Lecture 3 - Cells

Functions of the Plasma Membrane

• Mechanical barrier that separates the inside (intra-) and outside (extra-) cell fluids

• Selective permeability that controls what goes in and out

• Electrochemical gradient that maintains the inside of the cell being more negative than the outside

  • muscle and nerve cells can change their gradient

• Communication that allows for cell-to-cell recognition

• Cell signaling that interacts with extracellular chemical messengers and relays messages to the inside of the cell

Plasma Membrane Components

• Phospholipids: two layers with hydrophobic tails on the inside and hydrophilic heads on the outside

• Glycolipids: only on the outer membrane surface and contain a sugar group

• Cholesterol: adds stability

What are integral proteins?

• most span the entire membrane (transmembrane)

• have both hydrophobic (interact with lipid tails) and hydrophilic regions (interact w water)

• function as transport proteins, enzymes, or receptors

What are peripheral proteins?

• loosely attached to integral proteins

• function as enzymes and cell-to-cell connectors

Tight Junctions

• think of as a seal so things can’t pass through

• integral proteins on cells touching together that fuse to form an impenetrable junction that prevents fluids and most molecules from moving between cells

Desmosomes

• think of as velcro between cells that withstands tension so cells don’t rip

• formed when linker proteins (peripheral proteins) of neighboring cells interlock like the teeth of a zipper

• are anchored to its cell through thickened “button-like” areas on the inside of the membrane called plaques

Gap Junctions

• think of as a small tunnel between cells for small things to pass

• transmembrane proteins that form tunnels that allow small molecules to pass through from cell to cell

• spreads ions, simple sugars, and electrical signals to be passed quickly from one cell to the next

Passive Transport

• does NOT require energy

• ALL forms of diffusion are passive

  • simple, facilitated, osmosis

• filtration is a form —> it usually occurs through capillary walls

Diffusion

movement of molecules from an area of high concentration to low concentration (down their concentration gradient)

• smaller molecules = faster diffusion

• bigger molecules = slower diffusion

• higher temps = faster diffusion

• lower temps = slower diffusion

Simple diffusion

non-polar and lipid-soluble/hydrophobic substances diffuse through the lipid bilayer cell membrane

Carrier-mediated facilitated diffusion

substances bind to protein carriers

• think of a revolving door opening on one side to let the molecule in then turning and reopening on the other side to let it out

Channel-mediated facilitated diffusion

substances move through water-filled channels

• leakage channels are always open

• gated channels are controlled by chemical or electrical signals

Osmosis

• is a form of diffusion = does not require energy

• movement of water across the plasma membrane in 2 ways

  • via diffusion through the lipid bilayer

  • through small water channels called aquaporins

• water moves from low concentration to high concentration based on substance concentration

Water is _____

hypotonic

What happens to cells in an isotonic solution?

they retain their same size and shape because the solute and water concentrations are the same inside and outside

What happens to cells in a hypertonic solution?

the outside solution has a higher concentration of solutes so the cell shrivels up as fluid leaves the cell to try to balance the concentrations

What happens to cells in a hypotonic solution?

the inside of the cell has a higher concentration of solutes so the cell expands as fluid enters it to try to balance the concentrations

• may cause the cell to burst if it gets too full

Tonicity

the ability of a solution to change the shape/tone of cells by altering the cell’s internal water volume

Active Membrane Transport

• requires energy to move solutes across a plasma membrane when solute is too large/non-polar, not lipid soluble, or is moving against its concentration gradient

• uses carrier proteins but differs from carrier mediated facilitated diffusion

  • ATP hydrolysis changes the shape of the carrier protein to allow the solute to be pumped across

• Ex: Sodium-Potassium Pump

  • the enzyme Na⁺-K⁺ ATPase pumps Na⁺ out of the cell and K⁺ into the cell AGAINST concentration gradients

  • maintains electrochemical gradients = essential for muscle and nerve tissues

Vesicular Transport

• think as a carrier bubble that moves substances

• takes energy!!!

• 4 Processes

  • Endocytosis: transport into the cell

    • 1. Phagocytosis: for large things like bacteria

    • 2. Pinocytosis: small things like extracellular fluid

    • 3. Receptor-mediated endocytosis: only when something binds to a receptor on the plasma membrane to initiate endocytosis

  • Exocytosis: transport out of the cell

  • Transcytosis: transport into, across, then out of the cell

  • Vesicular Trafficking: transport from one organelle in a cell to another

Non-Membrane Organelles

ribosomes, cytoskeleton, centrioles

Mitochondria

• provides ATP for the cell to use as energy via cell respiration (requires oxygen)

• has a double membrane (inner membrane has folds called cristae)

• contain their own DNA, RNA, ribosomes

• resemble bacteria —> can do same cell division called fission

Ribosomes

• performs protein synthesis

• non membranous

• 2 globular subunits

• made of protein and ribosomal RNA (rRNA)

• 2 Forms

  • free ribosomes: free floating that synthesize proteins that function is cytosol or other organelles

  • membrane-bound ribosomes: attached to the rough ER and synthesize proteins that are incorporated into membranes or lysosomes

Endoplasmic Reticulum

• made of flattened membranous tubes that enclose fluid-filled interiors called cisterns

• continuous with the outer nuclear membrane

• rough and smooth parts

Rough ER

• rough surface bc has ribosomes attached

• synthesizes proteins that attach to plasma membrane

• synthesizes phospholipids

  • final protein is then enclosed in a vesicle and sent to golgi apparatus

Smooth ER

• continuous with rough ER

• contains enzymes within its membrane that function in:

  • lipid metabolism: synthesizing, breaking down, and transporting fats in cells to store energy, build cell membranes, and regulate body functions

  • cholesterol synthesis & making lipids for liver cells (lipoproteins)

  • synthesis of testosterone hormones in testes

  • absorption, synthesis, and transport of fats (in intestinal cells)

  • detoxification of chemicals and drugs (in liver and kidney cells)

  • breakdown of glycogen to glucose (in liver cells)

  • storage and release of calcium (in skeletal & cardiac muscle cells)

Golgi Apparatus

• stacked and flattened membranous sacs

• modifies, concentrates, and packages proteins and lipids from the rough ER

• 1. Transport vesicles from ER fuse with the inner side of Golgi

• Proteins or lipids taken inside are further modified, tagged, sorted, and packaged

• Golgi is “traffic director” controlling which of three pathways final products will take as new transport vesicles pinch off the outside side of Golgi

Lysosomes

• main function is to digest things like bacteria, viruses, and toxins, break down old organelles, break down and release glycogen, break down and release calcium from bones

• spherical membranous bags containing digestive enzyme

What is autolysis

when a cell digests itself

• often happens when a cell is injured and the lysosome ruptures

Peroxisomes

• spherical membranous sacs containing powerful enzymes

• main function is to neutralize toxic substances and neutralize free radicals (which are toxic, highly reactive molecules that are by products of cell metabolism)

  • Oxidase: uses oxygen to convert toxins into H₂O₂

  • Catalase: converts H₂O₂ to water

• a high amount found in liver and kidneys!!

Cytoplasm

• elaborate network of rods that run through cytosol

  • microfilaments

  • intermediate filaments

  • microtubules

Cellular extensions of cytoplasm

• Flagella: aid in the movement of the cell; sperm are the only cells w flagella

• Cilia: move materials across the surface of the cell - in respiratory tract

  • cells in respiratory tract push mucus up the tract to the nose to dispose of it

  • are bigger than microvilli in length & width

• Microvilli: fingerlike projections that increase surface area for better absorption - in digestive tract (small intestine) and kidneys

Nucleus

• largest organelle

• nuclei of all cells have the instructions for all cell types in them but they only use the one needed for their specific type of cell

• “control center” - responds to signals dictating the kind and amount of proteins that need to be synthesized

• most cells are uninucleate (1 nucleus)

• skeletal muscle, some bone cells, and some liver cells have multiple nuclei

• red blood cells have no nucleus

• 3 Structures

  • 1. nuclear envelope: double-membrane barrier that encloses the nucleoplasm

  • 2. nucleolus: contains the DNA codes for rRNA and synthesizes it

  • 3. chromatin: tightly packed DNA

Role of DNA Protein Synthesis

• DNA has the code for protein synthesis and dictates the AA sequences

• Gene: segment of DNA with the code for 1 polypeptide

What is a triplet code?

3 sequential bases that code for a particular amino acid

• ex: GCC, GCA, ACA, etc

What is RNA

• acts as the messenger for protein synthesis because DNA cannot leave the nucleus

• is a copy of the DNA code that is carried to ribosomes

  • RNA formed in the nucleus

• single stranded, has Uracil and not Thymine, has ribose sugar

What is mRNA?

Messenger RNA

• is an exact copy of the DNA we want to make (copied from template strand)

What is rRNA?

Ribosomal RNA

• forms ribosomes (location of protein synthesis)

What is tRNA

Transfer RNA

• carries the produced amino acids to ribosomes

Transcription - 1st Step of Protein Synthesis

• DNA info coded into mRNA

  • RNA polymerase unzips DNA and adds complementary nucleotides to the sequence of the template strand onto the new RNA strand so that it becomes the RNA version of the coding strand

• occurs in nucleus

Translation - 2nd Step of Protein Synthesis

• the language of mRNA is translated to the language of proteins

  • pairs pf base sequence translated into amino acid sequence

  • each triplet code in DNA = called codons in mRNA (are both 3 base sequences)

  • multiple codons code for the same AA to protect against transcription errors

• essentially mRNA codons are paired with tTNA anticodons to form amino acid sequences to synthesize proteins

  • anticodon sequence of tRNA is identical to DNA (except has U instead of T)

tRNA in Translation

Structure

• has an anticodon (3 bases) on one end and the opposite ends to the corresponding amino acid

• the anticodon end pairs with the mRNA codon that is complementary

  • ex: codon AUA bonds with anticodon UAU

Function

• tRNA binds codon on A site of ribosome

• ribosomal enzymes attach polypeptide chain from tRNA in P site to the A site where the amino acid of tRNA is attached

• ribosome displaces tRNA

  • A site moves to P site

  • P site moves to E site

  • E site is ejected from ribosome

How to differentiate between triplet code, codon, and anti-codon charts

• triplet code chart has T bases in it

• if the chart has U bases in it you know it is either codons or anti-codons but cannot differentiate unless told it is mRNA or tRNA

  • mRNA = codons

  • tRNA = anti-codons