Cell Theory

1. All living things are composed of one or more cells

2. A cell is the basic unit of life

3. All cells come from preexisting cells

• Archaea and bacteria are prokaryotic & the domains of life

• Eukarya is the third domain including fungi, protists, plants, and animals
  

Cell Similarities

• Prokaryotes and Eukaryotic cells both include:

  • Plasma membrane

  • Cytosol

  • DNA/Chromosomes

  • Ribosomes

Cell Differences

• Eukaryotic DNA is found in the nucleus & prokaryotic DNA is found in the nucleoid 

• Eukaryotic cells have membrane-bound organelles with specialized forms but prokaryotic cells do not 

• Eukaryotes are larger than prokaryotes
  

Endomembrane System

• Includes synthesis and transport of proteins within and outside the cell; metabolizes lipid movement and detoxes poisons

  • Nuclear envelope (NE)

  • ER (rough and smooth) 

  • Golgi Apparatus

  • Lysosomes

  • Vesicles

  • Plasma membrane (sometimes)

• Nucleus contains the DNA and is enclosed by the nuclear envelope

  • Nuclear envelope is the double layer of membranes with pores allowing movement in.out of the nucleus

  • Nucleolus is inside the nucleus where rRNA (ribosomes) are synthesized using instructions from the DNA

• Ribosomes carry out protein synthesis which is made from rRNA and protein in nucleolus

  • Found floating in the cytosol which remains to make protein within the cytosol

  • Found on the rough ER which makes proteins for membranes or will be exported out of the cell

• Endoplasmic reticulum is continuous with the nuclear envelope, two types of ER; smooth ER and rough ER

  • Smooth ER is not covered in ribosomes; synthesizes lipids such as cholesterol and phospholipids, metabolizes carbs, and detoxes drugs and poisons

  • Rough ER is covered in ribosomes, produces secretory proteins that will be released from the cell, signals molecule insulin and membrane-bound proteins

• Vesicles are tiny sacs made of lipid bilayer which transport materials within or out of the cell

  • Transports vesicles, secretory vesicles and lysosomes, such as moving proteins from the ER to golgi

• Golgi apparatus are proteins coming from the ER and are modified and stored here and shipped by the vesicles to the final destination

  • Sent from another part of the cell or the cell membrane and have a receiving end known as cis and a shipping end known as trans

• Lysosomes hold digestive enzymes in animal cells and are made by the rough ER and processed by the golgi apparatus

  • Plays a role in apoptosis, digesting food from food vacuoles, and digests old and damages organelles

• Vacuoles store vesicles with various functions

  • In animal and protists, food vacuoles fuse with lysosomes and food is digested

  • In protists, contractile vacuoles regulate water in the cell

  • In plants, central vacuole can holds cell sap, water, toxins, and pigments

• Cell membrane is semi-permeable allowing passage of oxygen, nutrients, and waste in/out of the cell, the membrane is a double layer of phospholipids

  • The interior nonpolar of the lipid bilayer allow small, nonpolar molecules to pass through 

• Mitochondria is the site of cellular respiration generates ATP from fats and sugars, membran-bound, not part of the endomembrane system, found in nearly all eukaryotic cells, has its own DNA (separate from nuclear DNA), and has thousands per cell

• Chloroplast is a plastid that is photosynthetic and holds the pigment chlorophyll found in plants and algae

  • Built similarly to mitochondria, inside are flattened sacs of thylakoids stacked into grana, the stroma (fluid outside the thylakoids) contain chloroplasts DNA, enzymes, and ribosomes

• Cell wall is made of cellulose; protecting cells, maintains shapes and prevents excessive water uptake, which can be found in plants, fungi, prokaryotes and some protists

• Peroxisomes contains enzymes that transfer hydrogen to oxygen, producing hydrogen peroxide but contains enzymes that can break hydrogen peroxide down into water

  • Similar to lysosomes but are not part of the endomembrane system, some aid mitochondria breaking down fatty acids into usable sizes, but works in detoxification

• Cytoskeleton has variety of roles includes supporting the shape of the cell, movement and regulations of biochemical activities, includes microtubules, intermediate filaments, and actin filaments

  • Microtubules shape and support the cell, acting as molecular highways for the transport of vesicles, and help in organelle movement, including centrosomes and centrioles which aid in cell division, cilia and flagella which aid movement

  • Intermediate filaments which anchor organelles like nucleus in place and reinforce cell shape

  • Actin(micro) filaments which are thin fibers of actin and myosin help in muscle contractions, are involved in cell motility, cell division (cleavage furrow)

• Extracellular Matrix made of glycoproteins and form strong fibers on the exterior of the animal cell, help in cell communication and framework for anchoring to nearby cells, the surface receptors embedded in plasma membrane called integrins used to communicate between the ECM and the cytoskeleton 

Surface Area to Volume Ratio

• The surface area to volume ratio of a cell is critical, as the surface area increases by a factor of n², the volume increases by a factor of n²

  • As a cell gets larger, the volume increases faster than the surface area resulting in a lower SA:V ratio, which harms diffusion

  • Cells must exchange substances with their environment at a rate that keeps up with its metabolism, high SA:V ratios allow surface area to be more efficient, such as exchanging materials with the environment and waste removal

    • If too large, cannot exchange material efficiently

    • If too small, not enough materials can fit inside

  • Inside an organism is made of a giant cell, it can increase its surface area by dividing into many smaller cells

  • Some cells possess highly convoluted (folded) membranes to increase surface area while minimally increasing volume, organelles like ER, Golgi apparatus, and mitochondria do this too by having many folds, their function depends on chemical reactions on cell membrane surfaces

• Membranes and organelles compartmentalize cellular functions allowing greater efficiency

  • Prokarttes isolate functions into cellular areas, while eukaryotes have organelles that compartmentalize different functions

• Mitochondria and chloroplasts contain shared features with prokaryotes, providing evidence of common ancestry

  • Both chloroplasts and mitochondria contain circular DNA, possess ribosomes, have double membranes, and are self-replicating

  • Endosymbiont theory states ancestral eukaryotic cells engulfed an ancestral mitochondrion establishing a mutualistic relationship

• The plasma membrane is semi-permeable, the fluid mosaic model describes the plasma membrane as composed of a phospholipid bilayer with an embedded “mosaic” of membrane proteins

  • Includes a phospholipid bilayer made of phospholipids' hydrophilic heads and hydrophobic tails, membrane proteins either integral or peripheral proteins, cholesterol, carbohydrates either glycoproteins and glycolipids

  • Lipids and proteins can shift within the membrane and temperature affects fluidity, cholesterol regulates the membrane, if it is too hot, the membrane stretches and if it is too cold the membrane stiffens, cholesterol helps regulate molecules that enter and exit the cell

  • Integral proteins are transmembrane proteins which go through the whole membrane whil some only extend midway, often act as pumps/channels involved in cell transport, some receptor proteins involved in cell signaling, have a hydrophobic region and many also have hydrophilic regions

  • Peripheral proteins are not embedded in the bilayer, but it is attached to the surface of the membrane or attached to exposed areas of integral proteins, can serve as cell identifiers such as antigens
    

Simple Diffusion

• Diffusion is random movement of molecules of any substance that spread out evenly in an available space

  • Concentration gradient results from unequal distribution of ions across the cell membrane

    • A substance will diffuse from high to low concentration until it reaches equilibrium

  • Molecule diffuse down their concentration gradient and does not use any energy

Passive Transport 

• Passive transport occurs when molecules move down the concentration gradient across a membrane and does not use energy

  • Simple diffusion allows small, nonpolar molecules move directly across lipid bilayer from high to low concentration,, since small and/or hydrophobic, they do not need assistance passing through the hydrophobic fatty acid interior of the lipid bilayer

    • Water can pass through because it is small, but it will move very slowly because it is polar

• Facilitated diffusion refers to molecules moving from high to low concentration across a membrane through transport proteins

  • Ions such as Na+, Cl-, calcium, etc, and polar/hydrophilic molecules are blocked by the hydrophobic core of the membrane so they need “assistance” of proteins to cross

  • Most water is transported this way known as osmosis with a specialized channel protein called an aquaporin

Transport Proteins Used in Facilitated Diffusion

• Channel proteins transport proteins with a hydrophilic channel that allow certain polar molecules and ions to pass

• Carries proteins change shape to specifically hold whatever is crossing, move larger molecules like glucose

  • Transport proteins are specific to each substance and will not allow other substances through that route
    

Active transport

• Active transport is the pumping of solute across a membrane against the concentration gradient, thus using energy (ATP)

• Enables cells to maintain solute concentrations that are different from their environment

• Primary active transport moves positively charged ions against gradient using energy from ATP directly

  • Example: sodium-potassium pump 

  
  

Secondary Active Transport

• Secondary active transport uses concentration gradient of an ion as its energy source

  • Typically moves one ion with its gradient to move another substance against its gradient and doesn’t use ATP directly

Coupled/Co Transport

• The transport of two substances across a biological membrane at the same time

Types of Transport

• Uniport is one substance moves at a time

• Symport is two substances moving in the same direction across a membrane

• Antiport is two substances moving in opposite directions across a membrane

Active transport (Bulk Transport) 

• Exocytosis is cell secretion is released through vesicles

• Endocytosis is when cells take in items from the outside of the cell by creating a vesicle

  • Phagocytosis is cellular eating

  • Pinocytosis is celular drinking the extracellular fluid engulfed also contains variety of solutes

  • Receptor-mediated endocytosis is receiving specific substances such as hormones, and proteins, through binding proteins in the plasma membrane forming vesicles

Passive Transport: Osmosis

• Osmosis is the diffusion of water through a selectively permeable membrane, which is a type of passive transport and do not require energy

  • Water diffuses across the membrane from the region of lower solute concentration to the higher solute concentration with less water

  • Water always moves to the side with more solute, osmosis occurs via facilitated diffusion using channel proteins called aquaporins

    • Osmosis can occur slowly through the lipid bilayer (simple diffusion)

  
  

Tonicity

• Tonicity is the ability of a solution to cause a cell to gain or lose water related to the concentration of solutes that cannot cross the membrane

  • Isotonic is no NET movement of water (equal solute concentration)

  • Hypertonic is a region with greater solute concentration, in which a cell in hypertonic solution loses water to the environment (water moves out of the cell and shrinks)

  • Hypotonic is a region with lower solute concentration which a cell in hypotonic solution gains water and may swell

Osmosis in Plant Cells

• Cell walls protect against excess water uptake and withstand turgor pressure

• Plant cells can be turgid (full of water), flaccid (isotonic), or plasmolyzed (lost water)

• Plasmolysis is a plant cell losing water causing the plasma membrane to pull away from the cell water and the plant to wilt

Osmoregulation & Water Potential

• Osmoregulation is the ability of organisms to maintain the balance of water with their environment and control of their solute concentration

• Water potential is the tendency or “potential” of water to move from one area to another, particularly in or out of cells

  • Water will always move from an area of high water potential to an area of low water potential