Organismal biology is roughly one-third of the CLEP exam, and plant items lean on structure-function reasoning and simple data interpretation. The classic transpiration data-table set (a shared table with 2–3 attached questions) shows up here, so this lesson gives you both the concepts and the reading skill.
A vascular plant has three organs: - Roots — anchor the plant and absorb water and minerals (root hairs increase surface area). - Stems — support the plant and connect roots to leaves; house vascular tissue. - Leaves — the main sites of photosynthesis; flattened for light capture and gas exchange.
Two vascular tissues run continuously through the plant:
| Tissue | Carries | Direction | Note |
|---|---|---|---|
| Xylem | Water + dissolved minerals | Roots → upward to leaves | Made of dead, hollow cells |
| Phloem | Sugars (made in photosynthesis) | Source → sink (both ways) | Made of living cells |
A memory hook: "Xylem = water up; phloem = food flows to where it's needed."
Transpiration is the loss of water vapor from a plant, mainly through stomata — pores on the leaf underside. Each stoma is flanked by two guard cells that swell to open the pore and shrink to close it.
Transpiration creates a pull: as water evaporates from leaf surfaces, it drags a continuous column of water up the xylem (the transpiration–cohesion–tension mechanism). This is how water reaches the top of tall plants.
The trade-off: open stomata let CO₂ in for photosynthesis but let water out. Under heat or drought, guard cells close the stomata to conserve water, even at the cost of slowing photosynthesis.
Factors that increase transpiration: dry air (low humidity), wind, heat, and bright light (which opens stomata). Factors that decrease it: high humidity, still air, and darkness.
[GRAPH: Leaf cross-section — waxy cuticle on top, palisade layer of photosynthetic cells, spongy layer with air spaces, and a lower epidermis dotted with stomata each bracketed by two guard cells.]
Worked transpiration table (used in Questions 4–5):
| Condition | Transpiration rate (mL/hr) |
|---|---|
| Still, humid air | 1.0 |
| Still, dry air | 2.5 |
| Windy, dry air | 4.0 |
| Dark, humid air | 0.5 |
A tropism is directional growth in response to a stimulus: - Phototropism — growth toward light. - Gravitropism (geotropism) — roots grow down, shoots grow up. - Thigmotropism — growth in response to touch (vines curling).
Two hormones to know at CLEP breadth: - Auxins — promote cell elongation; they drive phototropism. When light hits one side of a shoot, auxin accumulates on the shaded side, making those cells elongate more so the shoot bends toward the light. - Gibberellins — promote stem elongation and help break seed dormancy (stimulating germination); famously make dwarf plants grow tall.
Plants alternate between two multicellular forms: - Sporophyte — diploid (2n); produces haploid spores by meiosis. - Gametophyte — haploid (n); produces gametes (egg and sperm) by mitosis.
Fertilization (sperm + egg) restores the diploid sporophyte. In flowering plants the sporophyte dominates and the gametophyte is tiny (inside flower structures).
Flower parts:
| Part | Role |
|---|---|
| Sepals | Protect the flower bud |
| Petals | Often attract pollinators |
| Stamen (anther + filament) | Male; anther makes pollen |
| Carpel/pistil (stigma, style, ovary) | Female; ovary holds ovules |
Pollination transfers pollen to the stigma. A pollen tube grows down to the ovule, and double fertilization occurs: one sperm fertilizes the egg (→ diploid embryo); the other sperm fuses with two polar nuclei (→ triploid endosperm, the embryo's food supply).
After fertilization: - The ovule becomes the seed (embryo + endosperm + seed coat). - The ovary becomes the fruit, which protects seeds and aids dispersal.
Seed germination is the resumption of embryo growth. It requires water (imbibition), oxygen, and suitable temperature. Many seeds do not require light — some germinate best in darkness — so light is not a universal requirement.
1. A — Xylem. Fix: Xylem is the water-conducting tissue, moving water and minerals upward from the roots. - B: Phloem carries sugars, not water uptake. - C: Epidermis is a protective covering, not a conducting tissue. - D: Cambium is a growth (meristem) layer that produces new xylem/phloem, not the conductor itself. - E: Cortex is ground/storage tissue, not vascular. - Fix rule: "Xylem = water up."
2. B — Transporting sugars made in photosynthesis throughout the plant. Fix: Phloem moves dissolved sugars from sources (leaves) to sinks (roots, fruits). - A: Water transport upward is xylem's job. - C: Anchoring is a root function, not a tissue's transport role. - D: Light absorption is done by chlorophyll in leaf cells, not phloem. - E: Gas exchange happens through stomata, not phloem. - Fix rule: "Phloem = food flows to where it's needed."
3. D — The loss of water vapor from a plant, mainly through the stomata. Fix: Transpiration is evaporative water loss, chiefly through leaf stomata. - A: Mineral uptake by roots is a different process. - B: Sugar movement into fruit is phloem transport (translocation). - C: Carbon fixation is part of photosynthesis, not water loss. - E: Resumed growth of a seed is germination. - Fix rule: Transpiration = water vapor exiting the leaf.
Questions 4–5 refer to the following table, which shows the transpiration rate of a leafy plant under four conditions.
| Condition | Transpiration rate (mL/hr) |
|---|---|
| Still, humid air | 1.0 |
| Still, dry air | 2.5 |
| Windy, dry air | 4.0 |
| Dark, humid air | 0.5 |
4. C — Windy, dry air (4.0 mL/hr). Fix: The table's largest value is 4.0 mL/hr, under windy, dry air. - A: Still, humid air is only 1.0 mL/hr. - B: Dark, humid air is the lowest at 0.5 mL/hr. - D: Still, dry air (2.5) is high but below the windy condition. - E: "Cool, still night air" is not a row in the table and can't be selected from the data. - Fix rule: Read the column and pick the largest listed value.
5. C — Wind sweeping water vapor away from the stomata, steepening the water-vapor gradient. Fix: Moving air removes humid air at the leaf surface, so water evaporates faster. - A: Soil minerals don't drive transpiration rate. - B: Higher humidity would slow transpiration, not speed it. - D: Stomata closing would lower the rate, contradicting the increase shown. - E: Light was not reduced; dryness is constant across both rows — wind is the variable. - Fix rule: Wind removes the moist boundary layer, steepening the gradient and raising transpiration.
6. D — Closing the stomata to reduce water loss. Fix: Under water stress, guard cells lose turgor and close the pore to conserve water. - A: Opening wider would worsen dehydration — the opposite response. - B: Making chlorophyll doesn't address water loss. - C: Phloem sugar transport is unrelated to the immediate water crisis. - E: Root hairs form on roots, not leaves. - Fix rule: Water-stressed plant → guard cells close the stomata.
7. C — Auxin accumulating on the shaded side, causing those cells to elongate. Fix: Uneven auxin makes shaded-side cells grow longer, bending the shoot toward the light. - A: Gibberellins affect germination/elongation, not the directional light-bending mechanism. - B: Even auxin would produce straight growth, not bending. - D: Stomatal closure doesn't bend a stem. - E: Transpiration differences don't cause phototropic bending. - Fix rule: Auxin piles up on the dark side; that side elongates and the shoot leans toward light.
8. B — Gibberellin. Fix: Gibberellins promote stem elongation and can make dwarf plants grow tall. - A: Auxin governs phototropism and elongation but is not the classic dwarf-to-tall hormone here. - C: Abscisic acid promotes dormancy and stomatal closure — the opposite of growth. - D: Ethylene drives fruit ripening and leaf drop, not stem lengthening. - E: Cytokinins promote cell division, not the dramatic stem elongation described. - Fix rule: Dwarf plants shoot up → gibberellin.
9. A — Mitosis. Fix: The gametophyte is already haploid, so it makes gametes by ordinary mitosis (no ploidy change needed). - B: Meiosis is how the diploid sporophyte makes spores, not how the gametophyte makes gametes. - C: Binary fission is prokaryotic reproduction. - D: Fertilization fuses gametes; it doesn't produce them. - E: Budding is asexual reproduction, not gamete formation in the plant life cycle. - Fix rule: Haploid gametophyte → gametes by mitosis; diploid sporophyte → spores by meiosis.
10. D — Ovary. Fix: After fertilization the ovary matures into the fruit that encloses the seeds. - A: The anther produces pollen and does not become fruit. - B: The stigma receives pollen but is not the fruit-forming structure. - C: Petals typically wither after pollination. - E: Sepals protect the bud and do not develop into fruit. - Fix rule: Ovule → seed; ovary → fruit.
11. E — Triploid tissue formed when a sperm nucleus fuses with two polar nuclei. Fix: In double fertilization, a second sperm joins two polar nuclei, making 3n endosperm to feed the embryo. - A: Endosperm is not haploid, nor is it formed from the egg alone. - B: It is not diploid, and it comes from fertilization, not the ovary wall. - C: Endosperm forms as part of fertilization, not before pollination. - D: The seed coat is a separate, protective outer layer. - Fix rule: Double fertilization = diploid embryo (egg + sperm) + triploid endosperm (2 polar nuclei + sperm).
12. E — The claim is questionable, because many seeds germinate in darkness — light is not a universal requirement. Fix: Germination needs water, oxygen, and suitable temperature; light is not required for most seeds, so darkness alone is a weak explanation. - A: "All seeds require light" is false; many germinate underground in the dark. - B: Germination energy comes from stored seed reserves, not light. - C: The seeds were well-oxygenated; CO₂ deficiency is not the issue. - D: Seeds carry their own food reserves and don't need external nutrients to germinate. - Fix rule: Germination essentials are water, oxygen, and warmth — light is optional for most seeds.
1. A — Xylem. Fix: Xylem is the water-conducting tissue, moving water and minerals upward from the roots. - B: Phloem carries sugars, not water uptake. - C: Epidermis is a protective covering, not a conducting tissue. - D: Cambium is a growth (meristem) layer that produces new xylem/phloem, not the conductor itself. - E: Cortex is ground/storage tissue, not vascular. - Fix rule: "Xylem = water up."
2. B — Transporting sugars made in photosynthesis throughout the plant. Fix: Phloem moves dissolved sugars from sources (leaves) to sinks (roots, fruits). - A: Water transport upward is xylem's job. - C: Anchoring is a root function, not a tissue's transport role. - D: Light absorption is done by chlorophyll in leaf cells, not phloem. - E: Gas exchange happens through stomata, not phloem. - Fix rule: "Phloem = food flows to where it's needed."
3. D — The loss of water vapor from a plant, mainly through the stomata. Fix: Transpiration is evaporative water loss, chiefly through leaf stomata. - A: Mineral uptake by roots is a different process. - B: Sugar movement into fruit is phloem transport (translocation). - C: Carbon fixation is part of photosynthesis, not water loss. - E: Resumed growth of a seed is germination. - Fix rule: Transpiration = water vapor exiting the leaf.
4. C — Windy, dry air (4.0 mL/hr). Fix: The table's largest value is 4.0 mL/hr, under windy, dry air. - A: Still, humid air is only 1.0 mL/hr. - B: Dark, humid air is the lowest at 0.5 mL/hr. - D: Still, dry air (2.5) is high but below the windy condition. - E: "Cool, still night air" is not a row in the table and can't be selected from the data. - Fix rule: Read the column and pick the largest listed value.
5. C — Wind sweeping water vapor away from the stomata, steepening the water-vapor gradient. Fix: Moving air removes humid air at the leaf surface, so water evaporates faster. - A: Soil minerals don't drive transpiration rate. - B: Higher humidity would slow transpiration, not speed it. - D: Stomata closing would lower the rate, contradicting the increase shown. - E: Light was not reduced; dryness is constant across both rows — wind is the variable. - Fix rule: Wind removes the moist boundary layer, steepening the gradient and raising transpiration.
6. D — Closing the stomata to reduce water loss. Fix: Under water stress, guard cells lose turgor and close the pore to conserve water. - A: Opening wider would worsen dehydration — the opposite response. - B: Making chlorophyll doesn't address water loss. - C: Phloem sugar transport is unrelated to the immediate water crisis. - E: Root hairs form on roots, not leaves. - Fix rule: Water-stressed plant → guard cells close the stomata.
7. C — Auxin accumulating on the shaded side, causing those cells to elongate. Fix: Uneven auxin makes shaded-side cells grow longer, bending the shoot toward the light. - A: Gibberellins affect germination/elongation, not the directional light-bending mechanism. - B: Even auxin would produce straight growth, not bending. - D: Stomatal closure doesn't bend a stem. - E: Transpiration differences don't cause phototropic bending. - Fix rule: Auxin piles up on the dark side; that side elongates and the shoot leans toward light.
8. B — Gibberellin. Fix: Gibberellins promote stem elongation and can make dwarf plants grow tall. - A: Auxin governs phototropism and elongation but is not the classic dwarf-to-tall hormone here. - C: Abscisic acid promotes dormancy and stomatal closure — the opposite of growth. - D: Ethylene drives fruit ripening and leaf drop, not stem lengthening. - E: Cytokinins promote cell division, not the dramatic stem elongation described. - Fix rule: Dwarf plants shoot up → gibberellin.
9. A — Mitosis. Fix: The gametophyte is already haploid, so it makes gametes by ordinary mitosis (no ploidy change needed). - B: Meiosis is how the diploid sporophyte makes spores, not how the gametophyte makes gametes. - C: Binary fission is prokaryotic reproduction. - D: Fertilization fuses gametes; it doesn't produce them. - E: Budding is asexual reproduction, not gamete formation in the plant life cycle. - Fix rule: Haploid gametophyte → gametes by mitosis; diploid sporophyte → spores by meiosis.
10. D — Ovary. Fix: After fertilization the ovary matures into the fruit that encloses the seeds. - A: The anther produces pollen and does not become fruit. - B: The stigma receives pollen but is not the fruit-forming structure. - C: Petals typically wither after pollination. - E: Sepals protect the bud and do not develop into fruit. - Fix rule: Ovule → seed; ovary → fruit.
11. E — Triploid tissue formed when a sperm nucleus fuses with two polar nuclei. Fix: In double fertilization, a second sperm joins two polar nuclei, making 3n endosperm to feed the embryo. - A: Endosperm is not haploid, nor is it formed from the egg alone. - B: It is not diploid, and it comes from fertilization, not the ovary wall. - C: Endosperm forms as part of fertilization, not before pollination. - D: The seed coat is a separate, protective outer layer. - Fix rule: Double fertilization = diploid embryo (egg + sperm) + triploid endosperm (2 polar nuclei + sperm).
12. E — The claim is questionable, because many seeds germinate in darkness — light is not a universal requirement. Fix: Germination needs water, oxygen, and suitable temperature; light is not required for most seeds, so darkness alone is a weak explanation. - A: "All seeds require light" is false; many germinate underground in the dark. - B: Germination energy comes from stored seed reserves, not light. - C: The seeds were well-oxygenated; CO₂ deficiency is not the issue. - D: Seeds carry their own food reserves and don't need external nutrients to germinate. - Fix rule: Germination essentials are water, oxygen, and warmth — light is optional for most seeds.