Cell Physiology Lecture 1 - Intro, Membranes, Cell junctions, Organelles

What is a cell

Structural and functional unit of a living organism

All cells contain these 3 components:

• A plasma membrane

• cell organelles - compartmentalize the cell and perform specific functions

• cytoplasm - suspend the organelles

4 Functions of the plasma membrane

1. Physical barrier between ICF and ECF fluids - maintains difference in ion composition and homeostasis

2. Cell-to-cell communication - contains receptors that bind signaling molecules

3. Structural support - contain connections composed of proteins which anchor cells to each other or the ECM

4. Transport - selectively permeable

Biological membrane

Cell/plasma membrane that surrounds organelles

Composition of a biological membrane

Phospholipid bilayer - 2 layers of phospholipids with embedded prteins

Structure of a phospholipid

Ampipathic molecule

• Polar head group: phosphate attached to glycerol and a glycerol backbone

• Non-polar tail: 2 fatty acid chains composed of carbon and hydrogen

Unsaturated fatty acid chains

Contain double bonds, making a kink in the chain

Which part of the phospholipid is hydrophilic

The head group

Which part of the phospholipid is hydrophobic

The fatty acid tails

Structure of phospholipids in water

Forms a bilayer: Polar heads phase environment, non-polar tails form hydrophobic core. This forms the plasma membrane

Cholesterol

Amphipathic steroid that maintains proper membrane fluidity

• Almost 1 molecule of cholesterol per molecule of phospholipid

Glycolipids

Amphipathic lipids attached to carbohydrates, found on the outer leaflet of the plasma membrane

• Form the glycocalyx - Layer of carbs linked to lipids/membrane proteins, allows cells to identify and interact with each other

  
  

Integral (intrinsic) membrane proteins

Amphipathic Proteins inserted into the phospholipid bilayer and span across the membrane

• Often used as transported or channels

• Outer proteins are hydrophilic, internal proteins (in membrane) are hydrophobic

Peripheral (extrinsic) membrane proteins

non-amphipathic proteins that attach to the outside of the membrane, and do not associate with the non-polar tails inside the bilayer

• Mostly extracellular and associate with cytoskeletal elements related to shape and mobility

Glycoproteins

Carbohydrates attached to the peripheral membrane proteins that help form the glycocalyx

Cell Junctions

Specialized connections that stabilize interactions and promote communication betwen cells

3 types of cell junctions

Desmosomes, tight junctions, gap junctions

Desmosome function

Adhering junctions that anchor cells together that are subject to considerable stretching or mechanical stress

• Maintain structural integrity of tissues, preventing cells from tearing apart from one another

• Seen in heart muscle, uterus, and skin

What plaques (proteins) are desmosomes made out of

Plagues: On cytoplasmic surface - anchor cadherins

Cadherins: Link cells together by binding to plagues. Seen in the cell cleft

Intermediate filaments: Structural support Anchor cytoplasmic surface of desmosome to components inside the cell

TIght junction function

• Form nearly impermeable junctions

• Link cells together

• Limit movement of molecules between cells - molecules must go through cells or through the plasma membrane

• Limit integral protein and lipid movement to preserve function

  
  

  
  

What are tight junctions made out of

Occludins

Gap Junction functions

Transmembrane, water-filled channels that electrically and metabolically couple cells. Also known as communicating junctions as molecules + ions can move from one cell to the next

• Fast communication, low resistance

Electrical coupling

Allow ions to move between cells - size limited by diameter of connexons

Metabolic coupling

Allow small molecules such as nutrients to move between cells - size limited by diameter of connexons

What are gap junctions made out of

Connexons

Nucleus Function

Storage and transmission of genetic information required for protein synthesis

Parts of the nucleus

• Chromatin - DNA and associarted proteins condense during cell division to form chromosomes

• Nuclear envelope - doubled layered porous membrane to seperate nucleus from cytoplasm

• Nuclear pores - pores in envelope that allow molecules to move in and out of the nucleus

• Nucleolus - site of ribosomal RNA synthesis

  
  

Structure of the nucleus

Nuclear envelope made out of 2 phospholipid bilayers with nuclear pores

• DNA and RNA building blocks enter the nucleus

• RNA and ribosomal subunits exist the nucleus

Most cells have one nucleus, except for:

RBCs - no nucleus

Muscle cells - multiple nuclei to satisfy high protein needs (ribosome synthesis)

Ribosome function

Protein synthesis - read and mRNA and assemble AAs to form proteins

Ribosome composition

Made of small and large subunits. Subunits must come together to form functional ribosome

Location of ribosomes

Synthesized in nucleus

• Found free in cytoplasm

• Bound in the rough ER

Endoplasmic reticulum

Fluid filled membranous system, with rough and smooth ER

• Rough ER - flattened sacs with attached ribosomes

• Smooth ER - branched tubular structure with no ribosomes

Function of the smooth ER

Synthesizes lipids, stores calcium (sarcoplasmic reticulum in muscles), detoxifies drugs by converting into water soluble form

Function of the rough ER

synthesizes proteins and performs post-translational modifications needed to produce functional proteins

• Modifications include glycosylation and protein cleavage

• High levels in cells needed to make secretory proteins (pancreatic cells)

Golgi apparatus structure

Composed of cisternae (flattened sacs)

Golgi apparatus function

Receives vesicles containing proteins synthesized in RER.

• Post-translational modification

• Sorts and packages proteins into vesicles

Fate of proteins put into vesicles by the golgi

• Secreted from the cell

• Become integral membrane proteins

• Becomes proteins of the lysosome, ER, or golgi

Lysosome function

Contains hydrolytic enzymes that break large molecules into small subunits. Can recylce organelles or destroy viruses/bacteria brought into the cell by fusing to it.

• Acidic environment activates enzymes (pH 5)

• Breaks down proteins, nucleic acids, lipids, etc.

  

How does the lysosome have a lower pH than the rest of the cell

Protons pumps in membrane move protons inside the lysosome to lower pH

Peroxisosome function

Contains oxidative enzymes, which use oxygen to remove hydrogen from molecules for breakdown. Creates hydrogen peroxide as a byproduct

• Breaks down fatty acids, alcohol, and drugs

  

Hydrogen peroxide is toxic. How do peroxisomes remove hydrogen peroxide?

Peroxisomes have a catalase enzyme that converts hydrogen peroxide to water and oxygen

Function of mitochondria

Synthesizes energy by converting nutrients into ATP via the Kreb’s cycle, oxidative phosphorylation, and other metabolic pathways

Structure of the mitochondria

Contains double phospholipid membrane

• Outer membrane - separates mt from cytosol

• Inner membrane - folded into tubules called cristate

• Matrix - contains enzymes responsible for cellular respiration

  

Which cells have more mt than normal

Cells that require lots of energy - skeletal muscle, heart muscle, liver, sperm

Important feature of Mitochondria

Has it’s own DNA. MtDNA comes from circular genomes of bacteria that were engulfed by eukaryotic cells

Cytoskeleton function

Maintain cell shape, maintain the position of organelles in the cell interior, and mediate cell + organelle motility

Is the cytoskeleton membrane bound?

No. No PL bilayer (same with ribosomes)

Are ribosomes membrane bound?

No. No PL bilayer (same with cytoskeleton)

Cytoskeleton composition

Composed of protein filaments or cytoskeletal filaments

Cytoskeletal filament types

• Microfilaments - made of actin

  • Determine cell shape and movement

• Intermediate filaments - part of desmosomes

  • Support cell, and provide mechanical strength

• Microtubules - made of tubulin

  • Suport for cell, movement of organelles