Maintaining Life Chapter Notes | Year 9 Science IGCSE (Cambridge) PDF Download

Plants and Water

• Water is essential for plants, supporting absorption, transport, and various physiological processes critical for their survival and growth.

How Root Hairs Absorb Water

• Plants, composed mostly of water, continuously absorb water through their roots from the soil.
• Soil consists of irregularly shaped rock particles of varying sizes, with spaces between them filled with air and water.
• Root hairs, specialized cells extending from the root surface, significantly increase the surface area for absorbing water and mineral ions.
• Root hairs are microscopic but visible without a lens, each being part of a single cell.
• Water moves from the soil into the root hair cell through the cell wall and membrane, entering the cytoplasm.
• Minerals, such as magnesium and nitrate ions, dissolve in the soil water and are absorbed along with the water into the root hair cell.

How Water Moves Up the Plant

• A cross-section of a root reveals root hairs on the exterior and xylem vessels at the center, which form the plant’s water transport system.
• After absorption by root hair cells, water moves inward across the root to the xylem vessels.
• Xylem vessels are long, tube-like dead cells with no cytoplasm or nucleus, consisting only of cell walls and an empty interior.
• The end walls of xylem vessels disappear, allowing many vessels to stack into continuous tubes from roots to the plant’s highest parts.
• In trees, the wood in the trunk comprises xylem vessels that transport water from roots to the top.
• Xylem vessels, visible under a microscope, have diameters around 0.05 mm and vary in size.

Xylem in Roots and Stems

• Cross-sections of roots and stems show xylem vessels (in red), indicating their positions.
• In roots, xylem is located centrally, often in a cross-shaped area, facilitating water transport from root hairs to the stem.
• In stems, xylem is distributed to support water transport to leaves and other parts.

Transpiration

• Transpiration is the loss of water vapor from leaves, a key process in water movement through plants.

How Water Moves Through Leaves

• Water transported from roots through xylem vessels reaches the leaves.
• In leaves, water exits xylem vessels and enters leaf cells; cells with chloroplasts use some water for photosynthesis, but most water is not retained.
• Liquid water soaks into the cellulose cell walls, evaporates into water vapor, and diffuses into air spaces between leaf cells.
• Air spaces connect to the leaf’s exterior via stomata, tiny holes on the leaf underside, through which water vapor diffuses into the air.
• This evaporation and diffusion process constitutes transpiration.

Why Plants Need Water

• Water is vital for multiple plant functions, including support, transport, cooling, and photosynthesis.
• Water for Support: Water-filled plant cells, especially in vacuoles, are firm, pressing against each other to keep the plant upright; water-deficient cells become floppy, causing the plant to wilt.
• Water for Transport: Water in xylem vessels carries dissolved mineral salts from roots to other plant parts.
• Water for Cooling: Evaporation of water from leaf cell walls removes heat, cooling the plant, which is crucial in hot environments.
• Water for Photosynthesis: Water is a reactant in photosynthesis, combining with carbon dioxide to produce glucose and oxygen.

Excretion in Humans

• Excretion is the process of eliminating waste substances produced or accumulated within the body, essential for maintaining health.

What is Excretion?

• Excretion involves removing waste materials that have been part of the body, not all waste materials (e.g., faeces are not excreted).
• Food traveling through the digestive system (from mouth to anus) is not truly part of the body; only substances absorbed into blood or cells are.
• Faeces, waste from the digestive system, do not count as excretion since they were never part of the body.
• Excretion includes waste substances made in cells and excess substances in the body, such as carbon dioxide (from respiration), urea (made in liver cells), and excess water.

Excreting Urea

• Proteins in food are broken down into smaller molecules in the digestive system, absorbed into the blood, and transported to the liver.
• Excess protein is converted by the liver into urea, a poisonous substance that can cause illness if it accumulates.
• Urea is immediately carried away from the liver by the blood.
• The kidneys, part of the excretory (renal) system, filter urea and excess water from the blood.
• Urea dissolves in the excess water, forming urine, which flows from each kidney through a ureter to the bladder for storage.
• Urine exits the bladder via the urethra to the outside.

Keeping a Fetus Healthy

• A healthy pregnancy ensures the fetus, a developing baby, receives necessary nutrients and has waste removed, relying on the mother’s health.

A Healthy Pregnancy

• A fetus grows inside its mother for nine months, requiring nutrients, energy from respiration, and waste excretion, all provided by the mother.
• The mother’s and fetus’s blood systems do not mix but come close, allowing nutrient and waste diffusion between them.
• Nutrients diffuse from the mother’s blood to the fetus’s blood, while waste substances diffuse in the opposite direction.
• A healthy mother supports a healthy fetus through proper diet and lifestyle choices.

Diet

• A balanced diet during pregnancy is crucial, providing all necessary nutrients for the mother and fetus.
• Pregnant women need slightly more food to supply nutrients to the growing fetus.
• Protein: Essential for fetal cell growth and maternal muscle strength; extra protein supports increased haemoglobin production for oxygen transport to both mother and fetus.
• Carbohydrate: Supplies energy via glucose for respiration; adequate intake ensures energy for both mother and fetus, but excess can lead to unhealthy weight gain.
• Vitamins and Minerals: Iron is critical for haemoglobin production; calcium supports fetal bone growth and maternal bone/teeth health; vitamins are essential for overall health.
• Iron supplements may be recommended if dietary iron is insufficient.

Smoking Cigarettes

• Smoking during pregnancy allows harmful substances like carbon monoxide and nicotine to diffuse from the mother’s blood to the fetus’s, as if the fetus is smoking.
• Carbon monoxide reduces oxygen transport by haemoglobin, limiting fetal cell respiration and energy release, potentially affecting development and resulting in smaller babies.
• Nicotine, an addictive drug, damages blood vessels, harming the developing fetus.
• Cigarette packets often carry warnings about health risks, including specific warnings for pregnant women.

Drugs

• Drugs are substances that affect body function; medicinal drugs like antibiotics and painkillers (e.g., aspirin) can be beneficial, but unnecessary drugs can be harmful.
• Most drugs taken by a pregnant woman enter the fetus’s blood, affecting the fetus.
• Recommendations for pregnant women in some countries include:
  • Consulting a doctor or pharmacist before continuing prescribed drugs to ensure safety for the fetus.
  • Checking with a doctor before taking over-the-counter drugs like aspirin.
  • Stopping smoking completely.
  • Avoiding alcohol.
  • Never using illegal drugs, especially during pregnancy.
  • Limiting caffeine (in coffee and cola) due to evidence of potential harm to the fetus.