AQA Biology GCSE Topic 1: Cell Biology Detailed Study Notes

Eukaryotes and Prokaryotes (1.1.1)

All living things are composed of cells, which are categorized as either prokaryotic or eukaryotic.
Eukaryotic Cells: Animal and plant cells fall into this category. They are characterized by having: - A cell membrane. - Cytoplasm. - A nucleus which contains the DNA.
Prokaryotic Cells: Bacterial cells are prokaryotic. They are significantly smaller than eukaryotic cells and possess: - A cell wall. - A cell membrane. - Cytoplasm. - A single circular strand of DNA and plasmids, which are small rings of DNA found within the cytoplasm.
Organelles: The structures listed above (such as the cell membrane) are defined as organelles, which are structures within a cell that perform specific functions.
Orders of Magnitude: Cells are extremely small, necessitating the use of orders of magnitude to understand scale differences: - If an object is $10$ times bigger than another, it is described as being $10^1$ times bigger. - If an object is $1000$ times bigger than another, it is described as being $10^3$ times bigger. - If an object is $10$ times smaller than another, it is described as being $10^{-1}$ times smaller.
Prefixes in Measurement: Prefixes are used before units of measurement (e.g., metres) to indicate multiples of that unit: - Centi: Multiply unit by $0.01$ . - Milli: Multiply unit by $0.001$ . - Micro: Multiply unit by $0.000,001$ . - Nano: Multiply unit by $0.000,000,001$ .

Animals and Plants (1.1.2)

Sub-cellular structures within cells possess specific biological functions. While animal and plant cells share many structures, plant cells have unique additions. Bacterial cells (prokaryotes) share fewer similarities in organelle types compared to eukaryotic cells.
Shared Structures (Animal and Plant Cells): - Nucleus: Contains the DNA coding for specific proteins required to build new cells; it is enclosed within a nuclear membrane. - Cytoplasm: A liquid substance where chemical reactions occur; it contains enzymes, which are biological catalysts (proteins that increase the rate of reaction), and is the medium in which organelles are found. - Cell membrane: Responsible for controlling which substances enter and exit the cell. - Mitochondria: The site of aerobic respiration reactions, which provide the necessary energy for the cell. - Ribosomes: The site where protein synthesis takes place; these are located on a structure known as the rough endoplasmic reticulum.
Structures Unique to Plant Cells: - Chloroplasts: The site where photosynthesis occurs to provide food for the plant; they contain chlorophyll pigment, which makes the plant green and harvests the light needed for the process. - Permanent vacuole: Located within the cytoplasm, it contains cell sap and improves the cell's rigidity. - Cell wall: Also present in algal cells, it is made from cellulose and provides structural strength to the cell.
Bacterial Cell Specific Structures: - Cytoplasm: Same function as above. - Cell membrane: Same function as above. - Cell wall: In bacteria, this is made of a different compound called peptidoglycan. - Single circular strand of DNA: Because bacteria have no nucleus, this strand floats freely in the cytoplasm. - Plasmids: Small, independent rings of DNA.
Calculations of Size: To calculate the size or area of sub-cellular structures, one should identify a geometric shape that resembles the structure (e.g., a circle or rectangle) and apply standard formulas, such as $ext{length} imes ext{width}$ for a rectangle.

Cell Specialisation (1.1.3)

Differentiation: The process by which cells gain new sub-cellular structures to become suited to a specific role. Cells may differentiate only once early in their development or maintain the ability to differentiate throughout their lifespan (stem cells).
Animal Differentiation: Most animal cells differentiate only once at an early stage.
Plant Differentiation: Many plant cells retain the ability to differentiate throughout their entire life.
Specialised Animal Cells: - Sperm cells: Specialised for reproduction by carrying male DNA to the egg cell (ovum). - Features: Streamlined head and long tail for swimming; high concentration of mitochondria for energy; acrosome at the head containing digestive enzymes to penetrate the egg cell membrane. - Nerve cells: Specialised to transmit electrical signals rapidly across the body. - Features: Long axon to carry impulses over long distances; branched extensions called dendrites to form connections with other nerve cells; nerve endings rich in mitochondria to provide energy for transmitter chemicals called neurotransmitters. - Muscle cells: Specialised for quick contraction to move bones (striated muscle) or to squeeze (smooth muscle in blood vessels for pressure variation). - Features: Myosin and actin proteins that slide over each other; numerous mitochondria for respiration energy; storage of glycogen for use by mitochondria.
Specialised Plant Cells: - Root hair cells: Specialised for the uptake of water via osmosis and mineral ions via active transport from the soil. - Features: Large surface area due to hair-like projections; large permanent vacuole to manage water movement speed; mitochondria to provide energy for active transport of ions. - Xylem cells: Specialised for transporting water and mineral ions from roots to shoots. - Features: Lignin deposition causes cells to die and become hollow, forming a continuous tube; lignin is deposited in spirals to withstand water movement pressure. - Phloem cells: Specialised to transport the products of photosynthesis (food) throughout the plant. - Features: Cell walls break down into sieve plates to allow substance movement; energy is provided by the mitochondria of adjacent companion cells.

Cell Differentiation (1.1.4)

Differentiation involves switching specific genes on or off to produce different proteins, allowing the cell to acquire specific sub-cellular substances for its function.
Animals: Most cells differentiate early and lose the ability thereafter. Specialised cells typically reproduce via mitosis (producing two identical cells). Cells like red blood cells, which lose their nucleus and cannot divide, are replaced by adult stem cells.
Mature Animals: Cell division is primarily for the repair or replacement of damaged cells, as growth is limited.
Plants: Many cell types retain differentiation ability. They differentiate upon reaching their final position but can re-differentiate if moved.

Microscopy (1.1.5)

Historical Context: Robert Hooke first observed cork cells in $1665$ using a light microscope.
Light Microscopes: - Utilize two lenses: objective and eyepiece. - Usually illuminated from underneath. - Maximum magnification: approximately $imes 2000$ . - Resolving power (resolution): $200 ext{ nm}$ . Note: A lower resolving power indicates more detail can be seen. - Used for viewing tissues, cells, and large sub-cellular structures.
Electron Microscopes: - Developed in the $1930 ext{s}$ , using electrons instead of light. - Electrons have a much smaller wavelength than light waves, providing better resolution. - Scanning Electron Microscope (SEM): Creates $3 ext{D}$ images with a resolving power of $10 ext{ nm}$ . - Transmission Electron Microscope (TEM): Creates $2 ext{D}$ images of internal organelles with a resolving power of $0.2 ext{ nm}$ . - Maximum magnification: up to $imes 2,000,000$ .
Microscopy Calculations: - Total Magnification: $ext{Magnification of eyepiece} imes ext{Magnification of objective lens}$ . - Object Size: $rac{ ext{Size of image}}{ ext{Magnification}} = ext{Size of object}$ .
Standard Form: Useful for very large or small numbers. The leading number must be between $1$ and $10$ . - Example: $1.5 imes 10^{-5} = 0.000015$ . - Example: $3.4 imes 10^3 = 3400$ .

Culturing Microorganisms (1.1.6 - Biology Only)

Culture Medium: Contains carbohydrates (energy), minerals, proteins, and vitamins.
Growing Methods: - Nutrient Broth Solution: Bacteria suspended in sterile broth; flask stoppered with cotton wool to prevent air contamination and shaken for oxygen supply. - Agar Gel Plate: Bacteria form colonies on a medium of agar. Hot sterile agar is poured into a Petri dish, cooled, and inoculated using wire loops.
Aseptic Technique and Sterilisation: - Petri dishes and media must be sterilised (autoclave or UV) to prevent contamination by competing or harmful microorganisms/pathogens. - Inoculating loops are sterilised via flame. - Petri dish lids are taped (not sealed completely to avoid harmful anaerobic bacteria growth) and stored upside down to prevent condensation from disrupting growth. - Incubation Temperature: Stored at $25^ oundC$ in labs to prevent the growth of bacteria harmful to humans, which thrive at $37^ oundC$ (body temperature).
Binary Fission: Bacteria multiply by splitting into two, potentially every $20$ minutes given sufficient nutrients and temperature.
Population Calculation Formula: $ext{bacteria at beginning} imes 2^{ ext{number of divisions}} = ext{bacteria at end}$ .
Testing Antibiotics (Higher Tier): Paper discs soaked in antibiotics/disinfectants are placed on an agar plate. The size of the "inhibition zone" (clear area of dead bacteria) indicates effectiveness. - One disc must be a control (sterile water) to ensure only the antibiotic causes death. - Calculation of area: $ext{Area} = ext{\pi} r^2$ .

Cell Division (1.2)

Chromosomes (1.2.1): The nucleus contains DNA coiled into chromosomes. A gene is a short section of DNA coding for a protein. Body cells contain $23$ pairs of chromosomes ( $46$ total). Gametes (sex cells) contain half this amount ( $23$ total).
The Cell Cycle and Mitosis (1.2.2): - Stage 1 (Interphase): Cell growth, organelle increase (ribosomes, mitochondria), protein synthesis, DNA replication (forming 'X' shapes), and energy store increase. - Stage 2 (Mitosis): Chromosomes line up at the equator; cell fibres pull each half of the 'X' to opposite sides. - Stage 3 (Cytokinesis): Cytoplasm and cell membranes divide to form two identical daughter cells.
Importance: Mitosis is critical for growth, development, replacing damaged cells, and asexual reproduction.

Stem Cells (1.2.3)

Definition: Undifferentiated cells capable of dividing to produce more similar cells or differentiating into specialized cells.
Types: - Embryonic Stem Cells: Formed from zygotes; can differentiate into any cell type. Can be cloned for treating diabetes (insulin cells), Alzheimer's (neural cells), or paralysis (nerve cells). - Adult Stem Cells: Found in bone marrow; can form various cells including blood cells. - Meristems (Plants): Found in root/shoot tips; can differentiate into any plant type throughout life. Used for cloning desirable traits or saving rare species.
Therapeutic Cloning: Producing an embryo with the patient's genes so stem cells are not rejected by the patient's immune system.
Research Implications: - Benefits: Replacing damaged parts; using unwanted embryos from clinics; furthering differentiation research. - Problems/Ethics: Difficulty in controlling differentiation; destruction of embryos; religious/ethical objections; risk of viral transfer; high costs/time.

Transport in Cells (1.3)

Diffusion (1.3.1): The spreading out of particles in solution or gas, resulting in a net movement from an area of higher concentration to lower concentration. It is a passive process.
Factors Affecting Diffusion: - Concentration gradient: Greater difference increases rate. - Temperature: Higher temperature increases particle movement and collisions. - Surface area: Larger area allows more particles to pass through.
Surface Area to Volume Ratio ( $SA:V$ ): Calculated as $ext{Surface Area} : ext{Volume}$ . Large ratios (single-celled organisms) allow sufficient diffusion. Small ratios (multicellular) require specialized exchange surfaces.
Adaptations for Exchange: - Lungs: Alveoli provide large surface area ( $75 ext{ m}^2$ ) and thin membranes. - Small Intestine: Villi increase surface area for digested food absorption. - Fish Gills: Lamellae and filaments for gas exchange; counter-current flow (blood and water in opposite directions) maintains a steep oxygen gradient. - Plant Roots: Root hair cells increase surface area for water/minerals. - Leaves: Flattened shape, air spaces, and stomata (controlled by guard cells) manage gas exchange.
Osmosis (1.3.2): The passive movement of water from a dilute solution (high water potential) to a concentrated solution (low water potential) through a partially permeable membrane. - Isotonic: External concentration equals internal. - Hypertonic: External concentration is higher than internal (water leaves cell). - Hypotonic: External concentration is lower than internal (water enters cell). - Effects: Animal cells may burst or shrivel. Plant cells benefit from turgor pressure (rigidity) in dilute solutions; in concentrated solutions, they undergo plasmolysis (membrane pulls away from wall).
Active Transport (1.3.3): The movement of particles from low to high concentration against the gradient. Requires energy from respiration. - Examples: Mineral ion uptake in plant roots; glucose or amino acid uptake in the human gut.