Cell Structure and Function

• Living things are composed of cells which are the fundamental units of life, not all cells contain the same components

Cell Types

• Prokaryotic cells are less complex and lack a true nucleus or membrane-bound organelles
• Bacteria and archaea have a single prokaryotic cell
• Eukaryotic cells contain a nucleus and membrane-bound organelles
• Animals, plants, fungi, and protists are eukaryotic organisms
• Eukaryotic cells can be unicellular/multicellular, while prokaryotes are always unicellular

Cell Function

• Cells must obtain nutrients and expel waste to survive, facilitated by the cell membrane

Cell Membrane

• The cell membrane is composed of a phospholipid bilayer in a watery medium
• The membrane contains millions of phospholipid molecules with hydrophilic heads (facing outwards) and hydrophobic tails (facing inwards)
• The cell membrane controls what goes in and out of the cell
• Lipid and protein molecules surround the cell membrane

Fluid Mosaic Model

• Describes the structure of cell membranes, the structure keeps out damaging substances and allows needed chemicals in
• Phospholipids create the basic structure
• Lipid-soluble molecules like oxygen and carbon dioxide can easily pass through the membrane
• Larger molecules and water-soluble molecules need protein channels to enter

Passive Transport

• Does not require energy (ATP)
• Moves water through osmosis and ions through facilitated diffusion using protein channels
• Substances move down their concentration gradient (high to low concentration)
• Diffusion helps transport nutrients and wastes in and out of cells
• Osmosis is the diffusion of water across a semipermeable membrane

Active Transport

• Required for large, polar, and charged molecules like lipids and carbohydrates
• Requires both protein channels and ATP to move substances against the concentration gradient (low to high)

Additional Membrane Components

• Protein molecules are embedded in the lipid layers and are not fixed
• Carbohydrates attach to proteins/lipids on the outside, forming glycoproteins/glycolipids that act as hooks to capture molecules
• Carbohydrates play a role in the recognition of antibodies, hormones, viruses, etc
• Cholesterol provides stability to the cell membrane without affecting fluidity

Molecules In and Out of Cells

• Larger and polar molecules: need active transport due to size/concentration gradient
• Active protein pumps: pump substances in/out using ATP
• Carbohydrates, proteins, and fats: large molecules that cannot be readily absorbed, digestion is used to break these down
• Digestion breaks down large food molecules into smaller molecules, such as glucose, amino acids, and fatty acids
• Osmosis: diffusion of water molecules from a low concentration to a high concentration solution
• Osmosis: movement of water molecules from high to low concentration through a partially permeable membrane

Solutions

• Water molecules diffuse from pure water or a dilute solution to more concentrated solutions during osmosis
• Dilute solutions have a high concentration of water molecules
• Concentrated solutions have a low concentration of water molecules
• Plant/animal cells are surrounded by a semi-permeable plasma membrane, for water and small molecules to diffuse across
• Plant cells have a strong cell wall for support and protection

Homeostasis

• Animal cells need to maintain an isotonic water balance
• Water concentration inside and outside the cell are equal
• Kidneys control the concentration of water and salt in the blood, controlled by the hypothalamus

Vesicles

• Cell membranes can bulge outwards and pinch off to seal around substances, creating vesicles
• Vesicles transport things within, into, and out of the cell, formed from the cell membrane, endoplasmic reticulum, or Golgi apparatus
• Vesicle formation requires energy (active transport)
• Endocytosis: use of vesicles to transport substances into the cell (inside cell action)
• Exocytosis: use of vesicles to transport substances out of the cell (outside cell action)

Endocytosis Types

• Phagocytosis: endocytosis of solid particles
• Pinocytosis: endocytosis of liquid particles, importing liquid
• Receptor-mediated endocytosis: particles bind to a cell membrane receptor to initiate engulfment

Exocytosis

• Exports materials such as hormones and waste products outside of the cell
• Reverse of endocytosis: materials are expelled in vesicles that fuse with the membrane

Size Constraints

• Exchange of materials relies on diffusion/osmosis and is limited by cell size, cells are generally smaller than 1mm
• Cells must maintain a high Surface Area to Volume ratio (SA:V)

Autotrophs and Heterotrophs

• Autotrophs: supply their own food, convert inorganic to organic compounds storing chemical energy
• Producers: autotrophs start the food chains, not dependent on other organisms for nutrition
• Photosynthesis: most autotrophs use photosynthesis to make their own food (plants and cyanobacteria)
• Chemosynthesis: some autotrophs use chemosynthesis to obtain energy from inorganic substances
• Plants: autotrophs that perform photosynthesis within their chloroplasts
• Heterotrophs: depend on other organisms as sources of food, including animals, fungi, and heterotrophic bacteria
• Consumers: heterotrophs are the consumers in food chains and webs
• Decomposers: heterotrophs that break down the wastes and bodies of dead organisms

Evolution

• First cells were likely prokaryotic heterotrophic cells, used anaerobic respiration
• Prokaryotic autotrophs evolved later due to competition for inorganic molecules, had the advantage of producing their own food
• Eukaryotes evolved after autotrophic prokaryotes, endosymbiotic theory explains the evolution of eukaryotes (mitochondria and chloroplasts)

Photosynthesis

• Series of reactions that convert carbon dioxide and water into sugars and oxygen, requires chlorophyll and light
• Chloroplast: numerous flattened discs increase the surface area, allowing a higher rate of photosynthesis
• Chlorophyll molecules trap energy from sunlight and split water into oxygen
• Biochemical reactions controlled by enzymes

Photosynthesis Roles

• Transforms radiant energy from the Sun into chemical energy in organisms
• Releases oxygen and removes carbon dioxide from the atmosphere
• Drives the carbon/oxygen cycle and provides oxygen for respiration
• Autotrophs begin the food chain where energy is passed from one trophic level to the next trophic level

Cellular Respiration

• Series of chemical reactions that break down glucose in a cell to produce ATP that is used for energy
• Steps: Glycolysis, the Krebs Cycle (Citric Acid Cycle), and Oxidative Phosphorylation (Electron Transport Chain)
• Aerobic respiration: complete breakdown of molecules using oxygen to form carbon dioxide and water
• Oxidation: aerobic respiration commonly uses glucose to produce energy (ATP) for the cell in both autotrophs and heterotrophs
• Thirty-eight (38) ATP molecules are released per glucose molecule oxidized

Respiration Stages

• Glycolysis: glucose to two 3-carbon molecules (pyruvate), releases a small amount of energy and occurs in the cytoplasm
• Breakdown of the 3-carbon molecules into a 1-carbon molecule, carbon dioxide, uses oxygen and releases a larger amount of energy, occurs in the mitochondria
• Anaerobic respiration: occurs when there is no/insufficient oxygen, releases energy with 2 ATP produced from one glucose molecule via glycolysis

Chemiosmosis

• Process used to make ATP in anaerobic respiration
• Movement of ions across a selectively permeable membrane, down their electrochemical gradient

Enzymes

• Proteins that act as catalysts, speeding up chemical reactions
• Necessary for life as they enable cells to perform their necessary functions
• Enzymes are highly specific and act on only one type of substrate
• Made of protein constructed from amino acids (C, H, O, and N)
• 20 different amino acids exist in millions of different protein structures
• A chain of amino acids joined together is a polypeptide chain, then folds into a protein
• Catalysts speed up chemical reactions by lowering activation energy for the reaction, not used up in the reaction
• The substance the enzyme acts on is called the substrate, and once changed, it is called the product
• Enzymes function by binding to a substrate, causing a reaction that changes the substrate into a different molecule
• Enzymes are composed of amino acids arranged in a specific order

Enzyme Active Site

• Region where the substrate binds, its shape determines the substrate type that can bind
• Held together by weak bonds, allowing it to break apart when the substrate binds and forms a product
• The rate at which they catalyse a reaction is called the enzyme efficiency, measured as the amount of substrate used/product produced per second

Enzyme Efficiency

• Enzymes have a maximum rate at which they will catalyse their reaction when their conditions are at an optimum
• Denaturing: enzyme losing its shape and efficiency
• Denatured: if partially denatured, the enzyme can still work but its efficiency is reduced, can still return to its maximum if the conditions return (temperature and pH)

Enzyme Models

• Lock-and-key model: original model proposed that the active site is rigid and the small substrate molecule is reciprocally shaped, fitting exactly into the active site
• Induced-fit model: binding of a substrate to the active site induces the enzyme to alter its shape slightly, to fit more tightly around the substrate