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Cell Theory
Proposed to explain the observation that all organisms are composed of cells
Principles of Cell Theory
- All organisms are composed of one or more cells and the life process of a metabolism and heredity occur within these cells
- Cells are the smallest living things, the basis unit of organization.
- Cells only arise through the division of previously existing cells
Benefits of Cell Size
- Most cells are small because of the need for diffusion
- The rate of diffusion is effected by:
o Surface area
o Temperature
o Concentration of the substance
o Distance over which the diffusion occurs
- Benefits:
- Small cells have a larger volume-to-surface area ratio compared to larger cells
Cell Structure
All cells resemble one another in fundamental ways
4 Features that All Cells Share:
Centrally located genetic information, cytoplasm, ribosomes, and plasma membranes
Eukaryotic Cells
Cells in multicellular organisms (plants, animals, fungi, and protists) Achievement of compartmentalization through the endomembrane systems (existence of organelles), have membrane-bound organelle
1. The Plasma Membrane
a. Made up of a phospholipid bilayer with embedded proteins that separates the internal content of the cell from its surrounding environment.
b. Regulates the passage of substances like organic molecules, ions and water
1. The Cytoplasm
a. The contents of a cell between the plasma membrane and the nuclear envelope
b. Made up of organelles suspended in the gel-like cytosol
c. Cytosol- the aqueous component of the cytoplasm of a cell within which various organelles and particles are suspended
d. Function: medium for chemical reactions
1. The Cytoskeleton
a. organelles in place and allows the cytoplasm to move within the cell
b. Function: spatially organizes the contents of the cell, connects the cell physically and biochemically and generates coordinated forces that enable the cell to move and change shape
Three types of fibers in the cytoskeleton
actin filaments, intermediate filaments, and microtubules
Actin filaments
i. the thinnest of the cytoskeletal fibers that move cellular components and maintain the structure of the folding of the plasma membrane, responsible for muscle cell contraction.
Intermediate filaments
have structural function, maintain the shape of the cell and anchor organelles (ex Keratin)
Microtubules
i. thickest of the cytoskeletal fibers, hollow tubes that can dissolve and reform, give organelle movement and are structural components of flagella and cilia
The Endomembrane System
a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins
1. The Nucleus
a. Houses the cell’s DNA in the form of chromatin and directs the synthesis of ribosomes and proteins, acts as the information system
Chromatin
a. a mixture of DNA and proteins that form the chromosomes found in cells
Chromosomes:
a threadlike structure of nucleic acids and protein carrying genetic information in the form of genes
1. The Nuclear Envelope
a. A double membrane structure that constitutes the outermost portion of the nucleus (made of phospholipid bilayers)
1. Nuclear Pores
a. Pores on the nuclear envelope that control the passage of ions, molecules, and RNA
1. The Endoplasmic Reticulum
A series of interconnected membranous tubules that modify proteins and synthesize lipids
Rough Endoplasmic Reticulum (RER)-
a. ribosomes attached to the RER’s cytoplasmic surface give it a studded appearance; function- many roles in protein synthesis including post-translational modification (folding or addition of sugars), folding, and sorting
Smooth Endoplasmic Reticulum (SER)-
a. has few or no ribosomes on its cytoplasmic surface; synthesis carbohydrates, lipids, and steroid hormones; detoxifies medications and poisons
1. Golgi Apparatus
a. A series of flattened membranous sacs where the sorting, tagging, packaging, and distribution of lipids and proteins takes place
1. Lipid Droplets
a. Organelles with a neutral lipid core surrounded by a single layer of phospholipids
b. Form storage deposits for lipids that are used for energy storage
1. Vesicles and Vacuoles
a. Membrane-bound sacs that function in storage and transport; help transport material that an organism need to survive and recycle waste and maintains water balance
Ribosomes
a. Responsible for protein synthesis, organelles that freely float in the cytoplasm
1. Mitochondria
a. Powerhouse of the cell, responsible for making ATP through cellular respiration ( the process by which cells derive energy from glucose (glucose and oxygen are inputs, producing carbon dioxide, water, and ATP)
1. Peroxisomes
a. Small round organelles enclosed by single membranes that carry out oxidation reaction that break down fatty acids and amino acids
b. Detoxify poisons and create hydrogen peroxide
Animal Cell Specific Organelles
Centrosome, Lysosomes, Flagella, and Cilia
1. Centrosome
a. Control the internal spatial organization of cells, organize microtubules and provide structure to the cell
b. composed of centrioles
Lysosomes
a. The “garbage disposal” of the cell, where proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles are digested.
b. Use enzymes to destroy disease.
Flagellum (singular flagella)
a. long hair-like structures that extend from the plasma membrane and are used to move an entire cell (only a few in a cell)
Cilium (singular cilia)-
a. large in number and extend along the entire surface of the plasma membrane, short hair-like structures that are used to move cells or move substances along the outer surface of cells
Plant Specific Organelles
Cell Wall, Chloroplasts, and Tonoplasts
1. Cell Walls
a. Found immediately outside the cell membrane, provide protection, structural support, and maintain cell shape.
1. Chloroplasts
a. Serves as the site of photosynthesis, produces energy through photosynthesis and oxygen-release processes.
b. Responsible for the biosynthesis of active compounds into ATP
1. Tonoplast (vacuolar membrane)
a. The cytoplasmic membrane surrounding a vacuole, separating the vacuolar contents from the cell’s cytoplasm
b. Regulates the movement of ions around the cell
Prokaryotic Cells
a single-celled organism that has neither a distinct nucleus nor membrane bound organelles
Endosymbiosis
the hypothesis that eukaryotic cells evolved from a symbiotic association of prokaryotes
Prokaryotic Specific Cell Structures
Nucleoid
1. Nucleoid
a. The region in the cell that contains genetic material and chromosomes.
Cell Structure found in Prokaryotes
1. Plasma Membrane
2. Cytoplasm
1. Ribosomes
2. Cell wall
3. Cytoskeleton
4. Flagella
Glycolipids
- lipids with carbohydrate heads that act as cell-surface markers (Ex. MHC (major histocompatibility complex), a type of encoded gene that helps immune system cells recognize self and none-self or harmful cells)
Surface marker
1. variable, integral proteins or glycolipids in the plasma membrane that create cell identity (Ex MHC complexes)
Septate Junctions (tight junctions)-
1. tightly bound, leakproof, claudin protein seal that surrounds a cell, holds cells together such that material pass through, but not between cells (Ex. Cells in the gut) (unique to vertebrates)
Adhesive Junctions (desmosome)-
1. cadherins (trans-membrane proteins) bind to intermediate filaments of cytoskeletons creating strong flexible connection between cells (only in vertebrates)
Adhesive Junction (adherens junction)
1. cadherins bind to microfilaments of cytoskeleton connecting cells together (found in all multicellular organisms)
Adhesive Junction (hemidesmosome, focal adhesion)-
1. integrin proteins bind cell to extracellular matrix providing attachment to a substrate
Communicating junction (gap junction)-
1. six transmembrane connexon/pannexin proteins creating a pore that connects cells allowing passage of some molecules from cell to cell tissue (only in animals)
Communicating Junction (plasmodesmata)
1. cytoplasmic connection between gaps in adjoining plant cell walls