Cell Theory, Imaging Technology, and Cell Function

The Development of Cell Theory

• The development of cell theory illustrates the link between science, technology, and society.

• Social pressure influences the acceptance of new scientific ideas and technological advances.

• Scientific inquiry requires questioning, predicting, observing, and recording to provide unbiased information.

• Investigations must adhere to ethical guidelines, with reproducible results under controlled conditions.

• The microscope was crucial in exploring microscopic organisms, providing evidence to support or refute existing theories.

Early Microscopy

• Hans and Zacharias Janssen (c. 1595): Dutch lens-makers who invented the first compound microscope, using an eyepiece (ocular lens) and an objective lens.

  • A compound microscope uses more than one lens to magnify objects.

  • Their microscope had a magnification power of approximately 20X.

• Robert Hooke (1665): Used a handmade three-lens microscope in England.

  • Studied cork to understand its properties (lightweight, buoyant, compressible).

  • Observed empty chambers in thin slices of cork, which he called "cells".

  • These were remnants of living cells, the simplest functional units of life.

• Antoni van Leeuwenhoek: A Dutch businessman who used a simple single-lens microscope (similar to a magnifying glass).

  • First to observe the movement of single cells, including bacteria, sperm, and unicellular protozoa.

  • These observations demonstrated individual free-living cells surviving as independent systems.

  • Van Leeuwenhoek made detailed drawings of his discoveries and named them animalcules.

  • He achieved higher magnifications (up to 250X) due to his skill in making tiny lenses.

• Early compound microscopes produced blurry images with halos due to light scattering as it passed through multiple lenses.

• The achromatic lens (developed in the 18th century) combined lenses to control the halo effect and improve image detail.

Spontaneous Generation

• Spontaneous generation (abiogenesis) was the idea that life could arise spontaneously from non-living matter.

  • This belief was widely accepted from Roman times through the 19th century.

  • Example: the common belief was that mice could be produced by placing sweaty underwear and wheat husks in an open jar.

• Francesco Redi (1668): An Italian physician and poet, Redi questioned the spontaneous appearance of maggots on raw meat.

  • Hypothesized that flies laid eggs in the meat.

  • Experiment: flasks with raw meat were sealed, covered with gauze, or left open.

  • Maggots appeared only in open flasks, demonstrating that flies needed to lay eggs directly on the meat.

  • Manipulated variable: access of flies to the meat.

  • Responding variable: presence of maggots.

  • Controlled variables: type of meat, flask.

• Louis Pasteur (1864): A French chemist who disproved spontaneous generation.

  • Experiment: boiled meat broth in a flask with an S-shaped neck ( swan neck flask).

  • The S-shape allowed air to reach the broth but trapped microorganisms and particles.

  • No growth occurred unless the broth touched the S-bend or the neck was broken, allowing contamination.

  • Controlled variables: broth type, flask type, light, temperature.

  • Manipulated variable: access of dust to the flask.

  • Responding variable: mould growth in the broth.

  • Pasteur showed that microorganisms are present in the air.

• Biogenesis: The development of living things from other living things through reproduction.

Improvements in Lens Technology

• Improvements in lens technology during the 1830s led to a better understanding of the cell as the functional unit of life.

• Robert Brown (1833): A Scottish microscopist who identified the nucleus in orchids.

  • Observed an opaque granular spot within the cell and recognized its importance.

• M.J. Schleiden (1838): A German professor of botany, Schleiden observed that all plants were composed of cells.

  • Proposed that the nucleus was responsible for cell development.

• Theodor Schwann: Studied animal physiology and believed there were similarities between plant and animal tissues.

  • Schwann found structures resembling plant cells and nuclei in animal tissues.

• Schwann and Schleiden proposed the cell theory that all plants and animals are composed of cells and that the cell is the basic unit of all organisms.

• Rudolf Virchow (1859): Extended the cell theory by stating that all cells arise only from pre-existing cells.

The Cell Theory

1. All living things are made up of one or more cells and the materials produced by these cells.

2. All life functions take place in cells, making them the smallest unit of life.

3. All cells are produced from pre-existing cells through cell division.

• The cell theory applies to all living things.

• Subcellular particles like viruses and prions are neither living nor non-living but may exhibit certain characteristics of living cells.

Developments in Imaging Technology and Staining Techniques

• Advancements in microscope technology and staining techniques have directly enhanced knowledge of cell structure and function.

Light Microscopes

• Light microscopes magnify cells using curved lenses and a light source.

• Image quality depends on:

  1. Magnification: Increasing the image size.

  2. Contrast: The difference in light intensity between the image and the background.

     • Bright-field microscopy involves passing light directly through cells, resulting in colorless images.

     • Contrast can be improved by manipulating the light source.

  3. Resolution: The ability to distinguish between two closely spaced points.

Staining Experiments

• Stains or coloring agents improve contrast by attaching to specific cell parts.

• A disadvantage is that staining kills the cells, making it impossible to view living tissue.

Confocal Technology

• Confocal Laser Scanning Microscope (CLSM): Developed in the 1980s and uses laser beams and computers to view living, transparent cells in three dimensions.

  • A laser concentrates light onto the specimen.

  • The reflection passes through a confocal pinhole to an electronic detector that converts light into an image.

  • Only light from an exact plane of focus passes through the pinhole.

  • Images of thin sections are combined to produce a high-resolution 3D image.

Electron Microscopy

• Electron Microscope: Developed in the 1930s by James Hillier and Albert Prebus at the University of Toronto. It uses a beam of electrons instead of light waves to produce detailed images.

  • Image formation relies on the absorption or scattering of the electron beam.

  • Electron-dense materials prevent electrons from passing through.

  • Electromagnets are used for focusing instead of glass lenses.

Transmission Electron Microscope (TEM)

• Transmission Electron Microscope (TEM): It passes a beam of electrons through a thin section of fixed and stained tissue embedded in plastic.

  • Electrons that pass through the specimen fall on a fluorescent screen or photographic film, producing black-and-white photographs.

  • Magnification: up to 1,500,000X

  • Resolution: about 2.5 nm for biological specimens.

  • Specimens are chemically fixed and no longer living.

Scanning Electron Microscope (SEM)

• Scanning Electron Microscope (SEM): Developed in the 1940s and provides information about the surface features of a specimen.

  • Specimens are fixed and covered with an electron-dense material like gold to reflect electrons.

  • A sensor picks up the reflected electrons, forming a three-dimensional image.

  • Magnification: up to 300,000X

  • Resolution: 20 nm

  • Specimens are chemically fixed and no longer living.

  • The sample stage can be computer-controlled, allowing movement of the specimen in three dimensions.

• Photographs taken through electron microscopes are called electron micrographs.

Using a Compound Light Microscope

Parts of a Microscope

• Refer to Figure 8.1 in the textbook to label the diagram of a compound light microscope.

Tips for Using a Compound Light Microscope

1. Carry the microscope with two hands (one on the arm, one on the base).

2. Plug in the microscope and turn on the light.

3. Rotate the nosepiece to select the lowest power objective lens.

4. Place a microscope slide on the stage and secure it with stage clips.

5. Lower the nosepiece as far as possible using the coarse adjustment while watching from the side.

6. Look through the eyepiece and slowly move the stage upwards with the coarse adjustment until the image comes into view.

7. Use the fine adjustment to sharpen the image.

8. To view the object under higher magnification, rotate the nosepiece to the next higher power objective lens while watching from the side.

9. Use only the fine adjustment knob to focus the image at higher magnifications.

Calculating Total Magnification

• The total magnification is calculated by multiplying the magnification of the ocular lens by the magnification of the objective lens:

  • $total <br>ot{m}agnification = magnification <br>ot{o}f ocular lens × magnification <br>ot{o}f objective lens$

• Example: If the ocular lens is 10X and the objective lens is 25X, the total magnification is $10 <br>ot{X} × 25 <br>ot{X} = 250 <br>ot{X}$.

Calculating Field of View Diameter

• The field of view diameter is the distance across the viewing field under the objective lens.

• Typically, the low power field diameter is measured and used to calculate the field diameter under medium and high power.

• Formula:

  • ${MP <br>ot{o}r HP field diameter <br>ot{/} LP field diameter = LP magnification <br>ot{/} MP <br>ot{o}r HP magnification}$

  • $FOV1 × mag1 = FOV2 × mag2$

• Example: Low power FOV diameter is 4 mm. Objective lens magnifications: Low (4X), Medium (10X), High (40X). Ocular lens is 10X.

  • Medium power FOV: $(4 <br>ot{m}m × 4<br>ot{X}) <br>ot{/} 10<br>ot{X} = 1.6 <br>ot{m}m$

  • High power FOV: $(4 <br>ot{m}m × 4<br>ot{X}) <br>ot{/} 40<br>ot{X} = 0.4 <br>ot{m}m$

Calculating the Size of an Object Viewed Under a Microscope

• Estimate the number of times the object fits across the field diameter (fit number).

• Formula:

  • $Size <br>ot{o}f Object (SOO) = Field of View <br>ot{/} Fit Number$

• Example: 8 plant cells fit across the high power field of view. The field of view diameter is 400 µm.

  • Cell size: $400 <br>ot{\mu m} <br>ot{/} 8 = 50 <br>ot{\mu m}$

Calculating the Scale of a Drawing

• Determine the actual size of the specimen in millimeters.

• Measure the size of the drawing in millimeters.

• Formula:

  • $Scale = Size <br>ot{o}f Drawing (SOD) <br>ot{/} Size <br>ot{o}f Object (SOO)$

• Example: Specimen diameter is 200 µm and the drawing diameter is 3.0 cm.

  • Specimen diameter in mm: $200 <br>ot{\mu m} = 0.2 <br>ot{m}m$

  • Scale: $30 <br>ot{m}m <br>ot{/} 0.2 <br>ot{m}m = 150<br>ot{X}$

Practice Questions

These are several practice questions for using the equations and information provided above. Please solve them on your own.

Cell Research at the Molecular Level

Gene Mapping

• Molecular biology techniques have enabled gene mapping, including the Human Genome Project (draft published in 2001).

• Mapping involves breaking cells to release DNA, making copies, and sequencing chemical subunits using computer analysis.

• DNA analysis and gene mapping improve understanding of genetic material interactions.

• Potential applications include managing disease-causing abnormalities (e.g., cancer research).

• Gene mapping of crop plants may lead to pest-resistant or drought-tolerant varieties.

• Environmental, societal, and ethical concerns:

  • Resistance being incorporated into weed species.

  • Privacy issues related to genetic testing.

  • Ethical concerns with gene therapy and gene selection in humans.

  • Technology limits the science that can be done, and societal needs should guide scientific conduct.

Green Fluorescent Protein (GFP) Technology

• Fluorescence microscopy using green fluorescent protein (GFP) provides information about molecules on the cell surface.

  • Scientists attach GFP to specific cell parts.

  • When the specimen is exposed to ultraviolet light, GFP emits light of a different wavelength.

  • J.H. Coons (1941) used fluorescent-labelled antibody molecules to show antigens on cell surfaces (e.g., blood group molecules).

• GFP allows the study of living cells without staining techniques.

• Applications:

  • Studying Huntington's, Alzheimer’s, and Parkinson’s diseases at the molecular level.

    • Abnormal proteins clump together, inhibiting cell function.

    • GFP technology allows comparison of healthy and affected tissue.

  • Observing molecules (hormones, neurotransmitters) involved in cell-to-cell communication.

    • Messenger molecules attach to receptors, triggering a chain reactions.

  • Diagnosing diseases carried by viruses, bacteria, and protozoans and diseases of the immune system using fluorescent antibody techniques.

    • Binding between fluorescent-tagged antibody and the target antigen causes illumination when examined with a fluorescence microscope.

Three-Dimensional Structure of Molecules

• X-ray crystallography uses X-rays, sensors, and computer technology to determine molecular structure.

  • Essential in the model of DNA and our understanding of how DNA functions in the cell.

  • Currently, researchers study the 3D shape of proteins to understand how their activity is controlled.

The Cell

The Cell as an Efficient, Open System

• Cells are efficient, open systems that perform all life processes.

  • Open systems exchange matter and energy with their environment.

• Life processes include:

  • Intake of nutrients

  • Removal of waste

  • Movement

  • Growth

  • Reproduction

  • Response to stimuli

  • Exchange of gases

• Cells maintain these processes within specialized membrane-bound structures called organelles.

  • Ribosomes are the only organelle not enclosed by a membrane.

Chemical Composition of Cells

• Major elements: carbon, hydrogen, oxygen, and nitrogen.

• Organic compounds:

  1. Carbohydrates (e.g., sugar, starch, cellulose)

  2. Lipids (e.g., fats, oils)

  3. Proteins (e.g., hormones, enzymes, antibodies, muscle components)

  4. Nucleic acids (e.g., DNA and RNA)

• Inorganic substances: water and minerals (calcium, magnesium, sodium).

Cell Structures and Functions

• Cell membrane: protective barrier that regulates transport, interaction, communication, and molecule recognition.

• Nucleus: contains DNA, which directs cellular activities and is surrounded by a nuclear envelope with pores for transport.

• Cytoplasm: gel-like substance containing nutrients, supporting organelle suspension and cytoplasmic streaming.

• Cell wall (plants, bacteria, some protists, and fungi): a rigid frame that provides strength and support.

• Chloroplasts (plants and some protists): sites of photosynthesis, containing chlorophyll for converting carbon dioxide and water into sugars.

  • $6H<em>2O(l) + 6CO</em>2(g) \rightarrow C<em>6H</em>{12}O<em>6(aq) + 6O</em>2(g)$

• Vacuoles and vesicles: membrane-bound structures storing nutrients, secretions, and fats.

  • The central vacuole in plants stores water, maintaining turgor pressure.

  • Vesicles transport substances throughout and out of the cell.

• Ribosomes: dense granules composed of two parts, either free in the cytoplasm or attached to the endoplasmic reticulum where amino acids assemble into proteins (protein synthesis).

• Endoplasmic reticulum: interconnected tubes branching from the nuclear envelope, transporting materials.

  • Rough endoplasmic reticulum has ribosomes attached and is involved in protein synthesis.

  • Smooth endoplasmic reticulum is involved in lipid production.

• Lysosomes: membrane-bound sacs for digestion against bacteria and destruction of damaged cell organelles, and controlled tissue digestion during development.

• Golgi apparatus: flat, disc-shaped sacs involved in secretion that receive substances from the endoplasmic reticulum and packages them into vesicles for transport.

• Mitochondria: rod-like structures where cellular respiration occurs, converting chemical energy in sugars into usable energy.

  • $C<em>6H</em>{12}O<em>6(aq) + 6O</em>2(g) \rightarrow 6H<em>2O(l) + 6CO</em>2(g)$

Comparing Plant and Animal Cells

• Similarities:

  • Cell membrane

  • Cytoskeleton (proteins and lipids)

  • Genetic material (DNA: sugars, nitrogen bases, phosphate)

• Differences:

  • Centrioles (animal cells, involved in cell division; plant cells lack centrioles)

  • Cell wall (plant cells: cellulose; animal cells lack cell walls)

  • Chlorophyll (plant cells)

  • Specialized chemical compounds (hemoglobin/cholesterol in animal cells; starch/oils in plant cells)

  • Energy storage (glycogen/lipids in animal cells; starch/oils in plant cells)

  • Vacuoles (large central vacuole in plant cells; small vacuoles/vesicles in animal cells)

• Plant and animal cells are complementary.

  • Photosynthesis products (plant cells) are cellular respiration reactants (plant/animal cells), and vice versa.

  • This cycling of matter and flow of energy is important in the biosphere.

Cell Analogy Assignment

• Create an analogy for a plant or animal cell with a partner.

• Describe the function of each cell structure in relation to the analogy.

• Present the cell analogy in any format.

• Include the following cell structures:

  • Cell membrane

  • Cytoplasm

  • Nucleus

  • Lysosome

  • Vacuole

  • Vesicle

  • Mitochondrion

  • Smooth endoplasmic reticulum

  • Rough endoplasmic reticulum

  • Golgi apparatus

  • Ribosomes

  • Chloroplast (plant cells only)

  • Cell wall (plant cells only)

The Cell Membrane

Structure of the Cell Membrane

• The plasma membrane consists of a phospholipid bilayer with embedded proteins.

  • Phospholipids: double layer of lipids with a phosphate group.

    • Phosphates are hydrophilic (face outward into watery fluids).

    • Lipids are hydrophobic (face inward).

  • Proteins: suspend in the bilayer, attaching to the membrane's inside, outside, or running through.

  • Surface proteins: may have sugar molecules attached.

• Fluid-mosaic model (1972): the currently accepted structure.

  • The cell membrane looks like a mosaic of tiles (proteins) held together by fluid (lipid bilayer).

  • Molecules in the bilayer are in constant motion, allowing membrane protein arrangements to change.

Function of the Cell Membrane

• Important for maintaining equilibrium inside the cell.

• Selectively permeable (semi-permeable):

  • Allows passage of certain particles, but not all.

  • Determined by molecule size, charge, and lipid solubility

The Particle Model of Matter

1. All matter is made of particles, and particles differ in size and composition.

2. Particles are constantly moving (least in solids, most in gases). Energy affects particle movement.

3. Particles are attracted to or bonded together.

4. Particles have spaces between them (smallest in solids, largest in gases). Spaces can be occupied by other particles.

Concentration Gradients

• The difference in the concentration of a substance between two areas.

• Substances move along their concentration gradient until equilibrium is reached.

  • Particles maintain a balanced, even distribution at equilibrium.

• The steeper the concentration gradient, the faster the molecules will diffuse.

Transport Across the Cell Membrane

• The transport of gases, nutrients, and waste in and out of the cell is essential for the cell's survival.

• The cell membrane is responsible for transport.

• Substances may be ions, molecules, microorganisms, or other cells.

• Behavior can be examined with reference to the particle model of matter.

Diffusion

• The natural movement of particles from an area of higher concentration to an area of lower concentration.

• Passive transport: no added energy is required.

• The rate of diffusion can be increased by adding energy.

• In cells:

  • Particles soluble in lipids or small enough to pass through the pores will diffuse along their concentration gradient.

  • Carbon dioxide leaves the cell by diffusion.

  • Oxygen gas diffuses across the cell membrane into the cell.

• Diffusion also occurs within the cytoplasm if there is a concentration difference.

Osmosis

• The movement of water molecules along their concentration gradient from an area of higher water concentration to an area of lower water concentration.

• Passive transport type.

• If solute molecules cannot pass through the cell membrane there is net movement of water.

• Terminology:

  • Hypertonic: Solution with a higher solute concentration than that in a cell. Water will leave the cell.

  • Hypotonic: Solution with a lower concentration of solutes than that in a cell. Water will enter the cell.

  • Isotonic: Solution with the same concentration of solutes as a cell. No net water movement.

Facilitated Diffusion

• Substances that are soluble in water but not in lipids or are too large diffuse across cell membranes with facilitated diffusion.

• Passive transport form.

• Channel proteins create pores for water-soluble particles to move along the concentration gradient.

• Carrier proteins attach to molecules, change shape, and move the molecule across the membrane.

Active Transport

• The movement of particles across the cell membrane against the concentration gradient.

• Requires energy input (ATP).

• Used to concentrate materials inside the cell or expel waste materials.

• Carrier proteins work as a pump.

Endocytosis and Exocytosis

• Occurs when molecules are too large to pass across the cell membrane.

• Active transport form because it requires ATP energy for rearrangement of the cell membrane.

  • Endocytosis: A vesicle forms around the particle, and the cell membrane pinches off.

  • Exocytosis: A vesicle surrounds the particle, then fuses with the plasma membrane, releasing its contents outside the cell.

Applications of Knowledge of Semi-Permeable Membranes

• Research and compile information on how knowledge about semi-permeable membranes, diffusion, and osmosis is applied in various contexts.

• Summarize information in your own words and cite sources.

• Examples include:

  • Water purification

  • Peritoneal dialysis

  • Liposomes

  • Transdermal patches

  • Antibacterial agent by honey

Cell Size and Shape

The Ratio of Surface Area to Volume

• The transport of materials into and out of the cell is critical and determines cellular activity.

• As cell volume increases, more molecules need to be transported.

• A high surface area to volume ratio is needed for effective transport.

• As volume increases, the surface area-to-volume ratio decreases, resulting in small cells.

• A greater surface area to volume ratio results in efficient transport.

Surface Area to Volume Ratio Formulas

• Surface area of a cube: $SA = 6s^2$

• Volume of a cube: $V = s^3$

• Surface area of a rectangular prism: $SA = 2wl + 2hl + 2hw$

• Volume of a rectangular prism: $V = lwh$

• Surface area to volume ratio: $SA/V$

• $SA <br>ot{/} V= 6s^2<br>ot{/} s^3 = 6 <br>ot{/} s$

• If s=4, $SA <br>ot{/} V= 6 <br>ot{/} 4=1.5$

Cell Shape

• Many cells are specialized to increase surface area.

  • Intestinal cell membranes have microvilli.

  • Red blood cells have a biconcave shape for carbon dioxide and oxygen transport.

Practice Questions: Cell Size and Shape

Answer questions to prove the relations between all the parameters stated above.

Specialization of Cells, Tissues, and Systems in Plants

• Specialization is essential to deal with different functions as an organism grows.

• Multicellular organisms facilitate the movement of nutrients, gases, molecules, and wastes.

• Tissues: groups of cells performing the same function.

• Organs: tissues contributing to the same function.

• Organ systems: a system of contributing organs.

Specialization in Plant Systems

1. Shoot system: everything above ground (stem, leaves, buds, flowers, fruits, tubers).

2. Root system: everything underground, plus aerial roots.

Specialization in Plant Tissues

1. Dermal tissue/epidermis: outer layer of cells, typically one-cell-layer thick, covering herbaceous plants.

   • Shoot system's dermal tissue is involved in gas exchange.

   • Root system's dermal tissue is responsible for water and mineral uptake.

2. Ground tissue: makes up most of the plant found beneath the epidermis.

   • Provides strength and support in the stem.

   • Involved in food and water storage in the roots.

   • Photosynthesis occurs in the leaves.

   • Air spaces between cells allow gas diffusion.

3. Vascular tissue: transports materials throughout the plant.

   • Xylem tissue transports water and minerals from the roots.

     • Xylem vessels have thick-walled tubes.

     • Cells fuse together and end walls become perforated.

     • The contents of the cytoplasm break down, and the cells die, leaving the non-living cell walls attached together like a long straw.

   • Phloem tissue transports sucrose and sugars from the leaves.

     • Sieve tube cells have perforated end walls.

     • These cells remain alive but lose their nuclei.

     • Connected to companion cells that direct their activities.

     • Sugars provide energy, convert into cellulose, or store as starch in roots.

Specialization in Plant Cells

• Root cells produce root hairs to increase the surface area for absorption.

• Dermal cells produce a waxy cuticle to protect cells from pathogens and water loss.

• Guard cells form stomata for gas exchange.

• Cells that become part of the xylem specialize to conduct water and transport it to adjacent cells.

Specialization in the Leaf and Gas Exchange

• Leaves carry out and support photosynthesis.

Dermal Tissue

• Epidermis cells are clear to allow light to enter.

• Guard cells form stomata that allow gas exchange.

  • Most stomata are on the lower epidermis to prevent water loss.

  • Oxygen and carbon dioxide diffuse via the stomata.

  • Guard cells control whether the stomata are open or closed.

    • Light stimulates guard cells to accumulate potassium ions, increasing turgor pressure and opening the stoma; water enters to follow.

    • Darkness causes potassium to diffuse out, water follows, and stomata close.

• Transpiration: water is continually lost from the plant through stomata.

  • Water scarcity causes decreased turgor pressure and stomata closure.

• Lenticels: pores on woody stems that provide a pathway for gas exchange.

Ground Tissue

• Mesophyll: Ground tissues between the upper and lower epidermis.

  • Palisade tissue cells: long, rigid, rectangular cells tightly packed below the upper epidermis. They contain many chloroplasts for photosynthesis.

  • Spongy mesophyll tissue: loosely packed, irregularly shaped cells between palisade cells and the lower epidermis. The increased space between cells allows for the rapid gas exchange through the leaf. Cells contain some chloroplasts.

Vascular Tissue

• Xylem and phloem are bundled in a vascular bundle that forms leaf veins.

  • Vascular tissue provides water for transpiration and photosynthesis and removes sugars formed in photosynthesis.

  • Leaf vascular bundles are direct extensions of stem vascular bundles.

Transport Systems in Plants

Cohesion and Adhesion

• Cohesion: the attraction of water molecules to other water molecules due to their polar nature.

• Adhesion: the attraction of water molecules to molecules of other substances due to their polar nature.

Water Transport in Plants

• Root pressure pushes water upward.

  • Active transport results in dissolved minerals being present in the cells of the root, thus producing a higher solute concentration inside the cell.

  • Water is drawn into the cells via osmosis, creating positive pressure that forces fluid into the xylem.

• Transpiration pull: the evaporation of water through stomata and lenticels creates the tension, and it draws water up the stem.

  • Water from under the leaves is being pulled up into them.

  • Each water molecule creates a pull on adjacent molecules.

  • Combined with the forces of adhesion and cohesion.

The Effect of Tonicity on Plant Cells

• Plasmolysis: the loss of water from a plant cell in a hypertonic solution.

  • Water moves from the vacuole into the surroundings via osmosis.

  • Cell contents pull away from the cell wall, and the cell membrane becomes visible.

  • The result is a wilted or limp leaf.

• Deplasmolysis: water re-enters the cell in fresh water.

  • The vacuole swells, and internal pressure increases until the cell becomes turgid.

  • The pressure supports the plant.

Sugar Transport in Plants

• How phloem transports products of photosynthesis

• Pressure-flow theory: explains products moving from leaves, the source, to be used and stored, the sink.

  • Sugar is actively transported into the phloem, and water follows by osmosis at the leaf.

  • Increasing the water pressure will push the water and sugars throught the pholem into the rest of the plant.

  • Sugars are actively transported from sieve tube cells.

  • Water pressure decreases to the cells and the water is sucked into those cells.

  • This cycle sustains the water and nutrients being transfered to to cells.

Plant Control Systems

Phototropism

• The directional growth of a plant in response to light.

  • Stems show positive phototropism (grow toward light).

  • Roots show weak negative phototropism (grow away from light).

Investigations of Phototropism

• Charles and Francis Darwin (1880) determined that the tip of the stem detects and responds to light.

  • Plants with tip buried did not respond to light.

  • These scientists inferred that the cells of the tip communicate with other cells. They just didn't know how.

• Peter Boysen-Jensen (1913) investigated communication from the tip to the area of elongation.

  • Snipped grass seedlings had tips covered with gelatin, and growth happened as normal.

  • The same was done with the thin slice of mica, and growth was unobserved.

  • Area of elongation: the phototropic response was created by the elongation of cells on the side of the leaf facing away from the light, causing the leaf to bend toward the light.

  • The conclusion was that the factor can be diffused through gelatin but not mica.

• F. W. Went (1926) isolated the chemical substance called auxin.

  • This caused elongation for growth and more analysis indicated it's a hormone.

  • Hormones are a chemical comound that is manufactured somewhere and transported elsewhere that creates a physiological response by initiation.

Gravitropism

• The growth of plants in response to Earth’s gravitational force.

  • Stems show negative gravitropism (grow against gravity).

  • Roots show positive gravitropism (grow toward gravity).

  • Plants rely on heavy starch particles to indicate gravity.

    • When a plant is tipped, the starch grains trigger a response.

Practice Questions: Plant Control Systems

What are the differences and effects of things like positive phototropism and the roles, and what the observations and other results are.