AS Level Biology Notes (2015)

Chapter 01 - Cell Structure

01. Microscopy

Light Microscopes:

• Utilize visible light rays to magnify specimens, allowing cellular structures to be observed.

• Image is formed on the retina or screen, providing a view of living cells.

Electron Microscopes:

• Employ beams of electrons to achieve much higher magnifications than light microscopes.

• Specimens must be thinly sliced and placed under a vacuum to prevent scattering of electrons.

• Two types: Transmission Electron Microscope (TEM) for internal structures, and Scanning Electron Microscope (SEM) for surface topography.

02. Magnification and Resolution

• Magnification depends on resolution, which is the ability to distinguish between two close objects clearly.

• Electron microscopes provide superior resolution than light microscopes, enabling detailed observation of sub-cellular structures due to the shorter wavelength of electrons.

Units of Measurement: millimetre (mm), micrometre (µm), nanometre (nm).

03. Magnification Calculations

To calculate the real size of an object, use the formula:
ext{Real Size} = \frac{ \text{Image Size}}{ \text{Magnification}} .
Example: A drawing of a mitochondrion measuring 50mm at a magnification of 100,000x corresponds to a real size of 0.5 µm.

04. Measuring Cells Using a Graticule

• A graticule consists of a calibrated scale that overlays a microscope view, measured in graticule units, facilitating accurate size measurement.

• Example calibration: 80 graticule units equal 180 µm, allowing conversion between graticule units and actual micrometre measurements.

05. Cell Structure and Function

Differences between Animal and Plant Cells:

• Plant Cells: Possess rigid cell walls made of cellulose, chloroplasts for photosynthesis, and large vacuoles for storage and maintaining turgor pressure.

• Animal Cells: Contain lysosomes for digestion and centrioles that assist in cell division.

Organelles and their functions:

• Nucleus: Contains genetic material; is the command center regulating cellular activities and gene expression.

• Mitochondria: Known as the powerhouse of the cell, facilitates aerobic respiration and ATP production through the Krebs cycle and electron transport chain.

• Ribosomes: Sites of protein synthesis; may be free in the cytoplasm or attached to the rough endoplasmic reticulum.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Endoplasmic Reticulum (ER): Rough ER (studded with ribosomes) engages in protein processing; Smooth ER is involved in lipid synthesis, detoxification, and calcium storage.

06. Prokaryotic vs. Eukaryotic Cells

Prokaryotes:

• Typically smaller in size than eukaryotes; lack membrane-bound organelles and have circular DNA structure.

Eukaryotes:

• Larger, more complex cells with a defined nucleus housing linear DNA, along with various membrane-bound organelles like mitochondria and Golgi apparatus.

07. Summary of Cell Structure

• Cells are the fundamental units of life, visible through microscopy.

• Composed of genetic material (DNA), machinery for protein synthesis, and a variety of organelles.

• Eukaryotic cells can differentiate to form tissues, organs, and complex organ systems.

08. Cell Biology Questions

Includes a review of multiple-choice questions with detailed answers and explanations to reinforce learning outcomes and exam preparation strategies.

Chapter 02 - Biological Molecules

01. Carbohydrates - Monosaccharides and Disaccharides

• The backbone structure of carbohydrates is formed from carbon (C), hydrogen (H), and oxygen (O) atoms.

• Monosaccharides: Basic building blocks of carbohydrates; examples include trioses (glyceraldehyde), tetroses (erythrose), pentoses (ribose), and hexoses (glucose C₆H₁₂O₆).

• Disaccharides: Formed when two monosaccharides combine through a condensation reaction, creating a glycosidic bond; examples include sucrose (glucose + fructose) and lactose (glucose + galactose).

02. Carbohydrates - Polysaccharides

• Polysaccharides: Large macromolecules formed by extensive chains of monosaccharides.

• Common examples include glycogen (energy storage in animals), starch (energy storage in plants consisting of amylose and amylopectin), and cellulose (structural component in plant cell walls).

03. Testing for Carbohydrates

• Reducing Sugars: Identified using Benedict's test; a positive result displays a color change from blue to green/yellow/orange/red, indicating the presence of reducing sugars.

• Non-reducing Sugars: Require hydrolysis followed by Benedict's test to yield a result.

• Starch Testing: Iodine test yields a blue-black color, confirming the presence of starch in a sample.

04. Lipids

• Triglycerides: Composed of glycerol and three fatty acids; serve as energy storage molecules and insulation.

• Phospholipids: Consist of glycerol, two fatty acids, and a phosphate group; are essential components of biological membranes, facilitating membrane structure and function.

• Lipids Testing: The emulsion test produces a milky solution in the presence of lipids, indicating their presence in the sample.

05. Proteins - Structure and Function

• Proteins are polymers composed of amino acids linked through peptide bonds in a specific sequence that determines their structure and function.

• Levels of Protein Structure: Include primary (sequence of amino acids), secondary (alpha-helices and beta-sheets), tertiary (three-dimensional structure), and quaternary (combination of multiple polypeptide chains).

06. Water Properties

• Water is a polar molecule, resulting in unique properties such as being an excellent solvent, having a high specific heat capacity, and allowing ice to float on liquid water, which is vital for aquatic ecosystems.

07. Summary of Biological Molecules

• Overview of the roles that carbohydrates, lipids, proteins, and water play in various cellular processes, emphasizing their importance in biological systems.

Chapter 03 - Enzymes

01. Enzyme Functionality

• Enzymes act as biological catalysts that accelerate chemical reactions by lowering the activation energy required for the reaction to proceed.

02. Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature range; increasing temperature generally increases reaction rates until a point of denaturation is reached, where the enzyme's structure becomes irreversibly altered.

• pH: Enzymes have specific pH ranges for optimal activity; deviations from this range can lead to altered activity or denaturation.

03. Enzyme Inhibition

• Enzymes can be inhibited by substances that impede their activity; inhibitors can be classified into competitive (which bind to the active site) and non-competitive (which bind elsewhere on the enzyme, altering its function).

Chapter 04 - Cell Membranes

01. Fluid Mosaic Model

• The cell membrane is described by the fluid mosaic model, which depicts a phospholipid bilayer embedded with proteins. This structure allows for the fluidity and flexibility necessary for various cellular functions.

02. Transport Mechanisms

• Distinguishes between passive transport methods (diffusion, facilitated diffusion) that do not require energy and active transport mechanisms (such as pumped transport) that require ATP to move substances against their concentration gradient.

03. Surface Areas and Volume Ratio

• Discusses the implications of cell size on the surface area to volume ratio; as cell size increases, this ratio decreases, which can limit the efficiency of diffusion and transport processes.

Chapter 05 - Cell and Nuclear Division

01. Mitosis and Meiosis

• Mitosis: Process yielding identical diploid daughter cells through several phases (prophase, metaphase, anaphase, telophase).

• Meiosis: Reductive division producing haploid gametes; consists of two rounds of division, contributing to genetic diversity through processes like crossing over.

02. Cancer and Mutations

• Explores mechanisms of uncontrolled cell division, genetic mutations, and their role in the formation of tumors, emphasizing links to cancer.

Chapter 06 - Immunity and the Immune Response

01. Immune System Overview

• Overview of the immune system components including phagocytes, T lymphocytes (cell-mediated immunity), and B lymphocytes (humoral immunity) that work together to identify and eliminate pathogens.

02. Antibody Functionality

• Discusses the roles of antibodies in immune responses, including neutralization of pathogens, agglutination of bacteria, and opsonization facilitating phagocytosis by immune cells.

Chapter 07 - Ecology

01. Energy Flow in Ecosystems

• Describes energy flow through trophic levels, detailing producers (autotrophs), consumers (heterotrophs), and decomposers, along with energy transfer efficiency between levels.

02. Nitrogen Cycle Overview

• Overview of the nitrogen cycle processes, including nitrogen fixation, nitrification, assimilation, and decomposition, highlighting the importance of nitrogen in biological systems.

Chapter 08 - Plant Transport Mechanisms

01. Xylem and Phloem

• Examines the roles of xylem and phloem in water and nutrient transport; xylem conducts water and minerals from roots to leaves, while phloem transports sugars and other metabolic products throughout the plant.

Chapter 09 - Mammalian Transport System

01. Heart and Blood Vessel Structure

• Detailed examination of the mammalian circulatory system including the anatomy and function of the heart, arteries, veins, and capillaries, emphasizing the flow of blood and the significance of valves in maintaining unidirectional flow.

Chapter 10 - Smoking and Health

01. Effects of Smoking

• Discusses the numerous health effects associated with smoking, including chronic obstructive pulmonary disease (COPD), cardiovascular diseases, and various cancers caused by the toxic components found in tobacco smoke.