Cells: Structure, Theory, and Microscopy

• Cells are the most basic unit of life
• Robert Hooke first observed cells in 1665
• Antoni van Leeuwenhoek described blood, sperm, and pond water organisms, using more refined lenses

Cell Theory

• States cells are the basic units of life
• All living organisms consist of cells
• All cells originate from pre-existing cells

Microscopy

• Light microscopes (LM) are frequently used in biology labs
• Light passes through the specimen, glass lenses, and is projected into the viewer's eye in light microscopes
• Light microscopes can magnify specimens up to 1,000x their actual size
• Electron microscopes (EM) are very powerful microscopes used beginning in the 1950s to view cell ultrastructure
• Electron microscopes use electron beams instead of light
• Electron microscopes resolve biological structures as small as 2 nanometers
• Electron microscopes magnify up to 100,000x

Cell Size

• Cell size must be large enough to house necessary DNA, proteins, and structures for survival and reproduction
• Cell size must remain small enough to maintain a surface-to-volume ratio that allows adequate environmental exchange

Plasma Membrane

• The plasma membrane creates a flexible barrier between the cell and its surroundings
• Phospholipids create a two-layered structure called a phospholipid bilayer
• The phospholipid bilayer has hydrophilic heads facing out towards water
• The phospholipid bilayer has hydrophobic tails facing inward, away from water
• Membrane proteins attach to the membrane surface or embed within the phospholipid bilayer
• Some proteins create channels for ions and hydrophilic molecules through the membrane's hydrophobic center
• Some proteins act as pumps, using energy to move molecules in or out of the cell

Prokaryotic vs Eukaryotic Cells

• Bacteria and archaea are prokaryotic cells
• All other life forms are composed of eukaryotic cells
• Prokaryotic and eukaryotic cells both have a plasma membrane, chromosomes, and ribosomes
• Eukaryotic cells have a membrane-bound nucleus and various organelles
• Prokaryotes have a nucleoid, and lack true organelles

Prokaryotic Cell Details

• Prokaryotic cell DNA is coiled in a nucleoid region without a surrounding membrane
• Prokaryotic cell surfaces may have a chemically complex cell wall, a capsule around the cell wall, and short projections for attachment
• Prokaryotic cells may possess flagella, longer projections that help propel the cell in its liquid environment
• Prokaryotes possess ribosomes that might have precursors to chloroplasts called plastids

Eukaryotic Cell Functions

• Eukaryotic cell structures and organelles have four fundamental functions

  • Genetic control involving the nucleus and ribosomes
  • Manufacturing, distribution, and breakdown of molecules done by the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes
  • Energy processing involving mitochondria in all cells, and chloroplasts in plant cells
  • Structural support, movement, and communication facilitated by the cytoskeleton, plasma membrane, and cell wall

• Internal membranes within eukaryotic cells divide it into compartments

• Cellular metabolism, encompassing various cell activities, happens within organelles

• Animal and plant cells have almost all of the same organelles and other structures

• Plant cells, not animal cells, feature a rigid cell wall, chloroplasts, and a central vacuole, while animal cells do not feature lysosomes and centrioles

Nucleus

• Most of the cell's DNA is contained in the nucleus
• Directing protein synthesis through messenger RNA (mRNA) the nucleus manages cell activities
• DNA associates with proteins in chromosomes
• The nuclear envelope is a double membrane with pores, enabling material flow
• The nuclear envelope connects to the endoplasmic reticulum
• A nucleolus is a notable structure in the nucleus
• Ribosomal RNA (rRNA) synthesis occurs at the nucleolus

Ribosomes

• Protein synthesis happens in the ribosomes
• rRNA produced in the nucleolus creates ribosomes
• Cells that synthesize protein in large amounts have many ribosomes
• Free ribosomes are suspended the cytoplasm and
• Free ribosomes make proteins that function in the cytoplasm
• Free ribosomes commonly exist in prokaryotes
• Bound ribosomes attach to the endoplasmic reticulum (ER) associated with the nuclear envelope
• Bound ribosomes are associated with proteins packed in certain organelles or exported from the cell

Endomembrane System

• Encompasses eukaryotic cell membranes and is known as the endomembrane system
• Physically connected membranes and transfer of membrane segments by small sacs called vesicles relate some of these membranes
• These organelles work for synthesis, storage, and export of molecues
• Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane

Endoplasmic Reticulum Details

• A biosynthetic factory
• Has smooth and rough types
• Smooth ER lacks ribosomes
• Rough ER lines the outer surface of membranes
• Smooth and rough ER are physically connected, but differ in function and structure
• Smooth ER engages in various metabolic processes, synthesizing, including lipids, oils, phospholipids, and steroids
• Smooth ER enzymes help process drugs, alcohol, and other harmful substances, and some helps store calcium ions
• Rough ER makes more membrane for it and proteins destined for secretions

Golgi Apparatus

• The Golgi apparatus serves as a molecular warehouse and finishing factory for products manufactured by the ER
• Products travel in transport vesicles from the ER to the Golgi apparatus
• One side of the Golgi apparatus functions as a receiving dock for the product and the other as a shipping dock
• Products are modified as they go from one side of the Golgi apparatus to the other and travel in vesicles to other sites

Lysosomes

• Membranous sacs containing digestive enzymes

  • Enzymes and the membrane are products of the ER and processing is done in the Golgi apparatus
  • the membrane serves to isolate potent enzymes from the rest of the cell safely

• Help digest food particles engulfed by a cell

  • A food vacuole binds with a lysosome
  • the enyzmes then digest the food in the lysosome
  • nutrients are then released into the cell

• Help remove/recycle damaged parts of a cell

  • The damaged organelle is first enclosed in a membrane vesicle
    • Then a lysosome fuses with the vesicle -dismantles its contents -breaks down the damaged organelle

Vacuoles

• Large vesicles that are variety of functions
  • Some protists have contractile vacuoles help to eliminate water from the protist
  • In plants they may: -have digestive functions -contain pigments or -contain poisons that protect the plant

Energy Converting Organelles

• Mitochondria are organelles carrying out cellular respiration in nearly all eukaryotic cells
• Cellular respiration converts the chemical energy found in foods to chemical energy and then to ATP
• Mitochondria have two internal compartments
  • The intermembrane space is the narrow region between the inner and outer membranes
  • The mitochondrial matrix contains mitochondrial DNA, ribosomes, and many enzymes used to catalyze reactions of cellular respiration
• Chloroplasts are the photosynthesizing organelles in photosynthetic eukaryotes
• Photosynthesis is converting the suns light energy to chemical energy in sugar molecules
• Chloroplasts have compartments
  • Between the outer and inner membrane is a thin intermembrane space.

Inside the inner membrane have: -a thick fluid called stroma, containing the chloroplast DNA -ribosomes and many enzymes and a -network of interconnected sacs called thylakoids.

• In some regions, thylakoids are stacked like poker chips otherwise known as granum where green chlorophyll molecules trap solar energy

Endosymbiont Theory

• Mitochondria and chloroplasts have DNA and ribosomes
• The structure of DNA and the ribosomes are similar to prokaryotic cells
• Suggests mitochondria and chloroplasts were formerly small prokaryotes and then began living within larger cells

Cytoskeleton

• Is a network of protein fibers that function in structural support and motility
• Motility and cellular regulation happen through the cytoskeleton interacting with proteins called motor proteins

Cilia and Flagella

• Protists have flagella and cilia that help with locomotion

• Multicellular ogranisms have cells that contain them from different reasons

  • Cilia helps sweeps mucus of our lungs outside the body
  • Animal sperm are flagellated

• A flagellum, which is longer than cilia, propels a cell by undulating and performs whiplike motions

• Cilia more work like oars of a crew boat

• Although differences exist, flagella and cilia have a common structure and mechanism of movement.

Extracellular Matrix

• (ECM) Animal cells synthesize and secrete an elaborate extracellular matrix
  • ECM helps hold cells together in tissues
  • ECM protects and supports the plasma membrane

Cell Junctions

• Adjacent cells communicate, interact and adhere through them
  • Tight junctions prevent leakage of extracellular fluid across a layer of epithelial cells
  • Anchoring junctions fasten cells together into sheets
  • Gap junctions have channels that allow molecules to flow between the cells

Cell Walls

• Plant cells, but not animal cells, have a rigid cell wall
• The cell wall helps protect and provide skeletal support to keep the plant upright, against gravity and the wall
• The walls mostly consist of cellulose
• Plant cells have cell junctions called plasmodesmata for cell communication

Functions of Eukaryotic Cell Strucutres

• Can be categoried on 4 basic functions
  • Genetic control
  • Manufacturing, distribution, and breakdown
  • Energy processing
  • and support, movement and comunications between the cells