Cell Structure and Organization

• Living things are made of cells, the basic unit of life
• New cells come from pre-existing ones
• Biological levels of organization range from atoms to organisms, in this order: atoms, molecules, cells, tissues, organs, and organisms

Types of Cells

• Prokaryotes possess the prefix 'pro', which means "before"
• The term 'karyon' refers to nut, kernel, or nucleus
• Prokaryotes were the first successful organisms, existing 4 billion years ago, and were the only life form before eukaryotes
• Prokaryotes lack a true membrane-bound nucleus and membrane-bound organelles
• Prokaryotes contain a single loop of DNA located in the 'nucleoid' region
• Prokaryotes divide via binary fission
• Prokaryotes are present in all habitat types
• Prokaryotes are small, 1-2 um in diameter and 10 um long
• Bacteria, such as E. coli, are examples of prokaryotes
• Eukaryotes possess 'eu', which means "true"
• Eukaryotic DNA resides within a nuclear membrane
• Eukaryotes contain complex, membrane-bound organelles
• Eukaryotes include single-celled and all multicellular organisms, like plants and animals
• Eukaryotes first appeared 1.5 billion years ago
• Eukaryotes are about 10x larger than prokaryotes
• Eukaryotes divide through meiosis and mitosis
• All eukaryotic cells feature a cell/plasma membrane, nucleus, cytoplasm, and organelles
• Plant cells have a cell wall in addition to the cell/plasma membrane
• Cytoplasm comprises the contents between the nucleus and cell membrane
• Organelles are specialized structures that execute specific functions, enclosed by membranes

Nucleus

• A nuclear envelope, consisting of a double membrane, surrounds the nucleus, featuring pores for communication between the nucleus and cytoplasm
• The nuclear envelope consists of two lipid bilayers, with the outer membrane connected to the rough endoplasmic reticulum (RER)
• The nucleus regulates metabolic functions by controlling protein production and gene expression
• The nucleus controls protein synthesis
• Hereditary and genetic information (DNA) is contained in the nucleus
• Chromatin threads (uncoiled chromosomes) within the nucleus condense into chromosomes during cell division, containing DNA and histone proteins
• The fluid within the nucleus is the nucleoplasm
• The nucleus houses the nucleolus, which participates in the formation of ribosomal RNA (rRNA) and ribosomes, and is not surrounded by a membrane

Ribosomes

• Proteins are assembled in ribosomes
• Ribosomes consist of large and small subunits produced by the nucleolus
• Each subunit is made of rRNA and protein
• Ribosomes are present either freely in the cytoplasm or bound to the endoplasmic reticulum
• Free ribosomes synthesize proteins used in the cell, while bound ribosomes synthesize proteins for secretion or use in the cell membrane
• A group of ribosomes is known as a polyribosome or polysome

Endoplasmic Reticulum

• A vast membranous network continuous with the nuclear envelope's outer membrane, extending into the cytoplasm as tubular canals and vesicles
• The rough ER (RER), closest to the nucleus, has ribosomes attached, while the smooth ER (SER), further from the nucleus, lacks ribosomes
• The SER is more tubular
• The ER synthesizes,modifies, and transports macromolecules
• Ribosomes on the RER synthesize proteins that enter the ER, get modified if needed, and are transported to the Golgi Apparatus with transport vesicles that pinch off from the RER ends
• Insulin is produced by the RER in pancreatic cells
• Lipids, phospholipids, and steroids are synthesized in the smooth ER
• High amounts of Smooth ER is present in gland cells that produce steroid hormones
• The SER in liver cells detoxifies drugs
• Proteins and lipids are transferred, via transport vesicles formed by pinching off ER membranes, from the ends of RER and SER tubules to the Golgi Apparatus

Golgi Apparatus

• The Golgi apparatus processes, modifies, and packages macromolecules from the ER, distributing them via secretory vesicles and creating lysosomes
• The Golgi apparatus consists of flattened saccules called cisternae, arranged in stacks
• Proteins and lipids are modified in the Golgi, e.g. by adding carbohydrate chains to create glycoproteins and glycolipids
• Golgi products go to other cell parts and the cell membrane
• The Golgi Apparatus helps with secretion: secretory vesicles move to the cell membrane to discharge their contents outside the cell through exocytosis

Vesicles

• Membrane-bound sacs, either to transport substances within a cell (transport vesicles) or for secretion at the cell membrane (secretory vesicles)

Vacuole

• A large, single membrane-bound sac in the cytoplasm, filled with water and solutes
• Vacuoles serve as storage for water and salts
• Plant cells feature larger vacuoles filled with water and ions. This maintains turgor against the cell wall to keep the plant rigid
• Tonoplast is the membrane surrounding plant cell vacuoles
• Vacuoles are bigger than 100 nm, while vesicles are smaller than 100 nm, however, they share the same general structure

Lysosome

• Lysosomes are vesicles originating from the Golgi Apparatus, filled with hydrolytic enzymes
• Lysosomes facilitate intracellular digestion (by capturing and dismantling bacteria or damaged tissue) and autodigestion (by destroying debris and worn-out organelles)
• Lysosomes that contain debris degrade them
• Lysosomes merge with vesicles formed by endocytosis, digesting macromolecules into monomers released into the cytoplasm
• White blood cells are abundant in lysosomes

Peroxisome

• Vesicles containing enzymes that detoxify drugs and break down fats, especially in the liver
• Catalase, present in peroxisomes, breaks down hydrogen peroxide (H2O2), a product of fat and organic molecule breakdown
• Peroxisomes originate at the RER
• Free ribosomes create enzymes contained in a peroxisome

Endomembrane System

• The Endoplasmic Reticulum, Golgi apparatus, vesicles/vacuoles, and lysosomes collaborate to produce, transport, store, and secrete cellular products

Mitochondria

• The cell's energy-producing organelle, the mitochondria
• Mitochondria are double-membraned organelles, about the size of prokaryotes, featuring a smooth outer membrane and a folded inner membrane called cristae, which increases surface area where cellular respiration happens
• The fluid enclosed by the inner membrane, called the matrix, is a mixture of enzymes (that form ATP from glucose), DNA, and ribosomes
• Mitochondria are maternal in origin
• Cellular respiration converts the energy in glucose to energy in ATP
• Glycolysis in the cytoplasm converts glucose to pyruvate (producing 2 ATP, no oxygen needed, producing lactate)
• The citric acid cycle in the mitochondria produces 2 ATP, which requires oxygen
• The electron transport chain in the mitochondria produces 32-34 ATP and requires oxygen

Chloroplasts

• Chloroplasts produce carbohydrates by photosynthesis
• Chloroplasts are present in plant cells, but not in animal or fungal cells
• Chlorophyll imparts a green color
• Chloroplasts are double-membraned organelles enclosing a fluid-filled space called the stroma, which contains flattened sacs called thylakoids, arranged in stacks called grana. Chlorophyll is in the thylakoid membranes. Chloroplasts use sunlight to produce carbohydrates
• The stroma contains enzymes, DNA, and ribosomes

Cell Wall

• Plant cells are the only cells that have a cell wall
• Plant cell walls are made of cellulose and are rigid
• The cell wall is located outside of the cell membrane

Cytoskeleton

• The cytoskeleton is composed of filamentous protein structures that fill the cytoplasm
• Microtubules are 20-25 nm in diameter
• Intermediate filaments are 10 nm in diameter
• Microfilaments are 3-6 nm in diameter
• The cytoskeleton is a complex network facilitating cell movement (cilia and flagella components), shape maintenance, organelle movement and anchoring, vesicle movement, cytokinesis (via contractile ring constriction), chromosome movement (during mitosis and meiosis), and cell-to-cell communication
• Cytoskeleton structures can assemble and disassemble quickly due to their protein subunits

Centrosome

• The centrosomes is the origin and organizing centre of the cytoskeleton
• Centrioles are part of the centrosome in animal cells, replicating and positioning at right angles during cell division, organizing the spindle

Differences between Animal and Plant Cells

• Plant cells have cell walls, large central vacuoles, and chloroplasts, and they do not have centrioles