Know Your Cell-self and Pass the HESI A2 Biology Section

“I cannot think of a single field in biology or medicine in which we can claim genuine understanding, and it seems to me the more we learn about living creatures, especially ourselves, the stranger life becomes.” – Lewis Thomas

Let’s face it. Learning biology can be tough. Studying the inner workings of life down to the sub-cellular level is like learning a new language.
Don’t worry! We are here to help. In this article, we will cover big-picture topics so that when it’s time to master the minutia, you’ll know where to fit in the details within the big picture.

Ready? Take notes!

When undertaking a huge task, it’s best to start small. Let’s examine the building blocks of biology: Cells, Organelles, Membranes, and Tissues.

Cells are the basic unit of biology. All cells fall into two categories: Prokaryotes and Eukaryotes. Prokaryotes are single-celled Bacteria and Archaea. Prokaryotic cells have DNA located in the nucleoid. Prokaryotes are respectively less complex than Eukaryotes, however, they both do contain cellular membranes – a double lipid bilayer that separates the cell’s interior from the exterior.

Bacteria have cell structures which help them with their specific survival needs:

• Cell wall: rigid layer of protection, prevents dehydration
• Capsule: sticky layer for attachment
• Flagellum: tail like structure used for movement
• Fimbriae: hair-like structures used for attachment
• Pili: small hair-like structures used for movement and transfer of DNA

Eukaryotes are multicellular. Yeast, plants, fungi, animals (including humans) fall into this category. Eukaryotic cells have DNA in the nucleus and contain structures called organelles – specialized centers separated by layers of membrane.

Plants are eukaryotic and have organelles which help them with their specific survival needs:

• Cell wall: rigid cellulose regulates volume
• Plastids (chlorophyll): part of energy production, also contain pigments
• Vacuoles: storage compartments for material such as sap
• Apical meristem: growing tip – allows plants to continually grow

Animals do not have cell walls, plastids, vacuoles, or apical meristems. They have other organelles which help them with their specific survival needs:

• Cytoplasm: space inside the cell where organelles are located
• Mitochondria: used in energy production
• Endoplasmic Reticulum: used for protein transport
• Ribosomes & Golgi bodies: used in protein production
• Lysosomes (uncommon in plants): used for waste degradation/removal

Animals also have specialized cells which form tissues – a group of similar cells that carry out a specific function:

• Red blood cells: carry oxygen
• White blood cells: part of the immune system
• Nerve cells: carry electrical signals throughout the body
• Sperm/Egg: part of the reproductive system

There are 4 types of tissue:

• Epithelial: tightly packed layers which cover surfaces and line body cavities – examples are skin and intestinal lining
• Connective: cells are suspended in extracellular matrix which can be solid, liquid, or jelly-like. Loose connective tissue supports organs and blood vessels. Dense/fibrous connective tissue comprises tendons and ligaments.
• Muscle: Involuntary, smooth muscle is located in organs such as the stomach and intestines. Skeletal muscle is voluntary and used for motor activity. Cardiac muscle allows the heart to beat.
• Nervous: neurons and the spinal cord are used to sense and process stimuli, transmit information, and effect action.

The progression of biological units is cells to tissues to organs to organ systems to organisms.

Now that we know the basics of the cell, let’s talk proliferation! Which is basically just a fancy way of saying, let’s talk about how cells reproduce.

Prokaryotes reproduce through a process called binary fission. Eukaryotes reproduce through a cell cycle that varies depending on the number of chromosomes/type of cell. Diploid cells contain two homologous chromosome pairs. Haploid cells contain only a single set of chromosomes.

• DNA Replication: For diploid cells (non-gametic, somatic cells) the two pairs of homologous chromosomes separate during interphase and serve as the template for two new strands to be created. This can be denoted as 2n to 4n. Haploid cells (gametes) undergo one round of DNA replication which can be denoted by n to 2n.
• Mitosis/Meiosis: Somatic cells undergo mitosis – a series of events where replicated chromosomes prepare to separate into two new nuclei. The phases of mitosis are Prophase, Prometaphase, Metaphase, Anaphase, and Telophase. Gametes undergo meiosis. There are two cell divisions in meiosis which leave 4 identical daughter cells. The stages of meiosis are Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis, Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis.
• Cytokinesis: For somatic cells, cytokinesis begins after mitosis and is a series of events which lead to the division of the cytoplasm to form two new cells.

This subject matter is intense and I could use a breather! Speaking of breather, up next is respiration: Cellular Respiration & Anaerobic Respiration.

Anaerobic respiration is the main process of energy acquisition for Prokaryotes as they live in environments with low levels of oxygen. During anaerobic respiration, a starting “fuel” molecule provides electrons to an electron transport chain which creates a proton gradient. This proton gradient is used to drive the synthesis of ATP (the energy molecule).
Fermentation also occurs in the absence of oxygen and is used by prokaryotes, yeast (eukaryotes) and other multicellular organisms such as humans.

Cellular respiration requires the presence of oxygen and is a process which converts nutrients such as sugars, amino acids, and fatty acids into ATP. There are four main stages of cellular respiration: glycolysis, formation of Acetyl CoA, Citric acid cycle (also known as the Krebs cycle), and electron transport chain.

Alright. We know what they are, we know how they reproduce, and we know how they “breathe.” Let’s finally talk about what they do: Cellular Functions.

Exocytosis is a function of cells that involves waste removal where materials such as protein/waste are contained in membrane-bound vesicles, transported, and then released.

Passive transport is a function of cells which regulate the movement of particles and fluid (concentration). Passive transport does not require energy; it works based on a gradient.

• Diffusion is a type of passive transport where molecules move from areas of high concentration to areas of low concentration until equalization
• Selective permeability is a type of passive transport where some substances are able to pass through the membrane while others are excluded. Channel proteins make tunnels to allow transport or molecules.
• Facilitated diffusion utilizes carrier proteins or channel proteins to regulate the passage of molecules

These functions which regulate concentration are part of another important cellular function – tonicity. Tonicity involves water flow into and out of cells and is related to osmolarity.

If you made it through all of that, congratulations! Not only did you learn about cells, you actually changed your own! That’s right – as you learned from this article, the cells of your brain actually changed shape and connections to retain this information.