Lecture 7: Cytoskeleton, Mitochondria, Chloroplasts, Extracellular Matrix, and Peroxisomes Bio Sci 93

Mitochondria and chloroplasts (general)

Energy transformers of the cell. Enclosed in membranes but not part of the endomembrane system. Semiautonomous (grow and reproduce within the cell). Not fixed in location; they move through the cytoplasm along cytoskeletal tracks.

Mitochondrion

Organelle in eukaryotic cells where most of the cell's ATP is generated through cellular respiration.

Mitochondrial matrix

Interior of the mitochondrion. Contains the enzymes for the Krebs cycle, which oxidizes fuel molecules.

Cristae (inner mitochondrial membrane)

Folded inner membrane of the mitochondrion. Houses the enzymes that produce most of the ATP through oxidative phosphorylation.

Chloroplast

Site of photosynthesis. Found in leaves and other green structures of plants and in eukaryotic algae.

Thylakoid

Stacked disc-like membrane system inside the chloroplast. Chlorophyll is found in the thylakoid membrane.

Endosymbiont theory

Ancient host cells took up prokaryotes that evolved into mitochondria. At least one of these cells then took up a photosynthetic prokaryote that evolved into a chloroplast.

Evidence for the endosymbiont theory

Mitochondria and chloroplasts have a double membrane, contain free ribosomes and circular DNA molecules, and grow and reproduce somewhat independently in cells.

Mitochondria vs. chloroplasts: similarities

Both have a double membrane, their own circular DNA and ribosomes, and make ATP using an electron transport chain and chemiosmosis. Both likely evolved from endosymbiosis.

Mitochondria vs. chloroplasts: differences

Mitochondria perform cellular respiration, are found in almost all eukaryotic cells, and have no pigments. Chloroplasts perform photosynthesis, are found only in plants and green algae, contain pigments such as chlorophyll, and have a third membrane system of thylakoids.

Peroxisome

Specialized metabolic membrane-bound organelle that creates and uses hydrogen peroxide (H2O2) in many reactions.

Hydrogen peroxide (H2O2)

Toxic, highly reactive molecule made in peroxisomes. It is converted to water by the enzyme catalase.

Catalase

Peroxisome enzyme that converts toxic H2O2 to H2O.

Peroxisome functions

Breaks down very long fatty acids (beta-oxidation). Helps synthesize lipids such as plasmalogen, which makes up 80-90% of myelin sheaths (per the slide). In fungi, the site of the last steps of penicillin biosynthesis.

Cytoskeleton

Network of protein-based structures extending throughout the cytoplasm. Provides structural support (mechanical support, anchors internal components) and motility (movement of the cell, and of organelles, vesicles, and macromolecules with motor proteins).

Three types of cytoskeletal filaments

Microfilaments, microtubules, and intermediate filaments.

Microfilaments (actin filaments)

Two intertwined strands of actin, 7 nm in diameter. Functions: cell shape and shape changes, muscle contraction, cytoplasmic streaming (plants), cell motility, and cell division (animal cells).

Microtubules

Hollow tubes of tubulin (alpha and beta tubulin dimers), 25 nm in diameter with a 15 nm lumen. Functions: cell shape, cell motility, chromosome movement in cell division, and organelle movement.

Intermediate filaments

Fibrous proteins (e.g., keratins) coiled into cables, 8-12 nm. Not dynamic. Functions: cell shape, resisting tension, anchoring the nucleus and other organelles, and forming the nuclear lamina.

Dynamic vs. stable cytoskeletal filaments

Microfilaments and microtubules constantly polymerize and depolymerize (dynamic). Intermediate filaments are supercoiled into thick cables and are not dynamic.

Microfilament polarity

Plus (barbed) end and minus (pointed) end. Actin monomers can be added to both ends, but addition is favored at the plus end. ATP is coupled to polymerization.

Treadmilling

When monomers are added at the plus end at the same rate that they are removed from the minus end.

Actin cortex

Area near the plasma membrane with a high concentration of actin filaments, found in every cell. Allows shape changes that help with nutrient absorption, movement, and cell division.

Myosin

Motor protein of microfilaments. Used for short-distance transport, cell motility (crawling), and muscle contraction (with actin). Almost always moves toward the plus end.

Pseudopodia (pseudopodium)

Cell extensions driven by microfilaments, used in crawling and at the start of phagocytosis.

Crawling movement

Mediated by myosin motors interacting with microfilaments and by alternating polymerization and depolymerization of microfilaments in different regions of the cytoplasm.

Microtubule polarity

Plus and minus ends. Tubulin dimers can be added or removed at both ends, but much faster at the plus end. GTP is coupled to polymerization.

Microtubule functions

Structural support (resist compression), transport of organelles and vesicles, anchoring structures, chromosome movement during mitosis, and cell motility.

Microtubule organizing center (MTOC)

Site from which microtubules polymerize; where their minus ends are located. In animal cells there are two: the centrosome and the basal body.

Centrosome

Animal-cell MTOC usually found next to the nucleus. Contains two centrioles.

Centriole

Two sit perpendicular to each other in the centrosome. Each is nine sets of triplet microtubules in a ring (9+0).

Cilia and flagella

Microtubule-based appendages that provide movement and require ATP. In unattached cells they move the cell (e.g., sperm). In attached cells they move fluid over tissue (e.g., ciliated lining of the digestive tract).

Kinesin

Motor protein that moves cargo toward the plus end of microtubules. Uses ATP hydrolysis; long-distance transport.

Dynein

Motor protein that moves cargo toward the minus end of microtubules. Uses ATP hydrolysis; long-distance transport.

Lamin

Intermediate filament protein that forms the nuclear lamina. Lamin mutations cause accelerated aging (progeria) and abnormal nuclear architecture.

Extracellular matrix (ECM)

Complex network of proteins, glycoproteins, and sugars that surrounds and supports cells in tissue. In animals it includes collagen, elastin, and fibronectin.

Integrins

Plasma membrane proteins that link ECM components with the cell's cytoskeleton, allowing crosstalk between the cell and its environment.

ECM in cartilage

Hyaluronic acid is a polysaccharide that the proteoglycan aggrecan binds to, providing cushioning and structural support.

ECM and cancer

Collagen in the ECM is rearranged in cancerous tissue (e.g., colon) compared to healthy tissue. These changes can be both pro- and anti-invasive.

Plant cell wall

Extracellular structure of plants (also yeasts, some protists, and prokaryotes). Protects the plant, maintains its shape, and resists excess water uptake and turgor pressure. Mainly cellulose, whose beta glycosidic linkages form rigid fibers. Layers: middle lamella, primary and secondary cell walls.

Tight junction

Animal cell junction where neighboring membranes are pressed together, preventing leakage of extracellular fluid (e.g., blood-brain barrier, intestines).

Desmosome

Anchoring junction that fastens cells into strong sheets that resist mechanical forces (e.g., skin and cardiac muscle). Linked to intermediate filaments.

Gap junction

Communicating junction: cytoplasmic channels between adjacent animal cells that allow coordinated responses (e.g., smooth muscle, some neurons).

Plasmodesmata

Channels connecting plant cells. Water and small solutes (sometimes proteins and RNA) can pass from cell to cell.