Weber State University       Department of Botany

                                                    BTNY LS1203 - Plant  Biology

Attributes of Life

1.  The Cellular Basis of Life

“The key attribute of life is a boundary, a container that separates inside from outside and prevents desirable molecules from floating away, while keeping undesirable ones at bay.” – Julius Rebek

Cell Theory 
the cell is the smallest living unit
all organisms are composed of one or more cells
cells come from pre-existing cells
==> cells are fundamentally alike in their basic chemistry and processes

two types of cells:  prokaryotic and eukaryotic

2.  Metabolism

includes all of the chemical reactions of an organism  
most focus is on energy conversion reactions (respiration, photosynthesis), digestion, and assimilation

3.  Growth

irreversible increase in size:  increase in cell number, increase in cell volume 
limited by an organism's genetic heritage and environmental factors

4.  Reproduction

produce descendants:  sexual or asexual reproduction 
genetic variation in offspring from mutations, genetic mixing

5.  Response to Stimuli

on microscopic and macroscopic scales 
rapid and slow responses

6.  Movement

cellular level to whole organism 
plant movements in response to stimuli or part of developmental processes

7.  Adaptation to the Environment

organisms have features (structural, physiological, etc.) that enable a particular organism to do 2-6 in a particular environment at a particular time 
    variation in a population of organisms 
    at least some of that variation is inherited 
    differential survival and reproduction 
    → over time, evolution by natural selection occurs as the population changes and becomes adapted to the environment of a specific place at a specific time 
    example:  Saguaro cactus.  This cactus does fine in Tucson, but it is not adapted for all deserts.  Our local desert environment would kill it.
 

Molecules of Life

The four major classes of biological molecules:
 

polymers	monomers
carbohydrates (starch, cellulose)	sugars (glucose)
proteins	amino acids (20 to choose from)
lipids (fats or oils = triglycerides)	fatty acids (+ glycerol)
nucleic acids (DNA, RNA)	nucleotides (sugar, phosphate, N-base)

monomer vs. polymer
mer = unit
mono = one
poly = many
polymers are made many identical (starch) or similar (proteins) monomers

The monomers are joined by covalent bonds to make the polymers. Water is released as the covalent bonds form = condensation reaction (or dehydration synthesis).
(When the polymers are broken down - or digested - to release the monomers, water is added to break the covalent bond = hydrolysis reaction.)

all of the biological molecules contain C and H; most have O

Carbohydrates
monosaccharides:  hexoses (glucose, fructose), pentoses, trioses
disaccharides: sucrose
polysaccharides: starch, cellulose

Functions of carbohydrates:
1. Structural
        Cell wall
2. Storage of food (calories) in seeds, roots, stems, wood, bark, leaves
        Starch
        Sucrose
3. Transport of calories and carbon skeletons to make other molecules
        Sucrose

Proteins
contain N and some S (two of the 20 amino acids have S)
Polymers of amino acids
The bond between amino acids is called a peptide bond. Therefore, proteins are sometimes called polypeptides.
The 20 amino acids have the same basic format, differing only in what is known as the R group.

Functions of proteins
1. Structural
        Cell wall
        Cellular membranes
2. Storage
        Seeds, roots, stems: storage of calories and N
3. Enzymes
        catalysts: speed up chemical reactions but are not consumed or changed by the reactions
        As enzymes, proteins are responsible for all of the metabolic reactions and the synthesis of all of the other molecules found in organisms (carbohydrates, pigments, hormones, etc.)

Lipids
Functions of lipids
1. Storage
        triglycerides (triacylglycerides, TAGs): 
                for calorie storage, are usually found just in seeds. Exceptions: fruit of avocado, olive
2. Structural
        phospholipids in cell membranes
                     spontaneously form a bilayer in water: two hydrophilic surfaces with a hydrophobic interior; basis of membrane structure and function
        waxes:  long chain fatty acids + long chain alcohols
        cutin: in cuticle, which covers above ground plant parts
        suberin: in cell walls of the endodermis (roots, pine needles) and cork cells (bark)

Nucleic Acids 
DNA, RNA; important monomer = ATP

A single DNA molecule has:
a. many genes = information to make proteins (amino acid polymers)
b. genetic regulatory information = when to make a particular protein: development, response to environment; where to make a particular protein: seed, leaf

Overwhelmingly, the proteins made are enzymes. The actions of the enzymes (making pigments, making hormones, etc.) give an organism its physical attributes.

Secondary Metabolites

produced by a limited number of plants
generally not viewed as involved in essential metabolic and developmental processes 
historically, studied as "natural products"

As secondary metabolites are studied more extensively from a plant perspective rather than an economic botany perspective, we are finding that they serve the following functions for plants:
1. protective agents, such as antimicrobial agents and herbivory deterrents
2. attractants for animal pollination vectors or seed/fruit dispersers
3. allelopathy

The three most prominent types of molecules that are secondary metabolites:
phenolics
terpenes
alkaloids

Other Important Metabolites
organic acids
lignin
photosynthetic pigments

The Components of a Plant Cell

A eukaryotic cell contains many membrane bound compartments called organelles. Each organelle has several functions that it is responsible for. Key organelles found in a "typical" plant cell: 

1. nucleus = where genetic material (DNA) is stored in the form of chromosomes.  The initial steps of gene expression (DNA --->  RNA) occur here.

2. plastids = a group of organelles which include chloroplasts (photosynthesis), chromoplasts (lack chlorophylls but have carotenoids), and amyloplasts (store starch).  

3. mitochondria = convert stored energy in sugars (mostly) to useable cell energy in the form of ATP by the process of cellular respiration

4. vacuole = large, water-filled area that can occupy as much as 90% of the volume of a mature plant cell. It provides a place for water soluble pigments (anthocyanins), hydrolases, toxins (like alkaloids), wastes, storage proteins, sugars, and minerals. The water in vacuoles provides turgor pressure, which supports young, flexible plant structures.  The pH is usually acidic.

5. endoplasmic reticulum (ER) and Golgi apparatus = packaging system for export of materials to the cell exterior (including the cell wall) or to vacuoles.   

Other important cell structures that are not membrane bound:

6. plasma membrane = the selective membrane that surrounds a cell and controls entry and exit of solutes like sugars and mineral ions.  It defines the cell interior and exterior.  Some molecules, like water and carbon dioxide, pass freely through membranes.

7. ribosomes = follow instructions from nucleic acids to link amino acids together to make proteins. Found free in cytosol (see below) or attached to the ER.

8. cell wall = rigid support structure located to the outside of the plasma membrane. It provides a mechanical barrier to large things (like bacteria) but is very porous to small things (like soil mineral ions).  Cell specialization often involves elaborations to the cell wall, such as thickenings or addition of lignin or suberin.  The primary cell wall contains cellulose, hemicellulose, and pectin.  It tends to be more flexible than the secondary wall.  The secondary wall, if made, has more cellulose than the primary wall and is often further reinforced by the addition of lignin.  These additional materials make the secondary wall rigid.  For many of the cells that are dead at functional maturity (such as the water conducting cells of the xylem), the cell wall is the only cellular structure left.

Other cell-related terms:
protoplast = a cell whose wall has been removed
protoplasm = the entire cell contents inside the plasma membrane
cytoplasm = protoplasm minus the nucleus
cytosol = "the ground substance of the cytoplasm," which means everything inside a plasma membrane except the organelles.

Return to Botany 1203 Home Page.

Return to Botany 1203 Syllabus.

Return to Harley Home.

15 August 2009