BIOLOGY. Cell Structure

Cell Theory  Every living organism is made up of one or more cells  The smallest living organisms are single cells  Cells are the functional units of multi- cellular organisms  All cells arise from preexisting cells

Cell Structure  Basic Features of All Cells  Features of Prokaryotic Cells  Features of Eukaryotic Cells

Cells Share Certain Basic Features  A cell membrane  Nucleic acid  A cytoplasm  Ribosomes

CELLll Membrane

Function of Cell Membrane  Isolates the cell’s contents from external environment  Regulates the flow of materials into and out of the cell (i.e. selectively permeable )  Allows interaction among cells

Structure of the Cell Membrane Fluid-Mosaic Model  Consists of phospholipids and proteins  The head region of each phospholipid molecule “loves” water (i.e. is hydrophilic)  The tail region of each phospholipid “hates” water (i.e. is hydrophobic)  Phospholipids form a bilayer  Proteins are embedded in phospholipid bilayer

Fluid Mosaic Model of the Cell Membrane

Nucleic Acid  Deoxyribonucleic acid (DNA) has two functions:  DNA controls protein synthesis in the cell  DNA is replicated and passed on to progeny cells during reproduction

Cell Cytoplasm  Consists of all materials inside the plasma membrane  It is the “bulk” of the cell  It is the site of cell metabolism:  (1) breaking down complex molecules for energy and simple “building block” molecules  (2) synthesis of new cell structures

Animal Cell

Ribosome  Ribosomes are the sites of protein synthesis, where an RNA strand is translated into an amino acid sequence.  Protein synthesis is extremely important to cells, and so large numbers of ribosomes are found throughout cells (often numbering in the hundreds or thousands).

Ribosomes

ProkaryotesEukaryotes

Prokaryotic Cells  Lack a nucleus (though DNA is concentrated in a region called the cell’s nucleoid)  Lack membranous organelles  Are small in size ( < 5 um )  Many have cell walls  Belong to the Domain Archaea and Domain Bacteria

Eukaryotic Cell  DNA is housed in a membranous organelle called the nucleus  Membranous organelles exist in the cytoplasm and perform specific cell functions  A network of protein fibers called the cytoskeleton give the cell shape and stability  Cells are large in size (10 – 100 um)

Animal Cell

Plant Cell

Nucleus  Control center of the eukaryotic cell  Nucleus has three distinct parts:  (1) nuclear envelope: double membrane between nucleoplasm and cytoplasm  (2) nuclear pores: openings between nucleus and cytoplasm  (3) nucleoplasm: fluid portion of nucleus that contains DNA and nucleolus

Nuclear Envelope  Nuclear envelope is a double membrane  Isolates cytoplasm from nucleoplasm  Regulates flow of materials between cytoplasm and nucleoplasm  Outer nuclear membrane is continuous with cytoplasm’s endoplasmic reticulum

Nucleoplasm  Contains the hereditary molecule deoxyribonucleic acid (DNA) which always remains in the nucleus  Loose “non-condensed” strands of DNA are called chromatin (cell is not dividing)  Tight “condensed” strands of DNA are called chromosomes (cell is dividing)

Nucleolus  Present inside of nucleus as a distinct region  Consists of ribosomal RNA, protein, ribosomes and DNA  Nucleoli are the sites of ribosome synthesis in the cell

Nucleus of a Liver Cell

Endoplasmic Reticulum  A complex system of membranes present within the cytoplasm  Endoplasmic reticulum (ER) is continuous with the nuclear envelope and the plasma membrane  ER functions as an internal transport system for the cell

Endoplasmic Reticulum  There are two types of ER:  (1) smooth ER: lacks ribosomes on its surface  Involved in lipid synthesis  (2) rough ER: outside is studded with ribosomes  Involved in protein synthesis

Endo-membrane System

Golgi Complex  A set of specialized membranes derived from the ER  Looks like a stack of flattened sacs or vesicles  Receives contents from ER, modifies these products, and packages them for transport out of cell

Golgi Complex

Lysosome  A membranous sac (or vesicle) containing digestive enzymes  Enzymes used to break down complex molecules in the cell  Lysosomes’ enzymes derived from RER and lysosome’s sac derived from Golgi complex

Lysosome

Vacuole  Membranous sac used for storage of water, waste, nutrients in the cell  Plant cells tend to have one, large vacuole  Animal cells tend to have numerous, small vacuoles

Plant Cell Vacuole

Peroxisome  Similar to lysosome  Membrane-bound vesicle that contains enzymes  Enzymes are used to breakdown toxic substances to hydrogen peroxide  Hydrogen peroxide is broken down by catalase to produce water and oxygen

Mitochondrion  Double membrane organelle  Outer membrane is smooth  Inner membrane is highly folded (in order to increase inner surface area for cellular respiration)  Folds are called cristae (sing. crista)  Inner fluid portion of mitochondrion is called the matrix

Mitochondrion Structure

Mitochondrion  Space between inner and outer membrane is called the intermembrane compartment  Function of mitochondrion is the breakdown of complex molecules to release energy for the cell (cellular respiration)  All eukaryotic cells have mitochondrion

Muscle Cell Mitochondrion

Chloroplast  Present only in plants  Site of photosynthesis  Double membrane system encloses the fluid-filled space called the stroma  Membrane system within stroma is organized into interconnected, flattened sacs called thylakoids

Cytoskeleton  A network of protein fibers in the cell cytoplasm  Keeps organelles in place in cytoplasm and allows movement of organelles  Provides rigidity and structure to cell  Assists in cell reproduction (centrioles)

Cytoskeleton of Nerve Cell

Cytoskeleton Components  Actin filaments, a.k.a. microfilaments  Microtubules  Intermediate Filaments

Actin Filaments  Long, extremely thin fibers that occur in bundles or mesh-like networks  Structural role in cytoplasm, just beneath plasma membrane  Interact with protein molecules to allow movement of cell and its organelles

Microtubules  Larger than microfilaments  Microtubule production under the control of an organizing center called the centrosome  Centrioles, which are composed of microtubules, are also involved in microtubule assembly  Microtubules involved in maintaining shape of cell and movement of cell and its organelles  Cilia and flagella are produced from microtubules by the centrioles

Centrioles

Cilia: Arrangment

Flagella and Cilia

Intermediate Filaments  Size between microfilaments and microtubules  Provide support to nuclear and plasma membranes

Evolution of Eukaryotic Cells  Endosymbiotic Hypothesis  1) larger “host” prokaryotes formed a symbiotic relationship with smaller prokaryotes  eukaryote and it’s organelles  2) eg. aerobic heterotrophic bacteria  mitochondrion  3) eg. cyanobacteria  chloroplast