Lesson 05: The Structure of Cells

Organizational Hierarchy of Life

Atoms, molecules, macromolecules, organelles, cell

Organelles

Discrete internal macromolecular structures with specialized functions, assembled from macromolecules.

Cell Classification

Commonly classified as either prokaryotic or eukaryotic.

Human Eye (Microscopy)

Can image objects as small as \approx 200 \mu m.

Light Microscope

Illuminates with visible light, images objects as small as 200 nm. Can image all cells and many subcellular structures (e.g., mitochondria, chloroplasts).

Electron Microscope

Illuminates with an electron beam, images even smaller objects (e.g., virus particles, macromolecules). Provides better perspective of organelle structure.

Prokaryotic Cells

Comprise two of the three domains of life: Archaea and Bacteria. Unicellular, generally smaller than eukaryotic cells, and lack internal compartments.

Prokaryotic Cell Walls

Surround the plasma membrane, composed of carbohydrates and proteins (specific molecules differ between Archaea and Bacteria), explaining antibiotic specificity.

Capsule (Prokaryotic)

A polysaccharide coating produced by some prokaryotic cells.

Pili and Flagella (Prokaryotic)

Protein-based extensions produced by some prokaryotes for cell interaction and swimming.

Plasma Membrane

Separates the interior (cytoplasm) from the exterior environment in both prokaryotes and eukaryotes.

Genetic Material

DNA serves as the genetic material in both prokaryotes and eukaryotes.

Gene Expression

DNA is translated into proteins to carry out cellular functions, a process tightly controlled in both prokaryotes and eukaryotes.

Energy Metabolism

Both prokaryotes and eukaryotes generate and consume energy (e.g., glycolysis, cellular respiration, photosynthesis).

Ribosomes

Large RNA-protein complexes present in both, synthesize proteins by converting nucleotide sequence information into amino acid sequence information. Exist as two discrete units (large and small subunits) and are found in the cytoplasm of both prokaryotes and eukaryotes, as well as inside mitochondria and chloroplasts of eukaryotic cells.

Eukaryotic Cell Features

Have membrane-bound organelles that compartmentalize different functions.

Organelles present in Both Animal and Plant Cells

Nucleus, Endoplasmic Reticulum (ER), Golgi apparatus, Peroxisomes, Mitochondria.

Lysosomes (Eukaryotic)

Present in animal cells, presence in plant cells is debated.

Organelles Only in Plant Cells

Chloroplasts, Vacuole, Cell Wall.

Cytoplasm

Everything inside the plasma membrane; applies to both prokaryotes and eukaryotes.

Cytosol

All cellular material enclosed by the plasma membrane but outside of organelle membranes. It has a gel-like consistency and is filled with water-soluble molecules.

Lumen

The interior of each organelle, separated from the surrounding cytosol by the organelle membrane.

Nucleus

Typically the largest and most visible compartment in eukaryotic cells. Its interior is mostly chromatin, separated from the cytosol by a nuclear envelope with nuclear pores. It contains the nucleolus. Functions include protecting DNA and separating RNA synthesis from protein synthesis to provide more control over protein synthesis.

Interior (Nucleus)

Mostly chromatin (a complex of proteins and DNA).

Nuclear Envelope

Consists of an inner and outer membrane, separating the nucleus from the cytosol.

Nuclear Pores

Large openings in the nuclear envelope for protein and RNA movement between the nucleus and cytosol.

Nucleolus

A distinctive structure within the nucleus that synthesizes the RNA components of the ribosome.

Endoplasmic Reticulum (ER)

A network of membranes with a continuous lumen; accounts for \approx 10 \% of cell volume and 50 \% of total cell membrane. Critical functions: protein synthesis, lipid synthesis, calcium ion storage, detoxification.

Rough ER (RER)

Named for the presence of ribosomes on its cytosolic surface. Synthesizes roughly one-third of all types of cellular proteins, which enter the endomembrane system.

Smooth ER (SER)

Lacks ribosomes. Site of lipid synthesis and detoxification reactions.

Golgi Apparatus

Consists of a polarized stack of flattened membranes. Enzymes modify proteins and lipids arriving from the ER, preparing them for final destinations. Transport pathway: Vesicles from the ER arrive at the cis (entry) face, flow through the Golgi towards the trans (exit) face for sorting.

Exocytosis

The outward movement of proteins and lipids from the ER
ightarrow Golgi, and then to the plasma membrane, extracellular environment, or lysosomes. A transport vesicle fuses with the plasma membrane, releasing its inner contents and incorporating its phospholipids into the plasma membrane.

Lysosomes

Contain digestive enzymes that break down molecules, allowing the cell to recycle building blocks. Enzymes are synthesized on the RER, travel through the endomembrane pathway via the Golgi, and are sorted to the lysosomes. Degrade old or damaged organelles and material ingested by the cell. Have a highly acidic internal environment (pH \approx 5).

Peroxisomes

Enclose a variety of enzymatic reactions (oxidation of fatty acids, various biosynthetic reactions, and detoxification of compounds like hydrogen peroxide) that could damage molecules in the cytosol. Often include an internal crystal-like structure of digestive and detoxifying enzymes.

Vacuoles (Plants and Fungi)

Several types with functions including maintaining water balance, storage of small molecules, and storage of waste products. Some also function in macromolecule degradation, analogous to lysosomes.

Theory of Endosymbiosis

Scientists theorize that mitochondria and chloroplasts originated from bacteria engulfed by ancestral eukaryotes (mitochondria from oxidative metabolism, chloroplasts from photosynthesis). Evidence: Both organelles contain their own circular DNA, ribosomes, and divide within the eukaryotic cell via binary fission.

Mitochondria

Found in both animal and plant cells. Function: Synthesize ATP from sugars; often called the 'powerhouse of the cell.' Reproduce by binary fission. Structure: Two membranes (outer interacts with cytosol; inner for ATP synthesis). Genetic Material: Contain circular DNA that codes for some mitochondrial proteins, but most are coded by nuclear DNA. Ribosomes: Present within the matrix to facilitate DNA expression.

Chloroplasts

Found in plant cells and other photosynthetic eukaryotes. Function: Use light energy to synthesize ATP and sugar molecules. Reproduce by binary fission. Structure: Three membranes. Genetic Material: Contain circular DNA and ribosomes. Pigment: Readily visible due to the presence of light-absorbing chlorophyll.

Eukaryotic Cytoskeleton

A complex and dynamic internal structure, constantly changing, comprised of three main types of protein filaments: Actin filaments, Microtubules, Intermediate filaments.

Actin Filaments

Assembled from actin monomers (globular proteins). Roles: Muscle Contraction (with myosin) and control cell shape/cell crawling in non-muscle cells.

Microtubules

Assembled from tubulin dimers (globular proteins). Roles: Cell Organization (transport system for vesicles), Cell Division (anchor to DNA and help pull DNA apart during mitosis), form Cilia and Flagella, and drive Internal Vesicle Movement.

Cilia and Flagella (Eukaryotic)

Hair-like appendages on the outside of the cell that support cell swimming. Structure: Comprised of multiple microtubules bundled in a distinctive 9+2 arrangement (two central, nine pairs in a circle). Movement: Driven by motor proteins (e.g., dynein) along the microtubules, requiring ATP.

Intermediate Filaments

Formed from a variety of different types of fibrous subunits (not globular). Roles: Serve to support and protect the cell. Examples: Keratin (skin, hair, nails) and Lamins (around the nucleus). Structure: Arranged in an overlapping pattern, similar to burlap cloth.

Keratin

Major component of skin cells, hair, and nails, providing a waterproof barrier.

Lamins

Form a layer of intermediate filaments surrounding the nucleus.

Extracellular Structures (Eukaryotic)

Secreted by cells, providing support from the outside. Synthesized in the ER and sent to the cell surface via the endomembrane pathway.

Extracellular Matrix (ECM) - Animal Cells

A meshwork of secreted proteins and carbohydrates. Has a fibrous nature and is particularly abundant in connective tissue.

Cell Wall (Plant and Fungal Cells)

Located outside the plasma membrane, providing protective and structural support. Chemically and structurally distinct from prokaryotic cell walls.

Plant Cell Walls

Main structural component is the carbohydrate cellulose.

Fungal Cell Walls

Main component is chitin.