Elements: Substances that cannot be broken down into other substances through ordinary means.
There are 92 naturally occurring elements.
6 elements make up 99% of the weight of all living matter: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (CHNOPS).
Water (H_2O) makes up more than 50% of all living matter and >90% of plant tissue.
Electrically charged ions (K^+, Mg^+, Ca^+) are crucial for living tissues but make up ~1% of their weight.
The rest of a living organism is made up of organic molecules.
Organic molecules: Molecules that contain carbon.
A single bacterial cell contains 5,000 types of organic molecules.
Animal and plant cells contain more than twice that amount.
All are composed of CHNOPS.
Only a few of the thousands play major roles in living systems.
Carbon has special bonding properties, allowing it to bond to many things to form many types of organic molecules.
Four types of organic molecules make up most of the dry weight of living organisms:
Carbohydrates
Lipids
Proteins
Nucleic Acids
Carbohydrates
Most abundant organic molecule in nature.
Primary energy-storing molecule in most living organisms.
Simple carbohydrates are small molecules known as sugars.
Complex carbohydrates are sugars joined together.
Three main kinds of carbohydrates, classified by the number of sugar subunits they contain:
Monosaccharides: "Single" or "simple" sugars, such as ribose, glucose, and fructose. Consist of one sugar molecule.
Disaccharides: Contain two linked sugar units, such as sucrose, maltose, and lactose.
Polysaccharides: Contain many sugar subunits linked together.
Polymers (meaning "many parts")
Large molecules, like polysaccharides, that are made up of similar or identical small subunits.
Monomers: Individual subunits of polymers.
Polymerization: The stepwise linking of monomers into polymers.
Monosaccharides
Simplest type of carbohydrate.
Building blocks from which living cells construct disaccharides and polysaccharides.
Important monosaccharide: Glucose, one of the primary sources of chemical energy for plants and animals.
Disaccharides
Sugars are often transported in the plant body as disaccharides.
Sucrose: A disaccharide composed of monosaccharides glucose & fructose. Form in which sugar is transported in most plants - from photosynthetic source (usually leaves) to other parts of the plant body.
Sucrose is known as table sugar, harvested from sugar beets (roots) and sugar cane (stems).
Polysaccharides
Polymers made of monosaccharides linked together in long chains.
Starch: Primary storage polysaccharide in plants. Consists of chains of glucose molecules. Has 2 forms.
Polysaccharides must be broken down into monosaccharides and disaccharides before they can be used as energy sources or transported through living systems.
Plants break down their starch reserves when they need mono- and di- saccharides for growth and development.
Animals break down polysaccharides when our digestive systems break down starch in our foods, making monosaccharide glucose available as a nutrient for our cells.
Important structural compounds:
Cellulose: Principal component of the plant cell wall.
Most abundant organic compound known. Half of all organic carbon in the living world is contained in cellulose!
Chitin: Principal component of fungal cell walls, insect and crustacean exoskeletons.
Lipids
Fats and fat-like substances.
Generally hydrophobic ("water fearing"); insoluble in water.
Purposes:
Energy storing molecules, in the form of fats or oils.
Structural purposes: Phospholipids (component of biological membranes); waxes.
Barriers to water loss.
Plants typically store energy in the form of starches.
However, some plants store energy in the form of oils.
Especially in seeds and fruit (olive oil, peanut oil, sunflower oil, etc.).
Animals readily convert excess sugar to fat for storage.
Phospholipids
Important structural component of cellular membranes.
Consist of a hydrophobic fatty acid tail and a hydrophilic (soluble in water) phosphate head.
If added to water, they will form a film on the surface with the hydrophilic head submerged and the hydrophobic tails protruding above the surface.
If surrounded by water, like inside a cell, will form a double layer called a phospholipid bilayer.
Hydrophobic tails are oriented towards one another, and hydrophilic heads are oriented outwards.
Barrier Lipids
Cutin and suberin are unique lipids that are important structural components of cell walls.
They form a framework in which waxes – long-chain lipid compounds – are embedded.
Waxes are the most water-repellent of the lipids.
The waxes within the cutin & suberin framework form barrier layers that prevent water loss.
The cuticle covers the outer layer of plant epidermal cells that are exposed to air.
The cuticle is composed of wax embedded in cutin and acts to slow water loss from the epidermis.
Steroid Lipids
Steroids contain four interconnected hydrocarbon rings called a sterol.
Sterols are important components of membranes, where they stabilize the phospholipid tails.
Steroids may also function as hormones.
Steroid antheridiol serves as a sex attractant in water mold Achlya bisexualis.
Important sterols:
Sitosterol: Most abundant steroid found in green algae and plants.
Ergosterol: Most abundant steroid in fungi.
Cholesterol: Common in animal cells, only present in trace amounts in plant cells.
Proteins
In most living organisms, proteins make up >50% dry weight.
Only plants, with their high cellulose content, are less than half protein in their dry weight.
Perform an incredible variety of functions in living organisms.
All follow the same simple blueprint: They are all polymers of nitrogen-containing molecules called amino acids, arranged in a linear sequence.
A large variety of amino acids are theoretically possible, but only 20 kinds of amino acids are used by living organisms to form proteins.
Protein molecules are long and complex, often containing hundreds of amino acid monomers.
The possible number of different amino acid sequences, and thus protein types, is enormous.
A single cell of E. coli contains 600-800 different kinds of proteins.
A complex organism has several thousand, each with a special function.
In plants, the largest concentration of proteins is found in certain seeds (cereals, legumes, nuts…). These proteins function as stored amino acids to be used by the embryonic plant when the seed germinates.
Protein – Shapes & Functions
A chain of amino acids is also called a polypeptide chain. Proteins are assembled from one or more long polypeptide chains.
The sequence of the amino acids in the chain(s) determine the structural features and biological functions of the protein.
Even one small variation in the sequence may alter/destroy the way in which the protein functions.
As a polypeptide chain is assembled in the cell, interactions between the amino acids cause it to fold into a pattern.
Enzymes are Proteins
Enzymes are large, complex proteins that act as catalysts.
Catalyst: Substances that accelerate the rate of chemical reaction by lowering the energy of activation, but themselves remain unchanged in the process.
Because they remain unaltered, enzymes can be used repeatedly and are effective at low concentrations.
Nucleic Acids
Molecules responsible for encoding and translating the information that dictates the structures of proteins.
Consist of long chains of molecules called nucleotides.
Two types of Nucleic Acids are found in living organisms:
Ribonucleic acid (RNA)
Deoxyribonucleic acid (DNA)
Both consist of a long chain of nucleotides.
DNA and RNA play different biological roles.
DNA: Carrier of genetic messages. Contains the information that organisms inherit from their parents, organized in units called genes.
RNA: Use genetic information in DNA to synthesize proteins.
Nucleotides
In addition to their role as building blocks for nucleic acids, nucleotides have another crucial function.
When modified by the addition of 2 more phosphate groups, they become the carriers of the energy that powers the chemical reactions occurring within the cells.
Adenosine triphosphate (ATP): Principal energy carrier for most processes in living organisms.
Secondary Metabolites
Compounds that are produced by plants have two categories:
Primary metabolites: Molecules present in all plant cells and that are necessary for life – like simple sugars, amino acids, proteins, and nucleic acids.
Secondary Metabolites: Not evenly distributed throughout the plant body or between different plant species.
Serve a variety of important roles:
Chemical signals that enable plants to respond to environmental cues.
Function in plants' defense against herbivores, pathogens, and competitors.
Provide protection from the sun's radiation.
Aid in pollen and seed dispersal.
Production occurs in a specific organ, tissue, or cell type at specific stages of development (fruiting, flowering, juvenile stages, in response to wounding or presence of a pathogen, etc.).
Stored primarily in cell vacuoles.
Concentration varies greatly, even within the same plant in a 24-hour period.
Three major classes:
Alkaloids (cocaine, morphine, nicotine, caffeine)
Terpenoids (rubber, latex, essential oils)
Phenolics (tannins, salicylic acid)
Anatomy
Structure of a plant cell
Plant cells consist of a rigid cell wall on the outside and a protoplast on the inside.
Protoplast = cell contents
The Cell Wall
One of the biggest structural differences between plant and animal cells.
In a plant cell, the cell wall is thick, complex, and rigid.
This prevents the cell from bursting under the pressure of the water in the plant.
It also allows the plant to keep its shape while growing (sometimes very large) under the pressure of gravity.
Cellulose is the primary component of the plant cell wall.
Protoplast
Plant cells consist of a rigid cell wall on the outside and a protoplast on the inside.
Protoplast = cell contents
Protoplast consists of two parts:
Cytoplasm
Nucleus
The Cytoplasm
Consists of a variety of organelles suspended in a fluid called cytosol.
It is surrounded by a plasma membrane that:
Mediates the transport of substances into and out of the protoplast.
Detects and facilitates responses to hormonal and environmental signals.
In a living plant cell, the cytoplasm is always in motion!
The organelles and the cytosol move continuously in a pattern akin to an underwater current.
This movement is called cytoplasmic streaming and it continues as long as the cell is alive.
Organelles within the Cytoplasm
All eukaryotic cells:
Ribosomes: Site of protein formation
Mitochondria: Site of respiration
Peroxisomes: Responsible for photorespiration and enzyme storage
Endoplasmic Reticulum and Golgi Apparatus: Protein synthesis system, especially proteins for cell walls and other membranes
Specific to plant cells:
Chloroplasts and other plastids
Vacuole
Chloroplasts
Sites of Photosynthesis
Chloroplasts are the home of pigments:
Chlorophyll: Responsible for the green colors in plants
Carotenoid: Responsible for the orange/yellow colors in plants
They consist of an inner and outer membrane, stacks of thylakoids (called granum), and a stroma.
Chlorophyll and carotenoid are imbedded in the thylakoids.
Other Plastids
Storage organs
Chromoplasts are structures that only contain carotenoid, no chlorophyll.
They are storage organs for carotenoid – they are not photosynthetic.
In some instances, chloroplasts can become chromoplasts, for example during fruit ripening.
Other plastids, called leucoplasts, are pigment-less.
Vacuole
Storage organ
Liquid-filled cavity
Cell sap (mostly water, nutrients, and waste products)
Immature plant cells typically contain multiple small vacuoles that merge into one large vacuole as the cell enlarges.
Regulates internal pressure within the cell, which dictates the tissue rigidity of the plant.
The Nucleus
Controls the ongoing activities of the cell by determining which protein molecules will be produced by the cell and when.
Stores the DNA & RNA.
Movement of substances into and out of the cell
Plant Cell Membranes are semi-permeable
Cells are able to regulate the amount, kind, and sometimes the direction of movement of substances that pass across their membranes
Water (solvent)
Other molecules and ions that are dissolved in water (solutes)
Non-polar (no electrical charge) molecules such as water, oxygen, and carbon dioxide can permeate cellular membranes freely via simple osmosis
Many required substances are polar molecules and require transport proteins to transport them across membranes
Osmosis
Water can easily permeate cellular membranes.
Within the plant body water acts the same way it always acts – it moves from an area of high concentration to an area of low concentration until it reaches equilibrium.
Water will move from an area of low solutes (more water) to an area of high solutes (less water) until the water potential is the same on both sides of the membrane.
This is referred to as passive transport because it is not directly controlled by the plant.
Turgor Pressure and Plasmolysis
When a plant cell is existing in an area of high water concentration, the protoplast expands, and the plasma membrane will begin to exert pressure onto the cell wall (and vice versa).
This pressure is called turgor pressure, and it keeps the cell turgid, or stiff.
When a plant cell is existing in an area of low water concentration, water leaves the cell.
The vacuole and protoplast shrink, and the plasma membrane pulls away from the cell wall.
This loss of turgor pressure is called Plasmolysis and results in the wilting and drooping of plant leaves and stems.
Transport Proteins
Transport proteins facilitate the selective transport of polar molecules across membranes.
Highly specific – may accept one type of ion or molecule and exclude a nearly identical one.
Provides the specific solutes they transport with a continuous pathway across the membrane.
This type of transport is referred to as active transport because the plant controls what substances move, and when.