Cell Structure and Function Notes

AGR130: Biology in Agriculture - Topic 2.1: Cell Structure & Function

Subject Outline

The course covers:

• Topic 1: Introduction to Biology
• Topic 2: Cell Biology
• Topic 3: Genes and Inheritance
• Topic 4: Taxonomy, Diversity, and Evolution
• Topic 5: Plant Biology
• Topic 6: Animal Biology
• Topic 7: Agroecology

Cell biology will cover:

1. Structure & Function of Cells
2. Cellular Respiration
3. Cell Reproduction

Topic 2.1: Cell Structure & Function

Key areas of focus:

• History of cell theory
• Microscopy: seeing cells
• Eukaryotic cell structure
• Structure & function of major organelles
• (and non-organelles!)
• Plant cells
• Prokaryotes

Cell Theory

Cell theory is illustrated by:

• Figures 1, 5, and 6
• Micrographia, London, 1665
• Robert Hooke, 1635 - 1703

Antonie van Leeuwenhoek

• Antonie van Leeuwenhoek, 1632 - 1723
• Figures depicting his microscope and observations of "Animalcules."
• Royal Society of London, London, 1677-68

Louis Pasteur

• Louis Pasteur, 1822 - 1895
• Pasteur's experiment with swan-necked flask:
  1. Nutrient broth placed in swan-necked flask.
  2. Boiled to sterilize the flask (killing any living cells that were in the broth).
  3. Preexisting cells from air are trapped in swan neck.
• Germ theory

Cell Theory Principles

• All organisms are composed of one or more cells.
• Cells are the smallest unit of life.
• Cells arise by division of other cells.

Domains of Life

• Eukarya ("true nucleus")
  • Unicellular or multicellular
  • Includes land plants, green algae, amoebas, cellular slime molds, animals, fungi, red algae, forams, dinoflagellates, ciliates, diatoms, euglena, trypanosomes, and leishmania
• Prokaryotes ("before nucleus")
  • Unicellular
  • Includes Bacteria and Archaea
  • Bacteria: Green sulfur bacteria, spirochetes, chlamydia, green sulfur bacteria, cyanobacteria.
  • Archaea: Sulfolobus, thermophiles, halophiles, methanobacterium
• Common ancestor of all life
• Mitochondria and plastids (including chloroplasts) evolved through endosymbiosis.

Microscopy

• Microscopes provide higher magnification but are more expensive and difficult to use.
• Types: Dissecting microscope, compound microscope, electron microscope (transmission or scanning).

Cell Sizes

• Cell sizes range from 1 to 100 micrometres (\mum) (millionths of a metre) in diameter.
• 1 cm!

Cell Features

• Separated from surroundings by a plasma membrane.
• Encloses a variety of structures and chemicals in the cytoplasm.
• May contain organelles.
• Contain hereditary material (DNA, RNA).

Generalised Eukaryotic (Animal) Cell

• Nucleus (nuclear envelope, nucleolus, chromosomes)
• Rough endoplasmic reticulum
• Peroxisomes
• Smooth endoplasmic reticulum
• Mitochondria
• Golgi apparatus
• Lysosomes
• Plasma membrane
• Cytoskeleton
• Cytoplasm/cytosol
• Ribosomes
• Vesicles
• Membrane-bound organelles (“little organs”).

Not Organelles

• Cytoplasm & cytosol
• Cytoskeleton
• Plasma membrane
• Ribosomes

Cytoplasm and Cytosol

• Cytoplasm: All the fluid and structures inside the plasma membrane, except the nucleus.
  • Site of most biochemical reactions that support life.
• Cytosol: Fluid portion of cytoplasm
  • Water, salts, organic molecules.
• Cytosol + organelles = cytoplasm

Cytoskeleton

• Dynamic network of motor protein fibres:
  • Microtubules
  • Microfilaments
  • Intermediate filaments
• Functions in structural support.
• Also the intra-cellular highway!
• Motor proteins that attach to vesicles can “walk” along the cytoskeleton

Plasma Membrane

• Phospholipid bilayer with hydrophobic tails and hydrophilic heads.

Plasma Membrane: Fluid Mosaic Model

• Phospholipid bi-layer interspersed with cholesterol.
• Embedded with proteins: peripheral and integral.
• Selectively permeable.
• Function:
  • Barrier between interior and exterior of cell.
  • Regulate passage of molecules and ions

Diffusion and Osmosis

• Diffusion: Movement of molecules to equalize distribution.
  • Passive.
  • Rate is relative to concentration gradient.
  • Molecules move independently.
  • Fast over short distances, slow over long distances.
• Osmosis: Movement of water across a semipermeable membrane.
  • Passive.
  • Affected by pressure and solute concentration.
  • Rate is relative to concentration gradient.
  • Slow over large distances

Ribosomes

• Manufacture proteins.
• Structure: Made of RNA and protein, two-subunit structure.
• Function: Reads the sequence of the messenger RNA (mRNA) & translates the sequence of RNA bases into a sequence of amino acids.
• Can be freely scattered throughout the cytosol, or stuck to the endoplasmic reticulum

Membrane-Bound Organelles

• Nucleus
• Endoplasmic reticulums, Golgi apparatus & the endomembrane system
• Mitochondria
• Lysosomes & peroxisomes
• Vacuoles

Nucleus

• Information storage.
• Contains DNA as chromosomes.
• Nucleolus (plural, nucleoli; meaning “little nuclei”):
  • Site of ribosome synthesis.
  • At least one per nucleus.
• Surrounded by nuclear envelope Studded with pore-like openings for RNA transmission.

Endomembrane System

• A system of internal, membrane-bound compartments.
• Rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, lysosomes & peroxisomes.
• Products move between compartments via vesicles or are exported from the cell.
• Site of synthesis, processing, storage and recycling.

Rough Endoplasmic Reticulum

• Function: Protein synthesis.
  • New proteins end up inside the ER.
  • Move between compartments via vesicles.
• Structure:
  • “Labyrinth” of interconnected tubules and sacs.
  • Studded with ribosomes.
  • Note huge surface area to volume ratio!
• Vesicles "bud off" from ER for transport between compartments or to the PM, final destination tagged with "mail label" amino acids.

Smooth Endoplasmic Reticulum

• Function: Lipid synthesis, synthesis of new membranes (phospholipids).
• Structure:
  • No ribosomes.
  • More open (sacs >> tubules).
  • Lower SA to vol ratio.
  • Continuous compartment: Nuclear envelope → rough ER → smooth ER.

Golgi Apparatus

• Function: Protein processing, sorting and shipping.
  • Receives “cargo” from ER.
  • Ships them to other organelles or plasma membrane.
• Structure:
  • Flattened sacs called cisternae.
  • cis side and trans side.
  • Vesicles bud off.

Lysosomes & Peroxisomes

• Cleanup & recycling.
• Lysosomes: Contain digestive enzymes; old/faulty proteins get digested and recycled. (Animal cells only.)
• Peroxisomes: Contain detoxifying enzymes; reactive/toxic molecules get cleaned up. (Animal cells only.)

Mitochondria

• Function: Power-generators; the site of respiration (turning glucose into ATP).
• Structure:
  • Double membrane
  • Outer membrane defines mitochondrial surface
  • Inner membrane is the site of respiration reactions
• Fun fact: mitochondria contain their own DNA!

Organelles Review

• Organelles
  • Nucleus
  • Endoplasmic reticulums, Golgi apparatus & the endomembrane system
  • Mitochondria
  • Lysosomes & peroxisomes
• Not Organelles
  • Cytoplasm & cytosol
  • Cytoskeleton
  • Plasma membrane
  • Ribosomes

Plant Cells

• Unique features include cell walls, chloroplasts, and large central vacuoles.
• Other components: Nucleolus, rough ER, smooth ER, nucleus, microtubules, Golgi body, cell membrane, cytoplasmic strand traversing vacuole, vacuole, ribosomes, plasmodesmata, mitochondrion, peroxisome

Chloroplast

• Function: Photosynthesis; capture sunlight energy through chlorophyll.
• Structure:
  • Double membrane
  • Outer membrane defines chloroplast surface
  • Inner membrane is the site of photosynthesis reactions
• Fun fact: chloroplasts also contain their own DNA!

Vacuoles

• Plant cells have large “central vacuoles”. Animal and fungal cells also have digestive vacuoles
• Functions:
  • Storage
  • Digestion & recycling (alt. to lysosomes)
  • Water balance and cell turgor
• Structure:
  • Varies!
  • Often big (can take up 90% of the volume of plant cells)
  • May have internal structures (compartmentalisation of functions)

Endosymbiotic Origins of Organelles

• Are chloroplasts and mitochondria ancient bacteria?
• Evidence:
  • “Semi-autonomous”: replicate independently, have their own DNA (it’s circular like prokaryotes!) and ribosomes.
  • Divide by binary fission (like prokaryotes do).
  • Have double-membrane envelopes.

What’s Outside the Cell?

• Extracellular matrix (ECM)
  • In animals: supporting and adhesive proteins as well as chemical signals in a polysaccharide gel.
  • In plants: the cell wall, overlapping cellulose fibers perforated by plasmodesmata, connecting adjacent cells.

Prokaryotic Cells

• Always unicellular (e.g.: Bacteria & Archaea).
• No nucleus: DNA clumped in a “nucleoid region”.
• Relatively simple internal structure, no organelles.
• Nearly always have a cell wall.
• Can be up to 100x smaller than eukaryotic cells.
• Rod (bacilli), spiral (spirilla), or spherical (cocci) shaped.

Summary

• Three tenants of cell theory
• Microscopes help us see cells (and other cool, tiny stuff)
• Eukaryotic cells are complex & highly organised
• Structure and function of major organelles
• And non-organelles
• Plant cells have unique features
• Prokaryotes are pretty simple