3 Cell Structure and Function

Cell foundations and cell types

The cell theory provides a foundation for understanding life: all living organisms are made of one or more cells, cells are the basic units of structure and function, and new cells come from pre-existing cells. Cells share several essential features: a plasma membrane, cytoplasm, DNA, and ribosomes. These common features support basic cellular processes, while differences in organization allow cells to perform specialized roles.

Cells are grouped by how their internal structures are organized:

• Prokaryotic cells occur in Bacteria and Archaea. They lack a membrane-bound nucleus, so their DNA is concentrated in a region called the nucleoid. They also lack the membrane-bound organelles typical of eukaryotic cells. Many have a cell wall; some also have a capsule, flagella, or pili.

• Eukaryotic cells occur in animals, plants, fungi, and protists. They have a membrane-bound nucleus and other membrane-bound organelles, which create compartments for specialized tasks. They are generally larger than prokaryotic cells.

Both cell types have ribosomes, which build proteins. Cell walls are not exclusive to prokaryotes: plants and fungi also have them, though their composition differs from bacterial cell walls.

Takeaway: Both cell types share core features, but the presence or absence of a membrane-bound nucleus and organelles distinguishes their organization.

How organelles divide cellular work

Eukaryotic organelles divide cellular work into specialized areas. The nucleus stores most of the cell’s DNA and regulates gene activity. Mitochondria carry out much of cellular respiration and produce much of the cell’s ATP. In plants and some protists, chloroplasts capture light energy for photosynthesis. The cytoskeleton helps maintain cell shape, organize internal structures, and enable movement.

Other structures help build, process, and move cellular products:

• Ribosomes build proteins. They may be free in the cytoplasm or attached to the rough endoplasmic reticulum.

• The rough endoplasmic reticulum makes and begins processing many proteins destined for membranes or export.

• The smooth endoplasmic reticulum makes lipids and performs other functions, including detoxification in some cells.

• The Golgi apparatus modifies, sorts, and packages proteins and lipids for delivery.

• Lysosomes digest and recycle materials in many animal cells.

• Vesicles and vacuoles store or transport materials. Plant cells commonly have a large central vacuole.

A protein destined for export illustrates how organelles work together: a ribosome attached to the rough endoplasmic reticulum builds the protein; the protein is processed through the ER and Golgi apparatus; then a vesicle packages it for release.

Takeaway: Organelles are specialized, but many cellular tasks depend on several organelles working in sequence.

Membranes as selective boundaries

The plasma membrane surrounds the cell, and membranes also enclose many eukaryotic organelles. The membrane consists mainly of a phospholipid bilayer with embedded proteins. Each phospholipid has a water-attracting head and water-repelling tails. The heads face the watery environments inside and outside the cell, while the tails point inward, away from water.

Membrane proteins help transport substances, receive signals, and perform other tasks. Because the membrane is selectively permeable, some substances cross readily while others need transport proteins or cannot cross directly. Small nonpolar molecules such as oxygen can diffuse through the lipid bilayer. Charged ions and many large or polar molecules generally require membrane proteins.

This selective boundary helps the cell maintain suitable internal conditions while exchanging materials with its surroundings.

Takeaway: The bilayer forms a boundary, and its embedded proteins help control interactions across that boundary.

How substances cross membranes

Substances cross the plasma membrane in different ways depending on their properties and the direction they move relative to a concentration gradient.

Passive transport moves substances down their concentration gradient, from higher concentration toward lower concentration, without the cell directly using energy for that movement.

• Simple diffusion allows small molecules to move through the lipid bilayer. For example, oxygen can diffuse into a cell where its concentration is lower.

• Facilitated diffusion uses a channel or carrier protein to help a substance cross the membrane down its gradient. Glucose, for example, can enter cells through carrier proteins.

• Osmosis is the diffusion of water across a selectively permeable membrane. If a cell is placed in a solution with a higher concentration of nonpenetrating solutes, water tends to leave the cell; in a more dilute solution, water tends to enter. The effect on cell size depends on the cell type and the surrounding solution.

Active transport moves substances against a concentration or electrochemical gradient and requires energy, often supplied directly or indirectly by ATP. Membrane pumps can maintain different ion concentrations on opposite sides of a membrane, and these gradients support many cellular processes.

Very large materials can cross the plasma membrane inside membrane-bound vesicles. In endocytosis, the membrane encloses material and brings it into the cell. In exocytosis, a vesicle fuses with the plasma membrane and releases its contents outside the cell. Both processes require cellular energy.

Takeaway: Passive transport moves substances down a gradient without direct energy use; active transport and vesicle transport require cellular energy.