How plants work (pt.I. Function) ..As above, so below..
Its strange how we are surrounded by these beautiful, adaptable, resilient and precious things and are completley dependant on them for our survival- but know hardly anything about them. Most people will relate back to highschool biology and tell you about photosynthesis, but apart from that not much else is said. The fact that most people don't even know what a broccolli plant looks like, or how a pineapple grows seems really odd to me.
I myself never really delved into the world of biology until later in my life- I was always into the chemisty side of things (and once again, were taught in subjects rather than taught about how these things relate to one another!).
Here I will explore the basics of how plants work, to give the gardener, permaculturist or general inquisitive human a little more of a base platform to work from. I am hoping in time to fill these pages with more advanced plant biology and morphology, so keep posted and I will try and fine the thyme.
Hopefully the world of green that surrounds you will ake a little more sense after this.
First things first:
Photosynthesis: (means 'put together with light')
How plants obtain their energy from the sun by using chlorophyll to capture light and converting it to carbohydrates using carbon dioxide and water.
Plants can manufature their own food from the sun by using carbon from the air and water from the soil to form photosynthates (sugars, starches, carbohydrates and protiens). In turn they then release oxygen into the atmosphere as a biproduct.
The process is directly dependant on the amount of carbon dioxide, water and light.
Limiting any one of these factors on the left of the equation will lead to a reduction of photosynthesis and therefore a reduction in plant vigor, growth and overall health.
Chloroplasts are tiny sub cellular structures in the cells of leaves and stems which houses chlorophyll.
Respiration: This is the opposite of photosynthesis. This is a process where plants (and animals) convert sugars and carbohydrates (photosynthates) back into energy for growth and metabolic processes.
Respiration is similar to the oxidation process which occurs when wood is burned and gives off heat. Quite often when compounds are combined with oxygen it is said that they are 'burned'. Plants take in oxygen from the stomata in their leaves and also from their roots.
Respiration is the burning of photosynthates that creates energy for growth.
The difference between Photosynthesis and Respiration at a glance:
Produces sugars from light energy
Occurs only in cells with chloroplasts
Uses carbon dioxide
Respiration:Burns sugar for energy
Occurs in most cells
Produces carbon dioxide
Occurs in dark and light
Transpiration: This process occurs when water is brought through the roots of the plant to the leaves and is lost or transpires as water vapour through the stomata of the leaves.
About 90% of water is lost through this process, which shows that most of what is taken up by the plant through the water is the minerals within the soil. The other 10% is used for photosynthesis and cell growth.
Transpiration serves three functions; movement of minerals, cooling and what is called tugor pressure.
Minerals are moved through the plant in the xylem within the roots and sugars from photosynthesis are moved through the plant in the phloem. Water acts as a solvent and a mode of transport for these minerals and sugars.
Transpiration is responsible for the evaporative cooling that occurs with plants, much similar to the perspiration process in humans.
The tugor pressure is responsible for the turgidity of plants. It is what gives plants their form. Much like a balloon is blown up with air, the cells of a plant are made turgid or stiff by the water vapour carried through the plant. Being upright gives the plants advantage when it comes to competing for light.
Turgidity is also the force that governs roots being pushed through the soil.
Osmosis: Difussion is the movement of molecules from an area of higher conentration to an area of lower concentration. Once there are the same number of molecules on both sides there is no longer any net movement as this is because a gradient no longer exists between the two. Osmosis is a kind of diffusion, it is the movement of water through a selectivley permeable membrane from a higher water concentration to a lower water concentration. So the increase in particles on one side leads to a decrease in particles in the other.
Plant cells both have a cell wall and a plasma membrane, which is why they are selectively permeable. They can choose when to diffuse certain molecules. Osmatic pressure is caused within the cell by the rigidity of the cell wall and by turgor pressure (as explained above).
Phototropism: Tropism is a growth response between a plant and an external influence or stimulus. This stimulus could be touch, weather, light or gravity. A positive response means the plant grows towards the stimulus, whereas a negative response would have the plant growing away from the stimulus.
'Photo' means light, so phototropism means a response to light.
Auxin is a plant growth hormone. It is produced in the shoot apical meristem, the new leaves and the fruit. From these locations it is transported to the root tips.When light shines onto a plant the auxins move to the part of the plant that is in the darker or shadow region (think of the auxins shying away from the light). This hormone stimulates the cells on the dark side to elongate, while the cells on the light side remain the same. By doing this it means the plant then starts to bend in the direction of the light, allowing more light to reach the photosynthesising cells of the plant.
Gravitropism: This is the process of a plants response to gravity. This is also governed by the plant hormone auxin. The same principle as above applies. If a plant is laid on its side, the auxins will move toward the lower part of the roots and stem, elongate and cause the stem to bend upwards. The auxin ceases elongation in roots, which causes them to bend down into the soil.
Cytokinins: These are a group of plant hormones that stimulate cell division. They can prevent leaves form aging and support fruit and embryo development. In spring large amounts of this hormone are produced in the roots, which are then transported up the plant into the dormant buds that in turn become active.
Ethylene: This is the only gaseous hormone and is responsible for the ripening of fruit.
Life Cycles of Plants:
Annuals: Annual plants have a life cycle from when they germinate from seed to when they produce seed in one growing season, then they die.
Biannuals: Biannuals germinate from seed and produce vegative structures and food storage organs in the first season, and then in the second season flowers, fruit and seeds are produced before the plant dies. In the first season winter a hardy basal rosette of leaves persists before the plant is ready to put its energy into flowers and seed production.
Perennials: These plants, when they have reached maturity, typically produce flowers and seeds each season without dying. Perennials are classified as herbaceous if the top dies back to the ground each winter and new stems grow from the roots each spring. They are classified as woody if the top persists, as in shrubs or trees.
The Functions of Plant Parts
Roots: anchor and support for plants, absorb and conduct water and minerals, store products of photosynthesis (sugars, carbohydrates and proteins), aid plant survival in winter (for perennials),
Adventitious roots: These are roots that are produced from the plant leaf nodes in cuttings from the stem or from the basal point in cuttings of a leaf. It is also a practice of asexual reproduction. Auxin concentrations increase dramatically at the site of root production when this occurs.
Stem: Used for asexual and vegative propergation, supports leaves and fruits while connecting them to roots, conducts water and nutrients from the soil and products from photosynthesis to and from leaves and roots, water and nutrient storage.
Leaf: play a major role in photosynthesis, transpiration, gas exchange, food storage, plant support and defense.
Flower: the sole function of a flower is sexual reproduction through pollination.
Fruit: Part of the pollinated flower ripens and becomes the fruit, which usually encases or surrounds the seed and acts as protection, or as an invitation to be eaten by animals and in turn aid in seed dispersal.
Seeds: Storage of nutrients and protection from outside elements until conditions are correct for germination.
More coming on the parts of plants and their roles in a future post!