Cellular Organization

Homeostasis

• Dynamic state of balance within parameters that are compatible with life

• When a particular parameter gets too high or too low, disease or illness and sometimes death occur

Cell membrane

• Extremely pliable structure composed primarily of a bilayer of phospholipids

• Cholesterol + various proteins embedded within membrane giving membrane a variety of functions

Phospholipid bilayer

• Composed of two phospholipids

• Single phospholipid has phosphate group on one end, called the head, and two side by side chains of fatty acids that make up the lipid tails

• The tails of each phospholipid face each other

• One head faces outward to environment, and one faces inward into interior of cell

Receptor

Recognition protein that can selectively bind a specific molecule outside the cell → this binding induces a chemical reaction inside the cell

Integral protein

Embedded in the membrane

Channel protein

Integral protein that extends an opening through the membrane for ions to enter or exit the cell

Glycoprotein

Protein that has carbohydrate molecules attached

Glycocalyx

• Fuzzy appearing coating around the cell formed from glycoproteins and other carbohydrates attached to cell membrane

• Primary way a person’s immune defense cells know not to attack body’s own cells + may be reason why donated organs are rejected

Main ions

Ca++, Na+, K+, Cl-

Explain selective permeability in the cell membrane

• Only small, nonpolar materials can move through lipid bilayer (lipid tails are nonpolar)

• Ex. other lipids, oxygen, CO2, and alcohol

• Water-soluable materials (glucose, amino acids, electrolytes) need assistance to cross because they are repelled by hydrophobic tails

Active transport

Movement of substances across the membrane using energy from ATP

• Simple diffusion

• Explain this in terms of CO2 and O2

Molecules moving across membrane from high concentration to low

O2 diffuses into cells (more concentrated outside)

CO2 diffuses out of cells (more concentrated inside)

Facilitated diffusion

• Substances that cannot cross lipid bilayer due to size, charge and/or polarity

• Diffusion is facilitated by channels or carrier proteins

Osmosis

Diffusion of water through a semipermeable membrane from high to low water concentration.

Aquaporins

Protein channels that allow water to pass through cell membrane

Why is it important for cells to be in an environment where the concentration of solutes inside the cell matches outside? What is this called?

• Cells cannot regulate movement of water molecules across their membrane on their own

• Isotonic

Hypertonic solution

What happens to a cell in a hypertonic solution?

• Higher concentration of solutes

• Water tends to diffuse into hypertonic solutions

• Water leaves cell, causing it to shrivel

• Hypotonic solution

• What happens to a cell in a hypotonic solution?

• Solution that has lower concentration of solutes

• Water tends to diffuse out of hypotonic solutions

• Cell will take on too much water and swell or even burst

Hypertonic, isotonic, and hypotonic movement are examples of what type of transport?

Passive

Facilitated diffusion

• Passive transport through protein channels

• Down concentration gradient

• No ATP required

• Think of it like a slide

Primary active transport

• ATP required

• Against concentration gradient (low to high)

• Example: sodium-potassium pump

Sodium-potassium pump

The Na⁺/K⁺ pump uses 1 ATP to move 3 Na⁺ out and 2 K⁺ in, against their concentration gradients.

Functions:

• 🔋 Maintains resting membrane potential (keeps inside of cell negative)

• 🌊 Prevents cell swelling by controlling water movement

• 🚀 Creates Na⁺ gradient for secondary active transport (e.g., glucose absorption)

• 🔄 Resets ions after neurons and muscles fire

Summary:

It’s essential for cell stability, electrical function, and nutrient absorption. It quietly powers most of your body’s critical cell processes.

Secondary active transport

• Uses the gradient made by primary active transport to move other stuff

Symporter

• Secondary active transports that move two substances in the same direction

• Ex.: intestines use this to absorb glucose:

  • Na⁺ wants to rush into the cell (because of the Na⁺ gradient made by the sodium-potassium pump).

  • Symporter lets Na⁺ come in only if it brings glucose along for the ride.

  • Boom—glucose gets pulled in against its own gradient, without using ATP directly.

Antiporter

• Secondary active transport systems that transport substances in opposite directions

• Ex.: sodium-hydrogen ion antiporter

  • The Na⁺/H⁺ antiporter (a protein) opens and says:

    • “Alright Na⁺, you can come in—but only if I can use your energy to kick H⁺ out.”

  • Na⁺ moves into the cell (down its gradient)

  • → This movement releases energy

  • That energy is used to push H⁺ (protons) out of the cell (against their gradient)

Endocytosis

Process of cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane

Once pinched off, the portion of the membrane and its contents become an independent, intracellular vesicle

Exocytosis

Waste substances are packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases its contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane.

Ex: stomach and pancrease producing and secreting digestive enzymes

What are ion pumps used for?

Active transport

What is cytosol and its role?

The jelly-like fluid in cells that provides a medium for biochemical reactions

What is the main function of the endomembrane system?

Producing, packaging, and transporting cell products.

Difference between rough ER and smooth ER?

Rough ER has ribosomes for protein synthesis; smooth ER lacks ribosomes and synthesizes lipids, stores Ca++, and detoxifies.

What does the rough ER do?

Factory: studded with ribosomes (workers) that synthesize and modify proteins for the membrane or export —> sends them to the Golgi.

What are the key functions of the smooth ER?

Lab: Lipid and steroid hormone synthesis

Storage room: Ca++ storage

Carbohydrate metabolism (glycogen → glucose)

Cleaning room: detoxification.

What is the function of the Golgi apparatus?

Post office— Modifies, sorts, and packages proteins from the rough ER for delivery inside or outside the cell

What is the role of lysosomes?

Wrecking crew: contain digestive enzymes to break down damaged organelles, foreign particles, or initiate cell death (autolysis).

What is autophagy vs. autolysis?

Autophagy = self-digestion of parts; autolysis = lysosome-triggered cell death (apoptosis)

What is the function of mitochondria?

Convert nutrients into ATP (cellular energy) via cellular respiration.

Hepatocytes

Cells in the liver detoxify many of these toxins

Which cells have the most mitochondria and why?

Muscle and nerve cells—they require large amounts of ATP.

What are reactive oxygen species (ROS)?

Toxic byproducts like free radicals that can damage cells through oxidative stress.

How do antioxidants protect the cell?

They neutralize ROS by being oxidized themselves, stopping chain reactions of damage.

What are the three components of the cytoskeleton?

Microfilaments, intermediate filaments, and microtubules.

What are microfilaments made of and what are their functions?

Made of actin; they support cell shape, enable movement, and help in cell division and muscle contraction.

What do mitochondria do, and why do some cells have more?

• Mitochondria make ATP, the cell’s energy, through cellular respiration in the inner membrane folds called cristae.
  They need oxygen to work.
  Muscle and nerve cells have more mitochondria because they use more energy.

What do peroxisomes do?

• Peroxisomes break down lipids and detoxify harmful substances.
  They use enzymes to transfer hydrogen to oxygen, making hydrogen peroxide (H₂O₂), which helps neutralize poisons like alcohol.

What are reactive oxygen species (ROS or free radicals) and why are they harmful?

ROS (like H₂O₂, OH•, and O₂⁻) are unstable molecules with unpaired electrons that can damage DNA, proteins, and cell membranes, causing cell death, mutations, and diseases like cancer and Alzheimer’s.

How do peroxisomes protect cells from ROS?

Oversee reactions that neutralize free radicals

What is oxidative stress and how do antioxidants like fruits and veggies play a role?

• Oxidative stress happens when ROS outnumber the body’s defenses, damaging cells and speeding up aging and disease.

• Antioxidants (from fruits/veggies) help by being oxidized themselves to stop ROS chain reactions.

What are the three general phases of the cell cycle?

Interphase, mitosis (division of genetic material), cytokinesis (division)

What are the three subphases of the interphase stage of the cell cycle?

G0: cell not dividing, just doing its job

G1: growth phase

S: cell replicates DNA

G2: growth phase

Mitosis: division

4 stages of mitosis

Prophase, metaphase, anaphase, telophase

What leads to cancerous tumors?

Mutations or aberrations in the cell cycle control systems

Stem cell

Unspecialized; can divide without limit as needed and can differentiate into specialized cells