BIOL 1020- Lectures 8, 9 , 10: Cell Structure

Describe the limitations to cell size based on the surface area to volume ratio

The surface area-to-volume ratio imposes a fundamental limit on cell size because as cells grow, their volume outpaces their surface area, making nutrient and waste exchange insufficient to support the cell’s metabolic needs

Compare and contrast the structure of bacterial and eukaryotic cells

Bacterial cells:

• no nucleus

• no internal membrane-bound organelles

• single-celled

Eukaryotic cells:

• possess a nucleus

• many membrane-bound organelles

• single-celled

Where is DNA kept in prokaryotes?

Nucleoid (not bound by a membrane)

What protects prokaryotes?

• Cell wall outside their plasma membrane 

  • made of peptidoglycan

• Glycocalyx: a waxy protective coating made up of glycoproteins and glycolipids

Where is DNA kept in Eukaryotes?

• DNA is centralized in the nucleus (membrane bounded by the nuclear envelope)

What protects and supports Eukaryotes?

• Some have cell walls made up of cellulose (plants) and chitin (fungi)

• They have cytoskeleton for structure and support

Cytosol vs Cytoplasm

Cytosol: the part of the cytoplasm that is not held by any organellee

Cytoplasm: the material (protoplasm) within a living cell, excluding the nucleus (ie. the spaces and compartments between the plasma membrane and nuclear envelope)

What is in the cytoplasm of eukaryotes?

• Ribosomes for protein synthesis via translation

• Membrane bound organelles

Explain the endomembrane theory and the evolution of eukaryotes

• The origin of eukaryotes came from a heterotrophic prokaryotic cell

• The plasma membrane of a prokaryotic linage began invaginating (folding inwards) intoo the cytoplasm about 2 billion years ago

• The invaginated membranes eventually separated from the plasma membrane

  • it surrounded the DNA to form a nucleus and became the endomembrane system of the eukaryotic cell

Describe the structure and function of cellular organelles of the endomembrane system

The endomembrane system drives cell processes

• nuclear envelope

• endoplasmic reticulum

• golgi apparatus

• lysosomes

• vesicles and vacuoles

• plasma membrane

What are some of the functions of the Endomembrane System?

• protein synthesis (for both proteins staying in the cell or being exported)

• protein transport (into membrane bound organelles or out of the cell

• metabolism

• movement of lipids

• detoxifying the cell

Endoplasmic Reticulum (Factory)

• Composed of a rough and smooth area

Rough ER

• studded with ribosomes

• continuous with the nuclear envelope

• Produces glycoproteins (proteins with covalently bonded carbs)

• separates and transports proteins via transport vesicles

• produces phospholipids and other proteins

Smooth ER

• Lacks ribosomes

• Produces lipids (oils, steroids, phospholipids [main site])

• detoxifies the cell by adding -OH to drugs which → to liver cells

• Stores calcium ions to trigger muscle cell contractiono

Golgi Apparatus (Sorting)

• series of flattened sacs - sisternae

  • cis side faces RER (younger) → recieving end

  • trans side points outwards towards the rest of the cell (older) → shipping end

• Vesicles bring material from the RER to the cis face, fusing with the golgi membrane

• Materials are modified as they pass through (postal codes)

• Vesicles pinch off of the trans face and head to their final destination/outside the cell
  

Lysosome (digest and recycle)

• membranous sac of hydrolytic enzymes

  • can break down macromolecules into monomers

  • acidic interior

• when lysed, contents aren’t digested because the pH is too high for enzymee

• partakes in intracellular digestion

Define the two types of intracellular digestion

Phagocytosis:

• occurs between amoebas and other unicellular eukaryotes

• food vacuole merges with a lysosome

• digestive enzymes break down food molecules into monomers

• Macrophages (WBC) use this process to kill bacteria and other invaders

Autophagy:

• damaged organelles get surrounded by a double membrane

• merges with a lysosome which digests the materials with hydrolytic enzymes

• monomers and molecules are recycled by the cell

Vacuole

• Large vesicles made from RER and golgi

• Food and digestive vacuoles

• Contractile vacuoles

  • pump out excess water from freshwater single celled organisms

• Plant vacuoles

  • storage for small molecules

  • large central vacuole: contains inorganic ions and swells up due to osmosis

    • pushes on cell wall and provides turgor pressure

How do proteins know where to go?

Proteins have “built in postal codes” within their amino acid sequence, which get recognized by otheer proteins and direct the new proteins to their destination

Hydrophobic a.a in a specific order → endomembrane signalling sequences

Charged a.a in a specific order → mitochondrial signal sequences

No signalling sequence a.a. are destined for cytosol

Mitochondria

• in most eukaryotic cells

• site of cellular respiration

• contains two membranes

  • intermembrane space in between

• Inner membrane folded to make cristae which encloses the mitochondrial matrix

• contains DNA and ribosomes

Chloroplast

• type of plastid (closely related plant organelles such as amyloplasts and chromoplasts)

• only in plants in algae

• absorbs energy from photons

• site of photosynthesis

• contains two membranes

  • Intermembrane space in betweeen

  • inner membrane folded to make thylakoids which are stacked to form grana

• contains DNA and ribosomes in the stroma

  • space between thee thylakoid space and inner membrane

• contains chlorophyll (green)

Define heterotrophs, autotrophs, anaerobes, aerobes

Heterotrophs: organisms that obtain their energy through consuming other material

Autotrophs: organisms that obtain their energy themselves

Anaerobes: organisms that can’t survive in O2, and can’t use O2 to extract energy from molecules via aerobic respiratioon

Aerobes: organisms that can survive in O2 and can use O2 to extract energy from molecules via aerobic respiration

Explain the endosymbiont theory and the evolution mitochondria and chloroplasts

• A eukaryotic cell engulfed an aerobic heterotrophic prokaryotic cell that could use oxygen

  • the prokaryote was retained an become the mitochondria

  • became an endosymbiont: a cell living inside another cell in a beneficial relationship

This process occurred again in photosynthetic organisms:

• A eukaryotic cell with mitochondria engulfed an autotrophic prokaryote could do photosynthesis

  • Eventually became the chloroplast

Evidence to support;

• Mitochondria and Chloroplasts are double membraned

• contain their own set of DNA and ribosomes

• Autonomous organelles (able to grow and reproduce in the cell

Peroxisomes (detox)

• a specialized metabolic compartment bounded by a single membranee

• contains enzymes that remove hydrogen atoms from substances and transfers it to oxygen

  • produces hydrogen perioxide — toxic but peroxisomes have the enzymes to turn it into wateer

• Can be used for a variety of reactions

  • break down fatty acids

  • detoxify alcohol in the liver

• Plants have a specialized peroxisome calleed a glyoxysome

Cytoskeleton

• network of fibres that extend throughout the cytoplasm

  • in both bacteria and eukaryotes

Eukaryotic cytoskeletons are composed of

• microtubulees

• microfilaments

• intermediate fibree

• The cytoskeleton is dynamic — constantly rearranging

• structural support (esp. in animal cells which lack cell walls)  - resist tensile (pulling) and compressive forces

• movement

  • utilizes motor proteins to movee organelles and vesicles throughout thee cell on microtubule highways

  • cillia and flagella aree made from cytoskeletal proteins

Compare and contrast the structure and basic function(s) of the three cytoskeletal elements

Microtubules

• hollow protein tubes made of tubulin

• a dimer - made up of alpha and beta subunits

• undergoes polymerization (growth) by adding tubulin dimers

• undergoes depolymerization (dissassemble) by releasing tubulin dimeers to be recycled elsewhere

  • each end will add/release at different rates

• resist compressive forces

• organized by the centrosome

  • centriole: nine sets of triplet microtubules arranged in a ring

• serves as highways for vesicle transport and in cell division

Flagella:

• usually one per cell

• longer than cilia

• undulating motion (snake)

Cilia:

• usually many per cell

• short than flageella

• wave back and forth

• Large motor proteins are required to bend the flagella and cilia

  • made up of dyneins, which cross link between microtubulee doublets and have feet to walk (using ATP)

Microfilaments

• thinnest cytoskeleton proteins

• made up of actin, a globular protein

  • actin filaments are solid

• can be branched to form structural networks

• present in all eukaryotic cells

• actin can be added/removed from filament chains dynamically

• resist tension (pulling) forces and maintain cell shape

• myosin (motor proteins) interact with actin filaments to do muscle contractions

• Amoeboid movement (cell moving) is caused by actin polymerisation

• cytosolic streaming in plant cells

Intermediate Fibres

• animal specific fibres

• really resistant to tensile forces

• composed of other proteins depending on the type of intermediate filament (eg. keratin)

• more permenant (don’t often reassemble dynamically)

Roles:

• help maintain cell shape and integrity

• maintain organelle shape and integrity

• make up nuclear lamina

• maintain organelle position within the cells

What is the extracellular matrix

• stuff outside the cell

• connects cells together into tissues

• components of the ECM are made inside the cell and exportted outside via the EM system

Cell Wall

• extracellular structures in plants

• maintain cell shape and prevent uptake of excess water

• thicker than plasma membrane

• composed of cellulose

• composed of a primary cell wall with a middle lamella (composed of pectins)

  • some cells have a secondary cell wall between the plasma membrane and primary wall

    • deposited in layers

• Has pores called plasmodesmata to allow substances to travel through cell walls

Compare the cell walls of bacteria, fungi, and plants

Bacteria

• Main component: peptidoglycan (polysaccharide chains cross-linked with peptides).

• Provides shape and protection.

• Can be Gram-positive (thick peptidoglycan layer) or Gram-negative (thin layer + outer membrane).

Fungi

• Main component: chitin (a strong polysaccharide with nitrogen, also found in insect exoskeletons).

• Provides rigidity and protection.

• Also contains glucans and glycoproteins.

Plants

• Main component: cellulose (long chains of glucose).

• Primary cell wall: flexible, allows growth.

• Secondary cell wall: thicker, more rigid (with lignin).

Name the four cellular junctions

• Tight junctions

• Anchoring junctions

• Gap junctions

• Plasmodesmata (plant specific)

Tight junctions

• for the adhesion of cells

  • plasma membranes of adjacent cells are held together by specialized proteins

• creates a seal to prevent leakage for extracellular fluid, ions, etc

Anchoring junctions

• binding proteins

  • desmosomes form sheets of strongly connected cells

    • keratin anchors the desmosomes into the cytoplasm via cytoskeleton

Gap Junctions

• for communication of cells

• connects the cytosol of adjacent cells

  • kind of like pipes!

• allows for fast passage for ions and signalling molecules to coordinate activity

  • a reason why our heart beats as one

Plasmodesmata

• Plant specific gap junctions (for communication)

• channels that connect adjacent walls and cytosol

• water and small solutes can pass freely between cell walls via these channels

• can adjust diameter to allow large molecules pass through

  • move along microfibres of the cytoskeleton

Diagram the linkage between the extracellular matrix and intercellular cytoskeleton

The extracellular matrix (ECM) links to the intercellular cytoskeleton via transmembrane proteins called integrins. Integrins act as receptors for ECM components like fibronectin and laminin. Intracellularly, the cytoplasmic tails of integrins bind to focal adhesion proteins, such as talin and vinculin, which then connect to actin filaments. This multi-protein complex forms a physical connection, transmitting mechanical forces and transmitting signals, thereby anchoring the cell and facilitating processes like migration. 

Draw and label a typical bacterium, and plant and animal cell

What does the nucleus do?

• Information central: the nucleus contains the LINEAR DNA, transcription of genes for an individual cell

• surrounded by the nuclear envelope

  • a double membrane of lipids (each a lipid bilayer)

  • the inner membrane is lined by nuclear lamina, which helps the nucleus retain shape

  • the outer membrane is continuous with the rough ER

  • contains nuclear pores which regulate substances going in and out of the nucleus

What does the nucleolus do?

• a prominent feature in a non-dividing nucleus: densely stained region adjoininig the chromatin

• produces rRNA and assembles ribosomes using proteins from the cytoplasm

  • ribosome subunits are exported through the nuclear pores and assembled in the cytosol

  • ribosomes will then remain free floating or associate with the RER

Describe chromatin

• wound up into chromatin (a complex of DNA + histone proteins)

• Euchromatin: partially unwound DNA, accessible for DNA replication and transcription

• Heterochromatin: fully wound DNA, inaccessible

Describe the nucleoid in prokaryotes

• have circular DNA chromosome

• contains all the genes essential for prokaryote survival

• not associated with histones

• contain plasmids: non-essential DNA that aid in cell function

Which of the following represents the nucleoid?

A.

B.

C.

D.

A.

iclicker question: what features do plant cells have that animal cells do not? select all that apply
1. Plasma membrane

2. cell wall

3. lysosome

4. large central vacuolee

5. chloroplast

2,4,5

cell wall,

large central vacuole

chloroplast

iclicker question: which of the following regarding thee eukaryotic nucleus is true

a. the DNA is packed into chromatin using actin proteins

b. euchromatin is compact and contains genes that are not actively transcribed

c. the nuclear lamina controls what goes into and out of the nuclear envelope

d. the nuclear envelope is continuous with the golgi complex

e. rRNA genes are actively transcribed in the nucleolus

a. wrong. histines, not actin

b. wrong. heterochromatin, not euchromatin

c. wrong. pore, not lamina

d. wrong. ER, not nuclear envelopee

e. correct.

iclicker question: which of these statements properly describes the endosymbiosis theory of the origin of mitochrondria?
a. an anaerobic heterotrophic eukaryotic cell engulfed an aerobic heterotrophic prokaryotic cell

b. an aerobic autotrophic prokarytoic cell engulfed an anaerobic heterotrophic eukaryotic cell

c. an aerobic heterotrophic eukaryotic cell engulfeed an aerobic autotrophiic prokaryotic cell

d. an anaerobic heterotrophic eukaryotic cell engulfed an anaerobic autotrophic prokaryotic cell

a. an anaerobic heterotrophic eukaryotic cell engulfed an aerobic heterotrophic prokaryotic cell

iclicker question: a cell produces a glycoprotein which functions outside of the cell. What is the proper order of organelles in which this protein will be made in and then pass through to get to the exterior of the cell?

a. nucleus, RER, secretory vesicle, outside

b. cytoplasm, lysosome, SER, outside

c. SER, secretory vesicle, golgi apparatus, outside

d. RER, golgi apparatus, secretory vesicle, outside

d. RER, golgi apparatus, secretory vesicle, outside

iclicker question: which of the following are used in communication?

a. tight junctions

b. anchoring junctions

c. desmosomes

d. plasmodesmata

d. plasmodesmata

Which of the following is not a specialized cell connection

a. tight junctions

b. anchoring junctions

c. gap junctions.

d. plasmodesmata

e. nuclear pores

e. nuclear pores

quiz questions