Scaling up Study Guide

Mitosis is a fundamental biological process that results in the division of a single parent cell into two identical daughter cells. This process is crucial for the growth and repair of multicellular organisms. The entire series of events that cells go through as they grow and divide is known as the cell cycle. The cell cycle can be broadly divided into two main phases: interphase and the mitotic phase (M-phase).

Interphase, often called the 'resting phase', is actually a period of intense cellular activity. During interphase, the cell grows, carries out its normal functions, and prepares for division. A key event in interphase is DNA replication, where each chromosome in the nucleus is duplicated, resulting in two identical sister chromatids joined at a centromere. This ensures that when the cell divides, each new daughter cell receives a complete and identical set of genetic material.

The mitotic phase (M-phase) involves two main stages: mitosis and cytokinesis. Mitosis itself is a continuous process, but it is typically described in four main stages: prophase, metaphase, anaphase, and telophase.

1. Prophase: Chromosomes condense and become visible under a microscope. The nuclear envelope breaks down, and spindle fibres begin to form from structures called centrioles.
2. Metaphase: The condensed chromosomes line up along the equator (middle) of the cell. Each chromosome is attached to a spindle fibre at its centromere.
3. Anaphase: The sister chromatids of each chromosome separate and are pulled to opposite poles of the cell by the shortening spindle fibres. This ensures that each pole receives a full set of chromosomes.
4. Telophase: Chromosomes arrive at the poles and begin to decondense. New nuclear envelopes form around the two sets of chromosomes, and the spindle fibres disappear.

Following mitosis, cytokinesis occurs, which is the division of the cytoplasm and its organelles. This results in two separate, genetically identical daughter cells. In animal cells, the cell membrane pinches inwards to divide the cytoplasm, while in plant cells, a new cell wall forms between the two daughter nuclei. This continuous cycle of growth and division enables organisms to increase in size (growth), replace old or damaged cells (repair), and in some organisms, reproduce asexually.

Blood is a vital fluid within the circulatory system, responsible for transporting essential substances throughout the body. It consists of several components, primarily red blood cells and plasma, each uniquely adapted to its specific transport functions. Red blood cells are the most numerous blood cells and are specialised for oxygen transport. Their key adaptations include a biconcave disc shape, which increases their surface area to volume ratio, facilitating rapid diffusion of oxygen into and out of the cell. They also lack a nucleus and most other organelles when mature, making more space for haemoglobin, the protein that binds to oxygen. Furthermore, red blood cells contain a high concentration of haemoglobin, which gives them their characteristic red colour and enables efficient oxygen carrying capacity. The small, flexible size of red blood cells allows them to squeeze through narrow capillaries, ensuring oxygen delivery to all tissues. Plasma, the yellowish liquid component of blood, serves as the transport medium for a wide range of substances. It is primarily composed of water (around 90%), which acts as an excellent solvent, allowing many substances to dissolve and be transported. In addition to water, plasma contains dissolved proteins (like albumin, globulins, and fibrinogen), glucose, amino acids, mineral ions, hormones, antibodies, and dissolved waste products such as urea and carbon dioxide. This aqueous composition allows it to efficiently carry nutrients from the digestive system to cells, transport hormones from endocrine glands to target organs, move antibodies for immunity, and carry waste products like urea to the kidneys for excretion. Carbon dioxide, a waste product of respiration, is also transported in the blood, primarily dissolved in the plasma as hydrogencarbonate ions, or bound to haemoglobin, demonstrating another crucial transport role of this complex biological fluid.