Thousands of tiny mitochondria can be found in eukaryotic cells with high energy needs. For instance, mitochondria occupy 40 percent of the cytoplasm of a heart muscle cell, according to the British Society for Cell Biology. Through the process of cellular respiration (oxidative phosphorylation), mitochondria use oxygen and metabolize food energy to generate readily accessible ATP molecules that power up the cell. Athletes depend on plentiful mitochondria in their muscle cells for peak performance.

Muscle cells (myocytes) are snug bundles of microfibrils with specialized endoplasmic reticulum (sarcoplasmic reticulum). Muscle cells connect to form long muscle fibers. Muscles of an organism push, pull and contract in response to nerve cell stimulus from the brain or autonomic nervous system. Mitochondria are interspersed throughout the muscle cell to continually supply the cell with ATP molecules.

A muscle cell diagram looks quite unlike other types of cells in the human body because cell shape relates to cell function. Organelles of the muscle cell are also named slightly differently: the plasma membrane is called sarcolemma; the cytoplasm is sarcoplasm, and the endoplasmic reticulum is sarcoplasmic reticulum. Skeletal muscle cells have many nuclei along their membrane. The center of the cell contains alternating bands of proteins (myofibrils) that contract when nerve signals reach the cell.

Muscle tissue is made up of long, thin, cylindrical muscle cells containing closely packed organelles. Cells may be multi-nucleated and share cytoplasm. Numerous mitochondria are found in each muscle cell to provide metabolic energy for muscle contraction. The endoplasmic reticulum assists mitochondria in filtering molecules and maintaining homeostasis.

Mitochondria are essential organelles enclosed in a double membrane that has their own maternally inherited DNA. The outer membrane layer filters out large molecules. The inner membrane layer has several folds, called cristae, imbedded with proteins that transport molecules involved in ATP production. Eukaryotic cells can contain anywhere from one mitochondrion to thousands of mitochondria in their cytoplasm.

Recent studies suggest that mitochondria functions as a power plant by producing and distributing energy across a power grid, as reported by the National Institutes of Health. Mitochondria occur in proportion to cell function and purpose. For example, the plentiful mitochondria in muscle cells enable an organism to react quickly, which can be particularly helpful when fleeing a predator.

As the name implies, skeletal muscle is comprised of highly specialized cells that move the skeleton and certain other body parts like the tongue. Skeletal muscle is voluntary, meaning the brain can consciously signal when and how to move the arm to reach a library book on a shelf, for instance. Skeletal cells are uniquely structured to contract quickly and forcibly, as needed.

The two types of skeletal muscles are slow-twitch and fast-twitch. Slow-twitch muscles are reddish fibers that metabolize aerobically and contract continually to steadily perform such tasks as standing for hours or running a marathon. Mitochondria organelles and oxygen-binding molecules (myoglobin) are abundant in the cell.

Fast-twitch muscles may be further subdivided according to the amount of mitochondria and myoglobin present in muscle fiber. Muscle fibers with lots of mitochondria and myoglobin use aerobic respiration for energy, whereas muscles with fewer mitochondria use glycolysis. Fast-twitch muscles enable dramatic bursts of energy for activities like competitive sprinting.

Elongated smooth muscle contracts involuntarily under the influence of hormones, metabolites and the autonomic nervous system. Found in the digestive tract, ducts, arteries and lymph vessels, smooth muscle cells contract together. Smooth muscle cells have one centrally located nucleus like most other somatic cells.