Biology Chapter: The Cell and Plasma Membrane

The Cell

• Is the basic, living, structural and functional unit of the body
• Can be divided into 4 main parts:
  • Plasma (cell) Membrane
  • Cytoplasm
  • Organelles
  • Inclusions

Plasma Membrane

• Separates cell’s internal parts from the extracellular fluid and the external environment
• Is an exceedingly thin structure (6.5–10 nm in thickness).
• Is composed of:
  • Phospholipids
  • Proteins
  • Cholesterol
  • Glycolipids
  • Water
  • Carbohydrates
  • Ions
• Phospholipids are arranged in two parallel rows forming a lipid bilayer, and consist of a hydrophilic head-group and 2 long hydrophobic hydrocarbon chains
• Glycolipids are always located on the outer surface of the plasma membrane and have an oligosaccharide in the hydrophilic head projecting like hair into the surroundings
• Proteins are the main functional elements embedded in the lipid matrix, some extend through the whole membrane while others are anchored in the outer or inner layer
• Cholesterol molecules are almost equal in number to the phospholipids and are interspersed among the phospholipids, they stabilize the cell membrane

Functions of the Plasma Membrane

• Flexible boundary that encloses the cellular contents and separates them from the external environment
• Facilitates contact with other body cells or with foreign cells or substances
• Provides receptors for chemicals such as hormones, neurotransmitters, enzymes, nutrients and antibodies
• Mediates the entrance and exit of certain substances and restricts the passage of other substances

Plasma Membrane Permeability

• Is the selective permeability of the plasma membrane
• Is due to:
  • Size of molecules
  • Solubility in lipids
  • Charge on ions
  • Presence of carrier molecules

Cytoplasm

• Is the substance inside the cell’s plasma membrane and external to the nucleus
• Is the matrix or ground substance of the cell
• Is a thick, semitransparent, elastic fluid containing the cytoskeleton
• Consists of:
  • Water (75-90% of cytoplasm)
  • Proteins
  • Carbohydrates
  • Lipids
  • Inorganic substances

Functions of the Cytoplasm

• Site of synthesis of new substances
• Site where chemicals are packaged for transport to other parts of the cell
• Facilitates the excretion of waste materials

Organelles

• Are specialised portions of the cell that assume roles in:
  • Growth
  • Maintenance
  • Repair
  • Control
• Include:
  • Nucleus
  • Ribosomes
  • Endoplasmic Reticulum
  • Golgi Complex
  • Mitochondria
  • Lysosomes
  • Centrosome and Centrioles

Nucleus

• Is usually round or oval
• Is the largest structure in the cell
• Is the “control centre” of the cell
• Is separated from the cytoplasm by a nuclear membrane, which contains several small pores
• Contains the genetic material DNA as well as RNA that controls protein synthesis

Ribosomes

• Are tiny granules consisting of a type of RNA called ribosomal RNA (rRNA)
• May be found on the rough endoplasmic reticulum or scattered within the cytoplasm
• Are the sites of protein synthesis

Endoplasmic Reticulum

• Consists of a system of parallel membranes surrounding a network of channels
• Is continuous with the nuclear membrane
• There are 2 types of ER:
  • GRANULAR (rough) ER dotted with ribosomes
  • AGRANULAR (smooth) ER without ribosomes

Functions of ER

• Provides a surface area for chemical reactions to occur
• Acts as a transport channel through which various substances may pass from one part of the cell to another
• Serves as a storage site for synthesised molecules
• Plays a role in the packaging of molecules together with the Golgi Complex.

Golgi Complex

• Usually situated near the nucleus
• Is made up of four to eight flattened membranous sacs stacked upon one another

Functions of the Golgi Complex

• To process, sort and deliver proteins to various parts of the cell.
• Lipid packaging and secretion.

Mitochondria

• Are sausage-shaped structures scattered throughout the cytoplasm
• Each mitochondrion consists of two membranes:
  • The outer membrane – smooth
  • The inner membrane – folded upon itself forming cristae that provide a very large surface area where chemical reactions can take place.
• Provide energy for the cell by producing ATP

ATP Production

• Glucose
  • ↓ GLYCOLYSIS
  • Pyruvic Acid + fatty acids & amino acids
  • ↓
  • Acetyl-CoA
  • ↓
  • Krebs' Cycle
  • ↓
  • ADP  ATP

Lysosomes

• Are small spherical structures dispersed throughout the cytoplasm
• Contain powerful digestive enzymes
• Their function is to remove the debris from within the cell

Centrosome & Centrioles

• The centrosome is a dense area of the cytoplasm located near the nucleus
• It contains a pair of cylindrical structures called centrioles
• These play a part in spindle formation during cell division

Cell Inclusions

• Examples include:
  • Melanin: a pigment stored in cells of the skin, hair and eyes which protects the body by screening out harmful ultraviolet rays from the sun
  • Glycogen: a polysaccharide stored in the liver, skeletal muscle cells and the vaginal mucosa which is broken down by the liver into glucose.

Passive Processes

• Diffusion is the net movement of molecules or ions from an area of higher to lower concentration.
  • This occurs until equilibrium is reached, where movement is equal in both directions.
  • The difference between high and low concentrations is called the concentration gradient.
  • Substances can diffuse through the cell membrane in two ways:
    • By dissolving in the lipid bilayer and diffusing through it, such as oxygen, carbon dioxide, fatty acids, and alcohol.
    • By diffusing through channels within the integral proteins of the membrane, such as water and ions like sodium, potassium, and chloride.

Facilitated Diffusion

• Facilitated diffusion is the movement of larger molecules across a selectively permeable membrane with the help of integral proteins acting as carriers.
  • Large molecules like glucose are not soluble in lipids and cannot diffuse through the membrane on their own.
  • They combine with a carrier to become soluble in the phospholipid bilayer of the membrane.
  • This allows glucose to pass through the membrane from a high concentration to a lower concentration.
  • No energy is used in this process.

Osmosis

• Osmosis is the net movement of water molecules across a selectively permeable membrane from an area of higher to lower water concentration.
  • This happens until equilibrium is reached.
  • Water molecules pass through channels in integral proteins within the membrane.
  • The passage of water through a selectively permeable membrane generates osmotic pressure.
  • Osmotic pressure is the pressure required to prevent the movement of pure water into a solution containing solutes when separated by a selectively permeable membrane.
  • Higher solute concentration leads to higher osmotic pressure.
  • Osmotic pressure is crucial for water movement between body compartments.

Isotonic, Hypotonic, and Hypertonic Solutions

• Isotonic Solutions: These have the same total concentration of water molecules and solute molecules on both sides of the cell membrane.
• Hypotonic Solutions: These have a lower concentration of solutes and therefore a lower concentration of water. Water molecules enter the cell faster than they leave, causing the cell to swell.
• Hypertonic Solutions: These have a higher concentration of solutes and a lower concentration of water. Water molecules leave the cell faster than they enter, causing the cell to shrink.

Filtration

• Filtration is the movement of solvents like water and solutes like glucose across a selectively permeable membrane due to gravity or hydrostatic pressure.
  • This occurs from a higher to lower pressure area.
  • Most small to medium-sized molecules can be forced through the cell membrane.
  • Example: Filtration occurs in the kidneys where blood pressure forces water and small molecules through tiny blood vessels into kidney tubules.
  • Protein molecules are retained, while harmful substances like urea are eliminated.

Dialysis

• Dialysis is the diffusion of solute particles across a selectively permeable membrane where small molecules are separated from larger ones.

Active Processes

• Active transport is the movement of substances, usually ions, across a selectively permeable membrane from a lower to higher concentration.
  • This requires energy expenditure in the form of ATP.
  • It involves interaction with integral proteins in the membrane.

Active Transport of Sodium and Potassium (Sodium-Potassium Pump)

• This mechanism actively transports sodium from inside the cell to the outside and potassium from the outside to the inside.
  • Present in all cell membranes, it uses ATP as its energy source.
  • It typically transports 3 sodium ions to the outside for every 2 potassium ions transported to the inside.
  • This creates an electrical potential across the membrane with negativity inside and positivity outside.
  • The sodium-potassium pump is an electrogenic pump.

Endocytosis

• Endocytosis is the movement of large molecules and particles through plasma membranes where the membrane surrounds the substance, encloses it, and brings it into the cell.
  • Two types of endocytosis:
    • Phagocytosis: The cytoplasm extends outwards to form pseudopodia, which surround the object, enclose it within the cell membrane, and form a phagocytic vesicle. This vesicle then detaches from the cell membrane, and its contents are digested.
    • Pinocytosis: The engulfed material is liquid instead of solid. A small droplet of extracellular fluid is attracted to the cell membrane, which folds around it and forms a pinocytic vesicle. This vesicle then detaches from the cell membrane and travels into the cell.

Exocytosis

• Exocytosis is the export of substances from the cell by reverse endocytosis.
  • Commonly occurs in secretory cells.

Explore the fundamental unit of life in this quiz focused on the cell's structure and the plasma membrane. Discover how different components like phospholipids and proteins contribute to the cell's functionality. Test your understanding of cellular organization and membrane composition.