Organelles and the cell membrane
Inside a eukaryotic cell, organelles work like departments of a company. The membrane controls what enters and leaves.
The nucleus
The nucleus holds the cell's DNA and controls which genes are used. Its double membrane, the nuclear envelope, has pores that let messenger RNA out to the cytoplasm. Inside, the nucleolus builds the parts of ribosomes.
Ribosomes and the endoplasmic reticulum
Ribosomes read messenger RNA and build proteins. Rough endoplasmic reticulum (ER) is studded with ribosomes and makes proteins for export or for membranes. Smooth ER lacks ribosomes and makes lipids and breaks down some toxins; liver cells have lots of it.
The Golgi apparatus
The Golgi apparatus receives proteins from the ER, modifies them (for example by adding sugar chains), sorts them, and packages them into vesicles addressed to their destinations. It is the cell's post office.
Mitochondria
Mitochondria carry out cellular respiration, releasing energy from food to make ATP, the cell's energy currency. They have their own DNA and ribosomes, a clue to their origin as free-living bacteria engulfed long ago. Muscle cells have many mitochondria.
Lysosomes, vacuoles, and the cytoskeleton
Lysosomes contain digestive enzymes that break down worn-out parts and engulfed material. Vacuoles store water and substances. The cytoskeleton, a network of protein fibres, gives the cell its shape, anchors organelles, and moves materials and the whole cell.
The cell membrane
The membrane is a phospholipid bilayer: phospholipids have water-loving heads facing outward and water-fearing tails tucked inside. Proteins float in it, some acting as channels or pumps, others as receptors or identification tags. This fluid mosaic model describes a flexible, moving structure.
Passive transport
Diffusion moves substances from high to low concentration without energy. Small nonpolar molecules like oxygen pass straight through the bilayer; ions and larger polar molecules use channel or carrier proteins (facilitated diffusion). Osmosis is the diffusion of water across a membrane toward the side with more dissolved solute.
Active transport and bulk transport
Active transport uses energy, usually ATP, to pump substances against their concentration gradient. The sodium-potassium pump keeps nerve cells ready to fire. Large particles enter by endocytosis, the membrane wrapping around them, and leave by exocytosis, vesicles fusing with the membrane.
Key takeaways
The nucleus stores DNA; ribosomes make proteins; the ER and Golgi process and ship them; mitochondria make ATP; lysosomes digest. The membrane is a fluid phospholipid bilayer with proteins. Diffusion and osmosis need no energy; active transport and bulk transport do.
Flashcard
Holds DNA and controls the cell
Check your understanding
A red blood cell placed in pure water swells and may burst. Why?