specialised cells

• Erythrocytes (red blood cells) carry oxygen in the blood. The biconcave disc shape provides a large surface area for gas exchange. They have no nucleus so there's more room for haemoglobin, the protein that carries oxygen.
• Neutrophils (a type of white blood cell) defend the body against disease.

  Their flexible shape allows them to engulf foreign particles or pathogens. The many lysosomes in their cytoplasm contain digestive enzymes to break down the engulfed particles.
• Epithelial cells cover the surfaces of organs. The cells are joined by interlinking cell membranes and a membrane at their base. Ciliated epithelia (e.g. in the airways) have cilia that beat to move particles away.

  Other epithelia (e.g. in the small intestine) have microvilli - folds in the cell membrane that increase the cell's surface area. Squamous epithelia (e.g. in the lungs) are very thin to allow efficient diffusion of gases.
• Sperm cells (male sex cells) have a flagellum (tail) so they can swim to the egg (female sex cell). They also have lots of mitochondria to provide the energy to swim. The acrosome contains digestive enzymes to enable the sperm to penetrate the surface of the egg.
• structure of a erythrocyte

  A) large surface area

  B) no nucleus

  C) biconcave

  
• structure of a neutrophil

  A) flexible shape

  B) lots of lysosomes

  C) nucleus

  
• structure of a epithelial cell

  A) cilia

  B) microvilli

  C) cell membrane interlink

  D) nucleus

  
• structure of a sperm cell

  A) flagellum

  B) acrosome

  
• Palisade mesophyll cells in leaves do most of the photosynthesis.

  They contain many chloroplasts, so they can absorb a lot of sunlight.

  The walls are thin, so carbon dioxide can easily diffuse into the cell.
• Root hair cells absorb water and mineral ions from the soil. They have a large surface area for absorption and a thin, permeable cell wall, for entry of water and ions. The cytoplasm contains extra mitochondria to provide the energy needed for active transport
• Guard cells are found in pairs, with a gap between them to form a stoma. This is one of the tiny pores in the surface of the leaf used for gas exchange. In the light, guard cells take up water (into their vacuoles) and become turgid. Their thin outer walls and thickened inner walls force them to bend outwards, opening the stomata. This allows the leaf to exchange gases for photosynthesis.
• structure of a palisade mesophyll cell

  A) thin cell wall

  B) lots of chloroplasts

  
• structure of a root hair cell

  A) thin cell wall

  
• structure of guard cells

  A) cells turgid, stomata opens

  B) thickened inner walls

  C) thin outer walls

  D) vacuole

  E) cells flaccid, stomata closes