IB Biology UNIT 1 - Eukaryotic and Prokaryotic Organisms PDF

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Pearson 2nd level biology notes - First Unit focusing on the study of Eukaryotic and Prokaryotic Organisms...

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1.1 The evolution of multicellular organisms allowed cell specialization and cell replacement Cell Theory According to the cell theory: 1. Living organisms are composed of cells (or cell products) 2. The cell is the smallest unit of independent life 3. Cells can only arise from pre-existing cells Caveats (Certain organisms do not reproduce sexually) to the cell theory include: ➔ Striated muscle – composed of fused cells that are multinucleated ➔ Giant algae – unicellular organisms that are very large in size (~7 cm) ➔ Aseptate hyphae – lack partitioning and have a continuous cytoplasm

Functions of Life Living things obtain and use energy Living things reproduce Living things respond to their environment They are able to maintain homeostasis Living things are made of cells Living things grow and develop Living things adapt to their environment

< Diagram of a Paramecium -Single-celled organism -Demonstrates all the characteristics of life -Found in bodies of freshwater

< Diagram of a Chlorella A common freshwater organism. This organism has been used by many researchers to determine the details of, and the factors that affect, a process known as photosynthesis. The structure labeled chloroplast is especially important in this process. Cellular Organization

In multicellular organisms: • Cells may be grouped together to form tissues • Tissues may interact to form functional organs • Organs may combine to form body systems Typical levels of organization that one finds in the literature include the atomic, molecular, cellular, tissue, organ, organismal, group, population, community, ecosystem, landscape, and biosphere levels

Microscopes Light microscopes use lenses to bend light • Can view living specimens in natural colour • Have lower magnification and resolution Electron microscopes use electromagnets to focus electrons • Can only view dead specimens in monochrome • Have higher magnification and resolution • Can show cross-sections (TEM) or surface renderings (SEM)

Stem Cells Stem cells are unspecialized cells that have 2 key qualities:

1. Self-Renewal – They can continuously divide and replicate 2. Potency – They have the capacity to differentiate There are 4 main types of stem cells during human development: • Totipotent – Can form any cell type, as well as extra-embryonic tissue • Pluripotent – Can form any cell type (embryonic stem cells) • Multipotent – Can differentiate into closely related cell types • Unipotent – Cannot differentiate, but are capable of self-renewal

Stem Cell Therapy Stem cells can replace damaged or diseased cells with healthy ones. The therapeutic use of stem cells involves: • Harvesting stem cells from appropriate sources • Using biochemical solutions to trigger cell differentiation • Surgically implanting new cells into patient's own tissue • Suppressing the host immune system to prevent rejection • Monitoring new cells to ensure they do not become cancerous

1.2 Eukaryotes have a much more complex cell structure than Prokaryotes Prokaryotic Cell Structure Prokaryotes are organisms whose cells lack a nucleus • They belong to the kingdom Monera (i.e. bacteria) Prokaryotic cells share the following structures: • A single, circular DNA molecule (genophore) • A peptidoglycan cell wall and 70S ribosomes Prokaryotic cells may also contain the following: • Pili (for attachment or bacterial conjugation) • Flagella (a long whip-like tail for movement) • Plasmids (autonomous DNA molecules)

Binary Fission (Bacterial Cell Division) Prokaryotes divide via a process of asexual reproduction known as binary fission. In this process • The circular DNA is copied • The DNA loops attach to the membrane • The cell elongates, separating the loops • Cytokinesis occurs to form two cells Prokaryotic VS Eukaryotic Cells Prokaryotic and eukaryotic cells differ according to a number of key features: • DNA (composition and structure) • Organelles (types present and sizes) • Reproduction (mode of cell division) • Average Size (exceptions may exist)

Eukaryotic Cell Structure

Animal versus Plant Cells Organelles found only in specific cell types include: • Chloroplasts – Site of photosynthesis (plant cells only) • Lysosomes – Breakdown of macromolecules (animal cells)

Roles of organelles in the Cell Organelles are compartmentalized structures that serve specific purposes. Examples of eukaryotic organelles include: • 80S ribosomes – Responsible for protein synthesis (translation) • Nucleus – Stores genetic information (site of transcription) • Mitochondria – Site of aerobic respiration (ATP production) • Endoplasmic reticulum – Transports materials between organelles • Golgi complex – Sorts, stores, modifies & exports secretory products • Centrosomes – Involved in cell division (mitosis and meiosis)

1.3 The structure of biological membranes makes them fluid and dynamic Phospholipid Bilayer Structure of Phospholipids: • Contain a polar (hydrophilic) head composed of phosphate (+ glycerol) • Contain two non-polar (hydrophobic) tails, each composed of a fatty acid chain • Hence, phospholipids are amphipathic (have hydrophilic and hydrophobic parts) Arrangement in Membranes: • Phospholipids spontaneously arrange into a bilayer • The hydrophilic phosphate heads face out into the surrounding solution, while the hydrophobic fatty acid tails face inwards and are shielded from the polar fluids

Properties of the Phospholipid Bilayer: • The bilayer is held together by weak hydrophobic interactions between the tails • Individual phospholipids can move within the bilayer (fluidity and flexibility) • Amphipathic properties restrict the passage of certain substances (semi-permeable)

Cholesterol Cholesterol is a fundamental component of animal cell membranes • It is not present in plant cell membranes (as they have a rigid cell wall) Cholesterol reduces membrane fluidity and permeability to some solutes • It also anchors certain peripheral proteins and prevents crystallization

Membrane Proteins Membrane proteins are diverse in terms of their structure and position in a membrane Membrane proteins serve many functions: • Junctions • Enzymes • Transport • Recognition • Anchorage • Transduction

Fluid Mosaic Model Cell membranes are represented as a fluid-mosaic model • Fluid – membrane components can move the position • Mosaic – phospholipid bilayer is embedded with protein

This model was proposed by Singer-Nicolson in 1972, following the falsification of the Davson-Danielli model

1.4 Membranes control the composition of cells by active and passive transport Properties of Membranes Cell membranes have two key properties • Semi-permeable (only certain things can cross) • Selective (membranes can regulate material passage) Types of Membrane Transport Membrane transport can either be: • Passive (along a concentration gradient, no ATP expenditure) • Active (against a concentration gradient, ATP is required) Passive Transport Simple Diffusion The net movement of particles from a region of higher concentration to a region of lower concentration (i.e. along the gradient) until equilibrium is reached • Involves small/lipophilic molecules (e.g. O2, CO2, steroids) Facilitated Diffusion The passive movement of molecules across a cell membrane via the aid of a membrane protein (carrier / channel protein) • Involves large / charged molecules (e.g. ions, glucose, etc.) • E.g. Voltage-gated channels control the flow of ions in neurons

Osmosis The net movement of water molecules across a semipermeable membrane from a region of low solute

concentration to a region of higher solute concentration (diffusion of free water molecules)

Osmolarity Osmolarity is a measure of solute concentration • Hypertonic: High solute concentration (gains water) • Hypotonic: Low solute concentration (loses water) • Isotonic: Same solute concentration (no net flow)

1.5 There is an unbroken chain of life from the first cells on Earth to all cells in organisms alive today Abiogenesis The formation of living cells from non-living materials (abiogenesis) is theorized to involve 4 four key processes: • Non-living synthesis of simple organic molecules • Assembly of organic molecules into complex polymers • Formation of polymers that can self-replicate • Packaging of molecules into membranes to create internal chemistry different from the surroundings The Miller-Urey experiment replicated the conditions of a pre-biotic Earth in order to synthesize organic molecules Biogenesis Abiogenesis requires specific conditions in order to proceed • Including a reducing atmosphere (no oxygen) and either high temperatures (>100ºC) or electrical discharges As these conditions no longer commonly exist on Earth, cells can only be formed from the division of pre-existing cells This law of biogenesis was demonstrated by Louis Pasteur

• Broths were stored in sealed vessels that were sterilized • Bacterial growth occurred if the vessel was unsealed, but did not occur if the vessel stayed sealed (no contamination) Endosymbiosis Eukaryotic cells are believed to have evolved from aerobic prokaryotes that were engulfed by endocytosis The engulfed cell remained undigested and contributed new functionality to the engulfing cell (i.e. it became an organelle)

Chloroplasts and mitochondria arose via endosymbiosis: • Membranes (have a double membrane) • Antibiotics (show susceptibility) • DNA (have naked and circular DNA) • Division (occurs via a fission-like process) • Ribosomes (have 70S ribosomes)

1.6 Cell division Cell Cycle The cell cycle is an ordered set of events that culminates in cell division Interphase An active phase of the cell cycle where many metabolic reactions occur • Consists of G1, S, and G2 stages M phase The period of a cell cycle in which the cell and contents divide • Consists of mitosis (P, M, A, T) and cytokinesis Some cells may also enter a non-proliferative quiescent phase (G0)

Interphase Normal metabolism cannot occur during M phase, so key events must occur during interphase to prepare for division: • DNA replication (during S phase) • Organelle duplication • Cell growth • Transcription / translation • Obtaining nutrients • Respiration (cellular) Supercoiling During mitosis, chromatin condenses via supercoiling to become tightly packed chromosomes • Due to replication (S phase), chromosomes consist of identical sister chromatids (joined at a centromere) Mitosis Mitosis is the division of a diploid nucleus into two genetically identical diploid nuclei. This process of cell cloning is needed for many important processes: • Tissue repair • Organism growth • Asexual reproduction • Development of embryos

Cytokinesis Cytokinesis is the process of cytoplasmic division, whereby a cell splits in two occurring concurrently with telophase and differs in plants and animals Animals: • Microtubules form a concentric ring and contract towards the centre (centripetal)

Plants: • Vesicles form at the cell centre and fuse outwards to form a cell plate (centrifugal) Mitotic Index The mitotic index is a measure of the proliferative status of a cell population (i.e. number of dividing cells). The mitotic index will be elevated during growth and repair processes and acts as a prognostic tool for cancer.

*Mitotic cells have no nucleus and have visible chromosomes Stages of Mitosis http://bioninja.com.au/summaries/1.6%20Stages%20of%20Mitosis.pdf

Cell Cycle Regulation Checkpoints A cell cycle contains numerous checkpoints that ensure the fidelity and viability of continued cell divisions G1 checkpoint • Monitors potential growth conditions (nutrients, etc.) • Assesses level of DNA damage (from UV, etc.) G2 checkpoint • Monitors state of pre-mitotic cells (suitable size, etc.) • Identifies and repairs any DNA replication errors Metaphase checkpoint • Ensures proper alignment (prevents aneuploidy) Cancer Cancers are diseases caused by uncontrolled cell division • The resulting abnormal cell growths are called tumors Tumor cells may remain in their original location (benign) or spread and invade neighboring tissues (malignant) Metastasis is the spread of cancer from an original site to a new body location (forming a secondary tumor)

Cancer Development Cancers can be caused by many different factors: Mutagens Mutagens are agents that change the genetic material of cells • These agents may be either physical (e.g. UV), chemical (e.g. arsenic) or biological in origin (e.g. certain viruses) • Mutagens that cause cancer are classified as carcinogens Genetics Most cancers are caused by mutations to two classes of genes: • Proto-oncogenes stimulate cell growth and proliferation • Tumor suppressor genes repress cell cycle progression Proto-oncogene mutations create cancer-causing oncogenes

Cell Death The death of a cell may occur by one of two mechanisms: Necrosis (uncontrolled ‘cell homicide’) • The cell loses functional control due to injury, toxins, etc. • There is a destabilization of the membranes, leading to swelling • The cell bursts and releases its contents (causing inflammation) Apoptosis (programmed ‘cell suicide’) • It is a controlled event triggered by mitochondrial proteins • Cell contents are packaged in membranous protrusions (blebs) • The cell fragments into apoptotic bodies which are recycled NECROSIS APOPTOSIS Disintegration Fragmentation...