Cell Theory & Cell Size

Cell Theory

• All organisms are composed of cells.
• Cells are the smallest living things.
• Cells arise only from pre-existing cells.
• All cells today represent a continuous line of descent from the first living cells.

Cell Size

• Most cells are relatively small.
• Small size is due to reliance on diffusion of substances in and out of cells.
• Diffusion rate is affected by surface area available, temperature, concentration gradient and distance.

Surface Area-to-Volume Ratio

• As a cell gets larger, its volume increases at a faster rate than its surface area.
• Cell radius of 1 unit has a surface area of 12.57 units and volume of 4.189 units with a surface area/volume ratio of 3.
• Cell radius of 10 units has a surface area of 1257 units and volume of 4189 units with a surface area/volume ratio of 0.3.

Cell Size

• Most cells are microscopic in size, except for vertebrate eggs which can typically be seen with the unaided eye.
• Prokaryotic cells are generally 1 to 10µm across.

Cell Structure Similarities

• All cells resemble one another in three fundamental ways.
• All cells have centrally located genetic material (DNA).
• All cells have cytoplasm, which is a semifluid matrix of organelles and cytosol.
• All cells have plasma membrane, which consists mainly of Phospholipid bilayer.

Prokaryotic Cells

• Simplest organisms do not have a membrane-bound nucleus.
• DNA is present in the nucleoid and cells contain ribosomes.
• There are no membrane-bound organelles, but there is a cell wall outside of plasma membrane.
• The two domains of prokaryotes are Archaea and Bacteria.

Bacterial Cell

• Bacterial cells are encased by a strong cell wall made of peptidoglycan.
• Cell walls of plants, fungi, and most protists are different.
• The cell wall protects the cell, maintains its shape, and prevents excessive uptake or loss of water.
• Susceptibility of bacteria to antibiotics often depends on the structure of the cell walls.
• Some bacteria secrete a jelly-like protective capsule.

Archaea

• Do not have peptidoglycan cell walls.
• The walls are composed of polysaccharides and proteins.
• Membrane lipids are distinctly different from lipids in bacteria.
• DNA replication and protein is also different from bacteria.

Flagella

• Present in some prokaryotic cells and may be one or more or none.
• Used for locomotion with a rotary motion that propels the cell.
• Rotary motor uses energy stored in a proton gradient.

Eukaryotic Cells

• Possess a membrane-bound nucleus and are more complex than prokaryotic cells.
• Hallmark is compartmentalization achieved through use of membrane-bound organelles and endomembrane system.
• Possess a cytoskeleton for support and to maintain cellular structure.

Eukaryotic Cell Features

• Includes: plasma membrane, cytoplasm, organelles, cytoskeleton, and a nucleus.

Nucleus

• The Nucleus is the repository of the genetic information.
• Most eukaryotic cells possess a single nucleus.
• Nucleolus is a region where ribosomal RNA synthesis takes place and the nuclear envelope has 2 phospholipid bilayers.
• The DNA is divided into multiple linear chromosomes.
• Nuclear pores control passage in and out as Chromatin is DNA plus protein.

Ribosomes

• Ribosomes contain protein synthesis machinery.
• Ribosomes are found in all cell types in all 3 domains.
• Have Ribosomal RNA (rRNA)-protein complex.
• Protein synthesis also requires messenger RNA (mRNA) and transfer RNA (tRNA).
• Ribosomes may be free in cytoplasm or associated with internal membranes.

Endomembrane System

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

Endoplasmic Reticulum

• Rough endoplasmic reticulum (RER) has an attachment of ribosomes to the membrane gives a rough appearance.
• Synthesis of proteins to be secreted are sent to lysosomes or plasma membrane.
• Smooth endoplasmic reticulum (SER) has relatively few bound ribosomes.
• It has variety of functions including synthesis, storage, and detoxification
• The ratio of RER to SER depends on cell's function.

Golgi Apparatus

• Flattened stacks of interconnected membranes (Golgi bodies).
• Functions in packaging and distribution of molecules synthesized at one location and used at another within the cell or even outside of it.
• Has cis and trans faces.
• Vesicles (transport vesicles) transport molecules to destination.

Lysosome

• Membrane-bounded digestive vesicles that arise from Golgi apparatus.
• Enzymes catalyze the breakdown of macromolecules (P,C,L).
• Destroy cells or foreign matter that the cell has engulfed by phagocytosis.

Microbodies

• Variety of enzyme-bearing, membrane-enclosed vesicles or peroxisomes that contain enzymes involved in the oxidation of fatty acids.
• Hydrogen peroxide is produced as a byproduct but rendered harmless by catalase.

Proteosomes

• Large proteins cylindrical complexes engage in proteolytic activity.
• Misfolded, damaged, or no longer needed proteins.

Vacuoles

• Membrane-bounded structures in plants that have various functions depending on cell type.
• Central vacuole in plant cells.
• Contractile vacuole is found in some fungi and protists whereas storage vacuoles and food vacuoles are self-explanatory.

Mitochondria

• Found in all types of eukaryotic cells, bound by outer membrane, intermembrane space and inner membrane has cristae and matrix.
• On the surface of the inner membrane, and embedded within it, are proteins that carry out oxidative metabolism.

Chloroplasts

• Organelles present in cells of some plants and some other eukaryotes.
• Contain chlorophyll for photosynthesis.
• Surrounded by 2 membranes
• Thylokoids are membranous sacs within the inner membrane of chloroplast and Grana are stacks of thylakoids.

Endosymbiosis

• Proposes that some of today's eukaryotic organelles evolved by a symbiosis arising between two cells that were each free-living.
• One cell, a prokaryote, was engulfed by and became apart of another cell, which was then the precursor of modern eukaryotes.
• Mitochondria and chloroplasts evolved this way.

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 also a dynamic system that is constantly forming and disassembling.

Types of Fibers

• Actin filaments (Microfilaments) are two protein chains loosely twined together involved in movements like contraction, crawling, “pinching"
• Microtubules are the largest of the cytoskeletal elements with dimers of a- and ẞ-tubulin subunits that facilitate movement of cell and materials within cell.
• Intermediate filaments are between the size of actin filaments and microtubules and are very stable and not usually broken down.

Centrosomes

• The region surrounds centrioles in almost all animal cells.
• It is the microtubule-organizing center.
• Animal cells and most protists have centrioles-a pair of organelles.
• Plants and fungi usually lack centrioles.

Cell Movement

• Essentially all cell motion is tied to the movement of actin filaments, microtubules or both, while some cells crawl using actin microfilaments.
• Flagella and cilia have a 9 + 2 arrangement of microtubules, not like prokaryotic flagella.
• Cilia are shorter and more numerous.

Molecular Motors

• Vesicles can be transported along microtubules using motor proteins that use ATP to generate force.
• The vesicles are attached to motor proteins by connector molecules.
• The motor protein dynein moves vesicles along the microtubules

Flagella and Cilia

• Many protists use flagella and cilia to swim.
• Pairs of microtubules move past each other using arms composed of the motor protein dynein.
• The motor protein dynein, moves the connected vesicle along microtubules

Cell Wall

• Plants, fungi, and many protists have a Eukaryotic cell walls.
• These cell walls differ from prokaryote
• Plants and protists are made of cellulose.
• Fungi is made of of chitin.
• Plants have primary and secondary cell walls.