Unit 2

Cells

  • the basic structural and functional units of every organism

  • all cells:

    1. are bound by a plasma membrane → controls the entry and exit of substances in cells

    2. contain cytosol → contains certain important cell features

    3. contain chromosomes → condensed DNA

    4. contain ribosomes → to make proteins

Prokaryotes

  • single-celled organisms that have no nucleus

  • DNA is in the nucleoid region → has only one chromosome

  • generally smaller in size than eukaryotes

Eukaryotes

  • organisms that have a nucleus

  • can be single or multi-celled

  • DNA is in the nucleus → has multiple chromosomes

  • contains internal membrane organelles that compartmentalize the cells

  • cellular compartments allow for various processes and reactions to occur, increasing efficiency within the cell

  • internal membranes generally minimize the competing interactions and increase surface area at reaction sites

  • loss of/changes to intracellular compartments hinder cell function

Endosymbiont theory

  • states that an early eukaryotic cell engulfed a prokaryotic cell

  • prokaryotic cell became an endosymbiont (cell that lives in another cell) and became one functional organism

  • evidence:

    1. mitochondira and chloroplasts are the same shape and size as prokaryotes

    2. organelles have their own DNA, double membrane, and ribosomes

  • proves that eukaryotes and prokaryotes are similar in structure

Nucleus

  • contains chromosomes (genetic information)

  • enclosed by the nuclear envelope

  • has pores that regulate entry and exit of materials from the cell

  • contains a nucleolus

    • a region of the nucleus where rRNA is synthesized

    • rRNA is combined w/ proteins to form ribosomes, which then exit via nuclear pores

Ribosomes

  • synthesize proteins according to the mRNA sequence within the cell

  • made of rRNA and proteins

  • has a large and small subunit

  • can be found in 2 locations:

    1. cytosol

      • free ribosomes → produce proteins

    2. bound to the endoplasmic reticulum or nuclear envelope

      • bound ribosomes → exported via transport vacuoles

Endoplasmic reticulum (ER)

  1. rough ER

    • synthesizes w/ proteins that are generally secreted by the cell

    • has ribosomes attached to membrane

    • compartmentalizes the cell → packages newly synthesized proteins made by attached ribosomes for possible export from the cell → as proteins are produced by rough ER, polypeptide chains travel across ER membrane and into cisternal space

  2. smooth ER

    • functions in diverse metabolic processes and varis in cell type

    • has no attached ribosomes

    • processes include synthesis of lipids, metabolism of carbs, detoxification of drugs and poisons, and storage of calcium ions

Golgi complex

  • warehouse of receiving, sorting, manufacturing, and shipping proteins

  • has directionality

    1. cis face

      • receives vesicles from the ER

    2. trans face

      • sends vesicles back out into cytosol to other locations or to the plasma membrane for secretion

  • involved in the correct folding and chemical modification of newly synthesized proteins and packaging for protein trafficking

Lysosomes

  • trashcan of the eukaryotic cell

  • membranous sac of hydrolitic enzymes that are used to digest macromolecules or damaged cell parts

  • once the cell needs to self-destruct, the lysosomes will release its enzymes into the cytoplasm, leading to cell death and they recycle their own cell’s organic materials to allow the cell to renew itself

Peroxisomes

  • similar to lysosomes

  • found within the cytoplasm

  • catalyze reactions that produce H202 (hydrogen peroxide), which is toxic, into its non-toxic form, water

Vacuoles

  1. food vacuole

    • forms via phagocytosis (cell eating) and then are digested by lysosomes

  2. contractile vacuole

    • pumps excess water out of the cell to maintain concentration of ions + molecules and water balance

    • releases waste from a cell

    • stores water

  3. central vacuole

    • plays a major role in plant cell growth

    • can take up 80% of volume, typically the largest compartment of the cell

Mitochondira

  • site of aerobic respiration

  • functions in production of ATP that is used for cell work

  • has a double membrane

    • outer = smooth

    • inner = has folds called cristae → increases SA to allow more ATP to be made

Chloroplast

  • the site of photosynthesis

  • found in photosynthetic eukaryotic cells

  • has a double outer membrane → creates a concentration gradient and controls the flow of substances across it

  • captures energy from the sun and produces sugar for the organism

  • contains thylakoids that are stacked to increase the efficiency of light-dependent reactions and maximize SA

  • stroma is the fluid around the thylakoids and is the location for the calvin cycle

Cytoskeleton

  1. microtubules

    • hollow, rod-like structures made of protein tubulin

    • serve as structural support for the movement of organelles that are interacting w/ motor proteins

    • assist in the separation of chromosomes during cell division

  2. microfilaments

    • thin, solid rods made of the protein actin

    • maintains cell shape

    • assists in muscle contraction and cell motility → actin works w/ another protein called myosin to cause a contraction

    • division of animal cells

  3. intermediate filaments

    • fibrous proteins made up of varying subunits

    • permanent structural elements of cells

    • maintains cell shape

    • anchor nucleus and organelles

    • forms the nuclear lamina → lines the nuclear envelope

Cell size

  • cells are small because they have a high SA:V

  • high SA:V makes a more efficient exchange of materials w/ the environment

  • high value = high SA:V = smaller cell

Plasma membrane

  • separates internal cell environment from external environment

  • this is thanks to phospholipids, which form a bilayer in the environment → is ampiphatic w/ polar head and nonpolar tails

Fluid mosaic model

  • when temp increases, bond strength of the hydrophobic tails decreases, leading to more fluidity

  • cholesterol (embedded in the plasma membrane) helps maintain fluidity at high and low temps

  • high temp = less fluid membrane → protects bonds from breaking

  • low temp = more fluid membrane → reduces tightpacking of phospholipids

  • the fluidity of the plasma membrane maintains the positioning and stability of the proteins

Membrane proteins

  1. integral proteins

    • important proteins

    • embedded into the lipid bilayer

  2. peripheral proteins

    • not embedded into the lipid bilayer

    • loosely bonded to the surface

Membrane carbohydrates

  • important for cell-to-cell recognition

  1. glycolipids

    • carbohydrates bonded to lipids

  2. glycoproteins

    • carbohydrates bonded to proteins

    • most abundant

Cell wall

  • covers plants’ plasma membranes

  • provides:

    1. shape and structure

    2. protection from bursting

    3. regulation of water intake

  • composed of cellulose

  • contain plasmodesmata (hole-like structures in the cell wall filled w/ cytosol that connect adjacent cells

Passive transport

  • the movement of molecules that does not require ATP because they are moving with their concentration or electrochemical gradient

  1. diffusion

    • small nonpolar molecules (O2, N2, C02) pass freely → move with its concentration gradient

  2. osmosis

    • the diffusion of water with its concentration gradient across a semi-permeable membrane

  3. facilitated diffusion

    • molecules diffuse through the membrane via transport proteins

    • channel → provides a channel

    • carrier → undergoes conformational change

    • increases rate of diffusion for small ions, H20, and carbohydrates

    • still passive transport because the substances are moving with their concentration gradient and no ATP is required

Active transport

  • the movement of molecules against their concentration gradient; hence, ATP is required

  1. pumps

    • maintains membrane potential → unequal concentrations of ions across the membrane

    • embedded in the membrane

  2. phcotransport

    • the coupling of a favorable movement of one substance w/ and unfavorable movement of another substance

    • favorable = downfill diffusion

    • unfavorable = uphill transport

    • used for sugars and amino acids

Exocytosis

  • the secretion of molecules via vacuoles that fuse to the plasma membrane

  • vesicles form a bilayer in the membrane

  • once fused, the contents of the vesicle are released to the extracellular fluid

Endocytosis

  • the uptake (allowing substances to get inside) of molecules from vesicles fused from the plasma membrane

  • 3 types:

    1. phagocytosis

      • when a cell engulfs particles to be later digested by lysosomes

      • packages particles into a food vacuole → fuses w/ a lysosome to be digested

      • harmful

    2. pinocytosis

      • nonspecific uptake of extracellular fluid containing dissolved molecules

      • cell takes in dissolved molecules in a protein-coated vesicle

      • safe enough because it is inside of a vesicle

    3. receptor-mediated endocytosis

      • specific uptake of molecules via solute binding to receptors on the plasma membrane (glycoproteins)

      • allows cell to take up large quantities of a specific substance

      • specific = we need it

      • not harmful

Tonicity

  • the ability of an extracellular solution to cause a cell to gain or lose water

  • depends on the concentration of solutes that cannot pass through the cell membrane

Osmoregulation

  • cells must be able to regulate their solute concentration and maintain water balance

Cells can be in 3 types of solutions

  1. isotonic

    • have no net movement of water

    • the concentration of nonpenetrating solutes inside the cell is equal to that outside the cell

    • water diffuses into the cell at the same rate water moves out the cell

    • no gradient = no movement of water = equal amounts of solute and solvent

  2. hypertonic

    • lose water to their extracellular surroundings → becomes high in solute

    • the concentraiton of nonpenetrating solutes is higher outside the cell

    • water will move to the extracellular fluid

    • animal cells will shrivel and die → does not have a cell wall

    • movement of water from the inside to the outside = cell loses water

  3. hypotonic

    • gain water

    • the concentration of nonpenetrating solutes is lower outside the cell

    • animal cells will swell and lyse → eventually burst

    • plant cells work optimally → cell wall protects from bursting

    • cell has low water potential and high solute potential = water moves from outside to inside

Water potential

  • a physical property that predicts the direction water will flow

  1. high water potential to low water potential OR low solute concentration to high solute concentration

  2. high pressure to low pressure

Formulas

SA = 4pir²

V = 4/3pirÂł

SA:V = SA/V

water potential = solute potential + pressure potential (“open air” = 0 Mpa)

solute potential = (-) (ionization constant (no ions formed = 1)) (molar concentration) (pressure constant = 0.0831) (temperature in K = 273 + celsius)