Chloroplasts - Evolution, Characteristics, Functions

Chloroplasts are metabolically active, partially independent organelles in plants, algae, and cyanobacteria. Their primary job is to perform photosynthesis, which turns light energy into the energy of chemical bonds needed to synthesise organic substances.

Chemiosmotic Hypothesis

Evolution of Chloroplasts

Chicken Life Cycle

Plastids

• Chloroplasts: The photosynthetic pigment chlorophyll is found in chloroplast plant organelles. It absorbs sunlight, transforms it into usable energy, and causes water to release oxygen.
• Chromoplast: The plastids with colours other than green are referred to as chromoplasts, and they attract insects for pollination and provide various plant parts, such as petals, fruits, and so forth, different colours.
• Leucoplast: The term "leucoplast" refers to the colourless plastids. Leucoplast may change into chloroplast when exposed to sunlight and back to leucoplast when maintained in the dark for an extended period.

Aerobic and Anaerobic Respiration

Characteristics of Chloroplasts

• Like mitochondria, chloroplasts have an oval shape and two membranes: an outer membrane that creates the chloroplast's outside surface and an inner membrane that sits slightly below.
• A small intermembrane gap between the outer and inner membranes is 10–20 nanometers broad.
• The term "stroma" refers to the area inside the inner membrane. The inner membranes of chloroplasts are smooth, but the inner membranes of mitochondria include many folds known as cristae to absorb surface area. Instead, the stroma of chloroplasts contains a large number of tiny disc-shaped sacs known as thylakoids.
• Thylakoids are piled on top of one another in vascular plants and green algae, and this arrangement of thylakoids is referred to as a granum. Chlorophyll and carotenoids found in thylakoids are pigments that absorb light during photosynthesis.
• Photosystems are complexes of light-absorbing pigments and other compounds like proteins. Photosystems I and II are two distinct types of photosystems that play roles in many aspects of light-dependent processes.
• Enzymes create intricate chemical compounds required to store energy in the stroma, such as carbohydrates. Similar to the ribosomes seen in photosynthetic bacteria, stroma has its DNA.
• Because of this, it is believed that chloroplasts, like mitochondria, developed in eukaryotic cells from free-living bacteria.

Centriole

Functions of Chloroplasts

Central Dogma

Cell Wall and Cell Membrane

Photosynthetic Apparatus

Chemiosmosis

Cells Size Shape Count

Cell Division

DNA from Chloroplast

• The genome of chloroplasts and other plastids is distinct from the cell nucleus. Chloroplast DNA (cpDNA) was discovered biochemically in 1959 and was later verified by electron microscopy in 1962.
• The chloroplast is genetically semi-autonomous, as evidenced by the discovery that it synthesises proteins and houses ribosomes. In 1986, the chloroplast genome was first sequenced.
• Since then, hundreds of chloroplast DNAs from various species have been sequenced; nonetheless, they mostly come from land plants and green algae, with glaucophytes, red algae, and other algal groups being remarkably underrepresented.
• This could introduce bias in perceptions of "typical" chloroplast DNA structure and content.

Molecular Structure

• With some exceptions, most chloroplasts contain a single, lengthy circular DNA molecule that is generally 120,000–170,000 base pairs long and contains the whole chloroplast genome. They can weigh between 80 and 130 million daltons and have contour lengths of roughly 30 to 60 micrometres.
• There is some indication that chloroplast DNA molecules more frequently adopt a linear form, even though they are typically thought of as circular molecules.

Cell Organization

Replication of Chromosomal DNA

• Although the process for chloroplast DNA (cpDNA) replication has not been identified with certainty, two major hypotheses have been put forth. Since the 1970s, researchers have sought to use electron microscopy to examine chloroplast reproduction.
• The findings of the microscopy investigations supported the hypothesis that the double displacement loop (D-loop) is the mechanism through which chloroplast DNA repeats. The D-loop takes on a theta intermediary form, sometimes called a Cairns replication intermediate.
• It travels across the circular DNA and uses a rolling circle process to finish replication. At particular sites of origin, transcription begins. Replication machinery can transcribe DNA when many replication forks open. The forks expand and finally converge as replication proceeds. The daughter cpDNA chromosomes are formed when the new cpDNA structures split.