Notes on Unit 2: Organisation of the Organism (Cells, Organelles, and Levels of Organisation)

2.1 Structure of cells

• Learning outcomes: describe structures of plant and animal cells as seen under a light microscope; describe differences between plant and animal cells; state functions of structures visible in light microscopes in both plant and animal cells.

• Cells: basic idea

  • Cells are the small building blocks of all living organisms.

  • Very small organisms (e.g., bacteria) are unicellular.

  • Multicellular organisms (e.g., an insect) contain many cells.

• Differences between plant and animal cells (summary table concepts):

  • Plant cells typically have:

  • cellulose cell wall (present)

  • chloroplasts (present in many plant cells)

  • large central vacuole containing cell sap (present)

  • fixed/permanent shape largely determined by the cell wall

  • nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, nucleolus (present)

  • Animal cells typically have:

  • no cellulose cell wall (cell membrane only)

  • no chloroplasts (generally absent in animal cells)

  • small or no central vacuole (vacuoles exist but are small and not a central feature)

  • shapes vary; no rigid cell wall to fix shape

  • nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, nucleolus (present)

• Visual references mentioned:

  • Figure 2.1.1: cheek cell (stained with blue dye)

  • Figure 2.1.2: palisade mesophyll cell from a leaf

  • Figure 2.1.3: liver cells magnified (x40)

  • Figure 2.1.4: onion epidermis cells (magnified x250)

• Table 2.1.1 (differences, key features usable under light microscope):

  • Plant cell: cellulose cell wall present; chloroplasts present in some cells; large central vacuole present; nucleus present; cytoplasm; cell membrane; mitochondria present; ribosomes present.

  • Animal cell: cell wall absent; chloroplasts absent; large central vacuole absent or small; nucleus present; cytoplasm; cell membrane; mitochondria present; ribosomes present.

  • Note: Mitochondria and ribosomes are visible with an electron microscope; all other structures listed are visible with light microscopy.

• Key points:

  • A cell membrane, cytoplasm, and a nucleus are found in both plant and animal cells.

  • All plant cells have a cellulose cell wall; some also have chloroplasts and a vacuole containing cell sap. Animal cells do not have these parts.

  • Refer to Table 2.1.2 for the functions of cell structures.

• Functions of cell structures (Table 2.1.2):

  • Cell membrane: forms a barrier between the cell and surroundings; keeps contents inside; allows simple substances to enter/leave (O2, CO2, water); controls movement of other substances (e.g., glucose); often described as partially permeable.

  • Nucleus: controls all activities in the cell; controls how cells develop.

  • Cytoplasm: place where many chemical reactions take place (e.g., respiration) and where protein synthesis begins.

  • Chloroplasts (plant cells only): responsible for photosynthesis; store starch.

  • Cell wall (plant cells only): stops cells from bursting when they fill with water; provides shape to cells.

  • Vacuole (plant cells only): full of water to maintain shape and firmness of the cell.

  • Mitochondria: release energy during aerobic respiration.

  • Ribosomes: make proteins for the cell.

• Quick revision tips (from the exam tip):

  • When asked for differences, include features of both plant and animal cells.

  • Use examples (onion epidermis, cheek cells, leaf cells) to illustrate points.

2.2 Cell organelles

• Learning outcomes:

  • State that the cytoplasm of all cells contains vesicles and ribosomes.

  • Identify mitochondria in diagrams/photographs of cells.

  • State that aerobic respiration occurs in mitochondria.

  • Calculate magnification and actual size of biological specimens using micrometres as units.

• Key organelles and structures:

  • Mitochondria

  • Relatively large organelles in all eukaryotic cells; typical size around
    1 b5m (width) by 5 b5m (length).

  • Each mitochondrion has a double membrane: outer membrane controls entry/exit; inner membrane folds (cristae) where some chemical reactions of aerobic respiration occur.

  • Primary role: provide most of the cell's energy because aerobic respiration occurs there.

  • Cells with high respiration rates have many mitochondria (e.g., liver cells, flight muscle cells in insects).

  • Ribosomes

  • Small organelles, about
    20
    nm in diameter in eukaryotic cells (smaller in prokaryotes).

  • Function: synthesize proteins (e.g., enzymes involved in respiration).

  • Location: many attached to rough endoplasmic reticulum (RER); some free in cytoplasm.

  • Endoplasmic reticulum (ER)

  • Network of flattened sacs surrounded by membranes.

  • Rough ER (RER) has ribosomes on the surface; main function is to package and transport proteins made by ribosomes.

  • ER forms a network in the cytoplasm, enabling intracellular transport.

  • Vesicles

  • Small vesicles can bud off from the ends of the RER; they travel through the cytoplasm, fuse with cell membranes, and release contents outside the cell.

  • Vesicles also function at synapses at neurones (neural transmission).

  • Lysozyme (enzyme featured in Unit 2 materials)

  • Breaks down bacterial cell walls; part of antibacterial defense.

• Magnification and actual size calculations (supporting notes):

  • Formula to calculate actual size from a magnified image:

  • 
    ext{actual size} = rac{ ext{image size}}{ ext{magnification}}

  • Formula to calculate magnification from image size and actual size:

  • 
    ext{magnification} = rac{ ext{image size}}{ ext{actual size}}

  • Example workflow:

  • If a cell is measured at 13 mm on an image magnified 1000 times, the actual size is:

    • 
      ext{actual size} = rac{13 ext{ mm}}{1000} = 0.013 ext{ mm} = 13 ext{ μm}

  • There are 1000 μm in 1 mm, so the conversion is straightforward.

  • Note: When image size has been reduced (as in some photos), use the same formulas to determine actual size from the given magnification.

• Summary questions (structure):

  • a Which organelles carry out the following functions?

  • manufacture proteins

  • convert light energy into chemical energy

  • control the substances that pass into and out of a cell

  • carry out reactions in aerobic respiration

  • b Complete a table with structures (cell wall, chloroplast, mitochondrion, vesicle, bacterial cell, liver cell, palisade cell) to indicate their presence in various cell types.

  • c Measure the length of a mitochondrion in an electron micrograph magnified at
    ×80,000 and calculate its actual length in micrometres.

  • d Calculate the actual size of liver cells and onion cells from page 19.

• Key points:

  • Ribosomes in the cytoplasm synthesize proteins.

  • Aerobic respiration occurs in mitochondria.

  • Cells with high rates of respiration contain many mitochondria.

  • When converting measurements to micrometres, multiply millimetre measurements by 1000.

• Supplementary notes on magnification and size can be found in the example images:

  • Human cheek cells pictured at ×1000 magnification; actual size roughly 0.013 mm (13 μm) for the example cell.

  • Goldfish image example showing reduction or magnification factors; magnification shown as a fraction of actual size (e.g., ×0.33 actual size).

2.3 Different types of cell

• Learning outcomes:

  • Identify different cell types from diagrams/photos.

  • Relate cell features to their functions.

  • Calculate magnification and actual size of biological specimens using millimetres as units.

• Specialised (differentiated) cells and their features/functions:

  • Ciliated cells (airways in lungs and oviducts): have cilia on surfaces; cilia beat to move mucus and trap dust/pathogens in airways; move egg in oviducts toward uterus.

  • Root hair cells (plant): long extensions increase surface area to absorb water and ions from soil.

  • Xylem vessels (plant): cylindrical, hollow tubes arranged in columns; thickened cell walls with bands of cellulose and lignin; transport water and ions from roots to rest of plant; provide structural support.

  • Palisade mesophyll cells (plant): numerous chloroplasts; packed to absorb light efficiently; photosynthesis; part of the mesophyll tissue in leaves.

  • Neurones (nerve cells): highly specialised with long extensions to transmit electrical impulses around the body.

  • Red blood cells: flattened disc shape; contain haemoglobin to carry oxygen; large surface area to volume ratio for efficient oxygen uptake.

  • Sperm cells: tail for swimming; head carries paternal genetic information; nucleus contains paternal genes.

  • Egg cells: much larger; contain yolk as energy store; maternal genes inside nucleus.

  • Palisade mesophyll cells (example): leaf cells with lots of chloroplasts for photosynthesis; leaf tissue type called palisade mesophyll.

• Additional examples mentioned: muscle cell, goblet cell, white blood cell, xylem vessels (reiterated).

• Key points on development and adaptation:

  • During development, cells change their structure and often their shape to suit function.

  • Specialised cells have structures that enable their specific functions (e.g., cilia in ciliated cells for mucus transport).

• Summary questions (types and functions):

  • Match cell types to their shapes and functions (e.g., red blood cells, ciliated cells, root hair cells, neurones, xylem vessels).

  • Recognise how features like cilia, long extensions, thick-walled cellulose/lignin, chloroplasts, and haemoglobin support function.

• Example analysis prompts:

  • Describe how a palisade mesophyll cell is adapted for photosynthesis: high chloroplast density, large vacuole, thin cell walls, and dense cell packing facilitate light capture and gas exchange (as illustrated in Figure 2.4.2 and related notes).

• Key points:

  • During development, cells differentiate by changing structure to suit specialized functions.

  • Structural features directly relate to function in specialised cells.

2.4 Levels of organisation

• Learning outcomes:

  • Define tissue, organ, and organ system.

  • Describe plant and animal examples of tissues, organs, and organ systems.

  • Calculate magnification and actual size using millimetres as units.

• Definitions and relationships:

  • Tissue: a group of similar cells working together to perform a shared function.

  • Organ: a group of tissues working together to perform a specific function.

  • Organ system: a group of related organs working together to perform broader body functions.

• Example: human circulatory system

  • Muscle tissue in the heart (cardiac muscle) contracts to pump blood.

  • The heart and blood vessels form the circulatory system which transports blood around the body.

• Plant tissues and organs:

  • Leaf tissues: mesophyll is the tissue that carries out photosynthesis.

  • Palisade mesophyll cells are located in the upper part of the leaf; they are packed with chloroplasts to absorb light efficiently.

  • A leaf is an organ comprising several tissues, including the mesophyll and epidermis.

  • The epidermis includes upper and lower epidermal layers; some structures include stomata in epidermal tissue.

  • Other plant organs include roots and stems; flowers and fruits are modified leaves or specialized structures.

• Leaf structure and photosynthesis notes:

  • Palisade mesophyll tissue is responsible for photosynthesis due to high chloroplast content.

  • The leaf makes food (glucose) via photosynthesis and allows gas exchange (CO2 in, O2 out).

  • Spongy mesophyll cells contribute to gas exchange due to air spaces.

• Summary questions (structure):

  • Complete the sentences with: cells, tissues, organ, organ system, function.

  • Identify how organs correspond to different organ systems (e.g., lungs and trachea = gas exchange; heart and blood vessels = circulatory; brain and spinal cord = nervous; ovaries, oviducts and uterus = reproductive; gullet, stomach and intestines = digestive; kidneys and bladder = excretory).

  • Arrange words in order from smallest to largest: cell, tissue, organ, organ system, organism.

• Exam tips:

  • Be prepared to explain how a palisade mesophyll cell is adapted for photosynthesis by listing structural features and explaining how each feature supports the function.

• Key points:

  • Cells with the same function group together to form tissues.

  • Different tissues form organs which work together to perform a specific function.

  • Different organs work together as organ systems.

• Plant tissues and organs (recap):

  • In leaves, mesophyll tissue carries out photosynthesis; palisade mesophyll cells are enriched with chloroplasts to maximize light capture.

  • The leaf is an organ comprising tissues that enable photosynthesis and gas exchange.

  • The milkweed plant example illustrates how organs form organ systems to sustain life.

If you would like, I can convert these notes into a concise study sheet with a tighter question-and-answer section, or add more example problems for magnification/size calculations in your preferred format.