Anatomy and Physiology: Cell Structure

Structures of Cells

• Cells are the fundamental units of all living things, and every living entity begins with a single cell that divides at an exponential rate throughout time.
• The fundamental components of each cell are identical to one another, with certain cell types having extra components for specific functions.

Plasma Membrane

• The plasma membrane is a semipermeable barrier made up of a double layer of phospholipids with proteins, cholesterol, and carbohydrates scattered throughout.
• The phospholipid bilayer is the basic framework of a cell, consisting of hydrophilic heads that attract water and face the watery surroundings inside and outside the cell, while the hydrophobic tails reject water and face inward, away from water.
• The plasma membrane controls the transport of chemicals into and out of the cell, achieves selective permeability through different transport modes, and possesses receptor proteins for cell communication and signaling pathways.
• Cholesterol molecules maintain the flexibility and functionality of the membrane by ensuring its fluidity.

Cytoplasm

• The cytoplasm is a viscous colloidal solution that occupies the intracellular space, enveloping the nucleus.
• It consists primarily of water, ions, enzymes, and organelles, and functions as the medium for numerous cellular processes.
• The cytoplasm contains various organelles, including the endoplasmic reticulum, Golgi apparatus, and mitochondria, each serving essential activities necessary for the cell's survival.
• The cytoplasm plays a vital role in intracellular transport, facilitating the movement of molecules throughout the cell via processes such as diffusion and active transport.

Nucleus

• The nucleus is commonly known as the "control center" of the cell, governing cellular activity and housing genetic material in the form of DNA.
• The nucleus is enclosed by a dual-layered membrane known as the nuclear envelope, which regulates the transport of chemicals into and out of the nucleus.
• Chromosomes are structures within the nucleus that arrange genetic material in the form of DNA.
• The genetic information is transcribed into messenger RNA (mRNA) by the process of transcription, which is then translated into proteins in the cytoplasm.

Cilia

• Cilia are elongated, filamentous structures present on the outer surface of various cell types within the human body.
• Cilia are made up of microtubules and surrounded by the cell membrane, displaying regular and coordinated movements characterized by rhythmic pounding or waving motions.
• These movements enable different physiological processes based on their specific location, such as facilitating the removal of mucus and debris from the lungs, assisting in the migration of egg cells via the fallopian tubes, and contributing to the sense of smell.

Flagella

• Flagella are elongated appendages present in diverse unicellular species and specific cells of multicellular creatures, such as sperm cells.
• They play a crucial role in various cellular processes, such as autophagy, which involves the degradation and recycling of the cell's own components.

Cytoskeleton

• The cytoskeleton is an intricate and ever-changing system of protein filaments that spans the entire cytoplasm of eukaryotic cells.
• It has a vital function in preserving the structure of cells, allowing cells to move, and aiding in the movement and division of materials within cells.
• The cytoskeleton consists of three primary protein filaments: microfilaments, intermediate filaments, and microtubules, each possessing unique activities and characteristics.

Centrioles

• Centrioles are cylindrical organelles present in the majority of eukaryotic cells, playing a crucial role in cell division and the arrangement of the microtubule network.
• Centrioles consist of nine sets of microtubule triplets organized in a cylindrical shape, and are commonly observed in pairs, positioned perpendicular to each other, within a cellular area referred to as the centrosome.
• The centrosome serves as the primary microtubule-organizing center (MTOC) within the cell, playing a vital role in preserving the shape and functionality of the cytoskeleton.

Structures of Cells

• Cells are the fundamental units of all living things, with identical fundamental components.
• Certain cell types have extra components for specific functions.

Plasma Membrane

• The plasma membrane is a semipermeable barrier that preserves cellular integrity and facilitates cellular function.
• It is composed of a double layer of phospholipids with proteins, cholesterol, and carbohydrates scattered throughout.
• The phospholipid bilayer has hydrophilic heads that attract water and face the watery surroundings inside and outside the cell, while the hydrophobic tails reject water and face inward.
• The plasma membrane controls the transport of chemicals into and out of the cell through selective permeability, achieved through passive transport, active transport, and facilitated transport aided by membrane proteins.
• It also possesses receptor proteins that facilitate the cell's reception and response to chemical signals from its surroundings.

Nuclear Membrane

• The nuclear membrane, or nuclear envelope, functions as a safeguarding barrier that encloses the nucleus of a eukaryotic cell.
• It is composed of two lipid bilayers and regulates the movement of molecules in and out of its structure via specialized channels known as nuclear pores.
• The nuclear membrane offers structural reinforcement to the nucleus and aids in preserving its form.
• During cellular division, the disintegration of the nuclear membrane facilitates the segregation of chromosomes.

Nucleolus

• The nucleolus is a discrete organelle located within the nucleus of eukaryotic cells.
• It is responsible for the synthesis of ribosomal RNA (rRNA) and the initiation of ribosome assembly.
• The process of transcription occurs within the nucleolus, where rRNA molecules are synthesized from DNA.
• The nucleolus is involved in the regulation of the cell cycle and the response to cellular stressors.

Mitochondria

• Mitochondria are organelles enclosed by two membranes that are present in the majority of eukaryotic cells.
• They have a vital function in cellular respiration, producing energy in the form of adenosine triphosphate (ATP).
• Mitochondria possess their own genetic material, referred to as mitochondrial DNA (mtDNA), and have the ability to replicate and synthesize proteins to some extent independently.
• The mitochondria's inner membrane is composed of cristae, which are folded structures that enhance the available surface area for ATP synthesis.

Ribosomes

• Ribosomes are small yet powerful organelles present in both prokaryotic and eukaryotic cells.
• They are composed of ribosomal RNA (rRNA) and proteins and serve as the biological sites where the genetic information contained in messenger RNA (mRNA) is converted into proteins.
• Ribosomes can be found either freely in the cytoplasm or bound to the endoplasmic reticulum.

Flagella

• Flagella are structures made up of microtubules and surrounded by the cell membrane.
• They have a distinctive waving or spiraling movement that enables the cell to move forward in its surroundings.
• Flagella function as a method of propulsion, enabling unicellular organisms to navigate towards nourishment or away from detrimental substances.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is an intricate network of membranous tubules and sacs present in eukaryotic cells.
• It is divided into two distinct regions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).
• The RER is covered with ribosomes on its surface, responsible for the synthesis of proteins that will be either secreted, integrated into the cell membrane, or transported to other organelles.
• The SER is devoid of ribosomes and has a role in synthesizing lipids, detoxifying medicines and poisons, and storing calcium ions.

Golgi Apparatus

• The Golgi apparatus is an essential organelle present in the majority of eukaryotic cells.
• It consists of a sequence of flattened, membrane-bound sacs called cisternae.
• The cis face of the Golgi apparatus is usually positioned in close proximity to the endoplasmic reticulum (ER) and functions as the site where vesicles carrying newly produced proteins and lipids from the ER are received.
• The trans face, located on the opposite side, is directed towards the plasma membrane and serves as the department responsible for packaging processed molecules into vesicles for transportation to their ultimate destinations.

Lysosomes

• Lysosomes are created through the process of budding from the Golgi apparatus.
• They play a crucial role in the modification, organization, and packaging of proteins and lipids, whether for release outside the cell or for internal usage.

Cholesterol in Cellular Membranes

• Cholesterol molecules maintain the flexibility and functionality of the membrane by ensuring its fluidity in varying situations.

Cytoplasm

• The cytoplasm is a viscous colloidal solution that occupies the intracellular space, enveloping the nucleus.
• It consists primarily of water, ions, enzymes, and organelles, and functions as the medium for numerous cellular processes.
• The cytoplasm contains various organelles, including the endoplasmic reticulum, Golgi apparatus, and mitochondria, each serving essential activities necessary for the cell's survival.
• The cytoplasm plays a vital role in intracellular transport, facilitating the movement of molecules throughout the cell via processes such as diffusion and active transport.
• It offers essential support to the cell and aids in preserving its structural integrity.
• In eukaryotic cells, the cytoplasm plays a crucial role in facilitating many metabolic events and ensuring the functioning and survival of the cell.

Nucleus

• The nucleus is the control center of the cell, governing cellular activity and housing genetic material.
• It is a large, spherical body that is located near the center of the cell and contains genetic material in the form of DNA.
• The nucleus is enclosed by a dual-layered membrane known as the nuclear envelope, which is equipped with pores that regulate the transport of chemicals into and out of the nucleus.
• Chromosomes are structures within the nucleus that arrange genetic material in the form of DNA.
• The nucleus contains the cell's genetic information, which contains instructions for protein production and cell activity.
• The genetic information is transcribed into messenger RNA (mRNA) by the process of transcription, and the mRNA then migrates to the cytoplasm, where it is translated into proteins.

Flagella

• Flagella are structures made up of microtubules and surrounded by the cell membrane.
• They have a distinctive waving or spiraling movement that enables the cell to move forward in its surroundings.
• The motion is caused by the synchronized movement of motor proteins along the microtubules inside the flagellum.
• Flagella in unicellular organisms such as bacteria function as a method of propulsion, enabling the organism to navigate towards nourishment or away from detrimental substances.
• In multicellular organisms, flagella are essential for various functions, including fertilization, where sperm cells utilize their flagella to propel themselves towards the egg, enabling successful fertilization.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is an intricate network of membranous tubules and sacs present in eukaryotic cells.
• The ER is divided into two distinct regions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).
• The ER performs various crucial functions in cellular structure and function.
• The rough endoplasmic reticulum (RER) is covered with ribosomes on its surface, resulting in a coarse appearance when observed under a microscope.
• The ribosomes are responsible for the synthesis of proteins that will be either secreted, integrated into the cell membrane, or transported to other organelles.
• The smooth endoplasmic reticulum (SER) is devoid of ribosomes and has a role in synthesizing lipids, detoxifying medicines and poisons, and storing calcium ions.
• Both forms of endoplasmic reticulum (ER) are interconnected, enabling the efficient transportation of chemicals throughout the cell.

Golgi Apparatus

• The Golgi apparatus is an essential organelle present in the majority of eukaryotic cells.
• Structurally, it consists of a sequence of flattened, membrane-bound sacs called cisternae.
• The cisternae are typically organized in a vertical stack, displaying a clear orientation.
• The cis face of the Golgi apparatus is usually positioned in close proximity to the endoplasmic reticulum (ER) and functions as the site where vesicles carrying newly produced proteins and lipids from the ER are received.
• The trans face, located on the opposite side, is directed towards the plasma membrane and serves as the department responsible for packaging processed molecules into vesicles for transportation to their ultimate destinations.
• The Golgi apparatus plays a crucial role in the modification, organization, and packaging of proteins and lipids, whether for release outside the cell or for internal usage.
• Glycosylation is a process in which carbohydrates are added to proteins by enzymatic activities occurring within the cisternae, along with other changes.
• The Golgi apparatus also contributes to the production of intricate polysaccharides, which are vital constituents of the cell wall in plants and the extracellular matrix in animals.
• The Golgi apparatus acts as a major hub for trafficking cellular products, ensuring precise delivery of molecules to their proper destinations, thus preserving cellular order and function.

Lysosomes

• Lysosomes are created through the process of budding from the Golgi apparatus.
• They play a crucial role in various cellular processes, such as autophagy, which involves the degradation and recycling of the cell's own components.
• Autophagy is a process in which damaged organelles or misfolded proteins are enclosed within a double-membrane vesicle known as an autophagosome.
• The autophagosome then merges with a lysosome, resulting in the breakdown and reuse of its contents.
• In addition, lysosomes play a crucial role in the process of endocytosis, which involves the uptake of extracellular substances into the cell, as well as phagocytosis, which involves the engulfment of large particles like pathogens.

Cytoskeleton

• The cytoskeleton is an intricate and ever-changing system of protein filaments that spans the entire cytoplasm of eukaryotic cells.
• It has a vital function in preserving the structure of cells, allowing cells to move, and aiding in the movement and division of materials within cells.
• The cytoskeleton consists of three primary protein filaments: microfilaments, intermediate filaments, and microtubules, each possessing unique activities and characteristics.
• Microfilaments, or actin filaments, are the slenderest elements of the cytoskeleton, composed of two interwoven strands of actin.
• The main function of these filaments is to facilitate cell motility and morphological alterations, such as those observed during muscle contraction, cell division, and amoeboid movement.
• Intermediate filaments, characterized by their moderate diameter, contribute to the tensile strength of cells, enabling them to endure mechanical stress.
• Microtubules, which are part of the cytoskeleton, are the largest filaments and are made up of tubulin subunits organized in a tubular pattern.
• They play a crucial role in numerous cellular functions, such as the segregation of chromosomes during cell division, the transportation of vesicles and organelles along intracellular pathways, and the preservation of cell morphology.

Centrioles

• Centrioles are cylindrical organelles present in the majority of eukaryotic cells.
• They have a crucial function in cell division and the arrangement of the microtubule network.
• A centriole consists of nine sets of microtubule triplets organized in a cylindrical shape.
• Centrioles are commonly observed in pairs, positioned perpendicular to each other, within a cellular area referred to as the centrosome.
• The centrosome serves as the primary microtubule-organizing center (MTOC) within the cell, playing a vital role in preserving the shape and functionality of the cytoskeleton.
• Centrioles play a role in the creation of the mitotic spindle, which is a structure composed of microtubules that separates chromosomes into the daughter cells during cell division.

Nucleus

• The nucleus is a crucial organelle that preserves the integrity of the cell's genetic material and coordinates cellular function and survival.
• It has a compact area called the nucleolus, which is responsible for synthesizing ribosomal RNA (rRNA) and initiating ribosome assembly.

Nuclear Membrane

• The nuclear membrane, also known as the nuclear envelope, is a safeguarding barrier that encloses the nucleus of a eukaryotic cell.
• It is composed of two lipid bilayers and regulates the movement of molecules in and out of its structure via specialized channels called nuclear pores.
• The nuclear membrane offers structural reinforcement to the nucleus and aids in preserving its form.

Nucleolus

• The nucleolus is a discrete organelle located within the nucleus of eukaryotic cells, typically observable as a compact, spherical area.
• It is responsible for constructing ribosomes, which are the cellular apparatus responsible for protein synthesis.
• The process of transcription occurs within the nucleolus, where ribosomal RNA (rRNA) molecules are synthesized from DNA.

Mitochondria

• Mitochondria are organelles enclosed by two membranes that are present in the majority of eukaryotic cells.
• They have a vital function in cellular respiration, the cellular mechanism responsible for producing energy in the form of adenosine triphosphate (ATP).
• Mitochondria possess their own genetic material, referred to as mitochondrial DNA (mtDNA), and have the ability to replicate and synthesize proteins to some extent independently.

Ribosomes

• Ribosomes are small yet powerful organelles present in both prokaryotic and eukaryotic cells, with the primary function of synthesizing proteins.
• They are composed of ribosomal RNA (rRNA) and proteins and can be found either freely in the cytoplasm or bound to the endoplasmic reticulum.
• The process of translation consists of two primary stages: translation initiation, in which the ribosome forms around the mRNA, and elongation, in which amino acids are appended to the developing polypeptide chain based on the mRNA sequence.

Cilia

• Cilia are elongated, filamentous structures present on the outer surface of various cell types within the human body.
• They are structures made up of microtubules and surrounded by the cell membrane.
• Cilia facilitate the removal of mucus and debris from the lungs, assist in the clearance of respiratory passages, and provide protection against infections.

Flagella

• Flagella are elongated appendages present in diverse unicellular species and specific cells of multicellular creatures, such as sperm cells.
• They are structures made up of microtubules and surrounded by the cell membrane.
• Flagella have a distinctive waving or spiraling movement that enables the cell to move forward in its surroundings.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is an intricate network of membranous tubules and sacs present in eukaryotic cells.
• The ER is divided into two distinct regions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).
• The rough endoplasmic reticulum (RER) is covered with ribosomes on its surface, resulting in a coarse appearance when observed under a microscope.

Microtubules

• Microtubules are part of the cytoskeleton and are made up of tubulin subunits organized in a tubular pattern.
• They play a crucial role in numerous cellular functions, such as the segregation of chromosomes during cell division, the transportation of vesicles and organelles along intracellular pathways, and the preservation of cell morphology.

Centrioles

• Centrioles are cylindrical organelles present in the majority of eukaryotic cells.
• They have a crucial function in cell division and the arrangement of the microtubule network.
• Centrioles play a role in the creation of the mitotic spindle, which is a structure composed of microtubules that separates chromosomes into the daughter cells during cell division.