Biology Chapter on Prokaryotic Cells

Cell Structures

• Cells are the basic units of life
• All cells share basic structural similarities
• Prokaryotic cells lack a nucleus and membrane-bound organelles
• Eukaryotic cells have a nucleus and membrane-bound organelles

Prokaryotic Cells

• Prokaryotes include bacteria and archaea
• Single-celled organisms that can survive in hostile environments
• Bacterial cell walls are composed of peptidoglycan
• Archaeal cell walls are made of polysaccharides and peptides
• Archaeal membranes contain unusual lipids
• Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids forming mesh-like layer outside plasma membrane of most bacteria, forming the cell wall
•  Bacteria Gram Stains have two main types, Gram-Positive and Gram-Negative
• Gram-Positive cells have a thick peptidoglycan cell wall. More easily treated with antibiotics. Stains purple/violet after Gram Stain.
• Gram-Negative cells have a thin peptidoglycan cell wall and another plasma membrane (Outer membrane). Harder to treat with antibiotics. Stains red/pink after Gram Stain.
•  Prokaryotic cells lack interior organization, consisting of cytoplasm surrounded by plasma membrane, encased within a rigid cell wall.
• Prokaryotes play a very important role in ecology of living organisms
• Three shapes of Bacteria: sphere-shaped (cocci), rod-shaped (bacilli), spiral-shaped (spirochetes)
• Parts of a Bacterial Cell: capsule, cell wall, plasma membrane, cytoplasm, nucleoid (DNA), ribosomes, pili, flagellum

Eukaryotic Cells

• Eukaryotes have a nucleus (membrane-bound organelle) and other membrane-bound organelles
• DNA is organized in multiple linear chromosomes and packaged with proteins into a complex structure called chromatin
• Eukaryotes have an endomembrane system, a group of membranes and organelles working together to modify, package, and transport lipids and proteins
• Examples of membrane-bound organelles include: nucleus, rough ER, smooth ER, Golgi apparatus/body, vacuoles and vesicles, lysosomes, chloroplasts, mitochondria, peroxisomes.
• Examples of non-membrane-bound organelles include: ribosomes, cytoskeleton, centriole

Nucleus

• The nucleus is the largest and most easily seen organelle in a eukaryotic cell
• It acts as the cell's control center and stores, protects, and expresses most of the genetic information (DNA)
• The surface of the nucleus is bounded by two phospholipid bilayer membranes that make up the nuclear envelope
• The outer membrane is continuous with the endoplasmic reticulum
• The nuclear envelope contains nuclear pores which allow ions and small molecules to freely diffuse between nucleoplasm and cytoplasm while controlling the passage of proteins and RNA
• The inner surface of the nuclear envelope has a nuclear lamina; composed of intermediate filament fibers called nuclear lamins. Nuclear lamins provide shape and is involved in restructuring during cell division
• DNA is organized into a complex structure called chromatin
• The nucleolus is the site of ribosome production

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a large network of internal membranes
• It is specialized in the synthesis and transport of protein and lipid components of other cell organelles
• The ER has two forms: rough ER and smooth ER
• Rough ER has ribosomes on its outer surface and synthesizes proteins destined for export from the cell or to lysosomes and vacuoles, or to be embedded in the plasma membrane. 
• Smooth ER lacks ribosomes and synthesizes carbohydrates and lipids. Also detoxifies toxins and drugs
• Membrane proteins are inserted into the ER membrane
• Vesicles bud off ER to carry proteins to other locations

Golgi Apparatus/Body

• The Golgi apparatus functions as the cell's post office
• It collects, packages, and distributes molecules
• The cis face is the receiving end and is near the ER, and the trans face is the concave end facing the cytoplasm.

Vesicles

• Vesicles are large structures, enclosed by a lipid bilayer, that transport, secrete, or take up materials in the plasma membrane
• Types of vesicles include: transport vesicle, secretory vesicle, vacuoles, lysosomes

Vacuoles

• Vacuoles are membrane-bound organelles present in plant and fungal cells
• Stores water, inorganic and organic molecules, including enzymes
• Animal cells have vacuoles assisting in exocytosis and endocytosis
• Plant cells have central vacuoles with a tonoplast membrane
• Plant vacuoles can act as storage organelles for sugars, polysaccharides, organic acids and proteins and act as a micro-kidney, filtering and sequestering potentially toxic ions

Lysosomes

• Lysosomes are membrane-bounded digestive vesicles
• Contain high levels of degrading enzymes involved in the breakdown of molecules (proteins, nucleic acids, lipids, and carbohydrates).

Microbodies

• Microbodies are small, spherical, membrane-bound organelles
• Two main types: peroxisomes and glyoxisomes
• Peroxisomes carry out oxidation reactions producing hydrogen peroxide. They break down fatty acids, produce energy in the form of ATP, and synthesize lipids
• Glyoxisomes are specialized peroxisomes found in plant cells and break down fatty acids into sugars (important for germinating plant cells)

Mitochondria

• Mitochondria are "powerhouses of the cell" and are the site of cellular respiration for energy production
• They are typically tubular or sausage-shaped.
• Mitochondria are bounded by two membranes: a smooth outer membrane and a folded inner membrane called cristae.
• Cristae create two compartments: the matrix and the intermembrane space.
• Proteins found on the inner membrane carry out oxidative metabolism
• Mitochondria have their own DNA (circular) containing genes needed for oxidative metabolism. Most genes for these products are in the nucleus.

Plastids

• Plastids are a type of membrane-bound organelle found in plant, algae, and some eukaryotic organisms
• Surrounded by a double membrane
• The site of manufacture and storage of chemical compounds
• Three types of Plastids: chloroplasts, chromoplasts, leucoplasts (amyloplasts)
• Chloroplasts are involved in photosynthesis; contain chlorophyll which captures light energy for the plant; flattened sacs called thylakoids
• Chromoplasts synthesize pigments for storage in plant cells and store orange pigment carotene
• Leucoplasts lack pigment and some function as starch storage

Ribosomes

• Ribosomes are the cell's protein synthesis machinery
• They are large RNA-protein complexes outside the nucleus
• Composed of two subunits (large and small) that are composed of rRNA and different proteins
• Ribosomes join to form a functional ribosome when they are actively synthesizing proteins
• Two types of ribosomes: free ribosomes (synthesize cytoplasmic, nuclear, mitochondrial, and other proteins) and membrane-associated ribosomes (synthesize membrane proteins and proteins for export from the cell)

Centrosomes and Centrioles

• Centrosomes are microtubule organizing centers (MTOCs), found in animal cells with the centrioles
• Centrioles are barrel-shaped organelles found in animal and most protist cells
• Centrosomes are responsible for reorganization of microtubules during cell division
• Usually occur in pairs at right angles near the nuclear membranes
• Pericentriolar material is surrounding the centrioles, containing ring-shaped structures called microtubules

Cytoskeleton

• The cytoskeleton is a complex internal skeleton giving cells flexibility and support
• It maintains the cell's shape and internal organization
• Permits movement of substances and external projections such as cilia and flagella
• Comprised of three protein fibers: microtubules, intermediate filaments, and actin filaments

Cilia, Flagella, and Microvilli

• Cilia and flagella are hairlike projections capable of wavelike movement
• Human sperm are the only human cells with flagella enabling movement
• In humans, cilia move substances along the free cell surfaces (respiratory and reproductive)
• Micorvilli are smaller, more numerous than cilia and do not move but increase surface area of cell membrane to increase absorption (found in intestines)