates that have only a short distance to diffuse
Third, groups of enzymes that work together an be clustered on internal membranes instead of
floating free in the cytoplasm
When the product of a reaction is the substrate for a reaction catalyzed by another enzyme,
clustering the enzyme increases the speed and efficiency of a reaction sequence
Location of DNA
Internal Membranes
and Organelles
Cytoskeleton
Overall Size
Bacteria and Archae
In nucleoid (not
membrane bound)
Plasmids also
common
Extensive internal
membranes only in
photosynthetic
species
Limited types and
numbers of
organelles
Limited in extent
relative to
eukaryotes
Usually small
relative to
eukaryotes
Range from 1-10 μm
in diameter
Eukaryotes
Inside nucleus
(membrane bound)
Plasmids extremely
rare
Large numbers of
organelles
Many types of
organelles
Extensive usually
found throughout
volume of cell
Most are larger than
prokaryotes
Range from 5-100
μm in diameter
Nucleus
o Surrounded by a nuclear envelope (a double membrane)

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Bio 1A03
Nuclear Envelope is studded with pore-like openings, and its inside surface is associated with
fibrous proteins that form he nuclear lamina
Nuclear Lamina a lattice-like sheet that stiffens the structure and maintains its shape
o Chromosomes do not float free inside the nucleus each chromosome occupies a distinct area and is
attached to the nuclear lamina in at least one location
o Nucleolus rRNA synthesis and ribosome assembly
o Contains chromosomes (chromatin: DNA & histone proteins), which carry the cells genetic information
o Figure 7.7, page 130; Nucleus
Ribosomes
o Some are present in the cytosol (fluid compartment of the cell’s cytoplasm)
Figure 7.8, page 131
o Eukaryotic ribosomes are composed of RNA and protein
o Consists of small (one RNA molecule) and large (three RNA molecules) subunits
o When two subunits come together, they form a complex molecular machine that synthesizes proteins
o Not enclosed by membrane
Rough Endoplasmic Reticulum (rough ER)
o Figure 7.9, page 131; Rough ER
o Consists of a network of membrane-bound tubules and sacs
o Has ribosomes studded into the cytoplasmic surface of the membranes, where secreted and
transmembrane proteins are manufactured
o Enzymes, which fold and modify the proteins, are present inside the rough ER’s lumen
o From the nuclear envelope, the layers of sacs that make up the ER extend into the cytoplasm
o The ribosomes associated with the rough ER are responsible for synthesizing proteins that will be
inserted into the plasma membrane, secreted to the cell exterior or shipped to the lysosome
o As proteins are being manufactured by ribosomes, they move to the interior of the sac-like component
of the rough ER
o Rough ER products are destined for transport to various distant destinations often to the surface of
the cell or beyond
Golgi Apparatus
o Figure 7.10, page 131; Golgi Apparatus
o Products from the rough ER pass through the Golgi apparatus before they reach their final destination
o Consists of cisternae (flattened, stacked sacs); processes rough ER’s products
o The cis face receives products from the RER; closest to the rough ER and nucleus
o The trans face sends products to their destinations; oriented toward the plasma membrane
o Micrographs often show vesicles on either side of the Golgi stack, which carry proteins or other
products to and from the organelle
Smooth Endoplasmic Reticulum (smooth ER)
o Figure 7.11, page 132; Smooth ER
o Lacks ribosomes in the cytoplasmic surface of the membranes
o Enzymes are present, which are involved in several functions such as lipid synthesis, detoxification of
harmful materials, and calcium ion reserves
Endomembrane System consists of the Golgi Apparatus, lysosomes, and the endoplasmic reticulum
Peroxisomes
o Figure 7.12, page 133; Peroxisomes
o Are globular, single-membrane bond organelles
o Locations of oxidative reactions (remove electrons from atoms and molecules)
o Contain catalase, which is an enzyme that detoxifies hydrogen peroxide (H2O2) by converting it into
oxygen and water
o Eg/ Liver Cells peroxisomes oxidize an array of toxins, including ethanol, products are excreted or
used in other reactions
o Eg/ Enzymes that catalyze oxidation of fatty acids which produces molecules that include acetyl CoA,
which is used for the synthesis of important molecules elsewhere in the cell
o In plant leaves, glyoxisomes are packed with enzymes that convert one of the products of
photosynthesis into a sugar that can be used to store energy for the cell
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