Cell Structure and Function

Cell Structure & Function

The Fundamental Units of Life

• All living organisms consist of cells
• The cell is the simplest unit of living matter and is considered the "basic unit of life"
• Cells originate exclusively from pre-existing cells
• Cells exhibit substantial diversity while maintaining common fundamental characteristics

Common Features of all Cells

• Unifying characteristics of all cells include a plasma membrane, cytoplasm, genetic material, and ribosomes
• The plasma membrane is a phospholipid bilayer with embedded proteins that separate the cell from its environment
• It functions as a selective barrier for the material that goes into and out of a cell.
• It is considered a semi-permeable membrane because it allows some, but not all, substances to pass through
• Cytoplasm is the material contained within the plasma membrane, composed of cytosol and suspended particles
• Cytosol is the semifluid substance inside the cell (intracellular fluid)
• All cells house chromosomes which contain genetic material, containing instructions needed for the cell's growth, development, functioning, and reproduction
• Almost of all this genetic material is DNA; in some cases there are exceptions, however, in which RNA is the genetic material
• Ribosomes are tiny structures comprised of ribosomal RNA (rRNA) and protein
• Ribosomes consist of a small and a large subunit
• They build proteins based on instructions from genes in the DNA
• Cells that synthesize many proteins contain a large number of ribosomes, for example, a human pancreas cell has a few million ribosomes

Prokaryotic vs. Eukaryotic Cells

• All organisms on Earth are categorized as either prokaryotic or eukaryotic
• Prokaryotes: single-celled organisms composed of prokaryotic cells
• Prokaryotic cells are simple cells characterized by the absence of a nucleus
• Eukaryotes: single-celled and multi-celled organisms composed of eukaryotic cells
• Eukaryotic cells are complex cells characterized by a nucleus and other membrane-bound organelles

Prokaryotic Cells

• Prokaryotic cells are structurally smaller and simpler than eukaryotic cells
• Prokaryotic cell structures include:
• Cell membrane
• Cytoplasm
• Ribosomes
• Nucleoid: a region in the cell in which DNA is located in (not surrounded by a membrane); prokaryotic chromosomes are circular
• Cell wall: a rigid structure outside the cell membrane; in prokaryotes, the cell wall is made of the carbohydrate peptidoglycan
• Capsule: a jelly like outer coating found in various prokaryotes
• Flagella: locomotion organelles of some bacteria

Eukaryotic Cells

• Eukaryotic cells are larger and structurally more complex than prokaryotic cells
• Common features with prokaryotes include a cell membrane, cytoplasm, and ribosomes
• DNA is enclosed in a nucleus
• Eukaryotic chromosomes are linear
• Contain a diverse collection of specialized compartments called organelles
• Many organelles in eukaryotic cells are surrounded by a membrane (membrane-bound)
• Each organelle performs specific functions

The Three Domains of Life

• Taxonomy: the science of naming and classifying biological organisms based on shared characteristics
• The largest level of groupings are domains, which categorize all life on Earth into three groups:
• Archaea
• Bacteria
• Eukarya
• Eukarya consist entirely of eukaryotic organisms, whether single or multi-celled
• Include animals, plants, fungi, and protists (single-celled eukaryotes)
• Archaea and bacteria consist entirely of prokaryotes, which are all single-celled
• Bacteria and archaea are structurally very similar, however, they are genetically and biochemically very different
• Archaea include prokaryotes that primarily thrive in extreme environments like deep sea vents over 100 °C, hot springs, and basic, acidic, and salty waters
• Also common in neutral environments, in soil, wetlands, and in the intestines of cattle gas is produced while helping with digestion
• Bacteria: a diverse group of prokaryotes recognized for their roles in disease, health, ecological relationships with plants and animals, and decomposing organic matter

Animal vs. Plant Cells

• Both plant and animal cells are eukaryotic
• Structural differences include:
• Plant cells have a cell wall, a central vacuole, and chloroplast; these are absent in animal cells
• Animal cells have centrioles and lysosomes, which are absent in the majority of plant cells

The Endomembrane System

• The membrane-bound organelles found inside eukaryotic cells make up the endomembrane system
• The endomembrane system includes:
• The nucleus
• The endoplasmic reticulum
• The Golgi apparatus
• Lysosomes
• Various types of vesicles and vacuoles
• The plasma membrane

The Nucleus

• The nucleus holds the DNA, which is organized into discrete units called chromosomes; humans have 46 chromosomes which have one long DNA strand coiled around proteins which help condense the DNA
• Chromatin: complex of DNA and proteins
• The nucleus is enclosed by a double-membrane (two lipid bilayers) called the nuclear envelope
• The nuclear envelope has nuclear pores which allow material to enter and leave the nucleus
• The nucleolus is a DNA dense region inside the nucleus where ribosomes are synthesized

The Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is an extensive network of membrane continuous with the nuclear envelope
• This membrane vastly increases the surface area inside the cell, allowing many materials to be synthesized
• Two distinct regions:
• Rough ER: embedded with ribosomes
• Smooth ER: lacks ribosomes

The Rough ER

• A major function of the rough ER is to synthesize proteins (via embedded ribosomes)
• The embedded ribosomes build the polypeptide chain so that it is inserted into the lumen (inside) of the ER
• Then the polypeptide is modified, and the finished protein wrapped in a membrane that buds off from the rough ER
• The protein is carried to its destination by way of forming a transport vesicle
• Proteins made by the rough ER are ultimately destined to be
• Secreted from the cell, incorporating into the cell membrane or as hydrolytic enzymes in lysosomes
• Not all ribosomes are embedded on the rough ER
• Bound ribosomes: attached to the endoplasmic reticulum
• Free ribosomes: found suspended in the cytoplasm
• Proteins made by free ribosomes are destined to remain and be used inside the cytosol of the cell

The Smooth ER

• The smooth ER contains many important metabolic enzymes with diverse functions that depend on cell type
• A function of the smooth ER: synthesizes lipids
• This includes fats, steroids, and new membrane phospholipids
• In certain cell types, the smooth ER plays an important function in detoxifying cells

The Golgi Apparatus

• After leaving the ER, many transport vesicles travel to the Golgi apparatus, where proteins are modified, stored, and then sent to other destinations in new transport vesicles
• The Golgi is made of flattened membranous sacs called cisternae
• A Golgi stack has two opposite sides:
• cis face: (receiving end) vesicles from the ER fuse with the Golgi membrane, giving the Golgi its content
• trans face: (shipping end) after the contents are modified, they pinch off inside of vesicles to be transported to other parts of the cell
• While being transported from the cis to the trans face, the Golgi modifies the products of the ER, then sorts and targets them for various parts of the cell
• Most transport vesicles that leave the Golgi are exported from the cell or become incorporated into the plasma membrane.
• Some of the transport vesicles produced by the Golgi remain in the cell and become lysosomes.

Lysosomes

• A lysosome is a vesicle containing hydrolytic enzymes used to digest (break down) macromolecules.
• Lysosomes are not usually found in plants.
• The hydrolytic enzymes are produced by the rough ER and modified by the Golgi, which ultimately produces the lysosomes
• The enzymes work best in acidic environments found inside the lysosomes
• Lysosomes have a variety of functions
• They fuse with food vacuoles to break down the contents into simple sugars, amino acids, and other monomers, which are released to the cytoplasm to be used as nutrients
• They recycle cell components by breaking down worn and damaged organelles into material that can be reused by the cell
• Lysosomes play an important role in apoptosis, which is when a cell programs itself to die and be destroyed
• During apoptosis, lysosomes release their hydrolytic enzymes within the cell, which begin to break down important structures such as the cell membrane

Vacuoles

• Vacuoles are large vesicles created by the ER and Golgi that perform a variety of functions
• Food vacuoles form by phagocytosis, a process in which a cell engulfs materials from outside the cell
• Contractile vacuoles, found in many freshwater protists, pump excess water out of cells
• Large central vacuoles are commonly found in plant cells, and store water, nutrients, pigments, and waste products
• These vacuoles aren't found in animal cells.

Importance of the Endomembrane System

• Internal membranes facilitate cellular processes by reducing competing interactions and by increasing surface area where reactions can occur
• Membranes and membrane-bound organelles in eukaryotic cells localize (compartmentalize) processes and reactions occurring within the cell

Mitochondria

• Mitochondria are in nearly all eukaryotic cells, like plants, animals, fungi, and protists
• Mitochondria have a two phospholipid bilayer outer membrane
• There are also two compartments: the intermembrane space and the mitochondrial matrix
• The inner membrane contains foldings called cristae, which increases surface area for reactions related to cellular respiration
• Enzymes and other molecules in the mitochondria are responsible for aerobic (oxygen-requiring) cellular respiration
• Aerobic cellular respiration converts sugar molecules into carbon dioxide and water
• Releases energy stored during this process which produces ATP
• Overall reaction: sugar + oxygen è carbon dioxide + water + ATP
• Cells can contain many mitochondria
• Muscle cells have hundreds

Chloroplast

• The outside of the chloroplast is made up of a double membrane (two phospholipid bilayers), separated by a slim intermembrane space
• Inside the chloroplast is another membranous system in the shape of flattened, interconnected sacs called thylakoids
• A stack of thylakoids is called a granum
• The membranes of the chloroplast divide the space into three compartments:
• The intermembrane space
• The stroma: "fluid" space outside the thylakoids
• The thylakoid space: fluid inside the thylakoids
• Chloroplast contains the green pigment chlorophyll, along with enzymes and other molecules that function to perform photosynthesis
• Chloroplast is found only in plants
• Photosynthesis converts solar energy (from the sun) into chemical energy (such as sugar) that the plant uses for energy Uses light, water and carbon dioxide to synthesize carbohydrates
• Oxygen is also created in the process
• Overall reaction: light + carbon dioxide + water è carbohydrate + oxygen
• Then the plant cell's mitochondria can use the sugar created by photosynthesis for cell respiration
• Plant cells can contain many chloroplast

Origin of the Eukaryotic Cell

• An early evolutionary stage in eukaryotic cells began when an early prokaryotic ancestor engulfed an oxygen-using (aerobic) prokaryotic cell
• This was a mutually beneficial symbiotic relationship, where the aerobic prokaryotic cell could perform aerobic cell respiration while the large cell would provide nutrients and other materials to the cell
• The engulfed cell evolved into the modern-day mitochondria organelle found in all eukaryotes
• At a later time, one of these mitochondria containing cells engulfed a photosynthetic prokaryote, resulting in a second mutually beneficial symbiotic relationship
• The engulfed cell evolved into the modern-day chloroplasts found in plant cells
• A symbiotic relationship would explain and confirm why plant cells contain both mitochondria and chloroplasts
• This theory explains the evolution of eukaryotic plant and animal cells from prokaryotic ancestral cells

Evidence for the Endosymbiotic Theory

• Mitochondria and chloroplast have a double membrane (two membranes) surrounding them, unlike other organelles fund in the cell
• Relates to what happens when a host engulfs another cell
• Mitochondria and chloroplast contain their own ribosomes, as well as multiple circular DNA chromosomes, separate from ribosomes and the DNA that the rest of the cell contains
• The ribosomes and circular DNA inside this "endo cell" like the prokaryotes that were previously discussed
• The DNA inside them even contains genes that control some proteins used in these organelles
• Mitochondria and chloroplast are autonomous and can grow and reproduce within the cell they're in

Cytoskeleton

• The cytoskeleton is a network of fibers extending throughout the cytoplasm
• Functions include:
• Helping support the cell and maintain its shape
• Providing anchorage for organelles and molecules inside the cell
• Assisting in movement of the cell and movement of material within the cell
• Vesicles and molecules can "walk" along the tracks provided by the cytoskeleton inside the cell

Fibers of the Cytoskeleton

• Microtubules are the thickest of the three components of the cytoskeleton
• Microfilaments, also called actin filaments, are the thinnest fiber components
• Intermediate filaments are fibers with diameters in a middle range of thickness

Cilia and Flagella

• In eukaryotes, a specialized arrangement of microtubules creates protrusions from the cell called flagella and cilia
• Flagella are long protrusions limited to just one per cell
• Cilia are shorter protrusions occurring in large numbers on the cell surface
• Sometimes their function is to move the cell, such as propelling unicellular organisms through water
• Other times is to move liquid, such as the ciliated lining of the trachea moving debris trapped in mucous away from the lungs

The Cell Wall

• The cell wall protects plant cells, maintains its shape, prevents excessive water uptake, and helps hold up the plant against gravity
• In plants, the cell wall is composed of the polysaccharide cellulose
• In bacteria, the cell wall is made of peptidoglycan (a polymer of sugar and amino acids)

Cell Junctions

• In both animals and plants, cells are organized into tissues, enabling neighboring cells to adhere, interact, and communicate with each other via direct contact
• In plants, the cell walls are perforated with plasmodesmata, which are channels connecting the cells
• The plasma membrane of each cell lines the channels and are continuous (along with the cytosol of each cell)
• Water and materials can pass freely from one plant cell to another
• Animal cells have three main types of cell junctions:
• Tight junctions
• Desmosomes
• Gap junctions
• In tight junctions the plasma membranes of neighboring cells are tightly pressed against one another, bound together by proteins
• Creates a seal to block extracellular fluid from crossing a layer of epithelial cells (like skin)
• In desmosomes, fastening cells function like rivets, anchoring cells together into strong sheets; are also connected to anchoring keratin filaments
• Used to attach muscle cells to one another
• In gap junctions, membrane proteins create pores/channels between cells and allow ions and molecules to move across
• Gap junctions serve as a very important communication between cells in bodily tissues

This resource explores cell structure and function, covering the fundamental units of life and common cellular features. It discusses the plasma membrane, cytoplasm, genetic material, and ribosomes. It also highlights the role of the plasma membrane as a selective barrier.