Biology Chapter on Cell Walls and Signaling

Cell wall is an extracellular layer surrounding the cell membrane, providing structural support and protection.
Plant cell walls are primarily composed of cellulose, hemicellulose, and pectin.
Fungi cell walls contain chitin.
Bacterial cell walls contain peptidoglycan.
Algae cell walls contain cellulose.
Animal cells do not have a cell wall.
Primary cell walls are thin and flexible.
Secondary cell walls are thicker and rigid, formed later in cell development.
The middle lamella is a layer between adjacent cells, made of pectin.
Cell walls provide shape and strength to plant cells.

Cell communication occurs through signals between cells.
Substances involved in cell signaling are called ligands.
Hormones (proteins or steroids) are examples of ligands.
Receptors are proteins on the cell surface or inside of the cell, binding to ligands.
Ligand binding triggers a signaling cascade.
Protein hormones are hydrophilic and cannot directly cross the membrane. They bind to receptors on the cell surface, activating enzymes which convert ATP into cAMP, which triggers a cellular response.
Steroid hormones are hydrophobic and can cross the membrane directly into the cell to bind with intracellular receptors, initiating a response.
Signal transduction is the process of converting an extracellular signal into a cellular response.

Cells transport molecules across the membrane in three ways: passive transport, active transport, and bulk transport.
Passive transport requires no energy.
- Simple diffusion is the movement of molecules from a high concentration area to a low concentration area.
- Facilitated diffusion is the movement of molecules from a high concentration area to a low concentration area by way of carrier and channel proteins.
- Osmosis is the diffusion of water from a high water concentration area to a low water concentration area.
Active transport requires energy.
- Primary active transport uses ATP directly to move molecules against their concentration gradient.
- Secondary active transport uses the electrochemical gradient of one molecule to move another molecule against its concentration gradient.
Bulk transport move large amounts of molecules into and out of the cell via endocytosis and exocytosis.

Cytoplasm is the solution between the cell membrane and the nucleus.
Cytoplasm is a viscous liquid with suspended organelles.
It is composed of water and dissolved solutes, including organic molecules like proteins, carbohydrates, lipids, and salts.

The ER is a network of membranes extending through the cytoplasm.
ER is categorized as rough ER and smooth ER.
Rough ER has ribosomes attached, participating in protein synthesis, packaging, and modification.
Smooth ER is involved in lipid synthesis, carbohydrate metabolism, detoxification of drugs, and calcium storage.

Ribosomes are non-membrane-bound granular structures responsible for protein synthesis.
Ribosomes are found in both prokaryotic and eukaryotic cells.
Ribosomes are composed of RNA and proteins.
Ribosomes are divided into large and small subunits.

Stacks of cisternae and associated vesicles form the Golgi complex, involved in processing, sorting, packaging, and modifying proteins and lipids.
Processes secretions, forming glycoproteins/glycolipids.

Microbodies are small, single-membrane-bound organelles containing enzymes.
- Peroxisomes are in eukaryotes and perform various metabolic functions, including the breakdown of fatty acids and detoxification reactions.
- Glyoxisomes are found in plant cells and participate in converting lipids to carbohydrates during germination.

Mitochondria are double-membrane-bound organelles involved in cellular respiration and ATP synthesis.
Mitochondria have an outer membrane and an inner membrane with cristae, increasing surface area for ATP production.
The matrix contains enzymes and co-enzymes essential for the Krebs cycle.

Plastids are double-membrane-bound organelles in plant cells.
Chloroplasts are green plastids containing chlorophyll, essential for photosynthesis.
Chromoplasts have colorful pigments, involved in flower, fruit, and root coloring.
Leucoplasts are colorless plastids that store starch, proteins, and lipids.

Vacuoles are membrane-bound sacs involved in various cell functions, including storage and waste removal.
Plant cells usually have one large central vacuole, while animal cells have smaller vacuoles, or no vacuoles.

Centrioles are non-membrane-bound rod-shaped structures critical for cell division in animal cells.

The nucleus, a double-membrane-bound organelle, houses the cell's genetic material (DNA).
The nucleus has a nuclear envelope with pores enabling the exchange of materials.
The nucleolus, a dense region within the nucleus, is responsible for ribosome formation.
The nucleoplasm is the fluid substance within the nucleus.

Stem cells are undifferentiated cells with the ability to divide and differentiate into specialized cells.
Types of stem cells include: totipotent, pluripotent, multipotent, and unipotent.