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Flashcards · J247 · Topic 2

Scaling up flashcards

24 cards on Topic 2, subtopics B2.1 to B2.2. Pick a subtopic or study them all.

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B2.1 Supplying the cell

Diffusion
Net movement of particles from more to less concentrated, down a concentration gradient. Passive.
Osmosis
Net movement of water across a partially permeable membrane from higher water potential to lower water potential. Passive.
Active transport
Movement of particles from lower to higher concentration, against the gradient, using energy from respiration.
Water potential
A measure of how freely water can move. Pure water has the highest; more solute lowers it. Water moves from higher to lower water potential.
An animal cell and a plant cell are put in a solution with a lower water potential than the cell. What happens?
Both lose water by osmosis. The animal cell shrinks. The plant cell becomes flaccid and its membrane pulls away from the wall. (In a more dilute solution the animal cell may burst; the plant cell becomes turgid.)
A potato chip goes from 2.50 g to 2.75 g. Percentage change?
(2.75 − 2.50) ÷ 2.50 × 100 = +10.0%. Always divide by the initial mass.
The cell cycle in order
Cell growth (more ribosomes, mitochondria), DNA replication, more cell growth, movement of chromosomes (mitosis), then division of the cytoplasm to give two genetically identical cells.
A cell has 12 chromosomes. How many chromatids after replication, and chromosomes in each daughter cell?
After replication: 12 chromosomes, each of two chromatids (24 chromatids). Each daughter cell: 12 chromosomes.
200 cells are counted in a root tip and 18 are in mitosis. Percentage?
18 ÷ 200 × 100 = 9.0%. A high percentage means rapid growth.
Why does differentiation help a multicellular organism? Give two specialised cells.
Specialised cells each do one job efficiently, so the organism works better. Red blood cell: biconcave, no nucleus, haemoglobin, to carry oxygen. Palisade cell: many chloroplasts for photosynthesis.
Embryonic stem cells, adult stem cells and meristems
Embryonic (early embryo): can become any cell type. Adult (e.g. bone marrow): a limited range; repair and replacement, making blood cells. Meristems (root and shoot tips): divide throughout the plant's life, forming tissues such as xylem and phloem.
Why does a root hair cell need many mitochondria to take up nitrate?
Nitrate is often less concentrated in the soil than in the cell, so it is moved in by active transport, using carrier proteins and energy from respiration in the mitochondria.

B2.2 The challenges of size

Why does a large organism need exchange surfaces and a transport system?
As size increases, volume rises faster than surface area, so SA : V falls. The distance to inner cells is long and diffusion alone is too slow, so it needs a large, thin exchange surface and a transport system.
Surface area to volume ratio of a cube of side 10 mm
SA = 6 × 10² = 600 mm²; V = 10³ = 1000 mm³; ratio = 0.6 : 1. Doubling the side halves the ratio.
Double circulation, and why mammals have one
Blood goes heart → lungs → heart, then heart → body → heart. Blood loses pressure in the lung capillaries, so it returns to be pumped hard again to the body; the lungs get lower pressure, which protects their fragile capillaries.
Heart valves: names and positions
Tricuspid (right) and bicuspid (left) valves lie between the atria and ventricles. Semilunar valves are at the exits into the aorta and pulmonary artery. All stop backflow.
Arteries, veins and capillaries: structure linked to function
Artery: thick wall with muscle and elastic fibres, narrow lumen; stretches and recoils under high pressure. Vein: thin wall, wide lumen, valves for low pressure. Capillary: one cell thick, for a short diffusion distance.
Why does blood flow slowly in capillaries?
The total cross-sectional area of all the capillaries is far larger than that of the aorta, so flow is slow and there is time for diffusion. Narrow width alone is not the reason.
Red blood cell adaptations, and what plasma carries
Biconcave disc (large surface area, flexible); no nucleus (more haemoglobin); haemoglobin picks up oxygen in the lungs and gives it up in tissues. Plasma carries dissolved CO₂, glucose, amino acids, mineral ions, urea and hormones.
How do water and mineral ions enter a root hair cell?
Water: by osmosis from the dilute soil water. Mineral ions: mostly by active transport, against the gradient. The long thin hair gives a large surface area.
Transpiration, in order
Water evaporates from mesophyll cell surfaces into the leaf air spaces; water vapour diffuses out through open stomata; water is pulled up the xylem to replace it (the transpiration stream), carrying mineral ions.
Xylem vs phloem
Xylem: water and mineral ions, upwards only; dead hollow cells, no end walls, lignin. Phloem: dissolved sugars, both ways (translocation, from sources to sinks); living sieve tubes with perforated end walls and companion cells.
Effect of light, temperature, air movement and humidity on the rate of transpiration
Light up: rate up (stomata open wider). Temperature up: rate up (faster evaporation and diffusion). Air movement up: rate up (vapour swept away, gradient kept steep). Humidity up: rate down (smaller gradient).
A potometer bubble moves 63 mm in 7.0 min in a capillary of radius 0.40 mm. Volume of water taken up per minute?
Rate = 63 ÷ 7.0 = 9.0 mm per minute. Area = π × 0.40² = 0.503 mm². Volume = 9.0 × 0.503 = 4.5 mm³ per minute. A potometer measures uptake, which approximates transpiration.

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