Comparison of plant and animal cells
The key difference between a plant cell and an animal cell lies in the presence of plastids (chloroplasts, chromoplasts, leucoplasts) and a rigid cellulose cell wall located outside the plasma membrane. Animal cells lack these structures.
The mode of nutrition also differs: plants are autotrophs (phototrophs or chemotrophs), whereas animals are heterotrophs (saprotrophs or parasites).
- Energy metabolism: ATP synthesis in plants occurs in mitochondria and chloroplasts, whereas in animals it occurs only in mitochondria. ATP hydrolysis in both cases takes place in all parts of the cell where energy expenditure is required.
- Inclusions: Plants store starch grains, protein, and oil droplets, and also contain vacuoles with cell sap and salt crystals. Animals accumulate lipids and the carbohydrate glycogen, salt crystals, pigments, and metabolic waste products.
- Cell center: In animals, it is present in all cells, whereas in plants it is found only in lower forms.
Diversity and interconversion of plastids
Plastids are unique plant cell organelles capable of transforming into one another depending on external conditions (light, darkness) as well as the processes of maturation and aging.
All forms develop from proplastids — the initial undifferentiated structures. Subsequently, they can differentiate into the following types:
- Chloroplasts — green plastids responsible for photosynthesis.
- Leucoplasts — colorless organelles.
- Chromoplasts — contain carotenoid pigments that impart color.
- Etioplasts — form exclusively in the dark and contain a special prolamellar body.
- Gerontoplasts — form during tissue aging.
In addition, there are highly specialized plastids for nutrient storage: amyloplasts store starch, elaioplasts store lipids, and proteinoplasts store proteins.
Photosynthesis as the basis of plastic metabolism
In eukaryotes, photosynthesis is localized in chloroplasts. It is a pronounced anabolic process (plastic metabolism) that proceeds only in the presence of light and exclusively in cells containing chlorophyll.
During photosynthesis, complex carbohydrates are synthesized from simple inorganic compounds. Water and carbon dioxide are consumed, and oxygen is released into the environment. ATP energy is accumulated and securely conserved in the chemical bonds of the formed carbohydrates. The main outcome of this process is the continuous increase in the organic mass of the plant.
Response to light: etiolation and de-etiolation
Plant development critically depends on illumination. In the absence of light, the etiolation syndrome develops: the plant becomes white or yellowish, its internodes elongate significantly, leaf blades remain poorly developed, and etioplasts are present in the cells.
Upon receiving a light signal, deetiolation begins—a transition to normal development regulated by the phytochrome system. Internodes shorten, leaves unfold, and green coloration appears.
Photomorphogenesis (the conversion of etioplasts into chloroplasts) involves three stages:
- Synthesis of chlorophyll from protochlorophyllide and degradation of the prolamellar body.
- Accumulation of proteins, lipids, and pigments, and assembly of thylakoid membranes.
- Intensive formation of chlorophyll b, assembly of the light-harvesting complex II (LHCII), and final organization of thylakoids in chloroplasts.
Regulation of biogenesis and shade avoidance
Chloroplast biogenesis is under strict control:
- Photoregulation: light activates the synthesis of pigments and proteins via phyto- and cryptochrome systems.
- Hormonal regulation: cytokinins stimulate the synthesis of proteins and pigments, whereas ethylene deactivates these processes.
- Genetic regulation: the nucleus controls biogenesis at all stages of differentiation.
An interesting mechanism is shade avoidance. In a forest, due to vertical stratification (large trees → small trees → shrubs → herbs and dwarf shrubs → mosses → litter), a light gradient is created. If a plant receives far-red light but lacks the red spectrum, it enters an intermediate state between etiolated and deetiolated. In this process, regulated by phytochromes A and B, the plant's chlorophyll content decreases, leaves develop poorly, but internodes elongate.
The names of storage plastids speak for themselves: Proteinoplasts store proteins, amyloplasts store starch, and elaioplasts store lipids.
Class and exam questions
Where in the plant cell is ATP synthesized and broken down?
ATP synthesis occurs in mitochondria and chloroplasts. Breakdown takes place both within the chloroplasts themselves and in all parts of the cell that require energy expenditure.
What are etioplasts and when do they appear?
These are plastids that form in plants under conditions of darkness. They contain a prolamellar body and transform into chloroplasts upon exposure to light.
How do plant and animal storage substances differ?
Plant cells store carbohydrates in the form of starch grains, as well as protein and oil droplets. Animal cells accumulate glycogen and fats.
What else to study on this topic
- Mechanisms of the phytochrome system (red and far-red light)
- Structure of the light-harvesting complex II (LHCII) and thylakoids
- The role of phytochromes A and B in the shade avoidance mechanism
- Nuclear genetic control of chloroplast biogenesis
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