[AP Biology 2.2] Cell Structure and Function

The second section of Unit 2 in the AP Biology curriculum focuses on how the subcellular components of cells maintain the cell and gather energy. Specifically, this standard breaks down the two basic functional categories of cellular organelles: those that maintain and grow the cell, and those that capture and utilize energy. Thus, we will be looking at how the endomembrane system builds a cell, as well as seeing how chloroplasts and mitochondria work together to provide energy!

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Living systems are organized in a hierarchy of structural levels that interact.

Explain how subcellular components and organelles contribute to the function of the cell.

Describe the structural features of a cell that allow organisms to capture, store, and use energy.

Organelles and subcellular structures, and the interactions among them, support cellular function-

  1. Endoplasmic reticulum provides mechanical support, carries out protein synthesis on membrane-bound ribosomes, and plays a role in intracellular transport.
  2. Mitochondrial double-membrane provides compartments for different metabolic reactions.
  3. Lysosomes contain hydrolytic enzymes, which are important in intracellular digestion, the recycling of a cell’s organic materials, and programmed cell death (apoptosis).
  4. Vacuoles have many roles, including storage and release of macromolecules and cellular waste products. In plants, it aids in the retention of water for turgor pressure.

The folding of the inner membrane increases the surface area, which allows for more ATP to be synthesized.

Within the chloroplast are thylakoids and the stroma.

The thylakoids are organized in stacks, called grana.

Membranes contain chlorophyll pigments and electron transport proteins that comprise the photosystems.

The light-dependent reactions of photosynthesis occur in the grana.

The stroma is the fluid within the inner chloroplast membrane and outside of the thylakoid.

The carbon fixation (Calvin-Benson cycle) reactions of photosynthesis occur in the stroma.

The Krebs cycle (citric acid cycle) reactions occur in the matrix of the mitochondria.

Electron transport and ATP synthesis occur on the inner mitochondrial membrane.

2.2 Cell Structure and Function Overview

This section dives further into organelles and divides them into two functional groups – organelles that maintain and repair the cell, and organelles that function to collect and distribute the energy needed for biochemical reactions. Let’s start with organelles that maintain and repair cellular components.

Whether it is a prokaryotic organism or a eukaryotic organism, there is an ever-changing laundry list of tasks that must be completed in order for an organism to survive, grow, and reproduce. Before cells can replicate their DNA, they first need to collect and digest an energy source. This means the cell may need to move around, grow in size, fight off predators, or change environmental conditions. In a multicellular organism, even more functions are needed to prepare the organism for reproduction.

If we look at the cell membrane alone, we can start to understand the types of repair and maintenance tasks that need to be conducted. For instance, the phospholipids that create a cell membrane are constantly breaking down. These need to be replaced, and many more need to be created as a cell grows. Further, the many proteins embedded within the membrane and the other molecules that cells use to recognize each other, attach to surfaces, and communicate, also need to be created, repaired, or replaced almost constantly!

The endoplasmic reticulum and the Golgi complex are both a part of the endomembrane system – a series of membranes that form distinct chambers within a cell. These chambers can have completely different chemical properties than the cytosol surrounding them. The endoplasmic reticulum has two distinct portions: the rough ER and the smooth ER.

The rough ER is covered in ribosomes, which actively synthesize new polypeptide chains and deposit them into the various chambers created by the folded ER membrane. As it enters the lumen – the inside of the ER – the protein enters the proper environment needed to fold and become functional. Some of these proteins get placed directly in the membrane of the ER, which can then bud off as transport vesicles destined for the Golgi complex or the cell membrane itself. The rough ER is also able to synthesize phospholipids and new membrane, which it uses to replace the membrane lost by transport vesicles. The proteins created in the rough ER are kept out of the cytoplasm, where they may cause issues if they were allowed to function within the cell. Proteins needed in the cytosol are most often created directly in the cytosol.

As an example, the pancreas produces the protein hormone insulin – which acts to regulate blood sugar levels in the rest of the organism. Therefore, beta cells in the pancreas are packed with rough ER to produce all of the insulin an organism needs.

The smooth ER has a variety of functions and is also responsible for synthesizing phospholipids to replace what it loses to transport vesicles. However, the smooth ER also synthesizes a variety of other lipids – from fats to hormones – depending on the cell’s type. For instance, cells that produce the lipid-based sex hormones in animals are often loaded with smooth endoplasmic reticulum, because they are responsible for producing all of the sex hormones the organism needs to successfully reproduce.

Your body has an immune system, which continuously fights off bacterial infections. The most important organelles needed for this fight are lysosomes. The white blood cells that travel around your body are constantly looking for bacterial cells. When they find one, they “eat” it in a process known as phagocytosis. This essentially the same process that single-celled organisms use to obtain food.

However, enveloping the bacteria in a cellular membrane simply traps the bacteria in a food vacuole. Lysosomes are how the cell digests that material. Lysosomes attach to the food vesicle and merge with the lipid bilayer. As they do so, they dump their acid contents and hydrolytic enzymes into the food vacuole.

This digests the bacteria inside the cell by breaking apart all of the polymers with hydration reactions. The components the cell can recycle leach out of the food vacuole into the cytoplasm. Any waste products are dumped back into the bloodstream, where they will be removed by the kidneys and liver. The cell will then create more lysosomes with the Golgi complex, ready for the next bacterial cell it encounters.

Vacuoles are also a part of the endomembrane system, and they serve many storage functions in a variety of organisms. In general, the membrane of a vacuole is loaded with specific proteins that import specific substances into the vacuole. This takes the substances out of the cytoplasm, so the chemistry of the cytoplasm can remain consistent and reliable. The chemistry of the vacuole is not important, since no new molecules are synthesized here. There are two special types of vacuole that we should consider.

A contractile vacuole is found in many freshwater organisms. Freshwater organisms live in a hypotonic environment. This means that they have to maintain a higher concentration of solutes inside the cell, compared to the environment outside of the cell. Due to this gradient, water is constantly trying to flow into the cell. The contractile vacuole takes this water in and pumps it out of the cell at regular intervals. This allows the cell to remain at a consistent pH and water content, despite the constant influx of water.

Plants also rely on vacuoles, but for a much different reason. Plant cells have a large central vacuole, which fills with water. This vacuole pushes outward on the cell walls, which push against the walls of adjacent cells. Together, the many vacuoles within a plant create a rigid structure that allows the plant to stand tall in the face of gravity, wind, and other environmental conditions. When a plant does not have water, these vacuoles slowly empty – leading to a flaccid plant that usually dies. While this is the main function of the central vacuole, plants also use them to store a variety of substances. For instance, some plants store toxins in their vacuole that can kill herbivores or insects if they feed on the plant!

While the endomembrane system is highly specialized for maintaining and repairing a growing cell, other organelles are responsible for capturing, storing, and utilizing the energy needed to power the many reactions the endomembrane system completes. These organelles are chloroplasts and mitochondria. Both of these organelles have a double-membrane system, likely because they evolved from symbiotic bacteria billions of years ago.

The inner membranes of these organelles are highly convoluted, to increase the amount of surface area the organelle uses to complete important biochemical reactions. In mitochondria, the folds of the inner membrane are called cristae, and they house the electron transport chain that helps move energy from the bonds of glucose to the bonds of ATP. In chloroplasts, the inner membrane is distinctly folded into a large number of thylakoids. These disc-like structures are stacked into units called a granum, and each chloroplast is filled with a large number of grana. Thylakoids work to capture energy from sunlight, and also have a network of electron transport chains – known as photosystems – that capture the energy from sunlight and use it to form molecules of glucose.

Together, these two organelles provide energy for almost all of the life on Earth in one way or another!

Plant cells are packed full of chloroplasts, which produce sugar through the complex process of photosynthesis. This process starts in the membrane of the thylakoids. To maximize the amount of sunlight that can be captured, the thylakoids are stacked together tightly and fill up most of the internal space within the chloroplast. But, the real magic happens at the level of the photosystems. The photosystems consist of a series of proteins embedded into the membrane of the thylakoid. Though later sections of the AP Biology curriculum address this process further, these photosystems work by capturing energy with the pigment molecule chlorophyll. This energy is then used to split a water molecule. The energy released travels through an electron transport chain and through ATP synthase to create NADH and ATP, both of which can power other reactions.

These molecules are transferred to the stroma of the chloroplast, where the Calvin cycle takes place. This process – also known as carbon fixation – essentially uses the energy in ATP and NADH to generate sugar molecules from smaller carbon dioxide molecules. As we will see, this is essentially the exact opposite of what happens in mitochondria!

Once chloroplasts have created glucose, it can be used in the cell it was created – or it can be sent to other parts of the plant. Though animals don’t create glucose, herbivores eat plants to get the energy stored in glucose, and carnivores eat herbivores to get access to essentially the same energy that was first captured in plants. The first step of getting access to this energy is breaking down 6-carbon glucose – a process known as glycolysis – which happens in the cell’s cytosol. Then, a smaller 3-carbon molecule can be imported into the mitochondrial matrix.

This molecule then enters the Kreb’s cycle (also known as the citric acid cycle). Essentially, this process is the same as the Calvin cycle seen in photosynthesis, only in reverse! The 3-carbon molecule is added to another 3 carbon molecule to form a 6 carbon molecule. Through a series of biochemical reactions, the molecule is slowly torn apart – releasing carbon dioxide and creating NADH and FADH, as well as a tiny amount of ATP.

These electron carriers (NADH and FADH) make their way to the electron transport chain located on the inner membrane of the mitochondria. First, these electron carriers dump their electrons and energy into membrane-bound proteins. These proteins use the energy to pump hydrogens into the intermembrane space – between the inner and outer mitochondrial membranes. Then, ATP synthase uses the hydrogen gradient that has been created to make many more ATP molecules. These ATP molecules can be exported from the mitochondria to power reactions throughout the cell – from creating new lipids in the smooth endoplasmic reticulum to synthesizing new DNA molecules before cellular division!