Unit 2: Cell Structure and Function

Cells

Basic structural and functional units of every organism: all are bound by plasma membrane and contain cytosol, chromosomes, and ribosomes

Prokaryotes

Domains Bacteria and Archaea; DNA in the nucleoid region, generally smaller in size than eukaryotes, lack internal membrane-bound organelles

Eukaryotes

Protists, fungi, animals, and plants; DNA is in the nucleus; contain membrane bound organelles that compartmentalize cellular functions

Two classifications of organelles

Endomembrane system and energy organelles

Endomembrane system

Organelles and subcellular components that interact (directly or via vesicles) to modify, package, and transport polysaccharides, proteins, and lipids in the cell

Organelles in the Endomembrane system

Nuclear envelope, endoplasmic reticulum, Golgi complex, lysosomes, vesicles/vacuoles, plasma membrane

Energy organelles

Convert energy to forms that cells can use for work

Organelles that are energy organelles

Mitochondria and chloroplasts

Compartmentalization

Different metabolic processes and enzymatic reactions occur in different locations

-increases surface area for reactions to occur

-prevents interfering reactions from occurring in the same location

Unique cell components in plants

Chloroplasts, central vacuole, cell wall, plasmodesmata

Unique cell components in animals

Lysosomes, centrosomes, flagella

Nuclear envelope

Surrounds the nucleus; double membrane; has pores that regulate entry and exit of materials from the cell; continuous with the ER membrane

Nucleolus

Dense region of the nucleus where ribosomal RNA (rRNA) is synthesized

Ribosomes

Complexes made of ribosomal RNA and protein that synthesize proteins

Ribosomes found in the cytosol

Proteins produced here generally function only within the cytosol; they are known as free ribosomes

Ribosomes bound to the ER or nuclear envelope

Proteins produced here can be secreted from the cell and leave via transport vesicles

ER

A network of membranous sacs (cisternae) and tubules; provide mechanical support by helping cells maintain shape; intracellular transport

Rough ER

Contains ribosomes bound to the ER membrane; compartmentalization protein synthesis

Smooth ER

No ribosomes; synthesizes lipids and detoxifies the cell

Golgi complex structure

Contains flattened membranous sacs called cisternae; separates sacs called cisternae

Golgi complex directionality

Cis face: receives vesicles from the ER

Trans face: Sends vesicles back out into the cytosol to other locations or to the plasma membrane for secretion

Golgi Complex Functions

• receives transport vesicles with materials from the ER

• modifies the material (correctly folds and chemically modifies newly formed cellular products, like proteins)

• Sorts the materials

• Adds molecular tags

• Packages materials into new transport vesicles that exit the membrane via exocytosis

Lysosomes structure

Membranous sac with hydrolysis enzymes in animal cells

Lysosomes functions

• Hydrolyzes (digests/breakdowns) macromolecules

• Autophagy: lysosomes can recycle their own cell’s organic materials and organelles that are not functioning properly, allowing the cell to renew itself

• Apoptosis: membrane becomes permeable—> releases enzymes into cytoplasm—> breaks down cellular components

Peroxisomes

Membrane bound metabolic compartment (not apart of the Endomembrane system); catalyze reactions that produce H2O2

H2O2

Can detoxify certain compounds, but can damage the cell if any escapes; enzyme in peroxisomes (catalase) then breaks down H2O2 to water and oxygen

Peroxisomes functions

Break down fatty acid molecules, synthesize certain phospholipids, detoxify compounds

Vacuoles

Vesicles that stem from the ER and Golgi; selective in transport

Vacuoles in animal cells

Small in size, more per cell; assist in endocytosis/exocytosis processes; store cellular materials

Central vacuole in plant cells

Large in size; stores nutrients and water, which is important for turgor pressure; can function like lysosomes

Mitochondria

Site of cellular respiration

Double membrane in mitochondria

-Outer membrane is smooth; inner membrane is highly convoluted (has many folds), these folds are called cristae

-divides the mitochondria into two internal compartments and increases the surface area

Mitochondrial matrix

• enclosed by inner membrane

• Location for the Kreb’s cycle

• Contains enzymes that catalyze cellular respiration and produce ATP

• Contains mitochondrial DNA

• Contains Ribosomes

Number of mitochondria in a cell

Correlates with metabolic activity; cells with high metabolic activity have more mitochondria

Chloroplast

Specialized organelles in plants and photosynthetic algae

Thylakoids

Membranous sacs that can organize into stacks called grana: light dependent reactions occur in grana

Stroma

Location for the Calvin cycle; contains chloroplast DNA, Ribosomes, and Enzymes

Endosymbiont theory

• The theory that explains the similarities mitochondria and chloroplasts have too prokaryotic cells

• Mitochondria and chloroplasts evolved from once free-living prokaryotic cells via endosymbiosis

• Prokaryotic cell became an endosymbiont

Endosymbiosis

Engulfed by ancestral eukaryotic cell

Endosymbiont

Cell that lives in another cell

Evidence of endosymbiont theory

Double membrane, ribosomes, circular DNA, capable of functioning on their own

Cytoskeleton

• Network of fibers throughout the cytoplasm

• Give structural support and mechanical support

• Anchor organelles

• Allow for movement of vesicles and organelles and the whole cell

• Movement occurs when the cytoskeleton interacts with motor proteins

Microtubules structure

Thickest filaments in cytoskeleton, in animal cells they grow from the centrosome

Microtubules functions

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

• Assist in the movement of chromosomes during cell division

• Cell motility (cilia and flagella)

Microfilaments structure

Thin solid rods made of the protein actin

Microfilaments functions

• Maintain cell shape (bear tension)

• Assist in muscle contraction and cell motility

  • Actin works with another protein called myosin to cause a contraction

• Division of animal cells

  • Contractile ring of the cleavage furrow

Intermediate filaments structure

Fibrous proteins made up of varying subunits; permanent structural elements of cells

Intermediate filaments functions

• maintain cell shape

• Anchor nucleus and organelles

• Form the nuclear lamina

  • Lines the nuclear envelope

Cellular metabolism

• depends on cell size

• Cellular wast must leave

• Dissipate thermal energy

• Nutrients and other resources/ chemical materials must enter

What happens when the cell gets too big

It begins to be difficult for a cell to regulate what comes in and what goes out of the plasma membrane

Formulas for spherical cells

SA = 4πr2

V = 4/3πr3

Cells with a low SA:V ratio

Lose efficiency exchanging materials, rate of heat exchange decreases

Plasma membrane

Separates internal cell environment from external environment

What causes selective permeability

Hydrophobic interior of membrane bilayer

Fluid mosaic model

A model that describes the structure of cell membranes

Fluid

Membrane is held together by weak hydrophobic interactions and can therefore move and shift

• temperature affects fluidity

• Unsaturated hydrocarbon tails help maintain fluidity at low temps

• Kinked tails prevent tight packing of phospholipids

Cholesterol role in fluid mosaic model

Helps maintain fluidity at high and low temps

High temp: reduces movement

Low temp: reduces tight packing of phospholipids

Mosaic

Comprised of many macromolecules

Integral proteins

Proteins that are embedded into the lipid bilayer; can be hydrophilic, hydrophobic, or both; determined by R groups

Hydrophilic region of membrane proteins

Make up the interior of the channel or pore; hydrophilic regions are exposed to cytosol

Hydrophobic region of membrane proteins

Make up the protein surface and interact with fatty acids on the interior of the membrane

Peripheral proteins

Proteins that are not embedded into the lipid bilayer

• Loosely bonded to the surface

Membrane carbohydrates

Important for cell-to-cell recognition; glycolipids and glycoproteins

Glycolipids

Carbohydrates bonded to lipids

Glycoproteins

Carbohydrates bonded to proteins; Most abundant

• Easy passage across the membrane

• Small nonpolar, hydrophobic molecules

  • Examples:

    • Hydrocarbons

    • CO2

    • O2

    • N2

Difficult passage or protein assisted passage

• Hydrophilic polar molecules, large molecules, ions

  • Why? The nonpolar hydrocarbon tails

    • Note: small polar, uncharged molecules (like water and ammonia) can pass through in small amounts

Cell walls

Protection from osmotic lysis (cell bursting when excess water goes into cell)

Bacteria, archaea, fungi, and plants

Cell wall in plants

Composed of cellulose; thicker than plasma membrane; contain plasmodesmata

Plasmodesmata

Hole-like structures in the cell

wall filled with cytosol that connect adjacent cells

Passive transport

transport of a molecule that does not require energy from the cell because a solute is moving with its concentration or electrochemical gradient

Diffusion

spontaneous process resulting from the constant motion of molecules

Osmosis

the diffusion of water from areas of low solute concentration to areas of high solute concentration (hypotonic → hypertonic)

Facilitated diffusion

diffusion of molecules through the membrane via channel or carrier proteins

Channel proteins

• form an open pore/channel in the membrane

• Move charged ions like Na+ and K+

• Most are not permanently open

  • Gates can open/close in response to stimuli

• Ion channels are highly selective

• Movement can polarize the membrane

Aquaporins

Specialized channel proteins for water

Carrier proteins

Alternate between 2 conformations; large polar molecules (glucose/sugars, amino acids, nucleotides)

Active transport

transport of a molecule that requires energy because it moves a solute against its concentration gradient

ATP

can transfer the terminal phosphate group to the transport protein, which changes the shape of the transport protein to better move a substance

Pumps

maintain membrane potential

Membrane potential

unequal concentrations of Ions across the membrane results in an electrical charge (electrochemical gradient)

• The cytoplasm is relatively negative in comparison to the extracellular fluid

• Energy is stored in electrochemical gradients

Electrogenic pumps

proteins that generate voltage across membranes, which can be used later as an energy source for cellular processes

Sodium potassium pump

• animal cells will regulate their relative concentrations of Na+ and K+

  • 3 Na+ get pumped out of the cell

  • 2 K+ get pumped into the cell

  • Results in a +1 net charge to the extracellular fluid

Diagram of sodium potassium pump

Proton pumps

integral membrane protein that builds up a proton gradient across the membrane

○ Used by plants, fungi, and bacteria

○ Pumps H+ out of the celL

Cotransport

the coupling of a favorable movement of one substance with an unfavorable movement of another substance

• Uses the energy stored in electrochemical gradients (generated by pumps) to move substances against their concentration gradient

Favorable movement

Downhill diffusion

Unfavorable movement

Uphill transport

When do plants use cotransport

Sugars and amino acids

• sucrose-H+ cotransporter: Sucrose can travel into a plant cell against its concentration gradient ONLY if it is coupled with H+ that is diffusing down its electrochemical gradient

Exocytosis

the secretion of molecules via vesicles that fuse to the plasma membrane

• Vesicles can fuse to the membrane by forming a

  bilayer

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

    • Example: nerve cells releasing neurotransmitters

Endocytosis

the uptake of molecules from vesicles fused from the plasma membrane (think: opposite of exocytosis)

• the cell membrane folds inwards—> this forms a vesicle—> vesicle pinches off and moves into cell

Phagocytosis

when a cell engulfs particles to be later digested by lysosomes

• Cell surrounds particle with pseudopodia

  • Packages particles into a food vacuole

    • Food vacuole fuses with a lysosome to be digested

Pinocytosis

nonspecific uptake of extracellular fluid containing dissolved molecules

Receptor mediated endocytosis

specific uptake of molecules via solute binding to receptors on the plasma membrane

• Allows the cell to take up large quantities of a specific substance

• When solutes bind to the receptors, they cluster in a coated vesicle to be taken into the cell

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 concentrations and maintain water balance

• Animal cells will react differently than cells with cell walls, like plants, fungi, and some protists

Isotonic solutions

• cells have no net movement of water

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

Hypertonic solution

• cells lose water to extracellular surroundings

• Concentration of nonpenetrating solutes is higher outside of the cell

• Water will move to the extracellular fluid

• Cells shrivel and die

Plasmolysis

vacuole shrinks and the plasma membrane pulls away from the cell wall

Hypotonic solution

• cells will gain water

• The concentration of nonpenetrating solutes is lower outside of the cell

• The cell will gain water

  • Animal cells swell and lyse

  • Plant cells work optimally