Topic B2: Scaling up — OCR GCSE Study Guide

 ## Overview Welcome to Topic B2: Scaling Up. This topic is fundamentally about **logistics** in biology: how do organisms move the things they need (like oxygen and glucose) to where they are needed, and how do they get rid of waste (like carbon dioxide)? In this topic, you will learn the three core transport mechanisms: diffusion, osmosis, and active transport. You will then scale up to understand why single-celled organisms can rely on these basic mechanisms, while multicellular organisms—like humans and oak trees—require specialised transport systems such as the heart, blood vessels, xylem, and phloem. This is a high-yield topic in the exam; examiners frequently test your understanding of the Surface Area to Volume (SA:V) ratio and your ability to link cell structure to organ function. Listen to the full revision podcast here:  ## Key Concepts ### Concept 1: The Three Transport Mechanisms To survive, cells must constantly exchange substances with their environment. They do this via three main processes.  **1. Diffusion** Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It is a passive process, meaning it does not require energy from respiration. For example, oxygen diffuses from the alveoli in the lungs into the blood capillaries, while carbon dioxide diffuses in the opposite direction. **2. Osmosis** Osmosis is a specific type of diffusion. It is the net movement of **water molecules** from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution) through a **partially permeable membrane**. **Example**: If you place a piece of potato in pure water (high water potential), water will move into the potato cells by osmosis, causing them to swell and become turgid. If placed in concentrated sugar solution (low water potential), water moves out, and the cells become flaccid or plasmolysed. **3. Active Transport** Unlike diffusion and osmosis, active transport moves substances **against** the concentration gradient—from an area of lower concentration to an area of higher concentration. Because it works against the natural flow, it requires energy (ATP) released by respiration, and it relies on carrier proteins in the cell membrane. *Synoptic Link*: This is why cells that carry out a lot of active transport, such as root hair cells, contain many mitochondria. ### Concept 2: Surface Area to Volume Ratio (SA:V) As an organism gets larger, its volume increases much faster than its surface area.  Single-celled organisms like amoebas have a very large SA:V ratio. This means they have enough surface area to supply their small volume with oxygen simply by diffusion across their cell membrane. However, in multicellular organisms, the SA:V ratio is too small. Diffusion alone is too slow to supply all the cells deep inside the body. To solve this, large organisms have evolved specialised exchange surfaces (like lungs and intestines) and transport systems (like the circulatory system) to move substances efficiently. ### Concept 3: The Cell Cycle and Mitosis Growth and repair in multicellular organisms require the production of new cells. This happens via the cell cycle, which culminates in mitosis. 1. **Interphase**: The cell grows, increases its number of sub-cellular structures (like ribosomes and mitochondria), and replicates its DNA to form two copies of each chromosome. 2. **Mitosis**: One set of chromosomes is pulled to each end of the cell, and the nucleus divides. 3. **Cytokinesis**: The cytoplasm and cell membrane divide to form two genetically identical daughter cells. ### Concept 4: The Human Circulatory System The human heart is a double pump that drives blood through the circulatory system.  - The **right ventricle** pumps deoxygenated blood to the lungs for gas exchange. - The **left ventricle** pumps oxygenated blood around the rest of the body. The muscular wall of the left ventricle is much thicker because it must pump blood at a higher pressure to reach all extremities. Blood flows through three main types of vessels: - **Arteries**: Carry blood away from the heart. They have thick muscular and elastic walls to withstand high pressure. - **Veins**: Carry blood back to the heart. They have thinner walls, a larger lumen, and valves to prevent the backflow of blood. - **Capillaries**: Huge networks of tiny vessels. Their walls are only one cell thick, providing a short diffusion pathway for the exchange of substances between the blood and tissues. ### Concept 5: Plant Transport Systems Plants also need to move substances. They rely on two main tissues: - **Xylem**: Transports water and dissolved mineral ions from the roots to the stem and leaves. This upward movement is called the transpiration stream. Xylem vessels are made of dead cells strengthened by lignin. - **Phloem**: Transports dissolved sugars produced during photosynthesis from the leaves to the rest of the plant for immediate use or storage. This process is called translocation. Water enters the plant through **root hair cells**. These cells have long projections that greatly increase their surface area for the absorption of water (by osmosis) and mineral ions (by active transport). ## Mathematical/Scientific Relationships **Surface Area of a Cube**: $SA = 6 \times side^2$ **Volume of a Cube**: $V = side^3$ **SA:V Ratio**: Calculate SA and V separately, then express as a ratio simplified to $X:1$ (by dividing both sides by the volume). **Rate of Transpiration**: Often measured using a potometer. $Rate = \frac{Distance \; moved \; by \; bubble}{Time \; taken}$ ## Practical Applications **Required Practical: Investigating Osmosis** Students must investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue (usually potato cylinders). - **Method**: Cut identical potato cylinders, measure their initial mass, place them in different concentrations of sugar solution for 24 hours, dry them gently, and measure their final mass. - **Calculation**: Percentage change in mass = $\frac{Final \; Mass - Initial \; Mass}{Initial \; Mass} \times 100$ - **Examiner Tip**: Examiners love to ask why you must dry the potato cylinders before the final weighing (to remove excess surface fluid which would artificially inflate the mass).