Cell Theory & Discovery

• General Biology, prepared by Sadik Hussen at Samara University in Ethiopia, MAR, 2023.

Cell Discovery and Theory

• Robert Hooke, an English scientist, first discovered cells in 1665
• Hooke looked at dead cells
• Anton Van Leeuwenhoek, a Dutch scientist, observed the first living cells
• Leeuwenhoek called the tiny organisms he saw "Animalcules", meaning little animals
• In the 1830s, Mathias Schleiden and Theodore Schwann proposed all living things are composed of cells.
• Rudolph Virchow said cells arise only from other cells, or cell division of pre existing cells
• Schleiden (1838) was a German botanist who said all plants are composed of cells
• Schwann (1839) was a German zoologist who discovered all animals are made of cells
• Virchow (1855) was a German physician who said cells only come from preexisting cells

Cell Theory

• All living things are composed of one or more cells
• The cell serves as the basic, structural and functional unit of life
• All cells arise from pre-existing cells through cell division

Types of Cells

• There are two basic types of cells: prokaryotic and eukaryotic
• Prokaryotic cells lack a membrane-bounded nucleus and membranous organelles, examples are bacteria and cyanobacteria (blue-green algae)
• Eukaryotic cells membrane-bounded nucleus and membranous organelles, examples are protists, fungi, plants, and animals

Eukaryotic Cell Structure

• A typical eukaryotic cell has 3 major parts: plasma membrane, cytoplasm, and nucleus
• The plasma membrane is the outer boundary of the cell
• The cytoplasm contains intracellular fluid packed with organelles
• Organelles are small structures that perform specific cell functions
• The nucleus controls cellular activities and is typically located near the cell's center

Cellular Organelles

• An organelle is a specialized subunit within a cell with a specific function
• In eukaryotes, organelles are membrane-bound structures
• Prokaryotes do not have membrane-bound organelles

Organelle Structure and Function: Nucleus

• The nucleus is the largest cell part, surrounded by a double-layered membrane
• It governs most cellular activities and serves as the cell's master
• DNA makes up the chromosomes
• A double nuclear membrane (nuclear envelope) with nuclear pores surrounds the nucleus allowing materials to enter and leave to/from the cytoplasm

Organelle Structure and Function: Cytoplasm

• The cytoplasm includes the whole internal structure of the cell except for the nucleus
• It is filled with a jelly-like substance and consists of thousands of organelles
• It's the center for chemical reactions in the cell
• Major organelles found in the cytoplasm include mitochondria, endoplasmic reticulum, Golgi complex, and ribosomes.

Organelle Structure and Function: Endoplasmic Reticulum (ER)

• The ER is a network of internal membranes extending through the cytoplasm
• It connects the nucleus with the cell membrane
• There are two forms: rough and smooth endoplasmic reticulum
• Rough ER has ribosomes on its membrane surface and is involved in protein transport
• Smooth ER has no ribosomes on its surface and is concerned with lipid synthesis, carbohydrate metabolism, and detoxification

Organelle Structure and Function: Golgi Complex (Apparatus)

• The Golgi complex is a series of flattened sacs that modifies, packages, stores, and transports materials out of the cell ("shippers of the cell")
• It looks like a stack of plates that stores, modifies, and packages proteins
• It processes raw material into finished products (modifies proteins from the ER)
• Sorts and directs finished products to their final destination

Organelle Structure and Function: Lysosomes

• Lysosomes are membrane-enclosed sacs containing hydrolytic enzymes
• The enzymes digest and remove unwanted cellular debris and foreign materials
• Lysosomes serve as the cell's intracellular "digestive system"

Organelle Structure and Function: Ribosomes

• Ribosomes are found attached to the rough endoplasmic reticulum and are free in the cytoplasm
• They are composed of RNA and protein
• Ribosomes are involved in protein synthesis

Organelle Structure and Function: Mitochondria

• Mitochondria are the "power houses" of a cell
• They are responsible for the site of cellular respiration
• Energy is extracted from nutrients in food and transformed to ATP
• Contain respiratory enzymes that make ATP, releasing energy
• Mitochondria contain their own DNA called mitochondrial DNA (mDNA)
• Mitochondria are bound by a double membrane, inner and outer membranes

Organelle Structure and Function: Chloroplasts

• Chloroplasts participate in photosynthesis, the process by which plants synthesize their own food
• Located in outer surface of the cell to receive enough light
• They are green colored due to chlorophyll
• They are the site of photosynthesis (conversion of light energy to chemical energy)

Organelle Structure and Function: Vacuoles

• Vacuoles are membrane-bound cytoplasmic spaces containing materials
• They maintain water balance
• Plant cells usually have one large central vacuole
• Animal cells have many smaller vacuoles
• Vacuoles are storage containers for water, food, enzymes, wastes, and pigments.
• There are two types of vacuoles: food vacuoles that store and digest ingested food, and contractile vacuoles that pumps excess water from cells to maintain homeostasis

Organelle Structure and Function: Plasma/Cell Membrane

• The plasma membrane issemi-permeable and controls the movement of substances in and out of the cell.
• Carbon dioxide, oxygen, and water pass across the cell membrane, made of a protein and lipid bilayer, it is classified as a lipoprotein layer
• Maintains differences between the cytosol (intracellular fluid) and the extracellular environment
• Encloses the cell and defines cell boundaries
• Regulates the transport of substances in or out of cells
• In animal cells, the cell membrane is on the external layer, it plant cells, it is next to the cell well
• The plasma membrane is involved in signal transduction, enzymatic catalysis, cell interactions, and anchoring the cytoskeleton

Molecular Composition of Cell Membrane

• All plasma membranes are made of lipids and proteins plus a small amount of carbohydrates.
• Lipids consist of a phospholipid bilayer and cholesterol
• Proteins include peripheral and integral proteins
• Carbohydrates consist of oligosaccharides on glycoproteins and glycolipids

Membrane Proteins

• Enzyme proteins have catalytic activity.
• Carrier proteins help to transport materials in and out of the cell
• Structural proteins form the structure of the membrane
• Two classes of membrane proteins exist: peripheral (extrinsic) and integral (intrinsic/transmembrane)
• Peripheral proteins do not penetrate the lipid bilayer significantly and anchor integral proteins to the cytoskeleton
• Integral proteins include hydrophobic surfaces that penetrates the lipid bilayer and move substance across the membrane

Membrane Carbohydrates

• Short carbohydrate chains on the outer membrane surface serve in cell recognition and adhesion
• Act as structural roles as physical barriers

Membrane Lipids

• Lipids are a bilayer of many cell membranes
• Phospholipids are constructed from two fatty acid chains
• Include a phosphate group, alcohol (glycerol or sphingosine) and nitrogen-containing bases such as inositol
• Cholesterol reduces the fluidity of the membrane.
• Glycolipids serve as signaling molecules, have thee main components (fatty acid, sugar and cerebroside), contains glycan (carbohydrate), abundant in nervous tissue

Cellular Diversity: Cell Shape

• Cells are found in different organisms, with wide variances in size, shape, and internal structure
• This applies to cell found in the same organism
• Cell diversity is influenced by their roles and function within the body
• Have different shapes appropriate to their function

Cellular Diversity: Cell Size

• Cell size relates to its level of activity and the rate molecules move across its membranes
• Cells must have a constant supply of nutrients and oxygen, and be able to get rid of carbon dioxide and other waste products
• Smaller cells have a higher surface area:volume ratio than larger cells
• Small cells have advantages: support greater nutrient exchange in unit cell volume, and grow faster than larger cells
• The cell's nucleus can only control a certain amount of living, active cytoplasm

Transport Across Cell Membranes

• Not all particles can pass through a plasma membrane unaided
• Most biological molecules can't diffuse through the phospholipid bilayer
• The plasma membrane blocks exchange of molecules between the cytoplasm and external environment
• Specific transport proteins, carrier and channel proteins, mediate the selective passage of molecules across the membrane, allowing cell to control cytoplasm composition
• Small non-polar molecules (oxygen and nitrogen) dissolve in lipid bilayers and rapidly diffuse
• Small uncharged polar molecules (water and urea) diffuse too, but slower
• Lipid bilayers are highly impermeable to charged molecules (ions like Na+ and K+).

Types of Membrane Transport

• Passive transport moves substances from high to low concentration (down concentration gradient) without the expense of energy
• Three types of passive transport: simple diffusion through the phospholipid bilayer, facilitated diffusion through channel proteins or aided by carrier proteins, and osmosis
• Active transport moves substances from lower to higher concentration (against concentration gradient) with the expense of energy.
• Active transport Includes endocytosis and exocytosis

Diffusion

• Molecules move randomly from high to low concentration during diffusion
• Simple diffusion is involves molecules crossing the plasma membrane
• A molecule dissolves in the phospholipid bilayer and diffuses across it
• Requires no energy
• Does not need membrane proteins are involved
• The direction of transport is determined by the molecule's concentration inside and outside the cell
• The net flow of molecules always goes downhill from a higher to lower concentration
• Gases (O2 and CO2), hydrophobic molecules (benzene), and uncharged small polar molecules (H2O and ethanol) can diffuse across the plasma membrane
• Other biological molecules are unable to dissolve in the hydrophobic interior of the phospholipid bilayer.

Facilitated Diffusion

• Like simple diffusion, facilitated diffusion happens without energy
• The direction of transport is downhill
• The transported molecules do not dissolve in the phospholipid bilayer
• In facilitated diffusion, passage is proteins, allowing transported molecules to cross the membrane without interacting with its hydrophobic interior
• It allows the movement of polar and charged molecules
• The direction of transport depend on concentration and voltage (electrical) differences
• Two types of proteins that mediate transport: carrier and channel proteins

Carrier Proteins and Transporters

• Carrier proteins (transporters) transport a variety of ions and molecules across cell membranes
• There are three types of transporters: uniporters, symporters, and antiporters
• Uniporters transport a single type of molecule down its concentration gradient via facilitated diffusion
• Symporters (co-transporters) transport two different solutes simultaneously in the same direction
• Antiporters (counter-transporters) transport two different solutes simultaneously in opposite directions

Channel Proteins

• Channel proteins form a hydrophilic passageway for polar molecules or ions to move
• Molecules of the appropriate size and charge can pass freely
• Porins can move ions and polar molecules through the outer membranes of bacteria.
• Aquaporins permit water molecules to rapidly cross
• Ion channels are the best-characterized proteins, which mediate movement of ions across the plasma membrane

Rate of Diffusion Factors

• Diffusion happens faster when the molecules travel a shorter distance
• Smaller molecules diffuse faster
• Temperature: Higher temperature, faster diffusion
• Steepness of Concentration gradient: Steeper gradient, faster diffusion

Osmosis

• Osmosis is the diffusion of water through a selectively permeable membrane
• Water moves from an area of high water concentration to an area of low water concentration
• Water moves from dilute to concentrated solution
• With two solutions, the higher solute concentration is known as hypertonic and low solute concentration is hypotonic.
• At equal is said to be Isotonic
• Animal cells in a hypotonic solutions causes haemolysis
• In animal cells, the plasma membrane separates cytoplasm from extracellular
• In animal cells, the net movement of water is determined by the concentration of solute
• Animal calls in a hypertonic solutions causes crenation
• Animal cells in a isotonic doesn't change volume

Tonicity

• Tonicity measures the osmotic potential of two solutions separated by a semi-permeable membrane
• Solution = solute + solvent (water)
• Hypotonic solution means high water (there is net water movement)
• Hypertonic mean high solute (there is net movement of solute)
• Isotonic Solution means water and solute are equals (there is no net solute)

Direction of Osmosis

• Water will equalize to its surroundings by isotonic diffusion
• Animal cells will eventually lyse in a hypotonic conditions
• Animal cells will eventually shrivel in a hypertonic conditions
• Water will enter into the cell if outside is hypotonic.
• Water will leave cell if outside is hypertonic
• In isotonic condition then there is no water movement

Reaction of Solutions with Plant, Algal, Fungal, and Bacteria

• If solutions are hypotonic then there will be increase osmotic pressure
• Water will diffuse into the cell if outside is hypertonic solutions
• Solutions of algael and fungi will get turgid so it will maintain its volume in hyptonic solutions
• If solutions are hpertonic plasmoptysis is what to call its reaction

Factors that affect the rate of osmosis

• Temperature: Higher temperature, more diffusion of water
• Concentration gradient: Larger concentration gradient, faster osmosis
• Surface area to volume relationship: As SA:V gets higher then osmosis gets faster
• Permeability of membrane: As the membrane is permeable then osmosis can get faster

Active Transport

• Active transport pumps water into an equalized state
• It needs input
• A few sources of energy can come from ATP, from light, and gradients
• It could from direct (Primary), and from Indirect (Secondary)