AQA Biology GCSE: Cell Biology

AQA Biology GCSE: Cell Biology Notes

Cell Structure (1.1)

Eukaryotes and Prokaryotes (1.1.1)

• All living things are made of cells, which can either be prokaryotic or eukaryotic.

  • Eukaryotic Cells:

  • Found in animals and plants.

  • Structures include:

    • Cell membrane

    • Cytoplasm

    • Nucleus containing DNA

  • Prokaryotic Cells (Bacterial Cells):

  • Much smaller than eukaryotic cells.

  • Structures include:

    • Cell wall

    • Cell membrane

    • Cytoplasm

    • Single circular strand of DNA and plasmids (small rings of DNA found in the cytoplasm)

• The structures mentioned are examples of organelles, which are defined as the various structures within a cell that perform distinct functions.

Understanding Size with Orders of Magnitude

• Objects are compared in terms of size using orders of magnitude.

  • If an object is 10 times bigger, it is noted as being $10^{1}$ times larger.

  • If an object is 1000 times larger, it is referred to as $10^{3}$ times larger.

  • Conversely, being 10 times smaller can be expressed as $10^{-1}$ times smaller.

• Prefixes used before units (e.g., ‘metres’) to indicate multiples include:

  • Centi: Multiply unit by $0.01$

  • Milli: Multiply unit by $0.001$

  • Micro: Multiply unit by $0.000001$

  • Nano: Multiply unit by $0.000000001$

Animals and Plants (1.1.2)

• Subcellular structures in both animal and plant cells have specific functions.

  • Animal Cells:

  • Nucleus: Contains DNA codes for proteins required for building new cells; enclosed by a nuclear membrane.

  • Cytoplasm: Liquid substance for chemical reactions; contains enzymes (proteins that speed up reactions); organelles found within it.

  • Cell membrane: Regulates entry and exit of substances.

  • Mitochondria: Site for aerobic respiration, providing energy.

  • Ribosomes: Site of protein synthesis; found on rough endoplasmic reticulum.

  • Plant Cells (additional structures compared to animal cells):

  • Chloroplasts: Site of photosynthesis, containing chlorophyll pigment for light absorption.

  • Permanent vacuole: Contains cell sap; contributes to cell rigidity; located in cytoplasm.

  • Cell wall: Composed of cellulose, providing structural strength (also present in algal cells).

Bacterial Cells

• Bacterial cells are prokaryotic and generally exhibit fewer similarities in organelle types compared to eukaryotic cells.

  • Structure and Function in Bacterial Cells:

  • Cell wall: Composed of peptidoglycan.

  • Single circular strand of DNA: Floats in the cytoplasm due to lack of a nucleus.

  • Plasmids: Small rings of DNA.

Cell Specialisation (1.1.3)

• Cells undergo differentiation, gaining new structures that make them suited for specific roles.

  • Cells may either differentiate once early in development or retain the ability to differentiate throughout life (stem cells).

• Examples of Specialised Cells in Animals:

  1. Sperm Cells:

  • Function: Carries male DNA to egg cell for reproduction.

  • Characteristics:

    • Streamlined head and long tail optimize swimming.

    • Numerous mitochondria provide energy for movement.

    • Acrosome contains digestive enzymes to penetrate egg cell membrane.

  1. Nerve Cells:

  • Function: Transmits electrical signals throughout the body.

  • Characteristics:

    • Long axon for distance signal transmission.

    • Extensive dendrites for branching and connections.

    • Nerve endings house mitochondria to create neurotransmitters for impulse transmission.

  1. Muscle Cells:

  • Function: Contracts to move bones or invoke other types of movement.

  • Characteristics:

    • Specialized proteins (myosin and actin) engage in sliding mechanisms for contraction.

    • Abundant mitochondria supply energy for contraction from respiration.

    • Can store glycogen for respiration.

Examples of Specialised Cells in Plants

1. Root Hair Cells:

   • Function: Absorb water and minerals from soil.

   • Characteristics:

     • Large surface area due to root hairs to enhance water absorption.

     • Large permanent vacuole to help control water movement.

     • Mitochondria provide energy for active transport of minerals.

2. Xylem Cells:

   • Function: Transport water and minerals from roots to shoots.

   • Characteristics:

     • Lignin is deposited causing cell death for hollow structure.

     • End-to-end joining forms a continuous tube for water transport; lignin structure helps withstand pressure.

3. Phloem Cells:

   • Function: Carry photosynthesis products to all plant parts.

   • Characteristics:

     • Sieve plates developed from cell walls permit substance movement.

     • Despite lost sub-cellular structures, energy is provided by mitochondria from companion cells.

Cell Differentiation (1.1.4)

• Differentiation allows stem cells to acquire specialized functions.

  • Involves switching specific genes on or off to produce distinct proteins necessary for new functions.

  • Animals: Most cells differentiate at early stages and lose this ability.

  • Plants: Many cells maintain their differentiation ability throughout life.

Microscopy (1.1.5)

• Microscopic observations of cells require magnification.

• The light microscope's key characteristics are:

  • Developed by Robert Hooke in 1665 for cell observation.

  • Composed of two lenses (objective and eyepiece).

  • Maximum magnification of approximately x2000, resolving power of 200 nm (lower resolving power gives better detail).

  • Primarily used for tissues, cells, and large organelles.

• The electron microscope emerged in the 1930s, allowing more detailed observations.

  • Utilizes electrons instead of light due to smaller wavelength.

  • Two types:

  1. Scanning Electron Microscope (SEM): Creates 3D images at a lower magnification.

  2. Transmission Electron Microscope (TEM): Generates 2D images detailing organelles.

  • Magnification as high as x2,000,000 and different resolving power (SEM: 10 nm; TEM: 0.2 nm).

Common Calculations in Microscopy

1. Magnification of a Light Microscope:

   • Formula: Magnification = Magnification of eyepiece lens × Magnification of objective lens.

2. Size of an Object:

   • Formula: $( ext{Size of Image}) / ( ext{Magnification}) = ext{Size of Object}$.

   • Ensure consistent units are used.

Standard Form in Microscopy

• Useful for representing extremely large or small numeric values.

• Power of ten is used to scale numbers from $10^{-n}$ to $10^{n}$.

  • Example Values:

  • $1.5 imes 10^{-5} = 0.000015$

  • $3.4 imes 10^{3} = 3400$

Culturing Microorganisms (1.1.6 - Biology Only)

• Scientists cultivate microorganisms in labs using nutrient-rich mediums to study them.

Methods of Culturing Microorganisms:

1. Nutrient Broth Solution:

   • Involves creating a bacterial suspension in sterile nutrient broth, which must be stoppered with cotton wool and shaken regularly for oxygen supply.

2. Agar Gel Plate:

   • Hot sterilized agar is poured into petri dishes, cooled until set.

   • Bacterial inoculation is done using sterilized loops to evenly spread the microorganism.

   • Plates are then taped and incubated, inverted to prevent condensation.

Steps in Culturing Procedures and Their Importance:

Bacterial Growth

• Bacteria multiply through binary fission approximately every 20 minutes if conditions are suitable.

• Growth Calculation Formula: $ext{Bacteria at beginning} imes 2^{ ext{number of divisions}} = ext{Bacteria at end}$

  • To determine division count, divide total incubation time by the mean division time of that species.

• Results presented often in standard form, similar to mentioned microscopy examples.

Testing Antibiotic Effects

• Microorganisms can also evaluate antibiotic effectiveness through inhibition zones.

  • Process Overview:

  1. Discs soaked in various antibiotics are placed on bacteria-inoculated agar plates.

  2. Size of inhibition zone indicates the effectiveness of the antibiotic.

  • Control disc (soaked in sterile water) must indicate no bacterial death to confirm results solely reflect the antibiotic's impact.

Cell Division (1.2)

Chromosomes (1.2.1)

• The nucleus encases genetic information in the form of chromosomes, structured coils of DNA.

  • Gene Definition: A gene is identified as a short DNA section that codes for specific proteins, thereby influencing characteristics.

  • The human body comprises 23 pairs of chromosomes (46 in total), where gametes possess 23 single chromosomes.

Mitosis and the Cell Cycle (1.2.2)

• The cell cycle encompasses various stages leading to cell division, with mitosis representing the division phase.

1. Interphase:

   • Growth stage, organelle replication, protein synthesis, and DNA replication forming the characteristic 'X' appearance.

2. Mitosis:

   • Chromosomes align at the cell equator, and are pulled to opposite poles by spindle fibers.

3. Cytokinesis:

   • Cytoplasm and membrane physically divide to form two identical daughter cells.

• Importance of mitosis includes growth, development, and cellular repair in multicellular organisms, along with facilitating asexual reproduction.

Stem Cells (1.2.3)

• Stem Cell Definition: Undifferentiated cells capable of division, producing other specialist cells.

1. Embryonic Stem Cells:

   • Form from the zygote and can differentiate into any cell type.

   • Cloning and culturing possibilities for various therapeutic uses, such as diabetes, Alzheimer’s, or spinal cord injuries.

2. Adult Stem Cells:

   • Located in bone marrow; can form blood cells and numerous other cells.

3. Meristems in Plants:

   • Located in root and shoot tips with lifelong differentiation capacities, useful for cloning with desirable traits.

• Therapeutic Cloning Process:

  • Involves creating an embryo with identical genes as the patient for stem cell extraction, avoiding rejection.

Ethical Considerations

• Benefits of Stem Cell Research:

  • Potential for replacing damaged body parts.

• Problems Associated:

  • Ethical concerns regarding embryo destruction; understanding of differentiation processes is incomplete; resources could be allocated elsewhere in medicine.

Transport in Cells (1.3)

Diffusion (1.3.1)

• Diffusion Defined: The process by which particles spread out in solution or gas, moving from higher to lower concentration areas.

• Passive Process: No energy expenditure is necessary.

• Molecules Move:

  • Small molecules like oxygen, glucose, amino acids, and water traverse membranes via diffusion; larger molecules like starch and proteins cannot.

• Biological Examples:

  • Oxygen exchanged in alveoli for red blood cells for body respiration; urea moves from liver cells to blood for kidney excretion.

Factors Affecting Diffusion Rate

Adaptations for Efficient Transport

• Multicellular organisms show adaptations for effective transport:

1. In Lungs:

   • Alveoli with extensive capillary networks enhance gas exchange.

2. In Small Intestine:

   • Villi increase surface area for nutrient absorption.

3. In Fish Gills:

   • Countercurrent flow maximizes oxygen absorption across gill lamellae.

Osmosis (1.3.2)

• Osmosis Defined: Movement of water from a less concentrated solution to a more concentrated one across a semi-permeable membrane.

• Water Potential:

  • Dilute solutions possess high water potential; concentrated solutions demonstrate low water potential.

• Osmosis Models:

  • Demonstrated using a semi-permeable membrane bag containing sugar and varying solution concentrations.

  • Isotonic (equal concentration leading to no movement), Hypertonic (loss of water from cells), Hypotonic (water influx into cells).

Osmosis in Cells

• Animal Cells:

  • Dilute external solution causes bursting; concentrated solutions lead to shrinkage.

• Plant Cells:

  • Dilute external solutions contribute to turgidity (essential for plant structure); concentrated solutions cause plasmolysis and cell death.

Experimentation with Osmosis

• Effect of Sugar on Plant Tissue:

  • Investigate potato tubers in different sugar solutions; results yield mass changes based on water movement.

  • Calculate percentage mass change and graph findings.

Active Transport (1.3.3)

• Active Transport Defined: Movement of particles against concentration gradients, requiring energy input (respiration).

• Examples of Active Transport:

  • Root hairs use it to absorb minerals from soil where concentrations are typically higher inside the cell.

  • In the gut, glucose and amino acids move from lower gut concentrations into higher blood concentrations via active transport.