2.1 Cell structure and function
4 min read•august 14, 2024
Cells are the building blocks of life, with intricate structures that enable them to function. This section explores the components of eukaryotic and , highlighting their unique features and roles in cellular processes.
The cytoskeleton, plasma membrane, and organelles work together to maintain cell shape, facilitate movement, and carry out essential functions. Understanding these structures is key to grasping how cells operate and interact within living organisms.
Eukaryotic Cell Organelles and Functions
Membrane-bound Organelles and their Roles
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- contain specialized structures called organelles, each surrounded by a membrane and performing specific functions that contribute to the overall functioning of the cell
- The is the control center of the cell, housing the genetic material (DNA) and serving as the site of DNA replication and transcription into RNA
- The (ER) is a network of membranous channels and sacs involved in the synthesis, modification, and transport of proteins and lipids
- The rough ER is studded with ribosomes and is the site of protein synthesis
- The lacks ribosomes and is involved in lipid synthesis and detoxification
- The is a stack of flattened membrane sacs that modifies, packages, and sorts proteins and lipids for transport to various destinations (, plasma membrane, or secretion)
- Lysosomes are membrane-bound sacs containing digestive enzymes that break down and recycle damaged organelles, macromolecules, and foreign particles (bacteria, cellular debris)
Energy Production and Cellular Respiration
- are the powerhouses of the cell, generating through the process of
- The inner membrane of mitochondria is highly folded, forming that increase the surface area for energy production
- The , the innermost compartment of mitochondria, contains enzymes involved in the citric acid cycle and oxidative phosphorylation
- are small, membrane-bound organelles involved in the breakdown of fatty acids and the detoxification of harmful substances (hydrogen peroxide, alcohol)
- Peroxisomes contain enzymes such as and that neutralize toxic compounds
Prokaryotic vs Eukaryotic Cells
Structural Differences
- Prokaryotic cells (bacteria, archaea) lack membrane-bound organelles and a true nucleus, while eukaryotic cells (plants, animals, fungi) possess these structures
- The genetic material in prokaryotic cells is a single, circular DNA molecule located in the nucleoid region, whereas eukaryotic cells have multiple linear DNA molecules housed within a membrane-bound nucleus
- Prokaryotic cells are generally smaller (1-5 μm) than eukaryotic cells (10-100 μm) and have a higher surface area-to-volume ratio
- Prokaryotic cells have a composed of , while eukaryotic cells may have a cell wall made of (plants) or lack a cell wall entirely (animals)
Functional Differences
- Prokaryotic cells have ribosomes (70S) that are smaller than those found in eukaryotic cells (80S), reflecting differences in protein synthesis
- Eukaryotic cells have a more complex cytoskeleton, which includes , , and , while prokaryotic cells have a simpler cytoskeleton (FtsZ rings, MreB filaments)
- Prokaryotic cells lack membrane-bound organelles, so cellular processes (DNA replication, transcription, translation) occur in the cytoplasm, while in eukaryotic cells, these processes are compartmentalized within organelles
Structure and Function of Cells
Plasma Membrane and Cell Surface
- The plasma membrane is a selectively permeable barrier that controls the exchange of materials between the cell and its environment
- The large surface area-to-volume ratio of the plasma membrane facilitates efficient exchange of nutrients, waste products, and signaling molecules
- The describes the plasma membrane as a fluid phospholipid bilayer with embedded proteins that can move laterally within the membrane
- Membrane proteins perform various functions, such as transport (channels, carriers), enzymatic activity, cell signaling (receptors), and cell adhesion
Compartmentalization and Organelle Function
- The compartmentalization provided by membrane-bound organelles allows for the spatial separation of different cellular processes, enhancing efficiency and preventing interference between incompatible reactions
- The specific shapes and structures of organelles optimize their functions
- The cristae in mitochondria increase the surface area for energy production through cellular respiration
- The flattened sacs of the Golgi apparatus enable efficient modification, sorting, and packaging of proteins and lipids
- The arrangement and composition of the cytoskeleton provide structural support, enable cell movement, and facilitate the transport of organelles and macromolecules within the cell
The Cytoskeleton: Shape, Movement, and Organization
Components of the Cytoskeleton
- The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables cell movement, and organizes the internal components of the cell
- Microfilaments are thin, flexible filaments composed of actin monomers
- Microfilaments are involved in cell motility (pseudopodia formation, cytoplasmic streaming), muscle contraction, and the maintenance of cell shape
- Intermediate filaments are rope-like fibers that provide mechanical strength and resistance to shear stress
- Intermediate filaments help maintain cell shape and organize the internal structure of the cell (nuclear lamina, keratin filaments in epithelial cells)
- Microtubules are hollow, cylindrical tubes composed of α- and β-tubulin dimers
- Microtubules are involved in cell division (mitotic spindle), intracellular transport, and the maintenance of cell shape and polarity (centrioles, cilia, flagella)
Motor Proteins and Cellular Movement
- use ATP to drive the movement of organelles and macromolecules along the cytoskeleton
- motors are associated with microfilaments and are responsible for muscle contraction and cell movement (cytokinesis, phagocytosis)
- and motors are associated with microtubules and transport cargo (vesicles, organelles) within the cell
- Kinesin moves cargo towards the plus end of microtubules (cell periphery), while dynein moves cargo towards the minus end (centrosome)
- The cytoskeleton plays a crucial role in cell migration, allowing cells to move and change shape in response to external stimuli or during processes such as wound healing and embryonic development
- Actin polymerization drives the formation of protrusions (lamellipodia, filopodia) at the leading edge of migrating cells
- Myosin-mediated contraction of actin filaments at the rear of the cell propels the cell forward
Key Terms to Review (30)
Apoptosis: Apoptosis is a programmed cell death process that allows cells to self-destruct in a controlled manner, helping maintain tissue homeostasis and preventing the proliferation of damaged or unwanted cells. This mechanism is crucial for normal development, immune response, and the aging process, linking it to various biological functions such as cell signaling, division, and immune system regulation.
ATP: ATP, or adenosine triphosphate, is a nucleotide that serves as the primary energy currency of the cell, providing the necessary energy for various biological processes. This molecule is crucial for cellular activities, including muscle contraction, nerve impulse propagation, and biosynthesis. ATP is produced through cellular respiration and is utilized in a variety of cellular functions, making it essential for life.
Catalase: Catalase is an enzyme found in nearly all living organisms that catalyzes the decomposition of hydrogen peroxide into water and oxygen. This crucial enzyme helps protect cells from oxidative damage caused by hydrogen peroxide, which is a byproduct of various metabolic processes. By facilitating this reaction, catalase plays an essential role in cellular function and the overall health of organisms.
Cell differentiation: Cell differentiation is the process by which a less specialized cell becomes a more specialized cell type, enabling it to perform specific functions within an organism. This process is essential for the development and maintenance of multicellular organisms, as it allows cells to take on unique roles, such as muscle cells contracting or nerve cells transmitting signals, contributing to the overall complexity and functionality of tissues and organs.
Cell Wall: A cell wall is a rigid outer layer that surrounds the cell membrane in certain types of cells, primarily in plants, fungi, bacteria, and some protists. This structure provides support, protection, and shape to the cell while also playing a crucial role in regulating the movement of substances in and out of the cell. The composition of cell walls varies among different organisms, with plant cell walls primarily made of cellulose, fungal walls composed of chitin, and bacterial walls consisting of peptidoglycan.
Cellular Respiration: Cellular respiration is the biochemical process by which cells convert glucose and oxygen into energy, specifically ATP (adenosine triphosphate), while producing carbon dioxide and water as byproducts. This process is crucial for maintaining cellular functions, as ATP serves as the main energy currency of the cell, fueling various metabolic activities. The efficiency of cellular respiration is closely tied to the structure and function of cellular organelles, particularly mitochondria, which are known as the powerhouse of the cell.
Cellulose: Cellulose is a complex carbohydrate that serves as a primary structural component of plant cell walls, providing rigidity and strength. It is made up of long chains of glucose molecules linked together by β(1→4) glycosidic bonds, which create a stable and insoluble fiber that is crucial for maintaining the shape and integrity of plant cells. This polymer not only plays a vital role in plant structure but also influences various biological processes.
Cristae: Cristae are the fold-like structures found in the inner membrane of mitochondria, which play a crucial role in cellular respiration and energy production. These folds increase the surface area of the inner membrane, allowing for more efficient ATP synthesis through the electron transport chain. The presence of cristae is vital for the mitochondria's function as the powerhouse of the cell, providing energy that fuels various cellular processes.
Dynein: Dynein is a motor protein that plays a crucial role in the movement of cellular components along microtubules, which are part of the cell's cytoskeleton. This protein is essential for various cellular processes, including vesicle transport, organelle positioning, and cell division. Dynein functions by converting chemical energy from ATP hydrolysis into mechanical work, allowing it to 'walk' along microtubules in a directional manner, usually toward the minus end.
Endoplasmic Reticulum: The endoplasmic reticulum (ER) is a complex network of membranous tubules and sacs found in eukaryotic cells, playing a crucial role in the synthesis, folding, modification, and transport of proteins and lipids. It exists in two forms: rough ER, studded with ribosomes for protein synthesis, and smooth ER, which is involved in lipid synthesis and detoxification processes. The ER is essential for maintaining cellular homeostasis and facilitating communication between various cellular components.
Eukaryotic cells: Eukaryotic cells are complex cells that contain a nucleus and membrane-bound organelles, distinguishing them from prokaryotic cells, which lack these features. These cells are fundamental building blocks of multicellular organisms, including animals, plants, fungi, and protists. The presence of a nucleus allows for more sophisticated regulation of gene expression and cellular processes, enabling greater complexity in biological functions.
Fluid mosaic model: The fluid mosaic model describes the structure of cell membranes as a dynamic and flexible arrangement of various molecules, including phospholipids, proteins, cholesterol, and carbohydrates. This model emphasizes that the membrane is not a rigid structure but rather a fluid layer where components can move laterally, allowing for interactions essential for cell function, signaling, and transport mechanisms.
Golgi apparatus: The Golgi apparatus is a membrane-bound organelle in eukaryotic cells responsible for modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles. It acts as a processing and shipping center, receiving products from the endoplasmic reticulum (ER) and dispatching them to their final destinations, playing a crucial role in cellular function.
Intermediate Filaments: Intermediate filaments are a type of cytoskeletal component that provides structural support to cells, anchoring organelles in place and maintaining the cell's shape. They are thicker than microfilaments but thinner than microtubules, forming a resilient network that contributes to the mechanical stability of tissues. These filaments play a crucial role in cellular integrity and organization by connecting to desmosomes and other cell junctions, allowing cells to withstand stress and deformation.
Kinesin: Kinesin is a motor protein that transports cellular cargo along microtubules in eukaryotic cells. This protein plays a crucial role in cell structure and function by facilitating intracellular transport, which is essential for processes such as cell division, neurotransmitter release, and organelle movement. Kinesin operates through a walking mechanism, utilizing ATP hydrolysis to power its movement along the microtubules.
Lysosomes: Lysosomes are membrane-bound organelles found in eukaryotic cells that contain digestive enzymes responsible for breaking down waste materials and cellular debris. They play a crucial role in maintaining cellular homeostasis by degrading and recycling macromolecules, which helps prevent the accumulation of harmful substances within the cell.
Matrix: In biology, a matrix refers to the material or tissue in which cells are embedded, providing structural and biochemical support. This term is crucial for understanding how cells function and interact within their environment, as the matrix can influence cell behavior, growth, and communication. It is often made up of proteins, carbohydrates, and other molecules that together form a complex network essential for tissue integrity and function.
Microfilaments: Microfilaments are thin, thread-like structures made primarily of actin protein that form a part of the cytoskeleton in eukaryotic cells. They play a critical role in maintaining cell shape, enabling cellular movement, and facilitating various cellular processes such as division and intracellular transport. Microfilaments are crucial for muscle contraction and are involved in the movement of amoeboid cells, highlighting their importance in both structural support and motility.
Microtubules: Microtubules are cylindrical structures made of tubulin protein subunits that are a key component of the cytoskeleton in eukaryotic cells. They provide structural support, shape, and facilitate intracellular transport, cell division, and motility. Microtubules also play essential roles in organizing organelles and enabling the movement of chromosomes during cell division.
Mitochondria: Mitochondria are double-membraned organelles found in most eukaryotic cells, often referred to as the 'powerhouses' of the cell. They play a crucial role in generating adenosine triphosphate (ATP) through oxidative phosphorylation, which provides energy for various cellular processes. Beyond energy production, mitochondria are involved in regulating metabolism, cell signaling, and apoptosis.
Motor proteins: Motor proteins are specialized proteins that facilitate movement within cells by converting chemical energy into mechanical work. They play a crucial role in various cellular processes such as muscle contraction, intracellular transport, and cell division, by moving along cytoskeletal elements like microtubules and actin filaments.
Myosin: Myosin is a type of motor protein that plays a crucial role in muscle contraction and cellular movement. It interacts with actin filaments to facilitate muscle contractions, making it essential for various types of muscular tissues, including skeletal, smooth, and cardiac muscle. This protein not only contributes to the structural integrity of muscle cells but also supports numerous cellular functions, linking it to various physiological processes.
Nucleus: The nucleus is a membrane-bound organelle found in eukaryotic cells that contains the genetic material, or DNA, organized into chromosomes. It serves as the control center of the cell, regulating gene expression and mediating the replication of DNA during the cell cycle, which is crucial for cell function and reproduction.
Oxidases: Oxidases are a group of enzymes that catalyze oxidation-reduction reactions, specifically facilitating the transfer of electrons from a substrate to molecular oxygen, resulting in the formation of hydrogen peroxide or water. These enzymes play a crucial role in various metabolic pathways, influencing cellular respiration and energy production by aiding in the breakdown of organic compounds.
Peptidoglycan: Peptidoglycan is a polymer that forms a mesh-like structure in the cell walls of bacteria, providing both rigidity and shape. It is composed of glycan chains cross-linked by short peptide fragments, making it crucial for maintaining bacterial integrity against osmotic pressure. This unique structure helps differentiate between types of bacteria, particularly between Gram-positive and Gram-negative, which is important for understanding bacterial classification and antibiotic resistance.
Peroxisomes: Peroxisomes are small, membrane-bound organelles found in eukaryotic cells that play a critical role in cellular metabolism. They contain enzymes that are responsible for breaking down fatty acids and detoxifying harmful substances, such as hydrogen peroxide, which is produced during various metabolic processes. By efficiently managing oxidative stress and lipid metabolism, peroxisomes contribute significantly to overall cell function and health.
Prokaryotic Cells: Prokaryotic cells are unicellular organisms that lack a true nucleus and membrane-bound organelles, distinguishing them from eukaryotic cells. These cells are typically smaller and simpler in structure, with their genetic material organized in a single circular chromosome located in the nucleoid region. Prokaryotic cells include bacteria and archaea, and they play vital roles in various ecological processes and human health.
Rough Endoplasmic Reticulum (Rough ER): The rough endoplasmic reticulum (Rough ER) is an organelle in eukaryotic cells that is primarily responsible for the synthesis and processing of proteins. It is called 'rough' due to the presence of ribosomes on its cytoplasmic surface, which gives it a bumpy appearance. The Rough ER plays a crucial role in modifying proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle. Its structure allows for a large surface area, enhancing its capacity for protein synthesis.
Signal Transduction: Signal transduction is the process by which cells respond to external signals, converting these signals into a functional response. This intricate communication system involves various molecular pathways, allowing cells to interpret stimuli from their environment, which is crucial for maintaining homeostasis and coordinating physiological functions.
Smooth ER: Smooth endoplasmic reticulum (smooth ER) is a type of organelle found in eukaryotic cells that plays a vital role in the synthesis of lipids and the detoxification of harmful substances. Unlike rough ER, which is studded with ribosomes, smooth ER has a smooth appearance and is involved in various metabolic processes, including the production of hormones and storage of calcium ions. Its structure and function are essential for maintaining cellular health and metabolism.