Week_2_CTO_lecture_

Introduction

• Class Focus: Understanding cell structure, starting with prokaryotic cells.

• Upcoming Weeks: Comparison of prokaryotes and eukaryotes.

Minimal Requirements for a Cell

• Plasma Membrane:

  • Maintains selective (controls what comes in and out) barrier between the inside and outside of the cell.

  • Controls entry and exit of substances; protects cytoplasm.

• Cytoplasm:

  • Water-soluble area for chemical reactions and processes that make up cell happens.

• DNA:

  • Hereditary material needed for cell function; encodes instructions for the cell.

• Ribosomes:

  • Critical for translating DNA instructions into proteins; essential for cell machinery.

Constraints on Cell Size

• Lower Limit of Cell Size:

  • Must have sufficient space for genetic material and machinery.

• Upper Limit of Cell Size:

  • Surface Area to Volume Ratio:

    • Nutrient diffusion is constrained by cell size/ total area of the cell.

    • As cells increase in size, the surface to volume ratio decreases, limiting nutrient intake.

• Surface Area to Volume Ratio Calculation:

  • Example cube (1x1x1): Surface area = 6, Volume = 1, Ratio = 6.

  • Larger cube (5x5x5): Surface area = 150, Volume = 125, Ratio = 1.2.

  • Smaller cubes can maintain a better ratio, leading to more efficient nutrient uptake.

Cell Membrane Characteristics

• Fluid Mosaic Model:

  • Composed of lipids and proteins.

  • Divides space into hydrophilic (water-soluble) and hydrophobic (not water-soluble) - makes it very difficult for many particals to get through this plasma membrane

  • Exceptions: Extremely small molecules and small uncharged molecules eg) O2 and CO2

• Membrane Properties:

  • Small uncharged molecules (O2, CO2) pass freely; water passes slowly.

• Membrane Proteins:

  • Integral Membrane Proteins:

    • Permanently embedded in the membraine; as have hydrophobic regions ensure positioning. Some are fixed, but others can move freely throughout the plasma membrane , allowing for dynamic interactions and facilitating processes such as signaling and transport.

  • Peripheral Proteins:

    • Not permanently attached; as dont have hydrophobic region -bound with integral proteins or membrane components.

  • Phospholipids (the fluid part)

  • Fluid Phospholipid bilayer

  • Diverse phospholipids comprise the bilayer

  • Glycolipid - a type of lipid that contains a carbohydrate group, contributing to cell recognition and signal transduction through the plasma membrane.

  • Sterol - a type of lipid that helps to stabilize/ regulate membrane fluidity and integrity, playing a crucial role in maintaining the structure of the phospholipid bilayer. eg) collestrole

    • High temp: reduce fluidity of membrane

    • Low temp: reduce rigidity of membrane

Transport Mechanisms

• Diffusion:

  • Molecules moving from high concentration to low, achieving equilibrium when molecules are distributed equally throughout a container of space.

  • Movement can occur simultaneously across a selectively permeable membrane.

• Facilitated Diffusion: (selective membrane)

  • Permeable to water, impermeable to sugar

  • Eg) Water will go to the side where it is less concentrated (right). The sugar is not able to move to the left due to the selective membrane and so water rushes to the right. This may also happen in a cell (bad) causing it to burst due to all the water.

• Transport Proteins in facilitated diffusion

  • Solute will transport down electrochemical gradients with assistance from transporter proteins: (channels, open hydrophilic space where it can be transported across the membrane and carriers very specific part of protine that recognises specific solute, binds to it, then facilitates its transport across the membrane).

  • Above is passive transport mechanism as goes down electrochemical gradient.

  • Also simple diffusion, however cell has much less control over this solute

• Active Transport:

  • Moves solutes against their electrochemical gradient, requiring energy (usually ATP).

  • Example: Proton pump, uses ATP to push proton ion to outside membrane. This change of charge creates an electrochemical gradient, which can be used to transport other molecules through the membrane.

• Cotransport:

  • Symport (two molecules transported in same direction) and antiport (opposite direction) for ion transport across membrane (e.g., sodium-potassium pump).

Eukaryotic vs. Prokaryotic Cells

• Size Differences:

  • Eukaryotic cells can be 10-100 times larger than prokaryotes.

• Genetic Material:

  • Prokaryotes: One copy of circular genome (all genes of organism put together), no nucleus, haploid, functions carried out in the whole general area .

  • Eukaryotes: Multiple linear chromosomes housed within a defined nucleus, typically diploid, with distinct organelles performing specialized functions.

• Compartmentalization:

  • Eukaryotic cells have specialized organelles; prokaryotic functions occur in cytoplasm.

Endosymbiosis Theory

• Evolution of mitochondria and chloroplasts from symbiotic relationships between different prokaryotic cells.

• This theory suggests that these organelles were once independent bacteria that entered into a mutualistic relationship with ancestral eukaryotic cells, leading to the complex cellular structures we observe today.

• Evidence includes:

  • Presence of circular DNA in mitochondria and chloroplasts.

  • Double membranes indicative of engulfment.

  • Eukaryotic plasma membranes have cholesterol, while prokaryotic plasma membranes do not.

Mitochondria

• Different cells have diff no. of mitochondria

• Produces (ATP); thats why called power house of cell; number varies with cell energy needs.

• Convoluted inner membrane increases surface area so more area to carry out reactions that produce energy

• Much of the mitochondria is constructed from genes that are in the nucleus .

Chloroplasts

• Main purpose is for photosynthesis; contains chlorophyll - gives plants green colour and produces sugar, which can be converted into energy sorces such as ATP.

• Also have circular DNA and a membrane structure similar to mitochondria such as outer and inner membrane, own ribosomes, has own circular DNA and third complex membrane system called thylakoids, which are stacked into structures known as grana and are essential for the light-dependent reactions of photosynthesis. .

Eukaryotic Cell Components

• Nucleus:

  • Contains genetic material, execot for mitocondria and chloroplast, which have own DNA surrounded by a nuclear envelope with pores for transport.

  • Nucleolus: region within the nucleus responsible for ribosome production, where rRNA is synthesized and combined with proteins to form ribosomal subunits.

  • Nuclear pore complex:

    • Membrane blocks access to the nucleus except through nuclea pores

    • control transport of anything in or out of nucleus

• Ribosomes:

  • Synthesizes proteins based on mRNA from the nucleus.

  • DNA —> mRNA —> protein

• Endoplasmic Reticulum (ER):

• Wrapped around the nucleus as very closely associated with production of proteins

• Allows for production of much more complicated products (like adding sugar to protein) as they have a specific place where they can be added.

• Distinct from prokaryotes which have a single space where all functions have to take place

  • Rough ER: Has ribosomes; protein synthesis.

  • Smooth ER: Lipid synthesis and metabolism.

• Endomembrane System:

• Consists of various membranes within the eukaryotic cell that work together to modify, package, and transport lipids and proteins. This system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.

• Pieces of the membrane can break off and form seperate compartment which can go to the plasma membrane, fusing to make contents go to outside of cell or can travel to specific location to deposit protein or go to golgi apparatus

• Golgi Apparatus:

  • Modifies proteins and packages them into vesicles for transport.

  • Cis-golgi: (same side)

  • Trans-Golgi: (far side)

• Lysosomes:

  • Waste recycling center; break down food and damaged organelles

  • Phagocytosis: the process by which cells engulf large particles or even other cells, allowing for the ingestion of pathogens and debris.

  • Autophagy: Cell uses lysosomes to break down damaged organelles or proteins into smaller parts that they can use again

• Vacuoles:

  • Storage compartments for nutrients or water.

Cytoskeleton

• Unique to eukaryotes; provides structural support and motility.

• Microtubules: Arranged from tubulin dimers (two proteins, that always go together eg a and b tubulin) ; instead of growth/shrinkage, can provide more rigid structure that can be used for motility

  • Provide fixed structures for motility (e.g., flagella, cilia).

    • Flagella - one or only a few tails at the end of a cell, propell cell forward

    • Cilia - very small and cover the cell, work by beating back and forth

  • Classic 9+2 arrangement in eukaryotic flagella.

  • Blue: connect fixed parts of flagella

  • Red: dinan - protein. One side always fixed to microtubule and other side can move. Moving side will slide up and down and other side will carry put motion at other side, causing the motion