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Student Investigation: Organelles

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Student Investigation: Organelles

As living organisms, we are all composed of living cells, these cells are often referred to as eukaryotic cells. The majority of these cells contain a spherical structure called the “nucleus”. The nucleus is the most important organelle in the cell. It acts as the brain for the cell, directing and controlling all the activities within the cell. The exceptions are prokaryotic cells (e.g. bacteria) and red blood cells which are also termed as corpuscles (Study Read, n.d.). The nucleus holds the main power over the entire cell and works with other cell organelles throughout its life span. It contains large amounts of DNA, the genetic material.

All the RNAs needed for the cell are synthesised in the nucleus. Ribosomes are made within the nucleus. The lamin proteins that make up the structural framework of the nucleus take part in the instigation of apoptosis (or self-induced death) of the whole cell (Study Read, n.d.). According to Ravi Makhija (former science teacher, 2017), a eukaryotic cell cannot survive without a nucleus and dies instantly. (There are exceptions like the erythrocytes which on maturity loses their nucleus).

The nucleus, in appearance is circular for most of the time, it can also be other shapes (i.e. polymorphic) (Towle, 1999). In figure 1, it clearly depicts a double membrane (inner and outer membrane) which serves to protect the contents of the organelle.  Enclosed by the membrane, there is the nucleoplasm; which is similar to the cytoplasm. The nucleoplasm is mostly comprised of water, molecules and dissolved ions. It is an extremely viscous fluid that supports the chromosomes and nucleoli.  A component of the nucleoplasm is the hyaloplasm, which is soluble and fluidlike. The function of the nucleoplasm is to assist as a suspension medium for the organelles within the nucleus and helps preserve the shape and structure of the nucleus. (Softschools, n.d.). [pic 1][pic 2]

Laying within the nucleoplasm, the nucleolus is a round body that makes ribosomal subunits from proteins and rRNA. The subunits then combine to make ribosomes. Floating around in the nucleoplasm, lies chromatin. Their purpose is to package DNA. This allows for mitosis, meiosis, avoiding chromosome rupture, controls gene expression and DNA duplication (Gauthier, n.d.). Lastly, on the surface of the double membrane structure, the nuclear envelope is found. The pores located on the envelope are for connection with the external cytoplasm of the cell (Towle, 1999). Pores on the nucleus help transfer amino-acids and other substances required for formation of DNA, RNA, mRNA etc. mRNA aid in the creation and synthesis of proteins which is essential for the growth, physiology, multiplication and death of a cell (Study Read, n.d.).

The mitochondrion is an organelle that makes power for the cell by breaking down sugar. It’s also known as the ‘powerhouse’ of the cell. The power it releases is called ATP, an energy-loaded molecule (Andrew Rader Studios, n.d.). The cell uses the ATP to function. The mitochondria’s structure is perfectly catered to boost their productivity (Andrew Rader Studios, n.d.). The outer membrane serves as a protection and contains the organelle, much like the cell membrane. The inner membrane creases over numerous times and makes layered formations named cristae. The creasing of the membrane maximises the surface area so that the chemical reactions that occur within the mitochondria are multiplied and have more space to happen. The liquid in the mitochondria is called the matrix, it contains their own ribosomes and DNA. [pic 3][pic 4]

The chloroplast is only found in plant cells, because these cells make photosynthesis happen, which helps the plant live. Animal cells do not contain these organelles. The structure has a double membrane, the outer membrane is more permeable and allows things such as small organic molecules to go through. The inner membrane is less permeable and surrounds the stroma and the thylakoids. Chlorophyll molecules rest on the surface of the thylakoids and seize light energy emitted from the Sun. As energy-loaded molecules are made from reactions, they move to the stroma so that carbon can be secure, and sugars are synthesised (IvyRose Holistic, n.d.). The thylakoid stacks are interconnected by stroma lamellae, which act as a joint of the chloroplasts. It maximises the efficiency of the organelle. [pic 5][pic 6][pic 7]

When a cell needs to create proteins, the ribosomes are the organelles for the job. Ribosomes are found in many places such as floating in the cytosol and attached to the endoplasmic reticulum or nuclear envelope (Khan Academy, n.d.). The ribosomes are made of subunits which conjoin when it is time to make protein. The mRNA is sent out of the nucleus and the subunits latch onto it and start synthesis. To create protein the ribosome will connect with a tRNA; which then connects to an amino acid creating a long peptide chain which will eventually be part of a large protein (Andrew Rader Studios n.d.).[pic 8]

The origin of the eukaryotic cell began with 2 prokaryotic cells; which each has their own set of DNA molecules. As you can see in figure 5, they remain separate as the incorporated cell (blue) and the host cell genome (purple). Over time they replicate and due to errors, genetic material becomes separated from the incorporated cell and merges with the host cell genome. Eventually becoming mixed, it becomes enclosed in a membrane which then progresses into a nucleus (Nature Education, 2014).[pic 9][pic 10]

As for the mitochondria and chloroplasts are supposedly derived from when all living things were known as single-celled organisms; which can be seen in figure 6. A time when more flexible eukaryotes consumed molecules and smaller cells. Scientists deem that this where these two organelles came into being. These organelles likely grew from engulfed bacteria. Using the endosymbiotic theory, these prokaryotic cells gradually developed into what we know as mitochondria. These cells (containing newly formed mitochondria), engulfed photosynthetic prokaryotes which then evolved into chloroplasts (Nature Education, 2014).  [pic 11][pic 12]

As you can see, the figures 1.5 and 1.6 show that the origin of eukaryotic cells has a nucleus within the cell before any other organelles were introduced into the cell. The role of the nucleus is one that no other organelle can do. To send instructions to another. Only the nucleus has the power to instruct. It is responsible for governing and directing all the actions that occur within the cell. Both the mitochondria and chloroplast have their own DNA, that carries the code for some of their own proteins, they also still require on some proteins coded by genes in the nuclear DNA to help them function. As chloroplast is only apparent in plant cells and not animal cells, it deems the organelle as less important. In eukaryotic cells, ribosomes get orders only from the nucleus to begin protein synthesis. Without protein the cell wouldn’t function. Therefore, without the nucleus the eukaryotic cell would basically die. Hence, proving the nucleus as the most important organelle in the cell.



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