Lecture 5 Cell structure: Overview

Eukaryotic cells struture:

Eukaryotic cells have complex organization and structural compartmentalization

The Cell Theory

1.All organisms consist of one or more cells

2. The cell is the basic unit of structure for all organisms

3. All cells arise only from preexisting cells

The appearance of cells involved four phases:

1. Abiotic (nonliving) synthesis of simple organic compounds

2. Abiotic polymerization of these into macromolecules

3. Emergence of a macromolecule capable of replication and storing genetic information

4. Encapsulation of the first living molecule within a simple membrane

Abiotic formation of organic compounds:

• Stanley Miller (1953)

• Miller simulated the atmosphere of early Earth ▪ Hydrogen (H2 ), methane (CH4 ), ammonia (NH3 ), and water vapor (H2O)

• Lightning” as a catalyst (electrical discharge)

• Result: Produced of two simple amino acids

• Abiotic organic synthesis!

What were the parts of the device used in Abiotic formation of organic compounds: Stanley Miller (1953)?

• Electrical discharge

• sphere with the simulate atmosphere'

• place for cold water in and out

• trap

• sampling port

Predominant theory for the origins of cells

• RNA can carry genetic information

• RNA called ribozymes can perform enzymatic reactions ▪

• An “RNA world” may have predated DNA and proteins

• Lipids can self-organize into liposomes

• Liposomes and RNA could have been the earliest “protocells”

liposome

hollow membrane-bound vesicle of varying size that forms spontaneously when lipids are mixed with water

ribozyme

an RNA molecule with catalytic activity.

primative cell

• liposome

• lipid bilayer

• RNA

Last Universal Common Ancestor

Darwin: “probably all organic beings which have ever lived on this earth have descended from some one primordial form” ▪

This hypothetical “primordial” organism is though to have been the common ancestor of all life on Earth

Ancestral cell to:

• Bacteria

• Eukarya

• Archaea

Bacterial cells

• Simple organization

• A membrane encapsulates the cell’s components

• Little to no compartmentalization

whats in a bacterial cell?

• Ribosomes

• Plasma Membrane

• Cytosol

• Cell wall

• Nucleoid

• Flagella

ribosome

mall particle composed of rRNA and protein that functions as the site of protein synthesis in the cytoplasm of prokaryotes and in the cytoplasm, mitochondria, and chloroplasts of eukaryotes; composed of large and small subunits.

Eukaryotic cells: membrane-bound organelles

• Lysosome

• Peroxisome

• Mitochondrion

• Nucleus TWO PARTS:

• Nuclear envelope

• Nucleolus

• Rough endoplasmic reticulum

• Smooth Endoplasmic reticulum

• For all of these, compartmentalization is mediated by memebranes

lysosome

membrane-bounded organelle containing digestive enzymes capable of degrading all the major classes of biological macromolecules.

Peroxisome

single membrane-bounded organelle that contains catalase and one or more hydrogen peroxide–generating oxidases and is therefore involved in the metabolism of hydrogen peroxide.

What does the plasma membrane do?

The Plasma Membrane Defines Cell Boundaries and Retains Contents

▪ The plasma membrane surrounds every cell

▪ Ensures that cell contents are retained

▪ Consists of lipids (including phospholipids) and membrane proteins

▪ Organized into a bi-layer

Amphipathic Membrane Components

Phospholipids have two hydrophobic “tails” and a hydrophilic “head” (i.e., amphipathic)

Membrane proteins are also amphipathic

lipid bilayer

The lipid bilayer is formed when the hydrophilic heads face outward and the tails face inward

glycoproteins

Membrane proteins with polysaccharides attached to them are called glycoproteins

The Extracellular Matrix

• Outside the Plasma Membrane

• Most cells have extracellular structures that give physical support to cells

• Animal cells have an extracellular matrix (ECM) ▪

• Made of collagen (protein) and proteoglycans (glycoproteins) that were secreted from cells

• Specialized ECM structures include cartilage

• The ECM provides structural support and regulates cell migration, cell recognition, and cell adhesion

Cytoskeleton

• Structure for the Cytoplasm

• The cytoplasm contains an organized, 3D array of interconnected protein structures called the cytoskeleton

• The cytoskeleton gives a cell its distinctive shape and internal organization

• Microtubules, Microfilaments, Intermediate filaments

Nucleus

• Information Center of the Eukaryotic cell

• The nucleus contains the DNA

• Eukaryotic DNA is organized into chromosomes

Genome

sum total of genes on these chromosomes

▪ Human genome: instructions to make a human

46 chromosomes in human cells

nuclear envelope

double membrane around the nucleus that is interrupted by numerous small pores.

The nuclear envelope is composed of two lipid bi-layers

nucleolus

large, spherical structure present in the nucleus of a eukaryotic cell; the site of ribosomal RNA synthesis and processing and of the assembly of ribosomal subunits.

The nucleolus is the most prominent structure in the nucleus

Nuclear pores

protein channels in the nuclear envelope that allow transport between the nucleus and cytoplasm

nucleoplasm

the interior space of the nucleus, other than that occupied by the nucleolus.

chromatin

DNA-protein fibers that make up chromosomes; constructed from nucleosomes spaced regularly along a DNA chain.

Endomembranes and organelles

Eukaryotes use internal membranes to form subcellular compartments

These compartments are called organelles

Organelles have specific functions

organelle

any membrane-bounded, intracellular structure that is specialized for carrying out a particular function. Eukaryotic cells contain several kinds of membrane-enclosed organelles, including the nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes, peroxisomes, secretory vesicles, and, in the case of plants, chloroplasts.

endomembrane system

The endomembrane system is a network of organelles that transport membrane proteins and secreted proteins

Mitochondria and Chloroplasts Provide what?

• Energy for the Cell

• The mitochondrion and the chloroplast are organelles involved in energy production for cells

• Chloroplasts convert solar energy to glucose

• Mitochondria are the site of aerobic respiration, where nutrients are converted to cellular energy in the form of ATP

The Endosymbiont Theory:

• Bacterial origin of chloroplasts and mitochondria

• Both resemble bacteria in size and shape

• Both have their own DNA, which resemble bacterial DNA

• Circular chromosomes

• Different packaging proteins

• Both are surrounded by double membranes, the inner of which has bacterial-type lipids

Ribosomes

• The Site of Protein Synthesis

• Ribosomes are used by all living cells to translate mRNA and synthesize proteins

• Ribosomes are localized to the cytoplasm and are composed of RNA and protein

• Not enclosed by a membrane, so often not considered a true organelle

• ▪A single human cell contains millions of ribosomes

What does a A Virus Consist of?

• Consists of a DNA or RNA Genome Surrounded by a Protein Coat

Are viruses cells?

Viruses are not cells: they are obligate intracellular parasites that are incapable of a free-living existence

What are viruses made of?

• These parasitic macromolecular structures are composed of the same molecules as cells

• Nucleic acids, proteins (all); lipids, polysaccharides (some)

• All viruses consist of genetic material (DNA or RNA) that is protected by protein, and may or may not have a membrane

Are viruses alive?

Whether viruses are “alive” is debatable

▪ They do not synthesize their own molecules or make energy; viruses rely on a host cell for replication

Viruses turns cells into what?

• Viruses turns cells into virus factories

• Viruses are small e.g Adeno virus is 100nm but human neuron is 50,000nm

• Viruses invade and infect cells, and use the host’s synthetic capabilities to make more virus particles

human adenovirus

This human adenovirus consists of a 36kb dsDNA genome packaged in protein