|Biology 20 Fall 2001
Jan 9, 2001
What do you think of when you think of a plant?
Today: what is a plant, why are they so important and how they do the things they do.
Plants are living things and must do all the things that other living things do
Need to obtain food (i.e. energy and nutrients - they need to metabolize), need to get water, need to grow and develop, need to reproduce and disperse their offspring, need to protect themselves from danger and injury (avoid being eaten, avoid and fend off disease, survive harsh environmental conditions)
Plants are able to do all this yet remain rooted in the ground.
What is unique about plants (what makes a plant a plant)?
Plants are mostly photosynthetic: able to capture sun energy and use it to make complex organic (carbon based) molecules from simple molecules.
Most the plants you are exposed to daily are flowering plants (angiosperms): a very successful group of organisms that live on every continent and survive and thrive under nearly every environmental condition.
Plant success is tightly linked to ability to do photosynthesis.
Other features of plants:
Unable to move on a visible level: they do move but instead of via muscle motion, move via growth movements:
Because of this inability to run away, have a different way of responding to stress and injury than animals do - have their own way of dealing with environmental change.
Plants exhibit indeterminate growth: indeterminate growth means that they do not have a pre-programmed size, shape and often, even no set age. Animal growth is determinant or preprogrammed size, shape and even to some extent age (all adult animals look basically similar - for mammals like us, 4 limbs, one head, two eyes, etc - a cat is a cat is a cat). Plants do not have a set shape (your houseplant does not look identical to someone else’s).
Plants can reach incredible ages: Contrast with animals, where 100 to 200 year old organism get a ton of press (Oldest living animal tends to make a big splash), plants beat them hands down: some of the campus trees are at least 125 years old and other plants like the giant sequoia can easily reach 500 years and the bristle cone pines (a personal favorite of mine) can reach ages of 4000 to 5000 years.
Their ability to reach such ancient ages is due in part to no “set” death date (most animals have essentially a genetically determined life span and this is not true for most plant species).
Plants can be huge: Giant sequoia ~1000 tons (equivalent to 200 elephants)
Plants are incredibly productive: 90% of the earth’s biomass is plant derived:
Cellulose (the carbohydrate that makes up the cell wall of a plant cell) is the most abundant polymer (long chain molecule) on earth; RuBisco (a protein enzyme that is involved in photosynthesis) is the most abundant protein on earth - pretty darn amazing! This productivity allows for:
Plants can replace their parts: can hack a plant apart and it will survive - Loose a branch? - No problem, grow a new one whereas as an animal, if you loose a limb, it will not “grow back”.
Plants are absolutely essential for life: For aerobic (oxygen requiring) life- back to photosynthesis - a handy waste product of photosynthesis is O2 (oxygen) - you need it to break down complex molecules to capture energy. Plants are also essential for maintaining ordered life on earth - They are the point of entry of the energy of the sun into the living world - they capture it and ultimately convert it to a form that all living things can use.
Plants are essential for human beings: We like other animals depend on plants for food and a bunch of other things too. In addition, plants have helped shape human history:
Both of the big changes in human history (the agricultural and industrial revolutions) have been “fueled” by plant products:
The agricultural revolution changed the way society operates - change the social fabric of human society (shift from hunter gather to agrarian society = shift from nomadic to permanent settlement).
The industrial revolution driven first by wood then fossil fuels: coal and other fossil fuels are essentially fossilized plant material.
Changed human history (not always for the better): The search for spices, sugar and other plant products was the driving force behind the beginning of the Age of Exploration started by the Portuguese and Spanish in the 1400’s. Other examples as well on a smaller scale: The British shift to tea drinking society from coffee drinkers in the mid to late 1800s was due in large part to the destruction of the coffee plantations of Ceylon by coffee rust, a plant disease (the plantations were replaced by tea plantations and the British supported their imperial colonies).
Plants do all the things that a living thing needs to do but in their own unique way
Obtain food and water: Make their food with photosynthesis; mine the soil for water with roots or root like structures:
Getting water is a hard thing to do when you live on land: land plants have many adaptations associated with getting water, transporting water, and conserving water.
Reproduction: Plants do this both asexually that permits for a rapid increase in numbers of a clonal (genetically identical) population via a variety of specialized structures (plantlets, bulbs, corms, rhizomes, adventitious suckers, etc). Plants also reproduce sexually (for angiosperms - the flowering plants - via seeds) - this also can increase numbers but sexual reproduction leads to genetic diversity (which in biology is a really good thing).
They also need to disperse the offspring: plants have many clever ways of doing this.
Respond to the environment: Though they don’t move with muscles and nerves, they can respond to environmental stimuli with growth movements (trophic responses), turgor movements (rapid response system) and are able to respond to chemical signals. They can grow towards, away from and around stimuli both positive and negative and can fend off disease without an immune system (often by “walling it off” - grow specialized tissue to isolate damaged tissue).
As a group (population), plants can adapt and evolve in response to environmental changes:
Plants have been around for roughly 500 million years. This is a long time for changes to occur. One big step in evolution is the movement onto land, and plants did this earlier than animals.
Special Challenges of land life and how plants deal with them:
Have a waxy cuticle to prevent water loss, have stomata (specialize pores in the epidermis) for gas exchange, have vascular tissue for transport of water, have pollen and seed for “dry” reproduction (have gotten around the problem with having swimming sperm on dry land), have support tissue (lignin - a very rigid polymer - and lignified tissue to hold them upright), have roots for scavenging water and nutrients from the soil, and have other specialized organs for specialized function.