Untitled Quiz

Animal Cell Structure

• All cells are enclosed by a plasma membrane, acting as a selective barrier.
• The membrane controls which substances enter and leave the cell.
• Nutrients enter the cell, while waste products leave.
• The cell's interior is organized into specialized compartments called organelles, each surrounded by its own membrane.
• Examples of organelles include the nucleus, mitochondria, lysosomes, endoplasmic reticulum, and Golgi apparatus.
• Organelles are suspended in the cytoplasm, also known as the cytosol.

Plasma Membrane

• A thin membrane, approximately 0.005 µm thick.
• Encloses every living cell, separating it from the environment.
• Acts as a barrier, regulating the passage of substances in and out of the cell.
• Controls the entry of nutrients and the exit of waste products.

Chemical Composition of Plasma Membrane

• Composed of lipids and proteins.
• Lipids include phospholipids and sterols (e.g., cholesterol).
• Proteins include peripheral (extrinsic) proteins and integral (intrinsic) proteins.

Phospholipid Bilayer

• The fundamental structure of the plasma membrane.
• Composed of two layers of phospholipid molecules.
• Phospholipids have a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails.
• The hydrophobic tails face inward, while the hydrophilic heads face outward, interacting with the surrounding water.

Sterols

• Found in the plasma membrane.
• Have a complex ring structure, making them lipid-soluble.
• Contain a hydroxyl group, making them water-soluble.
• Cholesterol is a common type of sterol.

Membrane Proteins

• Intrinsic (integral) proteins are firmly embedded within the lipid bilayer.
• Some span the entire bilayer, forming channels or pores.
• Others are embedded in one leaflet, interacting with the lipid bilayer.
• Peripheral (extrinsic) proteins are loosely attached to the lipid bilayer.
• Their attachment is typically to ionic bonds or calcium bridges, and they may also attach other intrinsic proteins.

Transport Across the Plasma Membrane

• Substances cross the membrane via different methods.
• Permeation (simple diffusion) is the direct movement of substances across the membrane.
• Facilitated diffusion aids in the movement of substances, but doesn't require energy.
• Active transport moves substances against their concentration gradient, requiring energy, often from the hydrolysis of ATP.
• Types of transport include channel-mediated, carrier-mediated, and transport of particles.

Simple Diffusion

• Substances move from an area of high concentration to an area of low concentration.
• Lipid-soluble and small substances can readily permeate the membrane.
• Large, water-soluble, and charged substances are blocked due to the hydrophobic nature of the lipid bilayer.

Facilitated Diffusion

• Channels are integral proteins that provide pathways for ions and small molecules.
• These channels may be gated, opening and closing based on specific conditions.
• Many molecules, such as sugars and amino acids require transporter proteins, a type of carrier-mediated protein, which helps these substances fit through the bilayer.

Active Transport

• The movement of substances against their concentration gradient (from low to high concentration).
• Requires energy (often from ATP).
• Sodium-potassium pumps are a well-known example of active transport; maintaining gradients of sodium and potassium across the cell membrane.