Cell Structure and Prokaryotic Cells

Cell Structure

• Cells have three main components: nucleoid or nucleus where DNA is located, cytoplasm, and plasma membrane.
• Cytoplasm is a semifluid matrix of organelles and cytosol.
• Ribosomes synthesize proteins.
• Plasma membrane is a phospholipid bilayer.

Prokaryotic Cells

• Prokaryotic cells are the simplest organisms and lack a membrane-bound nucleus.
• DNA is present in the nucleoid.
• They have a cell wall outside of the plasma membrane.
• They do contain ribosomes, but not membrane-bound organelles.
• There are two domains of prokaryotes: Archaea and Bacteria.

Bacterial Cell Walls

• Most bacterial cells have a strong cell wall composed of peptidoglycan.
• Cell walls protect the cell, maintain its shape, and prevent excessive uptake or loss of water.
• Susceptibility of bacteria to antibiotics often depends on the structure of their cell walls.
• Archaea lack peptidoglycan.

Flagella

• Some prokaryotic cells have flagella, which are used for locomotion.
• Flagella have a rotary motion that propels the cell.

Eukaryotic Cells

• Eukaryotic cells possess a membrane-bound nucleus.
• They are more complex than prokaryotic cells.
• The hallmark of eukaryotic cells is compartmentalization, achieved through the use of membrane-bound organelles and the endomembrane system.
• They possess a cytoskeleton for support and to maintain cellular structure.

Nucleus

• The nucleus is the repository of genetic information.
• Most eukaryotic cells have a single nucleus.
• The nucleolus is the region where ribosomal RNA synthesis takes place.
• The nuclear envelope is composed of two phospholipid bilayers with nuclear pores that control passage in and out.
• In eukaryotes, DNA is divided into multiple linear chromosomes.

Ribosomes

• Ribosomes are the cell's protein synthesis machinery.
• They are found in all cell types in all three domains.
• Ribosomes are a ribosomal RNA (rRNA)-protein complex.
• Protein synthesis also requires messenger RNA (mRNA) and transfer RNA (tRNA).

Endomembrane System

• The endomembrane system is a series of membranes throughout the cytoplasm.
• It divides the cell into compartments where different cellular functions occur.
• One of the fundamental distinctions between eukaryotes and prokaryotes is the presence of an endomembrane system.

Endoplasmic Reticulum

• There are two types of endoplasmic reticulum: rough and smooth.
• Rough endoplasmic reticulum (RER) has ribosomes attached to the membrane, giving it a rough appearance.
• RER synthesizes proteins to be secreted, sent to lysosomes, or embedded in the plasma membrane.
• Smooth endoplasmic reticulum (SER) has relatively few bound ribosomes and performs various functions, such as synthesis, storage, and detoxification.

Golgi Apparatus

• The Golgi apparatus is a flattened stack of interconnected membranes.
• It functions in packaging and distributing molecules synthesized at one location and used at another within the cell or even outside of it.
• Vesicles transport molecules to their destination.

Lysosomes

• Lysosomes are membrane-bounded digestive vesicles that arise from the Golgi apparatus.
• They contain enzymes that catalyze the breakdown of macromolecules.
• They destroy cells or foreign matter that the cell has engulfed by phagocytosis.

Microbodies

• Microbodies are a variety of enzyme-bearing, membrane-enclosed vesicles.
• Peroxisomes contain enzymes involved in the oxidation of fatty acids and produce hydrogen peroxide as a byproduct.

Vacuoles

• Vacuoles are membrane-bounded structures in plants with various functions depending on the cell type.
• There are different types of vacuoles, including central vacuoles in plant cells, contractile vacuoles in some protists, and storage vacuoles.

Mitochondria

• Mitochondria are found in all types of eukaryotic cells.
• They are bound by two membranes, with the inner membrane having cristae.
• Mitochondria carry out oxidative metabolism and have their own DNA.
• They are involved in the synthesis of ATP.

Chloroplasts

• Chloroplasts are organelles present in cells of plants and some other eukaryotes.
• They contain chlorophyll for photosynthesis.
• They are surrounded by two membranes, with thylakoids being membranous sacs within the inner membrane.
• Chloroplasts have their own DNA.

Endosymbiosis

• Endosymbiosis proposes that some of today's eukaryotic organelles evolved from a symbiosis between two cells that were each free-living.
• One cell, a prokaryote, was engulfed by and became part of another cell, which was the precursor of modern eukaryotes.

Cytoskeleton

• The cytoskeleton is a network of protein fibers found in all eukaryotic cells.
• It supports the shape of the cell and keeps organelles in fixed locations.
• It is a dynamic system, constantly forming and disassembling.

Cytoskeleton Fibers

• There are three types of cytoskeleton fibers: microfilaments, microtubules, and intermediate filaments.
• Microfilaments (actin filaments) are composed of two protein chains loosely twined together.
• Microtubules are the largest of the cytoskeletal elements and are composed of dimers of α- and β-tubulin subunits.
• Intermediate filaments are between the size of actin filaments and microtubules and are very stable.

Centrosomes

• Centrosomes are regions surrounding centrioles in almost all animal cells.
• They are microtubule-organizing centers and can nucleate the assembly of microtubules.
• Animal cells and most protists have centrioles, which are a pair of organelles.

Cell Movement

• Essentially all cell motion is tied to the movement of actin filaments, microtubules, or both.
• Some cells crawl using actin microfilaments.
• Flagella and cilia have a 9 + 2 arrangement of microtubules.

Eukaryotic Cell Walls

• Eukaryotic cell walls are different from prokaryote cell walls.
• Plants, fungi, and many protists have cell walls.
• Plants and protists have cellulose in their cell walls, while fungi have chitin.

Extracellular Matrix (ECM)

• Animal cells lack cell walls.
• They secrete an elaborate mixture of glycoproteins into the space around them.
• Collagen may be abundant in the ECM.
• The ECM forms a protective layer over the cell surface and influences cell behavior.

Cell-to-Cell Interactions

• Surface proteins give cells identity.
• Cells make contact, "read" each other, and react.
• Glycolipids are most tissue-specific cell surface markers.
• MHC proteins are involved in the recognition of "self" and "nonself" cells by the immune system.

Cell Connections

• Cell connections can be categorized into three types based on function.

Learn about the components of a cell, including the nucleoid, cytoplasm, ribosomes, and plasma membrane, and discover the characteristics of prokaryotic cells, including their lack of a membrane-bound nucleus.