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 MembraneFluid-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

Prokaryotes

Eukaryotes

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

MicrotubulesLarger than microfilamentsMicrotubule production under the control of an

organizing center called the centrosomeCentrioles, 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: 9 + 2 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