Scaling up Revision Notes

The Cell Cycle: Interphase and Mitosis

The cell cycle is the series of events a cell goes through from its formation to its division into two daughter cells. It consists of two main phases: Interphase and the Mitotic (M) phase. Interphase is the longest part of the cell cycle and is a period of intense growth and metabolic activity. During interphase, the cell prepares for division by synthesising new organelles, growing in size, and importantly, replicating its DNA. The M phase includes both mitosis (nuclear division) and cytokinesis (cytoplasmic division). Mitosis is essential for growth, repair of tissues, and asexual reproduction, ensuring that new cells are genetically identical to the parent cell.

Stages of Mitosis

Mitosis is a continuous process, but it's typically divided into four main stages: prophase, metaphase, anaphase, and telophase. During prophase, chromosomes condense and become visible, and the nuclear envelope starts to break down. In metaphase, the condensed chromosomes line up along the equator (middle) of the cell. Anaphase is characterised by sister chromatids (identical copies of a chromosome) separating and moving to opposite poles of the cell. Finally, in telophase, the separated chromosomes arrive at the poles, decondense, and new nuclear envelopes form around each set of chromosomes. This results in two separate nuclei within the original cell.

Cytokinesis: Dividing the Cytoplasm

Following mitosis, cytokinesis occurs. This is the process where the cell's cytoplasm divides to form two distinct daughter cells. In animal cells, the cell membrane pinches inwards, forming a cleavage furrow that eventually splits the cell into two. In plant cells, a new cell wall is formed between the two nuclei, effectively dividing the original cell. Each daughter cell produced is genetically identical to the parent cell, receiving an exact copy of the DNA, which is crucial for maintaining genetic consistency in multicellular organisms and for successful asexual reproduction.

Observing Mitosis in a Practical Setting

To observe mitosis, root tips from plants like onion are typically used because they have regions of active cell division. A common practical involves preparing a squash slide of a stained root tip. The root tip is macerated (softened) and then stained with a dye like acetocarmine or toluidine blue, which binds to DNA and makes the chromosomes visible. The stained root tip is then squashed gently to spread the cells into a single layer, allowing for clear observation under a light microscope. By observing cells, different stages of mitosis and interphase can be identified. This allows for an estimation of the relative time spent in each stage and highlights the dynamic nature of the cell cycle in living organisms.

How a Simple Potometer Works

A simple potometer is a device used to estimate the rate of water uptake by a plant shoot, which is largely due to transpiration. The apparatus consists of a plant shoot inserted into a rubber bung, which then seals into a glass tube filled with water. A capillary tube, often with a scale, is connected to this glass tube. An air bubble is introduced into the capillary tube. As the plant transpires, it draws water from the apparatus, causing the air bubble to move along the scale. The distance the bubble travels in a given time period indicates the rate of water uptake. It is crucial that all connections are airtight to ensure accurate measurements.

Setting up a Potometer Experiment (PAG B6, B8)

When setting up a potometer, a healthy plant shoot should be cut underwater to prevent air bubbles from entering the xylem, which would block water transport. The shoot is then quickly and securely inserted into the potometer apparatus, ensuring a watertight seal. The entire apparatus should be filled with water, and an air bubble is introduced into the capillary tube. The starting position of the air bubble is recorded. The distance the bubble moves is measured over a specific time, for example, 5 minutes. This procedure can be repeated at least three times for reliability. Control variables such as the size of the shoot, light source, distance from light, and initial temperature should be kept constant when investigating the effect of a single factor. Safety considerations include careful handling of glassware and ensuring electrical equipment (like lamps) are used safely.

Investigating Factors Affecting Water Uptake

The rate of water uptake, and therefore transpiration, can be affected by several environmental factors: light intensity, temperature, humidity, and air movement. To investigate the effect of light intensity, the potometer can be placed at varying distances from a light source, or different light filters can be used. For temperature, the apparatus can be placed in temperature-controlled environments or near a heat source/fan. Humidity can be altered by placing the apparatus in a sealed bag with varying amounts of water vapour. Air movement can be investigated using a fan. For each investigation, only one factor is changed (independent variable) while all other factors (control variables) are kept constant. The dependent variable is the rate of water uptake, measured by the movement of the air bubble.

Interpreting Results and Calculations

The rate of water uptake is calculated as the distance the air bubble moves divided by the time taken. For example, if the bubble moves 10 mm in 5 minutes, the rate is 2 mm/minute. To calculate the volume of water taken up, the cross-sectional area of the capillary tube needs to be known (Area = πr²). If the tube has a radius of 0.5 mm, its area is π * (0.5 mm)² = 0.785 mm². If the bubble moved 10 mm, the volume would be 0.785 mm² * 10 mm = 7.85 mm³. When presenting results, line graphs are often suitable for showing how the rate of water uptake changes with a continuous independent variable like light intensity. It is important to identify sources of error, such as air leaks or variations in shoot size, and consider ways to improve the reliability and validity of the experiment.

What is Transpiration?

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is absorbed by the roots and transported through the xylem vessels to the leaves. In the leaves, water evaporates from the surface of spongy mesophyll cells into air spaces and then diffuses out of the leaf through small pores called stomata. This evaporation creates a 'transpiration pull' that draws water up the plant. Transpiration is crucial for transporting water and mineral nutrients from the soil to the rest of the plant and for cooling the plant itself.

Factors Affecting Transpiration Rate

Several environmental factors influence the rate of transpiration. Increased temperature leads to faster evaporation of water from the leaf surface. Lower humidity creates a steeper concentration gradient between the humid air inside the leaf and the drier air outside, increasing water loss. Wind removes water vapour from around the stomata, maintaining a steep concentration gradient and increasing transpiration. Higher light intensity stimulates stomata to open wider to allow more carbon dioxide for photosynthesis, which also increases water loss. These factors collectively determine how quickly water is lost from the plant.

What is Translocation?

Translocation is the process by which sugars (mainly sucrose), amino acids, and other organic substances, produced during photosynthesis in the leaves, are transported to other non-photosynthesising parts of the plant. These parts include roots, growing shoots, flowers, and fruits, where the sugars are needed for respiration, growth, or storage. This transport occurs through specialized vascular tissues called phloem. Unlike the transpiration stream, which is largely driven by physical forces, translocation requires energy (it is an active process) to move sugars into and out of the phloem tissue, ensuring their distribution to areas of demand.

Comparing Xylem and Phloem

Xylem and phloem are the two main transport tissues in plants. Xylem is responsible for the unidirectional transport of water and dissolved mineral ions from the roots up to the rest of the plant. This movement is part of the transpiration stream. Phloem, on the other hand, transports sugars (sucrose) and amino acids in a bidirectional manner, from 'source' regions (e.g., leaves where photosynthesis occurs) to 'sink' regions (e.g., roots, growing tips, fruits where they are used or stored). While xylem transport is largely passive, phloem transport is an active process requiring metabolic energy.

Source and Developmental Stage

Embryonic stem cells (ESCs) originate from the inner cell mass of a blastocyst, which is a very early-stage embryo, typically 4-5 days after fertilisation. These cells are removed when the embryo is still a ball of cells. Adult stem cells (ASCs), however, are found in specific tissues and organs throughout the body of a developed organism, including children and adults. They are present in bone marrow, skin, brain, gut, and other locations, acting as a repair system for the body.

Differentiation Potential

Embryonic stem cells are pluripotent. This means they have the remarkable ability to differentiate into almost any cell type within the body, effectively forming all three germ layers (ectoderm, mesoderm, endoderm). Their versatility makes them highly valuable for research into generating various tissues. In contrast, adult stem cells are generally multipotent. They can only differentiate into a limited range of cell types, typically those found within the tissue or organ they originate from. For example, bone marrow stem cells can form different blood cells but not brain cells.

Ethical Considerations

The use of embryonic stem cells raises significant ethical debates because their extraction involves the destruction of an embryo. This process leads to moral questions regarding the status of the embryo and the permissibility of its use for research. Adult stem cells, on the other hand, usually present fewer ethical issues since they are derived from adult tissues with the donor's consent and do not involve the destruction of an embryo.