Cell Size Basics

Cell size and the form are the focus of this portion of the AP Biology curriculum. More precisely, it examines how a cell’s surface-area-to-volume ratio influences how effective it is in exchanging macromolecules with the outside world. Due to unique restrictions of the cell membrane and the activities of the cell, cells fall within specified boundaries depending on the surface-area-to-volume ratio. In addition, this part examines how organisms modify cell size and structure to perform a variety of activities and execute tough tasks!

Cell Structure and Function Overview

The overall message of section 2.3 is that the intricate architecture of biological systems needs continual energy and macromolecule exchange. To comprehend how cells carry out these functions, we must first consider cell size and form. Surface area-to-volume ratios are critical for cells because they influence their capacity to acquire nutrients and collect energy from their surroundings.

Endomembrane system

The smallest known cells are roughly 1 micrometres in diameter. The size of these bacterial cells is comparable to that of chloroplasts and mitochondria. Bacterial cells may grow to be roughly 100 micrometres in diameter. However, due to their basic structure and absence of internal organelles, bacterial cells can only grow to roughly this size. Cells need an endomembrane system to grow because it aids in the effective distribution of nutrients and the excretion of waste materials. Eukaryotic cells may grow up to 1 centimetre in length, while the vast majority of them are closer to 100 micrometres.

Indeed, the blood cells of an elephant and a mouse are identical in size. This is due to the fact that blood cells are most effective at this size. Other cells, such as nerve cells that must travel considerably greater distances, may be larger in the elephant, but in general, cells achieve their maximal size depending on one factor: their surface area-to-volume ratio. Let’s take a closer look at this idea!

All cells, from bacteria to your body’s cells, must exchange macromolecules, gases, and water with the outside world. Many phospholipid molecules make up the cell membrane. These phospholipids include a large number of proteins, many of which are involved in the import and export of chemicals into and out of the cell. The volume of a cell grows more quickly than the surface area as it becomes bigger. As a result, the cell will need additional membrane proteins to meet the demands of the internal cytosol. A cell will eventually need more proteins than it can properly fit within the cell membrane. This is why cells have a maximum size.

Similarly, when a cell shrinks, the number of phospholipid molecules surrounding it decreases, limiting the number of proteins that can be kept inside the membrane. This is the smallest cell size possible. This is why a cell’s surface area-to-volume ratio is so important: it’s a good indicator of how well a cell can exchange the chemicals it requires to live!

The term “surface area-to-volume ratio” means “surface area to volume ratio.” It’s the cell’s total surface area divided by the cell’s entire volume. To determine a cell’s surface area-to-volume ratio, we must first choose a suitable model for the cell.

The form of most plant cells is cuboid. We can simply compute the volume and surface area of a plant cell with a height of 2 millimetres and a width and depth of 1 millimetre each using basic geometric calculations. The surface area is 10 and the volume is 2. As a result, the surface area-to-volume ratio is 10 divided by 2 – or 5.

Let’s explore what happens if we imagine a single-cell creature of equal size but with a spherical form. The radius of this cell is 1 millimetre if its diameter is 2 millimetres. As a result, the volume is about 4.2 mm3. The area of the surface is about 12.6 mm2. As a result, 12.6 divided by 4.2 equals the surface area-to-volume ratio. This is reduced to three. As a result of its spherical form, this cell has a somewhat lower surface area-to-volume ratio. This indicates that not only does size matter, but also that shape can affect the SA-to-V ratio!

Several examples demonstrate the constraints imposed by the surface area-to-volume ratio of cells. Let’s start with nature’s biggest cells: eggs!

The cell has a substantially greater volume than surface area at this end of the spectrum. In reality, if they were normal cells, the quantity of surface area available to exchange macromolecules would considerably outnumber the number of operations carried out in their volume. Eggs, on the other hand, are not ordinary cells. They are not actively producing products, excreting wastes, or otherwise exchanging macromolecules with the environment while they are asleep.

This is due to the fact that when eggs are created, they are filled with nutrients. The single cell within then divides quickly multiple times to become a creature. With each division, the cells become smaller and smaller, more efficient. As a result, the average cell size in an egg has dropped by orders of magnitude by the time it hatches as a fully fledged creature!

These bacteria, on the other hand, are at the other extreme of the size range. They’re only big enough to hold their DNA and carry out life’s essential processes. Their surface area is significantly larger than their volume, giving them an extremely high surface area-to-volume ratio. In fact, if these cells were any smaller, they wouldn’t have the space to correctly copy DNA, and their lipid bilayer wouldn’t be able to accommodate all the proteins required to import and export macromolecules!

While these surface area-to-volume restrictions apply to single cells, animals may improve the efficiency of collecting nutrients in bigger creatures in a variety of ways. Folding cells and cell membranes in ways that increase surface area is a recurring motif in cells that need to absorb a lot of nutrients or chemicals from the external environment. As a result, the surface area-to-volume ratio rises, allowing the cells to operate at maximum efficiency. Let’s have a look at a handful of instances.

Alveoli and Capillaries

Alveoli are tiny sacs found in the lungs that take oxygen and release carbon dioxide. These little sacs are lined with hundreds of tiny, rectangular cells instead of a single big cell at the end of each airway. It would take a long time and be inefficient if oxygen and carbon dioxide had to permeate through just one big cell. The numerous endothelial cells that make up the alveoli, on the other hand, can swiftly transport oxygen into capillaries and carbon dioxide out of the body.

Villi and Microvilli

The huge surface area growth of cells in the small intestine is similar to the folding pattern found in alveoli. In reality, there are three stages of folding that greatly improve the accessible surface area for absorbing vital nutrients from meals. The intestinal lining is folded first, with each fold increasing the surface area. More microscopic projections—called villi—sit on top of these folds, doubling the potential surface area.

Each villus is encased in a layer of cells. We can observe that each of these cells has its own microvilli if we look carefully. The surface area accessible to take in nutrients is greatly increased by these small expansions of the cell membrane. Overall, the folds of this system boost the intestinal lining’s effectiveness by hundreds of times!

Similarly, the human excretory system operates by creating convoluted folds in tissues, cells, and individual cellular membranes to greatly increase their surface area. Take, for example, the kidneys, which are the body’s primary excretory organ.

 Medulla

The kidney is a complex organ with numerous similar components. Each medulla is densely packed with cells and has both arteries and veins, which take blood from the heart, filter nitrogenous waste, and return it to the heart.

Malpighian Tubules

Each medulla contains the Malpighian tubules. These tubules are made up of many smaller cells, each of which serves a different purpose in the fold. Each of these cells is small, rectangular, and fairly flat, just like the cells in alveoli, allowing for maximum efficiency! A single kidney, in fact, can filter half a cup of blood per minute. Your kidneys can filter all the blood in your body every 40 minutes, assuming you have about 20 cups of blood in your body. Large organisms with trillions of cells that are constantly producing waste products require this level of efficiency!

While the primary reason for surface area-to-volume ratio limitations on some cells is to facilitate the import and export of chemicals, there are several additional tasks that require cells of a certain size. Certain creatures, for example, need heat dissipation.

They do so by transferring heat out of the body via a network of tiny cells on the skin and in the lungs. Seals swimming in the Arctic’s freezing waters, on the other hand, are coated in a coating of extremely massive fat cells. These cells store heat, enabling them to survive in water that would soon cause hypothermia in a person.