Unit 4 - The Cell (structure/function relationships)

How are prokaryotic different from eukaryotic cells?

• Size: Prokaryotic cells are smaller than eukaryotic cells (comparable to the size of mitochondria in eukaryotes).

• Nucleus: Prokaryotes lack a membrane-bound nucleus, while eukaryotes have one.

• DNA: Prokaryotic DNA is circular, while eukaryotic DNA is linear and stored within the nucleus.

• Organelles: Prokaryotes do not have membrane-bound organelles (like mitochondria or Golgi apparatus), but eukaryotes have these organelles. Ribosomes are an exception and are present in both, though eukaryotic ribosomes are larger.

• Cell Division: Prokaryotes divide by binary fission, while eukaryotes use mitosis.

• Cell Wall: Both types may have a cell wall, but the composition differs (cellulose in plant cell walls vs. chitin in fungi and bacteria).

• Cytoskeleton: Eukaryotic cells have a cytoskeleton, while prokaryotic cells do not.

How does the nucleus control the workings of the cell?

• The nucleus controls gene expression by organizing and managing DNA within the nuclear matrix.

• This organization affects how tightly or loosely the DNA is bound, influencing which proteins are made and when.

• It provides the right environment for the processes of DNA replication and mRNA production, ensuring that the necessary proteins are available for chemical reactions in the cell.

What is a system, and why is the cell a system? How would you describe how the parts work together to do the function of a system? Use bikes as an example.

• A system is a set of interconnected parts that work together to achieve a specific function.

• The cell is a system because its various organelles and structures cooperate to carry out life-sustaining processes.

• Like a bike, where each part (pedals, wheels, gears) has a role in enabling the bike to move, each cell part plays a specific function.

• For instance, the nucleus directs protein production (like a mechanic planning repairs), the ribosomes (as workers) make proteins, and the mitochondria (like an engine) provide the energy for all these activities.

• If any part of the bike is missing or broken, it won’t function properly, similar to how a disrupted cellular system impacts the cell’s performance.

How do the parts that are the main machinery of protein production work together to produce a working protein that goes to the right place?

The main machinery of protein production includes the nucleus, ribosomes, RER, Golgi apparatus, and vesicles.

• The nucleus contains DNA, which provides the instructions for making proteins. It organizes the DNA in such a way that it reads the right genes, making it match up with the right RNA to DNA, and the RNA leaves.

• The ribosomes which either float freely in the cytoplasm or attach to the RER, now have the RNA that left the nucleus. Ribosomes read the mRNA and build proteins by linking amino acids together. Once complete, they hook onto the RER.

• The proteins go from the ribosomes to the RER. The RER modifies these proteins (folding them and adding modifications like sugars), ensuring they are correctly structured. They then go from the RER to the Golgi apparatus by vesicles.

• The Golgi apparatus further modifies and packages proteins, adding signals that direct them to specific destinations. The proteins are then transported in vesicles to their final destination (e.g., the cell membrane, secretion, or other organelles).

Other organelles involved:

• Mitochondria provide ATP, the energy source required for protein synthesis.

• Lysosomes might be involved if proteins need to be broken down or recycled.

• Cytoskeleton helps in the transport of proteins to their proper location.

Why is protein making important to cells?

• Protein synthesis is vital because proteins carry out most of the cell's functions.

• They act as enzymes, structural components, transporters, and signaling molecules.

• Without protein synthesis, the cell cannot produce the necessary machinery for growth, repair, or responding to changes in its environment.

Cell Movement

• Cells can move through the action of the cytoskeleton and structures like flagella and cilia.

• Cytoskeletal filaments act as "tracks" for motor proteins that transport vesicles or move cell components.

• Flagella and cilia help cells move or move substances across their surfaces.

• These movements are vital for processes such as nutrient absorption, waste expulsion, and communication with other cells.

Epithelium Tissue

• Epithelial tissue is made of square/rectangular cells that fit together like blocks with tight junctions, which are proteins that hook the cells together tightly, acting as a barrier and preventing leakage between cells.

• They’re also hooked together by desmosomes, chains of proteins that hook the cytoskeletons of 2 cells together, which also helps in preventing leakage.

• This allows the tissue to selectively secrete and absorb based on the proteins in the cell membranes.

Nervous Tissue

• Nervous tissue is made of neurons and supporting cells. 

• Neurons are connected to each other by short incoming extensions and 1 long outgoing axon, which allows the dendrites and axon terminals of nerve cells to be close to each other. 

• Neurotransmitters are made in the large cell body of the neuron and are shipped down to the end of the axon where they wait.

• The signal to release the neurotransmitters is electrical, making it very fast.

• The axon almost touches the next cell it communicates with, so the neurotransmitter diffuses quickly across the gap, allowing rapid communication.

Skeletal Muscle

• Skeletal muscles are long, skinny, tube-shaped cells packed with contractile proteins and multiple nuclei.

• These muscles attach to bones and contract to move them.

• There is extracellular matrix (ECM) between the cells, and each muscle is connected to an axon for quick communication.

• The muscle's length allows for noticeable movement, while its thinness provides a high surface area-to-volume ratio for efficient diffusion.

• The striations, caused by the arrangement of strong fibrous proteins inside the cells, along with the bundles of connective tissue around them, make the muscle strong and resistant to tearing.

• There are no cell junctions, so each cell can be signaled individually, allowing for gradation of contraction.

• Additionally, each muscle cell is surrounded by connective tissue, which holds the cells together, making the muscle even stronger and harder to break.

Smooth Muscle

• Smooth muscle is made up of spindle-shaped, non-striated cells that surround hollow organs, allowing them to squeeze their contents and move them.

• Since the muscle fiber proteins are oriented in different directions, the cells can contract from all angles, creating a squeezing motion.

• These cells are shorter and connected by gap junctions, which allow sodium ions (Na+) to flow quickly from cell to cell, creating a wave of contraction.

• This enables the cells to work together efficiently to perform functions such as moving food through the intestine, grinding contents in the stomach, and squeezing urine out of the bladder.

• The presence of gap junctions and the unique structure of smooth muscle allow it to contract tightly and perform its squeezing function with efficiency.

Connective Tissue Bone

• Bone is made of concentric circles of bone cells separated by extracellular matrix (ECM). The amount and type of matrix give the tissue its characteristics.

• The matrix, which is always a mix of fibrous proteins and liquid, is the key element that defines the bone’s function, rather than the bone cells themselves.

• The matrix consists of super strong fibrous proteins that are hardened with calcium, allowing bone to support our weight.

• The bone cells sit in small spaces, which allow for diffusion, and these cells are responsible for making the ECM.

• Additionally, there are tiny canals through the ECM that provide blood to the cells.

Connective Tissue Adipose

• Adipose tissue is mostly fat cells surrounded by a small amount of matrix for strength. 

• The cells have a giant vacuole full of liquid fat and the cells are hooked together by the ECM. 

• This allows for a lot of storage and because the cells form a sheet of cells with some ECM between them - it makes for a great insulation and cushioning - it is like bubble wrap.

Connective Tissue Dense Fibrous

• Dense fibrous connective tissue is almost all ECM that’s densely packed fibrous proteins with very little liquid.

• The huge amount of matrix allows the tissue to support, protect, and hold muscles and other tissues and organs in place.

• The dense amount of fibrous proteins makes up fibers such as ligaments and tendons.

• The braided features of the fibers that make up the ECM create intracellular bonds, which is what makes the ECM so strong, making it optimal for structure and support.

• (There are cells here and there to make the ECM)

Why must living things be made of cells?

• To have enzyme/substrate concentrations high enough so that rates of reactions are reasonable to support life (Rates of Reaction)

• Rates of Diffusion

• Rates of  Intracellular Transport

What is the Advantage of cells in multi-cellular organisms?

• Reactions can occur without interference (conflicting reactions can happen simultaneously)

• Local environments can allow different reactions to happen

• Specialization (different cells can do different things) and cells then need to be able to communicate

• Damage can be contained

• Reactions can be ordered by embedding them into membranes