A Tour of the Cell

Posted: November 25, 2011 in Uncategorized

The cell is the basic functional unit of all life.  Modern cell theory dictates that all living things are composed of cells, and that all cells have arisen from other cells.

– the only “living things” that aren’t composed of cells are things like viruses, and it is quite debatable if those are truly living. More on that later.

– most organisms on earth consist of a single cell – unicellular

– e.g. bacteria, yeast, amoeba, paramecium

– other organisms consists of aggregations of specialized cells that work largely in concert with each other

– multicellular – e.g. plants, animals, most fungi

– cell theory was only possible with the advent of light microscopy – most eukaryotic cells are 10 ìm in dimension (Fig. 6.2) – most prokaryotic cells are 1-3 ìm in dimension – given that the resolving power of the human eye is only 100 ìm, we are therefore unable to see individual cells with the unaided eye

– [make sure you are familiar with SI (metric) units. A good review of them can be found in the Chapter 6 study area of Mastering Biology (Metric System Review)]

– interior details of cells really require electron microscopy (Fig. 6.3) – uses an electron beam with very low wavelength as the “illumination” source – this gives a functional resolving power of 1-2 nm

Prokaryotic cells

– Bacteria and Archaea (Fig. 6.5) – these are smaller, and much more simple than eukaryotic cells – unicellular – they do not have a nucleus

– they usually have only one circular piece of DNA (typically around 4 million basepairs in length) – a nucleoid region is sometimes visible, but it is not a properly defined entity- a prokaryotic cell has – a lot of ribosomes

– one or two phospholipid-based membranes – a rigid cell-wall

– in bacteria, the solid portion of the cell wall is comprised of a polymer of amino sugars known as peptidoglycan – in archaea, the cell wall is a bit more variable. Most have pseudomurein (which is very similar to peptidoglycan, but not quite the same), while others have polysaccharides and others have glycoproteins

– flagella and pili in many cases, but not all

– all prokaryotes that we are familiar with are the Bacteria; virtually all prokaryotic human pathogens are bacteria. The Archaea have an unbelievably broad ecological range, and are found in extreme environments (hot springs, salt flats, deep sea vents, etc.) that would kill any other life. We’ll take these up in more detail in the Microbial Diversity course (Bio 206).

– do the Prokaryotic Cell Structure and Function activity in the Chapter 6 study area of Mastering Biology

Eukaryotic cells

– the extra structures inside the eukaryotic cell allow functions to be compartmentalized (Fig. 6.8) – e.g. mitochondria take care of almost all ATP generation; in the prokaryotes, this happens at the plasma membrane.

– specialized structures are a response to the surface to volume ratio challenge that eukaryotic cells face

(Fig. 6.7)

– a LOT of key metabolic functions happen on membranes. Specialized structures allow for more membrane surface area per unit of volume. – this allows cells to be larger

Nucleus (Fig. 6.9) – defined by a double phospholipid bilayer nuclear membrane (envelope)

– several pores allow selective entry and exit of specific materials (such as messenger RNA) from the nucleus

– controlled by protein-based pore complex – the inner surface of the envelope is stabilized by a filamentous nuclear lamina

– contains all of the chromosomes, which is ostensibly it’s key function – linear strands of DNA wound around beads of histone proteins (Fig. 16.22)

– eukaryotes have much more DNA than in prokaryotes – humans have ~4 billion bp in the genome

– when chromosomes are wound up tightly, the individual chromosomes can be seen as discrete units under light microscope (Fig. 12.4)

– when wound loosely, they do not appear discrete, and are described as being chromatin (Fig. 12.7)

– a dark, unbound and non-discrete body within the nucleus is the nucleolus (Fig. 6.9) – it has the job of putting together the ribosomal subunits (Fig. 6.10)

– ribosomal subunits are comprised of ribosomal RNA (rRNA) and ribosomal proteins – subunits are exported to the cytoplasm, where they are put together into functional ribosomes

– ribosomes are where strands of messenger RNA (mRNA) are translated into protein – ribosomes are found either:

– free in the cytosol – used to make proteins for use in the cytosol

– bound to the endoplasmic reticulum (Fig. 6.11) – used to make proteins for export out of the cell, or for inclusion into membranes

– do the Role of the Nucleus and Ribosomes in Protein Synthesis activity in the Chapter 6 study area of Mastering Biology

Endomembrane system

– there are membranes and membrane-bound bodies throughout the cell – most, but not all, of them transfer materials between them, either directly or indirectly

– e.g. vesicles

– endoplasmic reticulum (ER) (Fig. 6.11) – is a membranous series of tunnels and sacs that runs throughout the cell – is continuous with the outer membrane of the nuclear envelope – two types of ER are typically recognized

– rough ER: studded with ribosomes – smooth ER: not studded with ribosomes

– roles of smooth ER:

– synthesis of lipids, phospholipids and steroids – adrenal gland cells that make steroid sex hormones have extensive smooth ER

– detoxify poisons and drugs in liver cells – smooth ER associated enzymes add -OH groups to toxins – makes them soluble in the cytosol, and thus can be more easily eliminated

– glycogen metabolism – smooth ER associated enzyme removes phosphate group from glucose, releasing it to the blood

– stores calcium for muscle contraction – Ca+2 pumped into the cisternae of the ER, then released into the cytosol when muscle contracts

– roles of rough ER:

– proteins for transport are translated and sent back into the cisternae – carbohydrates attached: glycoproteins – packed into vesicles that bud off from the ER

– synthesis of proteins that become part of the ER itself

Golgi apparatus (Fig. 6.12) – flattened membranous sacs, very similar to the ER – products of the ER are brought in via vesicles, are modified, and ultimately sent to other places, especially for secretion outside the cell

– e.g. amylase in saliva

– cis face

– facing toward the interior of the cell – receives vesicles from the rough ER

– the vesicles and the ER fuse like two soap bubbles – modifies products from the ER

– e.g. placing saccharides onto proteins to make glycoproteins

– modified products end up at the trans face – packaged into new vesicles, and pinch off – the vesicles containing products for secretion from the cell then fuse with the plasma membrane, and are released

– some Golgi-derived vesicles contain digestive enzymes and related proteins; become lysosomes (Fig. 6.13)

– hydrogen ions (H+) are pumped into the lysosome from the cytosol to keep the pH low – lysosomes have three functions:

– intracellular digestion – autolysis – programmed cell death (apoptosis)

– some cells can engulf (phagocytosis) particles by wrapping some plasma membrane around it and internalizing it as a food vacuole (phagosome)

– e.g. white blood cells, amoebae – lysosomes fuse with food vacuoles

– degrade the particle, and pump nutrients into the cytosol

– lysosome will engulf and destroy a cellular component when needed (autolysis) – damaged mitochondrion

– cellular health

– during development of multicellular organisms, some cells must be destroyed as part of normal tissue development (apoptosis)

– e.g. tissue between fingers – lysosomal enzymes released, killing the cell

– vacuoles

– large membrane-bound sacs with specific functions – food vacuole

– contractile vacuole – pumps water from cell (freshwater protozoa)

– central vacuole of mature plant cells – enclosed by tonoplast membrane (Fig. 6.14)

– has several functions – storage: organic or inorganic compounds, pigments, protective chemicals – sequestration of metabolic by-products – takes up water to help plant cell elongate

– takes up space, there is little cytoplasm per unit membrane surface

– do the Endomembrane System activity in the Chapter 6 study area of Mastering Biology – do the Comparing Prokaryotic and Eukaryotic Cells activity in the Chapter 6 study area of Mastering Biology

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