Cell Structure and Function

• Cells take in nutrients to grow, divide, and create needed materials.
• Cell structures like the cell membrane, nucleus, cytoplasm, chloroplasts, and mitochondria relate to basic cell functions.
• Cells organize into tissues, tissues into organs, organs into systems, and systems into organisms.
• Tissues, organs, and organ systems provide cells with oxygen, food, and waste removal.

Key Terms

• Concentration is the amount of solute in a solution.
• Solute refers to the dissolved substance in a solution.
• Solution is a mixture where two or more substances mix evenly.
• Concentration gradient is the gradual difference in solute concentration between two regions in a solution.

Cell Membrane and Cell Wall

• All cells have a cell membrane, which consists of proteins and lipids.
• Some cells have both a cell membrane and a cell wall, such as plants, fungi, and bacteria.
• Plant cells feature a cell wall made of cellulose, which is a fiber.
• Bacteria and fungi have cell walls but do not contain cellulose.
• Cell membranes and cell walls are porous, which helps water, carbon dioxide, oxygen, and nutrients pass through easily.

Function of the Cell Membrane

• The cell membrane separates the components of a cell from its surrounding environment.
• The cell membrane is a "gatekeeper," it regulates the flow of materials in and out of the cell.
• It is selectively permeable.
• The cell membrane helps cells maintain homeostasis to stabilize its internal balance.

Passive vs Active Transport

• Passive transport doesn't use energy.
• Active transport requires energy.

Types of Cellular Transport

• Passive transport doesn't use energy and includes:

  • Diffusion
  • Facilitated Diffusion
  • Osmosis

• Active transport uses energy, and this includes:

  • Protein Pumps
  • Endocytosis
  • Exocytosis

Diffusion

• Diffusion involves small particles moving across a selectively permeable membrane, like the cell membrane until balance is reached.
• Particles move from an area of high concentration to an area of low concentration.

Osmosis

• Osmosis involves water diffusing through a selectively permeable membrane, like the cell membrane
• Water diffuses across a membrane from high to low concentration.

Facilitated Diffusion

• Facilitated diffusion involves large molecules like glucose moving through the cell membrane.
• These molecules need assistance because of their size.
• Proteins in the cell membrane form channels for large molecules to pass through.
• Proteins that form channels or pores, are protein channels.

Hypertonic Solutions

• Hypertonic solutions have a high solute concentration relative to another solution, like a cell's cytoplasm.
• In a hypertonic solution, water goes out of the cell, causing it to shrivel.

Hypotonic Solutions

• Hypotonic solutions have a low solute concentration relative to another solution, like a cell's cytoplasm.
• In a hypotonic solution, the water goes into the cell, causing it to swell and potentially explode.

Isotonic Solutions

• Isotonic solutions have the same solute concentration as another solution, like a cell's cytoplasm.
• In an isotonic solution, water diffuses into and out of the cell at the same rate.
• The fluid surrounding body cells is isotonic.

Active Transport

• Active transport moves molecules from low to high concentration.
• It requires molecules to be pumped against the concentration gradient.
• Proteins that work as pumps are known as protein pumps.
• For example, body cells pump carbon dioxide into the blood vessels and into the lungs so energy moves it across the cell membrane from low to high concentration.

Endocytosis and Exocytosis

• Endocytosis and exocytosis are mechanisms for very large molecules, like food and wastes, to enter and exit the cell.
  • Food moves into the cell by endocytosis.
  • Wastes move out of the cell by exocytosis.
• White blood cells use endocytosis to surround and engulf bacteria.

Exocytosis

• Exocytosis forces material out of a cell in bulk.
• The membrane fuses with the cell membrane.
• The cell changes shape, which requires energy.
• Examples include hormones or wastes released from a cell.

Effects of Osmosis

• Osmosis is the diffusion of water through a selectively permeable membrane.
• Water is so small and there is so much of it, so the cell cannot control its movement through the cell membrane.

Hypotonic Solution

• Has a lower concentration of solutes and a higher concentration of water than inside the cell.
• High water, so the cell swells and bursts open (cytolysis).

Hypertonic Solution

• Has a higher concentration of solutes and a lower concentration of water than inside the cell.
• High solute, so the cell shrinks (plasmolysis).

Isotonic Solution

• The concentration of solutes in the solution is equal to the concentration of solutes inside the cell.
• Water moves equally causing a dynamic equilibrium.

Osmotic Pressure

• Bacteria and plants have cell walls to prevent over-expansion.
  • Plants exert pressure on the cell wall called tugor pressure.
• Protists like paramecium use contractile vacuoles to collect water, pump it out and prevent over-expansion.
• Saltwater fish pump salt out of specialized gills to prevent dehydration.
• Animal cells are bathed in blood, and kidneys maintain isotonic blood by removing excess salt and water.

Cells require nutrients for growth and division. Their structures, including the cell membrane, nucleus, and cytoplasm, are essential for basic functions. Cells are organized into tissues, organs, and systems, ensuring oxygen, food, and waste removal.