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The Cellular Level of Organization

Living things are constructed of cells.

Living things may be unicellular or multicellular.

Cell structure is diverse but all cells share common characteristics.

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The cell theory states:

All organisms are composed of one or more cells.

Cells are the basic unit of structure and function in organisms.

All cells come only from other cells.

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Eukaryotic Cells

All cells are surrounded by a plasma membrane made of phospholipids and proteins.

Eukaryotic cells have membrane bound intracellular organelles.

The most prominent is the nucleus that controls the workings of the cell.

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The plasma membrane regulates what enters and exits the cell.

Inside the plasma membrane, the nucleus is surrounded by cytoplasm.

Plant cells have a cell wall in addition to the plasma membrane.

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Animal and plant cells have organelles.

Organelles compartmentalize functions within the cell.

The organelles of animal and plant cells are similar to each other except that centrioles are present only in animal cells, and chloroplasts are present only in plant cells.

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Animal cell anatomy

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Plant cell anatomy

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Ribosomes

Protein synthesis occurs at tiny organelles called ribosomes.

Ribosomes are composed of a large subunit and a small subunit.

Ribosomes can be found alone in the cytoplasm, in groups called polyribosomes, or attached to the endoplasmic reticulum.

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The endoplasmic reticulum

The endoplasmic reticulum (ER) is a system of membranous channels and saccules.

Rough ER is studded with ribosomes and is the site of protein synthesis and processing.

Smooth ER lacks ribosomes and is the site of synthesis of phospholipids and the packaging of proteins into vesicles, among other functions.

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The Golgi apparatus

The Golgi apparatus consists of a stack of curved saccules.

The Golgi apparatus receives protein and also lipid-filled vesicles from the ER, packages, processes, and distributes them within the cell.

This organelle may also be involved in secretion.

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Lysosomes and vacuoles

Lysosomes are vesicles produced by the Golgi apparatus.

Lysosomes contain hydrolytic enzymes and are involved in intracellular digestion.

Vacuoles (large) and vesicles (small) are membranous sacs in the cell that store substances.

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Energy-Related Organelles

The two energy-related organelles of eukaryotes are chloroplasts and mitochondria.

Both organelles house energy in the form of ATP.

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Centrioles

Centrioles are short cylinders with a 9 + 0 pattern of microtubule triplets.

Centrioles may be involved in microtubule formation and disassembly during cell division and in the organization of cilia and flagella.