Cell Structure and Function: Comprehensive Notes

Cell Theory

  • All living things are composed of cells.

  • The cell is the basic unit of life.

  • Cells come from preexisting cells.

The Cell as the Basic Unit of Life

  • Smallest structural unit capable of using energy to sustain itself.

  • Life is an emergent property due to interactions at the cellular level.

  • In multicellular organisms, activity depends on the total activity of independent cells.

Cells Arise from Preexisting Cells

  • Prokaryotic cells divide by binary fission.

  • Eukaryotic cells divide through fission, mitosis, or meiosis.

  • Cells are formed by division of preexisting cells.

Microscopy

  • Microscopes produce magnified images of small objects.

  • Light microscopes are commonly used due to ease of use and affordability.

  • Electron microscopes offer higher magnification and resolution, but are less practical.

Magnification

  • Magnification is how much larger an object appears compared to its real size.

  • Total magnification = ocular magnification × objective lens magnification.

Preparing a Wet Mount

  • A drop of water suspends specimen between slide and cover slip.

  • Used for observing motile organisms or samples needing staining.

Staining

  • Chemicals that bind to structures, enhancing visibility.

  • Examples: Iodine (for starch), Methylene Blue (for cell nuclei), Gram Stain (for bacteria).

Microscope Field of View (FOV)

  • FOV is the diameter of the visible area through the microscope.

Determining FOV Diameter

  • Method 1: Use a transparent metric ruler under low power.

  • Method 2: Calculate using the equation: \text{High power FOV} = \frac{\text{Measured FOV at low power} \times \text{Low power magnification}}{\text{High or medium power magnification}}

Estimating Specimen Size

  • Method 1: Estimate fraction of FOV occupied by specimen, then multiply by FOV diameter.

  • Method 2: Estimate how many specimens fit across the FOV, then divide FOV diameter by that count; round to one significant digit.

Calculating Actual Size, Magnification and Scale

  • M = \frac{I}{A} (Magnification = Image size / Actual size)

  • A = \frac{I}{M} (Actual Size = Image size / Magnification)

Drawing Cells

  • Lines represent edges of structures.

  • When drawing a sample of a tissue, do not draw individual cells.

Guidelines for Drawing Cell Structures Seen with a Microscope

  • Use a sharp pencil on white, unlined paper.

    • Center drawing on the page, occupying at least half a page.

    • Labels: Use a ruler for straight, horizontal lines forming a vertical list; print labels.

    • Technique: Clear lines, no smudging, shading, or coloring.

    • Scale: Include a labeled scale bar with correct digits and unit.

    • Accuracy: Draw what is seen, not idealized; show only necessary details.

    • Title: State what has been drawn and lens power, include scientific name (italicized or underlined).

Developments in Microscopy

  • Microscopes magnify images of objects too small to be seen directly.

  • Improvements have led to significant advances in biology.

Benefits and Limitations of Microscopes

Compound Light Microscope

  • Benefits:

    • Ease of use.

    • Less expensive to buy.

    • Can observe dead or living cells in color.

    • Cell movement can be studied.

    • Quick specimen preparation (minutes to hours).

    • Limitations:

    • Maximum magnification of about 1500X.

    • Low resolving power (0.25µm to 0.3µm).

Electron Microscope

  • Benefits:

    • Magnification of 100,000X to 300,000X.

    • High resolving power (0.001µm).

  • Limitations:

    • Expensive to use.

    • Requires cells to be killed and chemically fixed.

    • No movement can be seen.

    • No color can be seen, without stain or dye.

    • High voltage electric current is required.

    • Specimen preparation usually takes few days.

Cell Structures Common to All Living Organisms

  • Plasma membrane.

  • Cytoplasm.

  • DNA.

  • Ribosomes.

Plasma Membrane

  • Barrier separating the interior from surroundings.

  • Bilayer formed from phospholipids.

Cytoplasm

  • Gel-like fluid with dissolved solutes.

  • Needed for metabolic processes.

DNA

  • Genetic material in all living organisms.

  • They are found in the cell nucleus in eukaryotes and wraps around histone proteins

  • They are not in a nucleus for prokaryotic.

Ribosomes

  • Catalyze synthesis of polypeptides during translation.

  • Composed of two subunits.

  • Prokaryotes have smaller (70s) ribosomes.

  • Eukaryotes have larger (80s) ribosomes.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic

  • Tiny (≈ 0.2 - 10 μm)

  • DNA in nucleoid (no nuclear membrane)

  • No membrane bound organelles

  • Flagella rotates

  • Cell wall with peptidoglycan

  • Smaller 70s ribosomes

  • DNA is circular and naked

  • Can have plasmids

  • Division by binary fission

  • All unicellular

Eukaryotic

  • Bigger (≈ 10 - 100 μm)

  • DNA in nucleus

  • Membrane bound organelles

  • Flagella moves laterally

  • Cell wall of cellulose or chitin

  • Larger 80s ribosomes

  • DNA is linear and associated with histone proteins

  • Does not have plasmids

  • Division by binary fission, mitosis or meiosis

  • Unicellular or multicellular

Prokaryotic Cell Structures

  • Cell (plasma) membrane: regulates movement of materials.

  • Cytoplasm (cytosol): gel-like fluid, site of metabolic reactions.

  • Ribosome: builds proteins during translation.

  • Cell wall: provides shape and withstands turgor pressure.

  • Pili: attach to surfaces, swap DNA.

  • Capsule: prevents dehydration, adheres to surfaces.

  • Flagellum: locomotion.

Prokaryotic Cell DNA

  • Nucleoid: main DNA, single loop, not enclosed in a membrane, naked DNA.

  • Plasmid: extra DNA, circular, replicates independently, not in all cells, can be shared, often contains genes for antibiotic resistance.

Eukaryotic Cell Structures

  • Nucleus

  • Free and bound 80s ribosomes

  • Rough endoplasmic reticulum

  • Smooth endoplasmic reticulum

  • Golgi apparatus

  • Vesicles

  • Lysosome

  • Mitochondria

  • Chloroplast

  • Vacuole

  • Microtubules and Centrioles

  • Cytoskeleton

  • Cilia and Flagella

Nucleus

  • Contains DNA, stores information, for making proteins.

  • Contains the nucleolus, which is where ribosome subunits are made

  • Eukaryotic DNA wraps around histone proteins

Ribosomes

  • Catalyzes the synthesis of polypeptides during translation

Rough Endoplasmic Reticulum (RER)

  • Synthesis and transport of polypeptides.

  • Has bound ribosomes.

  • Membrane continuous with the nuclear envelope.

Smooth Endoplasmic Reticulum (SER)

  • Synthesis of phospholipids and cholesterol.

  • Lacks ribosomes.

Golgi Apparatus

  • Modifies polypeptides into their functional state.

  • Sorts, concentrates and packs proteins into vesicles.

  • Vesicles are dispatched to one of three destinations:

    • lysosomes

    • The plasma membrane of the cell

    • Secretion to the outside of the cell via exocytosis

Lysosome

  • Contain enzymes that work in oxygen-poor areas and lower pH.

  • Digest pathogens that have been engulfed by phagocytes.

Mitochondria

  • Production of ATP by aerobic cellular respiration.

  • Surrounded by a double membrane.

Chloroplasts

  • Adapted for photosynthesis.
    *Light-absorbing pigments such as chlorophyll.

Vacuoles

  • Maintain turgor pressure against the cell wall.

Cytoskeleton

  • Helps cells maintain their shape, organizes cell parts and enables cells to move and divide.

  • Microtubules, actin filaments, and intermediate filaments.

Centrioles

  • Arrangement of the mitotic spindle during cell division

Cilia and Flagella

  • Cilia beat in coordination with each other.

  • Flagella move independently of each other.

Processes to Maintain Life

  • Homeostasis: Maintaining a stable internal condition

  • Metabolism: The sum of all chemical reactions in a cell.

  • Nutrition: Obtaining energy and matter

  • Movement: Adaptations for movement are a universal feature of living.

  • Excretion: Eliminating metabolic waste.

  • Growth: Is the increase in size and mass of an organism.

  • Response to Stimuli: The ability to recognize and respond to changes in environmental conditions.

  • Reproduction: Life will create more life.

Eukaryotic Kingdoms

  • Plant

  • Animal

  • Fungi

Similarities in Eukaryota Cell Structure

  • Nucleus

  • Free and bound 80s ribosomes

  • Rough endoplasmic reticulum

  • Smooth endoplasmic reticulum

  • Golgi apparatus

  • Vesicles

  • Lysosome

  • Mitochondria

  • Cytoskeleton

Differences in Eukaryotic Cell Structures

Plastids

  • Absent in Animalia and Fungi

  • Present in Plantae for manufacture and storing food

Cell Wall

  • Absent in Animalia

  • Present in Fungi (chitin)

  • Present in Plantae (Cellulose)

Vacuoles

  • Present in Animalia as small, temporary structures that expel excess water or other waste products

  • Present in Fungi as small, temporary structures that expel excess water or other waste products

  • Present in Plantae as a large, permanent organelle. Used to store water and to cause turgor pressure against the cell wall

Centrioles

  • Present in Animalia used to arrange the mitotic spindle during cell division

  • Absent in most Fungi, except a small number that have a swimming male gamete

  • Are present in the male gametes of moss and ferns. Absent in all conifers and flowering plants.

Cilia and Flagella

  • Present in many animal cells, including in the male gamete

  • Are absent from most fungi, except a small number that have a swimming male gamete.

  • Are present in the male gametes of moss and ferns. Are absent in all conifers and flowering plants.

Atypical Cell Structure in Eukaryotes

  • They all have a different number of nuclei from the typical eukaryotic
    cell, which has one nucleus.

Discrepancies

  • Red Blood Cell - Discard nucleus and mitochondria to increase surface area.

  • Aseptate Fungal Hyphae are not made of clearly defined individual cells, rather continuous structures with multiple nuclei

  • Multinucliated Skeletal Muscles are multi-nucleated

  • Phloem Sieve Tube Element don't have organelles

Eukaryotic Cell Micrographs

Many discrete structures are visible in eukaryotic cell (A2.2.6) micrographs. These structures are adapted to perform specific functions (B2.2.1).

  • Many discrete structures are visible in eukaryotic cell micrographs which perform specific functions.

  • Nucleus. Double membrane, stains darker.

  • Rough ER. Series of connected, flattened membranous
    sacs, Lined with dots (ribosomes)

  • Smooth ER. Series of connected, flattened membranous sacs Usually found further from the
    nucleus than the rough ER

  • Golgi Apparatus Flattened stacks, the Inside of the stack is clear

  • Lysosome Small, spherical organelles, Bound by a single membrane., Hard to distinguish from other vesicles

  • Mitochondrion Bound by a double membrane,
    Found in all eukaryotic cells,Circular or ovoid shape,Tend to stain darkish,Have infoldings of membrane.