Unit Two: Cell Structure and Function

What is a cell?

A cell is a membrane-bound unit containing hereditary information and other components, allowing it to metabolize, grow, and reproduce.

Prokaryote vs Eukaryote

Prokaryotes are bacterial cells with no defined nucleus and no membrane-bound organelles; eukaryotes are more complex, having a defined nucleus and organelles.

What is cytology?

Cytology is the study of cells.

Characteristics of all cells

All cells possess a plasma membrane, a nuclear region, and cytoplasm, which contains sugars, amino acids, proteins, and in eukaryotes, organelles.

Modern Cell Theory

1. All organisms are composed of cells. 2. Cells are the basic units of structure and function. 3. All cells come from other cells.

Cell Size Range

Prokaryotic cells range from 1-10μm, while eukaryotic cells range from 10μm to 5cm.

Why are cells small?

Cells are small because materials must diffuse into and out of them; larger cells take longer to transport materials.

Surface Area to Volume Ratio

As cell size increases, the surface area to volume ratio decreases making smaller cells more efficient in material transport.

Prokaryotic Cell Structure

Prokaryotic cells include a cell envelope (plasma membrane and cell wall), glycoalyx, cytoplasm containing DNA in a nucleoid region, and ribosomes that are scattered throughout.

What are the two domains of prokaryotes?

The two domains are Domain Bacteria and Domain Archaea. Bacteria are known for their shapes (bacillus, coccus, spirilla) and their role in disease and manufacturing, while Archaea have diverse shapes and live in extreme environments.

Endosymbiosis

Endosymbiosis refers to the origin of eukaryotic organelles that were once free-living bacteria incorporated into larger eukaryotic cells.

Fluid Mosaic Model

Proposed in 1972 by Singer & Nicolson, this model describes the plasma membrane as a mosaic of diverse components (phospholipids, proteins, carbohydrate chains) with fluidity allowing lateral movement.

Plasma Membrane Function

The plasma membrane separates the cell from the external environment and regulates the movement of materials; it is selectively permeable.

Types of Cell Transport

1. Passive Transport: does not require energy (Diffusion, Osmosis, Facilitated Diffusion). 2. Active Transport: requires energy (Active Transport, Vesicle Formation).

What is osmosis?

Osmosis is a type of diffusion that involves the movement of water across a selectively permeable membrane due to a concentration gradient.

Naming solutions for comparison

Isotonic: same solute concentration; Hypotonic: lower concentration of solute; Hypertonic: higher concentration of solute.

Filtration in the Excretory System

In filtration, small molecules exit the blood into the nephron due to blood pressure; it is not selective, allowing any small enough molecule to filter through.

ADH function

ADH (Antidiuretic hormone) increases water reabsorption in the kidneys, reducing urine output when blood osmolarity increases.

Aldosterone function

Aldosterone stimulates the reabsorption of sodium ions in the kidneys, leading to water retention and increasing blood pressure.

What is osmoregulation?

Osmoregulation is the maintenance of water and ion balance in the body, which is critical for homeostasis.

Cell Envelope Structure

The cell envelope in prokaryotic cells consists of a plasma membrane, a cell wall (which maintains shape), and a glycocalyx (a coating that prevents drying out).

Prokaryotic Appendages

Prokaryotes have flagella (for movement), fimbriae (bristles for attachment), and sex pili (for DNA exchange via conjugation).

Shapes of Bacteria

Bacteria can have three main shapes: bacillus (rod-shaped), coccus (spherical), and spirilla (spiral).

Cell Wall Composition in Bacteria

The cell wall of Domain Bacteria is made of peptidoglycan, while the cell wall of Domain Archaea is made of polysaccharides and protein, not peptidoglycan.

Eukaryotic Organelles

Eukaryotic cells contain various membrane-bound organelles, including mitochondria and chloroplasts, which resemble bacterial cells in size and appearance.

Plasma Membrane Components

The plasma membrane includes cholesterol (which stiffens and strengthens the membrane), peripheral proteins (for structure), and integral proteins (that can move laterally and span the membrane).

Fluidity of Plasma Membrane

At room temperature, the plasma membrane has the consistency of olive oil, where phospholipids can move laterally, but rarely flip-flop, allowing cellular flexibility.

Membrane Proteins Function

Integral proteins serve various roles, such as channel proteins (allowing molecules to pass through), carrier proteins (helping substances pass through), and receptor proteins (binding to specific molecules to signal changes in cell activity).

Selectively Permeable Plasma Membrane

The plasma membrane is selectively permeable; water and small non-charged molecules pass freely, while ions and polar molecules require carrier proteins to pass through.

Types of Passive Transport

Passive transport methods include diffusion, osmosis, and facilitated diffusion, which do not require energy.

Types of Active Transport

Active transport methods include sodium-potassium pump and proton pump, which require energy to move substances against their concentration gradient.

Vesicle Formation

Vesicle formation requires ATP and is used to transport macromolecules; types of vesicle transport include exocytosis and endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis).

Osmotic Pressure

Osmotic pressure is the pressure that develops in a system due to osmosis, influencing the movement of water across cell membranes.

Water Potential Equation

Water potential (Ψ) is calculated as Ψ = Ψs (solute potential) + Ψp (pressure potential), with pure water having a solute potential of 0.

Role of Kidneys in Osmoregulation

The kidneys control water excretion in urine, helping maintain water balance in the body through filtration, reabsorption, and secretion.

Hormonal Regulation of Water Balance

ADH and aldosterone are two hormones that regulate water and sodium balance in the body, affecting kidney function and urine output.