Cell Structure and Function WK 2

 Cell Structure and Function [1]

• The human body contains trillions of cells of more than 200 different types, varying in shape and size, each performing different functions [1, 2]. Most human cells average about 10–15 micrometres in diameter [1].

• Cell specialization occurs through differentiation or maturation [1, 2].

• Cell Shapes [1]:

• Disc-shaped (discoid) cells (red blood cells) [1]

• Squamous cells (thin, flat skin cells) [1]

• Biconcave cells (red blood cells) [1]

• Four Characteristic Cell Functions [2, 3]:

• Cell metabolism [2, 3]: Cells take in and use oxygen and nutrients to produce energy [2, 3].

• Excretion [2, 4]: Cells rid themselves of waste products from metabolic processes [2, 4].

• Communication [2, 4]: Internal and intercellular communication occurs via chemical and electrical signals to maintain homeostasis [2, 4].

• Reproduction [2, 5]: Cells replicate or reproduce, enabling growth and tissue maintenance [2, 5].

Cell Components [2, 6]

• Cells consist of a cell membrane, cytoplasm, and organelles [2, 7].

• Cell Membrane [7, 8]: The outer border that provides structural integrity [7, 8].

• Cytoplasm [7, 8]: The fluid within the cell that houses the organelles [7, 8].

• Organelles [2, 7]: Structures within the cell responsible for intracellular activities [2, 7].

Organelles [9]

• Specialized structures with distinct functions that may vary in number depending on the cell's primary function [2, 9].

• Membranous Organelles [10]:

• Nucleus [10]

• Endoplasmic reticulum [10]

• Golgi apparatus [10]

• Lysosomes [10]

• Peroxisomes [10]

• Mitochondria [10]

• Non-Membranous Organelles [10]:

• Ribosomes [10]

• Cytoskeleton [10]

• Nucleus [7, 11]:

• Control center containing genetic material [10, 11]

• Nuclear membrane (envelope) protects the contents and has nuclear pores for communication [11, 12].

• Nucleolus forms ribosome subunits for protein production [11, 13].

• Ribosomes [11, 14]:

• Small granules of protein and RNA responsible for protein production [11, 14]

• Can be free in the cytoplasm or form a complex with the rough endoplasmic reticulum [11, 14]

• Endoplasmic Reticulum [11, 15]:

• Network of tubules and cavities (cisternae) continuous with the nuclear membrane [11, 15]

• Rough ER produces proteins [11, 13].

• Smooth ER produces phospholipids and has other functions specific to the cell type [11, 13].

• Golgi Apparatus [16, 17]:

• Network of flattened membranes that modify, package, and redistribute lipids and proteins from the endoplasmic reticulum [16, 17]

• Forms secretory vesicles for transport to other organelles or release from the cell [16, 17]

• Lysosomes [17, 18]:

• Small storage vesicles containing powerful digestive enzymes [17, 18]

• Break down old or non-functioning organelles and digest foreign substances [17, 18]

• Peroxisomes [19, 20]:

• Similar to lysosomes but contain different enzymes [19, 20]

• Involved in the breakdown of amino acids and fatty acids [19, 20]

• Mitochondria [20, 21]:

• Powerhouses of the cell, producing ATP [20, 21]

• Have a double membrane, with the inner layer folding inwards to form cristae, increasing surface area for ATP production [20, 21]

• Cytoskeleton [20, 22]:

• Provides structural integrity, maintains cell shape, and allows intracellular transport [20, 22]

• Made up of protein filaments [20, 22]

• Cytoplasm [20, 23]:

• Aqueous solution filling the space between the nuclear membrane and the cell membrane [20, 23]

• Composed of water, proteins, sugars, and waste products [20, 23]

Cell Membrane [8, 24]

• Barrier between the outside and inside of the cell that controls the movement of substances [8, 24]

• Lipid Bilayer [8, 25]:

• Composed of phospholipids and cholesterol [8, 25]

• Hydrophobic tails and hydrophilic heads arrange into two layers [8, 25]

• Acts as a barrier to water and hydrophilic substances [8, 25]

• Proteins [8, 26]:

• Integral membrane proteins act as channels or receptors [8, 26]

• Peripheral membrane proteins provide support and join cells together [8, 26]

• Fluid Mosaic Model [8, 27]:

• The cell membrane is dynamic, with lipids and proteins moving laterally [8, 27]

• Allows the cell to protect itself against injurious agents [8, 27]

Cell Receptors and Communication [28, 29]

• Cell receptors are protein molecules on the cell membrane, in the cytoplasm, or in the nucleus that recognize and bind with specific molecules (ligands) [28, 29].

• Hormonal Signaling [30, 31]: Endocrine cells secrete hormones that travel via the bloodstream to target cells [30, 31].

• Neuron Signaling [30, 31]: Nerve cells communicate directly with target cells by releasing neurotransmitters at synapses [30, 31].

• Cells communicate via protein channels (gap junctions) and receptors on the cell membrane [29, 32].

Membrane Transport [31, 33]

• Substances move in and out of cells via various mechanisms [31, 33].

• Passive Transport [31, 33]:

• Does not require energy [31, 33]

• Water and small, uncharged molecules move through pores in the lipid bilayer [31, 33]

• Driven by osmosis, hydrostatic pressure, and diffusion [31]

• Filtration: Movement of water and solutes through a membrane due to pushing pressure [34, 35]

• Diffusion: Movement of a solute from an area of greater concentration to an area of lesser concentration [35, 36]

• Facilitated Diffusion: Substances move through specific channels or with the assistance of integral proteins down the concentration gradient without energy [35, 37]

• Osmosis: Movement of water across a semi-permeable membrane from a region of higher water concentration to one of lower concentration [38, 39]

• Active Transport [39, 40]:

• Requires energy (ATP) [39, 40]

• Protein transporters move molecules against the concentration gradient [39, 41]

• Sodium-potassium pump [39, 42]

• Endocytosis and Exocytosis [43, 44]:

• Transport of large molecules (macromolecules) via vesicle formation and fusion [43, 44]

• Endocytosis: A section of the cell membrane enfolds substances from outside the cell, invaginates (folds inwards) and separates from the cell membrane, forming a vesicle that moves into the cell [43, 45].

• Pinocytosis: Ingestion of fluids and solute molecules through small vesicles [44, 46].

• Phagocytosis: Ingestion of large particles, such as bacteria, through large vesicles (vacuoles) [44, 46].

• Exocytosis: Release of molecules produced by the cell into the extracellular space [44, 47].

Cell Metabolism [45, 48]

• Chemical tasks that maintain essential cell functions [45, 49]

• Anabolism: Energy-using process [45, 49]

• Catabolism: Energy-releasing process [45, 49]

• ATP (Adenosine Triphosphate) [45, 50]:

• Energy-transferring molecule [45, 50]

• Chemical energy is transferred from molecules of carbohydrate, lipid, and protein to ATP [45, 50]

• Aerobic respiration: ATP production in the presence of oxygen, yielding large amounts of ATP [51, 52]

• Anaerobic respiration: ATP production without oxygen, yielding small amounts of ATP and lactic acid [51, 52]

Tissues [52, 53]

• Groups of cells organized to perform complex functions [52, 53]

• Four Primary Tissue Types [52, 54]:

• Epithelial Tissue [52, 54]:

• Protective lining tissue for both inside and outside the body [52, 54]

• Functions include protection, absorption, secretion, excretion, and diffusion [52, 55]

• Connective Tissue [56, 57]:

• Connects or binds other tissues and organs [56, 57]

• Functions include protection, fat storage, support, and transport [56, 57]

• Muscle Tissue [57, 58]:

• Contracts and shortens when stimulated, resulting in movement [57, 58]

• Three types: skeletal, cardiac, and smooth muscle [57, 58]

• Nervous Tissue [57, 59]:

• Composed of neurons that receive and transmit electrical impulses [57, 59]

Aging and Cell Structure and Function [60, 61]

• Normal physiological process involving time-dependent loss of structure and function [60, 61]

• Cell Changes [61, 62]:

• Atrophy [61, 62]

• Decreased function and loss of cells [61, 62]

• Tissue Change: Progressive stiffness or rigidity [63]

• Total Body Changes: Decrease in height, reduction in circumference of the neck, thighs, and arms [64]

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