Cell Composition and Structure Lecture 1 PDF

Cell composition and structure Lecturer: Dr. Michelle Kuzma Adapted from: Dept head, Dr. Danuta Mielżyńska- Švach Molecular biology 2024/2025  Areas of cell study Cytology Cytochemistry Cytopathology Cytophysiology Cytogenetics ...

Cell composition and structure Lecturer: Dr. Michelle Kuzma Adapted from: Dept head, Dr. Danuta Mielżyńska- Švach Molecular biology 2024/2025  Areas of cell study Cytology Cytochemistry Cytopathology Cytophysiology Cytogenetics  The cell Cells are the smallest living structural and functional unit that comprise all organisms All cells are formed by the division of other cells (i.e., cell division) Cells contain genetic information that is passed on to daughter cells during cell division. All cells are made up of the same chemical compounds. All metabolic processes necessary for life occur in cells.  Types of Cells Prokaryotic Eurkaryotic Types of Cells  Eukaryotic organisms Single-cell: protozoa, Multi-cell some algae and fungi Fungi Plants Animals  Prokaryotic cell components Cell surface: ❑ cell membrane ❑ cell wall ❑ capsule (mucus) ❑ flagella, cilia ❑ pili, fimbriae Cell interior: ❑ cytosol ❑ nucleoid (equivalent of cell nucleus) ❑ ribosomes ❑ plasmids  Eukaryotic animal cell A eukaryotic cell consists of the following components each responsible for specific functions: ❑ cytoplasm (cytoplasmic matrix) ❑ cytoskeleton ❑ nucleus ❑ endoplasmic reticulum ❑ mitochondria ❑ Golgi apparatus ❑ lysosomes ❑ peroxisomes  Cell components All organisms are composed of two types of chemicals: inorganic and organic. Inorganic compounds mainly constitute the inanimate part of nature. Organic compounds occur almost only in living organisms or their remains.  Inorganic components Inorganic components that build cells include: ❑ Chemical elements: ❑ Macroelements - at least 0.01% of cell mass (0.1mg/1g) ❑ Microelements - between 0.01 - 0.00001% of cell mass (0.1 mg/1g to 0.1 µg/g) ❑ Trace elements – occur in the µg/g range in cells but intake required is on the scale of mg/g ❑ Ultratrace elements - occur in the µg/g range and require µg/g of dietary intake ❑ Water (~70%)  Chemical elements Macroelements Microelements Carbon (C) Iron (Fe) Hydrogen (H) Silicon (Si) Oxygen (O) Copper (Cu) Nitrogen (N) Manganese (Mn) Phosphorus (P) Fluorine (F) Sulphur (S) Iodine (I) Potassium (K) Boron (B) Sodium (Na) Molybdenum (Ml) Magnesium (Mg) Zinc (Zn) Ultraelements: radium (Ra), silver (Ag) and gold (Au)  Water The main component of every organism (on average 70 - 80% of the content of a living cell) Essential for proper functioning of the body Solvent for many chemical compounds (solutes) and an environment for all reactions Substrate and product of many chemical reactions Biological functions of water are due to its chemical structure and properties The structure of a water molecule A water molecule consists of one oxygen atom and two hydrogen atoms. A specialized dipole-dipole force known as a hydrogen bond exists between the oxygen and hydrogen A hydrogen bond is inherently polarized due to the difference in electronegativity between the two atoms. The uneven distribution of charges causes the water molecule to be dipolar. The attraction of hydrogen atoms (∂+) by oxygen atoms (∂-) causes water molecules to combine into larger groups (i.e., association) The structure of a water molecule The structure of the carbon atom The nucleus of a carbon atom contains: ❑ 6 protons (red) ❑ 6 neutrons (blue) The carbon atom has two electron shells: ❑ I (K shell) – contains 2 electrons, ❑ II (L shell) – contains 4 electrons.  The structure of the carbon atom A carbon atom contains four valence electrons and four vacancies for electrons from other elements. Carbon has a unique role in the cell because of its ability to form strong covalent bonds with other carbon atoms: C-C. Thus carbon atoms can join to form: ❑ chains ❑ branched structures ❑ rings Organic compounds consist of carbon atoms bonded to at least one other element. Namely, hydrogen and oxygen, as well as, nitrogen, sulfur and phosphorus, among others.  Organic components Cells contain four major families of small organic (containing carbon and hydrogen) molecules: ❑ saccharides ❑ fatty acids ❑ amino acids ❑ nucleotides The small organic molecules of the cell are carbon compounds that contain up to 30 or so carbon atoms. They are usually found free in solution in the cytosol. Monomer subunits construct the cell’s macromolecules (polymers). Organic components  Carbohydrates Carbohydrates consist of saccharides of which mainly contain carbon, hydrogen and oxygen. ⚫ one molecule - monosaccharides (glucose, fructose) ⚫ two molecules - disaccharides (sucrose, lactose, maltose) ⚫ several (up to 10) molecules – oligosaccharides (raffinose) ⚫ many molecules – polysaccharides (cellulose, starch) The longer the carbon chain, the less soluble the carbohydrate is in water Sucrose Raffinose Glucose  Carbohydrate function Energy storage / production: ❑ glycogen in animals ❑ starch in plants Structure: ❑ cellulose in cell walls of plants ❑ chitin in the cell walls in fungi ❑ ribose and deoxyribose sugars of DNA and RNA ❑ modifiers of proteins Transport: ❑ glucose in animals and humans ❑ sucrose in plants  Fatty acids Fatty acids usually contain an even number of carbon atoms (14 to 24) Fatty acids have a carboxyl group (acid) connected to a hydrocarbon chain (fat) The shorter the chain, the more fluid the fatty acid Saturated fatty acids only have single bonds Unsaturated fatty acids have one or more double bonds Fatty acids  Lipids Lipids are esters of fatty acids bonded to alcohols. Examples include: ❑ glycerol ❑ sphingosine ❑ higher monohydric alcohols (>6 C atoms) Insoluble in water due to the low ability to polarize under the influence of water  Types of lipids Steroids, i.e. a fats composed of 4 rings (sterane) Simple lipids are esters formed from alcohols and fatty acids: ❑ Fats and oils (triglycerides) ❑ Waxes (esters with non-glycerol alcohols) Complex lipids are made of an alcohol, fatty acids and another molecule, such as: ❑ phosphoric acid - phospholipid ❑ carbohydrate - glycolipid  Lipid functions Structural: building blocks of biological membranes (e.g., phospholipid bilayer, rigidity of plasma membrane) Energy storage: Energy reserves: ❑ in animals - stored as subcutaneous tissue, mainly in hibernators (e.g., squirrel, bear and badger) ❑ in plants - in seeds (e.g., sunflower, soybean and rapeseed), fruits and roots Signaling: steroid hormones, vitamins A and D.  Lipid functions Protection - Fat reserves protect: ❑ the eyeballs, kidneys and other abdominal organs from mechanical injuries in animals​, ❑ leaves and fruits of many plants from excessive water loss, in the form of wax coverings, ❑ marine mammals (e.g., seals, whales and walruses) from low temperatures. Cell composition  Cell structure The internal cell environment is separated from the extrernal environment by a cell membrane (plasma membrane) or by an additional cell wall (e.g., some bacteria, plant cells) The internal environment of the cell is known as the cytoplasm containing cytosol and organelles Organelles (little organs) are either membrane bound or non- membrane bound  Cell structure: cell membrane Every cell and all of its organelles are surrounded by a cell / plasma membrane All cell membranes, both extracellular and intracellular, consist of the following components: ❑ lipids ❑ proteins ❑ sugars  Cell structure: cell membrane Functions of the cell membrane: ❑ protect from physical, chemical and biological factors ❑ react to chemical, thermal and mechanical stimuli ❑ enzymatic - catalysis of various metabolic reactions ❑ regulate transport of substances into and out of the cell ❑ maintain the balance of osmotic pressure between the inside and the outside of the cell  Cell structure: cytoplasm The cytoplasm is a colloidal solution (i.e., a solution in which the particles of dissolved substance are: ❑ too small to settle under the influence of gravity, ❑ too large to dissolve in water and form a proper solution.) There are two phases in cytoplasm: ❑ dispersive - water (90% of the volume of cytoplasm) ❑ dispersed - substances suspended in water (approx. 9% organic compounds, and approx. 1% mineral compounds)  Cell structure: cytoplasm Cytoplasm functions: ❑ fill the cell and give it shape ❑ environment for suspending cell organelles ❑ site of metabolic reactions ❑ move organelles and transport substances in the cell thanks to movement of the cytoplasm  Cell structure: cytoplasm Cytoplasm is ductile and viscous (high protein content). Cytoplasm occurs in two states of aggregation: ❑ semi-liquid (sol), ❑ semi-solid (gel). Cytoplasm has the ability to move: ❑ rotationally - around a [usually] centrally located vacuole ❑ circulating - between organelles ❑ pulsating - in different directions ❑ fountaining - flows around two vacuoles in opposite directions  Cytoplasmic movement rotationally circulating pulsating fountaining vacuole cell membrane direction of cytoplasmic flow  Cell structure: cytoskeleton All cells have to be able to rearrange their internal components as they grow, divide, and adapt to changing circumstances. These spatial and mechanical functions depend on a system of filaments called the cytoskeleton. The three families of protein filaments are: ❑ intermediate filaments (diameter from 8 to 10 nm) ❑ microtubules (diameter of about 25 nm) ❑ actin filaments (diameter of about 7 nm) Cytoskeleton  Cytoskeleton actin filaments microtubules intermediate filaments  Cytoskeleton Intermediate filaments are made of tissue-specific proteins (keratin, vimentin, etc.) of which: ❑ give cells resistance to mechanical damage, stretching, and crushing ❑ help maintain a specific cell shape ❑ build the cell nuclear lamina  Cytoskeleton Microtubules are composed of tubulin (a globular protein). They: ❑ build centrioles and the mitotic spindle ❑ are responsible for transport within the cell ❑ form cilia and flagella (e.g. movement of microvilli) In cells that do not divide (i.e., neurons, myocytes, RBCs, WBCs), microtubules group together in a region called the centrosome  Cytoskeleton Actin filaments (microfilaments) are made up of actin. They: ❑ provide mechanical support for the cell and various cell organelles ❑ are involved in the movement of cytoplasm and organelles ❑ enable creeping movement and cell shape change ❑ participate in the contraction of muscle cells Cell organelles (eukaryotic, animal)  Cell structure: organelles Membrane-bound organelles are divided into: ❑ double membrane-bound ❑ single membrane-bound Organelles surrounded by a double membrane are the: ❑ nucleus - contains genetic information ❑ mitochondria - the site of cellular respiration ❑ chloroplasts - a group of organelles found in plant cells  Cell structure: organelles Organelles surrounded by a single cell membrane: ❑ Golgi apparatus – modifies proteins, secretes various substances ❑ lysosomes - contain digestive enzymes ❑ peroxisomes - vesicles containing various compounds to breakdown peroxides ❑ endoplasmic reticulum (ER) - consists of a network of channels and flattened cisternae site of protein production ❑ vacuoles ❑ in animal cells they sequester waste products ❑ in plant cells they are the "garbage bins and warehouses” of the cell and sustain water balance  Cell structure: organelles Non-membrane bound organelles: ❑ cell wall - outer covering of some non-animal cells ❑ cytoskeleton - provides cell structure ❑ ribosomes - location of protein synthesis ❑ centrosome / microtubule organizing center – contain centrioles and microtubules important in cell division ❑ centriole – cylindrical organelle involved in spindle fibers creation in cell division  Nucleus Amount in a human cell: ❑ monokaryocytes, bikaryocytes, polykaryocytes, ❑ zero: erythrocytes and cells of the stratum corneum of the epidermis Size and shape: ❑ depends on the type of cell, age and functional state ❑ spherical, ellipsoidal, fragmented ❑ ~10% of the cell volume of mammalian cells Position: ❑ in the middle of the cell ❑ along the cell membrane  States of the nucleus The cell nucleus can be in three different states: ❑ interphase – between/preparation of cell division ❑ mitotic - during cell division ❑ metabolic – present in cells in the resting or G0 phase; directs metabolic processes, maintenance functions  Nucleus structure during interphase Components of the nucleus in interphase: ❑ nuclear envelope (membrane), ❑ nuclear matrix (nucleoplasm), ❑ nucleolus ❑ chromatin: ❑ condensed chromatin (heterochromatin) ❑ dispersed chromatin (euchromatin) Structure nucleus during interphase  Nucleolus Nucleolus The nucleus usually contains one nucleolus, unseparated from the nucleoplasm (no membrane) It consists of fragments of five chromosomes, containing DNA responsible for the synthesis of ribosomal RNA (rRNA) and ribosomal subunits. These regions are called nucleolar organizers (NORs) In humans, there are 10 NORs, which are located on the short arms of chromosome pairs: 13, 14, 15, 21, and 22 Nucleolus in interphase  Functions of the nucleus Functions of the nucleus: ❑ site of DNA synthesis - replication of genetic information before nuclear division ❑ site of synthesis of RNA from DNA (transcription) ❑ site of formation of ribosomes – the structures responsible for protein synthesis (translation)  The endoplasmic reticulum The endoplasmic reticulum is a system of single-layer membranes that form a network of cisternae, channels and vesicles. It ensures: ❑ enlargement of the internal surface area of the cell ❑ division of the cytoplasm into compartments ❑ determines the route of transport of organelles, substrates and products  The endoplasmic reticulum Smooth endoplasmic reticulum (agranular endoplasmic reticulum): ❑ lacks ribosomes ❑ Place of synthesis of lipids and steroids, removal of toxic substances, internal transport Rough endoplasmic reticulum (granular endoplasmic reticulum): ❑ contains ribosomes for protein synthesis, modification, and quality control ❑ connects the outer nuclear membrane with the cell membrane and organelle membranes The endoplasmic reticulum Smooth endoplasmic reticulum Rough endoplasmic reticulum  Ribosomes Ribosomes are made up of ribosomal RNA (rRNA) and proteins. There are two types of ribosomes in eukaryotes: ❑ free ribosomes - freely float in the cytoplasm, which produce proteins that function in the cytosol ❑ ribosomes associated with the endoplasmic reticulum - produce proteins that undergo post-translational modification and are exported from the cell ❑ ribosomes found in the mitochondria and chloroplasts are smaller and similar to ribosomes found in bacteria  Ribosomes Each ribosome is made up of two subunits that fit together: ❑ small ❑ large Ribosomes are divided into: ❑ small - prokaryotic (70s) ❑ large - eukaryotic (80s) Ribosomes  Mitochondria ❑ the number of mitochondria in a single cell depends on the organism, type of cell and the energy requirements of a given cell ❑ vary in size (2 to 8 μm) ❑ they can quickly change shape and size (filamentous, granular and branched) ❑ new mitochondria are created by division of existing ones  Mitochondria The number of mitochondria in various cells: ❑ epidermal cells: 2 to 6 ❑ sperm cells: 20 to 50 ❑ liver cells: 1,000 to 2,500 ❑ skeletal muscle fibers: up to 1,600 ❑ skin cells: ~2,000 ❑ nerve cells: 10,000 ❑ ova: >100,000  Mitochondrial structure Mitochondrial structure: ❑ two-layer membrane ❑ the outer membrane is smooth and allows many substances to pass through on the basis of passive transport ❑ the inner membrane allows only selected compounds to pass through (facilitated diffusion) ❑ there is an intermembrane space between the outer and inner membranes ❑ the inner membrane separates the intermembrane space from the mitochondrial matrix and has many folds called cristae Mitochondrial structure Mitochondrial structure  Mitochondrial structure Inside a mitochondrion is the mitochondrial matrix, which contains: ❑ double-stranded, circular mitochondrial DNA (mtDNA) ❑ ribosomes (70S) ❑ enzymes necessary for the production of ATP A single human mitochondrion: ❑ contains four to ten mtDNA molecules ❑ a single mtDNA molecule is packed into mtDNA-protein complexes called nucleoids, which are ellipsoidal in shape Mitochondrial structure Green: nucleoids Gray-blue: inner membrane Gray: outer membrane  Mitochondrial functions Mitochondrial functions: ❑ aerobic respiration - the Krebs cycle and the electron transport chain ❑ production of adenosine-5′-triphosphate (ATP) - a carrier of chemical energy used in cell metabolism ❑ delivery of ATP to other parts of the cell (mitochondria can move in the cytoplasm) Mitochondrial functions  Mitochondrial division Mitochondria divide in a manner similar to that of bacteria cells mitochondrial Fragmentation  The Golgi apparatus The Golgi apparatus is composed of: ❑ highly flattened, arched cisternae (3 - 20) ❑ separating vesicles The Golgi apparatus is composed of: ❑ cis cisternae (beginning) ❑ medial cisternae ❑ trans cisternae (end)  The Golgi apparatus The cis cisterna face towards the nucleus/ER where vesicles with substrates intended for processing enter. The trans cisterna face the cell membrane where vesicles with the finished product are released to head towards organelles or the cell membrane The Golgi apparatus receives components from: ❑ the perinuclear endoplasmic reticulum ❑ the cell membrane ❑ endosomes The Golgi apparatus  The Golgi apparatus Golgi apparatus functions: ❑ post-translational modification of proteins and lipids for export ❑ linking carbohydrates to proteins, fats, and nucleosides ❑ sulfation of proteins and proteoglycans ❑ recycling of the cell membrane after endocytosis  The Golgi apparatus All secretory proteins, hydrolases and some integral membrane proteins are synthesized in their respective precursor forms The maturation of the precursor forms of proteins takes place in the Golgi apparatus coined controlled proteolysis The mechanism of proalbumin and preproinsulin maturation is well known The Golgi apparatus preproinsulin proinsulin insulin  Lysosomes Lysosomes have different shapes and sizes depending on cell type and function: ❑ macrophages - several microns ❑ hepatocytes and neurons - 0.5 to 1 μm Lysosomes are usually spherical or oval vesicles surrounded by a single membrane The number and location of lysosomes may differ even in cells of the same tissue. In hepatocytes and fibroblasts, lysosomes occupy up to ~0.5% of the cytoplasmic volume and up to 2.5% in macrophages  Lysosomes There are about 40 hydrolytic enzymes (acid hydrolases) in lysosomes, which catalyze intracellular digestion reactions Enzymes within lysosomes function in acidic environments (pH 5) A low pH environment is created by the transmembrane H + - ATPase / proton pump The lysosome membrane is resistant to acid hydrolases because it contains a range of unique proteins Structure of the lysosome  Types of lysosomes Lysosomes are categorized into: ❑ primary lysosomes: formed in the membranes of the rER and bud from the membrane of the Golgi apparatus ❑ secondary lysosomes: formed after the fusion of primary lysosomes with: ❑ endosomes ❑ autophagosomes  Types of lysosomes Secondary lysosomes are divided into: ❑ autolysosomes ❑ heterolysosomes Autolysosomes are formed by the fusion of a cell's own fragments (e.g., a damaged organelle) with a primary lysosome Autolysosomes participate in two processes: ❑ autophagy – destruction of damaged organelles ❑ autolysis – digestion of own dying or dead cells  Types of lysosomes Heterolysosomes (endosomes) are formed by the fusion of primary lysosomes with vesicles containing material taken in the cell by endocytosis (an endosome) Endosomes are subdivided depending on the type of material collected: ❑ phagosomes ❑ pinosomes In secondary lysosomes, decomposition byproducts are formed: simple sugars, amino acids, nucleotides of which can be used to synthesize other compounds in the cytosol Types of lysosomes  Perixosomes Peroxisomes are oval or spherical organelles surrounded by a single cell membrane Diameter ranges between 0.2 to 1.8 µm Number, morphology and physiological role depends on the type of cell, tissue and the stage of development, as well as, cellular stress Most abundant in liver, kidney and nervous tissue The granular matrix of peroxisomes may contain a crystalline core called the nucleoid, which may take on various forms depending on the species and type of tissue. Peroxisome structure matrix cell membrane nucleoid  Peroxisomes Peroxisomes are responsible for over 60 catabolic and anabolic processes and the enzymes enclosed in peroxisomes are responsible for: ❑ decomposition / reduction of toxic chemical compounds (detoxification) like ethanol to acetaldehyde ❑ β-oxidation reactions of fatty acids like oxidizing long- chain molecules (C22) to C8 molecules, ❑ α-oxidation reactions of branched fatty acids to create linear molecules composed of 8 carbon atoms  Peroxisomes ❑ cholesterol synthesis independent of the endoplasmic reticulum ❑ bile acid synthesis ❑ plasmalogen synthesis - part of the myelin sheath of neurons A byproduct of alcohol oxidation, β- and α-oxidation of fatty acids is hydrogen peroxide (H2O2), which is then broken down in peroxisomes by catalase or peroxidases.  Peroxisome formation Peroxisomes are formed: ❑ de novo from preperoxisomes ❑ as a result of division of pre-existing organelles De novo peroxisome formation involves the detachment of vesicles from the endoplasmic reticulum and mitochondria The resulting preperoxisomes recruit numerous enzymes, peroxins and integral membrane proteins The preperoxisomes fuse with each other and become a mature peroxisome  Peroxisome formation From pre-existing peroxisomes: ❑ the peroxisome takes the form of a tube ❑ a tightening ring is formed around the tube ❑ two daughter structures are created Peroxisome formation  Centrosome The centrosome (diplosome) is a structure found near the cell nucleus and the Golgi apparatus. It consists of two centrioles made of microtubules arranged in the form of cylinders. In the period preceding cell division, the centrosome duplicates itself and two centrosomes are formed (each with two centrioles), which move to opposite poles of the cell. Plant cells  Plant cells Compared to an animal cell, a plant cell contains additional components, such as: ❑ living (plasmic) components ❑ plastids ❑ dead (nonplasmic) components ❑ cell wall ❑ vacuole  Plastids Plastids are ovalur organelles surrounded by a double cell membrane They have plastid DNA and ribosomes All plastids arise from plastid precursors known as proplastids.  Types of plastids Chloroplasts contain the green pigment chlorophyll, which permits photosynthesis Chromoplasts contain xanthophyll and carotenoids Leucoplasts have an irregular shape and are colorless Leucoplasts perform storage functions and are divided into: ❑ proteinoplasts: contain proteins in the form of aleurone grains ❑ amyloplasts: contain carbohydrates - in the form of starch grains ❑ lipidoplasts: contain fats  Plastids Examples of plastids chromoplasts chloroplasts leucoplasts Chloroplast structure  Cell wall Plant eukaryotic organisms have a multi-layered cell wall made of cellulose or chitin Cellulose is a polymer of glucose composed of carbon, hydrogen and oxygen Chitin is a polymer of N-acetylglucosamine, which contains nitrogen in addition to carbon, hydrogen and oxygen There are two types of cell walls: ❑ Primary: made up of cellulose and pectin (a polysaccharide) that is formed during cell growth ❑ Secondary: made up of cellulose and lignin (a phenolic substance) that is produced after growth is completed  Functions of the cell wall Functions of the cell wall: ❑ gives shape and rigidity to the cell ❑ limits cell growth ❑ protects against: ❑ mechanical injuries ❑ bacterial, fungal and viral infections ❑ excessive evaporation  Plant vacuole The vacuole is surrounded by a single membrane, the tonoplast and the interior is filled with a solution called cell sap. Cell sap components: ❑ water (90%) ❑ ions (potassium, sodium, calcium, magnesium, zinc, sulfate, chloride) ❑ proteins (aleurone grains and amino acids) ❑ sugars (glucose, fructose in fruit, sucrose in sugar beets) ❑ organic acids  Vacuole functions Vacuole functions: ❑ maintain constant cell firmness (turgor pressure) ❑ store reserve materials ❑ gather unnecessary metabolic products Comparison  Endosymbiotic theory PROKARYOTIC cells (3.5 billion years ago) EUKARYOTIC cells (1.7 billion years ago) Explains the origin of mitochondria and chloroplasts in eukaryotic cells  Endosymbiotic theory The endosymbiotic theory assumes that eukaryotic organelles evolved from prokaryotic cells Evidence supporting the endosymbiotic theory: ❑ mitochondria and chloroplasts contain circular DNA molecules with a structure and size similar to that of bacterial DNA ❑ mitochondria and chloroplasts are formed by division; the cell does not create them de novo  Endosymbiotic theory ❑ mitochondrial and chloroplast ribosomes are similar to that of bacterial ribosomes, ❑ N-formylmethionine is the first amino acid in all proteins produced by the mitochondria and by chloroplasts like in bacteria Endosymbiotic theory Endosymbiotic theory  Cell metabolism Metabolism is the entirety of all biochemical reactions occurring in cells of living organisms It is the circulation of matter, energy and information that provides the organism reception of stimuli, growth, movement, reproduction, etc. There are two directions of metabolic changes: ❑ anabolic ❑ catabolic Directions of cell metabolism  Anabolism Anabolism involves the synthesis of complex organic compounds from simple compounds. Anabolic reactions require energy input More energy is stored in the products than in the respective substrates The energy is stored in the form of chemical bonds. energy substrate 1 + substrate 2 product  Catabolism Catabolism is the breakdown of complex organic compounds into simple products The products of catabolic reactions contain less energy than the respective substrates The energy released in catabolic processes is stored in bonds of energy carriers (e.g., adenosine triphosphate (ATP)) substrate product 1 + product 2 + ATP Cellular respiration  Cellular respiration Cellular respiration is the breakdown of organic compounds into inorganic compounds (i.e., CO2 and H2O) for energy In the cytoplasm, glycolysis occurs where glucose molecules are broken down into pyruvic acid molecules (1:2 ratio) with no net production of ATP – oxygen is present, first step of aerobic respiration In the mitochondria, pyruvic acid and intermediate compounds are oxidized into end products (e.g., water, carbon dioxide) Intracellular respiration  References Fundamentals of Cell Biology, Volumes 1 and 2, B. Alberts, D. Bray, K. Hopkin et all.