Notes on Cells, Organelles, and Membrane Transport

Size and Scale in Biology

• Cells are typically around 10 μm in diameter (example heading: "10 μm Cells").

• The slide contrasts sizes from molecules up to whole cells: molecules, cell membrane thickness, viruses, bacteria, organelles, and cells.

• Membrane thickness is used as an example of nanoscale structure (range discussed in other slides).

Measurement Units and Scale

• SI prefixes listed (from the transcript):

  • Kilo (10^3), Hecto (10^2), Deka (10^1)

  • BASE (10^0)

  • Deci (10^-1), Centi (10^-2), Milli (10^-3)

  • Micro (10^-6), Nano (10^-9), Pico (10^-12)

• Rule of thumb for decimal movement:

  • Move the decimal to the right to convert to larger units.

  • Move the decimal to the left to convert to smaller units.

• Common unit conversions mentioned (corrected for accuracy):

  • 1 centimeter (cm) = 0.01 meter (m)

  • 1 millimeter (mm) = 10^{-3} m

  • 1 micrometer (μm) = 10^{-6} m

  • 1 nanometer (nm) = 10^{-9} m

• Microscopy scales discussed: light microscope vs electron microscope; unaided eye for macroscopic observations.

Size Comparisons: Magnitudes and Examples

• Scales from the transcript include: human height (~1 m), chicken egg (~0.1 m), frog egg (~1 mm), nucleus (~10 μm), most bacteria (~0.5–2 μm), mitochondrion (~1 μm), viruses (~tens to hundreds of nm), ribosomes (~10 nm), proteins (~1 nm), lipids (~1 nm), small molecules (~0.1 nm).

• The transcript also notes that measurements range across several orders of magnitude from atoms (~0.1 nm) to cells (~10 μm) to organisms (~m).

Limitations to Cell Size and Surface Area to Volume Ratio

• Cells do not grow indefinitely; they eventually divide.

• If a cell becomes too large, its surface area to volume (SA:V) ratio becomes small, causing exchange with the environment to become inefficient.

• As organisms increase in size, volume grows faster than surface area, leading to a decreasing SA:V ratio as size increases.

• Consequences: Lower SA:V reduces the rate of exchange of nutrients, wastes, gases, and heat with surroundings, impacting viability.

Surface Area to Volume Ratio (SA:V)

• Key idea: As size increases, surface area increases but not proportionally with volume, reducing SA:V.

• Implication: Rate of exchange (diffusion/radiation) decreases with increasing size.

• All levels of biological organization rely on exchange with surroundings: organelles, cells, tissues, organs, and whole organisms.

Quantitative Look at SA:V (Conceptual Formulas)

• For a cube of side length a:

  • Surface area: SA = 6a^2

  • Volume: V = a^3

  • SA:V ratio: \text{SA:V} = \frac{SA}{V} = \frac{6a^2}{a^3} = \frac{6}{a}

• For a general rectangular prism with height h, width w, length l:

  • Surface area: SA = 2(hw + hl + wl)

  • Volume: V = hwl

  • SA:V ratio: \text{SA:V} = \frac{2(hw + hl + wl)}{hwl}

Consequences Across Scales

• As organisms get bigger, SA:V decreases, causing slower exchange with surroundings.

• Small organisms exchange with surroundings very quickly; large organisms exchange more slowly.

Organelles and Plant Cell Anatomy (Overview)

• Cells contain many tiny structures called organelles, each with specific functions; most are surrounded by membranes.

• Major organelles/functions:

  • Nucleus: houses genetic material; double-membrane nuclear envelope; contains chromatin and nucleolus (ribosome synthesis site).

  • Endoplasmic Reticulum (ER): rough ER (with ribosomes) synthesizes proteins for export; smooth ER synthesizes lipids and detoxifies substances; interconnected network of membranes.

  • Golgi apparatus: modifies, sorts, and packages cell products; vesicular transport to destinations.

  • Mitochondrion: site of cellular respiration and ATP production; double membrane; cristae increase inner-surface area; matrix inside.

  • Peroxisome: various metabolic functions, including hydrogen peroxide metabolism.

  • Lysosome: digestive organelle with hydrolytic enzymes (in animal cells).

  • Cytoskeleton: network of protein filaments (microfilaments, intermediate filaments, microtubules) providing shape and movement.

  • Ribosomes: synthesize proteins; may be free in cytoplasm or bound to rough ER/nuclear envelope.

  • Plasma membrane: phospholipid bilayer that controls entry/exit; membrane proteins perform transport, signaling, and adhesion roles.

  • Chloroplasts (plants): sites of photosynthesis; contain chloroplast DNA; double membrane; stacked thylakoids (grana) and stroma.

  • Vacuoles (plants): large central vacuole in plant cells for storage and turgor.

  • Plasmodesmata (plants): cytoplasmic channels through cell walls enabling transport and communication between plant cells.

  • Cell wall (plants): rigid support and protection; composed of cellulose.

  • Nucleolus: ribosome synthesis within the nucleus.

  • Nucleoplasm: interior of the nucleus.

Cell Theory and Life Characteristics

• Cell Theory:

  • Living organisms are composed of cells.

  • Cells are the smallest unit of life.

  • Cells arise from pre-existing cells.

• Characteristics of Life:

  • Organization: high level of cellular/molecular organization.

  • Response to stimuli (e.g., pupil dilation).

  • Homeostasis: maintenance of stable internal conditions.

  • Metabolism: chemical processes for energy and growth.

  • Growth and development: cell division and differentiation.

  • Reproduction: production of new organisms and transmission of DNA.

  • Evolution: changes leading to biodiversity.

Classification and Cellular Domains

• Three domains: Bacteria, Archaea, Eukarya.

• Prokaryotes: unicellular; lack membrane-bound organelles; first organisms on Earth (~3.5 billion years ago).

• Eukaryotes: organisms with membrane-bound organelles; evolved from prokaryotes via endosymbiosis.

Timeline of Life (Overview from Transcript)

• Precambrian to present highlights:

  • Origin of Earth (~4.5–4.6 billion years ago).

  • Oldest prokaryotic fossils (bacteria) among the earliest life forms.

  • Accumulation of atmospheric oxygen from photosynthetic cyanobacteria.

  • Oldest eukaryotic fossils appear later in the record.

  • Origin of multicellular organisms and oldest animal fossils.

  • Plants colonize land; reptiles and dinosaurs appear later.

  • First humans appear in the late Cenozoic.

• The timeline in the slides shows major markers with approximate eras such as Precambrian, Paleozoic, Mesozoic, and Cenozoic, and includes the sequence: origin of Earth, origin of life, oldest prokaryotic/eukaryotic fossils, atmospheric oxygen buildup, multicellular life, plants on land, dinosaurs, reptiles, and humans.

Prokaryotic Cells: Structure and Function

• General size: about 1–2 μm in length.

• Key features (no membrane-bound organelles):

  • Cell wall and plasma membrane.

  • Cytoplasm with nucleoid (naked DNA, no true nucleus).

  • Ribosomes (smaller than eukaryotic ribosomes).

  • Pili: adhesion and conjugation.

  • Flagella: locomotion.

  • Slime capsule: attachment, protection, sometimes food reserve.

  • Mesosome: membrane infolding potentially aiding ATP production and DNA movement during division.

  • Plasmids: small circular DNA elements that can transfer between bacteria.

• Ultrastructure example: Escherichia coli showing cell wall, plasma membrane, cytoplasm, pili, flagella, ribosomes, and nucleoid.

Prokaryotic Cell Functions (Key Roles of Structures)

• Cell wall: protective outer layer preventing damage and lysis from internal pressure.

• Plasma membrane: selective entry/exit; active transport includes pumping mechanisms.

• Mesosome: increases membrane surface area; potential role in ATP production and DNA movement during division.

• Cytoplasm: site of metabolic reactions; contains nucleoid where DNA is stored.

• Ribosomes: synthesize proteins (some exported, some stay in cell).

• Nucleoid: region with naked DNA controlling cell and passing to daughter cells.

• Pili: aid in adhesion; conjugation between bacteria.

• Flagella: movement.

• Slime capsule: adhesion and protection; food storage potential.

DNA Replication and Binary Fission in Prokaryotes

• Prokaryotes divide by binary fission.

• General steps (as depicted):

  • Origin replication begins.

  • Origins move toward opposite ends as replication proceeds.

  • Replication continues; one origin at each end.

  • Plasma membrane grows inward; new cell wall is deposited.

  • Two daughter cells result.

• This process yields two genetically identical daughter cells.

Conjugation in Prokaryotes

• Direct contact between bacterial cells with transfer of plasmid DNA from donor to recipient.

• Not fertilization or zygote formation; not sexual reproduction in the traditional sense.

Endosymbiotic Theory and Eukaryotic Cells

• Eukaryotic cells evolved from prokaryotic cells via Endosymbiotic Theory (Lynn Margulis).

• Endosymbiosis: one organism living inside another in a mutually beneficial relationship.

• Evidence: mitochondria and chloroplasts have their own DNA and double membranes.

• Evolutionary tree: ancestral prokaryote engulfed aerobic heterotrophic prokaryotes (mitochondria) and/or photosynthetic prokaryotes (chloroplasts) leading to eukaryotic organelles.

Eukaryotic Cell Organization (Key Organelles)

• Nucleus: contains most genetic material; nuclear envelope with pores; contains nucleolus for ribosome synthesis; chromatin exists as DNA-protein complexes.

• Endomembrane system: nuclear envelope, rough ER, smooth ER, Golgi apparatus; involved in protein synthesis, modification, and trafficking.

• Mitochondrion: site of cellular respiration; ATP production; cristae increase inner membrane surface area; matrix inside; double membrane; contains its own DNA (mtDNA).

• Chloroplast (plants): site of photosynthesis; double membrane; chloroplast DNA; thylakoids and stroma.

• Lysosome: digestive organelle with hydrolytic enzymes.

• Peroxisome: metabolism and detoxification functions; produces hydrogen peroxide.

• Golgi apparatus: packaging, sorting, and secretion of cell products; cis and trans faces with vesicular transport.

• Cytoskeleton: structural framework composed of microfilaments, intermediate filaments, and microtubules; involved in shape, transport, and movement.

• Ribosomes: protein synthesis; some attached to rough ER, others free in cytoplasm.

• Plasma membrane: phospholipid bilayer with embedded proteins; regulates passage of substances.

• Vesicles: transport carriers between ER, Golgi, and plasma membrane.

• Plasmodesmata (plants): cytoplasmic channels through cell walls for transport and communication between plant cells.

• Cell wall (plants): rigid cellulose-based structure providing support and protection.

• Vacuole (plants): large central vesicle for storage and turgor maintenance.

• Microvilli, ribosomes, and other minor components as noted in diagrams.

Nucleus and Nuclear Anatomy

• Nucleus: genetic control center; houses most DNA; directs cellular activities.

• Nuclear envelope: two membranes with nuclear pores; continuous with the ER.

• Nucleolus: ribosome synthesis site within the nucleus.

• Chromatin: DNA-protein complex that forms chromosomes during cell division.

Mitochondria and Chloroplasts: Endosymbiotic Evidence

• Mitochondria: energy producer in eukaryotes; have their own circular DNA; double membrane with inner cristae.

• Chloroplasts: photosynthetic organelles in plants and some algae; have their own DNA; double membrane with internal thylakoids (grana) and stroma.