Entry Four: Evolution of Plant and Photosynthesis

Researchers have identified green algae called charophytes as the closest relatives of land plants. But what is the evidence? Well, many key traits of land plants also appear in some protists, primarily algae. Both are multicellular, eukaryotic, and photosynthetic autotrophs that possess cell walls. However, the charophytes are the only algae that share the following four distinctive traits with land plants, suggesting their ancestry:

Both possess rings of cellulose-synthesizing proteins. The cells of both land plants and charophytes have distinctive circular rings of proteins in the plasma membrane that synthesize the cellulose microfibrils of the cell wall.

Both contain peroxisome enzymes. The perixosomes found in the cells of both land plants and charophytes contain enzymes that help minimize the loss of organic products resulting from photorespiration.

Both have structure of flagellated sperm. In species of land plants that have flagellated sperm, the structure of the sperm closely resembles that of charophyte sperm.

Both are involved in the formation of a phragmoplast. In  land plants and certain charophytes, a group of microtubules known as the phragmoplast forms between the daughter nuclei of a dividing cells during cell division.

Many species of charophyte algae inhabit shallow waters around the edges of ponds and lakes, where they are subject to occasional drying. In such environments, natural selection favors those algae that can survive periods when they are not submerged in water. In charophytes, a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out.  A similar chemical adaptation is found in the tough sporopollenin walls that encase the spores of plants. The accumulation of such traits by at least one population of charophyte ancestor most like enabled their descendents – the first land plants – to live permanently above water. These evolutionary novelties opened a new frontier: a terrestrial habitat that offered enormous benefits.

Whatever the precise age of the first land plants, those ancestral species gave rise to the vast diversity of living plants. One way to distinguish groups of plants is whether or not they have an extensive system of vascular tissue, cells joined into tubes that transport water and nutrients throughout the plant body. Plants with a complex vascular tissue system are called vascular plants. Plants that do not have an extensive transport system – liverworts, mosses, hornworts – are called nonvascular plants, or bryophytes. 

As we have all know, the chloroplasts of plants has the extraordinary power to capture light energy that has traveled one hundred and fifty million kilometers from the sun and convert it to chemical energy that is stored in sugar and other organic molecules through a process called photosynthesis. Photosynthesis involves two processes, each with multiple steps. These two stages are known as light reactions and the Calvin cycle.

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. During the reaction, water is split, providing a source of electrons and protons and giving off oxygen as a by-product. Light absorbed by chlorophyll powers the transfer of the electrons and hydrogen ions from water to an acceptor called NADP (nicotinamide adenine dinucleotide phosphate) where they are temporarily stored. The light reactions then use solar power to reduce NADP to NADPH by adding a pair of electrons along with a hydrogen atom. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Overall, light energy is converted into chemical energy in the form of two compounds, NADPH and ATP.

The Calvin cycle begins by incorporating carbon dioxide from the air into organic molecules already present in the chloroplast, a process known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH from the light reactions. To convert carbon dioxide into carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, also already provided by the light reactions. Thus, the Calvin cycle that makes sugar can only work with the help of the NADPH and ATP produced by the light reactions.

C4 plants are plants in which the Calvin cycle is preceded by reactions that incorporate carbon dioxide into a four-carbon compound, the end product o which supplies carbon dioxide for the Calvin cycle. CAM plants are plants that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. In this process, carbon dioxide entering open stomata during the night is converted to organic acids, which release carbon dioxide for the Calvin cycle during the day, when stomata are closed. Both C4 plants and CAM plants, as you can see, are plants that use an alternate mode of carbon fixation to gather carbon dioxide during photosynthesis. Using these alternative methods allow these plants to extract more carbon dioxide from a given amount of air, helping them prevent water loss and adapt in arid climate.