# Unit 2: Biology ## Cellular Biology A person contains roughly 100 trillion cells - Cells are roughly 20 `μm` (micrometre, 10-6 m), around 250 cells / cm ## Cell Theory 1. All living things are composed of cells 2. Cells are the basic units of living organisms 3. All cells came from pre-existing cells ## Eukaryotic versus Prokaryotic Cells `Prokaryotic cell`: Meaning before/lacking nucleus `Eukaryotic cell`: Means complete nucleus |Factors|Prokaryotic|Eukaryotic| |:-----------|:----------|:---------| |DNA|In nucleoid region|Usually in membrane-bound nucleus| |Size|Usually smaller|Usually larger| |Organelles|Not membrane-bound, smaller|Membrane-bound, more complex| |Organization|Usually singlecelled|Often form multicellular organisms| |Metabolism|May not need oxygen|Usually need oxygen| ## Cellular Organelles ### Business Analogy | In a business | In a cell | | ------------- | --------- | | Building | Cytoplasm & Cytoskeleton | | Department Head | Organelles | | Boss | DNA | | Workers | Ribosomes & Enzymes | | Waste management | Lysosomes | | Storage | Vacuoles | | Powerhouse | Chloroplasts & Mitochondria | | Security gate | Cell membrane controls entrance and exit from the cell | | Transportation Department | IMPORT: Cell membranes and vesicles, INTERNAL: Endoplasmic reticulum & vesicles, EXPORT: Golgi apparatus, vesicles, and cell membrane | ### Cell Membrane - Controls what substances enter/leave the cell selectively via various receptors/osmosis - Allows nutrients to enter - Allows waste products to leave - Surrounds and holds other organelles in cell - Interact with outside chemicals (e.g., hormones) ### Nucleus - Holds deoxyribonucleic acid (DNA) in form of `chromatin` - DNA is a double helix containing genes - `Genes` are a full set of instructions in DNA to make either RNA or a protein - `Chromatin` is DNA wrapped tightly in protein - `Chromosomes` are even more tightly wrapped chromatin used in cell division only - Surrounded by a double membrane - Substances enter and exit the nucleus via nuclear pores - Messenger RNA (mRNA) is encoded from DNA and sent to `ribosomes` to produce proteins - Humans have ~2 m of genes per cell per nucleus tightly wrapped ### Nucleolus - Dense region of DNA located in the nucleus - This area of DNA is specially for ribosomal DNA (rDNA), or DNA used to make ribosomes - Produce "large" and "small" subunits of ribosomes, which either form complete ribosomes in cytosol or mix with endoplasmic reticulum, forming rough endoplasmic reticulum (RER) ### Cytoplasm & Cytosol - `Cytosol` is the fluid cells contain - **All organelles** are suspended in `cytosol` - `Cytoplasm` is the `cytosol` along with everything in a cell, excluding the nucleus ## Endoplasmic Reticulum - The endoplasmic reticulum is a network of tubules and flattened sacs with a "rough" appearance because of the presence of `ribosomes` on the surface - Network of tubules and flattened sacs - **Transports** proteins via cytoskeleton in vesicles ## Specific to Rough ER - Appears "rough" due to the `ribosomes` attached to its outer surface - Located directly adjacent and attached to nucleus - Located next to `Golgi apparatus` - `Ribosomes` in rough ER **synthesize proteins** - Transports synthesized proteins to `Golgi apparatus` for packaging and distribution - About half the cell's proteins are produced here - Folds, fixes and **modifies both newly-created and pre-existing proteins** somewhat like **proof-reading** ## Specific to Smooth ER - Does not synthesize proteins - Appears "smooth" due to lack of `ribosomes` - Located directly adjacent and attached to nucleus - Synthesizes lipids (fats, e.g., cholesterol) - Metabolises carbohydrates ## Golgi Apparatus - Also known as Golgi body, Golgi complex, etc. - **Receives, modifies and transports** proteins that were produced by the rough ER - **Packages** proteins into `vesicles` and sends them `cell membrane` for export ## Lysosome - Spherical vesicle that containing `enzymes` - **Digests and kills** foreign matter which is then excreted - E.g., white blood cells use lysosomes to kill bacteria then spit it out - **Digests and breaks down** old and unused material/non-functional organelles as needed - If lysosome ruptures everything dies, hence they are known as "suicude sacs" ## Mitochondria - **Singular form is "mitochondrion"** - Contains an inner and outer membrane - Processes glucose + oxygen gas to form carbon dioxide + adenosine triphosphate (ATP) - ATP allows proteins to do things (e.g., spend 1 ATP break 1 molecule) - ATP cannot be stored ## Cytoskeleton - Made of protein filaments - Maintains and changes cell structure, much like a human skeleton + muscular system - Moves cells - Modifies and adjusts cell structure as needed - Chemicals can travel along cytoskeleton, e.g., organelles, vesicles, etc. ## Organelles specific to animal cells ### Centrioles and centrosomes - Made of same protein as cytoskeleton - A centrosome is a pair of centrioles - Important to cell division in animal cells - Create cilia and flagella (e.g., small hair-like things that swipe dirt out of lungs and tails of sperm, respectively) ### Lysosomes ## Organelles specific to plant cells ### Cell wall - Provides structure and prevents cell rupture - Can be tough, flexible, and/or rigid - Made of cellulose (type of sugar) - Also present in most bacteria, fungi, and protists ### Chloroplast - The `solar panel` of the plant cell - Conducts photosynthesis - All chlorophyll is located in chloroplasts - Looks green ### Central Vacuole - Extremely large, may take up to 90% of volume in cell - Contains water - Maintains `turgor` pressure against cell wall (pushes against cell wall in all directions) - Maintains cell shape and resistance - Plant cells that lack turgor pressure (e.g., celery left in fridge) become flaccid ## Cell division ### Purpose #### Growth - Cells have maximum size before transportation of substances within cell becomes inefficient - Cells transport chemicals (e.g., nutrients) via `diffusion` #### Reproduction - Single-cellular organisms reproduce via division asexually - Multicellular organisms reproduce via combining two germ cells ("sex cells") that contain half the DNA each of two organisms - This is sexual #### Repair - Cells naturally die and need to be replaced - e.g., red blood cells, hair cells, skin, injuries ### Cell cycle - Interphase - G1 - S - G2 - Mitosis - Occurs only in eukaryotic cells - Prophase - Prometaphase - Metaphase - Anaphase - Telophase - Cytokinesis - G0 - Cell no longer divides - Outisde of cell cycle