Cells: An overview
The cell is a basic unit of every live organism. The cells are basic building blocks of the organism, and give bodies their structure, absorb nutrients and create energy. Additionally, every cell contains the genetic material of the entire organism. Cells can make copies of themselves - this is how organisms grow, replace old or defective cells and heal wounds.
Cells are composed of organelles, which are like miniature versions of our organs - units within the cell that perform specific function. These include:
- The nucleus: this contains the cell's genetic material and controls the cell's activity. Operations with the genetic material happen here as well.
- The cytoplasm is a jelly-like isotonic substance which fills the inside of the cell. It houses the organelles and provides space for cellular activities such as manufacturing of proteins, metabolism of molecules or transport of molecules.
- Mitochondria are organelles responsible for generation of energy (thus referred to as the powerhouse of the cell). Energy is stored in the form of a molecule called adenosine triphosphate (ATP), which is generated by an enzyme ATP synthase and utilises a proton gradient across the mitochondrial membrane. Additionally, mitochondria also play a role in initiating cell death.
- The endoplasmic reticulum (ER) is a network of tubules, which is the site of synthesis of proteins and lipids. Here, the mRNA molecule, which is like a blueprint for protein building, is read by ribosomes, and amino acids are pieced together corresponding to the instructions encoded in the mRNA.
- The Golgi apparatus, a series of flattened sacs, modifies, sorts, and packages proteins and lipids for transport, as they exit the endoplasmic reticulum.
- Lysosomes break down waste materials and cellular debris into molecules which can be recycled by the cell. A similar structure, peroxisome, metabolises fatty acids by oxidation.
- The cytoskeleton, a network of microtubules, provides structural support to the cell, and plays roles in cellular transport (where molecules are dragged along the microtubule by specialised transport proteins), plasticity and movement of the cell, and cell division.
- The cell membrane (plasma membrane) is a thin lipid layer that surrounds the cytoplasm, providing a protective layer from the environment. The cell membrane also facilitates transport of substances into and out of the cell [1, 2, 3, 4, 5, 6, 7].
What is a chloroplast?
A chloroplast is a organelle which is found in the cells of plants and some algae, not animal cells. It is the place where photosynthesis occurs.
Chloroplasts are composed of a two-membrane envelope and stacks of units known as thylakoids. The space within the membrane is filled by a matrix called the stroma, where enzymatic reactions occur [8, 9, 10].
What is photosynthesis?
Photosynthesis is process of transforming light energy into chemical energy [11]. It is the main energy source for life on Earth. Briefly, UV light is absorbed by chloroplasts, specifically by the green pigment chlorophyll, and its energy is used to convert carbon dioxide and water molecules into glucose and oxygen.
Do animal cells have chloroplasts?
Animal cells do not have chloroplasts. There is no need for animal cells to have chloroplasts, as energy in animal cells is obtained through biochemical reactions including glycolysis, Krebs cycle and the electron transport chain.
Engineering photosynthesising animal cells
Photosynthesis is a very well-functioning and efficient system of energy production. Therefore, it is an intriguing research focus in modern engineering biology, with sustainability being the overarching objective.
A study demonstrated that the animal mitochondria is capable of a photosynthesis-like reaction, which results in ATP production using a light-sensitive metabolite of chlorophyll as one of the reagents [12]. Another study demonstrated creating a zebrafish capable of photosynthesis, which was achieved by injecting photosynthesising algae into the fish eggs. Following development of the fish, the algae were distributed throughout the organism, and appeared to have a symbiotic relationship with the fish [13].
However, it is important to note that this field of research is still in its early stages, and its utility is still uncertain.
Exceptions to the rule
There is one example in nature of a photosynthesising animal. Elysia chlorotica, a species of sea slug, is capable of sustaining itself using photosynthesis for several months. This is due to a symbiotic relationship from a type of algae which the slug consumes, Vaucheria litorea [14].
The sea slug harbours these chloroplasts in a process known as kleptoplasty. The chloroplasts continue to photosynthesize in the absence of the algal nucleus in the slug's body at rates sufficient to meet the nutritional needs of the slugs [15]. Interestingly, over the years of the symbiosis, the slug has not absorbed any of the algal DNA (this could happen, it is referred to as horizontal gene transfer) [15, 16]. This symbiotic interaction may be maintained by an array of undescribed bacteria, which were detected in the slug. These bacteria are capable of digesting complex saccharides, as well as nitrogen fixation, production of vitamin B12 and biosynthesise other compounds [14].
Summary
In summary, animal cells do not contain chloroplasts, as energy is not produced by photosynthesis in animal cells. However, there have been examples of animals in symbiotic relationships with bacterial or algal species, allowing them to supplement their metabolism with photosynthesis.