Study the illustration. the small blue spheres represent water molecules.

in which direction will water molecules move across the membrane?

there will be no net movement of water.

water will move from a to b.

water will move from b to a.

Water will move from B to A.

Explanation:

In passive diffusion, the small water molecules can move across the phospholipid bilayer, seen in pink. This layer acts as a semi-permeable membrane; its hydrophilic heads are attracted to water (seen facing outwards) while its water-repellent hydrophobic tails face towards each other- allowing  molecules of water to diffuse across the membrane down the concentration gradient.

Thus the water will move from an area of high concentration (more blue dots) to an area of low concentration  (i.e. from B to A), until the system reaches a steady state called equilibrium- after  which, there will be no net movement of water. Similarly via osmosis, the water passes through the membrane due to the difference in osmotic pressure on either side of the phospholipid by layer this means that the water moves from regions of high osmotic pressure/concentration to regions of low pressure/ concentration to a steady state.

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

explanation:

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