Cell Structure, Membranes, and Transport

Prokaryotic and eukaryotic cells share basics

A typical cell has a cell membrane, cytoplasm, DNA, and ribosomes. A prokaryotic cell, such as a bacterium, has no membrane-bound nucleus. Its DNA is in the cytoplasm. A eukaryotic cell has a nucleus and other membrane-bound structures.

Plant and animal cells are eukaryotic. Plant cells usually have a cellulose cell wall, chloroplasts, and one large central vacuole. Animal cells do not. As a cell grows, its volume grows faster than its surface area. This can make exchange across the membrane less efficient.

Use defining features, not size alone. DNA inside a nucleus identifies a eukaryote. DNA in a nucleoid with no membrane-bound organelles identifies a prokaryote. Ribosomes do not separate the groups because both have them.

Organelles specialize cellular work

An organelle is a cell structure with a specific job. The nucleus stores DNA. Ribosomes build proteins. Mitochondria release usable energy from food. Lysosomes digest materials. The cytoskeleton supports shape and movement.

Some proteins leave the cell. A ribosome on the rough ER builds the protein. The ER helps fold it. A vesicle carries it to the Golgi apparatus, which sorts and packages it. Another vesicle carries it to the cell membrane for release.

Worked example. A cell makes a protein for export. Follow the path: DNA is read in the nucleus, a ribosome on rough ER builds the protein, the ER folds it, the Golgi sorts it, and a vesicle releases it. If the Golgi fails, packaging and sorting fail even though the DNA remains intact.

Do not overgeneralize the ER. Free ribosomes commonly make proteins used in the cytosol. Ribosomes attached to rough ER make many proteins destined for secretion, membranes, or parts of the endomembrane system. Smooth ER lacks attached ribosomes and is associated with lipid synthesis and other specialized tasks. Thus, “a protein is made” does not by itself prove rough ER involvement; the protein’s destination matters.

A selective membrane regulates exchange

The plasma membrane is a phospholipid bilayer. Its water-loving heads face outward, and its water-fearing tails face inward. This design lets small nonpolar molecules cross more easily than ions or many large polar molecules.

Membrane proteins have different jobs. Channels form openings. Carriers bind and move certain substances. Pumps use energy. Receptors receive signals. The membrane is selectively permeable, meaning some substances cross more easily than others.

A concentration gradient is a difference in concentration between two places. Moving down the gradient means moving from high to low concentration. Oxygen can cross the lipid layer directly. Charged sodium needs a protein path because the oily membrane interior blocks it.

Passive transport follows gradients; active transport builds them

Diffusion is net movement from higher to lower concentration. Facilitated diffusion does the same job through a channel or carrier. Neither process uses cellular energy. Osmosis is diffusion of water across a selectively permeable membrane.

Active transport uses energy to move a substance, often from lower to higher concentration. The sodium-potassium pump uses ATP. Endocytosis brings large material into a cell in a vesicle. Exocytosis releases vesicle contents. Example: sodium moving against its gradient through an ATP-powered pump is active transport.

Do not classify transport only by the presence of a protein. A substance moving high to low through a channel uses facilitated diffusion. A pump moving it low to high uses energy. At equilibrium, particles still move, but there is no net change.

Read transport evidence in order. First identify the substance and whether a membrane protein is used. Next compare its starting and ending concentrations. Movement down a gradient without cellular energy is passive, even when a channel is present. Movement against a gradient or bulk vesicle movement requires energy. A protein is therefore not automatic evidence of active transport.

Setup 1 is facilitated diffusion: movement is down the gradient and continues when ATP use is blocked. Setup 2 is active transport: movement is against the gradient through a pump.

Predict cell behavior from gradients

Tonicity describes how a solution changes cell water. In a hypotonic solution, water enters an animal cell and it may swell. In a hypertonic solution, water leaves and the cell shrinks. In an isotonic solution, water enters and leaves at equal overall rates.

Plant cell walls resist swelling, so water entry creates helpful pressure. Always ask: What can cross the membrane? Which way is its gradient? Is energy needed? These questions identify the transport process.

Only solutes that cannot readily cross create a lasting tonicity effect. If an animal cell contains more noncrossing solute than the fluid outside, water enters and the cell swells. If the outside has more, water leaves and the cell shrinks.

Separate concentration from tonicity. For water movement, compare the concentrations of solutes that cannot cross the membrane. Water moves toward the side with more nonpenetrating solute. If both sides have equal effective solute concentration, water still crosses in both directions, but cell volume has no net change.

Summary

• All cells have a membrane, cytoplasm, DNA, and ribosomes.
• A nucleus and membrane-bound organelles identify eukaryotic cells.
• Organelles perform specific jobs.
• Diffusion and osmosis are passive.
• Active transport uses energy.
• Tonicity predicts whether a cell gains or loses water.

Passive goes with the gradient; active transport needs energy. Water follows the side with more nonmoving solute.

Sources

• OpenStax Anatomy and Physiology 2e
• National Human Genome Research Institute educational resources
• NIH National Institute of General Medical Sciences science education
• OpenStax Anatomy and Physiology — Erythrocytes: specialized mature cells
• NCBI Bookshelf — The Endoplasmic Reticulum