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AP Biology
Basic Cell Structure

Cells
Cells are considered to be the basic units of life.
The cell is an example of Emergent Properties. The organelles alone can do nothing, but if all of them are put
together inside a cell membrane, “life” can emerge.

Cytology is the study of cells; Cytologist – a person who works with cells.

Cell Types
Prokaryotic cells
These organisms (eubacteria and archae) evolved before the evolution of the nuclear membrane and
nucleus. These cells also lack membrane bound organelles. They are extremely small when compared
to eukaryotic cells.
It is believed that the first prokaryotic cells came into existence about 3. 5 Billion Years Ago (BYA).
The oldest prokaryotic fossils are found on stromatalites (bacterial mounds) in Shark Bay, Australia.
Eukaryotic cells (“Eu” means “true”)
These cells evolved after the evolution of the nucleus. All organisms on Earth, other than eubacteria
and archae are composed of eukaryotic cells. Eukaryotic cells have a membrane bound nucleus and
membrane bound organelles.
Endosymbiotic Hypothesis
The Endosymbiotic Hypothesis, proposed by Lynn Margulis in the 1960s, hypothesized that some
prokaryotes begin to live together in symbiotic relationships with the smaller prokaryotes living inside
larger ones. This gave the symbionts a survival advantage over other prokaryotes and eventually they
evolved into Eukaryotic cells.
Smaller organisms gained protection.
Larger organisms gained energy production or faster motility.
Over time DNA segments were “swapped” to create a more permanent existence. This
“swapping” is referred to as genetic annealing.
The smaller prokaryotes eventually became the organelles within the larger
prokaryotes.
Evidence for this hypothesis is found in mitochondria and chloroplasts.
Mitochondria and chloroplasts have their own single circular chromosome like
Bacteria.
Mitochondria and chloroplasts have ribosomes that are similar to those found in
Bacteria.
Mitochondria and chloroplasts are able to independently reproduce within the
larger eukaryotic cells. They use a process similar to binary fission.
Mitochondria and chloroplasts have a double phospholipid bilayer cell membrane.
This might be evidence of the phagocytosis of the original symbionts.
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The Surface Area- to- Volume Ratio is of GREAT Importance for all cells.
As cells grow, the surface area to volume ratio for the cell decreases. Think of the surface area as the cell
membrane and the volume as the internal contents of the cell. As the cell gets larger, it gets harder for cells to
transport in and out the required materials. This is one of the main reasons that cells must always be very small.
Smaller cells have a higher surface area to volume ratio and are better able to transport materials through the cell
membrane.

Adaptions for increasing the surface area without increasing the volume

Surface area to volume ratio can also be used to explain the shape of many cells / cellular surfaces. For
example the folds inside the mitochondria or the flat pan-cake like structures inside chloroplasts
provide a greater surface area on which specific reactions can occur. The folds in the lining of our
stomachs or the tiny cellular, finger-like projections that protrude from the wall of the small intestine
(villi and microvilli) all act to increase the surface area without increasing the overall size or volume of
the organ.
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Basic Prokaryotic Cell (Bacteria and Archae) Structure

All prokaryotes are unicellular. Prokaryotic cells possess a single circular chromosome (not enclosed in a nuclear
membrane), ribosomes (for making proteins), and cytoplasm. The cells do not possess membrane-bound
organelles like mitochondria or chloroplasts.

Three basic shapes of prokaryotes exist:

Cocci (Means “round”)
Bacilli (Means “rod”)
Helical (Means “spiral”)
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Most prokaryotes will have a cell wall. (This is NOT the same as a plant’s cell wall.)
This structure is primarily for protection of the underlying cell membrane.
It also helps prevent the prokaryotes from bursting in an aquatic environment.
In eubacteria, the cell wall is mainly composed a protein and sugar substance known as
peptidoglycan.

Scientists/doctors perform Gram stains to identify bacteria as either Gram positive or Gram negative.
Different antibiotics work against each group.
Gram positive bacteria stain blue. These bacteria possess a THICK peptidoglycan layer.
Gram negative bacteria stain red. These bacteria possess a THIN peptidoglycan layer
BETWEEN phospholipid layers.

Some bacteria produce a Capsule that covers the cell wall. The capsule is a sticky substance for adherence to
surfaces. The capsule can protect the bacteria from a host cell’s immune response.

Although we think of bacteria as germs that make us sick, most bacteria are beneficial. Bacteria are important in
the process of decomposition/mineral recycling. Other bacteria fix nitrogen, do photosynthesis, and help us
manufacture certain foods and chemicals.
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Basic Structure of Eukaryotic Cells

1. Plasma “cell” membrane --This holds the cell together. The eukaryotic cell membrane is very similar to the
prokaryotic cell membrane. The membrane is important for transporting substances into and out of the cell.

2. Nucleus--This structure controls the activities of a cell by holding the DNA. The DNA serves as the
instructions for building proteins. The DNA of eukaryotes is enclosed within a membrane called the nuclear
membrane or envelope.
Prokaryotic DNA floats in the cytoplasma and is sometimes referred to as nucleoid (nucleus-like).

3. Cytoplasm or cytosol—This fluid filled space contains the nucleus and the other organelles. This area
makes up most the volume of the cell.

4. Membrane-bound organelles—These enclosed structures specialize to carry out specific jobs within the cell.
Examples include: the nucleus, mitochondria, chloroplasts, the endoplasmic reticulum, and the Golgi
apparatus.
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AP Biology
Membrane Structure and Transport of Molecules

Selectively Permeable
The cell membrane (plasma membrane) is referred to as selectively permeable or semipermeable. This means
that the cell allows or actively transports certain materials into or out of the cell while not allowing other
materials to enter or exit the cell. Typically, small and nonpolar substances can enter the cell easily, while large
and/or polar/charged substances either cannot enter or have to be brought into the cell via some type of protein
channel.

Membrane Structure
A phospholipid bilayer makes up the majority of the cell membrane and also the organelle membranes.
Phospholipids are amphipathic molecules. They have both hydrophilic and hydrophobic
components/sides. These molecules form a bilayer because the hydrophilic portions of the molecule
(the phosphate groups) orient themselves toward the water that is located both inside and outside the
cell, while the hydrophobic portions of the molecule (the fatty acid tails) orient themselves away from
the water and toward each other in the center of the membrane.

Structure of a Phospholipid
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Proteins
Many different types of proteins are embedded into the phospholipid bilayer.
Integral Proteins run completely through the bi-layer from the outside to the inside and
function in the transport of molecules across the membrane. They also help to maintain the
INTEGRITY of the membrane.
Peripheral Proteins are located on one side of the membrane. They do not extend through the
bi-layer. Peripheral proteins can act as receptors for cell signals, as catalysts/enzymes, and as
structural components of the cytoskeleton.
The proteins of the cell membrane can also perform the following functions:
Molecule transport—Each transport protein aids the transport of a specific molecule, ion, etc…
across the membrane.
Act as enzymes to catalyze specific reactions.
Cell to cell communication and recognition—These proteins can help cells attach to each other
and work together and/or communicate with each other.
Signal Receptors—Act to receive hormones or other signaling molecules which circulate in the
blood or interstitial fluids.
Attachment points—Some proteins act as attachment points for the cytoskeleton.
Cholesterol
This lipid molecule functions to keep the membrane from being too fluid, and too permeable to some
small molecules. It also helps to secure the proteins that are embedded in the membrane.
Cholesterol helps to keep the cell membranes of plant cells from freezing solid in very cold
temperatures.

Fluid Mosaic Model of the Cell Membrane

The current scientific model of the cell membrane is referred to as the Fluid-Mosaic model because it looks like
a moving (Fluid) puzzle (mosaic). The phospholipids compose the fluid portion, while the proteins are embedded
within like a mosaic. All the pieces can move laterally, like students moving from seat to seat. The proteins
moving in this sea of phospholipids are like a teacher moving around the student desks. Imagine the ceiling and
floor are water molecules. The water molecules prevent vertical movement of the cell membrane
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components.
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AP Biology
Eukaryotic Cell Components

There are four main parts to Eukaryotic Cells:

Plasma “cell” membrane—This structure holds the cell together and helps to regulate which substances can
enter/exit the cell.
Nucleus--This controls the activities of a cell because it contains the DNA which acts as the instruction for
building the cell’s proteins and determining its traits.
Cytoplasm or cytosol—This fluid-filled space contains the organelles and makes up most of the volume of the
cell.
Organelles—These structures specialize to carry specific functions within the cell. By specializing, they divide
up the labor and make the cell more efficient. It is important to note that the number and distribution of
organelles differs from cell type to cell type.

Nucleus

This acts as a control center for all activities performed by the cell.
It is the source of the cell’s genetic information or DNA.
Nuclear Envelope
It is composed mainly of a double phospholipid bi- layer.
It encloses the DNA.
It also contains pores (tunnels) composed from proteins which allow certain specific materials to
enter/exit the nucleus. The messenger RNA must exit the nucleus and go to the ribosomes where it
acts as the directions for making proteins.
DNA
Chromatin phase—During most of the cell’s life cycle, the DNA is loose and spread-out throughout
the nucleus. During the chromatin phase, the DNA looks like a bowl of plain spaghetti noodles.
During the phase, the DNA can be transcribed and used to make proteins.
Chromosome phase—During this phase, the DNA coils around proteins called histones (in
eukaryotes and archae). The coiling helps to organize the DNA so that it can be corrected distributed
during the processes of nuclear and cell division
Nucleolus
This structure appears as a dark spot within the nucleus.
The nucleolus functions to make the ribosomal RNA (rRNA) and proteins which make up the cell’s
ribosomes.
Ribosomes
These are CELL PARTICLES made of ribosomal RN A(rRNA) and proteins. Ribsomes are not usually
considered to be organelles because they are not enclosed within a membrane. All cell types, both prokaryotic
and eukaryotic, have ribosomes.
Ribosomes are the sites of Protein Synthesis. The cell’s normal proteins and enzymes are ALL made
here.
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Two types of ribosomes exist based on location:
Free Ribosomes– These float “freely” in the cytoplasm of a cell. (They are found in ALL
TYPES of cells.) These ribosomes make proteins that will stay and function inside the cell that
made them.
Bound Ribosomes – These ribosomes are attached to the rough endoplasmic reticulum (RER).
(These are ONLY found in Eukaryotes ONLY because only they have the RER.) Bound
ribosomes make proteins that will leave the cell to be used elsewhere. Many of these proteins
acts as cellular communication signals or as antibodies to fight infections.

Endomembrane system
The endomembrane system (endo = “within”) is a group of membranes and organelles in eukaryotic
cells that work together to modify, package, and transport lipids and proteins. Once the bound
ribosomes make their proteins, the proteins enter the Rough ER and are eventually packaged into
phospholipid-based secretory vesicles. These vesicles transport the proteins to the Golgi apparatus
where they will be modified. After modification, the proteins are once again packaged into a lipid-
based vesicle and shipped to the cell membrane. The proteins are excreted from the cell while the
phospholipids that made up the vesicle become part of the cell membrane. The general pathway is
(RER Secretory vesicle Golgi secretory vesicle Membrane for release. In some cases, the packaged
proteins/enzymes become part of lysosome instead of being transport out of the cell.

Endoplasmic Reticulum (ER)

It is composed of a network of small tubes called cisternae. (“cisternae” means “tubes”)
The ER is ALWAYS found just outside and around the nucleus.
Two types of ER can exist inside EUKARYOTIC cells:
Smooth Endoplasmic Reticulum (SER)
This structure helps with the synthesis of lipids, phospholipids, and steroids.
It also helps with carbohydrate breakdown.
The smooth ER can also aid in the detoxification of the blood. (Liver cells are loaded with
SER.)
It also helps the storage of Ca++, needed for muscle contraction. (Muscle cells have lots of
SER.)
Rough Endoplasmic Reticulum (RER)
This structure helps with protein synthesis, modification, and transport.
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Ribosomes are bound to the outside of the organelle and deposit the newly constructed proteins
into the Rough ER. Inside the structure, the proteins are folded into the specific 3-D structure
needed to function.

Golgi Apparatus
The Golgi Apparatus or Golgi Body modifies proteins by attaching sugars to them. (called Glycoproteins). The
Golgi make act as a warehouse for storage of proteins, but eventually packages the proteins and ship them out in
vesicles.
The Golgi apparatus is usually located near the cell membrane.

Lysosomes - These organelles contain powerful hydrolytic enzymes and acids.

The lysosomes help to carry out the process of intracellular digestion. This process helps to breakdown materials
within a cell.
Once broken down, the components of some of these materials may be recycled for other purposes.

Vacuoles and Vesicles – These organelles act as phospholipid-based storage containers for the storage of
materials needed by the cell. Various types such as Food, Contractile, Central exist in different types of cells.

Mitochondria - Nicknamed the “Power House”.

The mitochondria perform the process of aerobic cellular respiration. During this process the energy from food is
transferred to the bonds of ADP and P to create ATP. ATP then serves as the source of energy for most of the
cell’s processes.
This organelle has its own DNA, its own bacteria-like ribosomes, its own enzymes and it can even reproduce
independently via binary fission. The inner membrane of the mitochondria are folded into structures known as
cristae. The folds increase the surface area and serve as the sites for the electron transport chain.
Evolutionary Significance—Mitochondria are believed to have descended from aerobic bacteria that entered into
a symbiotic relationship with a larger prokaryote cells that could provide protection in return for the ATP
produced by the mitochondria. Together they would have an evolutionary advantage over other bacteria. The
advantage allowed them to survive and reproduce and eventually led to the evolution of Eukaryotic cells.
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Chloroplasts

These organelles are the sites of Photosynthesis in plants and algae.

Chloroplasts are a type of Plastid or pigment container.
Like mitochondria, chloroplasts have their own DNA, ribosomes, and enzymes! They can also reproduce
independently via binary fission too!
The interior of a chloroplast is composed of stacks of sack-like structures known as thylakoids. This stack-like
arrangement increases the surface area needed to carry out the light-dependent stages of photosynthesis.
The stroma is mostly watery space in between the thylakoids and outer membrane. The stroma serves as the site
of the Calvin Cycle (the metabolic pathway in which sugar is made).
Evolutionary Significance—Chloroplasts are thought to have evolved from blue-green bacteria (cyanobacteria)
that entered into a symbiotic relationship with other bacteria for protection in return for sugar production.

Endosymbiont Hypothesis tries to scientifically explain the symbiotic relationships that led to the evolution of eukaryotic
cells from prokaryotic cells.

This hypothesis was proposed by Lynn Margulis in the 1960’s.

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It basically hypothesized that Prokaryotes came to live together in symbiotic relationships with smaller
prokaryotes living inside larger prokaryotes in order to gain a survival advantage over other prokaryotes. In
return, the larger prokaryotic hosts gained extra sources or energy or better motility. These symbiotic
partnerships eventually evolved into Eukaryotic cells over many generations that spanned hundreds of thousands
of years.

Cytoskeleton
These structures help to support and protect the cell.
The cytoskeleton also helps to keep inner organelles organized. The spindle fibers that help to move the
chromosomes during mitosis and meiosis are composed of elements of the cytoskeleton.
The cytoskeleton also helps to make up structures such as flagella and cilia which aid in cell motility or cell
organelle movement. (Much like your skeleton helps you move.)
The cytoskeleton is composed of various sized protein fibers known as either microtubules, microfilaments, or
intermediate filaments. (Your skeleton has different sized structures too. Largest – bones, middle – Ligament and
tendons, smallest- muscle fibers.)

Protective or weight bearing structures for cells:

Cell Wall of Plant Cells--It is composed primarily of the carbohydrate cellulose. It functions to provide support
and protection to the plant. It may also protect the cells from bursting if they are exposed to hypotonic
conditions.
Cell Walls of Fungus--Composed of the carbohydrate called Chitin. The functions of the fungal cell wall are
similar to those of the plant cell wall.
Extra Cellular Matrix (ECM)
The extracellular matrix consists of molecules that are secreted by a cell into the space out the cell’s
membrane. The extracellular matrix can form cell walls, bone, cartilage, etc… The ECM can
function to provide support, to segregate different tissues from one another, and to regulate
intercellular communication.

A CELL IS THE MORE THAN THE SUM OF ITS PARTS! Only when all the parts come together and work together
can “LIFE” happen.