Scaling up Revision Notes

What are Stem Cells?

Stem cells are unique cells that are undifferentiated, meaning they have not yet specialised to perform a specific function (like a muscle cell or nerve cell). They possess two key properties: self-renewal (they can divide to produce more stem cells) and differentiation (they can develop into specialised cell types). These properties make them crucial for growth, repair, and maintenance of tissues in multicellular organisms.

Embryonic Stem Cells (ESCs)

Embryonic stem cells are derived from the inner cell mass of a blastocyst, which is an early-stage embryo, typically 3-5 days old. A key feature of ESCs is their pluripotency; they have the potential to differentiate into virtually any cell type found in the body, making them medically significant for tissue regeneration. However, their use raises ethical concerns because the process usually involves the destruction of an embryo. They also pose a risk of immune rejection if used in a patient.

Adult Stem Cells

Adult stem cells are found in various mature tissues and organs throughout the body, including bone marrow, brain, and skin. Unlike embryonic stem cells, adult stem cells are usually multipotent, meaning they can only differentiate into a limited range of cell types within the tissue or organ they originate from. For example, blood stem cells in bone marrow can only form different blood cells. Their primary role is to repair and maintain the adult tissues. They pose fewer ethical issues as they don't involve embryo destruction, and they can be sourced from the patient, reducing immune rejection risks.

Key Differences Summarised

Embryonic stem cells are pluripotent (can become almost any cell type) and are obtained from early embryos, bringing ethical considerations. Adult stem cells are multipotent (limited range of cell types) and are found in developed tissues, generally raising fewer ethical concerns. Embryonic stem cells have greater developmental plasticity, while adult stem cells are involved in tissue repair and maintenance.

The Cell Cycle: Interphase and Mitosis

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It consists of two main phases: interphase and the mitotic phase (M-phase). During interphase, the cell grows, carries out its normal functions, and duplicates its DNA. This duplication ensures that each new cell will receive a complete set of genetic information. The mitotic phase involves nuclear division (mitosis) followed by cytoplasmic division (cytokinesis).

Stages of Mitosis

Mitosis is the process of nuclear division, ensuring that two daughter nuclei receive identical sets of chromosomes. It is divided into four main stages:

1. Prophase: Chromosomes condense and become visible. The nuclear envelope breaks down.
2. Metaphase: Chromosomes align at the cell's equator.
3. Anaphase: Sister chromatids separate and move to opposite poles.
4. Telophase: New nuclear envelopes form around the separated chromosomes at each pole.

Role of Mitosis in Growth

Mitosis is essential for growth in multicellular organisms. It allows an organism to increase its total number of cells, leading to an increase in size. For example, a zygote develops into a complex multicellular organism through repeated mitotic divisions. It is also vital for the repair of damaged tissues and the replacement of old cells, as new cells are continuously produced to substitute those that are worn out or injured.

Cytokinesis: Cytoplasmic Division

After the nucleus has divided during mitosis, the cytoplasm and its organelles also divide in a process called cytokinesis. This results in the formation of two distinct, genetically identical daughter cells. In animal cells, cytokinesis involves the formation of a 'cleavage furrow' that pinches the cell into two. In contrast, plant cells form a 'cell plate' in the middle, which develops into a new cell wall, separating the two daughter cells.

Red Blood Cell Adaptations for Oxygen Transport

Red blood cells are highly specialised to transport oxygen from the lungs to the body's tissues. Their biconcave disc shape is a crucial adaptation, as it greatly increases their surface area to volume ratio. This larger surface area allows for faster and more efficient diffusion of oxygen into and out of the cell. Another key adaptation is the absence of a nucleus and most other organelles in mature red blood cells. This lack of internal structures provides more space within the cell to pack in a high concentration of haemoglobin. Haemoglobin is the protein responsible for binding to oxygen, and its abundance ensures a high oxygen-carrying capacity. The small and flexible nature of red blood cells also allows them to easily deform and squeeze through the very narrow capillaries, ensuring that oxygen reaches even the most remote cells in the body.

Plasma Adaptations for Diverse Transport Functions

Plasma is the liquid matrix of blood, primarily composed of water, and it plays a central role in transporting a vast array of substances. Its high water content (approximately 90%) makes it an excellent solvent. This solvent property allows various substances to dissolve in it and be transported efficiently throughout the body. These dissolved substances include vital nutrients such as glucose and amino acids, which are carried from the digestive system to cells where they are needed for energy and growth. Hormones, produced by endocrine glands, are also transported by plasma to their target organs to regulate bodily functions. Antibodies, essential for the immune response, travel in the plasma to fight infections. Additionally, waste products like urea (from protein metabolism) and excess mineral ions are dissolved in the plasma and transported to the kidneys for excretion. Dissolved carbon dioxide, a waste product of cellular respiration, is also chiefly transported in the plasma, mostly as hydrogencarbonate ions, to the lungs to be exhaled.

Summary of Red Blood Cell Adaptations

1. Biconcave shape: Maximises surface area for oxygen diffusion. 2. No nucleus/organelles: Provides more room for haemoglobin. 3. High haemoglobin content: Enables efficient oxygen binding and transport. 4. Small and flexible: Allows passage through narrow capillaries. These adaptations ensure efficient delivery of oxygen to all body cells for cellular respiration.

Summary of Plasma Adaptations

1. High water content (approx. 90%): Acts as a universal solvent. 2. Transports dissolved substances: Carries glucose, amino acids, hormones, antibodies, mineral ions. 3. Transports waste products: Carries urea and carbon dioxide (as hydrogencarbonate ions) for excretion. Plasma's role is crucial for maintaining homeostasis by distributing nutrients, hormones, and removing waste.

Understanding the Potometer

A potometer is a scientific instrument designed to measure the rate of water uptake by a plant shoot. While often referred to as a transpirometer, it directly measures water absorption, which is primarily driven by the process of transpiration in the leaves. The apparatus typically consists of a cut plant shoot, a rubber bung to seal it, a glass tube, and a capillary tube with an air bubble. The movement of this air bubble along a scale provides a measurable indication of how quickly the plant is taking up water from the surrounding reservoir. Proper setup, especially ensuring no air bubbles are trapped within the xylem of the plant shoot, is crucial for accurate results.

Setting up the Potometer Experiment

To set up a potometer, a healthy, freshly cut plant shoot is used. The cut must be made underwater to prevent air from entering the xylem vessels, which would block water movement. The shoot is then inserted into a rubber bung, which is fitted into the potometer apparatus, ensuring a watertight seal. The entire apparatus is filled with water, again, taking care to avoid air bubbles. A small air bubble is introduced into the capillary tube, and the starting position of this bubble is recorded. The potometer can be reset by opening a tap or using a syringe to return the bubble to its starting point, allowing for multiple measurements.

Investigating Factors Affecting Water Uptake

The potometer can be used to investigate how various environmental factors influence the rate of water uptake. These factors include light intensity, temperature, humidity, and air movement (wind speed). To investigate the effect of one specific factor, it is essential to keep all other variables constant to ensure a fair test. For instance, if you're studying the effect of light intensity, the temperature, humidity, and air movement around the plant should remain the same across all experimental conditions. The rate of water uptake is calculated by measuring the distance the air bubble moves in a given time, and then dividing the distance by the time.

Interpreting Potometer Readings and Limitations

The movement of the air bubble in the capillary tube directly indicates the volume of water absorbed by the plant over time. A faster movement suggests a higher rate of water uptake. This rate is usually expressed in mm/minute or cm³/minute. It's important to remember that a potometer measures water uptake, which may not always exactly equal transpiration if the plant is also using some water for photosynthesis or storing it. However, for short experiments, the difference is often negligible. Potential sources of error include temperature fluctuations, air leaks, and the plant shoot's condition. Repeating measurements and calculating averages can help improve the reliability of the results.