Chapter 3: Cells

Cells- smallest unit of life

• All organisms are made of one or more cells

  • Trillions of cells in the human body

• Structural and functional building blocks of organisms

  • Different cell types have different functions

• Cells only arise from other cells

Extracellular material

1. Extracellular Fluid (ECF)

   • Examples: blood plasma, CSF

     • Countless roles

     • Surrounds cells

     • Contains thousands of ingredients

2. Cellular secretions

   • Substances that aid in digestion (ex. Gastric fluids)

   • Substances that lubricate (ex. Saliva, mucus)

3. Extracellular Matrix

   • 'cell glue' that helps binds cells together

   • Most abundant extracellular material

Cells- Three main parts

1. Plasma Membrane- barrier with semi permeable membrane

2. Cytoplasm- intracellular fluid with organelles

3. Nucleus- organelle that controls cellular activities

• Plasma Membrane

  • AKA cell membrane

  • Surrounds cell

  • Separates the intracellular fluid from extracellular fluid

  • Selectively permeable (allows some substances to pass will not others)

• Membrane Lipids

  • Phospholipids

    • Form basic bilayer structure

    • Hydrophobic tails prevent water soluble substances from crossing

    • Hydrophilic heads face intracellular fluid and extracellular fluid

  • Cholesterol

    • Platelike hydrocarbon rungs stiffen the membrane

    • Decreases water solubility

• Membrane Proteins

  • Allow communication with its environment

  • Responsible for most specialized membrane functions

  • Some float freely along the surface and others embedded in bilayer

• Membrane Carbohydrates

  • Identify Molecules

    • Glycocalyx- 'sugar covering'- have different patterns depending on cell type

    • Example: immunity cells identify friend or foe

  • Attached to membranes proteins or membranes lipids

Cell Junctions

• Join plasma membranes of adjacent cells

• Three types of cell junctions

  • Functions

    • Prevent molecules from passing between cells, 'impermeable' (tight junctions)

      • Tight Junctions

        • Prevent molecules from passing through extracellular space between cells

        • Example: Restrict digestive enzymes of digestive tract from getting in bloodstream

    • Anchoring junctions to keep cells from tearing apart, 'rivets' (desmosomes)

      • Desmosomes

        • Distribute tension

        • Abundant tissues with great mechanical stress

        • Example: heart, skin

    • Passage of molecules from one cell to another, communications (gap junction)

      • Gap Junction

        • Cells connected by cylinders

        • Limit what passes

        • Useful for ions to synchronize electrical activity

        • Example: heart

Membrane Transport Types

• Passive

• Active

• Passive

  • No added energy required

  • Move from high to low concentration

    • Simple diffusion

    • Facilitated diffusion

    • Osmosis

      • Concept of Concentration Gradient

        • Concentration Gradient- different concentrations of molecules/ substances between different areas

        • Gradients can be chemical, electrical, or both (electrochemical)

    • Diffusion (Passive)

      • Movement of molecules or ions from area of high concentrations to low concentration

      • Driving force diffusion - intrinsic kinetic energy of molecules

      • Diffusion influenced by three factors

        • Concentration- greater difference in concentration means faster diffusion

        • Molecule Size- small particles diffuse faster

        • Temperature- higher temp increases speed of molecules to faster diffusion

      • Plasma/ cell membrane barrier to diffusion because selectively permeable

  • What determines if substances can cross the cell membrane?

    • Lipid solubility- more lipid soluble, more readily it will diffuse

    • Size- smaller the molecule, more readily it will diffuse

      • Note: larger or less lipid soluble molecules cross membrane with assistance from carrier molecules

        • Example: ion channel or transport protein

  • Simple Diffusion

    • Substances diffuse through membrane (lipid bilayer)

    • Along the concentration gradient from high concentration to low concentration

      • Example: Lipids and gases (oxygen, carbon dioxide)

  • Facilitated Diffusion

    • Two types of facilitated diffusion: Carrier-mediated or channel-mediated

      1. Carriers are limited by the number of available carrier proteins

      2. Channels are selective based on pore size and charges

         • Ex. Leakage channels- always open and allows ions to move according to concentration gradient

         • Ex. Gated Channel- controlled (open or closed) by chemical or electrical signals

    • Osmosis

      • Diffusion of water through selectively permeable membrane

        • Ex. Water diffuses either

          • Across the plasma (lipid bilayer)

          • Through the aquaporins (specific channel protein)

        • Occurs whenever different in water concentration on two sides of membrane

Importance of Diffusion (Passive Transport)

• Passive Transport by diffusion represents huge savings of cellular energy

• Oxygen, water, glucose, and other ions critical to homeostasis, so passive transport is key

  • How is diffusion possible?

    • Concentration Gradients

      • Move from high concentrations to low concentrations

• Active

  • Requires added energy (ATP)

  • Move from low or high concentration

    • Needed because of one or more of the following substance:

      • Too large to pass through channel

      • Incapable of dissolving in lipid bilayer

      • Unable to move against concentration gradient

        • Two Types: Active Transport and Vesicular Transport

    • Active Transport

      • Requires transport proteins and energy

      • Active transporters move mostly ions against the concentration gradient

        • So that requires energy to be expended

      • Each membrane pump only transports specific substances

        • So no transporter = no transport

  • Primary Active Transport

    • Hydrolysis of ATP results in pump energized by ATP release of phosphate group, giving pump energy to transfer sodium out and potassium in

    • Transport proteins are called pumps

      • Primary active transport is the process in which solutes (mainly ions) are:

        • Moved across cell membranes

        • Against electrochemical gradients

        • Using energy supplied directly by ATP

      • The action of the Na -K pump is an important example of primary active transport

        • Na/ K Pump

          1. ATP

          2. Three Sodium

          3. Two potassium

          4. Positive (both)

          5. Out of the cell

          6. More negative, taking out more positives

          7. Out of the cell, constantly being pumped out

          8. Yeah, electrochemical

  • Secondary Active Transport (AKA Cotransport)

    • Transport driven by energy stored in ion gradients created by active transport pumps

    • Example:

      • Low sodium concentration that is maintained inside cell by Na-K pump strengthens sodium's drive to want to enter cell

      • As sodium moves back into cell, other substances get dragged along (ex. Glucose and sodium move together)

    • Always moves more than one substance at a time

      • Substances move in opposite direction or same direction

  • Vesicular Transport

    • Transport of large particles, macromolecules and fluid across plasma membrane in membranous sacs called vesicles

    • Requires cellular energy (Usually ATP)

      • Vesicular Transport includes:

        • Endocytosis

          • Transports into cell

            • Main route into cell

            • Three types of endocytosis:

              • Phagocytosis

                • 'cell eating'

                  • Step 1: Cell engulfs external particle (ex. Bacteria, cell debris) and encloses it in phagosome (vesicle)

                  • Step 2: Phagosome fuses with lysosome for digestion of contents

                • Example: macrophages and certain white blood cells

                • Function:

                  • Disposal of dying cells crucial so dead cell remnants don’t trigger inflammation

              • Pinocytosis

                • 'cell drinking'

                  • Step 1: cell gulps extracellular fluid into vesicle

                  • Step 2: Vesicle fuses with endosome for sampling content

                • Example: Kidneys for reabsorption

                • Function:

                  • Routine activity for most cells to sample ECF

              • Receptor-mediated endocytosis

        • Exocytosis

          • Transports out of the cell

            • Vesicular transport to eject substances from cell into the extracellular fluid

              • Step 1: Substances enclosed in membranous vesicle

              • Step 2: Vesicle fuses with plasma membrane

              • Step 3: releases substance to ECF

                • Function:

                  • Secretion of neurotransmitters, hormones, mucus, wastes, etc.

Resting Membrane Potential (RMP)

• Potential energy produced by separation of oppositely charged particles across plasma membrane in excitable cells (nerves and muscles)

• RMP occurs only at membrane surface

  • Rest of cell and extracellular fluid are neutral

  • Muscle/ Nerve cells have an RMP around -70 mV to -90 mV

    • Quick Na-K Pump Review…

      • Na-K pump creates chemical gradient

        • More Na outside the cell

        • More K inside the cell

      • Na-K pump creates small electrical gradient

        • More negative inside the cell membrane because with each pump, three positives leave and only two come back in

  • RMP generated by potassium

    • Remember high concentration of K in the cell

    • Cell membrane very permeable to K through leakage channel

    • K leaks down/ along chemical concentration gradient

  • RMP maintained by action of the Na-K pump, which continuously ejects 3 Na out of the cell and only brings

2 K back inside

• Result: maintains electrochemical gradient

• RMP necessary so excitable cells have potential to be excited (e.g., muscle contracting or nerve sending impulse)

K+ is a Key Player in RMP

1. K diffuses out of cell through K leakage channels down its concentration gradient

2. Negatively charged large proteins cannot leave

   • Result: cytoplasmic side of cell membrane becomes more negative

3. K is the pulled back by the more negative interior because of its electrical gradient

4. When drive for K to leave is balanced by its drive to stay, RMP is established

5. Electrochemical gradient of K sets RMP

Inside the cell- Cytoplasm

Cytoplasm

• Cellular material between plasma membrane and nucleus

• Location of most cellular activities

• Three elements of Cytoplasm

  • Cytosol (fluid)

    • Fluid in which other elements are suspended

  • Organelles

    • Metabolic machinery of the cell

    • Each one carries out specific function for the cell

  • Inclusions (chemicals)

    • Chemical substances presence depend on cell type

      • Ex. Glycogen in liver and muscles

• Organelles: Mitochondria

  • Function

    • Providing most of the ATP supply

    • Uses aerobic cellular respiration (energy used to attach phosphate group to ADP to for ATP)

    • Bean shaped membranous organelle

      • Cellular respirations, occurs in the cell

• Organelles: Ribosomes

  • Function: site of protein synthesis

  • Two Locations

    1. Float in cytosol making proteins there

    2. Attached to the rough endoplasmic reticulum

    3. Can switch locations

• Organelles: Endoplasmic Reticulum (ER)

  • Continuous with outer nuclear membrane

  • Two types of Er:

    1. Smooth ER

       • Structure

         • Continuous with the rough ER, free of ribosomes

         • Membranous system of sacs and tubules

       • Function: smooth ER enzymes catalyze reactions to accomplish

         • Synthesis of lipids

         • Detoxification of drugs (ex. Liver)

         • Breakdown of stored glycogen to free glucose (ex. Liver)

         • Ca+ storage (ex. Skeletal and cardiac muscle cells, know as sarcoplasmic reticulum)

    2. Rough ER (surface studded with ribosomes)

       • Structure

         • Surface studded with ribosomes

         • Continuous with nuclear envelope

         • Extensive system of parallel sacs

       • Functions:

         • Its ribosomes synthesize proteins secreted from cells

         • Cell membrane factory ( phospholipids and proteins)

         • Protein is enclosed in vesicle and sent to Golgi apparatus for further processing

• Organelles: Golgi Apparatus

  • Structure: Stack of flattened membranous sac and associated vesicles

  • Function: modify, concentrate and package the proteins and lipids made at the rough ER and destined for export

• Organelles: Peroxisomes

  • Spherical membranous sacs containing variety of powerful enzymes

  • Function: Detoxing enzymes used on toxins and free radicals

  • Numerous located in kidney and liver cells which are active in detoxification

• Organelles: Lysosomes

  • Spherical membranous organelles containing acidic enzymes

  • Function: cells that safely perform

    • Cell digestion of viruses, bacteria, toxins

    • Autophagy (eating stressed or dead cells)

  • Depicted as Vesicles with enzymes inside

    • Lysosomal storage diseases result when one or more lysosomal digestive enzymes are mutated and do not function properly

    • Tay-Sachs disease is a condition in which the patient lacks a lysosomal enzyme needed to break down glycolipids in brain cells

      •  Glycolipids build up as a result of this defect, interfering with nervous system functioning

      • Seen predominantly in infants of Central European Jewish descent

      • Causes seizures, mental retardation, blindness, and death before age 5

Cytoskeleton

• Function- Cell Skeleton

  • Cell's 'bones, muscles, and ligaments'

• Three types of rods in Cytoskeleton

  1. Microfilaments

  • Structure: semiflexible strands of protein actin

    • Cells move when they get their act(in) together

  • Function: involved in cell movement or changes in cell shape

  1. Intermediate Filament

     • Structure

       • Tough insoluble fibers resembling ropes

       • Most stable and permanent of cytoskeleton

     • Function

       • Strongly resists tension

       • Attach to desmosomes resister pulling forces on cell

  2. Microtubules

  • Structure

    • Hollow tubes

    • Dynamic (grow and disassemble)

  • Functions

    • Determine the overall shape of cell

    • Determine distribution of cellular organelles (attach to microtubules like ornaments on tree)

Centrioles

• Structure

  • Pinwheel array of microtubules

  • Orient in cells in pairs at 90 degree angle to each other

• Function

  • Play role during mitosis

  • Bases for cilia and flagella

Cellular extensions:

Three types:

• Cilia

  • Structure

    • Whiplike cell extensions

    • Made with microtubules

  • Function

    • Moves substances in one direction across cell surface

      • EX. Ciliated cells that line respiratory tract propel mucus laden with dust particles and bacteria up and out of lungs

• Flagella

  • Structure

    • Like cilia, but longer

  • Function

    • Propels cell

      • EX. Sperm

• Microvilli

  • Structure

    • Tiny finger like extensions of the plasma membrane

  • Function

    • Significantly increase surface area

      • Most often found on the surface of absorptive cells

        • EX. Intestinal cells

        • EX. Kidney tubule cells

Nucleus

• Genetic library

• Contains instruction to build nearly all the body's proteins

  • Most cells just have one nucleus

    • Not all cells have one nucleus

      • Ex. Skeletal muscle cells

      • Mature RBC's - no nucleus when entering the blood stream (anucleate)

• Three regions of Nucleus

  • Nuclear envelope

    • Covers nucleus

    • Double membrane barrier separated by fluid filled space

    • Outer membrane continuous with rough ER

    • Surface contain selectively permeable pores

  • Nucleolus (little nucleus)

    • Spherical bodies where ribosomal subunits are assembled

  • Chromatin

Cell Cycle

Interphase and Mitosis

Interphase

• Period of a cell's life when it carries out the normal metabolic activities and grows

• Includes DNA replication in preparation for cell division

  • Cell division- phase during which cell divides into two cells

    • Phases:

      • Mitosis- division of __ from parent cell and two daughter cells

      • Cytokinesis- division of __ to yields two daughter cells

• Function: Essential for body growth and tissue repair

  • Frequency of Division

    • Constant, slow, not at all

Phases of Mitosis

1. Prophase

   • Chromatin coils and condenses to form chromosomes

   • Nucleolus disappears

   • Spindle poled established

   • Nuclear envelope disappears

   • Microtubules attach to chromosomes

2. Metaphase

   • Chromosomes are at midline of cell with their centromere at the 'equator'

3. Anaphase

   • Centromeres split simultaneously

   • Microtubules shorten as they pull chromosomes toward pole of cell

   • Moving chromosomes looked V shaped with the centromeres leading and chromosomal "arms" dangling behind them

4. Telophase

   • Reverse of prophase

   • Chromosomes at opposite ends begin to unravel to chromatin

   • New nuclear envelope forms around each chromatin mass

   • Nucleolus reappears

• Cytokinesis

  • Starts during late anaphase when contractile ring forms cleavage furrow, separates cytoplasm and pinches cells apart

Genes and Protein Synthesis

Gene- segment of DNA molecule that carries instructions for one polypeptide chain (AKA protein)

Genetic Code- instructions in a gene that tell a cell how to make a specific protein

Four Nucleotide bases (A, C, G, T)

• A and T always paired together

• C and G always paired together

DNA- master blueprint for protein synthesis

Messenger RNA (mRNA)- 'half DNA,' carries coded info to cytoplasm

Ribosomal RNA (rRNA)-  along with proteins, forms ribosomes

Transfer RNA (tRNA)- small L-shaped molecules that bring amino acids to ribosomes

Protein (polypeptide) Synthesis

Two major steps for protein synthesis

1. Transcription- transfer base sequence from DNA to mRNA

2. Translation- info carried by mRNA is decoded and used to assemble the protein

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