Let us make an in-depth study of the structure and functions of cell. After reading this article you will learn about: 1. Comparison of Prokaryotic Cells and Eukaryotic Cells and 2. Structure and Components of a Human Cell.
Cell is a compartment where all the activities of life takes place. There are two basic types of cells in nature, viz., prokaryotic cells and eukaryotic cells.
Comparison of Prokaryotic Cells and Eukaryotic Cells:
Prokaryotic Cells:
1. Prokaryotes are the simplest cells without a nucleus and cell organelles.
2. Prokaryotic cells are the smallest cells (1-10 μm).
3. Unicellular and earliest to evolve (~4 billion years ago), still available.
4. The cell wall is rigid.
5. These cells reproduce asexually.
6. They include bacteria and archaea.
7. Some species are highly evolved pathogens e.g., Borrelia burgdorferi.
Eukaryotic Cells:
1. Eukaryotes are sophisticated cells with a well defined nucleus and cell organelles.
2. The cells are comparatively larger in size (10-100 μm).
3. Unicellular to multicellular in nature and evolved ~1 billion years ago.
4. The cell membrane is semipermeable and flexible.
5. These cells reproduce both asexually and sexually.
6. Include the animals, plants and fungi.
7. Size ranges from tiny yeasts to giant sequoias, dinosaurs, etc.
Structure and Components of a Human Cell:
A group of cells forms tissue, various tissues forms an organ and different organs make up the body.
The structure and components of a human cell are given below:
Cell membrane:
Thin layer of protein and fat that surrounds the cell is the cell membrane. It is semipermeable, allowing some substances to pass into the cell and blocking others.
Cytoplasm:
Jelly-like material present outside the nucleus in which the cell organelles are located. It is the site of protein synthesis and many metabolic events. The cytoplasm contains many enzymes for general metabolism. It contains fibre of the cytoskeletal system, which organizes cytoplasmic structure.
Mitochondria:
Spherical to rod-shaped organelles with a double membrane. The inner membrane is in-folded many times, forming a series of projections (called cristae). The mitochondrion is known as the power house of the cell as it generates ATP (adenosine triphosphate), the energy currency of the cell.
Ribosome:
Small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis. Ribosome size is measured in Svedberg (S) units; derived from sedimentation in ultracentrifuge (used before electron microscopes were available).
In prokaryotes the ribosomes are made of 30S and 50S subunits, assemble into 70S ribosome whereas in eukaryotes the ribosomes are made of 40S and 60S subunits, assemble into 80S ribosome. In bacteria they occupy 25% of cell volume and use 90% of cell energy. Less in many specialized eukaryotic cells but still are the dominant activity of almost all the cells.
Nucleus:
It is a spherical body containing many organelles, including the nucleolus. It controls many of the functions of the cell (by controlling protein synthesis) and contains DNA (in chromosomes). The nucleus is surrounded by the nuclear membrane. It is the locus of DNA/RNA synthesis and protein assembly. It contains chromatin i.e., DNA-protein complexes. Chromatin can condense into chromosomes during cell division.
Nuclear membrane:
The nuclear membrane is a double layered structure surrounding the nucleus containing many nuclear pores. These pores allow different materials to move in and out of nucleus. The pores have octagonal ‘doors’ made of protein which open and close on either side depending on specific signals. Pore diameter is about 10 nanometers (10 x 10-9 m), smaller than the diameter of a complete ribosome. They can open up to as much as 26 nm in response to certain signals. Some signals allow motion in but not out, other signals control reverse transport.
Nucleolus:
The nucleolus is present within the nucleus. Some cells have more than one nucleolus. It is the assembly plant for ribosomes. Ribosomal proteins are made in cytoplasm and transported back into the nucleus. Ribosomal RNA is made in the nucleus. These two elements are integrated inside nucleolus to create ribosomal subunits. These are then exported out of nucleus through nuclear pores.
Centrosome:
A small body located near the nucleus, also called the ‘microtubule organizing centre’. It has a dense center and radiating tubules. The centrosomes are where microtubules are made. During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell. The centriole is the dense centre of the centrosome.
Endoplasmic Reticulum:
Rough endoplasmic reticulum (rough ER):
A vast system of interconnected, membranous, in-folded and convoluted sacks that are located in the cell’s cytoplasm (the ER is continuous with the outer nuclear membrane). Rough ER is covered with ribosomes that give it a rough appearance. Rough ER transports materials through the cell.
It synthesizes proteins in sacks called cisternae for export or movement to different cell organelles like the Golgi body, or inserted into the cell membrane but not to cytoplasm. The transport proteins designated for export carry a peptide signal at growing end, causing growing protein to move to ER (docking), insert peptide into membrane, translocate growing polypeptide chain across ER membrane.
Smooth endoplasmic reticulum (smooth ER):
A vast system of interconnected, membranous, in-folded and convoluted tubes that are located in the cell’s cytoplasm (the ER is continuous with the outer nuclear membrane). The space within the ER is called the ER lumen. Smooth ER transports materials through the cell. It contains enzymes which produces and digests lipids (fats) and membrane proteins; smooth ER buds off from rough ER, moving the newly-made proteins and lipids to the Golgi body, lysosomes and membranes. It detoxifies drugs and poisons (in liver).
Golgi body:
A flattened, layered, sac-like organelle that looks like a stack of pancakes. It is also called the Golgi apparatus or Golgi complex. It is located near the nucleus. It produces the membranes that surround the lysosomes. The Golgi body packages proteins and carbohydrates into membrane-bound vesicles for export from the cell.
Functions as intracellular ‘post office’ for sorting new proteins made on rER. Vesicles containing protein pinch off from ER, fuse with cis face of Golgi. Inside Golgi, oligosaccharide chains on proteins are modified. Vesicles pinch off from trans face of Golgi, carry proteins to several possible destinations: export (out of cell), lysosomes, peroxisomes, cell membrane, etc.
Lysosomes:
These are round organelles surrounded by a membrane where the digestion of cell nutrients takes place due to presence of the digestive enzymes. They contain —40 hydrolytic enzymes such as lipases, proteases, nucleases, etc. which break down organic polymers of all types. Lysosomes continuously break down old proteins, foreign materials, and many wastes.
They also bring about phagocytosis, a process in which foreign materials are brought into the cell and ‘chewed up’. Sometimes lysosomes open up in cell itself causing death of the cell termed as apoptosis, hence are called suicide bags of the cell.
Vacuole:
Fluid-filled, membrane-surrounded cavities inside a cell. The vacuole fills with food being digested and waste material that is on its way out of the cell. There are specialized vacuoles which function to store fat as fat droplets (TAG).
Peroxisomes:
These are single membrane oval or spherical cellular organelles. They are also called as micro bodies. They contain catalase enzyme. Peroxisomes are involved in the oxidation of long chain fatty acids and synthesis of plasmalogens and glycolipids.
Cytoskeletal system:
It provides internal fibrous structure to cells because cell is not ‘just a bag in a bubble’, it contains lots of internal fibres or internal ‘skeleton’. It is not rigid like bone, instead it is capable of being assembled, allows cell movement, cell division, internal motion of the organelles and is broken down in minutes. The cytoskeletal system is composed of microtubules and microfilament.
Microtubules:
The microtubules have the largest diameter among the fibres found in the cytoplasm of all eukaryotes. It involves many structures: Cilia, flagella, spindle fibres that polymerize from centrioles during mitosis/meiosis. They are made of the protein called tubulin and polymerizes into hollow tubules of 25 nm diameter.
Cilia and flagella:
They are organelles of locomotion. Both of them contain 9 double rings of microtubules, 2 central microtubules, two motor proteins, i.e., motor protein 1-dynein and motor protein 2-kinesin, which allow motion along microtubules.
Microfilaments (actin):
Another kind of fiber found in cytoplasm of most eukaryotes. Involved in muscle contraction, cell support, pinching off of daughter cells after mitosis.
Extracellular matrix (ECM):
Animal cells do not have cell walls, but have ECM, i.e., a meshwork of macromolecules outside plasma membrane. It consists mainly of glycoproteins (proteins with oligosaccharide chains), especially collagen. Some cells are attached directly to ECM by bonding to collagen or fibronectin.
Intracellular junctions:
In multicellular organisms, adjacent cells are held together by several types of specialized junctions:
1. Tight junctions:
Specialized ‘belts’ that bind two cells tightly to each other, prevent fluid from leaking into intracellular space.
2. Desmosomes:
Intercellular ‘rivets’ that create tight bonds between cells, but allow fluids to pass through intracellular spaces.
Gap junctions:
Formed by two connecting protein rings embedded in cell membrane of adjacent cells. Allows passage of water, small solutes, but not proteins, nucleic acids.