The Architecture of Cells

By Keene Folwell

-Prokaryotes are ancient, small, and simple organisms.

Prokaryotes

-Share many essential subcellular structures with eukaryotes-Also different in numerous structural features.-Don’t have a nucleus or other membrane–enclosed structures-Their DNA is clustered in a region called the nucleoid.-Generally smaller then eukaryotes-Exist as either single-celled organisms, in chains, or in clusters-Further grouped into 2 major domains: Eubacteria and Archaebacteria.

Cell Structures of Prokaryotes

Cell Structures of Prokaryotes

-Cell wall

-Plasmids

-Cytoplasm-Ribosomes

-Plasma membrane

-Surface appendages

-Nucloid Region

Cell Structures of Prokaryotes-Nucloid Region• The cell’s DNA is stored in the nucleoid region.• In prokaryotes, there is only a single circular DNA molecule with a few thousand

genes that are responsible for reproduction and all cellular activities.

-Cell wall• Rigid wall outside of the plasma membrane that supports and protects the cell

from significant changes in the environment.• The Prokaryotic cell wall is made out of peptidoglycan (a peptides and

carbohydrates mixture); different from the cellulose (major component of cell walls in plant cells).

• The Prokaryotic cell wall is comparatively porous to allow nutrients in the environment to enter the cell.

• Some bacteria have a capsule (a thick layer of polysaccharides firmly attached to the cell wall).

• The capsule can also trap nutrients from the environment, and it provides another layer of protection from the host’s immune system.

Cell Structures of Prokaryotes-Surface Appendages• Some prokaryotes have a distinct appendages that allow them to move about or

adhere to solid surfaces.• These appendages consist of delicate strands of proteins and come in 2 main

forms.1. Flagella – long, thin extensions that allow bacteria to move about freely

in aqueous environments.2. Pili – shorter, finer appendages that surround the cells of some bacteria;

pili have no role in motility, but permit microbes to adhere to solid surfaces.

-Cytoplasm• A fluid-filled space where the majority of cellular activities take place.• Activities include: Synthesis of biological molecules, Production of energy,

Processing of waste products, and reproduction.• The fluid portion of the cytoplasm without the subcellular organelles is called the

cytosol.

Cell Structures of Prokaryotes-Ribosomes• Complexes of ribosomal RNA and about 3 dozen proteins found in cytoplasm.• Each of the thousands of ribosomes found in a prokaryotic cell is a self-contained

protein factory.

-Plasmids• Small, circular, double-stranded DNA pieces that are separate from the

chromosomal DNA found in the nucleoid region.• Can carry foreign genes and can self replicate independent of the larger

chromosomal DNA.• Many bacterial plasmids carry genes that benefit the survival of the host and can

move from one bacterium to another, even across species.

-Plasma membrane• A phospholipid bilayer that serves as the border that separates the cell from it’s

external environment.• Each layer is lined with an array of individual phospholipids composed of a

hydrophilic phosphate head “head” and two hydrophobic fatty acid “tails.”• Serves as a semi-impermeable barrier surrounding the cell.

Cell Structures of Prokaryotes-Plasma membrane (continued)• Biological membranes in both prokaryotes and eukaryotes are have numerous

species of proteins embedded in the lipid bilayer.• Serve as gates, tunnels, pumps, receptors, recognition molecules, and enzymes

that control traffic flow and signal transmission into and out of the cell and perform important metabolic reactions.

• Since the nonpolar fatty acids cannot exist in an aqueous solution such as water, these phospholipid molecules spontaneously orient themselves into double-layer with fatty acid tails pointing toward each other in the interior portion of the bilayer and phosphate heads exposed to the aqueous environment.

• These bilayered phospholipids further self-organize into round-up vesicles in an aqueous solution and establish an internal environment that is different from the external environment.

Eukaryotes-Larger and more complex then Prokaryotic cells.-Evolved much later in evolutionary history then Prokaryotes.-Many membrane-enclosed subcellular organelles-Can be divided into 3 distinct structures.

1. Outer selective barrier2. Internal membrane-bound structures3. A fluid-filled space

Cell Structures of Eukaryotes

-Golgi apparatus

-Rough endoplasmic reticulum

-Cytoplasm-Ribosomes

-Plasma membrane

-Endomembrane system

-Nucleus

-Lysosome-Vesicles-Smooth endoplasmic reticulum-Cytoskeleton elements-Centrioles

-Cilia & Flagella-Mitochondria & Chloroplasts-Central vacuoles

-Centrosomes

Cell Structures of Eukaryotes-Mitochondria & Chloroplasts• structures involved in transforming & harvesting energy (cellular respiration &

photosynthesis)• Mitochondria

the powerhouses of the cells. The final and most energy-productive step of metabolism take place here to

generate cellular energy in the form of ATP (adenosine triphosphate).• Chloroplasts

only found in plants and photosynthetic protists. organelles containing the pigment chlorophyll. Harness sunlight and turn it into the chemical energy for ATP for the

synthesis of sugar. (photosynthesis)• Both organelles are self-sufficient and can replicate by themselves.• Both have their own circular DNA; their own ribosomes (similar to prokaryotic

ribosomes); and they are surrounded by double membranes (2 layers of phospholipid bilayer with the internal layer structurally similar to prokaryotic membranes).

Cell Structures of Eukaryotes

-Rough endoplasmic reticulum (rough ER)• Got it’s name from the ribosomes that are studded on it’s membrane and give it a

rough or rugged appearance.• The first station in the endomembrane system after protein assembly.

-Golgi apparatus• A.K.A. the Golgi complex, or the Golgi body• A set of membranous sacs that receive proteins from the rough ER• Proteins are modified, concentrated, and packaged here, then sent out.• Highly ordered modification steps take place here.

-Nucleus• The main difference between eukaryotes & prokaryotes• Membrane-bound• Stores DNA, and spreads information to the rest of the cell• In Eukaryotes, that information is stored in genes.• Unlike prokaryotes, most eukaryotes have more than one DNA molecule inside

their nucleus.• Inside of the nucleus is the nucleolus, a ribosomal RNA factory.

Cell Structures of Eukaryotes-Endomembrane system• A system to ensure that each protein arrives at it’s intended destination• Consists of:

Nuclear envelope Endoplasmic reticulum Golgi apparatus Transport vesicles

-Cytoplasm• The cytoplasm of eukaryotic cells is filled with a large, complex collection of

organelles, many of them enclosed in their own membranes; unlike prokaryotic cells which have none.

• Located between the plasma membrane & the membrane-enclosed nucleus.• Cytoskeleton & cytoplasmic streaming

-Plasma membrane• Basically, the same as the prokaryotes with little differences.• In eukaryotic cells, the plasma membrane consists of sterols and carbohydrates;

whereas the prokaryotic cells don’t.• The composition of phospholipids & proteins varies greatly between eukaryotic

and prokaryotic cells

Cell Structures of Eukaryotes-Lysosome• The digestive organ of a cell.• Digests food via endocytosis• Also responsible for removing old & worn structures (autophagy{self-eating}) in

the cell to recycle molecules.• Plant cells have the central vacuole instead of the lysosome.

-Smooth endoplasmic reticulum (sER)• Ribosome-free• The synthesis of lipids, steroids, & carbohydrates happen in the sER’s membrane• Detoxifies drugs, alcohol, pesticides, & other toxins takes place.

-Centrioles• Paired organelles within the centrosomes• Duplicate before cell division & move to the opposite poles of the cell & serve as

anchors for spindle fibers.• Usually found around the nucleus in animal cells only.

-Centrosomes• Serves as the microtubule organizing center & plays an important role in cell

division.

Cell Structures of Eukaryotes-Central vacuoles• Found in plant cells & some protists• Exert pressure (turgor pressure) to help support the plant cell wall & maintains its

rigidity.• Store pigments, wastes & toxic byproducts.• In single-celled protists, they serve a function similar to lysosome.• Contractile vacuoles help eliminate excess water & restore the proper salt

balance in the cytoplasm.

-Ribosomes• Made up of 70 different types of protein combined with strands of ribosomal

RNA.• These particles are engaged in the synthesis of proteins under the instruction of

DNA.• There are 2 types of ribosomes

1. Those free-floating in the cystol.2. Those attached to the surface of rough endoplasmic reticulum.

Cell Structures of Eukaryotes-Vesicles• Small and membrane-enclosed compartments.• Acts as the cargo trucks of the endomembrane system for the proteins.

-Cilia & Flagella• Both are external projections of the plasma membrane that participate in cell

locomotion, adhesion, and the movement of materials on the outer surface.• The internal structure consists of microtubules bundled in a unique fashion.• The major difference, between cilia & flagella is that cilia has numerous small

projections from cell, while flagella usually exist as 1 or 2 large tails and propel a cell by undulating movement.

-Cytoskeleton elements• Comprised of a set of long, thin fibers • Found in the cytoplasm • The muscular, skeletal, anchoring, & conveyor belt system of the cells.• The jobs of the cytoskeletal elements include:

Supporting & maintaining the cell’s shape. Facilitating the cellular movement. Anchoring organelles at their needed location. Acting as tracks for “motor proteins” that transport cargo within the cell. Interacting with extracellular structures that anchor the cell.

• There are 3 major types of cytoskeleton elements: Microtubules. Intermediate filaments. Microfilaments (aka actin filaments).

• Each of these cytoskeletal elements is made of distinct protein building blocks.• All three cytoskeletal elements play important roles during cell division.

Cell Structures of Eukaryotes

-Cytoskeleton elements (continued)• Microtubules

Largest of the 3 major types Act as the skeletal system of the cell Provides intracellular support for cells Serve as “monorail” tracks to transport cargo throughout the cell Plays a prominent role as spindle fibers during cell division. Other important structures arising from microtubules are the cilia & flagella.

• Intermediate filaments Intermediate-sized fibers. Responsible for stabilizing & maintaining the position of the nucleus and other

organelles in the cell by forming a “cage” around them. Their molecular configuration resembles the steel cables found in suspension

bridges• Microfilaments

The thinnest of the 3. A strung bead-like structure. Generally located around the periphery of a cell. Provides shape, strength, and motility to the cell. Depending on the cell type, they may play a key role in cell eating, drinking, and

muscle contraction. Also responsible for cellular activities such as amoeboid movement.

Subcellular Organelles Involved in Cell Reproduction

-Cells involved in replicating DNA• Nucleus• Centrosomes & microtubules• Cell memebrane & cell wall

Life Cycle of an Organism

-Reproduction • Necessary to avoid extinction• Sexual and asexual reproduction Asexual

Binary fission for prokaryotes Mitosis for eukaryotes

The Cell Cycle

Most of the time during the cell cycle the cell prepares for mitosis, this preparation stage is called Interphase, which can be divided into 3 phases: G1 phase, S phase, & G2 phase in that order.