Scaling up Study Guide

Plants absorb water primarily through their roots, which then moves up through the stem to the leaves by a process called transpiration pull. Transpiration is the evaporation of water vapour from the leaves into the atmosphere through small pores called stomata. Several environmental factors influence the rate of transpiration, and thus the rate of water uptake.

Temperature affects the rate of water uptake because higher temperatures increase the kinetic energy of water molecules, leading to faster evaporation from the leaf surface. This steepens the water potential gradient between the leaf and the external air, pulling more water up the plant.

Humidity refers to the amount of water vapour in the air. When humidity is high, the air contains a lot of water vapour, reducing the water potential gradient between the inside of the leaf and the outside air. This slows down the rate of evaporation, and consequently, the rate of water uptake.

Wind speed influences water uptake by removing humid air from around the stomata. When the air around the leaf is still, it can become saturated with water vapour, reducing transpiration. A strong breeze sweeps away this humid air, maintaining a steep water potential gradient and increasing the rate of evaporation and water uptake.

The intensity of light affects water uptake because light stimulates the stomata to open. Stomata are pores on the leaf surface that allow for gas exchange, including carbon dioxide uptake for photosynthesis. When stomata are open, water vapour can escape more easily, increasing the rate of transpiration and water uptake. In darkness, stomata typically close, reducing water loss. Therefore, during the day with higher light intensity, water uptake is usually greater.

Cells are the fundamental units of life, and for them to function correctly, they need to acquire essential substances from their environment and remove waste products. This exchange of materials occurs across the cell membrane, which acts as a partially permeable barrier, controlling what enters and leaves the cell. There are three primary mechanisms for this transport: diffusion, osmosis, and active transport.

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. This process does not require energy from the cell and continues until the particles are evenly distributed, reaching equilibrium. Examples include the uptake of oxygen into cells and the release of carbon dioxide from cells. Factors affecting the rate of diffusion include temperature, concentration gradient, surface area, and distance.

Osmosis is a special type of diffusion that involves the net movement of water molecules across a partially permeable membrane from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution). Like diffusion, osmosis is a passive process and does not require energy. It plays a crucial role in maintaining cell turgor in plants and preventing osmotic lysis or crenation in animal cells.

Active transport is a process that moves substances across a cell membrane against their concentration gradient, meaning from an area of lower concentration to an area of higher concentration. This process requires energy, usually supplied by ATP generated during respiration, and involves specific carrier proteins within the cell membrane. Active transport is vital for cells to absorb nutrients, such as mineral ions by root hair cells or glucose by epithelial cells in the small intestine, even when their external concentration is lower.